In order characterize the effects of amodiaquine on CYP450-mediated metabolism in humans, Wennerholm et al. (2006) administered a single dose of amodiaquine (600 mg) in the presence or absence of a single oral dose of a cocktail of CYP450-selective probe substrates: 10 mg debrisoquine (CYP2D6), 20 mg omeprazole (CYP219), 25 mg losartan (CYP2C9), and 100 mg caffeine (CYP1A2) to 12 healthy Swedish subjects who were determined, via genotyping, to be wild-type metabolizers, using a prospective, cross over design where each subject served as their own control. The primary endpoint was the effect of amodiaquine on the metabolic ratios between each probe substrate and a selected metabolite (i.e. debrisoquine/4-hydroxydebrisoquine). However, the typical pharmacokinetic parameters (i.e. AUC, Cmax, t1/2, etc.) were not reported in the study, limiting further mechanistic interpretation of the data. The major finding from the study was that amodiaquine significantly elevated the metabolic ratios for debrisoquine (CYP2D6) and losartan (CYP2C9) by 1.4 and 1.7 fold, respectively, but had little effect on omeprazole and caffeine metabolism. The effects were reversible upon further washout and re-administration of probe substrates alone, supporting the validity of the interaction. The effects of amodiaquine on debrisoquine metabolism in this study is supported by the in vitro finding from Bapiro et al. (2001) where amodiaquine was shown to be a strong inhibitor of CYP2D6 activity. Likewise, the lack of effects of amodiaquine on omeprazole and caffeine metabolism is also consistent with its weak inhibitory effects toward their respective isoenzymes (i.e. CYP2C19 and CYP1A2 respectively) in vitro. On the other hand, amodiaquine was shown to be a weak inhibitor, in vitro, of CYP2C9, but had a significant effect on losartan metabolism in this study. The discrepancy, which remains to be clarified, may be due to an effect on alternative metabolic pathways not yet studied for amodiaquine and losartan, or simply the inability to extrapolate in vitro to in vivo findings. A few limitations should be considered while interpreting the findings from this study: although the authors suggested that these effects may be due to amodiaquine and/or its major metabolite N-desethylamodiaquine, the study was actually not designed to determine the relative contribution of either the parent or metabolite toward enzyme inhibition. Also, the dose of amodiaquine used (i.e. 600 mg orally × 1) is not reflective of the typical clinical approach, where a much higher dose is given at steady-state conditions. The inhibitory effects of amodiaquine may very well be different in these different settings, in the true target population, which remains to be studied (Table 5.1).