COMPOUNDS FOR PREVENTING MULTIDRUG RESISTANCE IN CANCER CELLS CONTAINING PROTEIN KINASE INHIBITORS

Abstract: This invention is directed to methods for preventing the emergence of multidrug resistance in tumor cells during cancer chemotherapy. In particular, it relates to the use of protein kinase inhibitors to prevent the induction of expression of the multidrug resistance gene (MDR1) encoding P-glycoprotein by chemotherapeutic drugs. MDR1 expression, which results in tumor cell resistance to subsequent treatment with certain chemotherapeutic drugs, is shown herein to be induced in response to treatment with various cytotoxic agents, including such agents that are and are not substrates for P-glycoprotein-mediated efflux from cancer cells. Inhibitors of protein kinases, in particular protein kinase C, are shown herein to suppress this cellular response. In addition, such protein kinase inhibitors are also shown herein to inhibit expression of a gene encoding a multidrug resistance-associated protein (the MRP gene). The instant invention provides methods for using such protein kinase inhibitors to both suppress induction of MDR1 gene expression by cytotoxic drugs, and to inhibit expression of MRP. The invention also provides methods for identifying protein kinase inhibitors that have either or both of these effects on MDR1 and MRP expression. Thus, the invention provides useful methods and reagents for preventing the emergence of multidrug resistance in tumor cells treated with cytotoxic and chemotherapeutic drugs in cancer patients undergoing chemotherapy, when such protein kinase inhibitors are administered prior to or simultaneous with cytotoxic drug treatment in such individuals.

Description: COMPOUNDS FOR PREVENTING MULTIDRUG RES ISTANCE IN CANCER CELLS CONTAINING PROTEIN KINASE INHIBITORS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is directed to methods for preventing the emergence of multidrug resistance in tumor cells during cancer chemotherapy. In paπicular, it relates to the use of protein kinase inhibitors to prevent the induction of the multidrug resistance (MDRl) gene by chemotherapeutic drugs. MDRl gene expression, which results in tumor cell resistance to subsequent treatment with certain chemotherapeutic drugs is shown herein to be induced in response to treatment with various cytotoxic agents. Inhibitors of protein kinases are also shown herein to suppress this cellular response. Therefore, protein kinase inhibitors are useful in preventing MDRl induction by chemotherapeutic drugs in a variety of tumor cells, when administered prior to and/or simultaneous with cytotoxic drug treatment in cancer patients. 2. Summary of the Related Art Chemotherapy is a primary form of conventional cancer treatment. However, a major problem associated with cancer chemotherapy is the ability of tumor cells to develop resistance to the cytotoxic effects of anti-cancer drugs during the course of treatment. It has been observed that tumor cells can become simultaneously resistant to several chemotherapeutic drugs with unrelated chemical structures and mechanisms of action. This phenomenon is referred to as multidrug resistance. The best documented and clinically relevant mechanism for multidrug resistance in tumor cells is correlated with the expression of P-glycoprotein, the product of the MDRl gene. P-glycoprotein is a broad specificity efflux pump located in the cell membrane, and functions by decreasing the intracellular accumulation of many lipophilic cytotoxic drugs, including some widely used anticancer agents such as anthracyclines, vinca alkaloids, epipodophyllotoxins. actinomycin D and taxol. thereby rendering cells resistant to these drugs (Pastan and Gottesman. 1991. Annu. Rev. Med. 42: 277-286; Roninson (edJ, 1991. Molecular and Cellular Biology of Multidrug Resistance in Tumor Cells. Plenum Press, New York: Schinkel and Borst, 1991 , Seminars in Cancer Biology 2: 213-226). Human P-glycoprotein is expressed in several types of normal epithelial and endothelial tissues (Cordon-Cardo et al. , 1990, J. Histochem. Cytochem. 38: 1277- 1287; Thiebaut et al. , 1989, Proc. Natl. Acad. Sci. USA 84: 7735-7738), as well as in hematopoietic stem cells (Chaudhary and Roninson. 1991. Cell 66: 85-94), and a subpopulation of mature lymphocytes (Neyfakh et al. , 1989. Exp. Cell Res. 185: 496-505). More importantly, MDRl mRNA or P-glycoprotein have been detected in most types of human rumors, both before and after chemotherapeutic treatment (Goldstein et al. , 1989, J. Natl. Cancer Inst. 81: 116-124; Noonan et al.. 1990, Proc. Natl. Acad. Sci. USA 87: 7160-7164). The highest levels of MDRl expression are usually found in tumors derived from MDRl -expressing normal tissues; e.g. , renal, adrenocortical or colorectal carcinomas. In other types of solid tumors and leukemias, MDRl expression prior to treatment is usually relatively low or undetectable, but a substantial fraction of such malignancies express high levels of MDRl after exposure to chemotherapy (Goldstein et al. , 1989. ibid.). Prior to the present invention, the increase in MDRl expression after chemotherapy was believed to result from in vivo selection for rare, pre-existing tumor cells that were already inherently resistant to chemotherapeutic drugs due to MDRl expression. Even low levels of MDRl expression have been correlated with the lack of response to chemotherapy in several different types of cancer (Chan et al.. 1990, J. Clin. Oncol. 8: 689-704; Chan et al. , 1991 , N. Engl. J. Med. 325: 1608-1614; Musto et al , 1991. Brit. J. Haematol. 77: 50-53), indicating that P-glycoprotein- mediated multidrug resistance represents an important component of clinical drug resistance. Whereas many clinical and pre-clinical studies have addressed pharmacological strategies for inhibiting P-glycoprotein function (Ford and Hait. 1990. Pharmacol. Rev. 42: 155-199), prior to the present invention, little was known about the factors that are responsible for the induction or up-regulation of P- glycoprotein expression in tumor cells under conditions relevant to cancer chemotherapy. Understanding such factors provides insight into the development of methods for preventing the appearance of P-glycoprotein in human tumors, thus reducing the incidence of multidrug resistance in cancer, and leading to more effective chemotherapy of cancer. Numerous gene transfer studies have demonstrated that elevated expression of the MDRl gene is sufficient to confer the multidrug resistance phenotype (Roninson, 1991 , ibid.). For instance, mouse NIH 3T3 cells infected with a recombinant retrovirus carrying human MDRl cDNA became multidrug-resistant in proportion to the density of human P-glycoprotein on their surface; the correlation was not affected by the presence or absence of cytotoxic selection (Choi et al. , 1991. Proc. Natl. Acad. Sci. USA 88: 7386-7390). Nevertheless, consistent association of other biochemical changes with multidrug-resistant cells suggested that these alterations may also play a role in multidrug resistance, possibly by affecting the expression or function of P- glycoprotein. The most prominent of such changes is the increased activity of protein kinase C (PKC), found in many, but not all, multidrug-resistant cell lines obtained after multiple steps of cytotoxic selection (Aquino et al., 1990, Cancer Commun. 2: 243-247; Fine et al., 1988, Proc. Natl. Acad. Sci. USA 85: 582-586; O'Brian et l. , 1989, FEBSLett. 246: 78-82; Posada et al. , 1989, Cancer Commun. 1: 285-292). PKC activation has been shown to increase the level of drug resistance in some drug-sensitive and multidrug-resistant cells lines (Ferguson and Cheng, 1987, Cancer Res. 47: 433-441; Fine et al. , 1988, ibid.; Yu et al, 1991 , Cancer Commun. 3: 181-189). Although PKC is reported to be capable of phosphorylating P-glycoprotein (Chambers et al , 1990, Biochem. Biophys. Res. Commun. 169: 253- 259; Chambers et al , 1990, J. Biol. Chem. 265: 7679-7686; Hamada et al , 1987, Cancer Res. 47: 2860-2865), it is not known whether such phosphorylation is responsible for the observed changes in drug resistance. While it has been shown that certain PKC inhibitors reversed multidrug resistance in some P-glycoprotein expressing cell lines (O'Brian et al , 1989. ibid. ; Posada et al , 1989. Cancer Commun. 1 : 285-292; Palayoor et al , 1987. Biochem. Biophys. Res. Commun. 148: 718-725), the available evidence suggests that at least some of the observed effects were due to direct inhibition of P-glycoprotein function by the tested compounds rather than inhibition of PKC-mediated phosphorylation (Ford et al. 1990, Cancer Res. 50: 1748-1756; Sato et α/. , 1990. Biochem. Biophys. Res. Commun. 173: 1252- 1257). These studies have provided no indication that PKC-interactive agents could have an effect on expression, rather than phosphorylation or function, of P- glycoprotein. Several laboratories have investigated the factors that regulate MDRl gene expression in normal and malignant cells. One example of normal physiological regulation of an MDRl homolog was found in mouse uterine endometrium. where the expression of a mouse mdr gene was induced by steroid hormones at the onset of pregnancy (Arceci et al , 1990, Molec. Repro. Dev. 25: 101-109; Bates et al , 1989, Molec. Cell. Biol. 9: 4337-4344). In rat liver, the expression of an mdr gene was found to be inducible by several carcinogenic or cytotoxic xenobiotics; similar induction was also observed during liver regeneration (Fairchild et al , 1987, Proc. Natl. Acad. Sci. USA 84: 7701-7705; Thorgeirsson et al , 1987. Science 236: 1120- 1122). Further, a rodent homolog of MDRl was induced in several cell lines in response to treatment with certain cytotoxic drugs (Chin et al . 1990, Cell Growth Diff. 1: 361-365). In contrast, no induction of the human MDRl gene by cytotoxic drugs was detected in any of the human cell lines tested in the same study. Other investigators have also failed to detect MDRl induction upon treatment with cytotoxic drugs (Schinkel and Borst, 1991, Sem. Cancer Biol 2: 213-226). Several studies have indicated, however, that the human MDRl gene may be susceptible to stress induction, under certain conditions. Thus MDRl expression in some human cell lines was increased by treatment with heat shock, arsenite (Chin et al, 1990, J. Biol. Chem. 265: 221-226) or certain differentiating agents (Mickley et al , 1989. J. Biol. Chem. 264: 18031-18040; Bates et al , ibid.). Some cytotoxic P-glycoprotein substrates were reported to stimulate transcription of a reporter gene from the human MDRl promoter (Kohno et al , 1989, Biochem. Biophys. Res. Commun. 165: 1415-1421 ; Tanimura et α/. , 1992. Biochem. Biophys. Res. Commun. 