Background and purpose: As low-dose metronomic cyclophosphamide (CTX) and hyperthermia (HT) both exert antitumour effects in part through antiangiogenic mechanisms, interactive effects of the two modalities were explored.
Materials and methods: Subcutaneously implanted rat tumours (BT4An) were treated with CTX 35 mg/kg i.p. three doses a week for two weeks, local water-bath HT yielding mean tumour temperature of 43 degrees C for one hour at day 0, both modalities combined (CTX-HT(0)), or saline. TUNEL assays, immunohistochemical staining of thrombospondin 1 (TSP-1) and real time RT-PCR of TSP-1 mRNA were analysed the first three hours after completed treatment day 0.
Results: Metronomic dosed CTX (p=0.006) and HT (p<0.001) both delayed tumour growth. The combined regimen was superior to either modality alone (p<0.001). Complete tumour regressions were observed in CTX (6%), HT (12%), and CTX-HT(0) (41%) treated rats. TSP-1 protein was specifically upregulated in the vascular matrix of tumours receiving CTX (weak), HT (moderate) and CTX-HT(0) (strong). In contrast, reduced expression of TSP-1 protein was observed in tumour cells after HT alone and CTX-HT(0). TUNEL assays indicated induction of apoptosis by HT and CTX-HT(0) 90 minutes after end of the first treatment.
Conclusion: A single session of local HT enhances the effects of low-dose metronomic CTX, possibly in part mediated through a differential effect on TSP-1 protein levels in tumour cells and tumour vasculature.