Abstract 3733: Heregulin as a novel resistant factor for cetuximab

Abstract

Abstract Background Epidermal growth factor receptor (EGFR) is a therapeutic target in colorectal cancer (CRC). Patients with CRC with wild type KRAS have significant tumor regressions with cetuximab. However cancers ultimately acquire resistance. The mechanisms of these are still unknown. We investigated mechanisms of acquired resistance to cetuximab. Methods The cetuximab-resistance cell line, A431ER, was established from cetuximab-sensitive parent A431 by exposing cetuximab. In vitro sensitivity was analyzed using MTS assay. Expression of ErbBs and heregulin (HRG) was analyzed with RTK array, immunoblotting (IB) and ELISA. Plasma samples were studied using ELISA among 44 pts with CRC, treated with cetuximab. Result A431 was so sensitive to cetuximab that the growth inhibition following 10μg/ml cetuximab (GI10) was about 50% in MTS. In contrast, A431ER was completely resistance (GI10 <10%). The RTK array showed that A431ER but not A431 had markedly phosphorylated ErbB2 and 3. Although ErbB2 and 3 were highly phosphorylated in A431ER, total protein level of those was equivalent to A431 in IB. Finally heregulin, a ligand for ErbB3, was detected significantly higher in A431 than A431ER using IB and also ELISA in culture mediums (355 vs 1025pg/ml). HRG inhibition with siRNA could restore the sensitivity to cetuximab in A431ER. And exogenous HRG induced A431 parent cell line resistant to cetuximab. The exogenous HRG also phosphorylated both Erb2 and 3 in A431, which means hetero-dimerization of those. So we did MTS assay with cetuximab and pertuzumab, which inhibited hetero-dimerization between ErbB2 and 3. Cetuximab alone or pertuzumab alone minimally effected in A431ER (GI10 was about 10%). But the combination of two drugs abundantly effected (GI10 was about 60%). About clinical samples, pts with progressive disease (PD, n=23) following cetuximab had higher expression of HRG in plasma than pts with non-PD (n=15) (median 1188 vs 4854pg/ml, p=0.0008). And low HRG group (<2000pg/ml, n=22) had longer progression free survival (HR 0.2371 (95%CI 0.095-0.593), p=0.0021) and overall survival (HR 0.2274 (0.075-0.693), p=0.0092) than high HRG group (>2000pg/ml, n=22). Finally, post- treatment samples had significantly higher HRG than pre-treatment in 6 pts, who acquired resistance after 5 months treatment (mean 904 vs 1890pg/ml, p=0.0161). Conclusion Our findings suggest that HRG could lead both de novo and acquired resistance to cetuximab. HRG-ErbB3-ErbB2 complex is a warranted target for overcoming resistance to cetuximab. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3733.