183: 917-924) and to increase P-glycoprotein expression in a mesothelioma cell line after prolonged exposure (Light et al , 1991. Int. J. Cancer 49: 630-637). Despite such reports of MDRl induction, however, it has never been determined whether short-term exposure to any agents used in cancer chemotherapy could induce expression of the MDRl gene in human cells, and whether MDRl induction could be prevented. Recently, Kiowa et al. (1992, FEBS Lett. 301: 307-309) have reported that the addition of a flavonoid, quercetin, can prevent an increase in MDRl expression in a hepatocarcinoma cell line induced by arsenite, a compound which is not used in cancer treatment, but is known to activate the transcriptional pathway mediated by the heat shock response element in the MDRl promoter. Although not disclosed by Kiowa et al , inhibition of PKC activity is one of the biological effects of quercetin (Gschwendt et fl/., 1984, Biochem. Biophys. Res. Commun. 124: 63). It is possible therefore that PKC inhibition by quercetin could be responsible, in part, for the observed inhibition of MDRl induction by arsenite. However, it is noteworthy that the ability of quercetin to inhibit a transcriptional response mediated by the heat shock response element is believed to those skilled in the art to be unrelated to PKC inhibition, (see, e.g. , Kantengwa and Polla, 1991 , Biochem. Biophys. Res. Commun. 180: 308-314). Furthermore, Kiowa et al provide no suggestion that non-flavonoid PKC inhibitors would be able to inhibit MDRl induction by arsenite. or that quercetin would be able to inhibit the induction of MDRl expression when used in combination with chemotherapeutic drugs or any other agents that are not known to activate the heat shock response element-mediated pathway. In addition to MDRl, another pleiotropic drug transporter has been recently discovered (Grant et al, 1994, Cancer Res. 54: 357-361)). This protein, termed the Multidrug Resistance-associated Protein (MRP), has been shown to confer a pattern of resistance to cytotoxic, particularly chemotherapeutic, drugs similar to the P- glycoprotein transporter encoded by the MDRl gene. No inhibitors of MRP expression have been previously reported. SUMMARY OF THE INVENTION The present invention relates to the use of protein kinase inhibitors for preventing the emergence of multidrug resistance in cancer cells, and an in vitro method for identifying protein kinase inhibitors which would be useful towards the same goal. This invention is based, in part, on the discovery that anticancer drugs, whether or not transported by P-glycoprotein. can induce the expression of the MDRl gene in human tumor cells of diverse tissue origins. The increase in MDRl gene expression is observed at both the RNA and protein levels. MDRl induction is also observed upon treatment of cells with PKC agonists. Further, this induction by either a cytotoxic drug or a PKC agonist can be prevented by treatment of cells with a protein kinase inhibitor, indicating that a protein kinase-mediated pathway is involved in MDRl gene induction, and that protein kinase inhibitors may be useful in preventing the expression of MDRl gene in cancer cells exposed to chemotherapeutic agents. More specifically, this inhibitory effect is associated with inhibition of PKC. since protein kinase inhibitors that are inactive against PKC fail to suppress MDRl induction, while protein kinase inhibitors which have potent effects on PKC efficiently inhibit the response. The ability of chemotherapeutic drugs to induce MDRl expression in human cells upon short-term exposure in vitro indicates that cancer chemotherapy induces multidrug resistance directly, rather than through selection of pre-existing rare variants. Such direct induction is likely to occur during a patient's course of drug treatment, and it would account, at least in part, for the observed increased incidence of MDRl expression in treated relative to untreated malignancies. Hence, administration of protein kinase inhibitors prior to and/or simultaneous with the chemotherapy involving cytotoxic drugs may be useful in prevent MDRl induction, and thus, prevent the emergence of multidrug resistant cancer cells, leading to a more favorable therapeutic outcome. The invention is illustrated by way of examples in which PKC agonists are shown to induce MDRl expression in normal peripheral blood lymphocytes (PBL) and tumor cells. Additionally, various cytotoxic anticancer drugs are also described to be capable of activating the MDRl gene. Most importantly, protein kinase inhibitors are shown to prevent this MDRl induction mediated by PKC agonist or cytotoxic drugs, especially in tumor cells that have little or no detectable P- glycoprotein prior to treatment. For the purposes of this invention, the term "little or no P-glycoprotein" is intended to describe low levels of mRNA or protein expression such as are found in certain, well-characterized drug-sensitive cell lines, for example, KB-3-1 (see Noonan et al , ibid.). A variety of uses are encompassed by the invention described herein, including but not limited to. the prevention of the appearance of multidrug resistant tumor cells during chemotherapy of cancer. In another aspect, the present invention provides a method for decreasing multidrug resistance in cancer cells, and an in vitro method for identifying protein kinase inhibitors which would be useful towards this goal. This aspect of the invention is based, in part, on the discovery that certain protein kinase inhibitors can inhibit expression of a multidrug-resistance associated protein, termed MRP (Grant et al. , 1994, ibid.), in cancer cells that express this protein in the absence of the inhibitor. This aspect of the invention is shown by way of examples in which protein kinase inhibitors are shown to inhibit MRP expression in tumor cells expressing this protein. A variety of uses are encompassed by this aspect of the invention as described herein, including but not limited to. decreasing expression levels of this protein in multidrug resistant tumor cells during cancer chemotherapy. Specific preferred embodiments of the present invention will become evident from the following more detailed description of certain preferred embodiments and the claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. Effects of phorbol ester (TPA), diacylglycerol (DOG) and staurosporine (Staur) on P-glycoprotein function and expression of H9 cell line. A: Three hr Rhl23 accumulation by untreated and TPA- or DOG-treated cells. B: Three hr RH123 accumulation by untreated cells and cells treated with TPA or DOG pretreatment with staurosporine. C: Staining of untreated and TPA- or DOG-treated cells with IgG2a isotype control. D: Same as in C. stained with UIC2 antibody. E: UIC2 staining of untreated cells or cells treated with staurosporine alone or with TPA or DOG pretreatment with staurosporine. Figure 2. cDNA-PCR analysis of the effects of TPA. DOG and staurosporine on MDRl mRNA expression indifferent cell lines. In each lane, the upper band (167 bp) corresponds to MDRl . and the lower band (120 bp) to /3 2 -microglobulin specific PCR products. A: Effects of TPA or DOG, with or without staurosporine treatment, on MDRl mRNA expression in H9 cells. B: Time course of induction of MDRl mRNA in H9 cells by TPA. The two negative control (neg. con.) lanes correspond to PCR carried out with water or reverse transcriptase mixture without RNA in place of cDNA. C: Induction of MDRl mRNA in K562 cells by TPA or DOG and in MCF-7 cells by TPA. Figure 3. Flow cytometric analysis of drug-induced MDRl expression. A. Efflux of P-glycoprotein-transported fluorescent dyes from K562 cells in the absence (left) or in the presence (right) of 30 μM verapamil (VER). Top panel: Rhl23 efflux from untreated cells (-) and from cells treated with 50 μM Ara-C (ARA) for 12 hours or with 10 μM Ara-C for 2 to 3 days. Bottom panel: DiOC 2 (3) efflux from untreated cells and from cells treated with 1 μg/mL vinblastine (VBL) for 36 hrs. B. Increased P-glycoprotein expression of Ara-C treated KG1 leukemia cells. Left: Rhl23 accumulation in untreated cells or cells treated with 10 μM Ara-C for 1.5 days. Right: indirect immunofluorescence labeling of the same cells with anti-P-glycoprotein UIC2 antibody or IgG2a isotype control. C. Contour density maps of K562 cells maintained in drug-free media after exposure to different drugs and analyzed by double labeling using DiOC : (3) (horizontal axis) and UIC2 antibody (left) or IgG2a isotype control (right) indirectly labeled with phycoerythrine (PE) (vertical axis). Top to bottom: untreated cells, cells treated with 60 ng/mL Adriamycin for 3 days and grown without drug for 5 weeks, cells treated with 30 μM chlorambucil (CHL) for 5 days and grown without drug for 2 weeks, cells treated with 10 μM Ara-C for 3 days and grown without drug for 5 weeks (this experiment utilized one-half the amount of the secondary antibody used in the other assays); R 23- dull population of cells treated with Ara-C as above and isolated by fluorescence-activated cell sorting six weeks after removal from the drug. Figure 4. cDNA-PCR analysis of MDRl mRNA expression in drug- treated cells. In each lane, the upper band (167 bp) corresponds to MDRl , and the lower band (120 bp) to β 2 - microglobulin specific PCR products, amplified in separate tubes. A. MDRl induction in K562 cells by Ara-C. Cells were exposed to the indicated concentrations of Ara-C for 4.5 days. Cell growth relative to untreated cells was determined by the MTT assay in parallel with RNA extraction. B. MDRl induction in K562 cells treated with different drugs. The times of drug exposure are indicated. The drugs and their concentrations are as follows: (-), untreated cells; DAU, 250 ng/mL daunorubicin; ADR, 500 ng/mL Adriamycin; VBL. 20 ng/mL vinblastine: VP, 1 μg/mL etoposide; MTX. 200 ng/mL methotrexate; CDDP. 3 μg/mL cisplatin; CHL. 50 μM chlorambucil; 5FU, 2 μg/mL 5-fluorouracyl; HU, 30 μM hydroxyurea. C. MDRl induction in KB-3-1 carcinoma cells, untreated or treated for 2 days with 200 ng/mL Adriamycin or 10 μM Ara-C. D. MDRl induction in EJ carcinoma cells, untreated (-) or treated for 4 days with 10 μM Ara-C. E. Maintenance of drug induced MDRl expression in K562 cells. Cells were treated for 3 days with 60 ng/mL Adriamycin. 10 μM Ara-C or 200 ng/mL methotrexate and cultured in drug-free medium for the indicated period of time. Figure 5. Effect of protein kinase inhibitors on MDRl mRNA induction by cytotoxic drugs in H9 cells. In each experiment, the inhibitors staurosporine (ST), H7, Iso-H7 (IH7) or HA1004 (HA) were added twice, the first time immediately prior to the addition of the corresponding drug and the second time after the specified period of time. A. H9 cells, untreated or treated with 50 μM Ara-C for 22 hr. The inhibitors were added at the indicated concentrations at the beginning of the experiment and 16 hr. later. B. H9 cells, untreated or treated with 200 ng/mL Adriamycin for 22 hrs. Equal amounts of inhibitors (0.03 μM staurosporine, 10 μM H7. HA- 1004 and Iso- H7) were added at the beginning of the experiment and 16 hrs later. C. H9 cells, untreated or treated with 40 ng/mL vinblastine or 200 ng/mL methotrexate for 36 hours. Equal amounts of inhibitors (0J μM staurosporine. 