EGFR and VEGF(R) signaling show extensive cross-talk providing a rational for joint targeting of the two pathways. However, combinations of monoclonal antibodies (mAbs) targeting EGFR and VEGF showed disappointing activity in patients with colorectal cancer (CRC). We speculated that inhibition of surface receptors and ligands might only partly prevent oncogenic signaling whereas small molecule tyrosine kinase inhibitors (TKIs) would also influence intracellular signaling.

Mice with CRC xenografts were treated with two TKIs, vargatef and afatinib, or with two mAbs, bevacizumab and cetuximab, and their influence on tumor growth, viability, in vivo DNA synthesis and the presence of phosphorylated EGFR and VEGFR was determined. The activity of the TKIs was further characterized in CRC cells with different KRAS status.

Vargatef and afatinib together showed strong tumor growth inhibition toward HT-29 xenografts, compared to either drug alone, which was associated with a 5-fold increase in apoptotic tumor cell death. In comparison, bevacizumab and cetuximab together were exclusively cytostatic with no more activity than either drug alone. Exposure to the two TKIs was accompanied by a marked decrease of tumor-associated intracellular phospho-VEGFR1 and phospho-EGFR while similar exposure to the two mAbs had no detectable effect. A synergistic activity of vargatef plus afatinib was observed in all eight CRC cell lines examined, independent of KRAS status.

Our results indicate that attenuation of intracellular EGFR and/or VEGF signaling is required for cytotoxic activity. These findings provide a rational for trials of the TKIs, even in patients with mutant KRAS.

PMID: 21880790 [PubMed - as supplied by publisher] Source: National Library of Medicine.