Structure-metabolism relationships of valine and tert-leucine-derived synthetic cannabinoid receptor agonists: a systematic comparison of the in vitro phase I metabolism using pooled human liver microsomes and high-resolution mass spectrometry

Synthetic cannabinoid receptor agonists, commonly referred to as ‘synthetic cannabinoids’ (SCs), gained popularity as recreational drugs due to their cannabis-like effects. The subclass of valine or tert-leucine-derived SCs has dominated the ‘designer drug’ market in recent years and has been associated with several severe intoxication cases. Most SCs are highly lipophilic compounds and are extensively metabolized prior to renal excretion. Hence, for drug detection in urine samples, the major metabolites of new compounds have to be identified first. The aim of this study was to elucidate structure-metabolism relationships (SMRs) of valine and tert-leucine-derived SCs enabling in-depth understanding of their phase I biotransformation and facilitating the prediction of suitable analytical targets for urine analysis. After incubation of 32 different valine/tert-leucine-derived SCs with pooled human liver microsomes (pHLM), the phase I metabolite profile of each compound was characterized using liquid chromatography–quadrupole time-of-flight mass spectrometry. By comparing chemical-structural analogs with the relative abundances of their metabolites, SMRs were studied. The terminal functionality (amide vs. methyl ester), the amino acid side chain (valine vs. tert-leucine), the core ring system (indole vs. indazole), and the N-alkyl side chain (cyclohexyl methyl vs. pentyl vs. 5-fluoropentyl vs. 4-fluorobenzyl) showed distinct differences of metabolic dehalogenation, dehydrogenation, formation of dihydrodiols, hydrolysis, hydroxylation, and N-dealkylation. The presented pHLM approach proved to be an effective tool for systematic investigation of SMRs. The information gained from this work may be useful for predicting potential SC metabolites for urine analysis.