Larry Witte
ImClone Systems Incorporated, New York, NY

Anti-Angiogenic and Anti-Tumor Activities of Antibodies Against VEGFR1 and VEGFR2

Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis and tumor growth. Two of its receptors, VEGFR2, also known as the kinase insert domain-containing receptor (KDR) and flk1 in mouse, and VEGFR1, also known as fms-like tyrosine kinase (flt1), were shown to be expressed on endothelial cells and hematopoietic cells. The level of the VEGFR2 (flk1/KDR) receptor has been shown to be up-regulated by VEGF in several tumors and confined to the vascular endothelial cells in close proximity to the tumors. These and numerous other studies suggest that the flk1/KDR/VEGF pathway plays an important role in tumor angiogenesis. This role is further supported by studies showing inhibition of tumor growth in nude mice by anti-VEGF antibodies, anti-flk1 antibodies, anti-VEGF antisense RNA expression, and VEGF-diphtheria toxin conjugate. We produced a rat anti-flk1 monoclonal antibody, DC101, and demonstrated that this antibody could specifically inhibit VEGF stimulation of flk1 receptor. Furthermore, DC101 has been shown to strongly inhibit tumor growth in numerous mouse tumor models. We have also produced anti-KDR (human form of the receptor) antibodies, including ones that block VEGF binding to KDR and that show potent inhibition of VEGF-induced receptor phosphorylation and proliferation of human umbilical vein endothelial cells. We have completed a Phase 1 clinical trial in colorectal cancer with one of these antibodies, IMC-1C11.

VEGFR1/flt1 is stimulated by VEGF and by placental growth factor (PlGF). VEGFR1 is expressed on endothelial cells, smooth muscle cells, monocytes, and some tumor types. We recently reported that VEGFR1 is also expressed in human breast cancer. VEGFR1 expression was detected with high frequency (55%) in breast tumor specimens by immuno-histochemical staining and was also detected in 10 of 12 human breast cancer cell lines. A neutralizing monoclonal antibody specific for human VEGFR1 (6.12) blocked PlGF-stimulated growth of DU4475 breast tumor cells and downstream signaling to MAP kinase. The growth of DU4475 xenograft tumors in nude mice was strongly inhibited by treatment with 6.12. Combination treatment of DU4475 tumor bearing animals with mAb 6.12 and a neutralizing anti-mouse VEGFR1 monoclonal (MF1), which is against host angiogenesis, resulted in a stronger anti-tumor response than treatment with either antibody alone. The anti-tumor effect of MF1 was also tested in the VEGFR1 positive 4T1 mouse metastatic breast tumor model where it significantly inhibited primary tumor growth (63%) and growth of pulmonary metastases (67%). These data suggest that anti-VEGFR1 antibodies may interfere with autocrine ligand stimulation of VEGFR1 positive tumor cells in addition to exerting a direct anti-angiogenic activity on VEGFR1-expressing tumor endothelium. Thus, anti-VEGFR1 antibodies may provide a new therapeutic strategy to treat tumors that express VEGFR1.