TREATMENT OF TOXOPLASMOSIS

Abstract: Pharmaceutical compositions comprising purine analogs and uses for the compositions in treating parasite infections and other diseases or conditions are described.

Description: Treatment of Toxoplasmosis CrQvernmen. Support The work resulting in this invention was supported in part by NIH Grant No. AJ-29848. The U.S. government may therefore be entitled to certain rights in the invention. Background of the Invention The parasitic protozoan Toxoplasma gondii is the etiologic agent for toxoplasmosis, a parasitic disease widespread among various warm-blooded animals, including man. Toxoplasma infection is common in man (e.g., 50% of the adult population in the U.S.), but is asymptomatic in the general population. By contrast, the disease represents a major health problem for immunocompromised individuals, such as AIDS patients, and for the unborn children of infected mothers. Toxoplasmic encephalitis h$s become the most common cause of intracerebral mass lesions in AIDS patients and possibly the most commonly recognized opportunistic infection of the central nervous system (Luft, BJ. and Remington, J.S. Toxoplasmic encephalitis. J Infect Dis 157: 1-6, 1988; McCabe, R. and Remington, JS, Toxoplasmosis: The time has come. N. Eng. J. Med. 318: 313-315, 1988). Congenital toxoplasmosis is as high as 1/1,000 live births, with approximately 50% of infected women giving birth to infected infants in the absence of treatment (Luft, B.J. and Remington, J.S. Toxoplasmic encephalitis. J Infect Dis 157: 1-6, 1988), and causes blindness, psychomotor or mental retardation, severe brain damage, or even death of infected children (McCabe, R. and Remington, JS, Toxoplasmosis: The time has come. N. Eng. J. Med.318: 313-315, 1988). In spite of the tragic consequences of toxoplasmosis, therapy for the disease has not changed materially in the last twenty years. The efficacy of the current therapy for toxoplasmosis (a combination of pyrimethamine and sulfadiazine) is limited, primarily by serious host toxicity and ineffectiveness against tissue cysts. Furthermore, as many as 50% of patients do not respond to therapy (Luft, B.J. and Remington, J.S. Toxoplasmic encephalitis. J Infect Dis 157: 1-6, 1988). The combination of trimethoprim and sulfamethoxazole is the only alternative therapy which has been shown to be effective in AIDS patients. However, patients who arc intolerant to pyrimethamine and sulfadiazine are invariably intolerant to this alternate therapy, since both therapies have the same mechanism of action. There is an urgent need for new and effective drugs for treating and preventing toxoplasmosis. Summary of the Invention In one aspect, the invention features therapeutic compositions comprising purine nucleoside analogs that are selective ligands of parasite salvage pathway enzymes and a pharmaceutically acceptable carrier. Preferred purine nucleoside analogs are ligands of a parasitic adenosine kinase, nucleosidase hydrolase or phosphorylase or a purine phosphoribosyltransferase (xanthine, guanine, hypoxanthine or adenine phosphoribosyl transferase) enzyme. Particularly preferred purine analogs are D and L enantiomers, α- and β - anomers of: 6- [4-nitrobenzyl thio]-9-β-D-ribofuranosyl purine (NBMPR), 6- [4- nitrobenzyl thio]-9-β-D-ribofuranosyl purine 5'-monophosphate (NBMPR-P), 6-chloropurine riboside, 3'-deoxysangivamycin, N^-(p-methoxybenzoyl)adenosine, nitrobenzyl-6- selenoriboside, N&. benzyladenosine, N^- azidobenzyladenosine and N >- nitrobenzyladenosine, 7-deazahypoxanthine, 7-deazaxanthine, 7-deaza-6-thiopurine, 7-deaza- 2-amino-6-thiopurine, ' and 7-de--za-2-oxo-6-thiopurines, with various substitutions on the pentose and purine moieties. In another aspect, the invention features methods for treating a subject for a disease, which is caused by or contributed to by cells, which are dependent on a parasite salvage pathway enzymes for their purine requirement by administering to the subject an effective amount of a pharmaceutical composition comprised of a selective purine nucleoside analog and a pharmaceutically acceptable carrier. In one preferred embodiment, the disease is toxoplasmosis and the cells are a toxoplasma sp.. In another preferred embodiment, the disease is a cancer or blood disorder and a parasitic gene(s) of purine salvage enzyme(s) has been introduced into the disease causing (e.g. malignant) mammalian cells. In a preferred embodiment, the parasitic genes for the purine salvage enzyme(s) is selected from the group consisting of adenosine kinase, purine nucleosidase (hydrolase or phosphorylase) or phosphoribosyltransferase genes. In yet another aspect, the invention features combination therapies and pharmaceuticals for treating or preventing: i) a disease or condition in which the causative agent is a parasite, which is dependent on a parasite salvage pathway for its purine requirement and ii) a condition that renders a subject immunocompromised. In one embodiment, the method comprises co-administering to the subject, i) an effective amount of a purine nucleoside analog; and ii) an effective amount of an agent for treating or preventing the establishment or growth (systemic or local) of a cyst or infection. In a preferred embodiment, the method is for treating toxoplasmosis and AIDS and comprises administering to the subject, a combination drug comprising a purine nucleoside analog and an AIDS drug. In another embodiment, the method comprises co-administering to a subject, i) an effective amount of a purine nucleoside analog; and ii) an effective amount of an agent that suppresses the immune system (e.g., to permit allotransplantation). The instant disclosed pharmaceutical compositions and methods of therapy exhibit high therapeutic indices, being highly toxic to disease causing cells, which are naturally or have been engineered to use the parasite salvage pathway enzymes for their purine requirement, but non-toxic to mammalian (host) cells. These compounds therefore represent safe and effective therapeutics for eliminating parasites in a subject's cells; or for eliminating other unwanted cells in a subject The pharmaceutical compositions can be administered alone or in combination with other drugs. Additional features and advantages of the invention will become more apparent from the following detailed description and claims. Detailed Description of the Invention In contrast to their mammalian hosts, T. gondii are incapable of de novo purine synthesis and are totally dependent on the salvage pathways for their purine requirements (Schwartzman, JD and Pfefferkorn, ER, Toxoplasma gondii: Purine synthesis and salvage in mutant host cells and parasites. Exptl Parisitol 53: 77-86, 1982; Krug, EC, Marr, JJ and RL Berens, Purine metabolism in Toxoplasma gondii: J. Biol Chem 264: 10601-10607, 1989). In addition, T. gondii replicate rapidly and require large amounts of purines and pyrimidines for the synthesis of nucleic acids and other macromolecules. Indeed, the current standard treatment for T. gondii infections (antifolates and sulfonamides) uses this approach by blocking folate metabolism, thereby depriving the parasite of the necessary cofactors required for nucleotide biosynthesis (Luft, BJ and Remington, JS, Toxoplasmic encephalitis. J Infect Dis 157: 1-6, 1988; McCabe, R. and Remington, JS, Toxoplasmosis: The time has come. N. Engl. J. Med. 318: 313-315, 1988). Enzymes involved with T. gondii nucleotide metabolism, therefore are potential targets for chemotherapeutic intervention. The present invention is based on the finding that certain purine nucleoside analogs selectively interfere with enzymes involved in the purine salvage pathways of Toxoplasma gondii. The high activity of these enzymes and the unique characteristics of nucleotide metabolism in parasites makes these enzymes excellent targets for chemotherapy. One of the significant enzymatic reactions in the purine salvage pathways in T. gondii is direct phosphorylation of adenosine to adenosine monophosphate (AMP), from which all other purine nucleotides can be synthesized (Krug EC, Marr JJ and Berens RL, Purine metabolism in Toxoplasma gondii J Biol Chem 264: 10601-10607, 1989). The reaction is catalyzed by the enzyme adenosine kinase (EC 2.7.1.20), which is approximately 10-fold more active than any other purine salvage enzyme in these parasites (Krug EC, Marr JJ and Berens RL, Purine metabolism in Toxoplasma gondii. JBiol Chem 264: 10601-10607, 1989). This contrasts sharply with mammalian cells where adenosine is predominantly deaminated to inosine, which is then cleaved to hypoxanthine by the sequential reactions of adenosine deaminase and purine nucleoside phosphorylase. T. gondii adenosine kina.se binding The binding of one hundred and thirty three purine nucleoside analogs were evaluated as potential ligands of T. gondii adenosine kinase as described in Example 1. Three types of purine nucleoside analogs were screened as follows: (1) analogs with either endocyclic or exocyclic substitutions on the purine moiety; (2) analogs with substitutions on the pentose moiety; and (3) analogs with substitutions on both the purine and pentose moieties. Table 1 shows the effect of various substitutions at the different positions of adenosine on binding to T. gondii adenosine kinase as compared to that of the reference compound adenosine. IΔBLEJ. Binding (AppK[) values for inhibition of T. gondii adenosine kinase. Compound Source AppA " j (μM)a β-D-Adenosine (9-(β-D-ribofuranosyl)adenine) SIG 7.0 ± 3.3 2 β-L-Adenosine RS t α-Adenosine (9-α-D-ribofuranosyl)adenine) SIG 8,400 ± 2,200 Modifications in the Purine Moiety 1-Posi on Substitutions Adenosine-Nl -oxide SIG 1,230 ± 10 1 -Deazaadenosine BWC t 1 -Methyladenosine SIG 88 ± 28 2-Position Substitutions 2-Chloroadenosine SIG 870 ± 50 2-Fluoroadenosine SC 3,000 ± 500 2-Phenylaminoadenosine RBI t 3-Postiion Substitutions 3-Deazaadenosine BWC t 6-Position Substitutions Purine riboside (Νebularine) SIG 170 ± 20 6-Bromopurine riboside NCI 28 ± 2 6-Chloropurine riboside SIG 9.5 ± 0.3 6-Iodopurine riboside NCI 18 ± 1 6-Oxopurine riboside (inosine) SIG t 6-Methoxypurine riboside SIG 370 ± 40 6-Thiopurine riboside (6-mercaptopurine riboside) SIG 5,300 ± 400 6-Methylmercaptopurine riboside SIG 22 ± 1 6-Ethylmercaptopurine riboside NCI 330 ± 3 6-Benzylmercaptopurine riboside NCI 140 ± 10 6-(p-Νitrobenzyl)mercaptopurine riboside (NBMPR) NCI 140 ± 10 6-(7-Nitrobenzofuroxan-4-yl)mercaptopurine riboside NCI l l ± l 6-Selenopurine riboside SIG 130 ± 20 6-(p-Nitrobenzyl)selenopurine riboside SIG 84 ± 46 TABLE 1 (continued) TABLE 1 (continued) TABLE 1 (continued) Compound Source Apptfj (μM) Other Substitutions 80 7-Carboxamido-7-deazaadenosine-Nl-oxide ΝCI 780 ± 70 81 8-Aza-9-deazaadenosine (formycin A) SIG 1,400 ± 200 82 8-Aza-9-deazainosine (formycin B) SIG 3J00 ± 200 83 8-Aza-6-methylmercaptopurine riboside BWC 130 ± 40 84 Benzimidazole riboside (" 1,3-dideazapurine riboside") BWC t 85 Coformycin CAL 2,000 ± 900 Modifications in the Pentose Moiety 2 '-Position Substitutions 86 Adenine arabinoside (ara-A) SCH 3,000 ± 700 87 2'-Deoxyadenosine SIG 4,200 ± 1,000 88 2'-0-Methyladenosine SIG t 89 2'Tosyladenosine SIG t 3 '-Position Substitutions 90 3'-Deoxyadenosine (Cordycepin) SIG lJOO ± lOO 91 3'-O-Methyladenosine SIG t 92 Adenine xyloside GG 1,400 ± 100 S'-Position Substitutions 93 5'-Deoxyadenosine SIG 40 ± 3 94 5'-Amino-5'-deoxyadenosine SIG 220140 95 5'-Chloro-5'-deoxyadenosine SIG 210 ± 10 96 5'-Iodo-5'-deoxyadenosine SIG 230 ± 10 97 5'-Sufonyl adenosine (adenosine 5'-monosulfate) SIG 210 ± 60 98 5'-Methylthio-5'-deoxyadenosine SIG 530 ± 230 99 adenosine 5'-Carboxylic acid (5'-Carboxyladenosine) SIG 1,100 ± 160 100 5'-Succinyladenosine (adenosine 5'-succinate) SIG 920 ± 600 101 5'-Isobutyl-5'-deoxyadenosine SIG 410 ± 90 102 5'-N-(Methyl)carboxamidoadenosine RBI t 103 5'-N-(Ethyl)carboxamidoadenosine RBI t 104 5'-N-(Cyclopropyl)carboxamidoadenosine SIG 1J00 ± 40 105 5'-(p-Fluorosulfonylbenzoyl)adenosine FLU 340 ± 40 Multiple Substitutions 106 2',3'-Dideoxyadenosine SIG 5,500 ± 2,900 107 Adenine lyxoside ΝCI 1,300 ± 70 TABLE 1 (continued) Compound Source Apptfj (μM) Multiple Substitutions (continued) 108 2',3'-0-p-Anisylidene adenosine SIG 1,400 ± 200 109 2',3'-Di-O-acetyladenosine SIG t 110 3',5'-Di-O-acetyladenosine SIG t 111 4'-Flouro-5'-O-sulfamoyIadenosine (nucleocidin) AMC 850 ± 220 Other Modifications 112 Carbocyclic adenosine JM 170 ± 30 113 (±)-EHNA SIG t 1 14 Neplanocin A ΝCI 37 ± 10 115 Adenine SIG 6,500 ± 2,600 Modifications in the Purine and Pentose Moities a Apparent K[ values ± the range were obtained from at least two separate estimations, b t — less than 10% inhibition at a concentration of 1 mM. Overall, the best ligands to bind to adenosine kinase from T. gondii were adenosine analogs with 6-position substitutions. Two 6-substituted compounds, N^ -(p- methoxybenzoyl) adenosine and 6-chloropurine riboside, were found to bind strongly to the enzyme with appKj values of 3.9 and 9.5 μM, respectively. Based on the inhibition data, a structure activity relationship for the binding of these analogs to the enzyme was formulated, using adenosine as a reference compound. It was concluded that the Nl , N3, N7 and N9 of the purine ring are all required for binding of T. gondii adenosine kinase; whereas, the C8 carbon can be replaced with a nitrogen with only a slight reduction in binding (3-fold). In general, exocyclic substitutions at the 1-, 2- or 8- position diminished binding. At the 6-position, hydrophobic substitutions were preferred over hydrophilic substitutions; however, most exocyclic substitutions diminished binding. In addition, there appears to be a "pocket" in the catalytic site of T. gondii adenosine kinase, adjacent to the 6-position, which is large enough to accomodate a substituted phenyl group. Two 7- substituted adenosine analogs, sangivanycin and toyocamycin, were found to bind with relatively good affinity (i.e. appKi < 60 μM). The presence of 2'-hydroxyl group in the ribo configuration appears to be critical for binding to the enzyme, as removal of this group from sangivamycin (3'-deoxysangivamycin) reduced binding by only 2-fold. The presence of the 5'-hydroxyI group, which is considered essential if the ligand is to become a substrate, seems not essential for binding, as its removal diminished binding by only 6-fold. However, substitution of the 5'-hydroxyl group with hydrophilic or hydrophobic groups decreased binding significantly. The following Table 2 summarizes the structure-activity relationship for the binding of purine nucleoside analog ligands to T gondii adenosine kinase. TABLE 2 Structure-activity relationship for the binding of nucleoside ligands to T. gondii adenosine kinase fta-ilion Substituent Effect ("numbers are with reference to the nucleoside analogs shown in Table 1) I-Position Endocyclic "pyridine-type" nitrogen required; replacement with endocyclic methine group (1-deazaadenosine, 5) abolishes binding. 2-Position Exocyclic hydrogen required; substitution (e.g. chloro, 7; fluoro, 8; or amino, 63) significantly decreases (120 to 430-fold) or abolishes (e.g. phenylamino, 9) binding. 3-Position Endocyclic "pyridine-type" nirogen required; replacement with endocyclic methine group (3-deazaadenosine, 10) abolishes binding. 6-Position Exocyclic amino group preferred; elimination of group (purine riboside, 11) decreases binding (25-fold). Exocyclic substituents in lactim tautomeric form strongly preferred; substitution of amino group with groups in lactim tautomeric form (e.g. bromo, 12; chloro, 13; or iodo, 14) diminishes binding slightly (1.4- to 4-fold); substitution of amino group with groups in lactam tautomeric form (e.g. oxo 15; thio, 17; or seleno, 23) decreases (19- to 750-fold) or abolishes binding. N 6 -Position Hydrophobic substituents preferred over hydrophilic substituents, e.g. ethyl (28) vs. 2-hydroxyethyl (29); straight chain aliphatic substituents preferred over cyclic aliphatic substituents, e.g. n-pentyl (35) vs. cyclopentyl (37); unsaturated substituents preferred over saturated substituents, e.g. 2-isopentenyl (36) vs. π-pentyl (35); aromtaic substituents preferred over non-aromatic substituents, e.g. phenyl (43) vs. cyclohexyl (40). Hydrophilic linkages (e.g. carbonyl, 49) between aromatic groups and N 6 preferred over hydrophobic linkages (e.g. methylene, 44). 7-Position Endocyclic "pyridine-type" nitrogen strongly preferred; replacement with endocyclic methine group (7-deazaadenosine, 51) decreases binding (49-fold). Hydrophobic exocyclic substituents (e.g. chloro, 52; or iodo, 53) attached to endocyclic carbon preferred over hydrophilic substituents (e.g. cyano, 54; carboximido, 55; or carbox amidoxime, 56). -Position Endocyclic methine group preferred but not required; replacement with endocyclic nitrogen (8-azaadenosine, 57) decreases binding slightly (3- fold). -Position Endocyclic nitrogen required; replacement with endocyclic carbon (9- deazaadenosine, 60) decreases binding significantly (490-fold). '-Position Exocyclic 2'-hydroxyl group in ribo configuration required for binding; elimination (2'-deoxyadenosine, 87) or substitution of group (e.g. methoxy, 88) significantly decreases (600-fold) or abolishes binding; change in configuration of group to ara configuration (86) significantly decreases binding (430-fold). '-position Exocyclic 3'-hydroxyl group in ribo configuration strongly preferred for binding; elimination (3 '-deoxy adenosine, 91) or substitution of group (e.g. methoxy, 92) significantly decreases (150-fold) or abolishes binding; change in configuration of grup to xylo configuration (90) significantly decreases binding (200-fold). '-Position Hydroxymethyl or methyl group strongly preferred for binding; elimination of hydroxyl group (5'-deoxyadenosine, 93) decreases binding slightly (5.7-fold); substitution of hydroxyl group (with variety of groups, e.e. 94-106) significantly decreases (at least 30-fold) or abolishes binding. Other A pentose or "pentose-like" moiety in a ribo configuration required for binding; elimination (i.e. adenine, 116) of pentose moiety or a change to lyxo configuration (adenine lyxoside, 107) significantly decreases binding (930- and 190-fold, respectively); replacement of pentose moiety with hydroxylated cyclopentene moiety (neplanocin A, 114) decreases binding slightly (5.3-fold). β-D-nucleoside configuration required; change in configuration to α-D- nucleoside (α-D-adenosine, 3) significantly decreases binding (1200- fold); change in configuration to β-L-nucleoside (β-L-adenosine, 2) abolishes binding Anti conformation around N-glycosidic bond preferred; change to syn confirmation (e.g. 8-bromoadenosine, 59) decreases binding (94-fold). Particularly preferred ligands of parasite adenosine kinase include substituted or unsubstituted D and L enantiomers, α- and β- anomers of: 6- [4-nitrobenzyl thio]-9-β-D- ribofuranosyl purine (NBMPR), 6- [4-nitrobenzyl thio]-9-β-D-ribofuranosyl purine 5'- monophosphate (NBMPR-P), 6-chloropurine riboside, 3'-deoxysangivamycin, N^-(p- methoxybenzoyl)adenosine, nitrobenzyl-6-selenoriboside, N"- benzyladenosine, N°- azidobenzyladenosine and N*>- nitrobenzyladenosine. The metabolism and fate of 6-substituted 9-β-D-ribofuranosylpurines (NBMPR or its nucleotide analog NBMPR-P) were studied as described in Example 3. NBMPR or its nucleotide analog NBMPR-P were found to kill T. gondii grown in human fibroblasts in a dose dependent manner without apparent toxicity to host cells. Doses up to 100 μM did not affect host cells. Another significant enzymatic reactions in the T. gondii purine salvage pathway is the conversion of purines, e.g. adenine, guanine, hypoxanthine and xanthine, to their respective nucleoside 5'-monophosphates by phosphoribosyltransferase activities (Schwartzman, JD and Pfefferkom, ER, Toxoplasma gondii: Purine synthesis and salvage in mutant host cells and parasites. Exptl Parisitol 53: 77-86, 1982; Krug, EC, Marr, JJ and RL Berens, Purine metabolism in Toxoplasma gondii: J. Biol Chem 264: 10601-10607, 1989; Pfefferkom ER and Pfefferkom LC, Toxoplasma gondii: Specific labeling of nucleic acids of intracellular parasites in Lesch-Nyhan cells. Exptl. Parasitol. 