In Vitro Inhibition of Udp Glucuronosyltransferases by Atazanavir and Other Hiv Protease Inhibitors and the Relationship of This Property to in Vivo Bilirubin Glucuronidation

Abstract

ABSTRACT:Several human immunodeficiency virus (HIV) protease inhibitors, including atazanavir, indinavir, lopinavir, nelfinavir, ritonavir, and saquinavir, were tested for their potential to inhibit uridine 5-diphospho-glucuronosyltransferase (UGT) activity. Experiments were performed with human cDNA-expressed enzymes (UGT1A1, 1A3, 1A4, 1A6, 1A9, and 2B7) as well as human liver microsomes.All of the protease inhibitors tested were inhibitors of UGT1A1, UGT1A3, and UGT1A4 with IC 50 values that ranged from 2 to 87 M. The IC 50 values found for all compounds for UGT1A6, 1A9, and 2B7 were >100 M. The inhibition (IC 50 ) of UGT1A1 was similar when tested against the human cDNA-expressed enzyme or human liver microsomes for atazanavir, indinavir, and saquinavir (2.4, 87, and 7.3 M versus 2.5, 68, and 5.0 M, respectively). By analysis of the double-reciprocal plots of bilirubin glucuronidation activities at different bilirubin concentrations in the presence of fixed concentrations of inhibitors, the UGT1A1 inhibition by atazanavir and indinavir was demonstrated to follow a linear mixed-type inhibition mechanism (K i ‫؍‬ 1.9 and 47.9 M, respectively). These results suggest that a direct inhibition of UGT1A1-mediated bilirubin glucuronidation may provide a mechanism for the reversible hyperbilirubinemia associated with administration of atazanavir as well as indinavir. In vitro-in vivo scaling with [I]/K i predicts that atazanavir and indinavir are more likely to induce hyperbilirubinemia than other HIV protease inhibitors studied when a free C max drug concentration was used. Our current study provides a unique example of in vitro-in vivo correlation for an endogenous UGT-mediated metabolic pathway.The HIV protease is an essential enzyme that cuts the viral gag-pol polyprotein into its functional subunits. Atazanavir, indinavir, saquinavir, lopinavir, ritonavir, and nelfinavir are HIV protease inhibitors. The structures of these HIV protease inhibitors are shown in Fig. 1. As opposed to atazanavir, which is an azapeptide protease inhibitor (Goldsmith and Perry, 2003), other listed HIV protease inhibitors are peptidomimetics sharing the same structural determinant, i.e., a hydroxyethylene or a hydroxyethylamine moiety, which makes them nonscissile HIV protease substrate analogs. These HIV protease inhibitors are metabolized primarily by hepatic CYP3A enzymes, and they are also inhibitors of CYP3A enzymes (Flexner, 2000;de Maat et al., 2003;Goldsmith and Perry, 2003;Ernest et al., 2005). All of these HIV protease inhibitors have a high protein binding (Ͼ98%), except indinavir and atazanavir, which have a protein binding of 60 and 86%, respectively. Most of the HIV protease inhibitors are bound to ␣1-acid glycoprotein instead of albumin (de Maat et al., 2003). There are no literature reports indicating that HIV protease inhibitors are good substrates for human UGT enzymes, although there is evidence to support indinavir as a substrate of UGTs (Balani et al., 1996).Glucuronidation represents a major pathway for the elimin...

Interaction between buprenorphine and atazanavir or atazanavir/ritonavir

“…ATV is also known to significantly inhibit bilirubin glucuronidation by UDPglucuronosyltransferase 1A1 (UGT 1A1) (Bristol Myers Squibb, 2006;Zhang et al, 2005). BUP and N-BUP glucuronidation is mediated by UGT 1A1 (King et al, 1996), although apparently using a different active site than is used for bilirubin (Rios and Tephly, 2002).…”

“…Increase in total bilirubin is a known adverse event associated with ATV and ATV/r treatment (Bristol Myers Squibb, 2006) and is probably related to ATV inhibition of UGT1A1 (Bristol Myers Squibb, 2006;Zhang et al, 2005) resulting in reduction of bilirubin conjugation. Increased total bilirubin was observed in both BUP-maintained and control study participants.…”

Long-Term Efficacy and Safety of Atazanavir/Ritonavir Treatment in a Real-Life Cohort of Treatment-Experienced Patients with HIV Type 1 Infection

“…uridine 5'-diphosphoglucuronosyltransferase, the enzyme responsible for glucuronidation of bilirubin. 27 Although ATVinduced hyperbilirubinemia is manageable, reversible, and independent of hepatocellular toxicity, the related symptoms of jaundice and ocular icterus could potentially have an impact on a patient's appearance and quality of life (QoL). 28 As expected with an ATV/r-based regimen, grade 3-4 hyperbilirubinemia occurred in 63% of patients in this study at any time point.…”

Automation and Miniaturization of the Bioluminescent UGT-Glo Assay for Screening of UDP-Glucuronosyltransferase Inhibition by Various Compounds

“…UGT1A1 with bilirubin as substrate 12 and a reported UG-T1A1 K m of 4.1 mM for indinavir 13 (Table 2). A compound such as indinavir that is a UGT substrate is also expected to act as a competitive inhibitor of UGT activity against a different substrate.…”

“…12 Raloxifene (Evista, Eli Lilly and Company, Indianapolis, IN), an antiestrogen drug used to treat osteoporosis, was found to be glucuronidated by UGT1A1, UGT1A8, UGT1A9, and UGT1A10, but not UGT1A4 and UGT2B7. 15 In accordance with this report, the screen found UGT1A1 inhibition by raloxifene, presumably because it is a competitive substrate (Fig.…”