Methods for identifying compounds for inhibition of neoplastic lesions

Abstract: There is disclosed a pharmaceutical composition for the treatment of neoplasia, comprising a pharmaceutically acceptable carrier and a compound selected by: determining the cyclooxyegenase (COX) inhibitory activity of the compound; determining the PDE inhibition activity of the compound wherein the PDE is characterized by: (a) cGMP specificity over cAMP; (b) positive cooperative kinetic behaviour in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase; and selecting the compound that has COX inhibitory activity lower than said PDE activity for treating neoplasia. Also provided by the invention is a method for selecting a compound the treatment of neoplasia, comprising determining the cyclooxygenase (COX) inhibitory activity of the compound; determining the PDE2 inhibition activity of the compound; and selecting the compound that has COX inhibitory activity lower than said PDE activity for treating neoplasia.

Description: BACKGROUND OF THE INVENTION This invention relates to the use of one or more forms of phosphodiesterase type 2 ("PDE2") and phosphodiesterase type 5 ("PDE5") and/or protein kinase G to identify compounds useful for the treatment and prevention of pre-cancerous and cancerous lesions in mammals. Currently, non-surgical cancer treatment involves administering one or more highly toxic chemotherapeutics or hormonal therapies to the patient after her cancer has progressed to a point where the therapeutic benefits of chemotherapy/hormonal outweigh its very serious side effects. Such side effects are well known to any oncologist, and vary from drug to drug. However, standard chemotherapeutics are typically used only for short periods of time, often alternating chemotherapy with periods off treatment, so as not to overwhelm the patient with drug side effects. Thus, given the risk-benefit trade-off, side effects typically preclude starting chemotherapy when patients exhibit precancerous lesions, or continuing chemotherapy or hormonal therapy on a chronic basis after frank cancer has been eliminated in an attempt to prevent its re-occurrence. Beginning a decade or so ago, a glimmer of hope began to appear from an unexpected source: non-steroidal anti-inflammatory drugs ("NSAIDs"). Cancer and precancer research is replete with publications that describe various biochemical molecules that are over-expressed in neoplastic tissue, leading one group after another to research whether specific over-expressed molecules are responsible for the disease, and whether, if such over-expression were inhibited, neoplasia could be alleviated. For example, in familial adenomatous polyposis ("FAP"), Waddell in 1983 (Waddell, W.R. et al., "Sulindac for Polyposis of the Colon," Journal of Surgical Oncology , 24 :83-87, 1983) hypothesized that since prostaglandins were over-expressed in such polyps, non-steroidal anti-inflammatory drugs ("NSAIDs") should alleviate the condition because NSAIDs inhibited prostaglandin synthetase (PGE 2 ) activity. Thus, he administered the nonsteroidal anti-inflammatory drug ("NSAID") sulindac (an inhibitor of PGE 2 ) to several FAP patients. Waddell discovered that polyps regressed and did not recur upon such therapy. PGE 2 inhibition results from the inhibition of cyclooxygenase (COX) by NSAIDs. The success by Waddell with sulindac and the PGE 2 /COX relationship seemingly confirmed the role of two other biochemical targets --PGE 2 and COX-- in carcinogenesis, and the subsequent literature reinforced these views. The glimmer of hope for patients suffering from neoplasia was that sulindac certainly exhibited far fewer side effects than conventional chemotherapeutics or hormonals, and opened up the possibility of treating cancer at earlier stages of the disease, and for longer periods of time as compared with conventional chemotherapeutics. However, such a hope had to be tempered with the open question of whether a compound such as sulindac could be used to treat frank cancer, given that Waddell had only administered sulindac to patients with a pre-cancerous condition, FAP. That hope was also tempered by NSAIDs own sets of side effects. Sulindac and other NSAIDs when chronically administered, aggravate the digestive tract where PGE 2 plays a protective role. In addition, when taken chronically, they exhibit side effects involving the kidney and interference with normal blood clotting. As Waddell unfortunately experienced, some of his sulindac patients stopped taking drug because of side effects (see Waddell, W.R. et al., "Sulindac for Polyposis of the Colon," The American Journal of Surgery , 157: 175-79, 1989), most likely returning to additional surgical interventions to control polyp formation. Thus, for neoplasia patients, such drugs are not a practical chronic treatment, e.g., for FAP, sporadic polyps or men post-prostatectomy with rising PSAs (a rising PSA in such men indicates the recurrence of disease, which may not yet present as a frank, visible cancer). These side effects also limit NSAIDs' use for any other neoplasia indication requiring long-term drug administration. More recently, some have suggested that the COX-2 specific NSAIDs such as celecoxib be used. However, the renal and other side effects of such compounds are believed to limit the dosing and length of treatment with such compounds for long-term anti-neoplastic indications. In addition, recently published data indicate that very high doses are needed for drugs like celecoxib to achieve a marginal effect on colon polyps in only pre-defined regions of the colorectum. Perhaps more significant to colon cancer treatment is that it has been reported that certain colonic neoplasias (e.g., HCT-116) do not express COX-2, and that such inhibitors are ineffective against such neoplasias (see, Sheng, et al., "Inhibition of Human Colon Cancer Cell Growth By Selective Inhibition of Cyclooxygenase-2," J. Clin. Invest., 99(9):2254-9, 1997). Recent discoveries have lead scientists away from the COX/PGE 2 targets, since those targets may not be the primary (or perhaps even secondary targets) to treat neoplasia patients successfully on a chronic basis. Pamukcu et al., in U.S. Patent No. 5,401,774, disclosed that sulfonyl compounds, that have been reported to be practically devoid of PGE2 and COX inhibition (and therefore not NSAIDs or anti-inflammatory compounds) unexpectedly inhibited the growth of a variety of neoplastic cells, including colon polyp cells. These sulfonyl derivatives have proven effective in rat models of colon carcinogenesis, and one variant (now referred to as exisulind) has proven effective in human clinical trials with FAP patients, and even more remarkably has shown effect in a frank cancer: prostate cancer itself, in a controlled clinical study presented below. Furthermore, very recent research has convincingly established that COX I and/or COX II are not expressed substantially in all neoplasias, diminishing the hope that a COX I or COX II specific inhibitor would be broadly therapeutically useful in neoplasia treatment (see, Lim et al., "Sulindac Derivatives Inhibit Growth and Induce Apoptosis in Human Prostate Cancer Cell Lines," Biochem. Pharmacology, Vol. 58, pp. 1097-1107 (1999) in press). Thus, like so many other proteins over-expressed in neoplasias, PGE 2 /COX over-expression may not be a cause of some neoplasias, rather a consequence of some of them. But the combination of such discoveries, however, has raised the question about how do compounds such as exisulind (that have a range of activity against both COX and non-COX expressing neoplasias) act? What do such compounds do to neoplastic cells? Piazza, et al. (in U.S. Patent Application Nos. 08/866,027 and 09/046,739) discovered that compounds (such as exisulind) inhibited cyclic-specific GMP phosphodiesterase (e.g., PDE5), and that other such compounds could be screened using that enzyme, which could lead to the discovery of still other compounds that could be developed and formulated into anti-neoplastic pharmaceutical compositions. Such pharmaceutical compositions can be highly anti-neoplastic, and can be practically devoid of side effects associated with conventional chemotherapeutics, or even the side effects of COX or PGE2 inhibition, if one wanted to avoid such side effects. In addition, anti-neoplastic cGMP-specific PDE-inhibiting compounds can induce apoptosis (a form of programmed cell death or suicide) in neoplastic cells, but not in normal cells. Thus, such new compounds have become referred to as a new class of antineoplastics known as selective apoptotic anti-neoplastic drugs ("SAANDs"). Accordingly, SAANDs have challenged several matters of conventional wisdom: (1) that anti-neoplastic compounds cannot be effective without also killing normal cells; (2) that COX's are responsible for neoplasia; and (3) that prevention of colonic neoplasia by NSAIDs is likely mediated by the inhibition of one or both types of COX. New research presented below has, however, shown that not all compounds exhibiting classic PDE5 inhibition induce apoptosis in neoplastic cells. For example, the well-known PDE5 inhibitors, zaprinast and sildenafil, do not singly induce apoptosis, or even inhibit neoplastic cell growth in our hands. However, because pro-apoptotic PDE5 inhibitors induced apoptosis selectively (i.e., in neoplastic but not in normal cells), and could do so without substantial COX inhibition, the usefulness of PDE5 as a screening tool for desirable anti-neoplastic compounds is unquestioned. However, an enhancement to the PDE5 screening method to find anti-neoplastic, pro-apoptotic but safe compounds is desirable so that new pharmaceutical compositions can be formulated for therapeutic use in the treatment of neoplasia, including pre-cancer and cancer. SUMMARY OF THE INVENTION In the course of researching why some PDE5 inhibitors singly induced apoptosis while others did not, we uncovered a form of cyclic GMP-specific phosphodiesterase activity, not previously described. This new phosphodiesterase activity was previously uncharacterized.. Without being limited to a specific theory, we believe this novel PDE activity may be a novel conformation of PDE2 that substantially lacks cAMP-hydrolyzing activity, i.e. it is cGMP-specific. Classic PDE2 is not cGMP-specific (it also hydrolyzes cAMP), classic PDE2 is also found in neoplastic cells. This new PDE and PDE2 are useful in screening pharmaceutical compounds for desirable anti-neoplastic properties. Basically, in neoplastic cells when PDE5 and the PDE2 activity (in its novel and conventional conformations) are inhibited by an anti-neoplastic PDE5-inhibiting compound, the result is apoptosis. When only PDE5 is inhibited (but not the several forms of PDE2), apoptosis does not occur. In its broadest aspects, this new PDE conformation has activity characterized by: (a) cGMP specificity over cAMP (b) positive cooperative kinetic behavior in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase Other characteristics of this novel PDE include: it has reduced sensitivity to inhibition by zaprinast and E4021, it can be separated from classical PDE5 activity by anion-exchange chromatography, it is not activated by calcium/calmodulin, and it is insensitive to rolipram, vinpocetine and indolidan. Another embodiment of this invention involves evaluating whether a compound causes an increase in cGMP-dependent protein kinase G ("PKG") activity and/or a decrease of β-catenin in neoplastic cells. It has been found that unexpected characteristics of SAANDs include the elevation of PKG activity and a decrease in β-catenin in neoplastic cells exposed to a SAAND. We believe that the elevation of PKG activity is due at least in part by the increase in cGMP caused by SAANDs inhibition of the appropriate PDEs, as described above. The other characteristics of SAANDs are (1) inhibition of PDE5 as reported in the '694 patent above, (2) inhibition of the novel cGMP-specific PDE conformation, (3) inhibition of PDE2; (4) the fact that they increase intracellular cGMP in neoplastic cells, and (5) the fact that they decrease cAMP levels in some types of neoplastic cells. Thus, one embodiment of the novel method of this invention is evaluating whether a compound causes PKG activity to elevate in neoplastic cells and whether that compound inhibits PDE5. Another embodiment of the novel screening method of this invention is evaluating whether a compound that causes PKG activity to elevate in neoplastic cells and whether that compound inhibits the novel cGMP-specific PDE described above and/or PDE2. Still a third embodiment is evaluating whether a compound causes PKG activity to elevate in neoplastic cells and whether that compound causes cGMP to rise in neoplastic cells and/or causes cAMP levels to fall. Compounds successfully evaluated in such fashions have application as SAANDs. Among other things, this invention relates to novel in vitro and in vivo methods for selecting compounds for their ability to treat and prevent neoplasia, especially pre-cancerous lesions, safely. In particular, the present invention is a method for selecting compounds that can be used to treat and prevent neoplasia, including precancerous lesions. The compounds so identified can have minimal side effects attributable to COX inhibition and other non-specific interactions associated with conventional chemotherapeutics. The compounds of interest can be tested by exposing the novel PDE described above to the compounds, and if a compound inhibits this novel PDE, the compound is then further evaluated (e.g., in vitro or in vivo animal or human testing models or trials) for its anti-neoplastic properties. One aspect of this invention, therefore, involves a screening/selection method to identify a compound effective for treating neoplasia that includes ascertaining the compound's inhibition of this novel PDE and/or PDE2 and its inhibition of COX. Preferably, the screening and selection methods of this invention further include determining whether the compound inhibits the growth of tumor cells in vitro or in vivo. By selecting compounds in this fashion, potentially beneficial and improved compounds for treating neoplasia can be identified more rapidly and with greater precision than possible in the past for the purposes of developing pharmaceutical compositions and therapeutically treating neoplasia. Further benefits will be apparent from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from SW480 neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column. Figure 2 is a graph of cGMP activities of the reloaded cGMP phosphodiesterases obtained from SW480 neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column. Figure 3 is a graph of the kinetic behavior of the novel PDE of this invention. Figure 4 illustrates the effect of the sulfide derivative of sulindac and the sulfone derivative of sulindac (a.k.a. exisulind) on purified cyclooxygenase activity. Figure 5 illustrates the effects of test compounds B and E on COX inhibition. Figure 6 illustrates the inhibitory effects of sulindac sulfide and exisulind on PDE4 and PDE5 purified from cultured tumor cells. Figure 7 illustrates the effects of sulindac sulfide on cyclic nucleotide levels in HT-29 cells. Figure 8 illustrates the phosphodiesterase inhibitory activity of compound B. Figure 9 illustrates the phosphodiesterase inhibitory activity of compound E. Figure 10 illustrates the effects of sulindac sulfide and exisulind on apoptosis and necrosis of HT-29 cells. Figure 11 illustrates the effects of sulindac sulfide and exisulind on HT-29 cell growth inhibition and apoptosis induction as determined by DNA fragmentation. Figure 12 illustrates the apoptosis-inducing properties of compound E. Figure 13 illustrates the apoptosis-inducing properties of compound B. Figure 14 illustrates the effects of sulindac sulfide and exisulind on tumor cell growth. Figure 15 illustrates the growth inhibitory and apoptosis-inducing activity of sulindac sulfide and control (DMSO). Figure 16 illustrates the growth inhibitory activity of compound E. Figure 17 illustrates the inhibition of pre-malignant, neoplastic lesions in mouse mammary gland organ culture by sulindac metabolites. Figure 18 A is a SDS protein gel of SW480 cell lysates from drug-treated cell lysates in the absence of added cGMP, where cells were treated in culture for 48 hours with DMSO (0.03%, lanes 1 and 2), exisulind (200, 400 and 600µM; lanes 3, 4, 5) and E4021 (0.1, 1 and 10µM, lanes 6, 7, 8). Figure 18 B is a SDS (X-ray film exposure) gel PKG assay of SW480 cell lysates from drug-treated cell lysates in the presence of added cGMP, where cells were treated in culture for 48 hours with DMSO (0.03%, lanes 1 and 2), exisulind (200, 400 and 600µM; lanes 3, 4, 5) and E4021 (0.1, 1 and 10µM, lanes 6, 7, 8). Figure 19 is a bar graph of the results of Western blot experiments of the effects of exisulind on β-catenin and PKG levels in neoplastic cells relative to control. Figure 20 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from HTB-26 neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column. Figure 21 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from HTB-26 neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column with low and high substrate concentration. Figure 22 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from LnCAP neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column_. Figure 23 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from LnCAP neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column