The effects of whole body hyperthermia on the pharmacokinetics and toxicity of the basic 2-nitroimidazole radiosensitizer Ro 03-8799 in mice

Abstract

Summary We have investigated the effects of 50 min whole-body hyperthermia (WBH; 15 min equilibration followed by 41°C for 35min) on the toxicity and pharmacokinetics of the radiosensitizer Ro 03-8799 in mice.WBH markedly reduced Ro 03-8799 LDSO,7d from 779 to 259 igg-1 (P<0.001). Pharmacokinetics were studied at 175 pgg 1 (-0.6WBHLDSO57d) with and without heat and 437,igg-1 (-0.6 control LDSO/7d) without heat. WBH increased Ro 03-8799 plasma concentrations and prolonged its elimination t112 by 26% (P<0.01). Total plasma area under the curve (AUC,,_-) was increased by 22%. but was still <50% of the unheated high-dose value. Ro 03-8799 concentrated 300-400% in tumour and brain relative to plasma. Absolute tumour and brain levels were unaltered by WBH, giving reduced tissue/plasma ratios. WBH greatly inhibited glomerular filtration (51Cr EDTA clearance) during heating, contributing to the increased plasma Ro 03-8799 concentrations. WBH increased peak plasma concentrations of the Ro 03-8799 N-oxide metabolite Ro 31-0313 by 61%, and the fl-phase AUC of i.v. administered Ro 31-0313 by 36%. Since Ro 31-0313 levels were increased to a greater extent after Ro 03-8799 and WBH than Ro 31-0313 and WBH, WBH must both increase metabolite production and decrease its plasma clearance. WBH had no effect on Ro 31-0313 tumour concentrations or its exclusion from brain. These complex effects of WBH on Ro 03-8799 pharmacokinetics may contribute to the enhanced toxicity, possibly through hyperthermia-stimulated bioreductive drug activation, but do not wholly explain it.The curability of many tumours by radiotherapy may be limited by the presence of radioresistant hypoxic tumour cells. Electron affinic compounds such as the nitroimidazoles act as hypoxic cell radiosensitizers and are also preferentially cytotoxic towards these cells (Adams et al., 1976). Hyperthermia enhances the tumour cytotoxicity of the 2-nitroimidazole radiosensitizer misonidazole (MISO; 1 -(2-nitroimidazol-1 -yl)-3-methoxy-2-propanol) both in vitro (Stratford and Adams, 1977) and in vivo (Bleehen et al., 1977;George et al., 1977) but also increases its acute lethality in mice (Overgaard, 1979).MISO is likely to produce sub-optimal radiosensitization in man because dose-limiting neurotoxicity limits achievable tumour concentrations (see Workman, 1983). Recent studies have shown that 2-nitroimidazoles substituted with basic side chains are more potent radiosensitizers than MISO in vitro (Smithen et al., 1980), with shorter half-lives and hence lower tissue exposures in vivo (Williams et al., 1982 Materials and methodsMice and tumours Adult C3H/He mice were obtained from our own breeding colony and from Olac Ltd (Bicester, UK). Males were used in most experiments but females were used occasionally. Mice were allowed food (PRD nuts; Labsure, Poole, Dorset, UK) and water ad lib, and were used at 25-35 g body weight.The KHT sarcoma was grown in the gastrocnemius muscle of the hind leg as previously described (Twentyman et al., 1979). Mice were treated bearing tumours...

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

“…Brain pimonidazole concentrations reached a peak of around 340 ugg-1 in both unrestrained controls and mice with sham-treated tumours, though it was much broader in the latter group (Figure 2b) and LTH treated mice respectively, values in good agreement with previous work (Walton et al, 1987a (Figure 2c). …”

“…Deaths were either rapid and convulsive, occurring during or immediately after drug injection and before jig loading, or non-convulsive and occurring during the subsequent 24 h. No deaths resulted during the period of either LTH or sham tumour treatments. Subsequent pharmacokinetic studies were carried out at the previously determined maximum tolerated dose (-60% LD50/7d) in unrestrained control mice of 437 pg g-(see Walton et al, 1987a) (Gomer & Johnson, 1979), and the dose of pimonidazole used in these LTH experiments has previously been shown to decrease mouse rectal temperature by 2-3°C for up to 1 h after administration (Walton et al, 1987a 37CC, and were consistently I°C higher than in unrestrained control mice given this dose of pimonidazole alone (Walton et al, 1987a). (with 95% confidence limits) of 31.3 (28.9-34) min.…”

“…Acute toxicity The effect of LTH on the acute toxicity (LDIo/7d) of pimonidazole was determined using tumour-bearing animals as described elsewhere (Walton et al, 1987a). Mice were allocated to one of the three different protocols described above.…”

“…Sample analysis Samples were prepared and analysed as described in detail elsewhere (Walton et al, 1987a). Briefly, whole blood was removed from anaesthetised animals and plasma obtained by centrifugation.…”

“…Drug concentrations were assayed by reverse-phase, paired-ion, isocratic HPLC using a modified version of the method of Malcolm et al (1983) as described elsewhere (Walton et al, 1987a Wagner, 1975) were calculated as described in detail elsewhere (White & Workman, 1980;Workman & Brown, 1981) …”