Heat-stimulated nitroreductive bioactivation of the 2-nitroimidazole benznidazole in vitro

Effects of elevated temperature on misonidazole O-demethylation by mouse liver microsomes: kinetic and stability studies of a model mixed-function oxidase reaction

We investigated the effects of a range of temperatures (33 degrees-44 degrees C) on the stability and kinetics of C3H mouse liver microsomal misonidazole (MISO) O-demethylase in vitro. Microsomal O-demethylase activity was stable for 60 min at 37 degrees C and for 30 min at 41 degrees C but was steadily inactivated with longer incubation times. Inactivation at 44 degrees and 47 degrees C was exponential, with half-lives of 41 and 11 min, respectively. MISO O-demethylation followed Michaelis-Menten kinetics from 33 degrees to 44 degrees C. The apparent Vmax for desmethylmisonidazole (Ro 05-9963) formation was decreased by 32% (from 2.14 to 1.47 nmol min-1 mg-1 protein) with a 4 degrees decrease from 37 degrees to 33 degrees C. An increase of 4 degrees from 37 degrees to 41 degrees C enhanced the Vmax by 47%, but there was only an additional 9% increase for a further 3 degrees rise to 44 degrees C. Apparent Km values were unaltered at about 1.6 mM. These results show that elevated temperatures in the clinically relevant hyperthermia range (41 degrees-44 degrees C) can enhance a model mixed-function oxidase reaction in vitro. Such effects may be important for the metabolism, activity and toxicity of anticancer drugs combined with hyperthermia in vivo.

Effects of localised tumour hyperthermia on pimonidazole (Ro 03-8799) pharmacokinetics in mice

We have investigated the effects of localised tumour hyperthermia (LTH; 43.5 degrees C x 30 min) on the acute toxicity and pharmacokinetics of the hypoxic cell sensitizer pimonidazole (Ro 03-8799) in mice. There were three treatment groups: unrestrained controls, sham-treated and LTH treated mice. LTH had minimal effects on the acute toxicity (LD50/7d) of pimonidazole with no significant difference between the three treatment groups. Pharmacokinetic studies were carried out at the maximum tolerated dose (MTD; approximately 60% LD50) of 437 micrograms g-1 i.v. in plasma, brain and tumour. Sham tumour treatment consistently increased plasma drug concentrations compared to unrestrained controls but had minimal effects on the elimination t1/2. The AUC0-infinitive was increased by 35% and the plasma clearance decreased by 26%. By contrast, LTH had minimal effects on these parameters compared to sham treatment. Brain pimonidazole concentrations were increased in restrained mice (sham and LTH treatments) compared to unrestrained controls, but average brain/plasma ratios were similar in all three groups at between 400 and 500%. Sham tumour treatment markedly reduced peak tumour pimonidazole concentrations compared to unrestrained controls giving a 29% lower AUC0-180min. Average tumour/plasma ratios were reduced from 236 to 129%. The most important finding was that LTH further reduced pimonidazole tumour concentrations, giving a 31% lower AUC0-180 min compared to sham treated tumours. Tumour/plasma ratios for pimonidazole were reduced by 41%. Plasma exposure to the pimonidazole N-oxide metabolite, Ro 31-0313, was unaltered by LTH. The markedly reduced drug concentrations in heated tumours may be a result of hyperthermia-stimulated bioreductive drug activation.