with low and high substrate concentration. Figure 24 is a bar graph illustrating the specificity binding of the non-catalytic cGMP binding sites of PDE5 for cyclic nucleotide analogs and selected PDE5 inhibitors. Figure 25 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from SW480 neoplastic cells, as assayed from the eluent from a DEAE-Trisacryl M column using ethylene glycol in the buffer. Figure 26 is a graph of the cGMP activities of the cGMP phosphodiesterases obtained from SW480 neoplastic cells grown in roller bottles, as assayed from the eluent from a DEAE-Trisacryl M column. Figure 27 A shows a time-dependent increase in the amount of histone-associated fragmented DNA in LNCaP cell cultures following treatment with 50 µM Compound I. Figure 27 B shows the course of treatment of PrEC prostate cells with Compound I (50µM) that did not affect DNA fragmentation for up to 4 days of treatment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS I. The Novel cGMP-Specific Phosphodiesterase And PDE2 From Neoplastic Cells A. The Isolation Of The Novel PDE Conformation The isolated cGMP-specific phosphodiesterase (which appears to be a novel conformation of PDE2) was first prepared from the human carcinoma cell line commonly referred to as SW480 available from the American Tissue Type Collection in Rockville, Maryland, U.S.A. SW480 is a human colon cancer cell line that originated from moderately differentiated epithelial adenocarcinoma. As discussed below, a similar conformation has also been isolated from neoplasias of the breast (i.e., HTB-26 cell line) and prostate (i.e., LNCAP cell line). By "isolated" we mean (as is understood in the art) not only isolated from neoplastic cells, but also made by recombinant methods (e.g., expressed in a bacterial or other non-human host vector cell lines). However, we presently believe isolation from the human neoplastic cell line is preferable since we believe that the target protein so isolated has a structure (i.e., a conformation or topography) that is closer to, if not identical with, one of the native conformations in the neoplastic cell as possible. This conformation assists in the selection of anti-neoplastic compounds that will inhibit the target enzyme(s) in vivo. The novel PDE activity was first found in SW480 colon cancer cell lines. To isolate the novel phosphodiesterase from SW480, approximately four hundred million SW480 cells were grown to confluence in and were scraped from 150 cm 2 tissue culture dishes after two washes with 10 mL cold PBS and pelleted by centrifugation. The cells were re-suspended in homogenization buffer (20 mL TMPI-EDTA-Triton pH 7.4: 20 mM Tris-HOAc, 5 mM MgAc 2 , 0.1 mM EDTA, 0.8% Triton-100, 10µM benzamidine, 10µM TLCK, 2000 U/mL aprotinin, 2 µM leupeptin, 2 µM pepstatin A) and homogenized on an ice bath using a polytron tissumizer (three times, 20 seconds/pulse). The homogenized material was centrifuged at 105,000 g for 60 minutes at 4°C in a Beckman L8 ultracentrifuge, and the supernatant was diluted with TMPI-EDTA (60 mL) and applied to a 10-milliliter DEAE-Trisacryl M column pre-equilibrated with TMPI-EDTA buffer. The loaded column was washed with 60 mL of TM-EDTA, and PDE activities were eluted with a 120 mL linear gradient of NaOAC (0-0.5 M) in TM-EDTA, at a flow rate of 0.95 mL/minute, 1.4 mL/fraction. Eighty fractions were collected and assayed for cGMP hydrolysis immediately (i.e. within minutes). Figure 1. shows the column's elution profile, revealing two initial peaks of cGMP PDE activity, peaks A and B, which were eluted by 40-50 mM and 70-80 mM NaOAC, respectively. As explained below, peak A is PDE5, whereas peak B is a novel cGMP-specific phosphodiesterase activity. Cyclic nucleotide PDE activity of each fraction was determined using the modified two-step radio-isotopic method of Thompson et al. (Thompson W.J., et al., Adv. Cyclic Nucleotide Res. 10: 69-92, 1979), as further described below. The reaction was in 400 µl containing Tris-HCl (40mM; pH 8.0), MgCl 2 (5mM), 2-mercaptoethanol (4 mM), bovine serum albumin (30 µg), cGMP (0.25µM-5µM) with constant tritiated substrate (200,000 cpm). The incubation time was adjusted to give less than 15% hydrolysis. The mixture was incubated at 30°C followed by boiling for 45 seconds to stop the reaction. Then, the mixture was cooled, snake venom (50µg) added, and the mixture was incubated at 30°C for 10 minutes. MeOH (1 mL) was added to stop the reaction, and the mixture was transferred to an anion-exchange column (Dowex 1-X8, 0.25 mL resin). The eluent was combined with a second mL of MeOH, applied to the resin, and after adding 6 mL scintillation fluid, tritium activity was measured using a Beckman LS 6500 for one minute. To fractionate the cGMP hydrolytic activities of peaks A and B further, fractions 15 to 30 of the original 80 were reloaded onto the DEAE-Trisacryl M column and eluted with a linear gradient of NaOAC (0-0.5 M) in TM-EDTA. Fractions were again immediately assayed for cGMP hydrolysis (using the procedure described above with 0.2, 2, 5µM substrate), the results of which are graphically presented in Figure 2. One observation about peak B illustrated in Figure 2 is that increasing substrate concentration of cGMP dramatically enhanced activity when contrasted to peak A. While this observation is consistent with its being a PDE2, the fact that the enzyme characterized in Figure 2 is cGMP-specific (see below) suggests that it has a novel conformation compared to the classic PDE2 reported in the literature. Peak A activity shows apparent substrate saturation of high affinity catalytic sites. B. The Isolation of Classic PDE2 From SW480 Two methods were found that allowed "peak B" to be isolated from SW480 so that the enzyme had the classical PDE2 activity (i.e. was not cGMP-specific, but was cGMP stimulated). The first method involved growing the SW480 in 850 cm 2 Corning roller bottles instead of 150 cm 2 tissue culture flasks. SW480 were grown in roller bottles at 0.5 rpm with each bottle containing 200 mL of RPMI 1640, 2 mM glutamine, and 25 mM HEPES. Cells were harvested by the following procedure. PBS media was warmed to 37°C for at least 15 minutes. 200 mL of 5% FBS/RPMI 1640 complete media is prepared and 5 mL of glutamine were added. 5 mL of antibiotic/antimycotic were also added. 70 mL of the PBS solution was added to 10 mL of 4X Pancreatin. The mixture was maintained at room temperature. The media was removed and the flask was rinsed with 4 mL of PBS being sure the bottom of the flask was covered. All solution was removed with a pipet. 4 mL of diluted Pancreatin was added to the flask, and the flask was swished to cover its bottom. The flask was incubated at 37°C for 8-10 minutes. After the incubation, the flask was quickly checked under an inverted microscope to make sure all cells were rounded. The flask was hit carefully on its side several times to help detach cells. 10 mL of cold complete media were added to the flask to stop the Pancreatin proteolysis. The solution was swirled over the bottom to collect the cells. The media was removed using a 25 mL pipet, and the cells placed in 50 mL centrifuge tubes on ice, The tubes were spun at 1000 rpm at 4°C for 5 minutes in a clinical centrifuge to pellet cells. The supernatant was poured off and each pellet frozen on liquid nitrogen for 15 seconds. The harvested cells can be stored in a -70°C freezer. The PDEs from the harvested SW480 cells were isolated using a FPLC procedure. A Pharmacia AKTA FPLC was used to control sample loading and elution on an 18 mL DEAE TrisAcryl M column. About 600 million cells of SW480 were used for the profiles. After re-suspending cells in homogenization buffer (20 mL TMPI-EDTA-Triton pH 7.4: 20 mM Tris-HOAc, 5 mM MgAc 2 , 0.1 mM EDTA, 0.8% Triton-100, 10µM benzamidine, 10µM TLCK, 2000 U/mL aprotinin, 2 µM leupeptin, 2 µM pepstatin A), samples were manually homogenized. FPLC buffer A was 8 mM TRIS-acetate, 5 mM Mg acetate, 0.1 mM EDTA, pH 7.5 and buffer B was 8 mM TRIS-acetate, 5 mM Mg acetate, 0.1 mM EDTA, 1 M Na acetate, pH 7.5. Supernatants were loaded onto the column at 1 mL per minute, followed by a wash with 60 mL buffer A at 1 mL per minute. A gradient was run from 0-15% buffer B in 60 mL, 15-50% buffer B in 60 mL, and 50-100% buffer B in 16 mL. During the gradient, 1.5 mL fractions were collected. The profile obtained was similar (Figure 26) to the profile for the novel PDE activity (see, e.g., Figure 1) obtained above, except that Peak B isolated in this manner showed cAMP hydrolytic activity at 0,25 µM substrate that could be activated 2-3 fold by 5 µM cGMP. A second method used to isolate classic PDE2 from SW480 was done using a non-FPLC DEAE column procedure described above (see Section IA) with the modification that the buffers contained 30% ethylene glycol, 10 mM TLCK and 3.6 mM β-mercaptoethanol. The addition of these reagents to the buffers causes a shift in the elution profile (see Figure 25) from low to high sodium acetate so that peak A moves from 40 to 150 mM, peak B from 75 to 280 mM and peak C from 200 to 500 mM Na acetate (see Figure 25). Peak B in Figure 25 was assayed with 2 µM cAMP substrate and showed a two-fold activation by 5 µM cGMP (see Figure -Y). The selective PDE2 inhibitor EHNA inhibited 2 µM cGMP PDE activity in this Peak B with an IC 50 of 1.6 µM and inhibited 2.0 µM cAMP PDE activity in Peak B with an IC 50 of 3.8 µM (and IC 50 of 2.5 µM with addition of 10 µM rolipram). C. cGMP-Specificity of PDE Peak A and The Novel Peak B Activity Each fraction from the DEAE column from Section IA was also assayed for cGMP-hydrolysis activity (0.25µM cGMP) in the presence or absence of Ca ++ , or Ca ++ -CaM and/or EGTA and for cAMP (0.25µM cAMP) hydrolysis activity in the presence or absence of 5µM cGMP. Neither PDE peak A and peak B (fractions 5-22, see Fig. 1) hydrolyzed cAMP significantly, establishing that neither had the activity of a classic cAMP-hydrolyzing family of PDE (i.e. a PDE 1, 2, 3). Ca ++ (with or without calmodulin) failed to activate either cAMP or cGMP hydrolysis activity of either peak A or B, and cGMP failed to activate or inhibit cAMP hydrolysis. Such results establish that peaks A and B constitute cGMP-specific PDE activities but not classic or previously known PDE1, PDE2, PDE3 or PDE4 activities. For the novel PDE peak B, as discussed below, cyclic GMP activated the cGMP hydrolytic activity of the enzyme, but did not activate any cAMP hydrolytic activity (in contrast with the Peak B from Section IB above). This reveals that the novel PDE peak B -- the novel phosphodiesterase of this invention -- is not a cGMP-stimulated cAMP hydrolysis ("cGS") or among the classic or previously known PDE2 family Activities because the known isoforms of PDE2 hydrolyze both cGMP and cAMP. D. Peak A Is A Classic PDE5, But The Novel Peak B--A New cGMP-Specific PDE―Is Not To characterize any PDE isoform, kinetic behavior and substrate preference should be assessed. Peak A showed typical "PDE5" characteristics. For example, the K m of the enzyme for cGMP was 1.07 µM, and Vmax was 0.16 nmol/min/mg. In addition, as discussed below, zaprinast (IC 50 =1.37µM) and E4021 (IC 50 =3 nM) and sildenafil inhibited activity of peak A. Further, zaprinast showed inhibition for cGMP hydrolysis activity of peak A, consistent with results reported in the literature. PDE Peak B from Section IA showed considerably different kinetic properties as compared to PDE peak A. For example, in Eadie-Hofstee plots of Peak A, cyclic GMP hydrolysis shows single line with negative slope with increasing substrate concentrations, indicative of Michaelis-Menten kinetic behavior. Peak B, however, shows the novel property for cGMP hydrolysis in the absence of cAMP of a decreasing (apparent K m = 8.4), then increasing slope (K m < 1) of Eadie-Hotfstee plots with increasing cGMP substrate (see, Fig. 3). Thus, this establishes Peak B's submicromolar affinity for cGMP (i.e., where K m < 1). Consistent with the kinetic studies (i.e., Fig. 3) and positive-cooperative kinetic behavior in the presence of cGMP substrate, was the increased cGMP hydrolytic activity in the presence of increasing concentrations of cGMP substrate. This was discovered by comparing 0.25 µM, 2 µM and 5 µM concentrations of cGMP in the presence of PDE peak B after a second DEAE separation to rule out cAMP hydrolysis and to rule out this new enzyme being a previously identified PDE5. Higher cGMP concentrations evoked disproportionately greater cGMP hydrolysis with PDE peak B, as shown in Fig. 2. These observations suggest that cGMP binding to the peak B enzyme causes a conformational change in the enzyme. This confirms the advantage of using the native enzyme from neoplastic cells, but this invention is not limited to the native form of the enzyme having the characteristics set forth above. E. Zaprinast- and Sildenafil-Insensitivity of PDE Peak B Relative to Peak A, and Their Effects on Other PDE Inhibitors Different PDE inhibitors were studied using twelve concentrations of drug from 0.01 to 100 µM and substrate concentration of 0.25 µM 3 H-cGMP. IC 50 values were calculated with variable slope, sigmoidal curve fits using Prism 2.01 (GraphPad). The results are shown in Table 1. While compounds E4021 and zaprinast inhibited peak A, (with high affinities) IC 50 values calculated against the novel PDE activity in peak B (Section IA) are significantly increased (>50 fold). This confirms that peak A is a PDE5. These data further illustrate that the novel PDE activity of this invention is, for all practical purposes, zaprinast-insensitive and E4021-insensitive. Comparison of PDE Inhibitors Against Peak A and Section IA Peak B (cGMP Hydrolysis) Compound PDE Family Inhibitor IC 50 Peak A (µM) IC 50 Peak B (µM) Ratio (IC 50 PeakA/Peak B) E4021 5 0.003 8.4 0.0004 Zaprinast 5 1.4 >30 <0.05 Compound E 5 and others 0.38 0.37 1.0 Sulindac sulfide 5 and others 50 50 1.0 Vinpocetine 1 >100 >100 EHNA 2,5 >100 3.7 Indolidan 3 31 >100 <0,31 Rolipram 4 >100 >100 Sildenafil 5 .0003 >10 <.00003 By contrast, sulindac sulfide and Compound E and competitively inhibited both peaks A and B phosphodiesterases at the same potency (IC 50 =0.38 µM for PDE peak A; 0.37 µM for PDE peak B). There is significance for the treatment of neoplasia and the selection of useful compounds for such treatment in the fact that peak B (either form of it) is zaprinast-insensitive whereas peaks A and B are both sensitive to sulindac sulfide and Compound E. We have tested zaprinast, E4021 and sildenafil to ascertain whether they induce apoptosis or inhibit the growth of neoplastic cells, and have done the same for Compound E. As explained below, zaprinast by itself does not have significant apoptosis-inducing or growth-inhibiting properties, whereas sulindac sulfide and Compound E are precisely the opposite. In other words, the ability of a compound to inhibit both PDE peaks A and B correlates with its ability to induce apoptosis in neoplastic cells, whereas if a compound (e.g., zaprinast) has specificity for PDE peak A only, that compound will not by itself induce apoptosis. F. Insensitivity of The Novel PDE Peak B To Incubation With cGMP-Dependent Protein Kinase G Further differences between PDE peak A and the novel peak B (Section IA) were observed in their respective cGMP-hydrolytic activities in the presence of varying concentrations of cGMP-dependent protein kinase G (which phosphorylates typical PDE5). Specifically, peak A and peak B fractions from Section IA were incubated with different concentrations of protein kinase G at 30°C for 30 minutes. Cyclic GMP hydrolysis of both peaks has assayed after phosphorylation was attempted. Consistent with previously published information about PDE5, Peak A showed increasing cGMP hydrolysis activity in response to protein kinase G incubation, indicating that Peak A was phosphorylated. Peak B was unchanged, however (i.e., was not phosphorylated and insensitive to incubation with cGMP-dependent protein kinase G). These data are consistent with Peak A being an isoform consistent with the known PDE5 family and Peak B from Section IA being a novel cGMP-specific PDE activity. G. Novel Peak B In Prostate and Breast Cancer Cell Lines The novel Peak B was also isolated from two other neoplastic cell lines, a breast cancer cell line, HTB-26 and a prostate cancer cell line, LnCAP by a procedure similar to the one above used to isolate it from SW480. The protocol was modified in several respects. To provide even greater reproducibility to allow comparison of different cell lines, a Pharmacia AKTA FPLC was used to control sample loading and elution on an 18 mL DEAE TrisAcryl M column. SW840 was run by this same procedure multiple times to provide a reference of peak B. 200-400 million cells of SW480 were used for the profiles. 