Inhibition of OATP1B1/3 Rather Than UGT1A1 May Be the Major Cause of the Bilirubin Elevation After Atazanavir Administration

Abstract

Atazanavir has been reported to increase total serum bilirubin level up to ninefold. It is widely believed that the observed total bilirubin elevation is primarily due to UGT1A1 inhibition. However, UGT enzymes are well-known as a low-affinity and high-capacity system, and the observed drug-drug interaction mediated by UGTs is usually less than twofold. There were discrepancies in the explanation of total bilirubin elevation due to UGT1A1 inhibition alone, suggesting the contribution of other mechanism(s) to the interaction. As atazanavir is a potent OATP1B1/3 inhibitor and the hepatic uptake of both unconjugated and conjugated bilirubin are mediated by OATP1B1/3, these transporters could be involved in the bilirubin-atazanavir interaction. To better understand the roles of UGT1A1 and OATP1B1/3 in this interaction, it would be useful to characterize the contribution of each individual pathway to the interaction. As multiple compounds, pathways, and potentially UGT1A1 polymorphism are involved, a thorough physiologically-based pharmacokinetic (PBPK) analysis was utilized to integrate the information from various relevant in vitro and clinical studies to quantitatively estimate the contribution of UGT1A1 and OATP1B1/3 inhibition to the interaction between bilirubin and atazanavir. The PBPK analysis indicated that UGT1A1 inhibition plays a modest role in bilirubin and atazanavir interaction contributing less than 33%. The results also suggested that unconjugated bilirubin is less sensitive than raltegravir upon UGT1A1 inhibition, therefore, unconjugated bilirubin may not be a useful endogenous biomarker for UGT1A1 inhibition. The analysis demonstrated that the metabolism of unconjugated bilirubin shares common features of other UGT enzyme-mediated reactions.

Figure 1

(a) Diagrams of unconjugated and conjugated bilirubin disposition. U‐BIL, unconjugated bilirubin; C‐BIL, conjugated bilirubin; UGT, UDP‐glucuronosyltransferases; OATP, organic‐anion‐transporting polypeptides; MRP, multidrug resistance‐associated protein. (b) Overall PBPK modeling strategy. (c) Overview of PBPK modeling validation workflow. [Correction added on 04 June 2025, after first online publication: Figure 1A has been corrected in this version.]

Figure 2

Validation of the bilirubin PBPK model. Plasma concentration–time profiles of total (a) and conjugated (b) bilirubin with and without 600 mg oral administration of 600 mg of rifampicin (RIF) (Appendix B , Figure B1 for additional rifampicin dose levels). Closed circles are observed in mean plasma concentration–time profiles of total bilirubin ((a); blue, baseline; pink, with rifampicin) and conjugated bilirubin ((b); green, baseline; yellow, with rifampicin) from the study reported by Mori et al. The continuous line represents the simulated mean plasma concentration–time profiles of total bilirubin ((a); blue, baseline; pink, with rifampicin) and conjugated bilirubin ((b); green, baseline; yellow, with rifampicin); the shaded areas represent the simulated 5th and 95th percentiles of the simulations. Results were from the virtual subjects with simulated total bilirubin baseline levels of ≤1.3 mg/dL. Summary of simulated vs. observed geometric means ratios of AUC (c) and C _max (d) for total (triangle), unconjugated (square), and conjugated (circle) bilirubin in the presence of rifampicin (blue, 150 mg; yellow, 300 mg; red, 600 mg) or cyclosporine (green, 20 mg; gray 75 mg). The observed AUCs of unconjugated bilirubin in the absence and presence of rifampicin were calculated by deducting the observed AUCs of conjugated bilirubin from total bilirubin with the data obtained in the corresponding administration. Then, the AUC ratios of unconjugated bilirubin were calculated with the corresponding values. The solid line is the line of unity; dashed lines represent the interval of 0.8–1.25; the shaded area represents the interval between 0.5 and 2. (e) Simulated (sim.) vs. observed (obs.) mean total bilirubin concentration in UGT1A1 EM, IM, and PM. Results shown in (a), (b), (c), and (d) were from virtual subjects with simulated total bilirubin baseline levels less than 1.3 mg/dL as the study was performed in healthy subjects. Results shown in (e) were from virtual subjects with simulated total bilirubin baseline levels of ≤1.3, 1.8, and 3.4 mg/dL for EM, IM, and PM,, respectively. The calculated fold of elevation of total, unconjugated, and/or conjugated bilirubin were similar between the results with or without excluding virtual subjects with total bilirubin greater than the cutoff values (Appendix B , Table B3, B4, and B5). AUC, area under the curve; C _max, maximum plasma concentration; EM, extensive metabolizer; IM, intermediate metabolizer; PM, poor metabolizer; obs., observed; sim., simulated.

