Pharmacokinetic interactions between sildenafil and saquinavir/ritonavir

To investigate the effect of the antiretroviral protease inhibitors saquinavir (soft gelatin capsule) and ritonavir on the pharmacokinetic properties and tolerability of sildenafil and to investigate the effect of sildenafil on the steady-state pharmacokinetics of saquinavir and ritonavir.

Two independent, 8 day, open, randomized, placebo-controlled, parallel-group studies (containing a double-blind crossover phase) were conducted at Pfizer Clinical research units (Canterbury, UK. and Brussels, Belgium). Twenty-eight healthy male volunteers entered each study. In each study, volunteers were randomized (n = 14 per group) to receive sildenafil on day 1 followed by a 7-day treatment period (days 2-8) with saquinavir or placebo (Study I) or ritonavir or placebo (Study II). Sildenafil or placebo (Study I and Study II) was administered alternately on day 7 or day 8, depending on initial randomization. The effect of saquinavir and ritonavir on the pharmacokinetics of sildenafil and its primary circulating metabolite (UK-103, 320) and the effect of single-dose sildenafil on the steady-state pharmacokinetics of saquinavir (1200 mg three times daily) and ritonavir (500 mg twice daily) were determined. The safety and tolerability of sildenafil coadministered with saquinavir or ritonavir were also assessed.

Both protease inhibitors significantly increased Cmax, AUC, tmax and t(1/2) values for both sildenafil and UK-103, 320. Ritonavir showed a significantly greater effect than saquinavir with increases in sildenafil AUC and Cmax of 11-fold (95% CI: 9.0, 12.0) and 3.9-fold (95% CI: 3.2, 4.9), respectively. This compared with increases of 3.1-fold (95% CI: 2.5, 4.0) and 2.4-fold (95% CI: 1.8, 3.3) for coadministration with saquinavir. In contrast, the steady-state pharmacokinetics of saquinavir and ritonavir were unaffected by sildenafil. The increases in systemic exposure to sildenafil and UK-103, 320 were not associated with an increased incidence of adverse events or clinically significant changes in blood pressure, heart rate or ECG parameters.

These results indicate that both saquinavir and ritonavir modify the pharmacokinetics of sildenafil presumably through inhibition of CYP3A4. The more pronounced effect of ritonavir may be attributed to its additional potent inhibition of CYP2C9. No change in safety or tolerability was observed when sildenafil was coadministered with either protease inhibitor. However, given the extent of the interactions, a lower sildenafil starting dose (25 mg) should be considered for patients receiving saquinavir and it is recommended not to exceed a maximum single dose of 25 mg in a 48 h period for patients receiving ritonavir.