Abstract B74: MET activation and Cetuximab resistance in patient-derived tumor xenografts

Abstract

Abstract Cetuximab (Erbitux), by targeting the epidermal growth factor receptor (EGFR), strongly inhibits tumor growth and is approved for the treatment of colon as well as head and neck cancer. Although expression of the EGFR is evident in a multitude of tumor histotypes, strong responses to therapy are only seen in selected patients and histotypes. In the present study we assessed the responsiveness to Cetuximab therapy given at 30mg/kg, qd7x3 and explored putative resistance mechanisms in 34 patient-derived xenografts established at Oncotest representing NSCLC, head and neck, gastric, colorectal and mammary cancer as well as melanoma. With a threshold of 35% as best T/C activity in vivo, 27 out of 34 tumor models (79.4%) were Cetuximab resistant despite strong inhibition of EGFR phosphorylation in tumors of treated animals. The mutational status for Kiras and B-raf was analyzed by exon sequencing with 10 and 4 out of 34 tumor models having Ki-Ras and B-Raf mutations, respectively. While all B-raf mutant tumors were resistant towards Cetuximab treatment, this was true for 8 out of 10 Ki-ras mutant tumors. As to EGFR family members, HER2 was highly expressed and activated only in the mammary adenocarcinoma MAXF1162, whereas phosphorylated HER3 was identified more broadly in Cetuximab-sensitive and resistant tumors. Interestingly, c-Met was highly activated only in the resistant LXFA526 and LXFA1647 lung adenocarcinoma. The function of c-Met was studied in more detail by siRNA-induced gene knock-down using the respective cell lines LXFA1647L and LXFA526L. In both lung adenocarcinoma, activation of c-Met was essential for anchorage-independent growth as analyzed using the tumor colony formation assay. Interestingly, LXFA526L cells with c-Met knock-down displayed EGF-dependent colony growth. As a further consequence of c-Met downregulation, EGFR phosphorylation was strongly reduced, indicating a functional interaction of c-Met and EGFR in LXFA526L cells, supported by co-immunoprecipitation of both receptors. In summary, constitutive activation of c-Met which is essential for oncogenic transformation in selected lung adenocarcinoma is a mechanism of primary Cetuximab resistance. Constitutively activated receptors like EGFR can support anchorage-independent, tumorigenic growth in vivo when c-Met is inhibited. Therefore, a combination therapy with c-Met and EGFR inhibitors (Cetuximab or a small molecule kinase inhibitor like Erlotinib) potentially have additive or even synergistic anti-tumor activity in c-Met and EGFR expressing NSCLC patients. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):B74.