Effect of Trimethoprim on Lamivudine Bioavailability

To the Editor. —Recent studies have shown encouraging antiretroviral and immunological benefit from the combination of lamivudine and zidovudine. 1-3 The cost of such combination therapy (with the possible addition of human immunodeficiency virus [HIV] protease inhibitors and nonnucleoside reverse transcriptase inhibitors) is considerable. A little publicized fact is that trimethoprim increases the bioavailability of lamivudine by approximately 40%. 4 It is not stated in any of these trials precisely how many patients were receiving trimethoprim-sulfamethoxazole for Pneumocystis carinii pneumonia prophylaxis. Trials of combination therapy of trimethoprim and lamivudine may show that lower doses of lamivudine are effective with significant cost benefits.