The Metabolism of the New Benzodiazepine Remimazolam

Remimazolam (RMZ) is a novel ultrashort-acting benzodiazepine used for sedation by intravenous administration. The pharmacophore of RMZ includes a carboxyl ester group sensitive to esterase- mediated hydrolysis, which is the primary path of metabolic elimination. However, for the sake of drug safety, a deeper and broader knowledge of the involved metabolic pathways and the evolving metabolites is required. Information is needed on both humans and experimental animals to evaluate the possibility that humans form harmful metabolites not encountered in animal toxicity studies.

The current study aimed at identifying the mechanisms of remimazolam's metabolism and any potential clinically significant metabolites.

Using tissue homogenates from various animals and humans, the liver was identified as the tissue primarily responsible for the elimination of RMZ. CNS7054, the hydrolysis product of remimazolam, was identified as the only clinically relevant metabolite. Using bacterial or eukaryotic over-expression systems, carboxylesterase 1 (CES1) was identified as the iso-enzyme predominantly involved in RMZ metabolism, with no role for carboxylesterase 2. Using a variety of inhibitors of other esterases, the contribution to elimination mediated by esterases other than CES1 was excluded.

Besides tissue carboxylesterases, rodents expressed an RMZ esterase in plasma, which was not present in this compartment in other laboratory animals and humans, hampering direct comparisons. Other pathways of metabolic elimination, such as oxidation and glucuronidation, also occurred, but their contribution to overall elimination was minimal.

Besides the pharmacologically non-active metabolite CNS7054, no other clinically significant metabolite of remimazolam could be identified.