The inhibitory effect of chloroquine and quinine on the formation and time-course of the PQ 5,6-orthoquinone generated from racemic PQ (a) and its individual enantiomers (b, c) by CYP2D6; and on the formation of primaquine alcohol from CYP2D6-catalyzed metabolism of racemic PQ (d) and its enantiomers (e, f). Each point represents values mean ± S.D. (n = 4)