LAUSR

PurposeHyperbilirubinemia has been observed in patients treated with tyrosine kinase inhibitor (TKI) drugs. Therefore, it would be beneficial to understand whether there is a relationship between inhibition of uridine-5′-diphosphate glucuronosyltransferase (UGT) 1A1 activity and observed bilirubin elevations in TKI drug-treated patients. UGT1A1 is responsible for the glucuronidation of bilirubin which leads to its elimination in the bile.MethodsTo examine this question, an in vitro glucuronidation assay was developed to determine the inhibitory effect of TKI drugs employing human liver microsomes (HLM) with varying UGT1A1 activity. Utilizing β-estradiol as the UGT1A1 probe substrate, 20 TKI drugs were evaluated at concentrations that represent clinical plasma levels. Adverse event reports were searched to generate an empirical Bayes geometric mean (EGBM) score for clinical hyperbilirubinemia with the TKI drugs.ResultsErlotinib, nilotinib, regorafenib, pazopanib, sorafenib and vemurafenib had IC50 values that were lower than their clinical steady-state Cmax concentrations. These TKI drugs had high incidences of hyperbilirubinemia and higher EBGM scores. The IC50 values and Cmax/IC50 ratios correlated well with EBGM scores for hyperbilirubinemia (P < 0.005). For the TKI drugs with higher incidence of hyperbilirubinemia in Gilbert syndrome patients, who have reduced UGT1A1 activity, six of eight had smaller ratios in the low UGT1A1 activity microsomes than the wild-type microsomes for drugs, indicating greater sensitivity to the drugs in this phenotype.ConclusionsThese results suggest that in vitro UGT1A1 inhibition assays have the potential to predict clinical hyperbilirubinemia.