Effects Of Temperature On The Therapeutic Efficacy And Pharmacokinetics Of Ifosfamide

The influence of tumor temperature (28, 32, 37, 39, 41, or 43-degrees-C for 1 h) on the therapeutic efficacy of i.v. single bolus injections of ifosfamide (IFO) (32, 65, 125, or 25 mg/kg body weight) in human tumor xenografts (MX1 breast carcinoma) grown in nude mice (n = 240) was studied. Tumor temperature was controlled by water bath immersion. Sixty days after treatment the percentage of tumor-free survival was determined. For example, at 37-degrees-C IFO in a dose of 6S mg/kg body weight led to 10% tumor-free survival in the treated animals. At 43-degrees-C the same dose resulted in 60% tumor-free survival. A clear drug dose- and temperature-dependent increase of the therapeutic efficacy of an active oxazaphosphorine compound was also demonstrated in vitro. The concentrations of IFO and of 4-hydroxyifosfamide in blood and tumors at different body temperatures (controlled by water bath immersion) were determined over 120 min and WBC counts were obtained. The half-lives and the areas under the curve for IFO in blood were not significantly different at 37-degrees-C and 41-degrees-C. Since the half-life of IFO depends mainly on hepatic metabolism, the similarity of half-lives and of areas under the curve for IFO at 37-degrees-C and 41-degrees-C indicates a constant activation rate. However, significantly lower plasma concentrations of the activated drug at a liver (body) temperature of 41-degrees-C, compared with 37-degrees-C, were found, indicating a higher elimination rate. The concentration of the activated drug in the tumors within the initial 60 min at 41-degrees-C, however, exceeded by >2-fold that at 37-degrees-C. The bone marrow toxicity of the same drug dose did not significantly increase with body temperature.