Influence of Uridine Diphosphate Glucuronosyltransferase Family 1 Member A1 and Solute Carrier Organic Anion Transporter Family 1 Member B1 Polymorphisms and Efavirenz on Bilirubin Disposition in Healthy Volunteers

Abstract

Chronic administration of efavirenz is associated with decreased serum bilirubin levels, probably through induction of UGT1A1. We assessed the impact of efavirenz monotherapy and UGT1A1 phenotypes on total, conjugated, and unconjugated serum bilirubin levels in healthy volunteers. Healthy volunteers were enrolled into a clinical study designed to address efavirenz pharmacokinetics, drug interactions, and pharmacogenetics. Volunteers received multiple oral doses (600 mg/day for 17 days) of efavirenz. Serum bilirubin levels were obtained at study entry and 1 week after completion of the study. DNA genotyping was performed for UGT1A1 [*80 (C>T), *6 (G>A), *28 (TA 7 ), *36 (TA 5 ), and *37 (TA 8 )] and for SLCO1B1 [*5 (521T>C) and *1b (388A>G] variants. Diplotype predicted phenotypes were classified as normal, intermediate, and slow metabolizers. Compared with bilirubin levels at screening, treatment with efavirenz significantly reduced total, conjugated, and unconjugated bilirubin. After stratification by UGT1A1 phenotypes, there was a significant decrease in total bilirubin among all phenotypes, conjugated bilirubin among intermediate metabolizers, and unconjugated bilirubin among normal and intermediate metabolizers. The data also show that UGT1A1 genotype predicts serum bilirubin levels at baseline, but this relationship is lost after efavirenz treatment. SLCO1B1 genotypes did not predict bilirubin levels at baseline or after efavirenz treatment. Our data suggest that efavirenz may alter bilirubin disposition mainly through induction of UGT1A1 metabolism and efflux through multidrug resistance-associated protein 2. SIGNIFICANCE STATEMENTEfavirenz likely alters the pharmacokinetics of coadministered drugs, potentially causing lack of efficacy or increased adverse effects, as well as the disposition of endogenous compounds relevant in homeostasis through upregulation of UGT1A1 and multidrug resistance-associated protein 2. Measurement of unconjugated and conjugated bilirubin during new drug development may provide mechanistic understanding regarding enzyme and transporters modulated by the new drug.

Mechanistic Modeling of the Drug‐Drug Interaction Between Efavirenz and Dolutegravir: Is This Interaction Clinically Relevant When Switching From Efavirenz to Dolutegravir During Pregnancy?

“…Few studies suggested that efavirenz inhibits UGT1A1, 53,54 a finding that contrasts with other studies indicating induction of UGT1A1. 13,16,18 In the presented PBPK model, it was assumed that efavirenz exclusively induces UGT1A1, but further studies may be needed to rule out a concomitant inhibition.…”

“…For example, Swart and Dandara 13 observed that efavirenz significantly increased the messenger RNA (mRNA) expression of UGT1A1 in HepaRG cells at clinically relevant concentrations. Several clinical studies have shown that the serum concentrations of total, conjugated, and unconjugated bilirubin, an exclusive substrate of UGT1A1, 14 are significantly decreased during efavirenz monotherapy in healthy volunteers, 15–17 an effect that was still significant after stratification by UGT1A1 phenotype, 18 which provides further evidence that efavirenz induces UGT1A1 at clinically relevant concentrations. Likewise, Krishna et al 19 and Iwamoto et al 20 investigated the effect of efavirenz on raltegravir, an HIV integrase inhibitor predominantly metabolized by UGT1A1 (estimated dose fraction metabolized by UGT1A1, ≈0.7 21 ), in clinical drug‐drug interaction (DDI) studies.…”