Bevacizumab, an anti-vascular endothelial growth factor (VEGF-A) antibody, is used in metastatic colorectal carcinoma (CRC) treatment, but responses are unpredictable. Vascular endothelial growth factor is alternatively spliced to form proangiogenic VEGF 165 and antiangiogenic VEGF 165 b. Using isoform-specific enzyme-linked immunosorbent assay and quantitative polymerase chain reaction, we found that over 90% of the VEGF in normal colonic tissue was VEGF xxx b, but there was a variable upregulation of VEGF xxx and downregulation of VEGF xxx b in paired human CRC samples. Furthermore, cultured colonic adenoma cells expressed predominantly VEGF xxx b, whereas colonic carcinoma cells expressed predominantly VEGF xxx . However, adenoma cells exposed to hypoxia switched their expression from predominantly VEGF xxx b to predominantly VEGF xxx . VEGF 165 b overexpression in LS174t colon cancer cells inhibited colon carcinoma growth in mouse xenograft models. Western blotting and surface plasmon resonance showed that VEGF 165 b bound to bevacizumab with similar affinity as VEGF 165 . However, although bevacizumab effectively inhibited the rapid growth of colon carcinomas expressing VEGF 165 , it did not affect the slower growth of tumours from colonic carcinoma cells expressing VEGF 165 b. Both bevacizumab and anti-VEGF 165 b-specific antibodies were cytotoxic to colonic epithelial cells, but less so to colonic carcinoma cells. These results show that the balance of antiangiogenic to proangiogenic isoforms switches to a variable extent in CRC, regulates tumour growth rates and affects the sensitivity of tumours to bevacizumab by competitive binding. Together with the identification of an autocrine cytoprotective role for VEGF 165 b in colonic epithelial cells, these results indicate that bevacizumab treatment of human CRC may depend upon this balance of VEGF isoforms.

Aims/hypothesis: Proliferative diabetic retinopathy results from excess blood vessel growth into the vit-