Sensitive and rapid UHPLC–MS/MS assay for simultaneous quantifications of calcipotriol and paclitaxel in rat whole blood and plasma samples

Vitamin-D analogues have emerged as potential stroma-modulating agents for the treatment of pancreatic ductal adenocarcinoma (PDAC). One such agent, calcipotriol (Cal) has shown significant activity in in vitro and in vivo models of pancreatic ductal adenocarcinoma. Attempts in our lab have been focused on establishing the therapeutic merits of co-formulating this agent with the chemotherapeutic drug paclitaxel (PTX) in animal models. Accurate and reliable quantifications of these agents is critical to successful pharmacokinetic/pharmacodynamic (PK/PD) projections from animals into humans. Herein, we developed a LC–MS/MS assay for measuring Cal and PTX in whole blood and plasma. A liquid-liquid analyte extraction procedure, using a mixture of water-MeOH (50:50, v/v) and hexane-dichloromethane- isopropyl alcohol (150:15:5, v/v/v) was used. Chromatographic separation was carried out on Kinetex C18 column (1.7 μm, 100 × 2.10 mm) under an isocratic elution at a flow rate of 0.4 mL/min with a total runtime of 3.5 min. The mobile phase was composed of ammonium acetate (pH 6.51; 5 mM)-methanol (15:85, v/v). The analytes were ionized by positive electrospray ionization using API 5500-Qtrap triple quadrupole mass spectrometer (Applied Biosystem/AB SCIEX). The linearity of calibration curves for both analytes were established at 0.5 (LLOQ)-500 ng/mL with correlation coefficients exceeding 0.99. Spiked whole blood and plasma samples were used as surrogates for matrix validation. For both analytes, the intra-day and inter-day accuracies were 90.5–105 % and 96.6–106 %, respectively, while the corresponding precisions were 3.09–10.7 % and 5.20–12.9 %. No carryover was observed for the analytes which also remained acceptably stable in the surrogate matrices under relevant conditions. The assay is robust, reliable and sensitive in rat whole blood and plasma. The analytes extraction procedure performs acceptably well in both matrices with high recoveries and minimal matrix effects. Additionally, only 20 μL of rat whole blood or plasma is required and the total run time per sample is 3.5 min. PK studies enabled by the assay revealed that when co-administered, PTX AUC_0→∞ and C_max increased while those of Cal decreased. This finding alerts a potential drug-drug interaction and warrants further investigation in studies using this combination regimen.