70 million cells of LnCAP were used for a profile (see Figures 22 and 23), and in a separate experiment 32 million cells of HTB-26 were used for a profile (see Figures 20 and 21). After re-suspending cells in homogenization buffer, samples were manually homogenized. FPLC buffer A was 8 mM TRIS-acetate, 5 mM Mg acetate, 0.1 mM EDTA, pH 7.5 and buffer B was 8 mM TRIS-acetate, 5 mM Mg acetate, 0.1 mM EDTA, 1 M Na acetate, pH 7.5. Supernatants were loaded onto the column at 1 mL per minute, followed by awash with 60 mL buffer A at 1 mL per minute. A gradient was run from 0-15% buffer B in 60 mL, 15-50% buffer B in 60 mL, and 50-100% buffer B in 16 mL. During the gradient 1.5 mL fractions were collected. Peaks of cGMP PDE activity eluted around fraction 65 that was at 400 mM Na acetate (see Figures 20-23). This activity was measured at 0.25 µM cGMP (indicating submicromolar affinity for cGMP). Rolipram, a PDE4-specific drug, inhibited most of the cAMP PDE activity (i.e. the cAMP activity was due to PDE4), indicating that the peak B's cGMP activity were specific for cGMP over cAMP. All three peak B's (from SW480, HTB-26, and LnCAP) did not show stimulation with calcium/calmodulin and were resistant to 100 nM E4021, a specific PDE5-specific inhibitor like zaprinast (see Figures 20 and 22). The peak B's also showed a dramatic increase in activity when substrate was increased from 0.25 µM to 5 µM cGMP (suggesting positively cooperative kinetics) (see Figures 21 and 23). Also, the three peaks show similar inhibition by exisulind and Compound I, below. II. Protein Kinase G and β-Catenin Involvement ― In General A series of experiments were performed to ascertain what effect, if any, an anti-neoplastic cGMP-specific PDE inhibitor such as exisulind had on cGMP-dependent protein kinase G ("PKG") in neoplastic cells containing either the adenomatous polyposis coli gene ("APC gene") defect or a defect in the gene coding for β-catenin. As explained below, such an inhibitor causes an elevation in PKG activity in such neoplastic cells. That increase in activity was not only due to increased activation of PKG in cells containing either defect, but also to increased expression of PKG in cells containing the APC defect. In addition, when PKG from neoplastic cells with either defect is immunoprecipitated, it precipitates with β-catenin. β-catenin has been implicated in a variety of different cancers because researchers have found high levels of it in patients with neoplasias containing mutations in the APC tumor-suppressing gene. People with mutations in this gene at birth often develop thousands of small tumors in the lining of their colon. When it functions properly, the APC gene codes for a normal APC protein that is believed to bind to and regulate β-catenin. Thus, the discovery that PKG in neoplastic cells containing either the APC gene defect or the β-catenin defect is bound to β-catenin indeed strongly implicates PKG in one of the major cellular pathways that leads to cancer. In addition, because of the relationship between cGMP-specific inhibition and PKG elevation upon treatment with SAANDs links cGMP to the PKG/ β-catenin/APC defect in such cells. This latter link is further buttressed by the observation that β-catenin itself is reduced when neoplastic cells containing the APC defect or the β-catenin defect are exposed to a SAAND. This reduction in β-catenin is initiated by PKG itself. PKG phosphorylates β-catenin -- which is another novel observation associated with this invention. The phosphorylation of β-catenin allows β-catenin to be degraded by ubiquitin-proteasomal system. This phosphorylation of β-catenin by PKG is important in neoplastic cells because it circumvents the effect of the APC and β-catenin mutations. The mutated APC protein affects the binding of the β-catenin bound to the mutant APC protein, which change in binding has heretofore been thought to prevent the phosphorylation of β-catenin by GSK-3b kinase. In the case of mutant β-catenin, an elevation of PKG activity also allows the mutant β-catenin to be phosphorylated. By elevating PKG activity in neoplasia with cGMP-PDE inhibition allows for β-catenin phosphorylation (leading to its degradation) in neoplastic cells containing either type of mutation. In short, these findings not only lead to new pharmaceutical screening methods to identify further SAAND candidate compounds, but also buttress the role of cGMP-specific PDE inhibition in therapeutic approaches to neoplasia. This observation may also explain the unexpectedly broad range of neoplasias SAANDs can inhibit since both neoplasia with and without the APC defect can be treated, as explained above. III. Screening Pharmaceutical Compositions Using The PDEs A. In General The novel PDE of this invention and PDE2 are useful with or without PDE5 to identify compounds that can be used to treat or prevent neoplasms, and that are not characterized by serious side effects. Cancer and precancer may be thought of as diseases that involve unregulated cell growth. Cell growth involves a number of different factors. One factor is how rapidly cells proliferate, and another involves how rapidly cells die. Cells can die either by necrosis or apoptosis depending on the type of environmental stimuli. Cell differentiation is yet another factor that influences tumor growth kinetics. Resolving which of the many aspects of cell growth is affected by a compound is important to the discovery of a relevant target for pharmaceutical therapy. Screening assays based on this technology can be combined with other tests to select compounds that have growth inhibiting and pro-apoptotic activity. This invention is the product of several important discoveries. First, the present inventors discovered that desirable inhibitors of tumor cell growth induce premature death of cancer cells by apoptosis (see, Piazza, G.A., et al., Cancer Research, 55 (14), 3110-16, 1995). Second, several of the present inventors unexpectedly discovered compounds that selectively induce apoptosis without substantial COX inhibition also inhibit PDE5. In particular, and contrary to leading scientific studies, desirable compounds for treating neoplastic lesions inhibit PDE5 (EC 3.1.4.17). PDE5 is one of at least ten gene families of phosphodiesterase. PDE5 and the novel PDE of this invention are unique in that they selectively degrade cyclic GMP and not cAMP, while the other families of PDE selectively degrade/hydrolyze cAMP and not cGMP or non-selectively degrade both cGMP and cAMP. Preferably, desirable compounds used to treat neoplasia do not substantially inhibit non-selective or cAMP degrading phosphodiesterase types. B. COX Screening A preferred embodiment of the present invention involves determining the cyclooxygenase inhibition activity of a given compound, and determining the cGMP specific PDE inhibitory activity of the compound. The test compounds are assessed for their ability to treat neoplastic lesions either directly or indirectly by comparing their activities against known compounds useful for treating neoplastic lesions. A standard compound that is known to be effective for treating neoplastic lesions without causing gastric irritation is 5-fluoro-2-methyl-1-(p-methylsulfonylbenzylidene)-3-indenylacetic acid ("exisulind"). Other useful compounds for comparative purposes include those that are known to inhibit COX, such as indomethacin and the sulfide metabolite of sulindac: 5-fluoro-2-methyl-1-(p-methylsulfinylbenzylidene)-3-indenylacetic acid ("sulindac sulfide"). Other useful compounds for comparative purposes include those that are known to inhibit (cGMP-specific PDEs, such as 1-(3-chloroanilino)-4-phenyphthalazine ("MY5445"). As used herein, the term "precancerous lesion" includes syndromes represented by abnormal neoplastic, including dysplastic, changes of tissue. Examples include dysplastic growths in colonic, breast, prostate or lung tissues, or conditions such as dysplastic nevus syndrome, a precursor to malignant melanoma of the skin. Examples also include, in addition to dysplastic nevus syndromes, polyposis syndromes, colonic polyps, precancerous lesions of the cervix (i.e., cervical dysplasia), esophagus, lung, prostatic dysplasia, prostatic intraneoplasia, breast and/or skin and related conditions (e.g., actinic keratosis), whether the lesions are clinically identifiable or not As used herein, the terms "carcinoma" or "cancer" refers to lesions which are cancerous. Examples include malignant melanomas, breast cancer, prostate cancer and colon cancer. As used herein, the terms "neoplasia" and "neoplasms" refer to both cancerous and pre-cancerous lesions. As used herein, the abbreviation PG represents prostaglandin; PS represents prostaglandin synthetase; PGE 2 represents prostaglandin E 2 ; PDE represents phosphodiesterase; COX represents cyclooxygenase; cyclic nucleotide, RIA represents - radioimmunoassay. COX inhibition by a compound can be determined by either of two methods. One method involves measuring PGE 2 secretion by intact HL-60 cells following exposure to the compound being screened. The other method involves measuring the activity of purified cyclooxygenases (COXs) in the presence of the compound. Both methods involve protocols previously described in the literature, but preferred protocols are set forth below. Compounds can be evaluated to determine whether they inhibit the production of prostaglandin E 2 ("PGE 2 "), by measuring PGE 2 . Using an enzyme immunoassay (EIA) kit for PGE 2 , such as commercially available from Amersham, Arlington Heights, IL U.S.A. Suitable cells include those that make an abundance of PG, such as HL-60 cells. HL-60 cells are human promyelocytes that are differentiated with DMSO into mature granulocytes (see, Collins, S.J., Ruscetti, F.W., Gallagher, R.E. and Gallo, R.C., "Normal Functional Characteristics of Cultured Human Promyelocytic Leukemia Cells (HL-60) After Induction of Differentiation By Dimethylsulfoxide", J. Exp. Med ., 149 :969-974, 1979). These differentiated cells produce PGE 2 after stimulation with a calcium ionophore, A23187 (see, Kargman, S., Prasit, P. and Evans, J.F., "Translocation of HL-60 Cell 5-Lipoxygenase", J. Biol. Chem ., 266 :23745-23752, 1991). HL-60 are available from the ATCC (ATCC:CCL240). They can be grown in a RPMI 1640 medium supplemented with 20% heat-inactivated fetal bovine serum, 50 U/mL penicillin and 50 µg/mL streptomycin in an atmosphere of 5% CO 2 at 37°C. To induce myeloid differentiation, cells are exposed to 1.3% DMSO for 9 days and then washed and resuspended in Dulbecco's phosphate-buffered saline at a concentration of 3x10 6 cells/mL. The differentiated HL-60 cells (3x10 6 cells/mL) are incubated for 15 minutes at 37°C in the presence of the compounds tested at the desired concentration. Cells are then stimulated by A23187 (5x10 -6 M) for 15 minutes. PGE 2 secreted into the external medium is measured as described above. As indicated above, a second method to assess COX inhibition of a compound is to measure the COX activity in the presence of a test compound. Two different forms of cyclooxygenase (COX-I and COX-2) have been reported in the literature to regulate prostaglandin synthesis. COX-2 represents the inducible form of COX while COX-I represents a constitutive form. COX-I activity can be measured using the method described by Mitchell et al. ("Selectivity of Nonsteroidal Anti-inflammatory Drugs as Inhibitors of Constitutive and Inducible Cyclooxygenase," Proc. Natl. Acad. Sci. USA ., 90 :11693-11697, 1993, which is incorporated herein by reference) using COX-I purified from ram seminal vesicles as described by Boopathy & Balasubramanian, "Purification And Characterization Of Sheep Platelet Cyclooxygenase" ( Biochem. J., 239 :371-377, 1988, COX-2 activity can be measured using COX-2 purified from sheep placenta as described by Mitchell et al., 1993, supra . The cyclooxygenase inhibitory activity of a drug can be determined by methods known in the art. For example, Boopathy & Balasubramanian, 1988, supra , described a procedure in which prostaglandin H synthase 1 (Cayman Chemical, Ann Arbor, Michigan) is incubated at 37°C for 20 minutes with 100 µM arachidonic acid (Sigma Chemical Co.), cofactors (such as 1.0 mM glutathione, 1.0 mM hydroquinone, 0.625 µM hemoglobin and 1.25 mM CaCl 2 in 100 mM Tris-HCl, pH 7.4) and the drug to be tested. Following incubation, the reaction can be terminated with trichloroacetic acid. After stopping the reaction by adding thiobarbituric acid and malonaldehyde, enzymatic activity can then be measured spectrophotometrically at 530 nm. Obviously, a compound that exhibits a lower COX-I or COX-2 inhibitory activity in relation to its greater combined PDE5/novel PDE/PDE2 inhibitory activities may be a desirable compound. The amount of COX inhibition is determined by comparing the activity of the cyclooxygenase in the presence and absence of the test compound. Residual (i.e., less than about 25%) or no COX inhibitory activity at a concentration of about 100 µM is indicative that the compound should be evaluated further for usefulness for treating neoplasia. C. Determining Phosphodiesterase Inhibition Activity Compounds can be screened for inhibitory effect on the activity of the novel phosphodiesterase of this invention using either the enzyme isolated as described above, a recombinant version, or using the novel PDE and/or PDE2 together with PDE5. Alternatively, cyclic nucleotide levels in whole cells are measured by RIA and compared to untreated and zaprinast-treated cells. Phosphodiesterase activity can be determined using methods known in the art, such as a method using radioactive 3 H cyclic GMP (cGMP)(cyclic 3',5'-guanosine monophosphate) as the substrate for the PDE enzyme. (Thompson, W.J., Teraski, W.L., Epstein, P.M., Strada, S.J., Advances in Cyclic Nucleotide Research , 10 :69-92, 1979, which is incorporated herein by reference). In brief, a solution of defined substrate 3 H-cGMP specific activity (0.2 µM; 100,000 cpm; containing 40 mM Tris-HCl (pH 8.0), 5 mM MgCl 2 and 1 mg/mL BSA) is mixed with the drug to be tested in a total volume of 400µl. The mixture is incubated at 30°C for 10 minutes with isolated PDE of this invention. Reactions are terminated, for example, by boiling the reactioll mixture for 75 seconds. After cooling on ice, 100 µl of 0.5 mg/mL snake venom ( O. Hannah venom available from Sigma) is added and incubated for 10 minutes at 30°C. This reaction is then terminated by the addition of an alcohol, e.g. 1 mL of 100% methanol. Assay samples are applied to 1 mL Dowex 1-X8 column; and washed with 1 mL of 100% methanol. The amount of radioactivity in the breakthrough and the wash from the column is combined and measured with a scintillation counter. The degree of phosphodiesterase inhibition is determined by calculating the amount of radioactivity in drug-treated reactions and comparing against a control sample (a reaction mixture lacking the tested compound but with drug solvent). Alternatively, the ability of desirable compounds to inhibit the phosphodiesterases of this invention is reflected by an increase in cGMP in neoplastic cells exposed to a compound being screened. The amount of PDE activity can be determined by assaying for the amount of cyclic GMP in the extract of treated cells using radioimmunoassay (RIA). In this procedure, HT-29 or SW-480 cells are plated and grown to confluency. As indicated above, SW-480 contains both PDE5 and the novel PDE of this invention, so when PDE activity is evaluated in this fashion, a combined cGMP hydrolytic activity is assayed simultaneously. The test compound is then incubated with the cell culture at a concentration of compound between about 200 µM to about 200 pM. About 24 to 48 hours thereafter, the culture media is removed from the cells, and the cells are solubilized. The reaction is stopped by using 0.2N HCl/50% MeOH. A sample is removed for protein assay. Cyclic GMP is purified from the acid/alcohol extracts of cells using anion-exchange chromatography, such as a Dowex column. The cGMP is dried, acetylated according to published procedures, such as using acetic anhydride in triethylamine, (Steiner, A.L., Parker, C.W., Kipnis, D.M., J. Biol. Chem ., 247(4) :1106-13, 1971. The acetylated cGMP is quantitated using radioimmunoassay procedures (Harper, J., Brooker, G., Advances in Nucleotide Research , 10 :1-33, 1979). Iodinated ligands (tyrosine methyl ester) of derivatized cyclic GMP are incubated with standards or unknowns in the presence of antisera and appropriate buffers. Antiserum may be produced using cyclic nucleotide-haptene directed techniques. The antiserum is from sheep injected with succinyl-cGMP-albumin conjugates and diluted 1/20,000. Dose-interpolation and error analysis from standard curves are applied as described previously (Seibert, A.F., Thompson, W.J., Taylor, A., Wilbourn, W.H., Barnard, J. and Haynes, J., J. Applied Physiol ., 72 :389-395, 1992). In addition, the culture media may be acidified, frozen (-70°C) and also analyzed for cGMP and CAMP. In addition to observing increases in the content of cGMP in neoplastic cells caused by desirable compounds, decreases in content of cAMP have also been observed. It has been observed that a particularly desirable compound (i.e., one that selectively induces apoptosis in neoplastic cells, but not substantially in normal cells) follows a time course consistent with cGMP-specific PDE inhibition as one initial action resulting in an increased cGMP content within minutes. Secondarily, treatment of neoplastic cells with a desirable anti-neoplastic compound leads to decreased cAMP content within 24 hours. The intracellular targets of drug actions are being studied further, but current data support the concept that the initial rise in cGMP content and the subsequent fall in cAMP content precede apoptosis in neoplastic cells exposed to desirable compounds. The change in the ratio of the two cyclic nucleotides may be a more accurate tool for evaluating desirable cGMP-specific phosphodiesterase inhibition activity of test compounds, rather than measuring only the absolute value of cGMP, only cGMP-specific phosphodiesterase inhibition, or only the level of cGMP hydrolysis. In neoplastic cells not treated with anti-neoplastic compounds, the ratio of cGMP content/cAMP content is in the 0.03-0.05 range (i.e., 300-500 fmol/mg protein cGMP content over 6000-8000 fmol/mg protein cAMP content). After exposure to desirable anti-neoplastic compounds, that ratio increases several fold (preferably at least about a three-fold increase) as the result of an initial increase in cyclic GMP and the later decrease in cyclic AMP. Specifically, it has been observed that particularly desirable compounds achieve an initial increase in cGMP content in treated neoplastic cells to a level of cGMP greater than about 500 fmol/mg protein. In addition, particularly desirable compounds cause the later decrease in cAMP content in treated neoplastic cells to a level of cAMP less than about 4000 fmol/mg protein. To determine the content of cyclic AMP, radioimmunoassay techniques similar to those described above for cGMP are used. Basically, cyclic nucleotides are purified from acid/alcohol extracts of cells using anion-exchange chromatography, dried, acetylated according to published procedures and quantitated using radioimmunoassay procedures. Iodinated ligands of derivatized cyclic AMP and cyclic GMP are incubated with standards or unknowns in the presence of specific antisera and appropriate buffers. Verification of the cyclic nucleotide content may be obtained by determining the turnover or accumulation of cyclic nucleotides in intact cells. To measure intact cell cAMP, 3 H-adenine pre-labeling is used according to published procedures (Whalin, M.E., Garrett Jr., R.L., Thompson, W.J., and Strada, S.J. "Correlation of cell-free brain cyclic nucleotide phosphodiesterase activities to cyclic AMP decay in intact brain slices", Sec. Mess. and Phos. Protein Research , 12 :311-325, 1989). The procedure measures flux of labeled ATP to cyclic AMP and can be used to estimate intact cell adenylate cyclase or cyclic nucleotide phosphodiesterase activities depending upon the specific protocol. Cyclic GMP accumulation was too low to be studied with intact cell pre-labeling according to published procedures (Reynolds, P.E., S.J. Strada and W.J. Thompson, "Cyclic GMP Accumulation In Pulmonary Microvascular Endothelial Cells Measured By Intact Cell Prelabeling," Life Sci ., 60 :909-918, 1997). The PDE inhibitory activity effect of a compound can also be determined from a tissue sample. Tissue biopsies from humans or tissues from anesthesized animals are collected from subjects exposed to the test compound. Briefly, a sample of tissue is homogenized in 500 µl of 6% TCA. A known amount of the homogenate is removed for protein analysis. The remaining homogenate is allowed to sit on ice for 20 minutes to allow for the protein to precipitate. Next, the homogenate is centrifuged for 30 minutes at 15,000g at 4°C. The supernatant is recovered, and the pellet recovered. The supernatant is washed four times with five volumes of water saturated diethyl ether. The upper ether layer is discarded between each wash. The aqueous ether extract is dried in a speed vac, Once dried, the sample can be frozen for future use, or used immediately. The dried extract is dissolved in 500 µl of assay buffer. The amount of cGMP-specific inhibition is determined by assaying for the amount of cyclic nucleotides using RIA procedures as described above. The amount of inhibition is determined by comparing the activity of the novel PDE (or PDE2) in the presence and absence of the compound. Inhibition of the novel PDE activity (or PDE2) is indicative that the compound is useful for treating neoplasia. Significant inhibitory activity greater than that of the benchmark, exisulind, preferably greater than 50% at a concentration of 10 µM or below, is indicative that a compound should be further evaluated for antineoplastic properties. Preferably, the IC 50 value for the novel PDE inhibition should be less than 50µM for the compound to be further considered for potential use. D. Determining Whether A Compound Reduces Tumor Cell Growth In an alternate embodiment, the method of the present invention involves further determining whether the compound reduces the growth of tumor cells. Various cell lines can be used in the sample depending on the tissue to be tested. For example, these cell lines include: SW-480 - colonic adenocarcinoma; HT-29 - colonic adenocarcinoma, A-427 - lung adenocarcinoma carcinoma; MCF-7 - breast adenocarcinoma; and UACC-375 - melanoma line; and DU145 - prostrate carcinoma. Cytotoxicity data obtained using these cell lines are indicative of an inhibitory effect on neoplastic lesions. These cell lines are well characterized, and are used by the United States National Cancer Institute in their screening program for new anti-cancer drugs. A compound's ability to inhibit tumor cell growth can be measured using the HT-29 human colon carcinoma cell line obtained from ATCC. HT-29 cells have previously been characterized as a relevant colon tumor cell culture model (Fogh, J., and Trempe, G. In: Human Tumor Cells in Vitro , J. Fogh (eds.), Plenum Press, New York, pp. 115-159, 1975). HT-29 cells are maintained in RPMI media supplemented with 5% fetal bovine calf serum (Gemini Bioproducts, Inc., Carlsbad, CA) and 2 mm glutamine, and 1% antibiotic-antimycotic in a humidified atmosphere of 95% air and 5% CO 2 at 37°C. Briefly, HT-29 cells are plated at a density of 500 cells/well in 96 well microtiter plates and incubated for 24 hours at 37°C prior to the addition of compound. Each determination of cell number involved six replicates. After six days in culture, the cells are fixed by the addition of cold trichloroacetic acid to a final concentration of 10% and protein levels are measured using the sulforhodamine B (SRB) colorimetric protein stain assay as previously described by Skehan, P., Storeng, R., Scudiero, D., Monks, A., McMahon, J., Vistica, D., Warren, J.T., Bokesch, H., Kenney, S., and Boyd, M.R., "New Colorimetric Assay For Anticancer-Drug Screening," J. Natl. Cancer Inst. 82 : 1107-1112, 1990. In addition to the SRB assay, a number of other methods are available to measure growth inhibition and could be substituted for the SRB assay. These methods include counting viable cells following trypan blue staining, labeling cells capable of DNA synthesis with BrdU or radiolabeled thymidine, neutral red staining of viable cells, or MTT staining of viable cells. Significant tumor cell growth inhibition greater than about 50% at a dose of 100µM or below is further indicative that the compound is useful for treating neoplastic lesions. Preferably, an IC 50 value is determined and used for comparative purposes. This value is the concentration of drug needed to inhibit tumor cell growth by 50% relative to the control. Preferably, the IC 50 value should be less than 100 µM for the compound to be considered further for potential use for treating neoplastic lesions. E. Determining Whether A Compound Induces Apoptosis In a second alternate embodiment, the screening method of the present invention further involves determining whether the compound induces apoptosis in cultures of tumor cells. Two distinct forms of cell death may be described by morphological and biochemical criteria: necrosis and apoptosis. Necrosis is accompanied by increased permeability of the plasma membrane; the cells swell and the plasma membrane ruptures within minutes. Apoptosis is characterized by membrane blebbing, condensation of cytoplasm and the activation of endogenous endonucleases. Apoptosis occurs naturally during normal tissue turnover and during embryonic development of organs and limbs. Apoptosis also is induced by cytotoxic T-lymphocytes and natural killer cells, by ionizing radiation and by certain chemotherapeutic drugs. Inappropriate regulation of apoptosis is thought to play an important role in many pathological conditions including cancer, AIDS, or Alzheimer's disease, etc. Compounds can be screened for induction of apoptosis using cultures of tumor cells maintained under conditions as described above. Treatment of cells with test compounds involves either pre- or post-confluent cultures and treatment for two to seven days at various concentrations. Apoptotic cells are measured in both the attached and "floating" compartments of the cultures. Both compartments are collected by removing the supernatant, trypsinizing the attached cells, and combining both preparations following a centrifugation wash step (10 minutes, 2000 rpm). The protocol for treating tumor cell cultures with sulindac and related compounds to obtain a significant amount of apoptosis has been described in the literature. (See, Piazza, G.A., et al., Cancer Research , 55 :3110-16, 1995). The novel features include collecting both floating and attached cells, identification of the optimal treatment times and dose range for observing apoptosis, and identification of optimal cell culture conditions. Following treatment with a compound, cultures can be assayed for apoptosis and necrosis by florescent microscopy following labeling with acridine orange and ethidium bromide. The method for measuring apoptotic cell number has previously been described by Duke & Cohen, "Morphological And Biochemical Assays Of Apoptosis," Current Protocols In Immunology, Coligan et al., eds., 3.17.1-3.17.16 (1992). For example, floating and attached cells can be collected by trypsinization and washed three times in PBS. Aliquots of cells can be centrifuged. The pellet can then be re-suspended in media and a dye mixture containing acridine orange and ethidium bromide prepared in PBS and mixed gently. The mixture can then be placed on a microscope slide and examined for morphological features of apoptosis. Apoptosis can also be quantified by measuring an increase in DNA fragmentation in cells that have been treated with test compounds. Commercial photometric EIA for the quantitative, in vitro determination of cytoplasmic histone-associated-DNA-fragments (mono- and oligonucleosomes) are available (Cell Death Detection ELISA okys , Cat. No. 1,774,425, Boehringer Mannheim). The Boehringer Mannheim assay is based on a sandwich-enzyme-immunoassay principle using mouse monoclonal antibodies directed against DNA and histones, respectively. This allows the specific determination of mono- and oligonucleosomes in the cytoplasmatic fraction of cell lysates. According to the vendor, apoptosis is measured in the following fashion. The sample (cell-lysate) is placed into a streptavidin-coated microtiter plate ("MTP"). Subsequently, a mixture of anti-histone-biotin and anti-DNA peroxidase conjugate are added and incubated for two hours. During the incubation period, the anti-histone antibody binds to the histone-component of the nucleosomes and simultaneously fixes the immunocomplex to the streptavidin-coated MTP via its biotinylation. Additionally, the anti-DNA peroxidase antibody reacts with the DNA component of the nucleosomes. After removal of unbound antibodies by a washing step, the amount of nucleosomes is quantified by the peroxidase retained in the immunocomplex. Peroxidase is determined photometrically with ABTS7 (2,2'-Azido-[3-ethylbenzthiazolin-sulfonate]) as substrate. For example, SW-480 colon adenocarcinoma cells are plated in a 96-well MTP at a density of 10,000 cells per well. Cells are then treated with test compound, and allowed to incubate for 48 hours at 37°C. After the incubation, the MTP is centrifuged, and the supernatant is removed. The cell pellet in each well is then resuspended in lysis buffer for 30 minutes. The lysates are then centrifuged and aliquots of the supernatant (i.e., the cytoplasmic fraction) are transferred into a streptavidin-coated MTP. Care is taken not to shake the lysed pellets (i.e. cell nucleii containing high molecular weight, unfragmented DNA) in the MTP. Samples are then analyzed. Fold stimulation (FS = OD max /OD veh ), an indicator of apoptotic response, is determined for each compound tested at a given concentration. EC 50 values may also be determined by evaluating a series of concentrations of the test compound. Statistically significant increases in apoptosis (i.e., greater than 2 fold stimulation at a concentration of 100µM) are further indicative that the compound is useful for treating neoplastic lesions. Preferably, the EC 50 value for apoptotic activity should be less than 100µM for the compound to be further considered for potential use for treating neoplastic lesions. EC 50 is herein defined as the concentration that causes 50% induction of apoptosis relative to vehicle treatment. F. Mammary Gland Organ Culture Model Tests Test compounds identified by the above methods can be tested for antineoplastic activity by their ability to inhibit the incidence of pre-neoplastic lesions in a mammary gland organ culture system. This mouse mammary gland organ culture technique has been successfully used by other investigators to study the effects of known antineoplastic agents such as certain NSAIDs, retinoids, tamoxifen, selenium, and certain natural products, and is useful for validation of the screening method of the present invention. For example, female BALB/c mice can be treated with a combination of estradiol and progesterone daily, in order to prime the glands to be responsive to hormones in vitro. The animals are sacrificed, and thoracic mammary glands are excised aseptically and incubated for ten days in growth media supplemented with insulin, prolactin, hydrocortisone, and aldosterone. DMBA (7,12-dimethylbenz(a)anthracene) is added to medium to induce the formation of premalignant lesions. Fully developed glands are then deprived of prolactin, hydrocortisone, and aldosterone, resulting in the regression of the glands but not the pre-malignant lesions. The test compound is dissolved in DMSO and added to the culture media for the duration of the culture period. At the end of the culture period, the glands are fixed in 10% formalin, stained with alum carmine, and mounted on glass slides. The incidence of forming mammary lesions is the ratio of the glands with mammary lesions to glands without lesions. The incidence of mammary lesions in test compound treated glands is compared with that of the untreated glands. The extent of the area occupied by the mammary lesions can be quantitated by projecting an image of the gland onto a digitation pad. The area covered by the gland is traced on the pad and considered as 100% of the area. The space covered by each of the non-regressed structures is also outlined on the digitization pad and quantitated by the computer. EXPERIMENTAL RESULTS A number of compounds were examined in the various protocols and screened for potential use in treating neoplasia. The results of these tests are reported below. The test compounds are hereinafter designated by a letter code that corresponds to the following: A - rac-threo-(E)-1-(N,N'-diethylaminoethanethio)-1-(butan-1',4'-olido)-[3',4':1,2]-6-fluoro-2-methyl-3-(p-methylsulfonylbenzylidene)-indan; B - (Z)-5-Fluoro-2-methyl-1-(3,4,5-trimethoxybenzylidene)-3-acetic acid; C - (Z)-5-Fluoro-2-methyl-1-(p-chlorobenzylidene)-3-acetic acid; D - rac-(E)-1-(butan-1',4'-olido)-[3',4';1,2]-6-fluoro-2-methyl-3-(p-methylsulfonylbenzylidene)-1S-indanyl-N-acetylcysteine; E - (Z)-5-Fluoro-2-methyl-1-(3,4,5-trimethoxybenzylidene)-3-indenylacetamide, N-benzyl; F - (Z)-5-Fluoro-2-methyl-1-(p-methylsulfonylbenzylidene)-3-indenylacetamide, N,N'-dicyclohexyl; G - ribo-(E)-1-Triazolo-[2',3';1",3"]-1-(butan-1',4'-olido)-[3',4';1,2]-6-fluoro-2-methyl-3-(p-methylsulfonylbenplidene)-indan; and H - rac-(E)-1-(butan-l ',4'-olido)-[3',4';1,2]-6-fluoro-2-methyl-3-(p-methylsulfonylbenzylidene)-1S-indanyl-glutathione). Example 1 - COX Inhibition Assay Reference compounds and test compounds were analyzed for their COX inhibitory activity in accordance with the protocol for the COX assay, supra . Figure 4 shows the effect of various concentrations of either sulindac sulfide or exisulind on purified cyclooxygenase (Type 1) activity. Cyclooxygenase activity was determined using purified cyclooxygenase from ram seminal vesicles as described previously (Mitchell et al, supra ). The IC 50 value for sulindac sulfide was calculated to be approximately 1.76 µM, while that for exisulind was greater than 10,000 µM. These data show that sulindac sulfide, but not exisulind, is a COX-I inhibitor. Similar data were obtained for the COX-2 isoenzyme (Thompson, et al., Journal of the National Cancer Institute, 87: 1259-1260, 1995). Figure 5 shows the effect of test compounds B and E on COX inhibition. COX activity was determined as for the compounds shown in Figure 4. The data show that neither test compound B and E significantly inhibit COX-I. Cyclooxygenase inhibitory activity for a series of compounds Reference compounds % Inhibition at 100 µM Indomethacin 95 MY5445 94 Sulindac sulfide 97 Exisulind <25 Test compounds % Inhibition at 100 µM A <25 B <25 C 87 D <25 E <25 In accordance with the protocol, supra , compounds A through E were evaluated for COX inhibitory activity as reported in Table 2 above. Compound C was found to inhibit COX greater than 25% at a 100 µM dose, and therefore, would not be selected for further screening. Example 2 - cGMP PDE inhibition assay Reference compounds and test compounds were analyzed for their cGMP PDE inhibitory activity in accordance with the protocol for the assay described supra. Figure 6 shows the effect of various concentrations of sulindac sulfide and exisulind on either PDE4 or cGMP PDE activity purified from human colon HT-29 cultured tumor cells, as described previously (W. J. Thompson et al., supra ). The IC 50 value of sulindac sulfide for inhibition of PDE4 was 41 µM, and for inhibition of cGMP PDE