Primaquine (PQ) metabolism by the cytochrome P450 (CYP) 2D family of enzymes is required for antimalarial activity in both humans (2D6) and mice (2D). Human CYP 2D6 is highly polymorphic, and decreased CYP 2D6 enzyme activity has been linked to decreased PQ antimalarial activity. Despite the importance of CYP 2D metabolism in PQ efficacy, the exact role that these enzymes play in PQ metabolism and pharmacokinetics has not been extensively studied in vivo. In this study, a series of PQ pharmacokinetic experiments were conducted in mice with differential CYP 2D metabolism characteristics, including wild-type (WT), CYP 2D knockout (KO), and humanized CYP 2D6 (KO/knock-in [KO/KI]) mice. Plasma and liver pharmacokinetic profiles from a single PQ dose (20 mg/kg of body weight) differed significantly among the strains for PQ and carboxy-PQ. Additionally, due to the suspected role of phenolic metabolites in PQ efficacy, these were probed using reference standards. Levels of phenolic metabolites were highest in mice capable of metabolizing CYP 2D6 substrates (WT and KO/KI 2D6 mice). PQ phenolic metabolites were present in different quantities in the two strains, illustrating species-specific differences in PQ metabolism between the human and mouse enzymes. Taking the data together, this report furthers understanding of PQ pharmacokinetics in the context of differential CYP 2D metabolism and has important implications for PQ administration in humans with different levels of CYP 2D6 enzyme activity. P rimaquine (PQ) is the only FDA-approved drug for treatment of relapsing infections with malarial strains, including Plasmodium vivax and P. ovale (1-3). PQ belongs to the 8-aminoquinoline (8AQ) class of antimalarial compounds, among which several molecules, including PQ, have potent antihypnozoite activity (2, 4, 5). Low doses of PQ are also recommended for malaria transmission-blocking efforts due to PQ's gametocidal activity (6, 7). PQ's utility in malaria treatment and potential use in malarial transmission reduction and malaria eradication efforts require an understanding of the molecular species involved in its mechanism of action.Recent reports have shown that PQ requires metabolic activation by the cytochrome P450 (CYP) 2D isoenzymes for liver-stage antimalarial activity in both mouse studies (CYP 2D) and human studies (CYP 2D6) (8-11). Pybus et al. demonstrated that PQ was active only in mice capable of metabolizing CYP 2D6 substrates. Deletion of the mouse enzyme closest to human CYP 2D6 (mouse CYP 2D22 via deletion of the CYP 2D gene cluster) in mice completely blocked liver-stage antimalarial activity in vivo (10). The study by Bennett et al. demonstrated a direct link between CYP 2D6 metabolizer status and PQ efficacy for P. vivax treatment in several human subjects (8). PQ therapy is of significant importance for P. vivax radical cure, presumptive antirelapse therapy (PART), and malaria eradication efforts, and the requirement of CYP 2D6 metabolism for PQ efficacy is problematic because CYP 2D6 is highly polymorp...

Primaquine (PQ), a racemic drug, is the only treatment available for radical cure of relapsing Plasmodium vivax malaria and blocking transmission of P. falciparum malaria. Recent studies have shown differential pharmacologic and toxicologic profiles of individual PQ enantiomers in rodent, dog, and primate animal models. This study was conducted in six healthy adult human volunteers to determine the plasma pharmacokinetic profile of enantiomers of PQ and carboxyprimaquine (cPQ), the major plasma metabolite. The individuals were orally administered PQ diphosphate, equivalent to 45-mg base, 30 minutes after a normal breakfast. Blood samples were collected at different time intervals, and plasma samples were analyzed for enantiomers of PQ and cPQ. Plasma PQ concentrations were low and variable for both parent enantiomers and peaked around 2-4 hours. Peak (2)-(R)-PQ concentrations ranged from 121 ng/ml to 221 ng/ml, and peak (+)-(S)-PQ concentrations ranged from 168 ng/ml to 299 ng/ml. The cPQ concentrations were much higher and were surprisingly consistent from subject to subject. Essentially all the cPQ detected in plasma was (2)-cPQ. The peak concentrations of (2)-cPQ were observed at 8 hours (range: 1104-1756 ng/ml); however, very high concentrations were sustained through 24 hours. (+)-cPQ was two orders of magnitude lower than (2)-cPQ, and in a few subjects it was detected but only under the limit of quantification. In vitro studies with primary human hepatocytes also suggested more rapid metabolism of (2)-PQ compared with (+)-PQ. The results suggest more rapid metabolism of (2)-PQ to (2) cPQ compared with (+)-PQ. Alternatively, (+)-PQ or (+)-cPQ could be rapidly converted to another metabolite(s) or distributed to tissues. This is the first clinical report on enantioselective pharmacokinetic profiles of PQ and cPQ and supports further clinical evaluation of individual PQ enantiomers.

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