Figure 3

Validation of the atazanavir PBPK model. Plasma concentration–time profiles of atazanavir after administration of atazanavir alone ((a), 400 mg q.d.) or atazanavir/r ((b), 300 mg/100 mg q.d.) in CYP3A5 expressors and non‐expressors. Closed circles are observed geometric mean plasma concentration–time profile of atazanavir. The continuous line represents the simulated geometric mean plasma concentration–time profile of atazanavir; the shaded area represents the 90% prediction intervals. Pink, expressor; blue, non‐expressor. Summary of simulated vs. observed AUC (c) and C _max (d) of atazanavir after atazanavir administration with various dosing regimens (Appendix C , Table C2, C3, and C4). (e) and (f) Summary of simulated vs. observed geometric mean ratios of AUC and C _max of clarithromycin, raltegravir, and drospirenone when administered in the presence and absence of atazanavir or atazanavir/r (Appendix C , Table C5). (g) Simulated vs. observed geometric mean ratio of AUC of atorvastatin in the presence and absence of atazanavir/c (300 mg/150 mg q.d.) or darunavir/c (800 mg/150 mg). in vitro K _i, in vitro K _i of atazanavir, darunavir, and cobicistat against OATP1B1/3; K _i of atazanavir optimized, K _i of atazanavir against OATP1B1/3 was optimized; CYP3A4 inh only, simulated without K _i of darunavir or cobicistat against OATP1B1/3 and assumed the interaction was by CYP3A4 inhibition only. In (c), (d), (e), and (f), the solid line is the line of unity; dashed lines represent the interval of 0.8–1.25; the shaded area represents the interval between 0.5 and 2. b.d., twice daily; q.d., once daily; SD, single dose.

Figure 4

(a) Summary of simulated vs. observed median total bilirubin levels in UGT1A1 EM, IM, and PM after atazanavir/r administration (300 mg/100 mg q.d.). The error bars represent the observed or simulated first (25th percentile, Q1) and third quartile (75th percentile, Q3). (b) Summary of simulated vs. observed mean unconjugated bilirubin increase in UGT1A1 EM, IM, and PM after administration of atazanavir alone (400 mg q.d.) or atazanavir/r (300 mg/100 mg q.d.). The error bars represent the observed or simulated 95% confidence interval (CI). Results shown in (a) and (b) were from virtual subjects with simulated total bilirubin baseline levels of less than 1.3, 1.8, and 3.4 mg/dL for EM, IM, and PM,, respectively. The average steady‐state concentration (C _ss,avg) of total or unconjugated bilirubin in the absence and presence of atazanavir administration was simulated. The calculated fold of elevation of total, unconjugated, and/or conjugated bilirubin was similar between the results with or without excluding virtual subjects with total bilirubin greater than the cutoff values (Appendix B , Tables B9 and B10).

Figure 5

(a) Comparison of the contribution of UGT1A1 and OATP1B1/3 inhibition to the simulated percent change of total bilirubin after administration of atazanavir alone (left, 400 mg q.d.) or atazanavir/r (right, 300 mg/100 mg q.d.). Control, simulated with all interaction pathways; w/o_UGT1A1_inh, simulated without UGT1A1 interaction by atazanavir or atazanavir/r; w/o_OAT1B1/3_inh simulated without OATP1B1/3 inhibition by atazanavir or atazanavir/r. (b) Comparison of simulated percent change in total bilirubin after administration of atazanavir/r (300 mg/100 mg q.d.) with (w/) and without (w/o) UGT1A1 induction effect by ritonavir in UGT1A1 EM, IM, and PM. Results shown in (a), (b), and (c) were calculated from the simulated C _ss,avg of total bilirubin in the absence and presence of atazanavir administration.

Figure 6

(a) Comparison of AUC ratios of unconjugated bilirubin, total bilirubin, and raltegravir in the presence and absence of atazanavir after administration of atazanavir alone (400 mg q.d.) with only UGT1A1 effect by atazanavir. All other inhibition effects of atazanavir except its auto‐inhibition via CYP3A4 were excluded in the simulation. (b) Comparison of AUC ratios of unconjugated, conjugated, and total bilirubin, atorvastatin, pravastatin, and simvastatin acid in the presence and absence of rifampicin after oral administration of a single dose of rifampicin (600 mg) with only OATP1B1/3 inhibition effect. All other inhibition effects by rifampicin were excluded in the simulation. Results of all AUC ratios shown in (a) and (b) were from virtual subjects with simulated total bilirubin baseline level of ≤1.3 mg/dL. The horizontal line in the box indicates the median value. The box edges represent the 1st and 3rd quartiles. The whiskers represent the 1st quartile −1.5 × IQR and the 3rd quartile +1.5 × IQR, where IQR is the interquartile range.