Simultaneous determination of l-tetrahydropalmatine and cocaine in human plasma by simple UPLC–FLD method: Application in clinical studies

•The new method allows for the simultaneous determination of cocaine and l-THP in human plasma.•The LLOQ of method is adequate for pharmacokinetic study.•The method was first applied in clinical study.

Currently, there are no FDA approved medications for treatment of cocaine addiction underscoring the dire need to develop such a product. There is an accumulating body of evidence that l-tetrahydropalmatine (l-THP), a non-selective dopamine antagonist, can be used for the treatment of cocaine addiction. Indeed, the FDA recently approved its usage in a Phase I study in cocaine abusers and it was indispensable to develop a simple and sensitive method for the simultaneous determination of l-THP and cocaine in human plasma. We developed a UPLC–FLD method for quantitation of these molecules using an ACQUITY BEH C18 column (2.1 mm × 50 mm, 1.7 μm) and a mobile phase that consisted of 10 mM ammonium phosphate (pH = 4.75), methanol, and acetonitrile (v:v:v, 78:16:6). Venlafaxine was used as the internal standard while hexane was used for the liquid–liquid extraction. The flow rate was 0.4 mL/min with fluorescence detection using an excitation wavelength of 230 nm and emission detection wavelength of 315 nm. This method was selective, linear and sensitive with a lower limit of quantification of 2.5 ng/mL for both cocaine and l-THP. The intra-day precision of cocaine and l-THP was <9.50% while the accuracy was <4.29%. The inter-day precision of cocaine and l-THP was <9.14%, and the accuracy was <12.49%. The recovery for cocaine and l-THP ranged from 43.95 to 50.02% and 54.65 to 58.31%, respectively. In comparison to forty reported cocaine quantitation methods this method is simple, sensitive and cost-effective and can be used for simultaneous quantitation of l-THP and cocaine. This method meets the FDA guidelines and can be used in current and future clinical studies.