Detection of clinical interactions between methadone and anti-retroviral compounds using an enantioselective capillary electrophoresis for methadone analysis

A capillary electrophoresis method was developed to detect interactions between methadone and anti-retroviral compounds. Eight subjects, who underwent methadone maintenance treatment in the Province of Alicante (Spain), consented to participate in the present study. Of those, one subject was followed up for 123 days to detect drug–drug interactions. The enantiomers of methadone and those of its main metabolite were conveniently resolved within 4 min using a chiral electrophoresis buffer mixture which consisted of phosphate buffer, pH 5, plus 0.2% highly sulphated-(beta)-cyclodextrin. The effective mobility of the analytes was in the 0.061–0.140 cm 2/(kV s) range at pH 5. The R-methadone plasma concentration range for seven patients was 91–318 ng/mL, it decreased from 186 to 46 ng/mL in a patient followed-up on commencement of the anti-retroviral therapy, returning to the previous higher levels after progressive dose increases. We conclude that monitoring R-methadone plasma levels can be a useful tool for the dose adjustment of methadone.