Comparison of dosages, intervals, and drugs in the prevention of Pneumocystis carinii pneumonia.

The efficacies of trimethoprim (TMP)-sulfamethoxazole (SMZ), TMP-dapsone, dapsone, and pentamidine were compared for the prevention of Pneumocystis carinii pneumonia in the corticosteroid-treated-rat model. While 11 (73%) of 15 untreated control animals had P. carinii pneumonia after 10 weeks of immunosuppression, none of the animals given 125 mg of dapsone per kg daily, weekly, biweekly, or monthly had evidence of infection. Of the 10 rats given a single dose of dapsone 23 and 50 days after immunosuppression was started, 5 (50%) had P. carinii pneumonia. When three drugs were given separately to groups of 10 rats in single doses biweekly, P. carinii pneumonia occurred in 40% of those treated with TMP-SMZ and in none of those treated with TMP-dapsone; although only 2 of those treated with pentamidine survived for evaluation, both had P. carinii pneumonia. The experiments showed that dapsone is highly effective in chemoprophylaxis for P. carinii pneumonia when given at monthly intervals or more frequently and that dapsone and TMP-dapsone are more effective than is TMP-SMZ when given at biweekly intervals. It seems reasonable to expect that biweekly doses of dapsone or TMP-dapsone would provide an effective and reasonably safe chemoprophylaxis regimen for patients at high risk for P. carinii pneumonia, and studies to test such a scheme are justifiable. Biweekly doses are preferred over monthly doses to allow for occasional inadvertent omission of doses expected from patients.