Anti-Pneumocystis carinii activity of PS-15, a new biguanide folate antagonist.

A newly synthesized biguanide inhibitor of dihydrofolate reductase in Plasmodium species was evaluated for its anti-Pneumocystis carinii activity. The compound N-3-(2,4,5-trichlorophenoxypropyloxy)-N'-(1-methylethyl)imidoca rbonimidic diamide hydrochloride, designated PS-15, was administered prophylactically and therapeutically to immunosuppressed rats latently infected with P. carinii. Doses of 5 and 25 mg of PS-15 per kg of body weight per day given orally during 7 weeks of dexamethasone immunosuppression prevented P. carinii infection in all (100%) 19 rats given the drug, while 6 of 9 (67%) untreated control rats developed P. carinii pneumonitis. A single weekly dose of 50 mg of PS-15 per kg also prevented the infection in all 10 rats. P. carinii pneumonitis was established after 4 weeks of immunosuppression and was then treated orally for 3 weeks with 25, 5, and 1 mg of PS-15 per kg/day. Complete resolution of the infection occurred in all (100%) 10 rats given 25 mg of PS-15, 6 of 9 (67%) rats given 5 mg of PS-15, and 6 of 8 (75%) rats given 1.0 mg of PS-15 per kg per day and in all (100%) 9 rats treated with trimethoprim-sulfamethoxazole. PS-15 was well tolerated at all doses. Because drug studies in the P. carinii rat model have been highly predictable of the effects of drugs on the disease in humans, these experiments suggest that PS-15 may have promise as a drug for the treatment of P. carinii pneumonitis in humans.