Hyperthermia-enhanced effectiveness of mitoxantrone in an experimental rat tumour

The influence of local hyperthermia (HT) on Mitoxantrone (MITOX) effectiveness was studied in an experimental rat tumour. R-1 rhabdomyosarcomas were treated with MITOX (5 mg/kg ip), HT (43 degrees C for 1 h) or combinations applied at various time intervals up to 24 h. Tumour growth delay and tumour cell clonogenicity were assessed in correlation with the pharmacokinetics in blood plasma and with MITOX-concentrations in tumour tissue. Combined treatments were more effective than expected on the basis of simple addition of effects of single treatments. With increasing time intervals between treatments up to 8 h, an increase in effectiveness was observed. Unfortunately, treatment with an 8-h interval resulted in a high mortality: 80% of the rats died with 5-10 days after treatment. Treatment with a 3-h interval between MITOX and HT was the most effective combination resulting in the highest therapeutic ratio. Even local tumour controls (14/18 rats) were observed. These enhanced effects were associated with a higher MITOX-concentration in the fraction of intact cells recovered from tumours. However, no differences were observed in MITOX-concentration in total tumour tissue nor in plasma concentrations. In conclusion, timing between MITOX and HT is important for drug availability, for interaction of the two modalities to increase damage in tumour cells and for limiting the toxicity to normal tissues.