Analysis of 4-hydroxycyclophosphamide by gas chromatography-mass spectrometry in plasma

A sensitive and specific method for the quantitative analysis of 4-hydroxycyclophosphamide/aldophosphamide has been developed using gas chromatography-mass spectrometry and a deuterium-labeled analogue as the internal standard. The labile 4-hydroxycyclophosphamide/aldophosphamide and the internal standard in plasma were first converted into the more stable cyanohydrin adducts before extraction. The isolated adducts were silylated and the products analyzed by gas chromatography-mass spectrometry. The assay was found to be linear from 50 to 5000 ng/ml in plasma with a routine detection limit of 50 ng/ml. The within- and between-run standard deviations at 100 ng/ml on eight replicate determinations were found to be 6.2 and 11.9%, respectively. The extraction recovery was ca. 80%. This analytical method was used to evaluate the stability of 4-hydroxycyclophosphamide/aldophosphamide in fresh rat and pooled human plasma and to measure plasma 4-hydroxycyclophosphamide/aldophosphamide concentrations in the rat.