LC-MS bioanalytical method for simultaneous determination of latanoprost and minoxidil in the skin

The association of minoxidil sulphate and latanoprost is currently emerging as a promising strategy for the treatment of androgenic alopecia, which is the most common type of scalp hair loss. In order to support the development of new pharmaceutical products containing such drugs combination, this study proposes a simple and efficient LC-MS bioanalytical method to simultaneously quantify minoxidil sulphate and latanoprost in different skin layers. Compounds separation was performed by liquid chromatography using a C₁₈ column with gradient elution of a mobile phase composed of 0.1 % formic acid in acetonitrile and water at a flow rate of 0.5 mL min^-1. Determinations were executed using mass spectrometry equipped with an ESI interface operating in a positive ionization mode. Quantification was performed using selective ion mode monitoring of m/z 210.1 for minoxidil sulphate and 433.3 for latanoprost. The matrix effect was very pronounced in samples containing some skin layers or electrolyte solution. Accordingly, a calibration curve for each contaminant group was built, leading to correlation coefficient values higher than 0.99. Additionally, lower limits of detection and quantification were obtained, and precision (repeatability and intermediate precision) achieved results with a coefficient of variation less than 15 %. Drug recovery from skin samples was higher than 70 %, fulfilling the recommendations. Also, the bioanalytical method was successfully tested in in vitro skin penetration studies proving its effectiveness in the development of topical formulations containing both drugs.