(M1430-10-69) Disposition and Metabolism of the Cardiac Myosin Inhibitor Aficamten in Humans

Purpose: Aficamten (also known as CK-3773274) is a small molecule, allosteric inhibitor of cardiac myosin being developed as a chronic oral treatment for patients with hypertrophic cardiomyopathy (HCM). The purpose of this study is to determine the absorption, metabolism, and excretion of [¹⁴C]aficamten and to identify and characterize the metabolites present in plasma, urine, and feces in healthy male subjects following single oral dose administration.

Methods: Eight healthy male subjects between ages of 22 and 43 years old were enrolled to ensure that least 6 subjects completed the study. On Day 1, all subjects received a single oral dose of 20 mg containing approximately 100 µCi of [¹⁴C]aficamten after an overnight fast of at least 10 hours. Blood, urine, and feces samples were collected until Day 19 if the subjects met the discharge criteria, and up to a maximum of Day 26. Whole blood and plasma were collected at predose, 0.25, 0.5, 1, 1.5, 2, 2.5, 3, 4, 6, 8, 12, 16, 24, 36, and 48 hours post-dose, then at 24‑hour intervals up to discharge. Urine samples were collected over the following intervals: predose (-12 to 0 hours), 0-4, 4-8, 8-12, and 12-24 hours post-dose, then at 24 hour intervals until discharge. Feces were collected at predose (from check-in to 0 hours), then at 24-hour intervals until discharge. Samples were processed and analyzed for total radioactivity by liquid scintillation counting. Profiling and identification of metabolites in plasma, urine, and feces were conducted by liquid chromatography (LC)‑radiometric and LC-tandem mass spectrometric (LC-MS/MS) analyses.

Results: Eight healthy participants completed all study assessments. A single 20 mg-dose of aficamten (100 - µCi) was well tolerated. The pharmacokinetics of aficamten were characterized by moderate absorption, with a median t_max of 2.00 hours postdose (range: 1.00 to 6.00 hours) and an apparent elimination half-life (t_1/2) of 99.6 hours (median, range 81.5 to 209 hours); 57.7% of the dose was recovered in feces and 32.0% in urine, resulting in overall mean recovery of 89.7%. Five metabolites were identified in plasma; the major metabolites were monohydroxylated metabolites M1a and M1b accounting for 10.5% and 36.4% of the total radioactivity AUC in plasma, respectively, and both with a median t_max of 5 hours. The other major plasma metabolite was M5 (a glucuronide conjugate of M1a/M1b), which accounted for 10.3% of the total plasma radioactivity exposure, with a t_max of 24 hours. Other metabolites, M3 (hydroxylated metabolite) and M64 (a sulfonated metabolite of M1a/M1b), accounted for less than 1.5% of the plasma radioactivity AUC. Twelve metabolites were detected in urine; none were >10% of dose. The only major metabolite in feces was M18 representing 44.1% of the radioactive dose.

Conclusion: Aficamten is eliminated by metabolism, and to a lesser extent, by fecal excretion with renal excretion playing a minor role. Aficamten metabolites account for approximately 80% of the recovered aficamten dose in human leading to the major metabolite M18 (44.1% of dose) being eliminated in feces. The main circulating metabolites in plasma are the hydroxylated enantiomeric metabolites M1a and M1b, followed by the subsequently formed ether-linked glucuronide metabolite M5.

Acknowledgements: This study was funded by Cytokinetics, Incorporated. All authors are employees of Cytokinetics, Incorporated, and hold stock in the company. The study was conducted in accordance with Good Clinical Practice and the Declaration of Helsinki and all subjects provided written informed consent.