41: 95-104, 1977; and O' Sullivan WJ, Johnson AM, Finney KG, Gero AM, Hagon E, Holland JW amd Smithers GW. Pyrimidine and purine enzymes in Toxoplasma gondii. AustJ Exptl Biol MedSci 59:763-767, 1981). In addition to the nucleobases, inosine and guanosine are first cleaved by purine nucleosidase (hydrolase or phosphorylase) to hypoxanthine and guanine, respectively, and then converted to nucleotides by the phosphoribosyltransferase activities. Purine nucleosidase (phosphorylase or hydrolases) converts a purine nucleoside to a purine, which can be sequentially phosphorylated by a phosphoribosyltransferase activity. Kinetic parameters of T. gondii phosphoribosyltransferases using hypoxanthine, guanine, xanthine or adenine as substrates were determined as described in Example 2. Hypoxanthine showed an apparent K m of 1.2 ± 0.1 mM and an apparent V max of 0.94 ± 0.01 mmol/min per mg protein. Guanine showed an apparent K m of 1.3 ± 0.4 mM and an apparent V max of 0.72 ± 0.07 mmol/min per mg protein. Xanthine showed an apparent K m of 3.0 ± 0.4 mM and an apparent V max of 1.1 ± 0.1 mmol/min per mg protein. Adenine exhibited an apparent K m of 2.9 ± 0.8 mM and an apparent V maχ of 0.099 ± 0.007 mmol/min per mg protein. The efficiencies of these activities (i.e. V maχ /K m ) were found to be in the order hypoxanthine> guanine > xanthine > adenine. This ranking differs from a previously reported order of specific activities which was xanthine > hypoxanthine > guanine > adenine (Krug EC, Marr JJ and RL Berens, Purine metabolism in Toxoplasma gondii J. Biol. Chem. 264: 1061-10607, 1989). The pH optima for T. gondii phosphoribosyltransferase activity using hypoxanthine (HPRTase), guanine (GPRTase), xanthine (XPRTase) or adenine (APRTase) as substrates were found to be 7.5, 7.5, 7.8 and 8.0-9.5 respectively. Thus, the order of efficiencies for these substrates may at least partially reflect the difference between the pH employed (i.e. 7.4) and the pH optima for the different substrates. Competition studies between the four substrates for T. gondii phosphoribosyltransferase activity were conducted in order to ascertain how many separate enzyme activities are present in these parasites. Adenine did not inhibit either GPRTase or XPRTase. Both hypoxanthine and xanthine were competitive inhibitors of GPRTase; whereas, hypoxanthine was a noncompetitive inhibitor of XPRTase and guanine was a competitive inhibitor. These results indicate that at least two phosphoribosyltransferases are present in T. gondii. One enzyme uses hypoxanthine, guanine and xanthine as substrates, while the other enzyme uses only adenine. However, the noncompetitive inhibition demonstrated by hypoxanthine towards XPRTase, suggests that there may be a third enzyme which uses xanthine. Sixty-eight compounds, mostly purine analogs, were evaluated as ligands of T. gondii XPRTase and GPRTase by examining their ability to inhibit these reactions in vitro. The purine nucleobase analogues that were screened included those having ring modifications (e.g. aza and deaza analogs) and/or exocyclic substitutions at various positions. Inhibition was quantified by determining apparent Kj values for compounds that inhibited these activities by greater than 10% at a concentration of 0.9 mM. The mean and range of the apparent Ki values for these compounds, determined from at least three separate estimations of the apparent KJS are presented in Table 3. TABLE 3 Apparent K γ - values for inhibition of T. gondii XPRTase and GPRTase Apparent K\ (mM)J Compound Source XPRTase GPRTase Hypoxanthine (6-oxopurine) SIG 3.5± 0.3 1.1 ± 0.1 2-Position substitutions 6-Position substitutions Purine CDC t t 6-Aminopurine (adenine) SIG t t 6-Methylaminopurine ( (N^-methyladenine) SIG 970 ± 170 770 ± 280 6-Benzylaminopurine (N*-benzyladenine) SIG 1100 ± 300 1400 ± 600 6-Methoxypurine CDC 380 ± 90 400 ± 80 6-Thiopurine (purine-6-thione) CDC 61 1 15 ± 1 6-Chloropurine CDC 1001 30 48 ± 3 6-Iodopurine CDC t t 6-Methylpurine CDC t 1100 ± 500 8-Position substitutions 8-Azahypoxanthine (8-aza-6-oxopurine) CDC 20001500 610 ± 190 8-Oxohypoxanthine (6,8-dioxopurine) CDC 41001 1000 t 8-Thiohypoxanthine (6-oxopurine-8-thione) CDC 4201 100 500 ± 150 1 ^-Position substitutions 2-Amino-l-methylhypoxanthine (1- SIG 401 5 150 ± 30 methylguanine) 2-Oxo- 1 -methy lhypoxanthine ( 1 -methy lxanthine) FLU 14001400 3700 1400 2,6-Position substitutions 2-Aminopurine CDC 67132 280196 2-Oxopurine (2-hydroxypurine) SIG 5701120 4301110 2,6-Diaminopurine (2-aminoadenine) CDC 8801280 4601210 6-Amino-2-oxopurine (isoguanine) NBC 53115 220160 6-Amino-2-chloropurine(2-chloroadenine) CDC 22001900 6301180 6-Amino-2-methylpurine(2-methyladenine) SIG 3701110 25001800 2-Aminopurine-6-thione (6-thioguanine) SIG 22112 610.3 2,6-Dithionepurine (dithioxanthine) CDC 420180 300130 2-Oxo-6-thiopurine (6-thioxanthine) SIG 246182 3181128 2-Amino-6-chloropurine CDC 230150 8901170 2,6-Dichloropurine CDC 410180 300011600 2,7-Position substitutions 2-Amino-7-deazahypoxanthine (7-deazaguanine) SIG 2615 9316 2-Amino-7-methylhypoxanthine (7- SIG 310011500 320012000 methylguanine) 2-Oxo-7-methylhypoxanthine(7-methylxanthine) CDC 19001400 570012300 2,8-Position substitutions 2-Amino-8-azahypoxanthine (8-azaguanine) CDC 260011000 450190 2-Amino-8-bromohypoxanthine (8- SIG 5401120 16001400 bromoguanine) ' 8-Aza-2-oxohypoxanthine (8-azaxanthine) CDC t t 6,8-Position substitutions 8-Aza-6-aminopurine (8-azaadenine) SIG 350011400 18001800 6-Amino-8-bromopurine (8-bromoadenine) CDC 75011000 6301140 6,9-Position substitutions 6-Amino-9-ethylpurine (9-ethyladenine) CDC t 10001400 6-Amino-9-cyclohexylpurine (9- CDC t 32001800 cyclohexyladenine) Other Di-Substitutions 6-Amino-l -methy lpurine (1 -methy ladenine) CDC 310190 590013000 2-Oxohypoxanthine-N J -oxide(xanthine-N 3 - SIG 13001300 550011200 oxide) 2-Oxo-3-methylhypoxanthine(3-methylxanthine) FLU t t 2-Amino-9-methylhypoxanthine (9- CDC 230150 6101150 methyl guanine) 8-Aza-7-deazahypoxanthine (allopurinol) SIG 3516 130120 Multiple substitutions 46 2- Amino- 1 -methy lpurine SIG 2901 90 7301 180 47 2,6-Dioxo-3-isobutyl- 1 -methylpurine SIG 1500 1 600 t 48 6-Amino-7-deazapurine (7-deazaadenine) BWC 1700 1 500 5800 ± 900 49 8-Aza-6-oxo- 1 ,3 -dimethylpurine SHC t t 50 1 ,2,4-Triazolo( 1 ,5 ,-a)pyrimidine ( 1 -deaza-5 - ALD t t azapurine) 51 8-Aza-2,6-diaminopurine (8-aza-2-aminoadenine) SIG 8501230 13001400 52 8-Aza-7-deaza-6-aminopurine (8-aza-7- CDC 81 1 52 t deazaadenine) 53 8-Aza-7-deaza-6-thiopurine BWC 8801330 9701 120 54 8-Aza-l,3-dideazapurine (benzotriazole) CDC 520012300 t 55 8-Aza-l-niuO-l,3-dideazapurine (5- ALD 390 1430 t nitrobenzotriazole) 56 2,6-Dioxo-7-(b-hyydroxypropyl)- 1 ,3- SIG 1900 1 1200 t dimethylpurine 57 2,6-Dioxo-l ,3 ,7-trimethy lpurine (caffeine) SIG 10001400 840 ± 280 58 2,6-Dioxo- 1 ,3 ,9-trimethylpurine (isocaffine) FLU 530011300 6100 1 2400 59 3-Methyl-2,6,8-trioxopurine FLU t t 60 1 ,3-Dimethyl-2,6,8-trioxopurine FLU t t Other substitutions 61 Benzonitrile CDC 8701290 5801 170 62 1 ,3-Dimethylpteridine SHC t t 63 6-Hydroxymemylpterin SIG 3400 1 1800 6600 14500 64 Cytosine (4-amino-2-oxopyrimidine) SIG 286160 476190 65 5-Fluorocytosine CDC 657 1 180 431 1 100 66 Isocytosine (2-amino-4-oxopyrimidine) NBC 1060 1270 616 1 200 67 Uracil (2,4-dioxopyrimidine) SIG 808 ± 20 699 1 140 68 4-Oxopyrimidine (4- hydroxypyrimidine) CDC 379 ± 100 445 1 130 t Apparent K t values 1 range were obtained from at least three separate estimations of the apparent K t . t Less than 10% inhibition at a concentration of 0.9 mM. A structure-activity relationship for the binding of ligands to T. gondii XPRTase and GPRTase was formulated using hypoxanthine (6-oxopurine) as a reference compound. Structural features of purine analogues required or strongly preferred for binding to both activities is detailed in Table 4. TABLE 4 Structure-activity relationship for the binding of nucleobase ligands to T. gondii XPRTase and GPRTase Position Substituent Effect -Position A pyrrole-type nitrogen strongly preferred or required for binding; substitution of exocyclic methyl group (e.g. 1-methylguanine, 18; or 1- methylxanthine, 19) decreases binding. -Position Exocyclic substitutions to purine (6) (e.g. oxo, 21 ; or amino, 20) generally increase binding; exocyclic substitutions to other compounds (e.g. hypoxanthine, 1) usually decrease (e.g. amino, 2; oxo, 4; or thio, 5) or abolish (e.g. methylamino, 3) binding. -Position Exocyclic substituents decrease (e.g. oxide, 42) or abolish (e.g. methyl, 42) binding. -Position Exocyclic oxo or thioxo group in the lactam tautomeric form is required for binding; substitution of 6-position oxo group of hypoxanthine (1) with thio group (6-thiopurine, 11) decreases binding slightly (1.7- and 14-foldJ, respectively); elimination (i.e. purine, 6) or substitution of the 6-position oxo group decreases (e.g. amino, 7; methylamino, 8; benzylamino, 9; or chloro, 12) or abolishes (e.g. iodo, 13 or methyl, 14) binding. -Position ■A pyridine-type nitrogen is preferred for binding; replacement with methine group (e.g. 7- deazaguanine, 31) decreases binding slightly (2.2.