Analysis of cyclophosphamide and carboxyethylphosphoramide mustard enantiomers in human plasma and application to clinical pharmacokinetics

This study describes for the first time a method for the sequential analysis of the enantiomers of cyclophosphamide (CY) and its metabolite carboxyethylphosphoramide mustard (CEPM) in human plasma. The CY and CEPM enantiomers were extracted from plasma using only ethyl acetate and separated on a Chiralpak® AD-RH column using a mixture of water:acetonitrile:ethanol (45:30:25, v/v/v) plus 0.1% trifluoroacetic acid as the mobile phase at a flow rate of 0.5 mL/min. No matrix effect was observed in the analysis of the enantiomers of both analytes and the analytical method was linear in the range of 0.05-25.0 μg and 250-1000 ng of each enantiomer/mL plasma. The coefficients of variation and relative errors obtained for the assessment of intra- and interassay precision and accuracy were less than 15%. CY and CEPM were found to be stable in human plasma after three successive freeze/thaw cycles, during storage for 4 h at room temperature, and after 24 h inside the autosampler at 4 °C, with deviations less than 15%. The method was applied to the study of the pharmacokinetics of CY and its metabolite CEPM in patients with multiple sclerosis (n = 10) who received a CY pretransplant conditioning regimen for hematopoietic stem cell transplantation. The pharmacokinetic parameters showed plasma accumulation of the (S)-(−)-CY enantiomer (S/R ratio = 1.3) and lack of enantioselective exposure to the CEPM metabolite (S/R ratio = 1.0).