Prediction of Drug-Induced Hyperbilirubinemia by In Vitro Testing.

Péter Tátrai,Péter Krajcsi

doi:10.3390/pharmaceutics12080755

Abstract

Bilirubin, the end product of heme catabolism, is produced continuously in the body and may reach toxic levels if accumulates in the serum and tissues; therefore, a highly efficient mechanism evolved for its disposition. Normally, unconjugated bilirubin enters hepatocytes through the uptake transporters organic anion transporting polypeptide (OATP) 1B1 and 1B3, undergoes glucuronidation by the Phase II enzyme UDP glucuronosyltransferase 1A1 (UGT1A1), and conjugated forms are excreted into the bile by the canalicular export pump multidrug resistance protein 2 (MRP2). Any remaining conjugated bilirubin is transported back to the blood by MRP3 and passed on for uptake and excretion by downstream hepatocytes or the kidney. The bile salt export pump BSEP as the main motor of bile flow is indirectly involved in bilirubin disposition. Genetic mutations and xenobiotics that interfere with this machinery may impede bilirubin disposition and cause hyperbilirubinemia. Several pharmaceutical compounds are known to cause hyperbilirubinemia via inhibition of OATP1Bs, UGT1A1, or BSEP. Herein we briefly review the in vitro prediction methods that serve to identify drugs with a potential to induce hyperbilirubinemia. In vitro assays can be deployed early in drug development and may help to minimize late-stage attrition. Based on current evidence, drugs that behave as mono- or multispecific inhibitors of OATP1B1, UGT1A1, and BSEP in vitro are at risk of causing clinically significant hyperbilirubinemia. By integrating inhibition data from in vitro assays, drug serum concentrations, and clinical reports of hyperbilirubinemia, predictor cut-off values have been established and are provisionally suggested in this review. Further validation of in vitro readouts to clinical outcomes is expected to enhance the predictive power of these assays.

Highlights

Bilirubin, a linear tetrapyrrole, is the catabolic end product of the heme prosthetic group found in hemoglobin, myoglobin, and cytochrome P450 enzymes
OATP1B1 inhibition was commonly assessed in screening-type in vitro studies
For compounds in the discovery phase, the OATP1B1 IC50 ≤ 6 μM cut-off is to be considered as a predictor that can be calculated without a knowledge of the actual or calculated Cmax

Summary

Bilirubin in Health

A linear tetrapyrrole, is the catabolic end product of the heme prosthetic group found in hemoglobin, myoglobin, and cytochrome P450 enzymes. OATP1B1- and OATP1B3-mediated transport of unconjugated bilirubin was shown first in vitro [18,19]; subsequent clinical and drug inhibition studies have confirmed the in vivo role of OATP1B transporters in hepatic bilirubin uptake [20]. The only study that attempted to link MRP3 mutations/variations to bilirubin homeostasis in humans focused on the natural c.3890G>A variant that introduces a conservative amino acid change (p.R1297H). This mutant, was at least as efficient as the wild type at transporting bilirubin glucuronides [41]. Cholestasis and conjugated hyperbilirubinemia that often accompany chronic treatment with fusidic acid are thought to arise from concomitant inhibition of OATPs and BSEP by this antimicrobial agent [46]

The Toxicity of Bilirubin

In Vitro Testing of Bilirubin Metabolism and Transport

Prediction of Drug-Induced Hyperbilirubinemia

Findings

Conclusions