Sildenafil 4- Rifampicin

Treiber, A., Schneiter, R., Hausler, S. and Stieger, B. (2007) Bosentan is a substrate of human OATP1B1 and OATP1B3 inhibition of hepatic uptake as the common mechanism of its interactions with cyclosporin A, rifampicin, and sildenafil. Drug Metabolism and Disposition The Biological Fate of Chemicals, 35, 1400-1407. 

On the basis of the 63% reduction in AUC seen with the moderate CYP3A4 indueer bosentan , (p.l274), the US manufacturer of sildenafil says that concurrent use with potent inducers of CYP3A4 such as rifampicin is predicted to cause a greater reduction in sildenafil levels. ... 

Rifampicin induces the activity of the cytochrome P450 isoenzyme CYP3A4, the principal enzyme concerned with the metabolism of sildenafil, tadalafil and vardenafii. 

Although the manufacturer of vardenafii does not mention CYP3A4 inducers, like tadalafil and sildenafil, vardenafii is principally metabolised by CYP3A4, and its levels are markedly raised by CYP3A4 inhibitors such as ketoconazole , (p. 1270). It is therefore very likely that vardenafii levels will be reduced by rifampicin and similar drugs, and concurrent use should be monitored. 

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