PRECLINICAL PHARMACOKINETICS AND STABILITY OF ISOPHOSPHORAMIDE MUSTARD

Stability and preclinical pharmacokinetics of isophosphoramide mustard (IPM), an active metabolite of ifosphamide, were investigated using a nalytical methods developed in this laboratory. For stability evaluati on of IPM we used a rapid, high-pressure liquid chromatographic (HPLC) method by which IPM is analyzed directly from aqueous solutions witho ut derivatization on a 10-mu m C-18 reversed-phase column with theophy lline as the internal standard. IPM in sodium phosphate buffers was fo und to undergo pH-dependent first-order degradations. At pH 7.4 and 38 degrees C, the IPM solution showed a half-life of 45 min. A gas chrom atographic-mass spectrometry (GC/MS) method for the analysis of IPM in plasma was also developed. This method utilized solid-phase extractio n with deuterium-labeled IPM as the internal standard. The routine det ection limit for the assay was 50 ng/ml with within-run and between-ru n coefficients of variation of 6% and 11%, respectively. By this metho d, stability of LPM in plasma and in RPMI 1640 tissue culture medium w as evaluated, and its pharmacokinetics in the Sprague-Dawley rat follo wing i.v. administration at 40 mg/kg were investigated. IPM was found to be more stable in these media, with half-lives in the range of 100 min. IPM plasma pharmacokinetics were found to decline monoexponential ly with terminal half-lives ranging from 6.8 to 18.7 min and total cle arance between 6.0 and 18.3 ml/min. Plasma protein binding of IPM was found to be 55%, and the partition ratio between plasma and red blood cells of 4.9 to 1, respectively. Cytotoxicity of IPM to L1210 cells wa s evaluated, and the results indicated that the IC50 with 1-h and 4-h exposure was 33 and 15 mu M, respectively. Based on these data, IPM pl asma levels in the rat declined below the IC50 in about 1 h at this do se. More frequent dosing or infusion may be necessary to maintain adeq uate drug levels for antitumor activity when IPM is administered direc tly.