- and 9.3-foldJ, respectively). Substitution of exocyclic methyl group (e.g. 7-methylguanine, 32; or 7-methylxanthine, 33) decreases binding by at least 136-fold. -Position Endocyclic methine group required for binding; replacement with nitrogen significantly decreases (e.g. 8-azahypoxanthine 15; or 8- azaguanine, 34) or abolishes (8-azaxanthine, 36)binding. Exocyclic substituents (e.g. oxo, 16; thio, 17; or bromo, 35) significantly decrease (at least 120-fold) or abolish binding. -Position Substitution of exocyclic methyl group (e.g. 9-methylguanine, 44) decreases binding (19- and 61-foldJ, respectively). Refers to XPRTase and GPRTase, respectively. Based on the structure activity relationship studies described above, purine nucleoside analogs that are ligands of a parasite salvage pathway enzyme comprise D (+) or L (-) enantiomers, α or β anomers of the following structural formula: wherein, K\ is a halogen, OR^, SR , SeR$ or CH2R6 and R^ is alkyl, alkene, arylalkyl, or aiyl; X is CH 2 , 0 or S; R 3 is H, OH, or a halogen; is H, OH, or a halogen; and R j is CH3, CF3, CH2OH, or CH2OY and Y is a carbon ester or phosphorus. The alkyl, alkene, arylalkyl and aryl groups (hereinafter hydrocarbon groups) comprising R $ can be either straight or branched chains, saturated or unsaturated. Unsaturated groups may have a single site of unsaturation or a plurality of unsaturated sites. The hydrocarbon groups preferably have up to about ten carbons, more preferably up to about six carbons, and most preferably up to about three carbons. A hydrocarbon group having three carbon atoms or less is considered to be a lower hydrocarbon group. For example, an alkyl group having three carbon atoms or less is a lower alkyl. Examples of lower hydrocarbon groups which may be used in the present invention include methyl, methylene, ethyl, ethylene, ethenyl, ethenylene, ethynyl, ethynylene, propyl, propylene, propenyl, propenylene, propynyl, and propynylene. Examples of higher hydrocarbon groups (from four to about ten carbons) include butyl, t-butyl, butenyl, butenylene, and butynyl, butynylene, nonyl, nonylene, nonenyl, nonenylene, nonynyl, and nonynylene. The alkyl or alkylene groups may be substituted with one or more oxygen or halogen atom to form alkoxy, haloalkyl, alkoxyene, and haloalkylene groups. The alkoxy and haloalkyl groups also may be straight or branched chain and preferably are made up of up to about ten atoms (including carbon, oxygen or halogen), preferably up to about six atoms, and most preferably up to about three atoms. The term halogen is art-recognized and includes chlorine, fluorine, bromine, and iodine. Examples of substituted hydrocarbon groups which are useful within this invention are similar to hydrocarbon groups set forth above except for the incorporation of oxygen(s) or halogen(s) into the groups. Purines and a number of purine analogs are available from commercial sources (Tables 1 and 3). Alternatively, a particular purine analog can be made from the corresponding purine or a purine analog using standard chemical procedures. Once prepared, candidate purine analogs can be screened for ability to bind to parasitic adenosine kinase, nucleosidase (hydrolase or phosphorylase), and/or parasitic phosphoribosyltransferase, for example as described in Examples 1 and 2. Alternatively, the efficacy by which a particular compound can prevent (e.g. inhibit growth) or treat ( e.g. decrease the number and/or size of a parasite) can be tested in the human fibroblast assay described in Example 3. Appropriate compounds, as well as combinations of compounds, can be prepared as pharmaceutical compositions upon admixture with a pharmaceutically acceptable carrier. Examples of such carriers include solutions (e.g. saline), solvents, dispersion media, delay agents, emulsions and the like. The identity and use of such media for pharmaceutically active substances are well known in the art. Pharmaceutical compositions comprising purine analogs can then be administered to a subject for treatment of a disease, which is caused by or contributed to by parasites, which are dependent on salvage pathway enzymes for their purine requirement. Examples of such parasites are Toxoplasma gondii, Entamoeba histolytica, Eimeria tenella, Leishmania donovani, Plasmodium falciparum, Tritichomonas foetus and Trypanosoma cnai. In order to treat a disease caused by such a parasite in a subject, an effective amount of a purine analog is administered to the subject. In addition, administration of an effective amount of a purine analog to a pregnant woman can prevent or decrease the transmission of a parasitic infection from the mother to her unborn child. For use in therapy, an effective amount of a purine nucleoside analog can be administered to a subject (e.g. a human or other mammal) by any mode, which allows the small molecule to be in contact with the parasite or transfected mammalian cell. Preferred routes of administration include oral and transdermal (e.g. via a patch). Examples of other routes of administration include injection (subcutaneous, intravenous, parenteral, intraperitoneal, intrathecal, etc.). The injection can be in a bolus or a continuous-infusion. Depending on the route of administration, the purine nucleoside analog can be associated with or disposed in a selected material to protect it from natural conditions which may detrimentally affect its ability to perform its intended function, increase its in vivo availability or increase its uptake by a specific organ (e.g. the central nervous system to treat toxoplasma induced encephalitis). An "effective amount" of a purine nucleoside analog refers to that amount necessary or sufficient to prevent, reduce or eliminate the symptoms of the parasitic infection. The effective amount can vary depending on such factors as the particular parasite or extent of infection, the particular adenosine analog being administered or the size of the subject. Doses of NBMPR or NBMPR-P of up to about 100 μM were found to effectively eliminate T. gondii parasites without affecting mammalian cells. One of ordinary skill in the art can empirically determine the effective amount of a particular purine analog which is administered alone or ' in conjunction with another active agent, without necessitating undue experimentation. The purine nucleoside-like, small molecule drugs disclosed herein can be administered alone or in conjunction with another active agent as a combination therapy. For example, since the initiation of a parasitic infection or the onset of symptoms can be triggered or enhanced when a subject is in an immunocompromised condition (e.g. from infection, cancer, stress etc.), an appropriate active agent for co-administration with a purine nucleoside analog can be an agent for treating or preventing the establishment or growth (systemic or local) of a tumor or infection. Examples of such agents include drugs (e.g. antibiotics, anti- virals, antifungals, anti-protozoals), toxins (e.g. ricin) , radionuclides (e.g. 1-131, Re-186, Re- 188, Y-90, Bi-212, At-211, Sr-89, Ho-166, Sm-153, Cu-67 and Cu-64), hormone antagonists (e.g. tamoxifen), heavy metal complexes (e.g. cisplatin), oligonucleotides (e.g. antisense), chemotherapeutic nucleotides, peptides, non-specific (non-antibody) proteins (e.g. sugar oligomers), boron containing compounds (e.g. carborane), photodynamic agents (e.g. rhodamine 123) and enediynes (e.g. calicheamicins, esperamicins, dynemicin, neocarzinostatin chromophore, and kedarcidin chromophore). As a combination therapy, a purine nucleoside analog can be administered prior to, simultaneously or after the administration of the other active agent. In a preferred embodiment for treating or preventing the establishment or growth of a tumor and a disease or condition which is caused by or contributed to by a parasite which is dependent on a salvage pathway for its purine requirement, the combination therapeutic compound can comprise a purine nucleoside analog; a radionuclide, toxin, hormone antagonist, heavy metal complex, oligonucleotide, chemotherapeutic nucleotide, peptide, non-specific (non-antibody) protein, boron compound or an enediyne; and a pharamaceutically acceptable carrier. In a preferred embodiment for treating or preventing the establishment or growth of a bacterial infection and a disease or condition which is caused by or contributed to by a parasite which is dependent on a salvage pathway for its purine requirement, the therapeutic compound can comprise a purine nucleoside analog; an antibiotic, radionuclide or oligonucleotide; and a pharmaceutically acceptable carrier. In a preferred embodiment for treating or preventing the establishment or growth of a viral infection and a disease or condition which is caused by or contributed to by a parasite which is dependent on a salvage pathway for its purine requirement, the therapeutic compound can comprise a purine nucleoside analog; an antiviral compound, radionuclide or oligonucleotide; and a pharmaceutically acceptable carrier. In a particularly preferred embodiment, the viral infection is by the Human Immunodeficiency Virus (HTV) and the antiviral is selected from the group consisting of: 3'- azido-2', 3'-dideoxythymidine (AZT), 2\3'-dideoxycytidine (ddC), 2 , ,3'-didehydro-2 , ,3 * -dideoxythymidine (d4T), 3'-thia-2 , ,3'- dideoxycytidine (3TC),2',3'-dideoxyinosine (ddI),2',3'-dideoxyguanosine (ddG), or 2'-β- fluoro-2',3'-dideoxyadenosine (F-ara-ddA), can be administered to a subject. In a preferred embodiment for treating or preventing the establishment or growth of a fungal infection, and a disease or condition which is caused by or contributed to by a parasite which can metabolize adenosine, the therapeutic compound can comprise a purine nucleoside analog; an antifungal compound, radionuclide or oligonucleotide; and a pharmaceutically acceptable carrier. In another embodiment, appropriate active agents for co-administration can be agents that suppress the immune system, for example to permit allotransplantation. Examples of suitable immunosuppressive agents to be co-administered with a purine nucleoside analog include: cyclosporine, azathioprine, methotrexate, immunoglobulin preparations (e.g. Rhogam, Gamulin), adrenocorticosteroids, sulfasalazine, FK-506, methoxsalen, rapamycin and thalidomide. - 23 - In another embodiment, the disease is a cancer or blood disorder and a parasitic gene(s) of purine salvage enzyme(s) has been introduced into the disease causing (e.g. malignant) cells. In a preferred embodiment, the parasitic genes for purine salvage enzymes are selected from the group consisting of adenosine kinase, purine nucleosidase (nucleoside hydrolase or phosphorylase) or a phosphoribosyltransferase. The present invention is further illustrated by the following examples which should not be construed a^ limiting in any way. The