50 μM H7) were added at the beginning of the experiment and 24 hrs later. Figure 6. Vinblastine resistance in Ara-C or Adriamycin-treated K562 cell. A. Growth inhibition by vinblastine in untreated and Ara- C (ARA)- or Adriamycin (ADR)-treated cells. Cells were treated as in FIG. 3C and grown in the absence of drugs for six weeks. Vinblastine inhibition assay was carried out for 10 days. B. Growth inhibition by vinblastine in untreated cells and Rhl23-dull and Rhl23-bright populations of Ara-C treated cells. Ara-C treated cells, six weeks after removal from the drug, were stained by the Rhl23 efflux procedure and separated into Rhl23-dull and Rh- 123 bright populations by fluorescence-activated cell sorting. The Rhl23-dull population was > 60% pure (FIG. 3C), and the Rhl23-bright population was 90- 95% pure. One week after sorting, vinblastine inhibition assay was carried out for 7 days. Figure 7. cDNA-PCR analysis of inhibition by protein kinase inhibitors of MDRl mRNA expression induced by treatment with Ara-C or TPA. In each lane, the upper band (167 bp) corresponds to MDRl , and the lower band (120 bp) to 3 2 -microglobulin specific PCR products, amplified in separate tubes. A. Effect of tyrphostins on MDRl induction in H-9 cells by Ara-C. Cells were incubated for 10 hours with 25μM Ara-C (Lanes 3 and 4) or with Ara-C and 50μM tyrphostin A25 (Lanes 5 and 6) or with Ara-C and 50μM tyrphostin B46 (Lanes 7 and 8). Each of the tyrphostins were added to their respective H-9 cell cultures 16 hours prior to the addition of Ara-C. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of Ara- C and either protein kinase inhibitor. B. Effect of neomycin sulfate on MDRl induction in H-9 cells by Ara-C. Cells were incubated for 10 hours with 25μM Ara-C (Lanes 3 and 4) or with Ara-C and 4mM (Lanes 5 and 6) or lOmM (Lanes 7 and 8) neomycin sulfate. Neomycin sulfate at each concentration was added to the respective H-9 cell 5 cultures 45 minutes prior to the addition of Ara-C. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of Ara- C and neomycin sulfate. C. Effect of an erbstatin analog on MDRl induction in H- 10 9 cells by Ara-C. Cells were incubated for 10 hours with 25μM Ara-C (Lanes 3 and 4) or with Ara-C and the erbstatin analog methyl-2.5-dihydroxycinnamate at a concentration of 32μM (Lanes 5 and 6) or 64μM (Lanes 7 and 8). The erbstatin analog at each 15 concentration was added to the respective H-9 cell cultures 45 minutes prior to the addition of Ara-C. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of Ara- C and either protein kinase inhibitor. 20 D. Effect of calphostin C on MDRl induction in H-9 cells by Ara-C. Cells were incubated for 10 hours with 25 μM Ara-C (Lanes 3 and 4) or with Ara-C and calphostin C at a concentration of OJμM (Lanes 5 and 6) or lμM (Lanes 7 and 8). Calphostin C at each 25 concentration was added to the respective H-9 cell cultures 45 minutes prior to the addition of Ara-C, and plates were incubated under direct illumination by white light. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the 30 absence of Ara-C and either protein kinase inhibitor. E. Effect of chelerythrine on MDRl induction in H-9 cells by Ara-C. Cells were incubated for 10 hours with 25μM Ara-C (Lanes 3 and 4) or with Ara-C and chelerythrine at a concentration of lμM (Lanes 5 and 6) or 5μM (Lanes 7 and 8). Chelerythrine at each concentration was added to the respective H-9 cell cultures 45 minutes prior to the addition of Ara-C. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of Ara- C and either protein kinase inhibitor. F. Effect of staurosporine and an erbstatin analog on MDRl induction in H-9 cells by TPA. Cells were incubated overnight with TPA (lOng/mL, Lanes 3 and 4) or with TPA and the erbstatin analog methyl-2,5- dihydroxycinnamate (32μM, Lanes 5 and 6) or TPA and staurosporine (30nM, Lanes 7 and 8). The erbstatin analog or staurosporine were each added to their respective H-9 cell cultures 45 minutes prior to the addition of TPA. Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of TPA and either protein kinase inhibitor. Figure 8. cDNA-PCR analysis of inhibition by protein kinase inhibitors of MRP mRNA expression. In each lane, the upper band (292 bp) corresponds to MRP, and the lower band (120 bp) to β 2 - microglobulin specific PCR products, amplified in separate rubes. A. MRP mRNA expression inhibition in drug-treated H-9 cells. Cells were incubated for 10 hours with 30nM staurosporine (Lanes 3 and 4). 5μM chelerythrine (Lanes 5 and 6), 32μM methyl-2.5-dihydroxycinnamate (erbstatin analog; Lanes 7 and 8) or lOmM neomycin sulfate (Lanes 9 and 10). Negative control lanes (Lanes 1 and 2) represent H-9 cell cultures incubated overnight in the absence of any drug. DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the use of protein kinase inhibitors to prevent the emergence of the multidrug resistance phenotype in cancer cells. The discovery of MDRl induction by cytotoxic drugs and the ability of protein kinase inhibitors to prevent such induction are fully described and exemplified in the Examples below. For clarity of discussion, the invention is described in terms of a variety of protein kinase inhibitors which have potent effects, in particular on PKC activity in a panel of human tumor cell lines. However, the invention can be analogously applied to a wide variety of in vitro cell lines and in vivo tumors treated with other chemotherapeutic drugs, using any protein kinase inhibitors. Specific embodiments of certain protein kinase inhibitors are exemplified as described below. It will be understood that any of a variety of analogues of the specific protein kinase inhibitors disclosed herein, known in the art or prepared using methods known in the art, are also encompassed by the invention as herein described. 1. Induction of MDRl Gene Expression TPA (12-o-tetradecanoylphorbol-13-acetate), an efficient PKC activator, and diacylglycerol, a physiological stimulant of PKC, are shown in Examples 1 and 2, below, to increase MDRl gene expression in normal human PBL. and in cell lines derived from different types of leukemias or solid tumors. The effect of TPA is observed in all the tested cell lines that expressed P-glycoprotein prior to treatment, and in some but not all other cell lines without detectable P-glycoprotein expression prior to treatment. It is possible, however, that MDRl expression could be induced in the non-responsive cell lines by higher concentrations of TPA than those tested as described herein. The observed effects of TPA and diacylglycerol indicate that MDRl expression in human cells may be regulated through a PKC-mediated signal transduction pathway. The increase in MDRl expression in cells treated with the PKC agonists is observed at the level of both P-glycoprotein and MDRl m NA. The increased stead- state level of MDRl mRNA may reflect either increased transcription or decreased mRNA degradation. It should be noted that the major downstream promotor of the human MDRl gene (Ueda et al , 1987, J. Biol. Chem. 262: 505-508) contains the AP-1 site responsible for the stimulation of transcription by TPA (Angel et al. , 1987, Cell 49: 729-739; Lee et al, 1987, Cell 49: 741-752). The AP-1 site and its surrounding sequences are conserved between the human MDRl gene and its rodent homologs (Hsu et al, 1990, Molec. Cell Biol. 10: 3596-3606; Teeter et al , 1991. Cell Growth Diff. 2: 429-437). The AP-1 sequence of the hamster pgpl gene was shown to be an essential positive regulator of its promoter (Teeter et al . 1991 , Cell Growth Diff. 2: 429-437), although the corresponding element of the homologous mouse mdrla (mdr3) gene may have a negative regulatory effect (Ikeguchi et al. , 1991 , DNA Cell Biol. 10: 639-649). Thus, it is possible that the AP-1 element of the human MDRl promoter is directly responsible for the stimulation of MDRl expression by PKC agonists. The induction of MDRl gene expression by a PKC-mediated pathway may explain previous findings that multidrug-resistant cell lines selected for increased P- glycoprotein expression frequently contained elevated levels of PKC (Aquino et al.. ibid.; Fine et al , ibid.; O'Brian et al , ibid. ; Posada et al, ibid.). An increase in PKC activity could represent an early event responsible for increased MDRl gene expression during the selection of such cell lines. This interpretation does not preclude, however, that the phosphorylation of the induced P-glycoprotein by PKC could further increase P-glycoprotein activity. Evidence for the latter hypothesis comes from the study of Yu et al. (1991 , Cancer Commun. 3: 181-189), who found that the level of drug resistance in a multidrug-resistant subline of MCF-7 cells, obtained after transfection with MDRl cDNA transcribed from a heterologous promoter, could be increased by the introduction of a vector expressing high levels of PKCα. The increased resistance in the PKCα transfectants was accompanied by increased P-glycoprotein phosphorylation, without apparent changes in expression levels. PKC plays a central role in various signal transduction pathways, associated with different adaptive, proliferative and differentiative processes. Even though PKC agonists have been found to induce MDRl expression in normal and malignant hematopoietic cells, the same result has not been achieved using hematopoietic growth factors that may also act through PKC-mediated pathways. Furthermore, PKC agonists induce MDRl expression in cell lines of not only hematopoietic but also epithelial origin, indicating that PKC-mediated regulation of MDRl expression may have a general physiological role. PKC-mediated mechanisms have been implicated in the transcriptional response to DNA damage by UV irradiation or alkylating agents (Kaina et al. , 1989, in M.W. Lambert and J. Laval (ed.), DNA Repair Mechanisms and Their Biological Implications in Mammalian Cells. Plenum Press, New York: Papathanasiou and Fornace, 1991, pp. 13-36 in R.F. Ozols (ed.), Molecular and Clinical Advances in Anticancer Drug Resistance. Kluwer Academic Publishers, Boston, MA). PKC activation has also been associated with cellular responses to other cytotoxic drugs, such as cytosine arabinoside (Kharbanda et al , 1991, Biochemistry Cancer Res. 49: 6634-6639). Thus, PKC-mediated induction of MDRl expression could be a part of a general stress response to different types of cellular damage, including the damage produced by cytotoxic chemotherapeutic drugs. The present invention discloses that MDRl expression in human leukemia and solid-tumor derived cell lines can be induced by short-term exposure to a number of different cytotoxic drugs that are used in cancer chemotherapy (see Example 3. below). MDRl induction, both at the mRNA and at the protein levels, was observed in a subpopulation of cells treated with either P-glycoprotein-transported agents (Adriamycin, daunorubicin. vinblastine, etoposide) or chemotherapeutic drugs that are not transported by P-glycoprotein (such as methotrexate. 5-fluorouracil, chlorambucil. cisplatinum. hydroxyurea and l- 3-D-arabinofuranosylcytosine(Ara-C), an effective anti-leukemic drug (Kharbanda et al . 