Apixaban, a direct oral anticoagulant and reversible inhibitor of factor Xa (FXa), is widely used for the prevention of venous thrombosis or thrombus formation in non-valvular atrial fibrillation (NVAF) and in the treatment of venous thromboembolism . In the ARISTOTLE trial, apixaban was shown to more effectively reduce the risk of stroke, systemic thromboembolism, and major bleeding in patients with NVAF, as compared to warfarin . Therefore, apixaban is increasingly prescribed to patients newly diagnosed with NVAF .
The dose of apixaban is typically determined based on the patient's age, weight, and serum creatinine level without routine anticoagulant testing or plasma drug concentration monitoring . However, achieving accurate dosing of apixaban is important, as it has been reported that increased plasma concentrations and areas under the plasma concentration-time curves (AUC) are associated with bleeding risk and thromboembolic events [4–6]. In addition, large interindividual variabilities in the pharmacokinetics (PK) of apixaban have been observed [7, 8]. For example, Gulilat et al. reported that the coefficient of variations of peak concentration (Cmax) and trough concentration (C0h) at the steady-state were 55.0% and 47.3%, respectively, in NVAF patients receiving 5 mg of apixaban twice daily .
These variations may be due to variables impacting the clearance pathways of apixaban. Orally administrated apixaban is absorbed mainly in the small intestine, where the bioavailability is approximately 50%. Apixaban is then metabolized by cytochrome P450 (CYP) in the liver and is excreted through the kidneys. Renal excretion as the unchanged form accounts for 27% of the total clearance, and excretion as metabolites in the urine and feces accounts for 25% of the administered dose. The elimination half-life of apixaban is reported to be approximately 12 h .
Apixaban is mainly metabolized by CYP3A4/5, with minor contributions from CYP1A2, CYP2C8/9/19, and CYP2J2 . Furthermore, apixaban is also a substrate of the efflux transporter P-glycoprotein (P-gp; gene code ABCB1) and breast cancer resistance protein (BCRP; gene code ABCG2) . These transporters are involved in absorption from the small intestine, in excretion from hepatocytes into bile, and in renal tubular secretion of substrate drugs .
Although several studies have investigated the effects of polymorphisms of genes encoding drug metabolic enzymes and drug transporters on the PK of apixaban, the results have been inconsistent. Ueshima et al. reported that the oral clearance of apixaban was lower in Japanese AF patients with the CYP3A5*3 allele than those with the CYP3A5*1/*1 genotype and was lower in those with ABCG2 421A/A genotype than those with ABCG2 421G allele [14, 15]. Dimatteo et al. reported that the Cmax, not the C0h, was higher in Caucasian patients taking apixaban with ABCB1 2482 − 2236 A/A genotype than those with the G allele . On the other hand, Roşian et al. reported that there were no significant differences in the trough or peak plasma concentrations of apixaban between the genotype groups of ABCB1 3435C > T and ABCB1 2482-2236G > A . In addition, Lenoir et al. reported that there were no significant differences in the AUC0-6h of apixaban between the genotype groups of ABCB1 1236C > T, 2677G > A/T, 3435C > T, and CYP3A, including CYP3A5*3 .
Several systems are known to be involved in the induction of drug-metabolizing enzymes or transporters, and variations in these genes may help to further explain differences in the PK of apixaban PK. For example, the nuclear receptor, pregnane X receptor (PXR), regulates the transcription of genes encoding several drug-metabolizing enzymes, such as CYP2 and CYP3A, and drug transporters, such as P-gp, and it thus facilitates the elimination of xenobiotics from the body [19, 20]. Accordingly, polymorphisms in PXR (also known as NR1I2) have been found to affect the induction of its target genes . The PXR*1B haplotype cluster, which is common in Asian populations , has been characterized by the combination of NR1I2 8055C > T, 11156A > C, and 11193T > C genotypes [22, 23] (Supplementary Table 1).
Another relevant system is P450 oxidoreductase (POR), which transfers electrons from NADPH oxidase to CYP enzymes, increasing CYP activity and affecting the metabolism of drug substrates [24, 25]. Among several single nucleotide polymorphisms (SNPs) of POR, the most common variant is POR*28 (c.1508 C > T, rs1057868), and its allele frequency in the Japanese population is approximately 40% . According to an in vitro study, this SNP is associated with increased activity of multiple CYPs, including CYP1A2, CYP2C19, and CYP3A4/5 [27, 28]. Therefore, the statuses of the PXR*1B haplotype and the POR*28 genotype may affect blood levels of drugs that are substrates for the above-mentioned metabolic enzymes and transporters, but the impacts of these polymorphisms on the PK of apixaban have not been investigated.
The first purpose of this study was to investigate the impact of polymorphisms of genes encoding the major metabolic enzymes and transporters, CYP3A5, ABCG2, and ABCB1, that directly affect the PK of apixaban, on steady-state C0h of apixaban in Japanese patients with NVAF. The second goal was to investigate the effects of polymorphisms of PXR and POR, which affect the expression or activity of CYPs and/or ATP-binding cassette (ABC) transporters, on this C0h.