Disposition and Metabolism of Semagacestat, a γ-Secretase Inhibitor, in Humans

Semagacestat is a functional γ-secretase inhibitor that has been shown to reduce the rate of formation of amyloid-β in vitro and in vivo. This study was conducted to characterize the disposition of semagacestat in humans. After a single 140-mg dose of [¹⁴C]semagacestat administered as an oral solution to six healthy male subjects, semagacestat was rapidly absorbed (T_max ∼0.5 h) and eliminated from the systemic circulation (terminal t_1/2 ∼2.4 h). The major circulating metabolites of semagacestat, M2 (hydrolysis of the amide bond proximal to the benzazepine ring) and M3 (benzylic hydroxylation of the benzazepine ring), accounted for approximately 27 and 10% of total radioactivity exposure, respectively, as calculated from relative area under the plasma concentration versus time curve from 0 to 24 h derived from the plasma radiochromatograms. The radioactive dose was almost completely recovered after 7 days postdose, with 87% of the dose in urine and 8% in feces. Unchanged [¹⁴C]semagacestat in urine accounted for approximately 44% of the dose, which indicates that renal excretion played an important role in elimination. Metabolites M2 and M3, with their related secondary metabolites, each accounted for approximately 20% of the dose in excreta. In vitro data indicate the formation of M3 is primarily mediated by CYP3A, with cDNA-expressed CYP3A5 approximately 2 times more efficient than CYP3A4 in forming M3. Thus, the relative content of CYP3A4 and CYP3A5 in humans will likely determine the formation clearance of M3 after exposure to semagacestat.