Human metabolism of zamicastat, an oral dopamine β-hydroxylase inhibitor

The absorption, metabolism, and excretion of zamicastat, a dopamine β-hydroxylase inhibitor, were investigated in healthy male subjects following a single oral dose of 400 mg [¹⁴C]-zamicastat. The mean recovery of total administered radioactivity averaged 86% after 336 hours. Feces was the major route of elimination, representing 69% of the administered dose (with 40% of unchanged compound), 14% were excreted in urine and 2.3% in expired air. The total radioactivity peaked in plasma between 4 and 48 hours, with unchanged zamicastat accounting for <1% of the total circulating radioactivity. The long terminal half-life of total radioactivity (153 hours) was primarily attributed to [¹⁴C]-thiocyanic acid, representing the main circulating metabolite (69.1% of the total circulating radioactivity). Few other minor metabolites were identified in circulation, resulting from multistep reactions involving the desulfuration of zamicastat followed by several phase I and phase II metabolic reactions, such as oxidation and methylation on the benzyl moiety and glucuronic acid conjugation. Although cytochrome P450 was responsible for some zamicastat metabolism, the phenotyping in human hepatocytes showed that the 2 main metabolites observed were not present in vivo. Additionally, only trace levels of thiocyanic acid were observed. This study provides insights into the human elimination routes of zamicastat [¹⁴C]-radiolabeled in the imidazole-thione ring, uncovering an unexpected major, multistep metabolite of zamicastat into thiocyanic acid, highlighting a major in vitro-in vivo difference in zamicastat metabolism. SIGNIFICANCE STATEMENT: The absorption, distribution, metabolism, and excretion of a single radioactive oral dose of zamicastat was evaluated. After oral administration, [¹⁴C]-zamicastat was extensively metabolized with the release of [¹⁴C]-thiocyanic acid that was responsible for the long-lasting circulating radioactivity. Few minor metabolites were identified in circulation. Across species in vitro studies have shown a completely different profile from human metabolism with only trace levels of [¹⁴C]-thiocyanic acid observed, precluding the prediction of the extensive zamicastat metabolism observed in humans.