contents of all cited references (including literature references, issued patents, published patent applications, and co-pending patent applications) cited throughout this application are hereby expressly incorporated by reference. Exam le 1; Structure - Activity Relationship of the Binding of Nucleoside Ligands to Adenosine Kinase from Toxoplasma Gondii Chemicals and supplies. The source of the compounds screened as inhibitors of adenosine kinase are indicated in Table 1 by the following abbreviations: ALD, Aldrich Chemical Co., Inc., Milwaukee, WI; AMC, American Cyanamid Co., Lederle Laboratories, Pearl River, NY; BW, Burroughs Wellcome Co., Research Triangle Park, NC; CAL, Calbiochem, San Diego, CA; FLU, Fluka Chemical Co., Ronkonkoma, NY; ICN, ICN Biomedicals, Inc., Costa Mesa, CA; GG, Dr. Gilles Gosselin, Universitέ de Montpellier, Montpellier, France; AM, Dr. John A. Montgomery, Southern Research Institute, Birmingham, AL; LBT, Dr. Leroy B. Townsend, University of Michigan, Ann Arbor, MI; NCI, Drug Synthesis and Chemistry Branch, Developmental Therapeutic Program, Division of Cancer Treatment, National Cancer Institute, Bethesda, MD.; RSK, Dr. Robert S. Klein, Montefiore Medical Center, Bronx, NY; SCH, Schweizerhall, Inc., Piscataway, NJ; SHC, Dr. Shih Hsi Chu, Brown University, Providence, RI; SIG, Sigma Chemical Co., St. Louis, MO; VEM, Dr. Victor E. Marquez, National Cancer Institute, Bethesda, MD [8- M C] Adenosine (45 mCi/mmol), [G- l3 H]NBMPR (36 Ci mmol), and [5,6- ,3 H]uracil (38 Ci/mmol) were purchased from Moravek Biochemicals, Brea, CA; Scintilene scintillation fluid was from Fisher Scientific, Pittsburgh, PA; silica gel G/UV 254 Polygram thin layer chromatography plates were from Brinkmann, Westbury, NY; and Bio-Rad protein assay kits were from Bio- Rad Laboratories, Richmond, CA. (+)-ervtΛro-9-(2-S-hydroxy-3-Λ-nonyl)adenine (EHNA) was a gift from Dr. Elie Abushanab, University of Rhode Island, Kingstown, RI. 5- iodotubercidin was from Research Biochemicals, Natick, MA; ^-(benzy adenosine and N 6 - (azidobenzyl)adenosine were generously provided by Dr. A. R. P. Paterson, University of Alberta, Edmonton, Alberta, Canada. All other chemicals and compounds were obtained from either the Sigma Chemical Co. or Fisher Scientific. Source of T. gondii. Tachyzoites of the RH strain of T. gondii were propagated by intraperitoneal passage in female Swiss- Webster mice (Sasco, Inc., Omaha, NE) as previously described [Iltzsch, M.H., Pyrimidine Salvage pathways in Toxoplasma gondii. J Euk. Microbiol., 40, pp. 24-28 (1993).] Preparation of cytosol extracts. For the structure activity relationships studies, approximately 5 x 10 8 T. gondii were suspended in 1.2 ml of 50 mM N-2- hydroxyethylpiperazine-N-2-ethane-,ulfonic acid (HEPES)-Cl (pH 8.0)/l mM dithiothreitol and homogenized for 30 seconds at setting 10, using a Brinkmann Instruments Polytron homogenizer fitted with a PTA 7K1 probe. The homogenate was then centrifuged at approximately 116,000 g for 1 hour at 5° and the supernatant (cytosol extract) collected and used as the enzyme source. Fresh cytosol extracts were prepared for each experiment. Adenosine kinase assay. T. gondii adenosine kinase activity was measured by following the formation of [ M C]AMP from [ M C]adenosine and ATP. Initial studies to set up the optimal assay conditions indicated that the apparent K^ for adenosine is approximately 5 μM, the optimal concentration of MgCl 2 /ATP is 0.5 mM each, and the optimal pH is 8.0. It should be noted that ATP containing vanadium (about 40 ppm) was used in all assays because the use of "vanadium-free" ATP resulted in low enzyme activity. The addition of 25 μM of (+)-EHΝA or 100 μM of (1)EHNA was also found to be required to inhibit any contaminating adenosine deaminase activity which may degrade the substrate adenosine to inosine. Therefore, the standard reaction mixture contained 50 mM HEPES-C1 (pH 8.0), 1 mM dithiothreitol, 5 μM [8-'<C]adenosine (45 Ci mol), 0.5 mM ATP, 0.5 mM MgCl 2 , 25 μM (+)-EHNA or 100 μM (±)-EHNA, 10-30 μl of cytosol extract (approximately 3-10 μg of protein), and either 0, 0.1, 0.25, 0.5 or 1.0 mM of the compound to be tested, in a final volume of 150 μl. In order to more accurately determine appKi values, lower concentrations of the compound to be tested Were used for very potent inhibitors or inhibitors that were poorly soluble, while higher concentrations were used for poor inhibitors. When compounds 9, 21, 22, 42, 47 and 48 (See Table 1) were tested, reaction mixtures (including those with no compound) contained 10% dimethyl sulfoxide ( in addition to the standard reaction mixture) in order to ensure the solubility of these compounds. Reactions were started by the addition of [8- 14 C]adenosine, incubated at 37° for 10 minutes and terminated by placing the reaction tubes in a boiling water bath for 2 minutes. Precipitated proteins were removed by centrifugation in a microcentrifuge (approximately 13,000 g) for 5 minutes, and a 15 μl aliquot of the resulting supernatant was mixed with 5 μl of a solution containing 10 mM each of AMP and adenosine. This mixture was then spotted on silica gel thin layer chromatography plates which were developed with a mixture of chloroform/methanol/acetic acid (16:3:1). The average R f values for AMP and adenosine were 0.0 and 0.37, respectively. The substrate and product spots (which accounted for all of the radioactivity on the plates) were identified by UV quenching, cut out, and the radioactivity quantitated by liquid scintillation counting in 20 ml of Scintilene, using a Packard 460 scintillation counter. Under these conditions, velocity was linear with respect to time and amount of cytosol extract. Enzyme velocity was calculated by multiplying the fraction of AMP formed from adenosine times the amount of adenosine in the assay, and dividing by the incubation time. Determination and significance of AppJT, values. AppK t values were used to determine the relative degree of binding of compounds to adenosine kinase as compared to adenosine. AppK t values were estimated from Dixon plots of the data (1/v versus [I]), using a computer program with least squares fitting according to the general principles of Cleland (Cleland, WW, The statistical analysis of enzyme kinetic data. Adv. Enzymol. 29:1-32, 1967). The program was written by Dr. Sungman Cha (Brown University, Providence, R.I.) and modified to fit IBM BASIC by Dr. Fardos N.M. Naguib. If a compound is a competitive inhibitor with respect to adenosine, appK x values are related to K, values by the following equation: appλT j = K (1 + [S] / K m ). In the present study, the concentration of adenosine (5 μ M) was approximately equal to the K m value. Thus, the appK. value determined for a competitive inhibitor would be about 2-fold higher than the K,. It should be noted, however, that the type of inhibition (i.e., competitive, noncompetitive, or uncompetitive) produced by the compounds was not determined, nor were the compounds evaluated as substrates for T gondii adenosine kinase. Protein determinations. Protein concentrations were determined by the method of Bradford, M.M.[a rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem., 72: 248- 254 (1976)], using the Bio-Rad Laboratories protein assay kit and bovine serum albumin as a standard. Example 2: Structure-Activity Relationship of the Binding of Nucleobase Ligands to Xanthine or Guanine Phosphoribosyltransferase from Toxoplasma gondii Chemicals and supplies. The source of the compounds screened as inhibitors of xanthine and guanine phosphoribosyltransferase are indicated in Table 3 by the following abbreviations: ALD, Aldrich Chemical Co., Inc., Milwaukee, WI; BWC, Burroughs Wellcome Co., Research Triangle Park, NC; CDC, Chemical Dynamics Corp., South Plainfield, NJ; SHC, Dr. Shih-Hsi Chu, Brown University, Providence, RI; FLU, Fluka Chemical Co., Ronkonkoma, NY; NBC, Nutritional Biochemicals Corp., Cleveland, OH; SIG, Sigma Chemical Co., St. Louis, MO. [8- 1 C] adenine (55 Ci/mol), [8- 14 C] guanine (55Ci/mol), [8- 14 C] hypoxanthine (55 Ci/mol) and [8- , 4 C] xanthine (55 Ci/molO were obtained from Moravek Biochemicals, Inc., Brea CA.; cellulose CEL 300 UV254 Polygram thin layer chromatography plates were from Brinkmann, Westbury, NY; and Bio-Rad protein assay kits were from Bio-Rad Laboratories, Richmond, CA. All other chemicals and compounds were obtained from the Sigma Chemical Company. Maintenance of T. gondii. Tachyzoites of the RH strain of T. gondii were propagated by intraperitoneal passage in female CD1 mice weighing 20-25g (Charles River Laboratories, Wellington, MA). Mice were injected intraperitoneally (i.p.) with an inoculum (10 6 cells) of T. gondii (\0& parasites) contained in 0.2 ml of sterile phosphate buffered saline (PBS), pH 7.2, and were sacrificed after 2-3 days by inhalation of ether. The parasites were harvested from the peritoneal cavity by injection, aspiration and reinjection of 3-5 mL of PBS (2-3 times). The peritoneal fluid was examined microscopically to determine the concentration of T. gondii and to ascertain the extent of contamination by host cells. Two- day transfers generally produced parasite preparations that contained very little contamination and had a viability of >97%. Preparation of enzyme extracts. T. gondii suspension in phosphate-buffered saline was washed 2-3 times in 50 mM Tris-Cl, pH 7.4, at room temperature. Enzyme extracts were prepared by sonicating live parasites in 3 volumes of 50 mM Tris-Cl, pH 7.4, using a Fisher Sonic Dismembrator (Model 300) on ice. The sonicate (enzyme extract) was used as the enzyme source. Phosphoribosyltransferase assays. Phosphoribosyltransferase activity from T gondii was measured by following the formation of [ M C] nucleoside 5'-monophosphate (and [ 14 C] nucleoside formed by phosphohydrolase acitivity from [ 14 C]nucleobase and 5- phosphoribosyl-1-pyrophosphate (PRibPP). The standard reaction mixtures contained 50 mM Tris-Cl (pH 7.4), 10 μM [8-' 4 C]xanthine (55 Ci mol) or 4 μM [8- ,4 C]guanine (55 Ci mol), 