Fig. 7. Relationship between methadone enantiomers clearance and CYP3A activity. (  A ) Intravenous (IV) methadone clearance and hepatic CYP3A activity (IV alfentanil clearance); (  B ) apparent oral methadone clearance and first-pass CYP 3A activity (oral alfentanil apparent clearance). CL = systemic clearance; CL/F = apparent oral clearance. Each data point is the result for a single subject. There were no significant correlations between methadone clearance and CYP3A activity.