was 17 µM. The IC 50 value of exisulind for inhibition of PDE4 was 181 µM, and for inhibition of cGMP PDE was 56 µM. These data show that both sulindac sulfide and exisulind inhibit phosphodiesterase activity. Both compounds show selectivity for the cGMP PDE isoenzyme forms over PDE4 isoforms. Figure 7 shows the effects of sulindac sulfide on either cGMP or cAMP production as determined in cultured HT-29 cells in accordance with the assay described, supra . HT-29 cells were treated with sulindac sulfide for 30 minutes and cGMP or cAMP was measured by conventional radioimmunoassay method. As indicated, sulindac sulfide increased the levels of cGMP by greater than 50% with an EC 50 value of 7.3 µM (Figure 7A). Levels of cAMP were unaffected by treatment, although a known PDE4 inhibitor, rolipram, increased cAMP (Figure 7B). The data demonstrate the pharmacological significance of inhibiting cGMP PDE, relative to PDE4. Figure 8 shows the effect of the indicated dose of test compound B on either cGMP PDE or PDE4 isozymes of phosphodiesterase. The calculated IC 50 value was 18 µM for cGMP PDE and was 58 µM for PDE4. Figure 9 shows the effect of the indicated dose of test compound E on either PDE4 or cGMP PDE. The calculated IC 50 value was 0.08 µM for cGMP PDE and greater than 25 µM for PDE4. cGMP PDE inhibitory activity among a series of compounds Reference compounds % Inhibition at 10 µM Indomethacin 34 MY5445 86 Sulindac sulfide 97 Exisulind 39 Test compounds % Inhibition at 10 µM A <25 B <25 C <25 D 36 E 75 The above compounds in Table 3 were evaluated for PDE inhibitory activity, as described in the protocol supra. Of the compounds that did not inhibit COX, only compound E was found to cause greater than 50% inhibition at 10 µM. As noted in Figure 8, compound B showed inhibition of greater than 50% at a dose of 20 µM. Therefore, depending on the dosage level used in a single dose test, some compounds may be screened out that otherwise may be active at slightly higher dosages. The dosage used is subjective and may be lowered after active compounds are found at certain levels to identify even more potent compounds. Example 3 - Apoptosis assay Reference compounds and test compounds were analyzed for their novel PDE inhibitory activity in accordance with the protocols for the assay, supra . In accordance with those protocols, Figure 10 shows the effects of sulindac sulfide and exisulind on apoptotic and necrotic cell death. HT-29 cells were treated for six days with the indicated dose of either sulindac sulfide or exisulind. Apoptotic and necrotic cell death was determined previously (Duke and Cohen, In: Current Protocols in Immunology, 3.17.1 - 3.17.16, New York, John Wiley and Sons, 1992). The data show that both sulindac sulfide and exisulind are capable of causing apoptotic cell death without inducing necrosis. All data were collected from the same experiment. figure 11 shows the effect of sulindac sulfide and exisulind on tumor growth inhibition and apoptosis induction as determined by DNA fragmentation. Top figure (11A); growth inhibition (open symbols, left axis) and DNA fragmentation (closed symbols, right axis) by exisulind. Bottom figure (11B); growth inhibition (open symbols) and DNA fragmentation (closed symbols) by sulindac sulfide. Growth inhibition was determined by the SRB assay after six days of treatment. DNA fragmentation was determined after 48 hours of treatment. All data were collected from the same experiment Figure 12 shows the apoptosis inducing properties of compound E. HT-29 colon adenocarcinoma cells were treated with the indicated concentration of compound E for 48 hours and apoptosis was determined by the DNA fragmentation assay. The calculated EC 50 value was 0.05 µM. Figure 13 shows the apoptosis inducing properties of compound B. HT-29 colon adenocarcinoma cells were treated with the indicated concentration of compound B for 48 hours and apoptosis was determined by the DNA fragmentation assay. The calculated EC 50 value was approximately 175 µM. Apoptosis-inducing activity among a series of compounds Reference compounds Fold induction at 100 µM Indomethacin <2.0 MY5445 4.7 Sulindac sulfide 7.9 Exisulind <2.0 E4021 <2.0 Zaprinast <2.0 Sildenafil <2.0 EHNA <2.0 Test compounds Fold induction at 100 µM A <2.0 B 3.4 C 5.6 D <2.0 E 4.6 In accordance with the fold induction protocol, supra , the compounds A through E were tested for apoptosis inducing activity, as reported in Table 4 above. Compounds B, C and E showed significant apoptotic inducing activity, greater than 2.0 fold, at a dosage of 100 µM. Of these three compounds, at this dosage only B and E did not inhibit COX but did inhibit cGMP-specific PDE. The apoptosis inducing activity for a series of phosphodiesterase inhibitors was determined. The data are presented in Table 5 below. HT-29 cell were treated for 6 days with various inhibitors of phosphodiesterase. Apoptosis and necrosis were determined morphologically after acridine orange and ethidium bromide labeling in accordance with the assay described, supra . The data show that the novel cGMP-specific PDE is useful for screening compounds that induce apoptosis of HT-29 cells. Apoptosis-Induction Data for PDE Inhibitors Inhibitor Reported Selectivity % Apoptosis % Necrosis Vehicle 8 6 8-methoxy-IBMX PDE1 2 1 Milrinone PDE3 18 0 RO-20-1724 PDE4 11 2 MY5445 PDE5 80 5 IBMX Non-selective 4 13 Example 4 - Growth inhibition assay Reference compounds and test compounds were analyzed for their PDE5 inhibitory activity in accordance with the protocol for the assay supra . Figure 14 shows the inhibitory effect of various concentrations of sulindac sulfide and exisulind on the growth of HT-29 cells. HT-29 cells were treated for six days with various doses of exisulind (triangles) or sulindac sulfide (squares) as indicated. Cell number was measured by a sulforhodamine assay as previously described (Piazza et al., Cancer Research , 55: 3110-3116, 1995). The IC 50 value for sulindac sulfide was approximately 45 µM and 200 µM for the exisulind. The data show that both sulindac sulfide and exisulind are capable of inhibiting tumor cell growth. Figure 15 shows the growth inhibitory and apoptosis-inducing activity of sulindac sulfide. A time course experiment is shown involving HT-29 cells treated with either vehicle, 0.1% DMSO (open symbols) or sulindac sulfide, 120 µM (closed symbols). Growth inhibition (15A top) was measured by counting viable cells after trypan blue staining. Apoptosis (15B bottom) was measured by morphological determination following staining with acridine orange and ethidium bromide as described previously (Duke and Cohen, in: Current Protocols in Immunology, 3.17.1-3.17.16, New York, John Wiley and Sons, 1992). The data demonstrate that sulindac sulfide is capable of inhibiting tumor cell growth, and that the effect is accompanied by an increase in apoptosis. All data were collected from the same experiment. Figure 16 shows the growth inhibitory activity of test compound E. HT-29 colon adenocarcinoma cells were treated with the indicated concentration of compound E for six days and cell number was determined by the SRB assay. The calculated IC 50 value was 0.04 µM. Growth-inhibitory activity among a series of compounds Reference compounds % Inhibition at 100 µM Indomethacin 75 MY5445 88 Sulindac sulfide 88 Exisulind <50 E4021 <50 sildenafil <50 zaprinast <50 Test compounds % Inhibition at 100 µM A 68 B 77 C 80 D 78 E 62 In accordance with the screening protocol of section supra, compounds A through E were tested for growth inhibitory activity, as reported in Table 6 above. All the test compounds showed activity exceeding a 100 µM single dose test. The growth inhibitory activity for a series of phosphodiesterase inhibitors was determined. The data are shown in Table 7 below. HT-29 cells were treated for 6 days with various inhibitors of phosphodiesterase. Cell growth was determined by the SRB assay described, supra. The data below taken with those above show that inhibitors of the novel PDE were effective for inhibiting tumor cell growth. Growth Inhibitory Data for PDE Inhibitors Inhibitor Reported Selectivity Growth inhibition (IC 50 , µM) 8-methoxy-IBMX PDE1 >200 µM Milrinone PDE3 >200 µM RO-20-1724 PDE4 >200 µM MY5445 PDE5 5 µM IBMX Non-selective >100 µM Zaprinast PDE5 >100 µM Sildenafil PDE5 >100 µM E4021 PDES >100 µM To show the effectiveness of this screening method on various forms of neoplasia, compounds were tested on numerous cell lines. The effects of sulindac sulfide and exisulind on various cell lines were determined. The data are shown in Table 8 below. The IC 50 values were determined by the SRB assay. The data show the broad effectiveness of these compounds on a broad range of neoplasias, with effectiveness at comparable dose range. Therefore, compounds identified and selected by this invention should be useful for treating multiple forms of neoplasia. Growth Inhibitory Data of Various Cell Lines Cell Type/ Tissue specificity IC 50 (µM) Sulindac sulfide Exisulind Compound E HT-29, Colon 60 120 0.10 HCT116, Colon 45 90 MCF7/S, Breast 30 90 UACC375, Melanoma 50 100 A-427, Lung 90 130 Bronchial Epithelial Cells 30 90 NRK, Kidney (non ras-transformed) 50 180 KNRK, Kidney (ras transformed) 60 240 Human Prostate Carcinoma PC3 82 0.90 Colo 205 1.62 DU-145 0.10 HCT-15 0.60 MDA-MB-231 0.08 MDA-MB-435 0.04 Example 5 - Activity in mammary gland organ culture model Figure 17 shows the inhibition of premalignant lesions in mammary gland organ culture by sulindac metabolites. Mammary gland organ culture experiment were performed as previously described (Mehta and Moon, Cancer Research, 46: 5832-5835, 1986). The results demonstrate that sulindac and exisulind effectively inhibit the formation of premalignant lesions, while sulindac sulfide was inactive. The data support the hypothesis that cyclooxygenase inhibition is not necessary for the anti-neoplastic properties of desired compounds. ANALYSIS To select compounds for treating neoplasia, this invention provides a rationale for comparing experimental data of test compounds from several protocols. Within the framework of this invention, test compounds can be ranked according to their potential use for treating neoplasia in humans. Those compounds having desirable effects may be selected for additional testing and subsequent human use. Qualitative data of various test compounds and the several protocols are shown in Table 9 below. The data show that exisulind, compound B and compound E exhibit the appropriate activity to pass the screen of four assays: lack of COX inhibition, and presence of effective cGMP-specific PDE inhibition, growth inhibition and apoptosis induction. The activity of these compounds in the mammary gland organ culture validates the effectiveness of this invention. The qualitative valuations of the screening protocols rank compound E best, then compound B and then exisulind. Activity Profile of Various Compounds Compound COX Inhibition PDE Inhibition Growth Inhibition Apoptosis Mammary Gland Organ Culture Exisulind - ++ ++ ++ +++ Sulindac sulfide ++++ +++ +++ +++ - MY5445 ++++ +++ +++ +++ + A - - +++ ++ ++ B - +++ +++ +++ ++ D - - ++ - - E - ++++ ++++ ++++ ++++ F - - ++ + - G - - +++ ++ +++ H - - ++ - - Table 9 Code: Activity of compounds based on evaluating a series of experiments involving tests for maximal activity and potency. - Not active + Slightly active + + Moderately active + + + Strongly active + + + + Highly active Also disclosed is a novel assay for PKG activity, which is used in the screening methods of this invention, but also has more general usefulness in assaying for PKG activity for other purposes (e.g., for studying the role of PKG in normal cellular function). For explanation purposes, it is useful to describe the PKG assay first, before describing how PKG activity can be useful in drug evaluation in ascertaining whether a compound is potentially useful in the treatment of neoplasia. The Novel PKG Assay The novel PKG assay of this invention involves binding to a solid phase plural amino acid sequences, each of which contain at least the cGMP binding domain and the phosphorylation site of phosphodiesterase type 5 ("PDE5"). That sequence is known and described in the literature below. Preferably, the bound PDE5 sequence does not include the catalytic domain of PDE5 as described below. One way to bind the PDE5 sequences to a solid phase is to express those sequences as a fusion protein of the PDE5 sequence and one member of an amino acid binding pair, and chemically link the other member of that amino acid binding pair to a solid phase (e.g., beads). One binding pair that can be used is glutathione S-transferase ("GST") and glutathione ("GSH"), with the GST being expressed as a fusion protein with the PDES sequence described above, and the GSH bound covalently to the solid phase. In this fashion, the PDE5 sequence/GST fusion protein can be bound to a solid phase simply by passing a solution containing the fusion protein over the solid phase, as described below. RT-PCR method is used to obtain the cGB domain of PDE5 with forward and reverse primers designed from bovine PDE5A cDNA sequence ( McAllister-Lucas L. M. et al, J. Biol. Chem. 268, 22863-22873, 1993) and the selection among PDE 1-10 families. 5'-3', Inc. kits for total RNA followed by oligo (dT) column purification of mRNA are used with HT-29 cells. Forward primer (GAA-TTC-TGT-TAG-AAA-AGC-CAC-CAG-AGA-AAT-G, 203-227) and reverse primer (CTC-GAG-CTC-TCT-TGT-TTC-TTC-CTC-TGC-TG, 1664-1686) are used to synthesize the 1484 bp fragment coding for the phosphorylation site and both low and high affinity cGMP binding sites of human PDE5A (203-1686 bp, cGB-PDE5). The synthesized cGB-PDE5 nucleotide fragment codes for 494 amino acids with 97% similarity to bovine PDE5A. It is then cloned into pGEX-5X-3 glutathione-S-transferase (GST) fusion vector (Pharmacia Biotech )with tac promoter, and EcoRI and XhoI cut sites. The fusion vector is then transfected into E. Coli BL21 (DE3) bacteria (Invitrogen). The transfected BL21 bacteria is grown to log phase and then IPTG is added as an inducer. The induction is carried at 20°C for 24 hrs. The bacteria are harvested and lysated. The soluble cell lysate is incubated with GSH conjugated Sepharose 4B (GSH-Sepharose 4B). The GST-cGB-PDE5 fusion protein can bind to the GSH-Sepharose beads and the other proteins are washed off from beads with excessive cold PBS. The expressed GST-cGB-PDE5 fusion protein is displayed on 7.5% SDS-PAGE gel as a 85 Kd protein. It is characterized by its cGMP binding and phosphorylation by protein kinases G and A. It displays two cGMP binding sites and the K d is 1.6±0.2 µM, which is close to K d =1.3 µM of the native bovine PDE5. The GST-cGB-PDE5 on GSH conjugated sepharose beads can be phosphorylated in vitro by cGMP-dependent protein kinase and cAMP-dependent protein kinase A. The K m of GST-cGB-PDE5 phosphorylation by PKG is 2.7µM and Vmax is 2.8 µM, while the K m of BPDEtide phosphorylation is 68µM. The phosphorylation by PKG shows one molecular phosphate incorporated into one GST-cGB-PDE5 protein ratio. To assay a liquid sample believed to contain PKG using the PDE5-bound solid phase described above, the sample and the solid phase are mixed with phosphorylation buffer containing 32 P-γ-ATP. The solution is incubated for 30 minutes at 30°C to allow for phosphorylation of the PDE5 sequence by PKG to occur, if PKG is present. The solid phase is then separated from solution (e.g., by centrifugation or filtration) and washed with phosphate-buffered saline ("PBS") to remove any remaining solution and to remove any unreacted 32 P-γ-ATP. The solid phase can then be tested directly (e.g., by liquid scintillation counter) to ascertain whether 32 P is incorporated. If it does, that indicates that the sample contained PKG since PKG phosphorylates PDE5. If the PDES is bound via fusion protein, as described above, the PDE5-containing fusion protein can be eluted from the solid phase with SDS buffer, and the eluent can be assayed for 32 P incorporation. This is particularly advantageous if there is the possibility that other proteins are present, since the eluent can be processed (e.g., by gel separation) to separate various proteins from each other so that the fusion protein fraction can be assayed for 32 P incorporation. The phosphorylated fusion protein can be eluted from the solid phase with SDS buffer and further resolved by electrophoresis. If gel separation is performed, the proteins can be stained to see the position(s) of the protein, and 32 P phosphorylation of the PDE5 portion of the fusion protein by PKG can be measured by X-ray film exposure to the gel. If 32 P is made visible on X-ray film, that indicates that PKG was present in the original sample contained PKG, which phosphorylated the PDE5 portion of the fusion protein eluted from the solid phase. Preferably in the assay, one should add to the assay buffer an excess (e.g., 100 fold) of protein kinase inhibitor ("PKI") which specifically and potently inhibits protein kinase A ("PKA") without inhibiting PKG. Inhibiting PKA is desirable since it may contribute to the phosphorylation of the PKG substrate (e.g., PDE5). By adding PKI, any contribution to phosphorylation by PKA will be eliminated, and any phosphorylation detected is highly likely to be due to PKG alone. A kit can be made for the assay of this invention, which kit contains the following pre-packaged reagents in separate containers: 1. Cell lysis buffer : 50 mM Tris-HCl, 1% NP-40, 150 mM NaCl, 1 mM EDTA, 1mM Na 3 VO 4 , 1 mM NaF, 500µM IBMX, proteinase inhibitors. 