Detection and Identification of Dizocilpine and its Major Urinary Metabolites in the Horse: A Preliminary Report

Abstract

Dizocilpine ([+]-10,11-dihydro-5-methyl-5H-dibenzo[a,d]cyclohepten-5,10-imine), is a potent and selective NMDA (N-methyl-D-aspartate) receptor antagonist, which acts by blocking receptor ion channels. Dizocilpine is pharmacologically related to ketamine and phencyclidine; as such, it has the potential to affect behavior and performance in horses, with particular efficacy at lower concentrations. We now report development of a sensitive method for the detection of dizocilpine and preliminary characterization of its urinary metabolites in the horse. Dizocilpine (MW 221) readily produces a protonated species [M+H]⁺ in formic acid, and yields a m/z 205 product ion in Multiple Reaction Monitoring (MRM), allowing highly sensitive detection of parent drug. The 17 AMU loss most likely represents an unusual loss of CH₅ from the exocyclic methyl group. No unchanged dizocilpine was identified in unhydrolysed urine, and the presence of hydroxymethyl and carboxydizocilpine glucuronide metabolites were supported by observation of m/z 414→238 and 428→235 transitions. Urine samples from horses dosed with dizocilpine (0.0132 and 0.0656 mg kg⁻¹, iv) were hydrolysed with glucuronidase and were found to contain dizocilpine and OH-dizocilpine. Tentatively identified phase I post-hydrolysis compounds include dizocilpine itself, an hydroxymethyl metabolite, two ring-hydroxylated metabolites, a di-hydroxy metabolite, and a carboxy-dizocilpine metabolite. Corresponding Phase II glucuronidated metabolites were also identified as well as a number of combination metabolites and a posssible n-glucuronide metabolite for a total of at least six identifiable urinary glucuronide metabolites. Among the phase I metabolites, the hydroxymethyl metabolite apparently predominated, especially at the 0.0132 mg kg⁻¹ dose. The goal of this research was to identify a target analyte for dizocilpine in post-administration equine urine, so that work may begin on development of a forensically validated qualitative method for this target analyte. Given the likelihood that the doses of dizocilpine used in attempts to influence the behavior or performance of horses, either alone or in combination with other agents, are expected to be in the order of 0.02 mg kg⁻¹ or less, these results suggest selection of the phase I hydroxymethyl metabolite of dizocilpine as the optimal target analyte for regulatory control of dizocilpine in performance horses.