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We have demonstrated previously that the kinase inhibitors (KIs) lapatinib, pazopanib, regorafenib and sorafenib are potent inhibitors of UGT1A1 and UGTs 1A7-1A10. The present study characterised the effects of four additional drugs in this class, dabrafenib, ibrutinib, nintedanib and trametinib, on human UGT enzyme activities in vitro. Dabrafenib, ibrutinib, nintedanib and trametinib were potent inhibitors of human liver microsomal UGT1A1; K i,u values ranged from 1.1 to 7.5 µM. Similarly, these KIs inhibited UGT 1A7, 1A8, 1A9 and 1A10, albeit less potently than UGT1A1. Despite the potent inhibition of UGT1A1, in vitro - in vivo extrapolation excluded the likelihood that dabrafenib, ibrutinib, nintedanib and trametinib would precipitate drug-drug interactions (DDIs) due to the low unbound plasma concentrations of these drugs observed in patients. The structures of dabrafenib, ibrutinib, lapatinib, nintedanib, pazopanib, regorafenib, trametinib and 22 other KIs overlaid well on that of sorafenib, a potent inhibitor of UGT1A1 and UGTs 1A7-1A10. Taken together, kinetic and computational modelling data suggest that all currently marketed KIs are likely to be potent inhibitors of UGT1A1, and are also likely to inhibit UGTs 1A7-1A10 to some extent due to the structural and chemical features shared in common by these drugs. By contrast, BIBF 1202, the major metabolite of nintedanib, did not appreciably inhibit human UGTs, due to the presence of a terminal electronegative group which appears to disfavor enzyme inhibition. Given the potent inhibition of several UGT enzymes, especially UGT1A1, by KIs, characterisation of the DDI potential of newly developed agents in this class is warranted., (Crown Copyright © 2019. Published by Elsevier Inc. All rights reserved.)

UGT1A9 contributes to the glucuronidation of numerous drugs and xenobiotics. There is evidence to suggest that the Met33Thr substitution, as occurs in the polymorphic variant UGT1A9*3, variably affects xenobiotic glucuronidation. The equivalent position in UGT1A4 is also known to influence enzyme activity, whilst an N-terminal domain histidine (His37 in UGT1A9) is believed to function as the catalytic base in most UGT enzymes. To elucidate the roles of key amino acids and characterise structure-function relationships, we determined the effects of amino acid substitutions at positions 33 and 37 of UGT1A9 on the kinetics of 4-methylumbelliferone (4-MU), mycophenolic acid (MPA), propofol (PRO), sulfinpyrazone (SFZ), frusemide (FSM), (S)-naproxen (NAP) and retigabine (RTB) glucuronidation, compounds that undergo glucuronidation at either a phenolic (4-MU, MPA, PRO), carboxylate (FSM, NAP), acidic carbon (SFZ) or amine (RTB) function. Substitution of Met33 with Val, Ile, Thr, and Gln, as occur in UGT1A1, UGT1A3, UGT1A4 and UGT1A6 respectively, variably affected kinetics and catalytic efficiency. Whilst K(m) values were generally higher and V(max) and CL(int) values were generally lower than for wild-type UGT1A9 with most substrate-mutant pairs, the pattern and the magnitude of the changes in each parameter differed substantially. Moreover, exceptions occurred; CL(int) values for MPA and FSM glucuronidation by the position-33 mutants were the same as or higher than that of UGT1A9. Mutation of His37 abolished activity towards all substrates, except RTB N-glucuronidation. The data confirm the importance of single amino acids for UGT enzyme activity and substrate selectivity, and support a pivotal role for residue-33 in facilitating substrate binding to UGT1A9., (Crown Copyright © 2012. Published by Elsevier Inc. All rights reserved.)