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The most important hepatic enzyme involved in the metabolism of protease inhibitors is cytochrome P450 3A4 (CYP3A4). Ritonavir (RIT) is a potent inhibitor of CYP3A4 and inhibits saquinavir (SQV) metabolism in healthy volunteers. In this study we investigated the kinetics of SQV when administered alone and in combination with RIT in HIV-infected patients.SQV pharmacokinetics were determined in seven patients who had advanced HIV disease. Steady-state SQV profiles were obtained on two occasions following treatment with SQV 600 mg three times daily alone and when administered with RIT 300 mg twice daily.Blood samples were obtained at times 0, 1, 2, 4, 6 and 8 h post-dosing. Following centrifugation, separated plasma was heated at 58°C for at least 30 min to inactivate HIV and stored at −80°C until analysis using high performance liquid chromatography.For patients treated with SQV alone there was a 12-fold variability in the area under the SQV concentration-time curve (AUC0–8h) ranging from 293 to 3446 ng•h/ml. When combined with RIT there was a marked increase in the maximum plasma concentration of SQV [median (range), 146 (57–702) versus 4795 (1420–15810) ng/ml; ∼95% confidence interval (CI), 2988–6819; P = 0.0006, Mann-Whitney U test]. The AUC0–8h for SQV was also significantly increased in the presence of RIT [median (range), 470 (293–3446) versus 27 458 (7357–108 001) ng•h/ml; ∼95% CI, 16 628–35 111; P= 0.0006].For some patients, administration of SQV 600 mg three times daily results in very low SQV plasma levels and possibly little antiviral effect. Combination of SQV with RIT results in a significant drug interaction mediated by enzyme inhibition which exposes patients to very high SQV concentrations and potential toxicity. If combination therapy with SQV plus RIT is considered then the dose of SQV should be greatly reduced.