Fenton, B. M. and Paoni, S. F. Alterations in Daily Sequencing of Axitinib and Fractionated Radiotherapy do not Affect Tumor Growth Inhibition or Pathophysiological Response. Radiat. Res. 171, 606–614 (2009).
A variety of antiangiogenic strategies have proven effective in preclinical tumor models, either as single agents or in combination with radiation. Clinical gains have been relatively modest, however, and questions remain regarding optimal scheduling. The objectives of the current work were to evaluate whether the sequencing of acute treatment critically affects tumor pathophysiological and therapeutic response. Axitinib (Pfizer Global Research & Development), an inhibitor that predominantly targets vascular endothelial growth factor receptors, was administered either before or after each daily radiation fraction in two human prostate xenograft tumor models. Tumors were frozen at sequential times to monitor changes in (1) vascular spacing, (2) pericyte and basement membrane coverage, and (3) hypoxia. Although similar reductions in blood vessel counts were observed with each tumor model, tumor vasculature was not functionally normalized. Instead, tumor hypoxia increased, accompanied by a progressive dissociation of pericytes and basement membranes. Ultimately, tumor growth inhibition was found to be equivalent for each of the combination schedules. These studies illustrate a clear advantage to combining axitinib with fractionated therapy but argue against an acute radiosensitization or radioprotection of either the tumor cells or tumor vasculature. Instead, post- and preirradiation daily drug administration serve equally well in supplementing the response to radiotherapy.