Drug Interaction Report

In a study with 14 HIV-positive patients, coadministration of trimethoprim/sulfamethoxazole (trimethoprim/sulfamethoxazole DS once a day for 5 days) and lamivudine (300 mg single dose on day 5) resulted in a mean decrease of 35% in lamivudine renal clearance and a mean increase of 43% in lamivudine area under the plasma concentration-time curve. The mechanism of interaction is thought to be competitive inhibition of tubular secretion by trimethoprim. Lamivudine did not affect the pharmacokinetic profile of trimethoprim/sulfamethoxazole. Given the favorable safety profile of lamivudine, this interaction is unlikely to be of clinical significance. However, patients with renal dysfunction should be monitored carefully and the lamivudine dose adjusted if necessary. In addition, it should be noted that the effect of higher dosages of trimethoprim/sulfamethoxazole on lamivudine pharmacokinetics has not been investigated.