Highly Sensitive LC–MS/MS Method for Determination of Dexamethasone in Rat Plasma and Brain Tissue: An Application to Pharmacokinetic Study in Rats

A highly sensitive and rapid LC-MS/MS method was developed and validated for the quantification of dexamethasone in rat plasma and brain tissue. Protein precipitation method was used for sample preparation. The separation of dexamethasone and the IS (labetalol) was achieved on an Atlantis dC₁₈ column using an isocratic mobile phase (10 mM ammonium formate and acetonitrile, 25/75, v/v) delivered at 0.7 mL/min flow-rate. Dexamethasone and the IS were eluted at 1.03 and 1.06 min, respectively. The MS/MS transitions monitored were m/z 393.100 → 373.100 (dexamethasone) and 329.100 → 91.100 (IS). Method validation was performed as per FDA guidelines and all parameters met the acceptance criteria. The assay was validated with a quantification range of 0.05-1046 ng/mL in both matrices. The intraday and interday precision for were in the range of 2.62-7.28 and 2.76%-6.98% and 2.24-6.85 and 2.97%-6.37%, in plasma and brain tissue, respectively. Dexamethasone was stable in a series of stability conditions in both matrices. Post-intravenous administration to rats, dexamethasone concentrations in plasma and brain tissue were quantifiable up to 24 and 10 h, respectively. Dexamethasone half-life was ~2.30 h. Dexamethasone exhibited low clearance and moderate volume of distribution in plasma but in brain tissue the clearance and volume of distribution were high.