2. Protein kinase G solid phase substrate: recombinant GST-cGB-PDE5 bound Sepharose 4B (50% slurry). 3. 2x Phosphorylation buffer: 32 P-γ-ATP (3000 mCi/mmol, 5∼10 µCi/assay), 10 mM KH 2 PO 4 , 10 mM K 2 HPO 4 , 200 µM ATP, 5 mM MgCl 2 4. PKA Protein Kinase I Inhibitor Disposable containers and the like in which to perform the above reactions can also be provided in the kit. From the above, one skilled in the analytical arts will readily envision various ways to adapt the assay formats described to still other formats. In short, using at least a portion of PDE5 (or any other protein that can be selectively phosphorylated by PKG), the presence and relative amount (as compared to a control) of PKG can be ascertained by evaluating phosphorylation of the phosphorylatable protein, using a labeled phosphorylation agent. SAANDs Increase PKG Activity In Neoplastic Cells Using the PKG assay described above, the following experiments were performed to establish that SAANDs increase PKG activity due either to increase in PKG expression or an increase in cGMP levels (or both) in neoplastic cells treated with a SAAND. Test Procedures Two different types of PDE inhibitors were evaluated for their effects on PKG in neoplastic cells. A SAAND, exisulind, was evaluated since it is anti-neoplastic. Also, a non-SAAND classic PDE5 inhibitor, E4021, was evaluated to ascertain whether PKG elevation was simply due to classic PDE5 inhibition, or whether PKG elevation was involved in the pro-apoptotic effect of SAANDs inhibition of PDE5 and the novel PDE disclosed in United States Patent Application No. 09/173,375 to Liu et al filed October 15, 1998. To test the effect of cGMP-specific PDE inhibition on neoplasia containing the APC mutation, SW480 colon cancer cells were employed. SW 480 is known to contain the APC mutation. About 5 million SW480 cells in RPMI 5% serum are added to each of 8 dishes: 2 - 10cm dishes --- 30 µL DMSO vehicle control (without drug), 3 - 10cm dishes ―200 µM, 400 µM, 600 µM exisulind in DMSO, and 3 - 10cm dishes --- E4021; 0.1 µM, 1 µM and 10 µM in DMSO. The dishes are incubated for 48 hrs at 37°C in 5% CO 2 incubator. The liquid media are aspirated from the dishes (the cells will attach themselves to the dishes). The attached cells are washed in each dish with cold PBS, and 200 µL cell lysis buffer (i.e., 50 mM Tris-HCl, 1% NP-40, 150 mM NaCl, 1 mM EDTA, 1mM Na 3 VO 4 , 1 mM NaF, 500µM IBMX with proteinase inhibitors) is added to each dish. Immediately after the cell lysis buffer is added, the lysed cells are collected by scraping the cells off each dish. The cell lysate from each dish is transferred to a microfuge tube, and the microfuge tubes are incubated at 4°C for 15 minutes while gently agitating the microfuge tubes to allow the cells to lyse completely. After lysis is complete, the microfuge tubes are centrifuged full speed (14,000 r.p.m.) for 15 minutes. The supernatant from each microfuge tube is transferred to a fresh microfuge tube. A protein assay is then performed on the contents of each microfuge tube because the amount of total protein will be greater in the control than in the drug-treated samples, if the drug inhibits cell growth. Obviously, if the drug does work, the total protein in the drug-treated samples should be virtually the same as control. In the above situation, the control and the E-4021 microfuge tubes needed dilution to normalize them to the high-dose exisulind-treated samples (the lower dose groups of exisulind had to be normalized to the highest dose exisulind sample). Thus, after the protein assays are performed, the total protein concentration of the various samples must be normalized (e.g., by dilution). For each drug concentration and control, two PKG assays are performed, one with added cGMP, and one without added cGMP, as described in detail below. The reason for performing these two different PKG assays is that cGMP specifically activates PKG. When PKG activity is assayed using the novel PKG assay of this invention, one cannot ascertain whether any increase the PKG activity is due to increased cGMP in the cells (that may be caused by cGMP-specific PDE inhibition) or whether the PKG activity level is due to an increased expression of PKG protein. By determining PKG activity in the same sample both with and without added cGMP, one can ascertain whether the PKG activity increase, if any, is due to increased PKG expression. Thus, if an anti-neoplastic drug elevates PKG activity relative to control, one can establish if the drug-induced increase is due to increased PKG protein expression (as opposed to activation) in the drug-treated sample if (1) the drug-treated sample with extra cGMP exhibits greater PKG activity compared to the control sample with extra cGMP, and (2) the drug-treated sample without extra cGMP exhibits greater PKG activity relative to control. After, parallel samples with and without added cGMP are prepared, 50 µL of each cell lysate is added to 20 µL of the PDE5/GST solid phase substrate slurry described above. For each control or drug cell lysate sample to be evaluated, the reaction is started by adding phosphorylation buffer containing 10 µCi 32 P-γ-ATP solution (200 µM ATP, 4.5 mM MgCl; 5 mM KH 2 PO 4 ; 5 mM K 2 HPO 4 ;) to each mixture. The resultant mixtures are incubated at 30°C for 30 minutes. The mixtures are then centrifuged to separate the solid phase, and the supernatant is discarded. The solid phase in each tube is washed with 700 µL cold PBS. To the solid phase, Laemmli sample buffer (Bio-Rad) (30 µL) is added. The mixtures are boiled for 5 minutes, and loaded onto 7.5% SDS-PAGE. The gel is run at 150 V for one hour. The bands obtained are stained with commassie blue to visualize the 85 Kd GST-PDE5 fusion protein bands, if present. The gel is dried, and the gel is laid on x-ray film which, if the PDE5 is phosphorylated, the film will show a corresponding darkened band. The darkness of each band relates to the degree of phosphorylation. As shown in Figures 18A and 18B, the SAAND exisulind causes PKG activity to increase in a dose-dependent manner in both the samples with added cGMP and without added cGMP relative to the control samples with and without extra cGMP. This is evidenced by the darker appearances of the 85 Kd bands in each of the drug-treated samples. In addition, the SW480 samples treated with exisulind show a greater PKG phosphorylation activity with added cGMP in the assay relative to the samples treated with exisulind alone (i.e. no added cGMP). Thus, the increase in PKG activity in the drug-treated samples is not due only to the activation of PKG by the increase in cellular cGMP when the SAAND inhibits cGMP-specific PDE, the increase in PKG activity in neoplasia harboring the APC mutation is due to increased PKG expression as well. Also the fact that the E4021-treated SW480 samples do not exhibit PKG activation relative to control (see Figures 18A and 18B) shows that the increased PKG activation caused by SAANDs in neoplasia containing the APC mutation is not simply due to inhibition of classic PDE5. As an analytic technique for evaluating PKG activation, instead of x-ray film exposure as described above, the 85 Kd band from the SDS page can be evaluated for the degree of phosphorylation by cutting the band from the gel, and any 32 P incorporated in the removed band can be counted by scintillation (beta) counter in the 32 P window. To test the effect of cGMP-specific PDE inhibition on neoplasia containing the β-catenin mutation, HCT116 colon cancer cells were employed. HCT116 is known to contain the β-catenin mutation, but is known not to contain the APC mutation. The same procedure is used to grow the HCT116 cells as is used in the SW480 procedure described above. In this experiment, only exisulind and controls were used. The exisulind-treated cells yielded PKG that was phosphorylated to a greater extent than the corresponding controls, indicating that PKG activation occurred in the drug-treated cells that is independent of the APC mutation. Thus, for the purposes of the present invention, we refer to "reducing β-catenin" in the claims to refer to wild type and/or mutant forms of that protein. Confirmation of Increased PKG Expression and Decreased β-Catenin In SW 480 By Western Blot As demonstrated above, SAANDs cause an increase in PKG expression and an increase in cGMP level, both of which cause an increase in PKG activity in SAANDs-treated neoplastic cells. This increase in PKG protein expression was further verified by relatively quantitative western blot, as described below. SW480 cells treated with exisulind as described previously are harvested from the microfuge tubes by rinsing once with ice-cold PBS. The cells are lysed by modified RIPA buffer for 15 minutes with agitation. The cell lysate is spun down in a cold room. The supernatants are transferred to fresh microcentrifuge tubes immediately after spinning. BioRad DC Protein Assay (Temecula, CA) is performed to determine the protein concentrations in samples. The samples are normalized for protein concentration, as described above. 50 µg of each sample is loaded to 10% SDS gel. SDS-PAGE is performed, and the proteins then are transferred to a nitrocellulose membrane. The blotted nitrocellulose membrane are blocked in freshly prepared TBST containing 5% nonfat dry milk for one hour at room temperature with constant agitation. A goat-anti-PKG primary antibody is diluted to the recommended concentration/dilution in fresh TBST/5% nonfat dry milk. The nitrocellulose membrane is placed in the primary antibody solution and incubated one hour at room temperature with agitation. The nitrocellulose membrane is washed three times for ten minutes each with TBST. The nitrocellulose membrane is incubated in a solution containing a secondary POD conjugated rabbit anti-goat antibody for 1 hour at room temperature with agitation.. The nitrocellulose membrane is washed three times for ten minutes each time with TBST. The detection is performed by using Boehringer Mannheim BM blue POD substrate. As graphically illustrated in Figure 19, exisulind causes the drop of β-catenin and the increase of PKG, which data were obtained by Western blot. SW480 cells were treated with exisulind or vehicle (0.1% DMSO) for 48 hours. 50 pg supernatant of each cell lysates were loaded to 10% SDS-gel and blotted to nitrocellulose membrane, and the membrane was probed with rabbit-anti- β-catenin and rabbit anti-PKG antibodies. SAANDs Reduce β-Catenin Levels in Neoplastic Cells This observation was made by culturing SW480 cells with either 200, 400 or600 µM exisulind or vehicle (0.1% DMSO). The cells are harvested 48hours post treatment and processed for immunoblotting.. Immuno-reactive protein can be detected by Western blot. Western blot analysis demonstrated that expression of β-catenin was reduced by 50 % in the exisulind-treated cells as compared to control. These results indicate that β-catenin is reduced by SAANDs treatment. Together with the results above establishing PKG activity increases with such treatment and the results below establishing that β-catenin is phosphorylated by PKG, these results indicate that the reduction of β-catenin in neoplastic cells is initiated by activation of PKG. Thus, using PKG activity in neoplasia as a screening tool to select compounds as anti-neoplastics is useful. The Phosphorylation of β-catenin By PKG In vitro, PKG phosphorylates β-catenin. The experiment that established this involves immunoprecipitating the β-catenin-containing complex from SW480 cells (not treated with any drug) in the manner described below under "β-catenin immunoprecipitation" The immunoprecitated complex, while still trapped on the solid phase (i.e., beads) is mixed with 32 P-γ-ATP and pure PKG (100 units). Corresponding controls with out added PKG are prepared. The protein is released from the solid phase by SDS buffer, and the protein-containing mixture is run on a 7.5%SDS-page gel. The running of the mixture on the gel removes excess 32 P-γ-ATP from the mixture. Any 32 P-γ-ATP detected in the 93Kd β-catenin band, therefore, is due to the phosphorylation of the β-catenin. Any increase in 32 P-γ-ATP detected in the 93 Kd β-catenin band treated with extra PKG relative to the control without extra PKG, is due to the phosphorylation of the β-catenin in the treated band by the extra PKG. The results we obtained were that there was a noticeable increase in phosphorylation in the band treated with PKG as compared to the control, which exhibited minimal, virtually undetectable phosphorylation. This result indicates that β-catenin can be phosphorylated by PKG. The Phosphorylation of Mutant β-catenin By PKG The same procedure described in the immediately preceding section was performed with HCT116 cells, which contain no APC mutation, but contain a β-catenin mutation. The results of those experiments also indicate that mutant β-catenin is phosphorylated by PKG. Thus, for the purposes of the present invention, we refer to the phosphorylation of β-catenin in the claims to refer to the phosphorylation of wild type and/or mutant forms of that protein. β-Catenin Precipitates With PKG Supernatants of both SW480 and HCT116 cell lysates are prepared in the same way described above in the Western Blot experiments. The cell lysate are precleared by adding 150 µl of protein A Sepharose bead slurry (50%) per 500 µg of cell lysate and incubating at 4°C for 10 minutes on a tube shaker. The protein A beads are removed by centrifugation at 14,000 x g at 4°C for 10 minutes. The supernatant are transferred to a fresh centrifuge tube. 10 µg of the rabbit polyclonal anti-β-catenin antibody (Upstate Biotechnology, Lake Placid, New York) are added to 500 µg of cell lysate. The cell lysate/antibody mixture is gently mixed for 2 hours at 4°C on a tube shaker. The immunocomplex is captured by adding 150 µl protein A Sepharose bead slurry (75 µl packed beads) and by gently rocking the mixture on a tube shaker for overnight at 4°C. The Sepharose beads are collected by pulse centrifugation (5 seconds in the microcentrifuge at 14,000 rpm). The supernatant fraction is discarded, and the beads are washed 3 times with 800 µl ice-cold PBS buffer. The Sepharose beads are resuspended in 150 µl 2 x sample butter and mixed gently. The Sepharose beads are boiled for 5 minutes to dissociate the immunocomplexes from the beads. The beads are collected by centrifugation and SDS-PAGE is performed on the supernatant. A Western blot is run on the supernatant, and the membrane is then probed with an rabbit anti β-catenin antibody. Then the membrane is washed 3 times for 10 minutes each with TBST to remove excess anti β-catenin antibody. A goat, anti-rabbit antibody conjugated to horseradish peroxidase is added, followed by 1 hour incubation at room temperature. When that is done, one can visualize the presence of β-catenin with an HRPO substrate. In this experiment, we could clearly visualize the presence of β-catenin. To detect PKG on the same membrane, the anti-β-catenin antibody conjugate is first stripped from the membrane with a 62 mM tris-HCl buffer (pH 7.6) with 2 % SDS and 100 µM 2β-mercaptoethanol in 55°C water bath for 0.5 hour. The stripped membrane is then blocked in TBST with 5% non-fat dried milk for one hour at room temperature while agitating the membrane. The blocked, stripped membrane is then probed with rabbit polyclonal anti-PKG antibody (Calbiochem, LaJolla, CA), that is detected with goat, anti-rabbit second antibody conjugated to HRPO. The presence of PKG on the blot membrane is visualized with an HRPO substrate. In this experiment, the PKG was, in fact, visualized. Given that the only proteins on the membrane are those that immunoprecipitated with β-catenin in the cell supernatants, this result clearly establishes that PKG was physically linked to the protein complex containing the β-catenin in the cell supernatants. The same Western blot membrane was also probed after stripping with anti-GSK3-β antibody to ascertain whether it also co-precipitated with β-catenin. In that experiment, we also detected GSK3-β on the membrane, indicating that the GSK3-β precipitated with the GSK3-β and PKG, suggesting that the three proteins may be part of the same complex. Since GSK3-β and β-caten form part of the APC complex in normal cells, this that PKG may be part of the same complex, and may be involved in the phosphorylation of β-catenin as part of that complex. Anti-Neoplastic Pharmaceutical Compositions Containing cGMP PDE Inhibitors As explained above, exisulind is one compound that exhibits desirable anti-neoplastic properties. Its efficacy and use as an anti-neoplastic was discovered before it was understood that the compound acted by inhibiting cGMP-specific PDE activity in neoplastic cells. Among other things, the verification that the selection process of this invention could be used to select compounds for human treatment was obtained in human clinical trials in patients with neoplasias. By understanding after the fact that exisulind was anti-neoplastic (in vitro), that it had the profile of a desirable compound meeting the selection criterion of this invention, the success of the compound in two human clinical trials establishes that other compounds can be selected meeting the selection criterion of this invention. As indicated above, a number of neoplasias harbor the APC mutation, Among other things, the verification of the selection process of this invention was established in human clinical trials in patients with neoplasia harboring the APC mutation. The APC mutation was first discovered in patients with the hereditary neoplasia, adenomatous polyposis coli ("APC"). The APC disease is characterized by the appearance in the teen years of hundreds to thousands of polyps in the colon, and the common therapy is surgical removal of the colon before the age of 20. The first clinical trial involved patients with APC using exisulind. In that study, each patient had already had his/her colon removed, except for a small section of colon adjacent the rectum (where the small intestine was attached) to preserve rectal function. However, such a patient commonly forms polyps in the small remaining colonic section, which polyps require periodic removal (e.g., by electrocautery). That trial where exisulind was selected was a prevention trial designed to evaluate the anti-neoplastic characteristics of the drug by comparing the cumulative number of new polyps formed over twelve months by the drug and placebo groups. Eligible patients were those who form between 9 and 44 polyps per year. Patients were fully ablated (had all polyps removed) at the start of the study, at the end of 6 months and at the end of 12 months. The study enrolled thirty-four eligible patients. Based on the estimated mean number of polyps formed over a year in APC patients who had historically produced 9 to 44 polyps per year, exisulind was clinically and statistically significantly better than placebo in decreasing the rate of polyp formation. Based on the median number of polyps produced in the first six months of the study, patients treated with exisulind developed approximately one-third the number of polyps as patients treated with placebo (median values 9 polyps/year and 26 polyps/year, respectively; p = 0.013). Based on the median number of polyps produced over the entire 12 months of the study, patients treated with exisulind produced approximately half the number of polyps as patients treated with placebo (median values 18 polyps/year and 38 polyps/year, respectively; p = 0.020). A separate clinical trial was also performed on male patients who had prostate cancer, and as a result had their prostates removed. The study was conducted in patients with detectable PSA (prostate specific antigen) levels that were rising following radical prostatectomy, indicating recurrence of prostate cancer. 