The influence of pre-treatment temperature on the thermal sensitivity of a mouse tumour

The influence of tumour temperature on ischemia-induced cell death: potential implications for the evaluation of vascular mediated therapies

We have evaluated the influence of tumour temperature on the kinetics and extent of tumour cell death following induction of ischemia. Induction of ischemia in SCCVII tumours implanted subcutaneously in the back of syngeneic C3H mice results in a rapid cooling of the tumour from a resting value of 35.2 degrees C, towards ambient temperature. In the SCCVII tumour no significant cell kill is detected in the first hour of ischemia. At longer times cell kill was detected and a survival level of 2 x 10(-2) was attained after 6 h of ischemia. However, if the tumours were maintained at a temperature of 37 degrees C following induction of ischemia a more rapid and dramatic reduction in cell survival is observed with a survival level of approximately 10(-5) being attained after 4 h of ischemia. Qualitatively similar results are obtained using the Lewis lung tumour implanted in C57BL mice. Similar rapid cooling following occlusion of the blood supply is observed in the C3H/TIF tumour implanted in the foot. For the C3H/TIF induction of ischemia for up to 6 h resulted in no significant growth delay provided the tumours were kept at room temperature. However, if the tumours were maintained at a temperature of 37 degrees C during the ischemic insult significant growth delay was observed for all clamping times exceeding 1 h. These results show the importance of tumour temperature on the kinetics and extent of tumour cell kill during ischemia. This finding has particular relevance for studies in which agents known to reduce tumour blood flow are used in animals bearing superficially located tumours.

Temperature differentials between treatment and pretreatment temperatures correlate with local control following radiotherapy and hyperthermia

PURPOSE

To evaluate the influence of pretreatment tumor temperatures and the temperature differential between treatment and pretreatment temperatures on local tumor control in patients who underwent combined radiation therapy and hyperthermia.

METHODS AND MATERIALS

Mapped intratumoral temperatures were measured immediately prior to and during hyperthermia in 138 hyperthermia fields among 59 patients with nodular (60 fields) or diffuse (78 fields) superficially-located tumors. In the nodular subgroup there were 40 fields with adenocarcinomas (31 breast, two prostate, seven other primary sites), six melanomas, nine squamous cell carcinomas, and five other histologies. The fields with diffuse tumor involvement consisted of 77 adenocarcinomas (67 breast, 10 other) and one melanoma. The maximum, minimum, and average temperatures were determined for both the pretreatment (pTmax, pTmin, pTave) and treatment (Tmax, Tmin, Tave) distributions and the differences, Dm = Tmin-pTmax, and Da = Tmin-pTave, computed. These quantities were averaged over treatments to produce the corresponding mean quantities for each hyperthermia field. Univariate and multivariate analyses were performed to determine treatment and pretreatment parameters which best correlated with the duration of local control.

RESULTS

Pretreatment tumor temperatures were significantly lower than the oral temperatures with mean pTmax, mean pTmin, and mean pTave of 36.2 degrees C, 34.2 degrees C, and 35.4 degrees C, respectively. For the adenocarcinomas with diffuse involvement within the hyperthermia field, the covariates best correlating with local control duration on univariate analysis were concurrent radiation dose (p = 0.0026), Dm (p = 0.009), pTmax (p = 0.012) and Da (p = 0.036). Lower pTmax and larger Dm and Da were predictive for longer local control. In multivariate analyses, all thermal parameters lost power, however, the best model included Dm which was significant at the p = 0.040 level. For the nodular subgroup, nonthermal parameters and dichotomized thermal parameters were of prognostic significance for local control.

CONCLUSION

For fields diffusely involved with adenocarcinoma significant correlations with duration of local control have been demonstrated both for a) low pretreatment temperatures and b) large differentials between treatment and pretreatment intratumoral temperatures. These correlations were also found in a dichotomized description for fields with nodular tumors. The results support the concept that pretreatment hypothermic conditions can lead to an increase in thermal sensitization and may help explain the excellent clinical results noted in the treatment of superficial tumors with radiation and hyperthermia. Further exploitation of this approach by planned cooling of superficially-located recurrent tumors prior to hyperthermia treatment warrants investigation.

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

Hyperthermia enhances nitroimidazole cytotoxicity, possibly through increased nitroreductive bioactivation. Using C3H/He mouse liver microsomes and KHT tumour homogenates, we have investigated the effects of temperature (33-44 degrees) on the anaerobic nitroreduction of benznidazole (BENZO) to its amine metabolite in vitro. Microsomal nitroreductase activity was unaltered after 2 hr anaerobic incubation at 37 and 41 degrees. However at 44 degrees and 47 degrees, inactivation occurred with half-lives of 68 and 17 min respectively. At 33 degrees microsomal reduction rates were 45% lower than at 37 degrees. Reduction rates were increased by 22% at 41 compared to 37 degrees, and by 0-54% depending on substrate concentration at 44 degrees. Microsomal amine formation followed Michaelis-Menten kinetics up to 41 degrees. The 4 degrees rise from 33 to 37 degrees increased the apparent Vmax by 45% (from 0.54 to 0.98 nmol min-1 mg-1 protein) with a further increase of 32% occurring at 41 degrees. Apparent Km values were unaltered. Deviation from Michaelis-Menten kinetics was seen for amine formation at 44 degrees. The kinetics of parent drug disappearance exhibited deviation from the Michaelis-Menten relationship at all temperatures studied. KHT tumour BENZO amine formation rates were also markedly increased at elevated temperatures, e.g. by 26% at 37 degrees compared to 33 degrees and by a further 35% from 42.5 to 57.4 pmol min-1 mg-1 protein over the range 37-41 degrees. In contrast to the microsomal results, tumour reduction rates were enhanced by an average of 54% (range, 26-79%) at 44 degrees compared to 37 degrees at low as well as high substrate concentrations. These results support the hypothesis that hyperthermia-enhanced nitroimidazole cytotoxicity may be a result of increased nitroreductive bioactivation.