Table 1.

Specific CYP inhibitors used to investigate inhibition of sildenafil N-demethylation by various CYP enzymes. The values are mean ± s.d. of triplicate determinations in microsomes prepared from a pool of four human livers.

Isoform	Inhibitor	Concentration (µM)	% Effect on probe substrates
CYP1A2	Furafylline	1	59 ± 6% inhibition
		10	95 ± 9% inhibition
CYP2C9	Sulphaphenazole	2.5	53 ± 4% inhibition
		25	85 ± 2% inhibition
CYP2D6	Quinidine	2.5	82 ± 3% inhibition
		25	95 ± 1% inhibition
CYP3A4	Ketoconazole	2.5	79 ± 1% inhibition
		25	88 ± 1% inhibition