The possible role of chemotherapy in antiangiogenic drug resistance

The use of antiangiogenic drugs for cancer treatment was welcomed because of the hypothesis that they would be much less likely to lose their therapeutic activity as a result of tumor-acquired resistance over time. Unfortunately, the clinical experience has shown that acquired resistance to antiangiogenic therapeutic strategies is possible since many patients whose tumors initially respond to drugs such as bevacizumab (a monoclonal antibody against VEGF), sorafenib, or sunitinib (tyrosine kinase inhibitors targeting VEGF receptors and PDGF receptors) or metronomic chemotherapy (e.g. low dose cyclophosphamide) become nonresponsive, often within months of therapy initiation. Indeed, the role of associated antineoplastic chemotherapy in antiangiogenic resistance seems to be ignored by the previous studies and the real part played by these drugs has to be written yet. The studies undertaken on antiangiogenic resistance mainly involved mechanisms directly related to the antiangiogenic drugs alone and as such lead one to ask whether the acquired resistance to angiogenesis pathway-targeting might also be mediated by the chemotherapeutic drugs usually associated (at least into the clinic) with these types of drugs. The proposed hypothesis is concerning the possibility that the acquired resistance to antiangiogenic therapy could be actively and heavily modulated by the choice of the associated chemotherapeutic drug. The chemotherapeutic compounds may delay or accelerate the process through the induction, upregulation or downregulation of pro-angiogenic or anti-angiogenic factors or their receptors in the tumor, endothelial and other type of cells of the tumor microenvironment. In conclusion, the consequences of our hypothesis could be promptly translated into the preclinical studies and verified in clinical trials, involving cancer patients resistant to chemotherapy plus antiangiogenic drug schedules.