4 mM PRibPP, 20 mM MgCl 2 , 50 mM KCI, and 50 μl of enzyme extract (4-8 μg of protein) in a final volume of 100 μl. When analogues were being tested, various concentrations (0 -0.9 mM) of the analogue were also included. Reactions were incubated at 37°C and terminated in boiling water bath for 2 minutes. Precipitated proteins were removed by centrifugation. A 10 μl aliquot of the resulting supernatant was spotted on cellulose thin layer chromatography plates which were developed with 5% dibasic potassium phosphate. The average Revalues were as follows: hypoxanthine, 0.49; inosine, 0.65; IMP, 0.77; guanine, 0J4; guanosine, 0.57; GMP, 0.70; xanthine, 0.46; xanthosine, 0.61; XMP, 0.74; adenine, 0.33; adenosine, 0.46; AMP, 0.67. The radioactivity was quantified using a Berthold TLC Linear Analyzer (Wallac Inc., Gaithersburg, MD). All assays were run under conditions in which velocity was linear with respect to time and amount of enzyme extract. Exzyme velocity was calculated by multiplying the fraction of nucleotide plus nucleoside formed from nucleobase by the amount of nucleobase in the assay divided by the incubation time. Specific activity was estimated by dividing enzyme velocity by the amount of protein in the assay. Determination of apparent K m values. The assay conditions were the same as for the standard assay except for the substrate concentrations used. The range of substrate concentrations was 1-7 μM. Apparent V max and K m values were calculated using a computer program written by Dr. Sungman Cha (Brown University, Providence, RI) and fitted into IBM BASIC by Dr. Fardos N.M. Naguib. This program employs the Wilkinson-Cleland procedure [Wilkinson, G.N., Statistical Estimations in Enzyme Kinetics, Biochem. J., £Q:324- 332, 1961; and Cleland, W.W., The Statistical Analysis of Enzyme Kinetic Data, Adv. Enzymol, 22J-32, 1967] for the estimation of V max and K m . Determination and significance of apparent K ( values. Apparent K t values were estimated from Dixon plots of the data (1/v versus (I)) using a computer program that employs least-squares fitting according to the general principles of Cleland (Cleland, WW The statistical analysis of enzyme kinetic data. Adv Enzymol 29: 1-32, 1967). This program was developed by Drs. S. Cha and F.N.M. Naguib. If a compound is a competitive inhibitor with respect to the substrate, apparent K. values are related to K. values by the following equation: Apparent K, - K, (1 + (S)/K m ). In the present study, the concentration of the purine nucleobase (10 μM and 4 μM for xanthine and guanine, respectively) was about 3- fold greater than its approximate apparent K m value (Table 1) and the concentration of PRibPP (4 mM) was at a saturating concentration (results not shown). Thus, the apparent K, value determined for a competitive inhibitor would be about 4-fold higher than the K, . It should be noted, however, that the type of inhibition (i.e. competitive, noncompetitive, or uncompetitive) produced by the compounds was not determined. Protein determinations. Protein concentrations were determined by the method of Bradford, M.M.[a rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal. Biochem., 72: 248- 254 (1976)], using the Bio-Rad Laboratories protein assay kit and bovine serum albumin as a standard. Exam le 3: Metabolism and Toxicitv of 6-Substituted 9-β-D-Ribofuranosylpurines in Toxoplasma Gondii Chemicals and supplies. [G- 13 H]NBMPR (36Ci/mmol), [5,6- 13H]uracil (38 Ci/mmol) and [8-' 4 C]adenosine (45 Ci/mol) were purchased from Moravek Biochemicals, Brea, CA, 5- iodotubercidin was from Research Biochemicals, Natick, MA; N6- (benzyl)adenosine and N6-(azidobenzyl)adenosine were generously provided by Dr. A.R.P. Paterson, University of Alberta, Edmonton, Alberta, Canada; NBMPR-P was synthesized from NBMPR as described in Example 1 ; all other chemicals were purchased from Sigma, St. Louis, MI. Maintenance of T. gondii. The wild type RH, the adenosine kinase deficient mutant ara-A R (R), and the phosphoribosyltransferase deficieut mutants (HGPRT" and Tnx R - ') strains of T. gondii were propagated by intraperitoneal passage in female CD1 mice (20- 25g). Mice were injected intraperitoneally (i.p.) with an inoculum (10 6 cells) of T. gondii contained in 0.2 ml of sterile phosphate buffered saline (PBS), pH 7.2, and were sacrificed after 2-3 days by inhalation of ether. The parasites were harvested from the peritoneal cavity by injection, aspiration and reinjection of 3-5 mL of PBS (2-3 times). The peritoneal fluid was examined microscopically to determine the concentration of T. gondii and to ascertain the extent of contamination by host cells. Two-day transfers generally produced parasite preparations that contained very little contamination and had a viability of >95%. While T. gondii were used for in vitro incorporation studies, the procedure was performed aseptically and the parasites were washed and resuspended in Dulbecco's modified Eagle medium (Gibed BRL) containing 100 units/mL penicillin G, 100 μg/mL streptomycin sulfate, and 3% FBS (fetal bovine serum) (HyClone Laboratories, Logan, UT). For the purpose of enzyme studies, to ascertain the purity of T. gondii for enzyme isolation and purification, the parasites were purified from host cells and debris by coagulating the host cells by addition of 50 μL phytohemagglutinin P (Difco) to the suspension of the parasites in 5 mL of medium, shaking slowly for 15 minutes, filtering slowly through a 5 μm Nucleopore polycarbonate filter (Costar, Cambridge, MA) using trypan blue exclusion method and counting on a hemacytometer. These procedures produce parasite preparations that are >99% pure. The parasites were then pelleted by centrifugation, washed twice with PBS and resuspended in 1 mL of the appropriate homogenization buffer required by the enzyme to be assayed. Parasites that were not immediately used were stored frozen in PBS at -70°C. Preparation of extracts. Extracts of T. gondii were prepared by homogenizing parasites or tissues in ice cold (3:1, v/w) 50 mM Tris-Cl buffer (pH 7.4) containing 1 mM EDTA, 1 mM dithiothreitol with a polytron homogenizer (Brinkmann, Westbury, NY) and the homogenates were centrifuged at 105,000 x g for 1 hour at 4°C. The supernatant fluids (cytosol) were collected and used as the enzyme source. Conversion assays. To measure the percent conversion of NBMPR to NBMPR-P and nucleobase (NBMP) by extracts of Toxoplasma gondii and mouse spleen, 50 mM Tris-Cl (pH 7.4), 5 mM MgCl 2 , 2.5 mM ATP, 5 mM creatine phosphate, creatine phosphokinase, 50 μM [ 3 H-(G)] NBMPR (1.95 μCi/nmol), and 10 μl of cytosol extract were reacted in a final volume of 20 μl and incubated at 37°C. Reaction was initiated by addition of extract and terminated by boiling for 3 minutes. Results are reported in Table 5. In a second reaction, 25 mM KCN was added to the reaction mixture. Results are reported in Table 6. Enzyme assay. Assays were run under conditions where activity was linear with time and enzyme concentrations. Activities were determined by following the formation of radiolabeled nucleotides and nucleobases from their respective nucleosides (adenosine or NBMPR). The assay mixture contained 50 mM Tris-Cl (pH 7.4), 5 mM MgCl 2 , 2.5 mM ATP, 5 mM creatine phosphate, 50 μM [G-Η] NBMPR (1.95 Ci mmol) or 5 μM [8- ,4 C]adenosine (45 Ci/mol), and 50 μL of enzyme preparation in a final volume of 100 μL. When adenosine was used as a substrate, 25 μM of EHNA ((±)erytλrσ-9-(2-S-3-R- nonyl)adenine), an inhibitor of adenosine deaminase, was included in the reaction mixture to inhibit any adenosine deaminase activity present. Incubation was carried out at 37°C. The reaction was terminated by boiling for 3 minute followed by freezing for at least 20 minutes. Proteins were removed by centrifugation and 10 μL of the supernatant fluid were spotted on Silica Gel TLC plates (Brinkmann, Westbury, NY). The plates for adenosine kinase assay were then developed in a mixture of chloroform methanol/acetic acid (102:12:6, v/v/v). The R f values were: adenosine, 0.27; adenine, 0.36; AMP, 0J7. The plates for the assays of NBMPR were developed in π-butanol/ethyl acetate/NH 4 /methanol (49:28:28:21, v/v/v/v). The R f values were: NBMPR, 0.76; NBMPR-R, 0.50; NBMP, 0.89. The amounts of radioactivity in the substrate (nucleoside) and product (nucleobase and nucleotides) were calculated on a percentage basis using a Berthold LB-284 Automatic TLC-Linear Analyzer. Phosphoribosyltransferase activity was measured by following the formation of PH] NMMPR-R from [ 3 H] NBMPR and 5-phosphoribosyl-l-pyrophosphate (PRPP). The standard reaction mixtures contained 50mM Tris-Cl (pH 7.4), 10 μM [ 3 H] NBMPR (55 Ci/mol), 4mM PRibPP, 20 mM MgCl 2 , 50 mM KCI, and 50 μl of enzyme extract (4-8 μg of protein) in a final volume of 100 μl. Reactions were incubated at 37°C and terminated in boiling water bath for 2 minutes. Precipitated proteins were removed by centrifugation. A 10 μl aliquot of the resulting supernatant was spotted on cellulose thin layer chromatography plates which were developed with 5% dibasic potassium phosphate. The average R f values were as follows: NBMPR, 0.28; NBMP, 0.16; NBMPR-R, 0.44. The radioactivity was quantified using a Berthold TLC Linear Analyzer (Wallac, Inc., Gaithersburg, MD). All assays were run under conditions in which velocity was linear with respect to time and amount of enzyme extract. Enzyme velocity was calculated by multiplying the fraction of nucleotide formed from nucleoside by the amount of nucleoside in the assay divided by the incubation time. Specific activity was estimated by dividing enzyme velocity by the amount of protein in the assay. Preparation of Samples for HPLC Analysis. The metabolism of NBMPR was also monitored in the acid-soluble fraction of the enzyme assays preparations. At the end of each incubation period, proteins were removed by centrifugation. The supernatant was collected and stored frozen at -20°C until analyzed by HPLC. HPLC Analysis. The analysis was performed on two Hypersil C lg reverse phase (25 x 0.4 cm, ODS 5 μm) columns (Jones Chromatography, Littleton, CO) in tandem on a Hewlett-Packard 1050 liquid chromatograph system equipped with diode-array multiple wave length uv monitor. The system was operated by a computer program which handles data analysis, comparison and storage after each run. Elution was performed stepwise, using 2 mobile phases: 50 mM ammonium acetate in 0.5% acetonitrile, pH 4.8, (Buffer A), and 50 mM ammonium sulfate in 60% acetonitrile, pH 4.8, (Buffer B). A 25 μL sample was injected with Buffer A for 10 minutes, followed by a 120 minute linear gradient to 60% Buffer B, then a 10 minute isocratic elution by 40% Buffer A-60% Buffer B. Flow rate was 1 ml/min, except, from 5-10 minutes and 120-132 minutes it was 0.5 mL/min. The eluent was monitored at 254 nm and λ^ of the compound under study. Under these conditions, NBMPR-R eluted at 86-87 minutes, NBMPR eluted at 102-103 minutes; the nucleobase, NBMP (pk 6), eluted at 106-107 minutes; and a fourth, unknown, peak eluted (pk 7) at 111- 112 minutes. Mass Spectrometry Analysis. Analysis of NBMPR metabolites was performed on an API III triple quadrupole Perkin-Elmer SCIEX LC/MS/MS mass spectrometer. The metabolites of NBMPR, pk 6 and pk 7, were separated by reverse phase HPLC on 100 x 2.1 mm Aquapore C g column at a flow rate of 0.2 mL/min, using a 0-50% acetonitrile gradient (5% per minute) in aqueous ammonium acetate. The elute was split 1 : 1 with 100 μL/min going to the electrospray interface. Positive and negative ion mass spectra were recorded in this mode. The LCMS analysis of the pk 6 (presumed to be the nucleobase, NBMP) produced two molecular species of weights 287 and 403. The LCMS analysis of unknown pk 7 produced a molecular weight of 358. Incorporation of Radiolabeled Uracil. Uracil uptake assays are highly specific to T. gondii as mammalian cells do not incorporate uracil into either their nucleoside and nucleotide pool, or nucleic acids. An exponential increase in radiolabel incorporation closely correlated with the exponential growth of the parasite. Uptake and incorporation of radiolabeled uracil into nucleic acids of T. gondii were carried out, at least in triplicates, in tissue culture, using monolayers of human foreskin fibroblasts infected with T. gondii. The „ OT „,„ PCT/US95/16343 - 31 - fibroblasts were cultured in no more than 30 passages in Dulbecco's modified Eagle medium, containing 100 units/mL penicillin G and 100 μg/mL streptomycin sulfate, and 10% FBS, pH 7.2. Briefly, confluent cells (4-5 day incubation) were cultured for 24 hours in the 24-well flat bottom microtiterplates (~ 5 x 10V1 mL/well) and incubated at 37°C in 5% CO 2 , 95% air to allow the cells to attach. The medium was then removed and the cells were infected with isolated T. gondii in medium with 3% FBS (1 parasite/cell). After 1 hour incubation, the cultures were washed with media with 10% FBS to remove extracellular parasites. FBS was maintained at a final concentration of 10%. Five concentrations of the compound were then added to cultures of the parasite-infected cells to give a final concentration of 0, 5, 10, 25 and 50 μM. Drugs were dissolved in 50% ethanol to give a final concentration of 2.5% ethanol when added to the wells. After an additional 18 hours incubation, the medium was replaced with 1 mL drug free media containing [5,6- l3 H]uracil (2 μCi/mL) and incubated for another 6 hours, after which the media was removed. The fibroblasts were released from the wells by trypsinization with the addition of 200 μL trypsin/EDTA (2.5X) to each well. After 10 minutes incubation, 1 mL of ice cold 10% trichloroacetic acid (TCA) was added to each well. The plates were then placed on a shaker to insure the detachment of the cells. The suspended contents of each well were filtered through GF/A 2.4 cm glass microfiber filters (Whatman, HiUsboro, OR), which were prewashed each with 1 mL double distilled H 2 O and dried. After filtration, the filters were washed with 10 mL methanol, left to dry, and then placed in scintillation vials containing 5 mL of Econo-Safe scintillation fluor (Research Products International Corp., Mount Prospect, IL), and radioactivity was counted using an LS5801 Beckman scintillation counter. Toxicity of compounds. Toxicity of the different doses of the various analogues were performed at least in triplicates, using a modification of the Microculrure Tetrazolium (MTT) assay and uninfected monolayers of human foreskin fibroblasts. Briefly, confluent cells were incubated for at least 24 hours in the 96-well flat bottom microtiterplates (-10V200 μL/well) and incubated at 37°C in 5% CO 2 , 95% air to allow the cells to attach. The medium was then replaced with 200 μL of fresh medium. The appropriate concentration of the compounds was dissolved in 50 μL of medium and added to each well to give final concentrations of 0, 5, 10, 25, and 50 μM. The cultures were then incubated for 48 hours, after which 50 μL of sterile MTT solution (2 mg/1 mL PBS) was added to each well. MTT solution was sterilized by filtration through 0.22 μm filters (Costar, Cambridge, MA). After 4 hours incubation, the medium was removed and 100 μL of dimethylsulfoxide (DMSO) was added to each well and the plates were shaken gently for 2-3 minutes to dissolve the formed formazan crystals. The absorbance was measured at 540 nm, using a computerized microtiterplate reader (Themomax, Molecular Division) Chemotherapy of Toxoplasma-Infected Mice. Female CD-I mice, approximately 22 g (Charles River Laboratories, Wilmington, MA), were infected with 200 RH strain tachyzoites/mouse by i.p. injection. The treatment group received 100 mg/kg NBMPR-P i.p. injections every 8 hours on Days 0, 2, 4 and 5, in proportion of 0J ml/10 g. Mice were maintained under controlled temperature (25°C) and light (12/12 hours, light/dark), food and water ad libitum, and survival was monitored daily. R≤sulls Table 5 shows that extracts of T gondii, unlike extracts of mouse spleen, is capable of converting nitrobenzylthioinosine or 6-[(4-nitrobenzyl)thio]-9-β-D- ribofuranosylpurines (NBMPR) to its nucleoside 5'-monophosphate, NBMPR-P, and its nucleobase, nitrobenzylmercaptopurine (NBMP). The formation of these metabolites increased with the incubation time. The authenticity of the nucleotide and nucleobase formation was verified by HPLC and MS/MS LC analyses. XΔB -Ef Percent conversion of NBMPR to NBMPR-P and nucleobase (NBMP) by extracts of Toxoplasma gondii and mouse spleen Period of Incubation fminl Product 15 30 60 120 240 NBMPR-P T. gondii 2J ± 0.5 3.2 ± 0.4 7.8 ± 1.5 9.4 ± 1.0 15J ± 4.4 Mouse 0 0 0 0 0 NBMP T. gondii 8.0 ± 0.4 7.8 ± 3.7 14.8 ± 1.2 9.4 ± 2.8 10.4 ± 6.2 Mouse 0 0 0 0 0 Since it is known that NBMPR could be oxidized to 6-mercaptopurine riboside which is cleaved to 6-mercaptopurine, which is then converted to 6-mercaptopurine 5'- monophosphate by hypoxanthine-guanine phosphoribosyltransferase [EC2.4.2.8], the conversion of NBMPR to NBMPR-P and NBMP was tested in the presence of KCN, an inhibitor of oxidation. The results in Table 6 indicate that KCN had no significant effect on the formation of these metabolites. TABLE 6 Percent conversion of NBMPR to NBMPR-P and NBMP by extracts of Toxoplasma gondii in the presence of 25 mM KCN. Period of Incubation (mint ElΩdllCl 15 30 60 120 240 NBMPR-P 0.5 ± OJ l.l ± OJ 3.7 ± 2.5 5.8 ± 0.7 11.8 ± 4.8 NBMP 0.9 ± 0.3 2.8 ± 3.8 4.0 ± 1.3 9.5 ± 1.3 14.3 ± 3.3 To investigate by which enzyme NBMPR is converted to NBMPR-P, the effect of various natural substrates and inhibitors of several enzymes of the purine salvage pathways which could be involved in the metabolism of NBMPR were tested. Tables 7 and 8 show that adenosine inhibits the formation of NBMPR-P from NBMPR by extracts of T. gondii, while inosine has a minimal effect. Adenine or hypoxanthine at 1 mM had no effect on the phosphorylation of NBMPR. These results suggest that adenosine kinase is the enzyme responsible for the direct phosphorylation of NBMPR. TABLE 7 Percent inhibition by different concentrations of adenosine on the formation of NBMPR-P from NBMPR by extracts of Toxoplasma gondii AdcnQsine [μM] NBMPR [μM] 50 75 150 25 66 96 100 50 85 91 98 TABLE 8 Percent inhibition by different concentrations of NBMPR on the formation of nucleosides S'-monophosphate from adenosine and inosine by extracts of Toxoplasma gondii NBMPR [μM] Compound 10 100 1000 Adenosine 77 100 100 Inosine 15 19 ND » » ND, not determined. Therefore, the effect of NBMPR, iodotubercidin (an inhibitor of adenosine kinase), formycin A (an inhibitor of 5'-methylthioadenosine phosphorylase, EC 2.4.2.28), and 8-aminoguanosine (an inhibitor of purine nucleoside phosphorylase) on the phosphorylation of adenosine were investigated. Table 6 shows that formycin A and 8-aminoguanosine had no effect on the phosphorylation of adenosine by extracts from T. gondii, human liver, or mouse liver. Only NBMPR affected the phosphorylation of adenosine in T. gondii, but not in human or mouse livers. Iodotubercidin, on the other hand, inhibited the phosphorylation of adenosine by extracts from all three tissues. TABLE 9 Percent inhibition by different concentrations of various concentrations of NBMPR and Iodotubercidin on the phosphorylation of adenosine by extracts from Toxoplasma gondii, human and mouse liver. % Inhibition Compound T. gondii Human Liver Mouse Liver NBMPR 10 μM 3.4 0 0 25 μM 37.7 0 0 50 μM 56.0 0 0 Iodotubercidin 10 μM 82.9 95.9 97.2 25 μM 85.6 100 97.5 Similarly, the data on Table 10 shows that only iodotubercidin inhibited the phosphorylation of NBMPR in T. gondii. TABLE 10 Percent inhibition by different concentrations of various inhibitors on the phosphorylation of NBMPR by extracts of Toxoplasma gondii and human liver. % Inhibition Compound Table 11 shows that only the RH wild type strain can phosphorylate adenosine and NBMPR to their respective nucleoside 5'-monophosphate. Extracts from the adenosine kinase deficient mutant, ara-A R , had little activity with either substrate. These results indicate that adenosine