Radiolabel Uncovers Nonintuitive Metabolites of BIIB104: Novel Release of [¹⁴C]Cyanide from 2-Cyanothiophene and Subsequent Formation of [¹⁴C]Thiocyanate

Abstract

BIIB104 (formerly PF-04958242), N- ((3S,4S)-4-(4-(5-cyanothiophen-2-yl)phenoxy)tetrahydrofuran-3-yl)propane-2-sulfonamide, is an AMPAR potentiator investigated for the treatment of cognitive impairment associated with schizophrenia. Preliminary in vitro metabolism studies with non-radiolabeled BIIB104 in rat, dog, and human liver microsomes (RLM, DLM, and HLM) showed O-dealkylation in all 3 species, tetrahydrofuran hydroxylation dominating in DLM and HLM, and thiophene hydroxylation prevalent in RLM. However, a subsequent rat mass balance study with [nitrile-14 C]BIIB104 showed incomplete recovery of administered radioactivity (~80%) from urine and feces over 7 days following an oral dose, and an exceptionally long plasma total radioactivity half-life.Radiochromatographic metabolite profiling and identification, including chemical derivation, revealed that [ 14 C]cyanide was a major metabolite of [nitrile-14 C]BIIB104 in RLM, but a minor and trace metabolite in DLM and HLM, respectively. Correspondingly in bile duct-cannulated rats, [ 14 C]thiocyanate accounted for ~53% of total radioactivity excreted over 48 h postdose and it, as an endogenous substance, explained the exceptionally long plasma radioactivity half-life. The release of [ 14 C]cyanide from the 2-cyanothiophene moiety is postulated to follow an epoxidation-initiated thiophene-opening based on the detection of non-radiolabeled counterpart metabolites in RLM. This unusual biotransformation serves as a lesson regarding placement of the radioactive label on an aryl nitrile when material will be used for evaluating the metabolism of a new drug candidate.Additionally, the potential cyanide metabolite of nitrile-containing drug molecules may be detected in liver microsomes with LC-MS following a chemical derivatization, so to be informed early during drug discovery.