Zamicastat is a noncompetitive Dopamine‐β‐hydroxylase inhibitor that modulates sympathetic nervous system activity

Abstract

Dopamine‐β‐hydroxylase (DβH; EC 1.14.17.1) is an enzyme responsible for the conversion of dopamine (DA) into norepinephrine (NE). It is present in the chromaffin cells of adrenal medulla and in noradrenergic nerve terminals of the central and peripheral sympathetic nervous system. DβH inhibition has been proposed as an alternative mechanism for the treatment of hypertension and chronic heart failure. Zamicastat ((R)‐5‐(2‐(benzylamino)ethyl)‐1‐(6,8‐difluorochroman‐3‐yl)‐1H‐imidazole‐2(3H)‐thione) is a novel DβH inhibitor. The aim of this study was to characterize the pharmacodynamics of Zamicastat in vivo, both in Wistar Han and in spontaneously hypertensive rats (SHR), and on DβH in human cells. DβH activity was measured by a modification of the Nagatsu and Udenfriend (1) spectrophotometric method, based on the conversion of tyramine into octopamine either in adrenal tissues of rats administered orally with Zamicastat (30 mg/Kg) or vehicle, or in human neuroblastoma (SK‐N‐SH) cells. Zamicastat pharmacodynamic characterization in vivo was performed in Wistar rats by evaluating NE and DA levels in tissues using HPLC with electrochemical detection at 9h post‐administration. Additionally, systolic and diastolic blood pressure, heart rate and activity were measured continually by telemetry in SHR rats (a model of essential hypertension) and their controls, Wistar Kyoto (WKY). Zamicastat was found to be a reversible noncompetitive inhibitor of DβH towards tyramine, with a Ki value of 43 (21; 66) nM and either noncompetitive or uncompetitive towards ascorbic acid. Zamicastat (30 mg/Kg, p.o.) fully inhibits adrenal DβH activity from 2 to 8h recovering to baseline values after 24h. This is accompanied with a decrease in NE and an increase in DA in peripheral tissues, namely left ventricle and kidney but not significantly in the central nervous system (brain stem or prefrontal‐cortex). Zamicastat at 3, 30 and 100 mg/Kg displayed a dose‐dependent decrease on systolic and diastolic blood pressure with no effect in heart rate or overall activity when evaluated in SHR rats by telemetry. In conclusion, Zamicastat is a DβH inhibitor displaying binding affinity for both the free enzyme and the complex enzyme‐substrate, that when orally administered at 30 mg/Kg to rats acts peripherally and significantly reduces sympathetic activity by decreasing NE levels and consequently systolic and diastolic blood pressure in an animal model of essential hypertension.