Methadone implants for methadone maintenance treatment. In vitro and in vivo animal studies

Methadone implant formulations elaborated with polylactide-co-glycolide (PLGA) and polylactic acid (PLA) for 1 week and 1 month release duration, respectively, were evaluated in vitro and in vivo. One-week implants prepared with methadone clorhydrate, methadone clorhydrate/methadone base blend or methadone base were tested in vitro. Results showed that the methadone release rate decreased as the methadone base increased. The best release profile was achieve when the methadone base implants, made by compression of a 50:50 PLGA (12 kDa) and methadone base mix, were coated with PLA (30 kDa). For 1-month implants, the methadone base load was increased to 65% and PLA of 30 kDa was used as a matrix component. In this case the implants were coated with the same polymer. Deconvolution methods could not be used for in vivo release estimation because an increase in methadone clearance was observed with methadone clorhydrate solution multiple-dose treatment. Therefore the amount of drug remaining within the implants was evaluated and the deconvolution was only used to establish the release profile range. The upper limit was estimated applying the absorption-disposition function obtained after multiple-dose administrations while the lower curve was estimated using the single-dose function. Methadone serum levels were maintained around 200 ng/ml during 1 week and approximately 5 weeks with the optimised implants. In vivo-in vitro correlations were always very good with slopes near 1.