1991. Biochemistry 30: 7947- 7952)). Since MDRl expression does not provide resistance to drugs of the second group, and because MDRl induction could be achieved after short times of drug exposure (less than one cell generation in many cases), these findings indicate that cytotoxic selection for MDRl -expressing cells was not responsible for the observed increase in MDRl expression. MDRl induction became detectable at the same time as visible cell damage, indicating that expression was more likely to be an indirect consequence of such damage, rather than a direct response to specific agents. Most importantly, MDRl expression induced by treatment with cytotoxic drugs did not disappear after the removal of the drug, but was maintained for at least several weeks in cells cultured in drug-free media. P-glycoprotein-positive cells, growing in the absence of the drugs, showed no change in differentiation state. This result indicates that drug-induced MDRl expression is a stable phenomenon which is not limited to dying or terminally differentiating cells. In addition to increased MDRl expression, drug-treated cells displayed a 2-3 fold increase in resistance to vinblastine, a P-glycoprotein transported drug; such resistance was specifically associated with MDRl -expressing cells. Drug-treated cells also showed increased resistance to chlorambucil, a chemotherapeutic drug which is not transported by P- glycoprotein. The latter finding suggests that some other clinically relevant mechanisms of drug resistance may be co-induced with MDRl expression after treatment with cytotoxic drugs. Taken collectively, these findings suggest that treatment of human tumor cells with various drugs used in cancer chemotherapy can induce MDRl expression directly, rather than by selection of preexisting genetic variants, as previously believed. The resulting increase in multidrug resistance is stable and may be sufficient to reduce the response to chemotherapeutic drugs both in vitro and in vivo. It seems likely that drug-mediated induction of MDRl expression would occur during cancer chemotherapy, and may account, at least in part, for the observed increase in the incidence of MDRl expression in drug-treated human tumors. This invention therefore provides the first documentation of MDRl induction under clinically relevant conditions and suggests that PKC may play a central role in such induction. The latter hypothesis provides a basis for chemotherapeutic protocols that would prevent MDRl induction during cancer chemotherapy through the inhibition of PKC. 2. Use of Protein Kinase Inhibitors to Prevent MDRl Induction The present invention demonstrates that protein kinase inhibitors, especially those with potent activity against PKC. are capable of preventing the induction of MDRl gene expression in cancer cells. For example, staurosporine, a potent but non-selective inhibitor of PKC (Ruegg and Burgess, 1989, Trends Pharmac . Sci. 10: 218-220), is shown herein (when used at a concentration of 30nM) to prevent MDRl induction in P-glycoprotein-negative cells treated with TPA. diacylglycerol and a number of chemotherapeutic cytotoxic drugs, including Ara-C, vinblastine, methotrexate and Adriamycin. A variety of other protein kinase inhibitors, including H7 (50μM), calphostin C (at a concentration of lμM, used in conjunction with white light illumination), and chelerythrine (5μM). are also shown to prevent MDRl induction by chemotherapeutic drugs. Inhibition of MDRl expression induced by treatment with Ara-C has also be demonstrated using inhibitors that have not previously been understood to have specificity for protein kinase C. Examples of such inhibitors include erbstatin and analogues of erbstatin, and neomycin sulfate. Erbstatin and its analogues, such as methyl-2,5-dihydroxycinnamate, are known to be capable of inhibiting the epidermal growth factor receptor tyrosine kinase (Umezawa and Imoto, 1991 , Meth. Enτymol. 201: 379-385). This erbstatin analog was found to completely inhibit Ara-C induced MDRl expression at a concentration of 32μM (see, Example 4, below). This erbstatin analog was also found to inhibit MDRl expression induced by treatment of sensitive cells with TPA, a known PKC agonist, consistent with anti-PKC activity (see Example 4 and Figure 7F, infra). In addition to the erbstatin analogue, two other compounds known as inhibitors of protein tyrosine kinase, tyrphostin A25 and tyrphostin B46, were also found to inhibit Ara-C induced MDRl expression, at concentrations of 50-100μM. The antibacterial compound, neomycin sulfate, was found to inhibit Ara-C induced MDRl expression, when contacted with induced cells at a concentration of lOmM. This concentration of neomycin showed no cellular toxicity on human H9 cells, and appears to be within the range of clinically-achievable concentrations. This compound has the advantage of being of general benefit to patients undergoing chemotherapy, and has been widely used for the treatment of bacterial intestinal infections in humans. Interestingly, both erbstatin (Bishop et al , 1990, Biochem. Pharmacol. 40: 2129-2135) and neomycin sulfate (Chauhan et al , 1990, FASEB J. 4: A1779) have been reported to be protein kinase C inhibitors. These findings provide evidence that protein kinase inhibitors, and in particular PKC inhibitors, are involved in MDRl induction and suggest the possibility of using protein kinase inhibitors to prevent MDRl gene activation. Other protein kinase inhibitors, however, were found to be ineffective in inhibiting Ara-C induced MDRl expression. These include the PKC inhibitor D,L- tΛreo-sphingosine (when used at 5 μM concentration; higher concentrations induced cellular toxicity). This was the only PKC inhibitor tested that did not inhibit MDRl induction, suggesting that the observed cellular toxicity prevented administration of this compound at an effective inhibitory concentration. The tyrosine kinase inhibitor, herbimycin A. was also found to be ineffective at inhibiting MDRl induction by Ara- C at concentrations ranging from 0.35-3μM. Another tyrosine kinase inhibitor, genistein, only weakly inhibited Ara-C induced MDRl expression, at a concentration of 150μM. Staurosporine is a P-glycoprotein inhibitor that may bind directly to P- glycoprotein (Sato et α/., 1990, Biochem. Biophys. Res. Commun. 173: 1252-1257). In some P-glycoprotein-positive cell lines, however, staurosporine. when used alone, significantly increased P-glycoprotein expression. Additionally, two other P- glycoprotein binding compounds, cyclosporine A and verapamil, also known PKC inhibitors, increased P-glycoprotein expression in some ofthe P-glycoprotein-positive cell lines. These agents and staurosporine may act through a common, presently unknown mechanism. These results suggest that protein kinase inhibitors may be more effectively used to prevent an increase in MDRl in P-glycoprotein-negative or nearly negative tumors than in tumors already expressing P-glycoprotein in a large fraction of tumor cells. It should be noted, however, that the finding that staurosporine increased P-glycoprotein expression in a small number of hematopoietic cell lines does not indicate that augmentation of P-glycoprotein expression by PKC inhibitors is a general property of P-glycoprotein-positive tumor cells, and that patients with P-glycoprotein-positive tumors cannot benefit from the use of protein kinase inhibitors to prevent further drug-induced increase of multidrug resistance in tumor cells. P-glycoprotein negative solid tumors or leukemias can be identified by the analysis of biopsy material, surgical or hematological specimens of patients' tumors using techniques well known in the art (Roninson, ibid.). These techniques include but are not limited to immunocytochemical, immunohistochemical or immunofluorescent assays with P-glycoprotein-specific antibodies; vital staining with P-glycoprotein transported fluorescent dyes: Northern dot blot or slot blot hybridization with MDRl -specific nucleic acid probes; or cDNA-PCR analysis of MDRl mRNA. Working examples of some of the above assays are described in Examples 1 and 3, below. It should be noted that some cell lines that appear to be P-glycoprotein negative by protein or function-based assays may still show detectable levels of MDRl mRNA when assayed by cDNA-PCR (see Table I). This indicates that protein or function-based assays would be preferable as the primary criterion for the identification of tumors that are likely to benefit from the use of protein kinase inhibitors. Alternatively, cDNA-PCR or other methods for MDRl mRNA measurement may be used with the understanding that MDRl mRNA expression at the level of K562 cells or slightly (e.g. 2-fold) higher may still be indicative of P- glycoprotein negative tumors. Although the protein kinase inhibitors tested herein are known to be non- selective in their inhibitory activities, i.e. , their action is not specific for PKC, the studies described herein provide evidence that their ability to inhibit PKC activity may be a critical factor in the prevention of MDRl induction. For example, a number of potent PKC inhibitors, including staurosporine. H7, chelerythrine. neomycin sulfate and calphostin C, are capable of inhibiting MDRl induction by cytotoxic drugs. In contrast, HA 1004, a protein kinase inhibitor that is inactive against PKC, is shown to be totally ineffective in preventing MDRl induction. Hence, it is likely that any protein kinase inhibitor that is capable of inhibiting PKC. irrespective of its specificity for PKC, would be useful in preventing MDRl induction in cancer cells. Accordingly, any protein kinase inhibitor capable of preventing the induction of MDRl by chemotherapeutic drugs as measured by any method described in Example 3, below, such as fluorescent dye accumulation, cDNA-PCR for MDRl mRNA or staining with P-glycoprotein specific antibody, may be used in the practice of the method of the invention. Such inhibitors may be administered in a cancer patient bearing a solid tumor or leukemia prior to and/or simultaneous with treatment by chemotherapeutic drugs. Any anti-cancer drug commonly used in cancer chemotherapy is encompassed within the scope of this regimen, including, but not limited to, Ara-C. Adriamycin, daunorubicin, vinblastine, etoposide, methotrexate, 5-fluorouracyl, chlorambucil, cisplatin. and hydroxyurea. Prior to the present invention, a number of compounds capable of inhibiting PKC have been investigated in vitro and in vivo for potential use in cancer chemotherapy. However, it should be noted that while such compounds were found to show selective growth inhibition for tumor relative to normal cells (Powis and Kozikowski, 1991 , Clin. Biochem. 24: 385-397; Grunicke et al . 1989, Adv. Enzyme Regul 28: 201-216) they have not been shown or suggested to be capable of preventing MDRl expression in cancer cells. In vitro studies have shown that the anti-proliferative effects of PKC inhibitors occurred at approximately the same dose as their PKC inhibitory activity (Grunicke et al, ibid.). Compounds tested in vivo include staurosporine and its benzoyl derivative CGP 41 251 , which were found in nude mice to show anti-tumor effect at 1/10 of their maximum tolerated doses (MTD) (MTD was 1 mg/kg for staurosporine and 250 mg/kg for CGP 41 251) (Meyer et al, 1989, Int. J. Cancer 43: 851-856). Other staurosporine analogs shown to have antitumor activity in vivo include UCN-01 (Takahashi et al . 