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We examined several characteristics of reduced forms of folate antagonists. Dihydro and tetrahydro derivatives of aminopterin and methotrexate (MTX) were chemically prepared. When titrated with dihydrofolate reductase, reduced derivatives were equipotent with the parent compounds and titrated stoichiometrically with the enzyme at pH 6 and nonstoichiometrically at higher pH conditions (pH 7.4 and above). When incubated with L1210 cells in vitro, rates of uptake of tritiated reduced compounds were significantly less in L1210 and L1210/MTX cells compared with the respective oxidized parent compounds and significantly less in L1210/MTX than in L1210 cells. Although dihydroaminopterin and tetrahydromethotrexate increased the survival rate of mice bearing L1210 tumors, these compounds had no such effect on the life-spans of animals with L1210/MTX tumors.

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Pentamidine is an aromatic diamidino compound synthesized originally for the therapy of trypanosomiasis. The pharmacologic effects of pentamidine vary, depending on its route of administration. In animals, the dominant effects have been a precipitous, transitory drop in blood pressure after injection and renal toxicity following repeated administration. To avoid the possibility of immediate toxic reactions associated with iv administration, we now usually give the drug im to humans. Further interest in pentamidine has been stimulated by its usefulness in the treatment of interstitial pneumonia caused by Pneumocystis carinii. In some patients receiving antineoplastic or immunosuppressive therapy who have superimposed P. carinii pneumonia, pentamidine may cause serious renal toxicity. Distribution and excretion studies in animals indicate pentamidine is deposited in tissues, with the greatest concentration in the kidneys, and gradually eliminated over a prolonged period. The mechanism of action of pentamidine against P. carinii or the means whereby fixation in tissues and subsequent toxicity occur have not been elucidated. Recent investigations to help clarify these points indicate that pentamidine inhibits dihydrofolate reductase in all tissues studied both in vitro and in vivo. In addition, pentamidine interacts and forms water-insoluble products with specific nucleotides and nucleic acids.

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Interaction between aromatic diamidines (pentamidine, propamidine, and stilbamidine) and nucleic acids were studied to elucidate the mechanism underlying renal toxicity included by pentamidine in patients. Pentamidine, propamidine, and stilbamidine precipitated with RNA and DNA of all species and types. Furthermore, the diamidines interacted and precipitated with all nucleoside triphosphates and nucleoside diphosphates, but not with nucleoside monophosphates, nucleosides, or free bases. Nucleoside diphosphate is the minimum unit necessary for interaction with the diamidines. This interaction between diamidines and nucleotides or nucleic acids may be implicated in the etiology of renal damage and skin reactions associated with these drugs.

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Triazinate (TZT), a potent inhibitor of dihydrofolate reductase, was selected for detailed investigation to determine its mechanism of selective action as well as its metabolic fate in mice, rats, dogs, and monkeys. The serum disappearance of TZT in normal and tumor-bearing mice was similar, with a rapid tissue equilibration phase and a slower elimination phase. Serum disappearance in normal and tumor-bearing rats was 1.5 to 2.2 hr. Serum disappearance in dogs and monkeys was similar, with half-lives of 3 to 4 and 2 to 4 hr, respectively. Urinary excretion of TZT at 24 hr was only 5 to 6% of the injected dose in mice and rats; in contrast, the dogs excreted 60% of the injected dose in 8 hr. TZT accumulated to comparable degrees in the organs of rats and mice, with progressively lesser concentrations in liver, kidney, spleen, and brain. Dihydrofolate reductase activity became almost undectectable in all tissues studied within 15 min after drug adminsitration. An important difference in drug accumulation was in the ascites cells of tumor-bearing animals: in mice, the drug level was consistently lower in the L1210 cells than in the ascites fluid; in contrast, by 30 min after treatment with TZT the drug level in Walker 256 cells was 10-fold higher than the level in the ascites fluid. No evidence for drug metabolism was found in extracts of urine, feces, or organ tissues from either mice or rats. TZT and two related triazines were studied for their ability to accumulate in the cerbrospinal fluid of dogs after i.v. administration. TZT achieved a cerebrospinal fluid level of approximately 15% of the serum concentration at 1 hr; in contrast, the other two triazines reached maximum cerebrospinal fluid values of 1% at 1 hr.

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