Interaction of sildenafil and indinavir when co-administered to HIV-positive patients

The prevalence of erectile dysfunction in HIV-infected men is estimated to be 33%. Sildenafil citrate (Viagra; Pfizer Ltd, Sandwich, Kent, UK) is the first oral drug for this condition. Since sildenafil and the protease inhibitors are both metabolized by, and act as inhibitors of cytochrome P450 3A4, we evaluated the pharmacokinetics of the combination sildenafil plus indinavir in HIV-infected patients.

Six patients at steady state in treatment with indinavir participated in the study. On the first day blood samples for indinavir assay were drawn at times 0, 1, 2, 3, 4, 6 and 8 h after dosing. On the second study day patients received a single dose of 25 mg of sildenafil in addition to their routine morning medication. Blood samples were taken as described. Separated plasma was stored at -80 degrees C until analysis by high performance liquid chromatography. In a parallel study, the effect of indinavir, ritonavir, saquinavir and nelfinavir on the in vitro hepatic metabolism of sildenafil was assessed.

The geometric mean area under the concentration curve for 0-8 h (AUC0-8h) and maximum plasma concentration (Cmax) for indinavir were 19.69 microg/ml h (range, 9.19-31.99 microg/ml h) and 7.02 microg/ml (range, 2.33-16.17 microg/ml), respectively, on the first study day. In the presence of sildenafil, the mean AUC0-8h and Cmax of indinavir were 22.37 microg/ml h [range, 10.08-37.25 microg/ml h; 95% confidence interval (CI) for difference between means, -15 to 13.25) and 9.11 microg/ml (range, 3.41-22.78 microg/ml; 95% CI, -13 to 6.37), respectively. The geometric mean AUC0-8h and Cmax for sildenafil were 1631 ng/ml h (range, 643-2970 ng/ml h) and 384 ng/ml (range, 209-766 ng/ml) respectively. The AUC for sildenafil was 4.4 times higher than data from historical controls given either 50 mg or 100 mg of sildenafil and dose normalized to 25 mg. Indinavir was a potent inhibitor of sildenafil hepatic metabolism in vitro [concentration producing 50% inhibition of control enzyme activity (IC50) = 0.39 +/- 0.17 microM, mean +/- SD].

Co-administration of sildenafil 25 mg did not significantly alter the plasma indinavir levels. However, plasma sildenafil AUC was markedly increased in the presence of indinavir compared with historical controls. From the in vitro data, the mechanism of increase is indinavir inhibition of the hepatic metabolism of sildenafil. The magnitude of this interaction suggests a lower starting dose of sildenafil may be more appropriate in this clinical setting.