Pharmacokinetic analysis of mephedrone and methylone after intravenous and oral administration in rats.

R. Lopez-Arnau1, J.M. Martinez-Clemente1, M. Carbo2,3, D. Pubill1, J. Camarasa1, E. Escubedo1. 1University of Barcelona, Pharmacology and Therapeutic Chemistry, 08028, Spain, 2University Pompeu Fabra, CEXS, 08003, Spain, 3Hospital del Mar-IMIM, Human Pharmacology and Clinical Neuroscience Research Group, 08003, Spain

In recent years, a new class of designer drugs has appeared in the abuse drugs market in many countries, namely, the so-called beta-keto designer drugs such as mephedrone and methylone. In the present work we have studied plasma pharmacokinetics of these compounds in rats. Male Sprague-Dawley rats weighting 215-230g were administered either intravenously with mephedrone or methylone (10mg/kg) or orally (30 and 60mg/Kg for mephedrone and 15 and 30mg/Kg for methylone). Animals were anesthetized with isoflurane and blood samples were collected from the jugular vein at times from 0.08 to 8 hours. Blood proteins were precipitated by the addition of methanol, and the extract was purified by ultrafiltration. Samples were analysed by LC-ESI-MS. Plasma metabolites and their fragmentation characteristics were identified based on MS2 and MS3 scan by LC-MS/MS. Values of plasma concentration after intravenous administration of mephedrone and methylone were fitted to an open bicompartmental model using SAM II v.1.1.1 package software. The pharmacokinetic parameters of mephedrone and methylone following intravenous administration were: AUCinf 1327 ± 17.2 µg.h/l and 4251.9 ± 82.5 µg.h/l respectively. Mephedrone showed a short terminal half-life with high total clearance and a little volume of distribution. The values of t½β, Vc and CL were 0.37 ± 0.01 h, 1.38 ± 0.11 l, and 7.53 ± 0.10 l/h·kg, respectively. Methylone showed a longer t1/2 β (0.95 ± 0.07 h), a similar Vc (1.90 ± 0.09 l) and a lower value of plasmatic Cl (2.53 ± 0.05 l/h·kg). After oral administration, for both compounds, we have obtained a dose-dependent variation in the Tmax value. Bioavailability was 65% for methylone, significantly higher than that of mephedrone. Consequently, after 30 mg/kg, maximum methylone concentrations (Cmax) were 10-fold higher than mephedrone concentrations, although both compounds exhibited similar Tmax (45-60 min). Mephedrone metabolic pathways revealed a side-chain degradation by N-demethylation to the corresponding primary amine 4-methylcathinone; two hydroxylations were identified, one in the benzilic position of the ring to produce 4-(hydroxymethyl)methcathinone and another in β-position of ketone group to give 3’-hydroxy-4-methylmethcatinone. Finally a oxidation of 4-(hydroxymethyl)methcathinone gives 4-carboxyhydromethcatinone. Similarly for methylone, a side-chain degradation by N-demethylation to the corresponding primary amine methylenedioxycathinone was observed. Also drug demethylenation followed by O-methylation renders 4-hydroxy-3-methoxymethcathinone or 3-hydroxy- 4-methoxymethcathinone. A hydroxylation in β-position of ketone group, 3’-hydroxy-3,4-methylenedioxymethcathinone, was identified. After intravenous study, elimination kinetics for mephedrone was faster than for methylone. In the oral route, a flip-flop kinetic model was observed for both compounds and the elimination half-life was controlled by the absorption phase. According to the bioavailability, a hepatic first-pass effect is suggested. When a high dose of methylone was assayed, Clp was reduced, pointing to a metabolic saturation; for mephedrone a non-linear kinetic was achieved after a 60mg/Kg dose.

This work was supported by grants from Ministerio de Ciencia e Innovación (SAF2010-15948); Plan Nacional sobre Drogas (2010/005) and Generalitat de Catalunya (SGR977).