Biotransformation of cyclizine in greyhounds. 2: N 1 -dealkylation and identification of some neutral and phenolic metabolites in canine urine by gas chromatography-mass spectrometry

Abstract

1. The in vivo enzymatic Phase I biotransformation of cyclizine (Marezine ®) in the racing greyhound has been shown to proceed via several different pathways. Aromatic and heterocyclic oxidation and the N 4 -demethylation of cyclizine lead to the formation of unconjugated and conjugated (Phase II) basic metabolites excreted in canine urine. 2. Enzymatic N 1 -dealkylation of cyclizine and its basic metabolites leads to the formation of the neutral and phenolic Phase I metabolites containing the diphenylmethane/methylene substructures. Further, Phase I metabolism of the neutral metabolites could also lead to the formation of several secondary phenolic products. These neutral and phenolic compounds are then excreted as unconjugated and Phase II conjugates in greyhound urine. 3. Following enzymatic deconjugation of selected post-Marezine administration urine samples from two greyhounds, the total aglycones were extracted and separated into neutral/acidic and basic fractions using copolymeric mixed-mode solid-phase extraction cartridges. 4. The neutral/acid isolates were further separated into neutral and phenolic fractions by column chromatography on a lipophilic strong anion-exchanger gel, triethylaminohydroxypropyl Sephadex LH-20 in OH - form. 5. The individual neutral and phenolic fractions obtained from the acid/neutral isolate were derivatized as trimethylsilyl ethers and analysed by positive-ion electron ionization gas chromatography-mass spectrometry (EI + -GC-MS). 6. Three compounds, diphenylmethane (M1), benzophenone (or diphenyl ketone, M2) and benzhydrol (M3), were identified in the neutral isolates by comparison of their EI + mass spectra with authentic standards. At least seven secondary compounds containing the functionalized diphenylmethylene substructure were detected in the phenolic isolates. As no authentic compounds are available, the structures of these putative metabolites (M4 → M10) were elucidated from an interpretation of the EI + -GC-mass spectra of their TMS derivatives.