1987, J. Antibiot. 40: 1782-1784) and 8-N-(diethylaminoethyl) rebeccamycin (BMY 27557) (Schurig et al, 1990, Proc. Amer. Assoc. Cancer Res. 31: Abs. 2469). For the latter compound, optimal doses for i.p. administration ranged from a total of 108 mg/kg administered over nine injections/day at 12 mg/kg/injection to a single dose of 64 mg/kg. Another group of PKC inhibitors actively investigated as anticancer agents comprises ether lipid analogues, including hexadecylphosphocholine, ET-18-OCH3. ilmofosine, SRI 62-834 and BM 41440 (Powis and Kozikowski, 1991, Clin. Biochem. 24: 385-397; Grunicke et al , ibid.). Some of these agents have been used in clinical trials. The MTDs established in these trials for oral administration are 200 mg/day for ilmofosine (Berdel et al . 1988. Proc. Amer. Assoc. Cancer Res. 29: Abs. 2050) and 5 mg/kg body weight for BM41440 (Hermann et al , 1987, Lipids 22: 962-966). Hexadecylphosphocholine was also used topically for treatment of skin metastases of breast cancer, at a dose range of 0.2 to 38.5 g per patient, administered over 3 to 128 weeks (Unger et al . 1990, Cancer Treat. Rev. 17: 243- 246). Compounds of this group were also tested as purging agents for autologous bone marrow transplantation (Vogler et al , 1991 , Exp. Hematol 99: 557 AbsJ. Another PKC inhibitor, suramin. has been used in the treatment of parasitic diseases, and is being evaluated in clinical trials as an antineoplastic agent. Continuous infusion of suramin at a rate designed to reach a peak of 300 μg/mL at the end of 14 days has shown activity in hormone-refractory prostate cancer (Myers et al , 1992, J. Clin. Oncol 10: 875-877). A member of another class of PKC inhibitors, the flavonoid quercetin, was shown to potentiate the anti-tumor effect of cisplatin. a drug which is not transported by P-glycoprotein. in nude mice when administered i.p. at 20 mg/kg (Grunicke et al . ibid.). The epidermal growth factor receptor tyrosine kinase inhibitor erbstatin has been reported to have antitumor activity in vivo (Imoto et al. , 1987, Japanese J. Cancer Res. 78: 329-332; Toi et al , 1990, Eur. J. Cancer 26: 722-724). No evidence has been reported that erbstatin has any capacity to inhibit MDRl induction by cytotoxic drugs. While none of the above compounds, with the exception of staurosporine described in Examples 2 and 4, below, have been tested for the ability to prevent MDRl induction by cytotoxic drugs, the results disclosed in the present invention strongly indicate that they are likely to possess such an effect, since all of them are capable of inhibiting PKC. The availability of in vivo animal and clinical trial data for these and other PKC inhibitors enables those skilled in the art to use such compounds in combination with conventional anticancer drugs to prevent the emergence of multidrug resistance during chemotherapy. These compounds may be administered with chemotherapeutic drug treatment at a dose range of about 1-250 mg/kg body weight, either by repeated injections, by continuous infusion, or as topical treatment. In addition to these aspects of the present invention, an in vitro assay is disclosed for rapid identification of any compound which is capable of preventing the induction of MDRl gene expression by chemotherapeutic drugs. For example, H-9 or K562 leukemia cell lines are treated with a test compound for about 30 minutes for 10-36 hours (though any time of culture over 1 hour may be sufficient, see FIG. 2B) prior to exposure to 10-25μM Ara-C or 200 ng/mL vinblastine under standard tissue culture conditions, followed by evaluation of MDRl induction by the drugs in cultures treated with the test compound, as compared to controls. Test compounds identified by such assays as being capable of preventing MDRl induction by chemotherapeutic drugs, are used for patient treatment in the same manner as protein kinase inhibitors described herein. 3. Use of Protein Kinase Inhibitors to Inhibit MRP Expression The present invention demonstrates that protein kinase inhibitors are also capable of inhibiting expression of a protein, termed the multidrug resistance- associated protein (MRP; Grant et al, ibid.). In contrast to MDRl , certain human tumor cells, such as H9 human T-cell leukemia cells, display robust expression of the MRP gene, as evidenced by cDNA-PCR experiments as disclosed in Example 5 and Figure 8, below. Treatment of such cells with certain protein kinase inhibitors results in a dramatic reduction of MRP expression. Compounds shown to be capable of inhibiting MRP expression in H-9 cells include staurosporine (at a concentration of lOOnM). neomycin sulfate (lOmM), chelerythrine (l-5μM). and the erbstatin analogue. methyl-2.5-dihydroxycinnamate (32μM). The present invention thus provides a number of agents having two related activities of benefit to cancer chemotherapy patients. These agents prevent the activation of the MDRl gene by chemotherapeutic drugs, thereby suppressing the emergence of MDR 1 -mediated multidrug resistance during chemotherapy of patients bearing MDR 1 -negative tumors. In addition, these agents reduce MRP expression, thereby decreasing multidrug resistance mediated by MRP in -\/RP-positive tumors. One of these agents, neomycin sulfate. has the additional advantage of being a commonlv-used antibiotic, which has been well-characterized clinically and whose use may have added benefits for combating adventitious bacterial infections in cancer chemotherapy patients. The Examples which follow are illustrative of specific embodiments of the invention, and various uses thereof. They set forth for explanatory purposes only, and are not to be taken as limiting the invention. EXAMPLE 1 A Protein Kinase Inhibitor prevents Protein Kinase C Agonist-mediated MDRl Induction in Normal and Tumor Cells 1. Materials and Methods 1J Cell Lines and Drug Treatment Normal human PBL were obtained from healthy volunteers by venipuncture after informed consent, followed by the isolation of low-density mononuclear cells by density gradient centrifugation in Histopaque-1077 (Sigma Chemical Co., St. Louis. MO). KG1 cell line was maintained in Iscove ' s modified Dulbecco medium with 20% fetal calf serum (FCS) and 2 mM L-glutamine, 100 units/mL penicillin and 100 μg/mL streptomycin (GIBCO Laboratories. Grand Island, NY). MCF-7, EJ, KB- 3-1. HeLa. and HT-1080 cell lines were maintained in DMEM with 10% FCS and 2mM L-glutamine. 100 units/mL penicillin and 100 μg/mL streptomycin. A stock solution of ImM Ara-C (Sigma) was prepared in sterile phosphate buffered saline (pH 7.2) and stored at -20°C until use. Stock solutions of 100-1000 μg/mL TPA (Sigma) and 30 mM 1.2-dioctanoylglycerol (DOG or DiC 8 ) (Molecular Probes. Eugene. OR) were prepared in dimethylsulfoxide (DMSO) and stored at - 30°C. Stock solutions of chelerythrine. calphostin C, methyl-2.5-dihydroxycinnamate. tyrphostin A25, tyrphostin B46, herbimycin A and genistein (all obtained from Calbiochem. LaJolla. CA) and D.L-t/zreosphingosine. staurosporine and H7 (all obtained from Sigma) were prepared in DMSO at appropriate concentrations. Neomycin sulfate (Calbiochem) was dissolved in sterile, deionized water. Control experiments with DMSO solutions showed that DMSO had no effect on P- glycoprotein function or expression. Different concentrations of TPA were used for the treatment of different cell lines, depending on the observed cytotoxicity. PBL were treated with 1 ng/mL TPA: H9 and K562 cells with 10 ng/mL TPA: KG la and KG1 cells with 100 ng/mL TPA and the other cell lines with 10 ng/mL TPA. Similarly, different concentrations of DOG were used for different cell lines. Thus. PBL were treated with 75 μM DOG. whereas H9 and K562 cells were treated with two 75 μM doses of DOG give 2 hr apart. Cells were exposed to TPA or DOG for 8-12 hr before flow cytometric analysis or RNA extraction. Staurosporine (Sigma) was used at 100 nM concentration for KG la cells and 30 nM for the other cell lines: it was added to the cells 30 minutes prior to the addition of TPA or DOG. 1.2. Flow Cytometric Assays P-glycoprotein activity was assayed by the rhodamine 123 (Rhl23) accumulation assay. For this assay, drug-treated or untreated cells were washed three times and incubated for 1.5-2 hours at 37°C in media containing 100 ng/mL Rhl23 (Sigma). Cells were then washed, stained with propidium iodide (PI) and kept on ice until analysis. Cells growing in monolayer were suspended with 20 mM ethylene- diaminetetraacetic acid (Sigma) in phosphate buffered saline (PBS) at pH 7.4 and washed three times prior to Rhl23 staining. In some experiments a Rhl23 efflux assay (Chaudhary and Roninson. 1991 , Cell 66: 85-94) was used instead of Rhl23 accumulation. P-glycoprotein expression on the cell surface was analyzed using a P- glycoprotein-specific mouse IgG2a monoclonal antibody (mAB). UIC2 (Mechetner and Roninson. 1992. Proc. Natl. Acad. Sci. USA 89: 5824-5828). Mouse IgG2a isotype control antibody was obtained from Sigma. For staining of PBL with UIC2 mAB or isotype control. 10 6 cells were stained at 4°C with 10 μg of the antibody for 30 minutes and. after two washes, stained for 30 minutes with 10 μg of FITC- conjugated goat anti-mouse IgG2a antibody (Fisher Scientific. Fairlawn. NJ). diluted 1 :2 with PBS plus 2% FCS. Cells were then washed twice with ice-cold PBS plus 2% FCS. stained with PI and kept on ice until analysis. Essentially the same protocol was used for staining of other cell types except that 2 μg of the secondary antibody were used per 10 6 cells. In some experiments, phycoerythrine (PE)-conjugated goat anti-mouse IgG2a was used as the secondary antibody: no PI was added in such cases. Flow cytometric analysis was conducted on a Coulter Epics 753 Flow Cytometer. 1.3 RNA Extraction and cDNA-PCR Analysis RNA was extracted from approximately 10 6 cells by a small-scale sodium dodecyl sulfate extraction procedure (Peppel and Baglioni. 1990. BioTechniques 9: 71 1-713). In the alternative. RNA was isolated from cells using the TRIzol method (commercially available from GIBCO/BRL). cDNA synthesis and polymerase chain reaction (PCR) amplification of MDRl and β-,-microglobulin cDNA sequences were carried out essentially as described (Noonan et al, 1990. Proc. Natl. Acad. Sci. USA 87: 7160-7164; Noonan and Roninson. 1991. in Roninson (ed.), Molecular and Cellular Biology of Multidrug Resistance in Tumor Cells, Plenum Press. New York, pp. 319-333). Briefly, cDNA was prepared using the method of Krug and Berger ( 1987. Meth. Enzymol. 