Microenvironmental transformations by VEGF- and EGF-receptor inhibition and potential implications for responsiveness to radiotherapy.

The microregional distribution and dynamics of tumor cell hypoxia and proliferation are important determinants of tumor aggressiveness and resistance to treatment. Modulation of these elements by biological targeted drugs such as EGFR- and VEGFR-inhibitors may improve the effect of radiotherapy significantly. These combinations are being evaluated in clinical trials and evidence of their effectiveness is accumulating. However, the mechanistic basis of this cooperative effect and the role and behavior of the microregional tumor phenotype under EGF- and VEGF-blockage is poorly understood. Unfolding of these interactions and effects further downstream is necessary to exploit these biological modifiers most profitably to unravel questions such as: (1) can microregional phenotypes be modulated by EGFR- or VEGFR-blockage and how do downstream effects in the signaling pathways relate to these changes? (2) How do the microregional changes induced by EGFR- and VEGF-blockage affect the responsiveness of tumors to ionizing radiation? Answering these questions will improve our understanding of tumor growth related phenotypic transformations at the microregional level and how these can be influenced by modulation of the EGF- and VEGF-signaling pathways. This knowledge can be used to identify and improve therapeutic combinations with the novel biological modifiers and test a variety of biological-based treatment approaches.