Quantification of Tacrolimus in Human Skin Samples and Ointment by LC-MS

A sensitive and simple liquid chromatography-mass spectrometry method was developed to determine the immunosuppressant tacrolimus in human skin samples after treatment with the commercially available ointment. Utilizing diffusion cell experiments according to Franz, human skin samples were treated with ointment containing tacrolimus and the extraction procedure of the drug was optimized. The analytical assay was performed using an LC system consisting of a reversed phase C-18 column and an isocratic mobile phase, coupled with electrospray ionization mass spectrometry. The detection was performed in the positive selected ion monitoring mode. Mycophenolate mofetil was used as internal standard to control the stability of the electrospray ionization. The calibration curve was linear for tacrolimus over the range of 5-1,000 ng mL(-1) (average correlation coefficient of r (2) = 0.9941) with a limit of detection (LOD) of 2 ng mL(-1), with a limit of quantification (LOQ) of 5 ng mL(-1) and with a precision of 8.70%. The analytical assay described in this paper was successfully applied in order to quantify tacrolimus in human skin samples as well as in the commercially available ointment.