152: 316-324). with the modification that random hexamers were substituted for oligo(dT) as primers for cDNA synthesis. (Commercially- available kits (for example, from GIBCO/BRL. Gaithersburg. MD) can be advantageously modified for making cDNA using random hexamers.) PCR is then performed using the following primers: β 2 M (sense) 5'-ACCCCCACTGAAAAAGATGA-3' (SEQ ID No.: 1) β 2 M (antisense) 5'-ATCTTCAAACCTCCATGATG-3' (SEQ ID No.: 2) MDRl (sense) 5'-CCCATCATTGCAATAGCAGG-3' (SEQ ID No.: 3) MDRl (antisense) 5'-GTTCAAACTTCTGCTCCTGA-3' (SEQ ID No.: 4) (where β 2 M represents β 2 microglobulin). PCR was performed for β 2 M experiments under a regime consisting of 1 cycle comprising denaturation at 94°C for 3 minutes, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 1 minute, followed by 19 cycles comprising denaturation at 94°C for 30 seconds, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 1 minute. followed by a final cycle comprising denaturation at 94°C for 30 seconds, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 5 minutes. MDRl cDNA sequences were amplified using the identical protocol as for β 2 M. but extending the amplification from 19 cycles to 25 cycles. In addition, the cDNA-PCR amplification protocol contained the following modifications: (i) Taq DNA polymerase was added to the PCR mixtures after initial heating of the samples to 94°C. (ii) The yield ofthe β : -microglobulin-specifιc band, obtained after 22-28 cycles of PCR. was used as the primary criterion for equalizing the starting amounts of the cDNA templates in different preparations, in order to account for differential RNA degradation in cells subjected to different types of treatment. 3 P-labeled PCR products were detected by autoradiography. EXAMPLE 2 Functional Assays of P-Glycoprotein Expression in Cells Treated with Lvmphoid Cell Differentiation Inducing Agents A functional assay was used to detect changes in P-glycoprotein activity in human PBL treated with different agents that induce lymphoid differentiation or activation, based on flow cytometric analysis of cellular accumulation of Rhl23. which is a P-glycoprotein-transported fluorescent mitochondrial dye. In this assay, cells expressing little or no P-glycoprotein stain brightly with Rhl23. whereas cells with higher levels of P-glycoprotein activity appear Rhl23-dull. No apparent effects on P-glycoprotein were observed in cells treated with calcium ionophore A23187. IL- lα or IL-2. In contrast, treatment of PBL with the phorbol ester TPA caused a significant increase in the number of Rhl23-dull cells. The increase in the Rhl23- dull population could be prevented by the addition of 30 μM verapamil. a P- glycoprotein inhibitor. Since the best-known cellular effect of TPA is the stimulation of PKC. DOG. a cell-permeable diacylglycerol and a physiological stimulant of PKC. was also tested to determine its effects on Rhl23 accumulation in PBL. Treatment of PBL with DOG was found to decrease Rhl23 accumulation by PBL. To determine if the observed effect of PKC stimulants on P-glycoprotein activity was due to an increase in P-glycoprotein expression, untreated and TPA- treated PBL were stained by indirect immunofluorescence labeling with mAB UIC2 that recognized an extracellular epitope of P-glycoprotein encoded by the human MDRl gene. TPA treatment markedly increased the levels of P-glycoprotein on the cell surface. The increase in P-glycoprotein was accompanied by a corresponding increase in MDRl mRNA levels in the total population of TPA-treated PBL. as detected by polymerase chain reaction (PCR) amplification of MDR 1 cDNA sequence. Thus, the TPA-induced increase in P-glycoprotein activity was due at least in part to the activation of MDRl gene expression at the mRNA and protein levels. Since PBL comprise a heterogeneous population of many different subtypes, a series of leukemia-derived clonal cells lines were tested for changes in P- glycoprotein expression after treatment with TPA. As summarized in Table I. all the cell lines that were positive for P-glycoprotein prior to TPA treatment showed a major increase in their P-glycoprotein expression after exposure to TPA. This group included human KG1 and KG la (stem-cell like leukemia cell lines), whose relatively high level of P-glycoprotein was likely to reflect the expression of this protein in normal hematopoietic stem cells (Chaudhary and Roninson. ibid.), as well as murine E14 thymoma and LBRM 33 lymphoma cell lines. Among the cell lines that expressed no detectable P-glycoprotein. H9 and K562 leukemia cell lines showed clear-cut induction of MDRl mRNA and P- glycoprotein by either TPA or DOG. Flow cytometric assays showed that the treatment of these cell lines with TPA or DOG resulted in the appearance of a major cell population that expressed P-glycoprotein (FIG. 1 ). These changes were paralleled by an increase in steady-state levels of MDR] mRNA in TPA- or DOG-treated cells (FIGS. 2A and 2B). As shown in FIG. 2B. MDRl mRNA became detectable in H9 cells 2 hrs after the addition of TPA and continued to increase until at least 5 hr, indicating a rapid response to TPA, consistent with transcriptional activation of MDRl by TPA in these cells. The increase of MDRl expression after TPA treatment was not limited to hematopoietic cells, but was also observed in some solid tumor-derived cell lines, including EJ bladder carcinoma cells that expressed a low level of P-glycoprotein. and MCF-7 breast carcinoma cells where MDR 1 expression was undetectable without TPA treatment (FIG. 2C). As summarized in Table I. most of the tested P-glycoprotein negative cell lines were only treated with a fixed concentration (20 ng/mL) of TPA. and were not tested for their ability to respond to higher TPA concentrations. In an attempt to interfere with the induction of MDR \ gene expression by PKC agonists, a potent protein kinase inhibitor, staurosporine. was used to treat various cell lines. Unexpectedly, staurosporine alone caused a significant increase in P-glycoprotein expression in the cell lines that were already positive for P- glycoprotein (KG1. KGla. mouse EL4 and LBRM 33 cell lines). Two additional compounds, cyclosporine A and verapamil. which are known to be P-glycoprotein TABLE I. EFFECT OF TPA ON MDRl EXPRESSION Cell l ine Untreated TPA-Treated Assays Human Hematopoietic Cell Lines KG1 (acute myelogenous leukemia) ++ +++ F.A KG la (acute myelogenous leukemia) ++ +++ F,A 10 K562 (chronic myelogenous leukemia) ++ F,A,R 119 (J-cell leukemia) ++ F.A.R HL-60 (promyelocytic leukemia) F JI IP- 1 (promyelocytic leukemia) 1 Jurkat, clone L6-I (T-cell leukemia) F 15 Molt-4 (T-cell leukemia) F U937 (histiocylic leukemia) F Mouse Hematopoietic Cell Lines EIA (thymoma) ++ + + + LBRM 33. clone 4A2 (lymphoma) + ++ 20 Human Solid Tumor Cell Lines EJ (bladder carcinoma) F MCF-7 (breast carcinoma) + R HeLa (cervical carcinoma) F.R B-3-1 (HeLa subline) F.R 25 I I I ' 1080 (llbrosarcoma) I .R MDRl gene expression was evaluated by a functional assay for R l23 accumulation (F), U1C2 antibody staining (A) or cDN PCR assay for MDRl mRNA (R), and expressed as relative values. Cells were considered negative if they expressed no glycoprotein detectable by the Rhl 23 or UIC2 staining assays and had MDRl mRNA level no higher than that of KB-3-1 cel inhibitors as well as inhibitors of PKC. have also been found to increase P- glycoprotein expression and/or dye efflux in K.G 1 and EL4 cells. The effect of PKC inhibitors on P-glycoprotein expression in the P-glycoprotein-positive cell lines made it difficult to analyze the interactions between staurosporine and PKC agonists in such cells. However, staurosporine did not induce .MDRl expression in P-glycoprotein- negative H9 cells. The addition of staurosporine to H9 cells 30 minutes prior to TPA or DOG treatment completely abolished MDRl induction by these agents, as evidenced by flow cytometric (FIG. 1 ) and cDNA-PCR assays (FIG. 2A). Staurosporine also inhibited the effects of TPA and DOG in normal PBL. EXAMPLE 3 Protein Kinase Inhibitors prevent Cytotoxic Drug-mediated MDRl Induction in Tumor Cells 1. Materials and Methods 1.1 Flow Cytometric Assays K562 cells were stained for 10 minutes at 37°C with 100 ng/mL Rhl23 or with 10 ng/mL of DiOC : (3). another P-glycoprotein-transported dye (Chaudhary and Roninson. ibid.), in 5 mL of DMEM supplemented with 10% fetal calf serum. After two washes, the cells were allowed to efflux the dye for 3 hrs (for Rh 123) or 2 hrs (for DiOC,(3)) at 37°C in 5 mL of dye-free media, as previously described (Chaudhary and Roninson. ibid.). In double-labeling experiments. 3 ng/mL of DiOC : (3) in 5 mL of media were used for staining. Each efflux assay was carried out in the presence and in the absence of 30 μM verapamil. KG1 cells were stained with 100 ng/mL Rhl23 in 5 mL of media for 3 hours at 37°C and analyzed without efflux. Indirect immunofluorescence labeling (as described in Chaudhary and Roninson. ibid.) was carried out using 2 μg of the primary antibody (UIC2 or mouse IgG2a isotype control from Sigma) and 10 μg of the secondary antibody. PE- conjugated F(ab') ; fragments of sheep anti-mouse IgG (Sigma) per 2 x 10' KG1 cells. Flow cytometric analysis and flow sorting were carried out as described (Chaudhary and Roninson. ibid.): non-viable cells were excluded from analysis on the basis of abnormal size or granularity or. in experiments not utilizing phycoerythrine. by accumulation of propidium iodide. 1.2. Growth Inhibition Assays Cells were plated in duplicate in 96-well microtiter plates at 3.000 cells per well, and allowed to grow in increasing concentrations of different drugs. Cell growth after 7-10 days was analyzed by MTT assay (Pauwels et al. 1988. J. Virol. Meth. 20: 309-321). 1.3. MDRl Induction Inhibition Assays Cells were plated in 6-well Falcon tissue culture plates at 3.300 cells per well, and incubated in the appropriate concentrations of drugs. Each of the protein kinase inhibitors to be tested were added at the appropriate final concentration 45 minutes prior to MDR 1 induction by the addition of Ara-C or TPA. with the exception of herbimycin A. which was added 12 hours prior to Ara-C or TPA. and tyrphostins A25 and B46. which were each added 16 hours prior to MDRl induction by the addition of Ara-C or TPA. Cells were incubated overnight (TPA) or for 10 hours (Ara-C) at 37°C/ 5% CO : after the addition of the MDRl inducing agents before harvesting for cDNA-PCR analysis. In addition, the analysis of certain drugs imposed added requirements. For experiments using calphostin C, cells were incubated under direct white light illumination. For experiments using D,L-/ .reo-sphingosine. the cell culture medium was pre-treated with the appropriate concentration of the drug for one hour at 37°C with continuous gentle mixing. After this pre-treatment. cells were resuspended in media supplemented with drug and incubated for 45 minutes as described above. After the completion of the incubation period, each culture was treated with 25μM Ara-C and incubated for an additional 10 hours. Alternatively, each culture was treated with 16nM (lOng/mL) TPA and incubated for 12 hours or overnight. cDNA-PCR analyses on each culture treated with drugs, or in control cultures incubated in the absence or presence of Ara-C or TPA alone, were performed as described in Example 1 above. EXAMPLE 4 Functional Assays of P-Glycoprotein Expression in Cells Treated with Protein Kinase Activators and Inhibitors The studies described in Examples 1 and 2. above, demonstrate that PKC agonists can induce MDRl expression, suggesting an important role in PKC in the activation of the multidrug resistance response in tumor cells. PKC has also been implicated in cellular responses to different types of cytotoxic stress (Papathanasiou and Fornace. 