Overcoming acquired resistance to cetuximab by dual targeting HER family receptors with antibody-based therapy

Background

Cetuximab, an anti-EGFR monoclonal antibody, is used to treat several cancers. However, many patients who initially respond to cetuximab acquire resistance. To examine mechanisms of acquired resistance, we developed a series of cetuximab-resistant (Ctx^R) clones derived from the cetuximab sensitive (Ctx^S) non-small cell lung cancer (NSCLC) cell line H226. Previous studies characterizing this model revealed that: 1) EGFR was robustly overexpressed in Ctx^R clones due to decreased EGFR ubiquitination and degradation and 2) Ctx^R clones expressed increased HER2 and HER3 activation resulting in constitutive activation of the PI3K/AKT signaling axis. These findings suggest that dual targeting HER family receptors would be highly beneficial in the Ctx^R setting.

Results

Since HER3 has been implicated in resistance to EGFR inhibitors, the efficacy of dually targeting both EGFR and HER3 in Ctx^R models was evaluated. First, EGFR and HER3 expression were knocked down with siRNAs. Compared to the Ctx^S parental cell line (HP), all Ctx^R clones exhibited robust decreases in cell proliferation upon dual knockdown. Analysis of Ctx^R clones indicated that neuregulin-1 was highly overexpressed compared to HP cells. Incubation of HP cells with neuregulin-1 rendered them resistant to cetuximab. Next, dual treatment of Ctx^R clones with cetuximab and the HER3 neutralizing monoclonal antibody (mAb) U3-1287 led to potent anti-proliferative effects. Blockade of EGFR with cetuximab resulted in inactivation of MAPK, while blockade of HER3 with U3-1287 resulted in the inactivation of AKT. Treatment with both mAbs resulted in knockdown of both signaling pathways simultaneously. HER2 was also strongly inactivated upon dual mAb therapy, suggesting that this treatment regimen can diminish signaling from three HER family receptors. De novo Ctx^R H226 mouse xenografts were established to determine if dual therapy could overcome acquired resistance to cetuximab in vivo. Tumors that had acquired resistance to cetuximab were significantly growth delayed upon dual treatment of U3-1287 and cetuximab compared to those continued on cetuximab only. Combinatorial-treated xenograft tumors expressed decreased Ki67 and increased cleaved caspase-3 levels compared to tumors treated with either monotherapy.

Conclusions

These studies demonstrate that dually targeting HER family receptors with antibody-based therapies can overcome acquired resistance to cetuximab.

Targeting AKT with the allosteric AKT inhibitor MK-2206 in non-small cell lung cancer cells with acquired resistance to cetuximab

The epidermal growth factor receptor (EGFR) is a central regulator of tumor progression in human cancers. Cetuximab is an anti-EGFR monoclonal antibody that has been approved for use in oncology. Despite clinical success the majority of patients do not respond to cetuximab and those who initially respond frequently acquire resistance. To understand how tumor cells acquire resistance to cetuximab we developed a model of resistance using the non-small cell lung cancer line NCI-H226. We found that cetuximab-resistant (Ctx (R) ) clones manifested strong activation of EGFR, PI3K/AKT and MAPK. To investigate the role of AKT signaling in cetuximab resistance we analyzed the activation of the AKT pathway effector molecules using a human AKT phospho-antibody array. Strong activation was observed in Ctx (R) clones for several key AKT substrates including c-jun, GSK3β, eIF4E, rpS6, IKKα, IRS-1 and Raf1. Inhibition of AKT signaling by siAKT1/2 or by the allosteric AKT inhibitor MK-2206 resulted in robust inhibition of cell proliferation in all Ctx (R) clones. Moreover, the combinational treatment of cetuximab and MK-2206 resulted in further decreases in proliferation than either drug alone. This combinatorial treatment resulted in decreased activity of both AKT and MAPK thus highlighting the importance of simultaneous pathway inhibition to maximally affect the growth of Ctx (R) cells. Collectively, our findings demonstrate that AKT activation is an important pathway in acquired resistance to cetuximab and suggests that combinatorial therapy directed at both the AKT and EGFR/MAPK pathways may be beneficial in this setting.

