Case Study 5. Deconvoluting Hyperbilirubinemia: Differentiating Between Hepatotoxicity and Reversible Inhibition of UGT1A1, MRP2, or OATP1B1 in Drug Development

Abstract

New molecular entities (NMEs) are evaluated using a rigorous set of in vitro and in vivo studies to assess their safety and suitability for testing in humans. Regulatory health authorities require that therapeutic and supratherapeutic doses be administered, by the intended route of administration, to two nonclinical species prior to human testing (ICH Expert Working Group. The international conference on harmonization of technical requirements for registration of pharmaceuticals for human use (ICH); Multidisciplinary guidelines; Nonclinical safety studies (M3). http://www.ich.org/fileadmin/Public_Web_Site/ICH_Products/Guidelines/Multidisciplinary/M3_R2/Step4/M3_R2__Guideline.pdf , 2009). The purpose of these studies is to identify potential target organ toxicity and to determine if the effects are reversible. Liver is a potential site for toxicity caused by orally administered NMEs due to high exposure during first pass after oral administration. A range of clinical chemistry analytes are routinely measured in both nonclinical and clinical studies to evaluate and monitor for hepatotoxicity. While bilirubin itself circulates within a wide range of concentrations in many animal species and humans, without causing adverse effects and possibly providing benefits (Sedlak and Snyder. Pediatrics 113(6):1776-1782, 2004), bilirubin is one of the few readily monitored circulating biomarkers that can provide insight into liver function. Therefore, any changes in plasma or urine bilirubin levels must be carefully evaluated. Changes in bilirubin may occur as a result of adaptive nontoxic changes or severe toxicity. Examples of adaptive nontoxic changes in liver function, which may elevate direct (conjugated) and/or indirect (unconjugated) bilirubin above baseline levels, include reversible inhibition of UGT1A1-mediated bilirubin metabolism and OATP1B1-, OATP1B3-, or MRP2-mediated transport (Keogh. Adv Pharmacol 63:1-42, 2012). Alternatively, hepatocellular necrosis, hypoalbuminuria, or cholestasis may also lead to elevation of bilirubin; in some cases, these effects may be irreversible (FDA/CDER. Guidance for industry drug-induced liver injury: premarketing clinical evaluation. http://www.fda.gov/downloads/Drugs/…/Guidances/UCM174090.pdf , 2012).This chapter aims to demonstrate application of enzyme kinetic principles in understanding the risk of bilirubin elevation through inhibition of multiple processes-involving both enzymes and transporters. In the sections that follow, we first provide a brief summary of bilirubin formation and disposition. Two case examples are then provided to illustrate the enzyme kinetic studies needed for risk assessment and for identifying the mechanisms of bilirubin elevation. Caveats of methods and data interpretation are discussed in these case studies. The data presented in this chapter is unpublished at the time of compilation of this book. It has been incorporated in this chapter to provide a sense of complexities in enzyme kinetics to the reader.

Protein Abundance of Hepatic Drug Transporters in Patients With Different Forms of Liver Damage

“…The changes observed in the levels of drug transporters are consistent with altered handling of bilirubin, which is a substrate for the uptake transporters OATP1B1 and OATP1B3 and for the efflux transporter MRP2 . The gradual decrease found in OATP1B1, OATP1B3, and a more intense drop of MRP2 protein abundance as liver function declined could explain the increased serum total bilirubin levels associated with progression of hepatic insufficiency.…”

Extrahepatic Drug Transporters in Liver Failure: Focus on Kidney and Gastrointestinal Tract

“…On the contrary, the function of uptake carriers, namely, NTCP, OCT1, OATP1B1 and OATP2B1 decrease [13]. The decline in levels of the transporters implicated in bilirubin (the uptake transporters OATP1B1 and OATP1B3 and the efflux transporter MRP2) and bile salts handling correspond with the observed concentration changes of these endogenous transporter levels in liver failure [38]. Deterioration of liver function entails an increase in bilirubin blood concentrations, due to reduced hepatic uptake (OATP1B1 and OATP1B3) and sinusoidal elimination (MRP2) [39].…”

Selective Inhibition of the Mitochondrial Permeability Transition Pore Protects against Neurodegeneration in Experimental Multiple Sclerosis

“…5 C ). Activity against organic anion transporters OATP1B1 and OATP1B3 has been linked to drug-induced unconjugated hyperbilirubinemia in patients receiving alisoporivir ( 80 ). JW47 demonstrated 7–8-fold less inhibition of β-estradiol-17-β- d -glucuronide uptake for the OATP1B1 transporter than CsA (2.56 ± 0.47 μ m versus 0.44 ± 0.084 μ m for CsA).…”