Development and Validation of S-Allyl-L-Cysteine in Rat Plasma Using a Mixed-Mode Reversed-Phase and Cation-Exchange LC-ESI-MS/MS Method: Application to Pharmacokinetic Studies

S-Allyl-l-cysteine (SAC), the most abundant organosulfur compound derived from garlic, has multifunctional biological activities that occur via different mechanisms. A sensitive, rapid and simple LC–ESI–MS/MS method using a mixed-mode reversed-phase and cation-exchange column containing C18 silica particles and sulfonic acid cation-exchange particles has been developed and validated for the analysis of SAC in rat plasma. The mobile phase was optimized at 2 mM ammonium acetate buffer (pH = 3.5) and acetonitrile (75:25, v/v). The assay utilized 0.6% acetic acid in methanol to achieve simple and rapid deproteinization. Quantification was conducted using multiple reaction monitoring (MRM) of the transitions of m/z 162.0 → 145.0 for SAC. The standard curve for SAC was linear (r2 ≥ 0.999) over a range from 5 to 2,500 ng/mL. The intra- and interday precision (relative standard deviation) of the method was not >6.0% at three quality control levels. The limit of quantification (LOQ) was 5.0 ng/mL. After being fully validated, the method was successfully applied to the pharmacokinetic monitoring of SAC in rat plasma.