Differential CYP 2D6 Metabolism Alters Primaquine Pharmacokinetics

Abstract

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...

Differential Cytochrome P450 2D Metabolism Alters Tafenoquine Pharmacokinetics

“…The results presented above provide in vivo evidence that tafenoquine pharmacokinetics differ depending on CYP 2D metabolizer status in mice. The changes in pharmacokinetic profiles for tafenoquine in the KO strain versus WT mice were of lower magnitude than what was observed previously for primaquine (25). These differences in the KO versus WT mouse strains for both molecules in the liver are summarized in Fig.…”

“…Experiments were conducted as described by Potter et al (25). Briefly, male 6-to 14-week-old C57BL/6 and 2D knockout C57BL/6 mice (Taconic, Hudson, NY) were used for PK evaluations.…”

“…Pharmacokinetic parameter determination was performed as described by Potter et al (25). Briefly, pharmacokinetic parameters for tafenoquine and the 5,6-orthoquinone in plasma and liver were determined using noncompartmental analysis via the Phoenix-WinNonlin software package (version 6.3; Phar- sight Corp., Mountain View, CA).…”

“…1B for tafenoquine corresponds to the stable oxidation product of the 5-dearylation pathway. A related metabolic pathway (C5 hydroxylation) was recently shown to be affected by mouse CYP 2D metabolism for primaquine (25). The existence of such a phenolic metabolite for tafenoquine has not been demonstrated in vivo.…”

“…This CYP 2D6 requirement has unknown pharmacokinetic (PK) and pharmacodynamic implications in humans for the 8-aminoquinolines. Recently, the studies of Potter et al and Bennett et al demonstrated clear differences in primaquine pharmacokinetic profiles in both mouse and human models of differential CYP 2D6 metabolism (7,25). Diminished CYP 2D6-mediated clearance resulted in higher concentrations of the parent primaquine in vivo.…”