96 patients were enrolled in the prostate cancer evaluation: a double-blind, placebo-controlled, multi-center trial involving exisulind administered to the drug-receiving patients at 500 mg/day. As presented below, the data show a statistically significant difference in PSA levels between the exisulind-treated group and the placebo-treated group. PSA levels in the exisulind-treated group were significantly reduced as compared with the PSA levels of the placebo-treated group. Although a rising level of PSA is not itself a disease condition, it is widely regarded in the medical community as a surrogate marker indicative of the presence of recurrence of prostate cancer in such men. In addition to performing an evaluation based on the differences in mean PSA levels between the exisulind and placebo groups as a whole, the interim analysis included subgroup analysis. The patients in the study were classified into high, intermediate and low risk groups in terms of their risk of developing metastatic disease. This classification was performed using the methodology published in the Journal of the American Medical Association ( JAMA May 5, 1999, pp. 1591-97). To ascertain which study patients fell into which risk group, medical histories were supplied to a researcher who was blinded as to whether patients were on drug or placebo; he assigned study patients to the appropriate risk groups according to the above referenced published methodology. The statistical analysis revealed statistically significant differences in mean PSA levels between exisulind and placebo patients in both high and intermediate risk groups. The data from the prostate study are as follows: Effect of Exisulind On Mean PSA Level In Men Post-Prostatectomy With Rising PSA Group Placebo Exisulind "p" value Overall 4.49 2.85 0.0004 High Risk 4.98 2.91 0.0002 Intermediate Risk 6.24 2.95 0,0053 In these exisulind trials and several others involving the drug in other indications, safety was evaluated by monitoring adverse events (AEs), clinical laboratory tests (hematology, serum chemistry, and urinalysis), vital signs (blood pressure, pulse rate, respiratory rate, temperature, and weight), physical examination, and upper endoscopy. No outstanding safety issues have been demonstrated in the clinical trials conducted with exisulind to date in over 400 patients. Exisulind did not demonstrate any blood dyscrasia, dose-limiting vomiting, or neurological or renal toxicities associated with convention chemotherapeutics. It also did not cause any clinically significant changes in vital signs. In fact, in paired biopsies of polyp and normal colonic tissues in APC patients, it was found that exisulind increased apoptosis rates in polyp, but not normal colonic tissues, suggesting minimal effects on normal tissues. At doses above the maximum tolerated dose (MTD = 600 mg in patients with subtotal colectomy; 400 mg in patients with intact colons; 350 mg in pediatric patients), the only dose-limiting adverse events found were elevations in liver function tests (LFTs) that are seen early during treatment. When experienced, LFT elevations were rapidly reversible, and do not recur when the dose has been lowered. Other events (e.g., occasional abdominal pain) were typically short lasting and of mild to moderate intensity, and did not necessitate discontinuing or lowering of the exisulind dose. In short, these trials demonstrated that exisulind is an effective, well-tolerated chronic therapy for the clinical management of neoplasia. Thus, these results illustrate that selecting an additional compound that inter alia inhibits cGMP-specific PDE activity (as well as meeting the other selection criteria of this invention) can result in a therapeutically effective drug, in vivo . A second drug that was also invented before its mechanism of action was found to involve cGMP inhibition and before it was known to meet the selection criterion of this invention is (Z)-5-fluoro-2-methyl-(4-pyridylidene)-3-(N-benzyl)indenylacetamide hydrochloride (Compound I). It has been demonstrated in in vitro and in vivo evaluations as anti-neoplastic having activities against a broad range of neoplasias. It is also safe in animal studies and in a single, escalating dose human study. As one skilled in the art will recognize from the data presented below, Compound I can safely be given to animals at doses far beyond the tolerable (and in many cases toxic) doses of conventional chemotherapeutics or anti-neoplastic NSAIDs. For example, in an acute toxicity study in rats, single oral doses of Compound I administered (in a 0.5% carboxy-methylcellulose vehicle) at doses up to and including 2000 mg/kg resulted in no observable signs of toxicity. At 4000 mg/kg, body weight gains were slightly reduced. A single dose of 1000 mg/kg administered intraperitoneally resulted in reduced body weight gain, with mesenteric adhesions seen in some animals from this group at necropsy. In dogs, the administration of Compound I in capsules at 1000 mg/kg resulted in no signs of toxicity to the single group of two male and two female dogs. Due to the nature of Compound I capsules, this dose necessitated the use of at least 13 capsules to each animal, which was judged to be the maximum number without subjecting the animals to stress. Therefore, these dogs were subsequently administered seven consecutive doses of 1000 mg/kg/day. At no time in either dosing phase were any obvious signs of drug-related effects observed. Thus, on a single-dose basis, Compound I is not acutely toxic. Based on the findings of these studies, the oral LD 50 of Compound I was considered to be greater than 1000 mg/kg in dogs and 4000 mg/kg in rats, and the intraperitoneal LD 50 was considered to be greater than 1000 mg/kg in rats. A seven-day dose-range finding study in rats, where Compound I was evaluated by administering it at doses of 0, 50, 500 or 2000 mg/kg/day resulting in no observable signs of toxicity at 50 mg/kg/day. At 500 mg/kg/day, treatment-related effects were limited to an increase in absolute and relative liver weights in female rats. At 2000 mg/kg/day, effects included labored breathing and/or abnormal respiratory sounds, decreased weights gains and food consumption in male rats, and increased liver weights in female rats. No hematological or blood chemistry changes nor any microscopic pathology changes, were seen at any dose level. A 28-day study in rats was also carried out at 0, 50, 500 and 2000 mg/kg/day. There were no abnormal clinical observations attributed to CP-461, and body weight changes, ophthalmoscopic examinations, hematological and blood chemistry values and urinalysis examinations were unremarkable. No macroscopic tissue changes were seen at necropsy. Organ weight data revealed statistically significant increase in liver weights at 2000 mg/kg/day, and statistically significant increases in thyroid weights for the 2000 mg/kg/day group. The slight increases at the lower doses were not statistically significant. Histopathological evaluation of tissues indicated the presence of traces of follicular cell hypertrophy, increased numbers of mitotic figures (suggestive of possible cell proliferation) in the thyroid gland and mild centrilobular hypertrophy in the liver. These changes were generally limited to a small number of animals at the 2000 mg/kg/day dose, although one female at 500 mg/kg/day had increased mitotic figures in the thyroid gland. The findings in the liver may be indicative of a very mild stimulation of microsomal enzymes, resulting in increased metabolism of thyroid hormones, which in turn resulted in thyroid stimulation. Thus, one skilled in the art will recognize that these effects are extremely minimal compared to what one would expect at similar doses of conventional chemotherapeutics or NSAIDs. To further establish the safety profile of Compound I, a study was performed to evaluate whether Compound I-induced apoptosis of prostate tumor cell lines was comparable to its effects on prostate epithelial cells derived from normal tissue. The androgen-sensitive prostate tumor cell line, LNCaP (from ATCC (Rockville, MD)) was propogated under standard conditions using RPMI 160 medium containing 5% fetal calve serum and 2 mM glutamine. Primary prostate epithelial cell cultures (PrEC) derived from normal prostate (from Clonetics Inc. (San Diego, CA)) were grown under the same conditions as the tumor cell line except a serum-free medium optimized for the growth of such cultures was used (Clonetics Inc). For the experiments, LNCaP or PrEC cells were seeded in 96 well plates at a density of 10,000 cells per well. After 24 hours, the cells were treated with either vehicle (0.1% DMSO) or 50 µM Compound I (free base) solubilized in DMSO. After various drug treatment times (4, 24, 48, 72, or 99 hours) the cells were lysed and processed for measurement of histone-associated DNA as an indicator of apoptotic cell death (see, Piazza et al., Cancer Research 57: 2452-2459, 1997). Figure 27 shows a time-dependent increase in the amount of histone-associated fragmented DNA in LNCaP cell cultures following treatment with 50 µM Compound I(free base). A significant increase in fragmented DNA was detected after 24 hours of treatment, and the induction was sustained for up to 4 days of continuous treatment. By contrast, treatment of PrEC ("normal"" prostate) cells with Compound I (50 µM) did not affect DNA fragmentation for up to 4 days of treatment. These results demonstrate a selective induction of apoptosis in neoplastic cells, as opposed to normal cells. This is in marked contrast to conventional chemotherapeutics that induce apoptosis or necrosis in rapidly growing normal and neoplastic cells alike. Finally as to safety, in a single, escalating dose human clinical trial, patients, human safety study in which the drug was taken orally, Compound I produced no significant side effects at any dose, including doses above the level predicted to be necessary to produce anti-cancer effects. As indicated above, Compound I also exhibits potent anti-neoplastic properties. The growth inhibition IC 50 value obtained for Compound I was 0.7 µM in the SW-480 cell line. This result has been confirmed by evaluating Compound I in rodents using aberrant crypt foci ("ACF") as an indicator of carcinogenesis (see, Bird, Cancer Lett. 37: 147-151, 1987). This established rodent model of azoxymethane ("AOM")-induced carcinogenesis was used to assess the effects of Compound I (free base and salt) on colon cancer development in vivo . ACF are precursors to colonic tumors, and ACF inhibition is predictive of chemo-preventive efficacy. In the rats in this experiment, ACF initiation was achieved by two consecutive weekly injections of the carcinogen. Compound I was administered one week prior to ACF initiation and for the duration of the experiment. ACFs were scored after 5 weeks of treatment. Compound I was administered orally to male Fisher 344 rats in the rat chow. Daily food consumption (mg/kg body weight) varied over the course of the study, and therefore Compound I dose was expressed a grams per kg of diet to provide a basis of comparison between doses. To determine if Compound I had an adverse effect on growth and/or feeding behavior, body weight was determined throughout the course of the experiment. The experimental groups gained less weight than the controls, which was indicative of bioavailability. However, the weight differences were less than 10% and not considered to affect ACF formation. The free base of Compound I inhibited ACF formation as measured by a reduction of crypts per colon. The data are summarized in Table 11. With the exception of the low dose group (only 0.5 g/kg diet), the differences between treatment and control groups were substantial, and statistically significant in the case of the 1.0 and 2.0 g/kg diet group. Inhibition of Aberrant Crypt Foci by Compound I Compound n Mean ACF/colon % Control p (t-test) Dose (g/kg diet) (+ SE) vs. control Control 10 149 ± 9 - - 0.5 7 149 ± 14 100 0.992 1 10 111±9 75 0.008 1.5 10 132±4 89 0.101 2.0 10 107±15 72 0.029 Also, Compound I retrospectively met the selection criterion of this invention, and was one of the compounds used to establish the validity of this selection criteria. For example, using the protocols described previously, Compound I has a cGMP-specific PDE IC 50 value of 0.68 µM utilizing cGMP-specific PDE from HT29 cell extracts. Its COX I inhibition (at 100µM) was less than 25%. As for being pro-apoptotic, Compound I's DNA fragmentation EC 50 was 15 µM. In addition, the percent apoptosis for Compound I in SW-480 is shown in Table 12 at various drug concentrations. Apoptosis Induction of HT-29 Cells of SW-480 Colon Adenocarcinoma Cells by Compound I as Determined by Morphology Treatment Dose %Apoptosis Vehicle (0.1%DMSO) - 1 Compound I 0.35 µM 16 Compound I 0.7 µM 27 Compound I 1.5 µM 88 56 Compound I's activity is not confined to activity against colon cancer cell lines or animal models of colon cancer. It has a broad range of anti-neoplastic effects in various neoplastic cell lines. Various types of human cancer cell lines were propagated under sterile conditions in RPMI 1640 medium with 10% fetal bovine serum, 2 mM L-glutamine and sodium bicarbonate. To determine growth inhibitory effects of Compound I, cells were seeded in 96-well plates at a density of 1000 cells per well. Twenty-four hours after plating, the cells were dosed with various concentrations of the free base of Compound I solubilized in DMSO (final concentration 0.1%). The effect of the drug on tumor cell growth was determined using the neutral red cytotoxicity assay following five days of continuous treatment. Neutral red is a dye that is selectively taken up by viable cells by an ATP-dependent transport mechanism. As summarized in Table 13, Compound I (free base) displayed potent growth inhibitory activity when evaluated against a panel of cultured human cell lines derived from various tissue origins. Compound I displayed comparable growth inhibitory effects regardless of the histogenesis of the tumor from which the cell lines were derived. The GI 50 value (concentration of drug to inhibit growth by 50% relative to vehicle control) calculated for all cell lines was 1-2 µM. In addition to the data in the table below, we observed comparable sensitivity of human leukemia cell lines (CCRF-CEM, K562, and Molt-4), a myeloma cell line (RPMI8226), a pancreatic tumor cell line (PAN-1), and an ovarian tumor cell line (OVCAR-3) to Compound I (HCl salt). Growth Inhibition of Various Human Tumor Cell Lines by Compound I Cell Line Tumor origin GI 50 µM GI 90 µM Colo 205 Colon 1.6 2.4 HCT-15 Colon 1.7 3.0 HT-29 Colon 2.1 8.0 SW-620 Colon 1.7 2.5 DU145 Prostate 1.6 2.8 PC-3 Prostate 1.7 82.5 NCI-H23 Lung 1.7 2.5 NCI-H322M Lung 2.1 13.2 NCI-H460 Lung 1.9 30.0 NCI-H82 Lung 1.7 5.8 MDA-MB-231 Breast 1.8 77.6 MDA-MB-435 Breast 1.6 2.3 UISO-BCA-1 Breast 1.5 4.7 Molt-4 Leukemia 1.6 ND CCRF-CEM Leukemia 1.4 ND K-562 Leukemia 1.8 ND RPMI-8226 Myeloma 1.2 ND OVCAR Ovary 1.2 ND PANC-1 Pancreas 2.2 ND Given the animal and human safety characteristics, and the animal and very broad cell culture efficacy of Compound I, it is clear that compounds meeting the selection criteria of