Quantitative determination of D₄-cystine in mice using LC-MS/MS and its application to the assessment of pharmacokinetics and bioavailability

Cystine is the primary source material for the synthesis of glutathione. However, the pharmacokinetics and tissue distribution of cystine are largely unknown. A surrogate analyte D₄-cystine was employed to generate calibration curves for the determination of levels of D₄-cystine and endogenous cystine in mice by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Validation assessments proved the sensitivity, specificity and reproducibility of the method with a lower limit of quantification (LLOQ) of 5 ng/mL over 5–5000 ng/mL in plasma. The pharmacokinetics of D₄-cystine were evaluated after administering injections and oral solutions, both of which minimally impacted endogenous cystine levels. The absolute bioavailability of cystine was 18.6%, 15.1% and 25.6% at doses of 25, 50 and 100 mg/kg, respectively. Intravenously injected D₄-cystine resulted in dramatically high plasma levels with reduced levels in the brain and liver. Intragastrically administered D₄-cystine resulted in high levels in the plasma and stomach with relatively low levels in the lung, kidney, heart and brain.