Current status of angiogenesis inhibitors combined with radiation therapy

Angiogenesis inhibitors combined with cytotoxic chemotherapy have recently entered routine oncological practice. Several rationales exist for combining these agents with ionizing radiation, a primary curative cancer treatment, either in bimodal or trimodal fashion, i.e. with or without additional chemotherapy. More than 20 different anti-angiogenic agents have been studied in preclinical animal tumor models. This systematic review compares the results of preclinical studies published before February 2006. The combination of vascular endothelial growth factor (VEGF) inhibitors with irradiation consistently resulted in improved tumor growth delay (at least additive effects), despite different radiation schedules, drugs and doses, and combination regimens. Only two studies evaluated tumor control dose (TCD)₅₀ as a measure of tumor cure (radiation dose yielding permanent local control in 50% of the tumors). While anti-VEGF receptor (VEGFR) antibody treatment improved the outcome, a VEGFR tyrosine kinase inhibitor showed negative results. For agents interfering with other pathways, the results are also not consistent, although most studies were positive. Trimodal approaches seem to improve tumor growth delay even further. Importantly, both radiotherapy schedule and sequence of the modalities in combined treatment may impact on the outcome. Hence, further preclinical studies examining these parameters need to be conducted. While preclinical research is ongoing, phase I and II clinical trials with bevacizumab, combretastatin A-4, thalidomide and different receptor tyrosine kinase inhibitors, usually combined with radio- and chemotherapy, have been designed. Early results suggest that acute toxicity is acceptable, planned surgery after such treatment is feasible, and that further evaluation of such combined modality treatment is warranted.