Enantioselective determination of plasma protein binding of common amphetamine-type stimulants

Amphetamine-type stimulants (ATS) like amphetamine ('speed'), methamphetamine ('crystal meth') and 3,4-methylenedioxy-N-methylamphetamine (MDMA, 'ecstasy') represent some of the most frequently abused drugs worldwide. Another less frequently abused ATS is 4-fluoroamphetamine (4-FA). The enantiomers of these four compounds exhibit different pharmacokinetic and pharmacodynamic properties. According to the free drug theory, the pharmacological properties of a substance are dependent on its plasma protein binding (PPB). However, data on PPB of stimulant enantiomers in humans are rare or non-existent. Human plasma samples were spiked with racemic mixtures of the stimulants and subjected to ultrafiltration to extract the unbound fraction. Enantioselective liquid chromatography - tandem mass spectrometry (LC-MS/MS) methods were applied using a chiral Phenomenex® Lux3 μm AMP column. Method validation showed satisfactory selectivity, linearity (0.5 250 ng/mL), accuracy and precision. Enantiomers were quantified before and after ultracentrifugation to determine PPB. For all analytes, low to medium plasma protein binding was found. For (R)-amphetamine a slightly but significantly higher PPB was found compared to the (S)-enantiomer (31.7 % vs 29.0 %). (R)-MDMA also showed only slightly but significantly significantly higher PPB than (S)-MDMA, although the mean difference was negligible (21.6 % vs 21.3 %). For the enantiomers of methamphetamine and 4-FA, no significant differences in PPB were found. In summary, there were no or only minor differences in PBB for the enantiomers of all investigated compounds. The different pharmacological properties of the stimulant enantiomers can therefore not be explained by differences in PPB.