A Comprehensive Gene Expression Analysis of Resistance Formation upon Metronomic Cyclophosphamide Therapy.

Resistance formation is one of the major hurdles in cancer therapy. Metronomic anti-angiogenic treatment of xenografted prostate cancer tumors in severe combined-immunodeficiency (SCID) mice with cyclophosphamide (CPA) results in the appearance of resistant tumors. To investigate the complex molecular changes occurring during resistance formation, we performed a comprehensive gene expression analysis of the resistant tumors in vivo. We observed a multitude of differentially expressed genes, e.g., PAS domain containing protein 1, annexin A3 (ANXA3), neurotensin, or plasminogen activator tissue (PLAT), when comparing resistant to in vivo passaged tumor samples. Furthermore, tumor cells from in vivo and in vitro conditions showed a significant difference in target gene expression. We assigned the differentially expressed genes to functional pathways like axon guidance, steroid biosynthesis, and complement and coagulation cascades. Most of these genes were involved in anti-coagulation. Up-regulation of anticoagulatory ANXA3 and PLAT and down-regulation of PLAT inhibitor serpin peptidase inhibitor clade A were validated by quantitative real-time polymerase chain reaction. In contrast, coagulation factor F3 was upregulated, accompanied by the expression of an altered gene product. These findings give insights into the resistance mechanisms of metronomic CPA treatment, suggesting an important role of anti-coagulation in resistance formation.