1991. in R.F. Ozols (ed.). Molecular and Clinical Advances in Anticancer Drug Resistance, Kluwer Academic Publishers. Boston. MA, pp. 13-36). In particular PKC is activated by treatment with Ara-C. Therefore, experiments were performed to test whether Ara-C. which is not transported by P-glycoprotein. would have any effect on P-glycoprotein function in K562 leukemia cells. As illustrated in FIG. 3 A. exposure to K562 cells to Ara-C for 12-72 hours led to the emergence of a subpopulation of 3-17% cells that effluxed Rhl23. Rhl23 efflux was sensitive to the P-glycoprotein inhibitor verapamil. The appearance of Rhl23-dull cells was paralleled by a dose-dependent increase in MDR 1 mRNA expression relative to β 2 - glycoprotein in Ara-C treated K562 cells, as detected by polymerase chain reaction amplification of cDNA sequences (FIG. 4A). A number of other chemotherapeutic drugs were also tested for their ability to induce MDRl expression in K562 cells. Adriamycin, daunorubicin. vinblastine. cisplatin and hydroxyurea were all found to induce MDRl mRNA expression (FIG. 4B) and Rhl23 or DiOC 2 (3) efflux by 3-10% of the treated cells (FIG. 3A). Only the first four of these drugs are transported by P-glycoprotein (Roninson. ibid.). This result, together with the short times of drug exposure required for MDRl induction. indicates that cytotoxic selection for MDR 1 -expressing cells was not responsible for the emergence of the P-glycoprotein-positive subpopulations. The ability of cytotoxic drugs to induce MDRl expression was not limited to K562 cells. Ara-C increased P-glycoprotein expression in KG1 leukemia cells, which contained a significant amount of P-glycoprotein prior to drug treatment, as detected by Rhl23 accumulation or immunoreactivity with monoclonal antibody UIC2 (FIG. 3A). Ara-C also activated MDRl mRNA expression in H9 T-cell leukemia (FIG. 5). KB-3-1 epidermoid carcinoma (FIG. 4C). and EJ bladder carcinoma (FIG. 4D). though the magnitude of induction was somewhat lower in carcinoma cell lines. In addition. MDRl mRNA expression was induced in H9 cells by treatment with Adriamycin. vinblastine and methotrexate (FIG. 5), and in KB-3-1 cells with Adriamycin (FIG. 4C). However. P-glycoprotein induction was not detected in HL60 leukemia cells treated with these drugs. In all cases. MDRl induction became detectable at the same time as visible cell damage, which was evidenced by cell swelling, increased granularity, altered cell shape, and growth inhibition (FIG. 4A). In addition, continuous passage of some cell lines in the absence of drugs for several months also led to a small increase in MDRl expression, accounting for the variability in the basal levels of MDRl mRNA in untreated cells. Also assayed was whether drug-induced MDRl expression was maintained after cytotoxic drug treatment. For this purpose. K562 cells were treated with cytotoxic concentrations of Ara-C, Adriamycin. chlorambucil or methotrexate for 3-5 days, and then allowed to grow in the absence of the drugs. At different times, MDRl expression in the surviving cells was analyzed by dye efflux and immunofluorescence labeling with UIC2 (FIG. 3C) or by cDNA-PCR (FIG. 4E). MDRl expression in a subpopulation of treated cells was maintained for a least several weeks after the removal of drug (up to 1 1 weeks in the Ara-C treated population). P-glycoprotein-positive K562 cells showed no significant changes in their size, granularity and expression of differentiation-related antigenic markers. The presence of multidrug-resistant cells six weeks after the removal of Ara-C or Adriamycin was also demonstrated by a growth inhibition assay with vinblastine. a P-glycoprotein substrate. Vinblastine resistance, characterized by approximately 2-3 fold increase in the ID, 0 value, was specifically associated with the Rhl23-dull subpopulation of cells (FIG. 6). Thus, drug treatment leads to sustained induction of MDRl expression and its associated drug resistance in a subpopulation of treated cells. It was also found that Ara-C and Adriamycin-treated K562 cells were more resistant than the untreated cells to the cytotoxic effect of chlorambucil. a chemotherapeutic alkylating agent which is not transported by P-glycoprotein. This result indicates that other pathways or mechanism of clinically relevant drug resistance are co-induced with MDRl expression after treatment with chemotherapeutic drugs. To demonstrate that PKC was involved in MDRl induction by cytotoxic drugs, a variety of PKC inhibitors were used to block MDRl mRNA induction in H9 cells. The addition of certain these compounds effectively blocked MDRl induction by Ara- C. Adriamycin. methotrexate and vinblastine. as detected by cDNA-PCR (FIG. 5) and dye efflux assays with Ara-C treated cells. To investigate the specificity of the observed inhibition for PKC. effects of increasing doses of H7 (IC 50 =6.0 μM for PKC. 3.0 μM for protein kinase A) and HA 1004. a non-PKC specific protein kinase inhibitor (IC J0 =40 μM for PKC. 2.3 μM for protein kinase A) (Hidaka et al. 1984. Biochemistry 23: 5036-5041) were compared. As shown in FIG. 5A. H7 inhibited MDRl induction by Ara-C at 10 μM or higher concentration, but HA 1004 showed no significant inhibition even at 60 μM. These results were consistent with a role for PKC in MDRl induction by cytotoxic drugs. To further evaluate the capacity of protein kinase inhibitors to suppress MDRl induction by cytotoxic drugs and PKC agonists. H-9 human T-cell leukemia cells were used in cDNA-PCR assays to examine induced MDRl mRNA expression levels in the presence of protein kinase inhibitors. The results of these assays are shown in FIG. 7 and are summarized in accompanying Table II. MDRl was induced in cells represented in FIG. 7F by overnight incubation in lOng/mL TPA: 10 hour incubation with 25μM Ara-C was used to induce MDRl expression in all other cell cultures analyzed as in FIG. 7A through 7E. Two erbstatin analogues (the tyrphostins A25 and B46) were tested for the ability to inhibit Ara-C induced MDRl mRNA expression, as shown in FIG. 7A. Both of these compounds (tyrphostin A25. Lanes 5 and 6; tyrphostin B46. Lanes 7 and 8) demonstrated the capacity to strongly inhibit Ara-C induced MDRl expression when incubated with H-9 cells at concentrations of 50-1 OOμM. The antibacterial agent neomycin sulfate was tested for inhibitory capacity against Ara-C induced MDRl expression. This compound was found to inhibit MDRl expression in cells induced with 25μM Ara-C at concentrations of lOmM (FIG. 7B. Lanes 7 and 8). These results demonstrate that neomycin sulfate possesses MDRl inhibitory capacity at clinically relevant and achievable concentration. The concentration dependence ofthe ability of the erbstatin analog methyl-2.5- dihydroxycinnamate to inhibit MDRl expression induced by treatment of H-9 cells with 25μM Ara-C is shown in FIG. 7C. These results demonstrate that this erbstatin analog inhibits MDRl expression at both 32μM and 64μM to essentially the same degree. Inhibition of Ara-C induced MDRl expression by the combination of calphostin C and direct illumination with white light in shown in FIG. 7D. This compound was found to be capable of inhibiting MDRl expression when present at concentration of lμM (Lanes 7 and 8); the inhibitory capacity of this compound was greatly reduced when the concentration used was reduced to OJμM (Lanes 5 and 6). Progressive dose-dependent inhibition of Ara-C induced MDRl expression was demonstrated for the PKC inhibitor chelerythrine. shown in FIG. 7E. The dose- dependence of inhibition using this compound is seen by a comparison ofthe intensity of the MDRl -specific band in Lanes 5 and 6 ( lμM chelerythrine) and Lanes 7 and 8 (5μM chelerythrine). FIG. 7F shows the results of incubation of H-9 cells in the presence of 30nM staurosporine (Lanes 7 and 8) or 32μM methyl-2.5-dihydroxycinnamate (erbstatin analog) on the induction of MDRl expression by TPA, a known PKC agonist. TPA alone (Lanes 3 and 4) efficiently induced readily detectable levels of MDRl expression, consistent with the results disclosed above. Both staurosporine. a PKC inhibitor, and methyl-2,5-dihydroxycinnamate. completely inhibited TPA-induced MDRl expression. These results are in agreement with previous observation that erbstatin analogues, which are known to inhibit epidermal growth factor tyrosine kinase. also have PKC inhibitory activity (Bishop et al.. ibid.). A number of other protein kinase inhibitors were found to have little or no capacity to inhibit Ara-C induced MDRl expression. These compounds include D.L- tAreø-sphingosine (tested at 5μM; higher concentrations induced cellular toxicity) and herbimycin A (tested at concentrations from 0.35-3μM). No significant inhibition was observed with Iso-H7. a structural analog of H7 with 10 fold weaker effect on protein kinases (Pelosin et al, 1990. Biochem. Biophys. Res. Commun. 169: 1040-1048). Similar results were observed with Ara-C treated K562 cells. Another protein kinase inhibitor, genistein (150μM) was found to only weakly inhibit Ara-C induced MDRl expression (Table II). These results indicated that certain among a number of protein kinase inhibitors, and particularly protein kinase C inhibitors, were capable of inhibiting MDRl expression induced by treatment of cancer cells with cytotoxic drugs or protein kinase C agonists. The data presented herein demonstrated that different chemotherapeutic drugs, including those that are not transported by P-glycoprotein. induced MDRl expression directly, rather than by selection of preexisting genetic variants. Drug-induced MDRl expression was limited to a subpopulation of treated cells and was associated with a moderate increase in the resistance of P-glycoprotein- transported drugs (approximately 2-3 fold in the case of K562 cells). This increase may be sufficient to reduce the response to chemotherapy in vivo and to enhance the selection of genetic mutants with higher levels of drug resistance. Drug-mediated induction of MDR 1 expression may occur during cancer chemotherapy, and it may largely account for the increased incidence of MDRl expression in treated tumors. Hence, the demonstration that PKC inhibitors can prevent MDRl induction suggested using such agents in combination with cytotoxic drugs in cancer chemotherapy in order to achieve a higher degree of eradication of cancer cells. TABLE II Inhibition of Ara-C Induced MDRl Expression bv Protein Kinase Inhibitors Compound Cone, tested Results herbimycin A ( 12 h) 0.3-3 μM - tyrphostin A25 ( 16 hr) 50-100 μM +-H- tyrphostin B46 (16 hr) 50-100 μM +-H- D.L-tΛreo-sphingosine 5 μM - genistein 150 μM + staurosporine 30 nM -H-t- H7 50 μM +++ calphostin C (+ light) 1 μM -H-+ chelerythrine 5 μM — H- methyl- 1.5-dihydroxycinnamate 32 μM -+-H- neomycin sulfate 10 mM +++ MDRl induced by incubation of H-9 cells with 25μM Ara-C for 10 h: +++ = complete inhibition of Ara-C mediated induction: + = weak inhibition of Ara-C mediated induction: = no inhibition of Ara-C mediated induction EXAMPLE 5 Protein Kinase Inhibitors prevent MRP Expression in Tumor Cells 1. Materials and Methods 1.1 MRP Expression Inhibition Assays Cells were plated in 6-well Falcon tissue culture plates at 3.300 cells per well, and incubated in the appropriate concentrations of drugs. Each of the protein kinase inhibitors to be tested were added at their appropriate final concentrations, and the cells then incubated for 10 hours at 37°C under an atmosphere of 5% CO 2 . cDNA-PCR analyses on each culture treated with drugs, or in control cultures incubated without drugs, were performed as described in Example 1 above. PCR was performed using the following primers: β 2 M (sense) 5'-ACCCCCACTGAAAAAGATGA-3' (SEQ ID No.: 1 ) β 2 M (antisense) 5'-ATCTTCAAACCTCCATGATG-3' (SEQ ID No.: 2) MRP (sense) 5'-GGACCTGGACTTCGTTCTCA-3' (SEQ ID No.: 5) MRP (antisense) 5'-CGTCCAGACTTCCTTCATCCG-3' (SEQ ID No.: 6) (where β 3 M represents β 2 microglobulin). PCR was performed for β 2 M experiments under a regime consisting of 1 cycle comprising denaturation at 94°C for 3 minutes, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 1 minute, followed by 19 cycles comprising denaturation at 94°C for 30 seconds, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 1 minute, followed by a final cycle comprising denaturation at 94°C for 30 seconds, primer annealing at 60°C for 30 seconds, and primer extension at 72°C for 5 minutes. MRP cDNA sequences were amplified using a protocol consisting of 1 cycle comprising denaturation at 94°C for 3 minutes. 