Background <p>Fezolinetant is a non-hormonal selective neurokinin-3 receptor antagonist for the treatment of moderate to severe vasomotor symptoms associated with menopause. The objective of this study was to characterize the pharmacokinetics, mass balance, and metabolism of fezolinetant in postmenopausal women.</p> Methods <p>A single dose of 180 mg ¹⁴C-fezolinetant solution was administered to healthy postmenopausal women (<i>n</i> = 5) to evaluate mass balance and pharmacokinetics. Quantitative metabolite profiling and metabolite structure elucidation were performed on samples collected from the mass balance study by high performance liquid chromatography with radioactivity detector or liquid chromatography–tandem mass spectrometry analyses.</p> Results <p>Following a single administration of ¹⁴C-fezolinetant, the average recovery of radioactivity was 90.9%, where the majority of radioactivity was recovered in urine (mean: 76.9%) and to a lesser extent in feces (mean: 14.0%). Fezolinetant was well absorbed and primarily metabolized to the hydroxylated metabolite, ES259564, which was eliminated mainly in urine. Fezolinetant accounted for nearly 29% of exposure for total radioactivity in plasma. In addition to the parent drug, only ES259564 was detected as a circulating metabolite and accounted for approximately 52% of total drug-related exposure. Additional minor metabolites (&lt; 3.5% of dose for each metabolite) were only detected in urine or feces. Fezolinetant metabolic pathways included hydroxylation of the methyl group of the 3-methyl-1,2,4-thiadiazole moiety (M9, ES259564), further oxidation of M9 to the carboxylic acid metabolite M4, further glucuronidation of the hydroxyl group of M9 to the glucuronide metabolite M5, direct glucuronidation of fezolinetant to the glucuronide M6, and cleavage of the 1,2,4-thiadiazole moiety to the ring-opened metabolite M1.</p> Conclusion <p>This study successfully characterized the overall pathways of metabolism and excretion of fezolinetant, identified the circulating metabolites, and provided key data to support the development of fezolinetant. Fezolinetant is mainly metabolized to yield ES259564, and primarily excreted into urine as ES259564. In plasma, only fezolinetant and ES259564 were detected, accounting for approximately 81% of total radioactivity, indicating that the majority of circulating drug-related material was quantitatively characterized, with the remaining radioactivity attributable to multiple low-abundance components below the threshold for a major circulating metabolite.</p> <p>Trial registration number: EudraCT Number 2017-004911-38</p>