Initial dosing regimens of ZDV were based on the plasma half-life of the drug. However, recent studies of the intracellular metabolism of ZDV have demonstrated that the active anabolite, ZDV-TP, is present within the cell for an extended period of time relative to the drug in the plasma. This suggests that antiviral activity may be present for a sufficient time frame with less-frequent dosing of the drug. Careful comparison of the rate and extent of intracellular phosphorylated ZDV metabolites as a function of schedule will determine whether less-frequent dosing has a sound pharmacological basis. Also, the intracellular metabolism of 3TC is via different enzymes than that of ZDV and there are quantitative differences in the amount of triphosphate formed from both drugs. This study will provide information about intracellular metabolites when both ZDV and 3TC are concurrently administered.

This is a study of 2 schedules of Combivir therapy. At study entry or Part I, all patients take Combivir twice daily for the 7-day adherence assessment.

Patients who have demonstrated 70 percent or greater adherence [AS PER AMENDMENT 7/20/01: 70 percent compliance with the study regimen for Combivir. This corresponds to taking at least 10 of the prescribed 14 Combivir tablets during the 7 days prior to an adherence assessment, including all scheduled doses in the 24-hour period prior to that assessment.], and have taken all scheduled Combivir doses in the previous 24 hours, have pharmacokinetic samples obtained and are randomized to Group A or Group B in Part II. Group A patients take 2 Combivir tablets once daily; Group B patients take 1 Combivir tablet twice daily. After patients have completed the targeted duration of Part II (7 days for Group A and 7-14 days for Group B), they are assessed for adherence. Patients who have demonstrated 70 percent or greater adherence, and have taken all scheduled Combivir doses in the previous 24 hours, have pharmacokinetic samples obtained and then change to the alternate dosing schedule. Group A patients take 1 Combivir tablet twice daily; Group B patients take 2 Combivir tablets once daily. After patients have completed the targeted duration of Part III (7-14 days for Group A and 7 days for Group B), they are assessed for adherence. All patients who meet the adherence criteria have pharmacokinetic samples obtained. After completion of Part III pharmacokinetic studies, patients have completed the study. (Note: Combivir will not be provided in this study.)