26 cycles of denaturation at 94°C for 30 seconds, primer annealing at 62°C for 30 seconds, and primer extension at 72°C for 1 minute, followed by 1 cycle comprising primer extension at 72°C for 5 minutes, resulting in an MRR-specific amplified fragment of 292 bp. 32 P-labeled PCR products were detected by autoradiography. EXAMPLE 6 Functional Assays of MRP Expression in Cells Treated with Protein Kinase Activators and Inhibitors H-9 cells were used to analyze the effect of various protein kinase inhibitors on the expression of the multidrug resistance associated protein MRP (Grant et al. ibid.). The results of these experiments are shown in FIG. 8. When MRP mRNA expression was evaluated in H-9 cells in the absence of any drugs, these cells were found to express the MRP gene robustly (FIG. 8. Lanes 1 and 2). In contrast to MDRl . incubation of H-9 cells in 25 μM Ara-C resulted in a slight decrease in MRP expression (data not shown). Following incubation with staurosporine ( lOOnM: Lanes 3 and 4), chelerythrine ( l-5μM; Lanes 5 and 6); methyl-2.5-dihydroxycinnamate (32μM; Lanes 7 and 8) or neomycin sulfate (lOmM: Lanes 9 and 10). MRP expression was strongly inhibited. These results demonstrate that a variety of protein kinase inhibitory compounds have the capacity to inhibit both cytotoxic drug-induced MDR 1 expression and expression of the multidrug resistance phenotype-related MRP gene. These results strongly suggest that such protein kinase inhibitors have great utility in preventing the emergence of multidrug resistance in cancer patients receiving chemotherapeutic treatment. TABLE III Inhibition of MRP Expression bv Protein Kinase Inhibitors Compound Cone, tested Results staurosporine 100 nM ++++ chelerythrine 1-5 μM methyl- 1.5-dihydroxycinnamate 32 μM ¥+++ neomycin sulfate lOmM ++++ complete inhibition of MRP expression The present invention is not to be limited in scope by the exemplified embodiments, which are intended as illustrations of individual aspects of the invention. Indeed, various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims. All publications cited herein are incorporated by reference in their entirety. SEQUENCE LISTING (1 ) GENERAL INFORMATION: (i) APPLICANT: (A) NAME: Board of Trustees of the University of Illinois (B) STREET: 352 Henry Administration Building. 201 Wright Street (C) CITY: Urbana (D) STATE: Ilinois (E) COUNTRY: USA (F) POSTAL CODE (ZIP): 61801 (G) TELEPHONE: (H) TELEFAX: (ii) TITLE OF INVENTION: Methods of Preventing Multidrug Resistance in Cancer Cells (iii) NUMBER OF SEQUENCES: 6 (iv) COMPUTER READABLE FORM: (A) MEDIUM TYPE: Floppy disk (B) COMPUTER: IBM PC compatible (C) OPERATING SYSTEM: PC-DOS/MS-DOS (D) SOFTWARE: Patentin Release #1.0. Version #1.25 (EPO) (v) CURRENT APPLICATION DATA: APPLICATION NUMBER: PCT/US95/ (2) INFORMATION FOR SEQ ID NO.J : (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NOJ : ACCCCCACTG AAAAAGATGA 20 (2) INFORMATION FOR SEQ ID NO:2: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2: ATCTTCAAAC CTCCATGATG 20 (2) INFORMATION FOR SEQ ID NO:3: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3: CCCATCATTG CAATAGCAGG 20 (2) INFORMATION FOR SEQ ID NO:4: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4: GTTCAAACTT CTGCTCCTGA 20 (2) INFORMATION FOR SEQ ID NO:5: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 20 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5: GGACCTGGAC TTCGTTCTCA 20 (2) INFORMATION FOR SEQ ID NO:6: (i) SEQUENCE CHARACTERISTICS: (A) LENGTH: 21 base pairs (B) TYPE: nucleic acid (C) STRANDEDNESS: single (D) TOPOLOGY: linear (ii) MOLECULE TYPE: DNA (genomic) (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6: CGTCCAGACT TCCTTCATCC G 21

Claims: WHAT IS CLAIMED IS: 1. A method of inhibiting MDRl induction in a cancer cell by treatment with a cytotoxic drug, comprising contacting the cell with a protein kinase inhibitor coincident with treatment with the cytotoxic drug. 2. The method according to Claim 1. wherein the protein kinase inhibitor is erbstatin. tyrphostin A25. tyrphostin B46. staurosporine. H7. neomycin sulfate. chelerythrine or calphostin C, or derivatives thereof. 3. The method according to Claim 1. wherein the cytotoxic drug is a chemotherapeutic drug. 4. The method of Claim 1 wherein the cancer cells contain little or no detectable MDR 1 -encoded P-glycoprotein. as determined by immunoreactivitv with anti-P-glycoprotein antibodies, accumulation or efflux of P-glycoprotein transported dyes, or MDRl mRNA expression assay. 5. The method of Claim 1 in which the cancer cells are derived from hematopoietic tumors. 6. The method of Claim 1 in which the cancer cells are derived from solid tumors. 7. A method of inhibiting MRP expression in a cancer cell, comprising contacting the cell with a protein kinase inhibitor. 8. The method according to Claim 7. wherein the protein kinase inhibitor is erbstatin. staurosporine. neomycin sulfate. or chelerythrine. or derivatives thereof. 9. The method of Claim 7. wherein the cytotoxic drug is a chemotherapeutic drug. 10. The method of Claim 7 in which the cancer cells are derived from hematopoietic tumors. 1 1. The method of Claim 7 in which the cancer cells are derived from solid tumors. 12. A method for screening a multiplicity of compounds to determine which of said compounds are capable of inhibiting MDRl induction in a cancer cell by treatment with a cytotoxic drug, the method comprising: a. contacting a first sample of cancer cells with each of the multiplicity of compounds for a time and at a concentration sufficient to have an inhibiting effect on MDRl expression, and treating the cancer cells with a cytotoxic drug capable of inducing MDRl gene expression for a time and at a concentration sufficient to have an MDRl expression inducing effect: b. treating a second sample of cancer cells with a cytotoxic drug capable of inducing MDRl gene expression for a time and at a concentration sufficient to have an MDRl expression inducing effect, in the absence of any of the compounds to be tested; c. comparing MDRl gene expression of the cancer cells of subpart (a) with MDRl gene expression of the cancer cells of subpart (b): and d. identifying a compound of the multiplicity of compounds tested wherein the induced level of MDRl gene expression in the cancer cells of subpart (a) is lower than the level of MDRl gene expression in the cancer cells of subpart (b). 13. The method of Claim 12. wherein the cytotoxic drug is a chemotherapeutic drug. 14. The method of Claim 13. wherein the chemotherapeutic drug is cytidine arabinoside, vinblastine. Adriamycin. or methotrexate. 15. The method of Claim 12 in which the cancer cell comprises the cancer cell line H-9 leukemia. 16. The method of Claim 12 in which the cancer cells comprise the cancer cell line K562 leukemia. 17. A method for screening a multiplicity of compounds to determine which of said compounds are capable of inhibiting MRP expression in a cancer cell, by comparing MRP expression in the presence or absence of each of the multiplicity of compounds to be screened. 18. The method of Claim 17 in which the cancer cell comprises the cancer cell line H-9 leukemia. 19. Use of a protein kinase inhibitor for preparing a medicament for suppressing induction of MDRl expression in an animal bearing a malignant tumor and undergoing cancer chemotherapy, the medicament comprising a therapeutically- effective amount of a protein kinase inhibitor. 20. A use according to Claim 19 wherein the protein kinase inhibitor is a protein kinase C inhibitor. 21. A use according to Claim 20 wherein the protein kinase inhibitor is neomycin sulfate. 22. A use according to Claim 20 wherein the protein kinase inhibitor is erbstatin or an analog thereof. 23. Use of a protein kinase inhibitor for preparing a medicament for suppressing induction of MDRl expression in an animal bearing a malignant tumor and undergoing cancer chemotherapy, the medicament comprising a therapeuticallv- effective amount of a protein kinase inhibitor identified according to the method of Claim 12. 24. A use according to Claim 23 wherein the protein kinase inhibitor is a protein kinase C inhibitor. 25. A use according to Claim 23 wherein the medicament also comprises a chemotherapeutic drug. 26. Use of a protein kinase inhibitor for preparing a medicament for suppressing MRP expression in an animal bearing a malignant tumor and undergoing cancer chemotherapy, the medicament comprising a therapeuticallv-effective amount of a protein kinase inhibitor. 27. A use according to Claim 26 wherein the protein kinase inhibitor is a protein kinase C inhibitor. 28. A use according to Claim 26 wherein the protein kinase inhibitor is neomycin sulfate. 29. A use according to Claim 26 wherein the protein kinase inhibitor is erbstatin or an analog thereof. 30. A use according to Claim 26 wherein the medicament also comprises a chemotherapeutic drug. 31. Use of a protein kinase inhibitor for preparing a medicament for suppressing MRP expression in an animal bearing a malignant tumor and undergoing cancer chemotherapy, the medicament comprising a therapeuticallv-effective amount of a protein kinase inhibitor identified according to the method of Claim 17. 32. A use according to Claim 31 wherein the protein kinase inhibitor is a protein kinase C inhibitor.

Inventors: CHAUDHARY PREET, RONINSON IGOR B, SHTIL ALEXANDER A, CHAUDHARY, PREET, RONINSON, IGOR, B., SHTIL, ALEXANDER, A.
Applicants: UNIV ILLINOIS, BOARD OF TRUSTEES OF THE UNIVERSITY OF ILLINOIS
Application year: 1996
Main classification: A61K45
IPC codes: A61K31, A61K45, A61P35, B05B7, C07H15