this invention (including cGMP-specific PDE inhibition) are useful anti-neoplastic therapeutics. As to identifying structurally additional cGMP-specific PDE inhibiting compounds that can be effective therapeutically as anti-neoplastics, one skilled in the art has a number of useful model compounds disclosed herein (as well as their analogs incorporated by reference) that can be used as the bases for computer modeling of additional compounds having the same conformations but different chemically. For example, software such as that sold by Molecular Simulations Inc. release of WebLab® ViewerPro™ includes molecular visualization and chemical communication capabilities. Such software includes functionality, including 3D visualization of known active compounds to validate sketched or imported chemical structures for accuracy. In addition, the software allows structures to be superimposed based on user-defined features, and the user can measure distances, angles, or dihedrals. In this situation, since the structures of other active compounds are disclosed above, one can apply cluster analysis and 2D and 3D similarity search techniques with such software to identify potential new additional compounds that can then be screened and selected according to the selection criteria of this invention. These software methods rely upon the principle that compounds, which look alike or have similar properties, axe more likely to have similar activity, which can be confirmed using the selection criterion of this invention. Likewise, when such additional compounds are computer modeled, many such compounds and variants thereof can be synthesized using known combinatorial chemistry techniques that are commonly used by those of ordinary skill in the pharmaceutical industry. Examples of a few for-hire combinatorial chemistry services include those offered by New Chemical Entities, Inc. of Bothell Washington, Protogene Laboratories, inc., of Palo Alto, California, Axys, Inc. of South San Francisco, California, Nanosyn, Inc. of Tucson, Arizona, Trega, Inc. of San Diego, California, and RBI, Inc. of Natick, Mass. There are a number of other for-hire companies. A number of large pharmaceutical companies have similar, if not superior; in-house capabilities. In short, one skilled in the art can readily produce many compounds for screening from which to select promising compounds for treatment of neoplasia having the attributes of compounds disclosed herein. To further assist in identifying compounds that can be screened and then selected using the criterion of this invention, knowing the binding of selected and-neoplastic compounds to PDE5 protein is of interest By the procedures discussed below, it was found that preferable, desirable compounds meeting the selection criteria of this invention bind to the cGMP catalytic region of PDE5. To establish this, a PDE5 sequence that does not include the catalytic domain was used. One way to produce such a sequence is to express that sequence as a fusion protein, preferably with glutiathione S-transferase ("GST"), for reasons that will become apparent. RT-PCR method is used to obtain the cGB domain of PDE5 with forward and reverse primers designed from bovine PDE5A cDNA sequence ( McAllister-Lucas L. M. et al, J . Biol. Chem. 268, 22863-22873, 1993 ) and the selection among PDE 1-10 families. 5'-3', Inc. kits for total RNA followed by oligo (dT) column purification of mRNA are used with HT-29 cells. Forward primer (GAA-TTC-TGT-TAG-AAA-AGC-CAC-CAG-AGA-AAT-G, 203-227) and reverse primer (CTC-GAG-CTC-TCT-TGT-TTC-TTC-CTC-TGC-TG, 1664-1686) are used to synthesize the 1484 bp fragment coding for the phosphorylation site and both low and high affinity cGMP binding sites of human PDE5A (203-1686 bp, cGB-PDE5). The synthesized cGB-PDE5 nucleotide fragment codes for 494 amino acids with 97% similarity to bovine PDE5A. It is then cloned into pGEX-5X-3 glutathione-S-transferase (GST) fusion vector (Pharmacia Biotech )with tac promoter, and EcoRI and Xhol cut sites. The fusion vector is then transfected into E. Coli BL21 (DE3) bacteria (Invitrogen). The transfected BL21 bacteria is grown to log phase, and then IPTG is added as an inducer. The induction is carried at 20°C for 24 hrs. The bacteria are harvested and lysed. The soluble cell lysate is incubated with GSH conjugated Sepharose 4B (GSH-Sepharose 4B). The GST-cGB-PDE5 fusion protein can bind to the GSH-Sepharose beads, and the other proteins are washed off from beads with excessive cold PBS. The expressed GST-cGB-PDE5 fusion protein is displayed on 7.5% SDS-PAGE gel as an 85 Kd protein. It is characterized by its cGMP binding and phosphorylation by protein kinases G and A. It displays two cGMP binding sites, and the Kd is 1.6±0.2 µM, which is close to K d =1.3 µM of the native bovine PDE5. The GST-cGB-PDE5 on GSH-conjugated sepharose beads can be phosphorylated in vitro by cGMP-dependent protein kinase and cAMP-dependent protein kinase A. The K m of GST-cGB-PDE5 phosphorylation by PKG is 2.7µM and Vmax is 2.8 µM, while the K m of BPDEtide phosphorylation is 68µM. The phosphorylation by PKG shows molecular phosphate incorporated into GST-cGB-PDE5 protein on a one-to-one ratio. A cGMP binding assay for compounds of interest (Francis S. H. et al, J. Biol. Chem. 255, 620-626, 1980) is done in a total volume of 100 µL containing 5 mM sodium phosphate buffer (pH=6.8), 1 mM EDTA, 0.25 mg/mL BSA, H 3 -cGMP (2µM, NEN) and the GST-cGB-PDE5 fusion protein (30 µg /assay). Each compound to be tested is added at the same time as 3 H-cGMP substrate, and the mixture is incubated at 22°C for 1 hour. Then, the mixture is transferred to Brandel MB-24 cell harvester with GF/B as the filter membrane followed by 2 washes with 10 mL of cold 5 mM potassium buffer( pH 6.8). The membranes are then cut out and transferred to scintillation vials followed by the addition of 1 mL of H 2 O and 6 mL of Ready Safe™ liquid scintillation cocktail to each vial. The vials are counted on a Beckman LS 6500 scintillation counter. For calculation, blank samples are prepared by boiling the binding protein for 5 minutes, and the binding counts are < 1% when compare to unboiled protein. The quenching by filter membrane or other debris are also calibrated. PDE5 inhibitors, sulfide, exisulind, Compound B, Compound E, E4021 and zaprinast, and cyclic nucleotide analogs, cAMP, cyclic IMP, 8-bromo-cGMP, cyclic UMP, cyclic CMP, 8-bromo-cAMP, 2'-O-butyl-cGMP and 2'-O-butyl-cAMP are selected to test whether they could competitively bind to the cGMP binding sites of the GST-cGB-PDE5 protein. The results were shown in Figure 24. cGMP specifically binds GST-cGB-PDE5 protein. Cyclic AMP, cUMP, cCMP, 8-bromo-cAMP, 2'-O-butyl-cAMP and 2'-O-butyl-cGMP did not compete with cGMP in binding. Cyclic IMP and 8-bromo-cGMP at high concentration (100 µM) can partially compete with cGMP (2 µM) binding. None of the PDES inhibitors showed any competition with cGMP in binding of GST-cGB-PDE5. Therefore, they do not bind to the cGMP binding sites of PDE5. However, Compound E does competitively (with cGMP) bind to PDE 5 (i.e., peak A). (Compound E also competitively (with cGMP) binds to PDE peak B.). Given that Compound E does not bind to the cGMP-binding site of PDE5, this the fact that there is competitive binding between Compound E and cGMP at all means that desirable compounds such as Compound E bind to the cGMP catalyic site on PDE5, information that is readily obtainable by one skilled in the art (with conventional competitive binding experiments) but which can assist one skilled in the art more readily to model other compounds. Thus, with the chemical structures of desirable compounds presented herein and the cGMP binding site information, one skilled in the art can model, identify and select (using the selection criteria of this invention) other chemical compounds for use as therapeutics. accordance with the desired method of administration ( i.e ., oral or rectal). The selected dosage level therefore depends upon the nature of the active compound administered (e.g., its IC 50 , which can be readily ascertained), the route of administration, the desired duration of treatment, and other factors. If desired, the unit dosage may be such that the daily requirement for active compound is in one dose, or divided among multiple doses for administration, e.g. , two to four times per day. For IV administration, an initial dose for administration can be ascertained by basing it on the dose that achieves the IC 50 in the plasma contents of the average adult male (i.e., about 4 liters). Initial doses of active compound selected in accordance with this invention can range from 0.5-600 mg. The pharmaceutical compositions of this invention are preferably packaged in a container ( e.g., a box or bottle, or both) with suitable printed material ( e.g ., a package insert) containing indications, directions for use, etc.

Claims: A method for selecting a compound for the treatment of neoplasia, comprising: determining the cyclooxygenase (COX) inhibitory activity of the compound; determining the PDE2 inhibition activity of the compound; and selecting the compound that has COX inhibitory activity lower than said PDE activity for treating neoplasia. The method of claim 1 wherein said compound is further selected by determining whether the compound inhibits tumor cell growth in a neoplastic cell culture; and selecting the compound that wherein inhibition of neoplastic cell growth for treating neoplasia. The method of claim 2, wherein growth inhibition is ascertained by evaluating whether the compound induces apoptosis. A method for selecting a compound for the treatment of neoplasia, comprising: determining the neoplastic cell growth inhibitory activity of the compound; determining the PDE2 inhibition activity of the compound; and selecting the compound that exhibits neoplastic cell growth inhibitory activity and activity inhibitory of said PDE. The method of claim 4, further comprising determining whether the compound induces apoptosis in neoplastic cells; and selecting the compound that induces apoptosis. The method of claim 5, further comprising: determining the COX inhibitory activity of the compound; and selecting the compound with lower COX inhibitory activity relative to the inhibitory activity of the compound against said PDE. A method for selecting a compound for the treatment of neoplasia, comprising determining the PDE2 inhibition activity of the compound and selecting the compound with such inhibitory activity. The method of claim 7 wherein the compound is further selected by determining whether the compound induces apoptosis in a neoplastic cell, and selecting the compound with apoptosis-inducing activity. A method for identifying a compound for treating neoplasia, comprising determining the cyclooxygenase (COX) inhibitory activity of the compound; and determining the PDE inhibition activity of the compound wherein the PDE is characterized by : (a) cGMP specificity over cAMP; (b) positive cooperative kinetic behavior in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase selecting the compound that has COX inhibitory activity lower than said PDE inhibitory activity. The method of claim 9, further comprising determining whether the compound inhibits tumor cell growth in a culture; and selecting the compound that inhibits tumor cell growth for treating neoplasia. The method of claim 9, further comprising: (a) determining whether the compound induces apoptosis of a tumor cell; and (b) selecting the compound that induces apoptosis for treating neoplasia. A method of selecting a compound for treating neoplasia, comprising determining the growth inhibitory activity of the compound; determining the PDE inhibition activity of the compound wherein the PDE is characterized by : (a) cGMP specificity over cAMP; (b) positive cooperative kinetic behavior in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase; and selecting the compound that exhibits growth inhibitory activity and activity inhibitory of said PDE. A method for screening a compound with potential for treating neoplasia comprising determining the PDE inhibition activity of a test compound wherein the PDE is characterized by : (a) cGMP specificity over cAMP; (b) positive cooperative kinetic behavior in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase; and selecting the compound with such inhibitory activity. The method of claim 13 further comprising determining whether the compound induces apoptosis in a cell and further selecting the compound with apoptotic inducing activity. The method of claim 13 further comprising determining whether the selected compound inhibits prostaglandin synthesis and further selecting the compound with insubstantial prostaglandin inhibitory activity. The method of claim 13 wherein said PDE activity is ascertained by isolating cGMP-specific PDE activities from a neoplastic cell line containing a PDE5 and said novel PDE, inhibiting the PDE5 activity by exposing said combined PDE isolate to a PDE5 inhibitor at a concentration that does not inhibit the novel PDE, and ascertaining the inhibitory activity of the test compound against the remaining cGMP activity, whereby the inhibitory activity of the test compound against the novel PDE can be determined. An isolated phosphodiesterase characterized by having: (a) cGMP specificity over cAMP; (b) positive cooperative kinetic behavior in the presence of cGMP substrate; (c) submicromolar affinity for cGMP; and (d) insensitivity to incubation with purified cGMP-dependent protein kinase. The isolated phosphodiesterase of claim 17 further characterized in that it can be separated from classical PDE5 activity by anion-exchange chromatography. The isolated phosphodiesterase of claim 18 further characterized in that it is not substantially activated by calcium/calmodulin. The isolated phosphodiesterase of claim 19 further characterized in that it is insensitive to rolipram, vinpocetine and indolidan. A method of selecting a compound for treatment of a neoplasia to be treated, comprising: (a) evaluating the anti-neoplastic activity of the compound against the neoplasia to be treated; (b) evaluating whether the compound increases PKG activity in the neoplasia to be treated; and (c) selecting the compound that exhibits anti-neoplastic activity and the ability to cause an increase PKG activity in the neoplasia to be treated. The method of claim 21 further comprising evaluating whether the compound inhibits PDE5, and selecting the compound that inhibits PDE5. The method of claim 21 further comprising evaluating whether the compound reduces b-catenin in the neoplasia to be treated, and selecting the compound that so reduces b-catenin. The method of claim 21 further comprising evaluating whether the compound inhibits cGMP-specific phosphodiesterase ("PDE") and selecting the compound that inhibits said PDE. The method of claim 21 further comprising evaluating whether the compound increases PKG expression, and selecting the compound if it increases PKG expression. The method of claim 21 further comprising evaluating whether the compound increases PKG activation, and selecting the compound if it increases PKG activation. The method of claim 21 further comprising evaluating whether the compound inhibits PDE2, and selecting the compound that inhibits PDE2. A method of selecting a compound for treatment of a neoplasia to be treated, comprising: (a) evaluating whether the compound increases PKG activity in intact neoplastic cell in the neoplasia to be treated; (b) evaluating whether the compound reduces b-catenin in neoplastic cells; and (c) selecting the compound that causes an increase PKG activity in intact neoplastic cell and causes a decrease in b-catenin in the neoplasia to be treated. A method for identifying a compound with potential for treating neoplasia, comprising: selecting a compound that increases PKG activity in the neoplasia; and evaluating the neoplasia growth inhibiting activity of the compound wherein a compound that increases PKG activity and has neoplasia growth inhibiting activity has the potential to inhibit neoplasia without substantially inhibiting the growth of normal cells. A method for identifying a compound with potential for treating neoplasia, comprising determining the cyclooxygenase (COX) inhibitory activity of the compound; and determining whether the compound increases PKG activity in neoplastic cells; wherein low COX inhibitory activity and an increase in PKG activity is indicative that the compound has potential for treating neoplasia. A method of selecting a compound for treating neoplasia, comprising determining the neoplastic cell growth inhibitory activity of the compound; determining whether the compound increases PKG activity in neoplastic cells; and selecting the compound that exhibits neoplastic cell growth inhibitory activity and an increase in PKG activity in neoplastic cells. A method for selecting a compound for treatment of a neoplasia to be treated, comprising evaluating whether PKG causes β-catenin to be phosphorylated in the neoplasia to be treated with the compound, and selecting the compound that causes PKG to phosphorylate β-catenin as the compound to treat the neoplasia to be treated.

Inventors: HAN LI, LIU LI, PAMUKEU RIFAT, PIAZZA GARY A, THOMPSON JOSEPH W, ZHU BING, HAN, LI, LIU, LI, PAMUKEU, RIFAT, PIAZZA, GARY A., THOMPSON, JOSEPH W., ZHU, BING
Applicants: CELL PATHWAYS INC, CELL PATHWAYS, INC.
Application year: 1999
Main classification: A61K31
IPC codes: G01N33, A61K31, A61K38, A61P35, C12N9, C12Q1, A61K45, A61P43