Absorption, Metabolism, and Excretion of [14C]-Tolebrutinib After Oral Administration in Humans, Contribution of the Metabolites to Pharmacological Activity

Background and Objective Tolebrutinib is a covalent inhibitor of Bruton's tyrosine kinase, an enzyme expressed in B lymphocytes and myeloid cells including microglia, which are thought to be major drivers of inflammation in multiple sclerosis. This excretion balance and metabolism study evaluated the metabolite profile of tolebrutinib in healthy male volunteers. Methods Six healthy volunteers received a 60-mg oral dose of [C-14]-tolebrutinib, and metabolite profiling of C-14-labeled metabolites was performed using a combination of liquid chromatography, mass spectrometry, and radioactivity assay methods. Results Tolebrutinib was rapidly and completely absorbed from the gastrointestinal tract, followed by rapid and extensive metabolism. Excretion via feces was the major elimination pathway of the administered radioactivity (78%). Tolebrutinib was highly metabolized, with 19 metabolites identified in human plasma. Phase 1 biotransformations were primarily responsible for the circulating metabolites in plasma. Seven metabolites that achieved exposure in plasma similar to or higher than the parent compound were characterized biochemically for inhibition of Bruton's tyrosine kinase activity. Metabolite M8 exceeded the exposure threshold of 10% (18%) of the total radioactivity but had little if any pharmacological activity. Metabolite M2 (4% of circulating radioactivity) retained the ability to irreversibly and potently inhibit Bruton's tyrosine kinase in vitro, similar to the parent compound. Tolebrutinib and metabolite M2 had short (3.5-h) half-lives but durable pharmacodynamic effects as expected for an irreversible antagonist. Conclusions Tolebrutinib was extensively metabolized to multiple metabolites. The hydroxylated metabolite M2 demonstrated similar inhibitory potency toward Bruton's tyrosine kinase as the parent compound. Both tolebrutinib and metabolite M2 likely contributed to pharmacological activity in vivo.