kinase in toxoplasma is the enzyme responsible for the direct phosphorylation ofNBMPR. TABLE 11 Phosphorylation of Adenosine and NBMPR by extracts from RH wild type strain and adenosine kinase deficient ara-A R mutant strain of T. gondii. Substrate Strain Adenosine NBMPR RH 309.8 ± 3.2 a 248.9 ± 15.7 ara-A^ 4.3 ± 1.2 2.5 ± OJ ■ fmol/min/mg protein. To demonstrate that NBMPR can be metabolized by adenosine kinase and/or a phosphoribosyltransferase, the formation of NBMPR-P from NBMPR was assayed in the presence of ATP (kinase route) or PRPP (phosphoribosyltransferase route) in two types of mutants. These mutants either lack adenosine kinase (Ara-A R ) or hypoxanthine-guanine phosphoribosyltransferase (HGPR or Thx^ 1 ). The results (Table 12) indicate that in the presence of ATP and absence of the phosphoribosyltransferase, i.e. HGPRT" or Thx*- 1 , NBMPR-P was formed. Similarly, in the presence of PRPP and absence of adenosine kinase, i.e. Ara R , NBMPR-P can still be formed indicating that the phosphoribosyltransferase route is active. This is a strong indication that NBMPR-P can be formed by both the kinase and phosphoribosyltransferase reactions in T. gondii. TABLE 12 Percent conversion ofNBMPR to NBMPR-P in the presence of ATP or PRPP by extracts from different strains of Toxoplasma Strain + ATP + PRPP RH 0.7 ± 0.2 4.3 ± 0.3 Ara-A R 0 3.8 ± 0.2 HGPRT- 2.6 ± 1.3 0 Thx R - ] 2.0 ± 0.6 0 To investigate the effect of metabolizing NBMPR on the survival of T. gondii, the number and size of plaque formation by T. gondii on human fibroblasts was investigated. NBMPR decreased the number and size of toxoplasma in human fibroblasts (Table 13). TABLE 13 Effect of different concentrations ofNBMPR on number of plaques formed by Toxoplasma gondii grown in human fibroblasts in culture. NBMPR [μM] Number of Plaques 0 21 ± 3.5 33 23 ± 0.7 100 3 ± 1.4 The growth of toxoplasma was also inhibited by NBMPR and NBMPR-P and the inhibition was dose dependent (Table 14). These two compounds had no significant toxic effect on the survival of host cells (Table 15). These results indicate that NBMPR or NBMPR-P could be used as therapeutic agents against toxoplasma. Indeed, administration of NBMPR-P to mice infected with toxoplasma was found to increase their life span from 6 to 8 days (Table 13). TABLE 14 Effect of treatment with NBMPR-P on the survival of mice infected with Toxoplasma gondii. Survival Treatment Each mouse was infected with 200 tachyzoites and the treatment group received 100 mg/kg NBMPR-P i.p. injections every 8 hours on Days 0, 2, 4 and 5 in proportion of 0J ml/10 g. The survivorship of the treated mice can be improved by manipulation of the dose and regimen ofNBMPR. Because of the potentiality ofNBMPR in the treatment of toxoplasma, several 6-substituted 9-β-D-ribofuranosylpurines were tested as anti-toxoplasmic agents in vitro. Among the analogues tested, nitrobenzyl-6-selenopurine riboside, -benzyladenosine, N 6 - (azidobenzyl)adenosine and N°-(nitrobenzyl)adenosine were promising (Table 14). These four compounds were comparable or better agents than sulfadiazine, or pyrimethamine, the standard chemotherapeutic agents for the treatment of toxoplasmosis. TABLE 15 Effect of various 6-substituted 9-β-D-ribofuranosylpurines and other therapeutic compounds on percent survival of Toxoplasma gondii grown in human fibroblasts in culture. Concentration [μM] Compound 0 5 10 25 50 ΝBMPR 100 74.3 55.5 23J 6.7 ΝBMPR-P 100 75.3 53.3 21.4 10.2 Νitrobenzyl-6-selenopurine riboside 100 67.8 3.03 0 0 N6-Anisoyladenosine 100 94.2 93J 104.2 125.4 N6-Benzoyladenosine 100 96.4 94.6 111.7 100.5 N"-Benzyladenosine 100 62.8 41.8 52.9 26.3 N6-(Aminobenzyl)adenosine 100 100.0 95.6 108.3 119.9 N6-(Azidobenzyl)adenosine 100 36.9 17.0 15J 24.5 N -(Νitrobenzyl)adenosine 100 35.9 27.6 21.6 8.4 Sulfadiazine 100 74.6 64.8 66.3 66.3 Pyrimethamine 100 25.9 12.9 10.2 13.8 Survival was measured by incorporation of [5,6- 3 H]uracil from at least 2 experiments of 4 replica each. However, -benzyladenosine, unlike the other three compounds, was toxic to host cells at therapeutic doses (Table 16). TABLE 16 Effect of various 6-substituted 9-β-D-ribofuranosylpurines and other therapeutic compounds on percent survival of uninfected human fibroblasts grown in culture. Concentration TuMI Compound 0 5 10 25 50 NBMPR 100 98.8 97J 96.3 95.4 NBMPR-P 100 103.3 94.6 93.6 96J Nitrobenzyl-6-selenopurine riboside 100 89.3 90J 63.4 39.6 N"-Anisoyladenosine 100 92.9 92.6 99.2 97.4 >-Benzyladenosine 100 46.3 21.8 36.5 31.0 N^-Benzoyladenosine 100 94.4 94J 97.6 99.3 N"-(Aminobenzyl)adenosine 100 93.3 91.9 100.6 101.4 N"-(Azidobenzyl)adenosine 100 97.3 90.3 88.9 81.5 N6-(Νitrobenzyl)adenosine 100 104J 102J 92.3 106.8 Sulfadiazine 100 98.2 99.7 99.8 102.5 Pyrimethamine 100 103.6 98.8 108.2 118.4 Survival was measured by absorbance at 540 nm of MTT treated cells from at least two experiments each of 4 replica. Equivalents Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents of the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims: Claims 1. A pharmaceutical composition comprising a ligand of a parasite salvage pathway enzyme; and a pharmaceutically acceptable carrier. 2. A pharmaceutical composition of claim 1, wherein the parasite salvage pathway enzyme is adenosine kinase, a purine nucleosidase or a parasitic purine phosphori bosy ltransferase . 3. A pharamaceutical composition of claim 2, wherein the parasite is Toxoplasma gondii. 4. A pharmaceutical composition comprising a compound which is an α or β anomer, a D (+) or L (-) enantiomer of the following structural formula: wherein, Rj is a halogen, OR^, SR$, SeR$ or CH2R6 and R$ is alkyl, alkene, arylalkyl, or aryl; X is CH 2 , O or S; R 3 is H, OH, or a halogen; R 4 is H, OH, or a halogen; and R j is CH3, CF3, CH2OH, or CH2OY and Y is a carbon ester or phosphorus; and a pharmaceutically acceptable carrier. 5. A pharmaceutical composition of claim 4, wherein the compound is at least one α or β anomer, D (+) or L (-) enantiomer selected from the group consisting of: 6-[4- nitrobenzyl thio]-9-β-D-ribofuranosyl purine (NBMPR), 6-[4-nitrobenzyl thio]-9-β-D- ribofuranosyl purine 5'-monophosphate (NBMPR-P), 6-chloropurine riboside, 3'- deoxysangivamycin, N^-(p-methoxybenzoyl)adenosine, nitrobenzyl-6-selenoriboside, N^- benzyladenosine, N^-azidobenzyladenosine and N*>- nitrobenzyladenosine, 7- deazahypoxanthine, 7-deazaxanthine, 7-deaza-6-thiopurine, 7-deaza-2-amino-6-thiopurine, and 7-deaza-2-oxo-6-thiopurines. 6. A method for treating a subject for a disease or condition, which is caused by or contributed to by a parasite which primarily employs a purine salvage pathway for generating purines, comprising administering to the subject an effective amount of a pharmaceutical composition of claim 1. 7. A method of claim 6, wherein the disease is toxoplasmosis. 8. A method of claim 6, wherein the disease is a cancer or a blood disorder and a parasitic purine salvage enzyme had been previously introduced into disease causing cells. 9. A method of claim 8, wherein the purine salvage enzyme is selected from the group consisting of: an adenosine kinase enzyme, a purine nucleosidase and a phosphoribosyl transferase enzyme. 10. A pharmaceutical composition comprising: i) a purine nucleoside analog; ii) an agent for treating or preventing the establishment or growth of a cyst or infection; and iii) a pharmaceutically acceptable carrier. 11. A method for treating or preventing the establishment or growth of a tumor or infection by a parasite, which is dependent on a purine salvage pathway for its purine requirement, comprising administering to the subject an effective amount of a composition of claim 10. 12. A pharmaceutical composition comprising: i) a purine nucleoside analog; ii) an anti-bacterial agent; and iii) a pharmaceutically acceptable carrier. 13. A method for treating or preventing infection by a bacteria and a parasite, which is dependent on a purine salvage pathway for its purine requirement, comprising administering to the subject an effective amount of the pharmaceutical composition of claim 12. 14. A pharmaceutical composition comprising: i) a purine nucleoside analog; ii) an antiviral agent; and iii) a pharmaceutically acceptable carrier. 15. A pharmaceutical composition of claim 14, wherein the antiviral agent is selected from the group consisting of: 3'- azido-2', 3'-dideoxythymidine (AZT), 2',3'- dideoxycytidine (ddC), 2 , ,3 , -didehydro-2',3 , -dideoxythymidine (d4T), 3'-thia-2',3'- dideoxycytidine (3TC),2',3'-dideoxyinosine (ddI),2',3'-dideoxyguanosine (ddG), or 2'-β- fluoro-2' ,3 '-dideoxyadenosine (F-ara-ddA) . 16. A method for treating or preventing infection by the human immunodeficiency virus and a parasite, which is dependent on a purine salvage pathway for its purine requirement, comprising administering to the subject an effective amount of the pharmaceutical composition of claim 14. 17. A pharmaceutical composition comprising: i) a purine nucleoside analog; ii) an antifungal agent; and iii) a pharmaceutically acceptable carrier. 18. A method for treating or preventing infection by a fungus and a parasite, which is dependent on a purine salvage pathway for its purine requirement, comprising administering to the subject an effective amount of the pharmaceutical composition of claim 17. 19. A pharmaceutical composition comprising: i) a purine nucleoside analog; ii) an immunosuppressive agent; and iii) a pharmaceutically acceptable carrier. 20. A pharmaceutical composition of claim 19, wherein the immunosuppressive agent is selected from the group consisting of: cyclosporine, azathioprine, methotrexate, immunoglobulin preparations (e.g. Rhogam, Gamulin), adrenocorticosteroids, sulfasalazine, FK-506, methoxsalen, rapamycin and thalidomide

Inventors: EL KOUNI MAHMOUD H, GUARCELLO VINCENT, NAGUIB FARDOS N M, EL KOUNI, MAHMOUD, H., NAGUIB, FARDOS, N., M.
Applicants: UNIV ALABAMA RES FOUND, THE UNIVERSITY OF ALABAMA AT BIRMINGHAM RESEARCH F
Application year: 1995
Main classification: A61K31
IPC codes: A61K31, A61K45