Istradefylline forty mg/day amplified peak levels and AUC of CYP3A4 substrates in clinical trials. This effect wasn't observed with istradefylline 20 mg/working day. Consider dose reduction of sensitive CYP3A4 substrates.lumacaftor/ivacaftor will reduce the level or effect of fentanyl by affecting hepatic/intestinal enzyme CYP3A4 metabolism.nafcill