Nuclear EGFR as a molecular target in cancer

The epidermal growth factor receptor (EGFR) has been one of the most targeted receptors in the field of oncology. While anti-EGFR inhibitors have demonstrated clinical success in specific cancers, most patients demonstrate either intrinsic or acquired resistance within one year of treatment. Many mechanisms of resistance to EGFR inhibitors have been identified, one of these being attributed to alternatively localized EGFR from the cell membrane into the cell's nucleus. Inside the nucleus, EGFR functions as a co-transcription factor for several genes involved in cell proliferation and angiogenesis, and as a tyrosine kinase to activate and stabilize proliferating cell nuclear antigen and DNA dependent protein kinase. Nuclear localized EGFR is highly associated with disease progression, worse overall survival in numerous cancers, and enhanced resistance to radiation, chemotherapy, and the anti-EGFR therapies gefitinib and cetuximab. In this review the current knowledge of how nuclear EGFR enhances resistance to cancer therapeutics is discussed, in addition to highlighting ways to target nuclear EGFR as an anti-cancer strategy in the future.

Abstract 4276: Mapping C-terminal transactivation domains of nuclear HER family receptor tyrosine kinases

The HER family of receptor tyrosine kinases consist of four receptors, epidermal growth factor receptor (EGFR/ErbB1/HER1), HER2 (ErbB2), HER3 (ErbB3), and HER4 (ErbB4). Collectively, this family of receptors plays a critical role in initiating proliferation and survival signals in several human cancers. It is well established that the HER family receptors rely on two distinct compartments of signaling: 1) Classical membrane bound signaling and 2) nuclear signaling. In the nucleus, HER family receptors can serve as co-transcription factors, mediated by their C-terminal domains, to promote transcription of several genes essential for cell proliferation and cell cycle regulation, including regulation of the gene cyclin D1. However, the domains within the C-terminus of EGFR, HER2 and HER3 that confer this transcriptional potential (transactivation domains) have yet to be defined.

In the current study we aimed to minimally map the regions of the C-terminal domains of EGFR, HER2 and HER3 that function as transactivation domains. We first demonstrate that EGFR, HER2, and HER3 are nuclear localized in their full-length forms in various breast and lung cancer cell lines. Next, we fused various intracellular cytoplasmic domain regions of each receptor to the DNA binding domain of the yeast transcription factor Gal4, and measured the ability for each construct to transactivate Gal4 UAS-luciferase activity. This analysis demonstrated that the C-terminal region distal to the tyrosine kinase domain (CTD) of all HER family receptors have strong transactivation potential, with HER2 exhibiting the highest transactivation potential. Further deletion mapping analysis of each receptor's CTD identified two regions (bipartite) of approximately 25-40 amino acids in length that harbored the majority of the receptors transactivation potential. To understand how the identified bipartite C-terminal regions of each HER family receptor influenced their transcriptional functions we deleted these regions from each full-length receptor and performed cyclin D1 promoter-luciferase assays. While wild type EGFR, HER2, and HER3 overexpression was capable of transactivating cyclin D1-luciferase, bipartite deleted receptors were severely hindered in their ability to regulate the cyclin D1 promoter. Collectively, the findings presented herein suggest that the EGFR, HER2 and HER3 contain a bipartite C-terminal transactivation domain that may be responsible for their ability to function as co-transcription factors.

Citation Format: Toni M. Brand, Mari Iida, Matthew J. Wleklinski, Neha Luthar, Megan M. Starr, Deric L. Wheeler. Mapping C-terminal transactivation domains of nuclear HER family receptor tyrosine kinases. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4276. doi:10.1158/1538-7445.AM2013-4276

Abstract 5646: Overcoming acquired resistance to cetuximab by dual targeting of HER family members using antibody based therapy

The EGFR is recognized as a key mediator of proliferation and progression in many human tumors and strategies to inhibit EGFR signaling have emerged as highly promising cancer therapy approaches. However, many tumors do not show response to EGFR inhibition and some of the responders eventually manifest resistance to treatment. In an effort to examine mechanisms of acquired resistance to anti-EGFR monoclonal antibody we developed a series of cetuximab-resistant clones using the NSCLC line NCI-H226. During efforts to elucidate molecular pathways that are involved in acquired resistance to cetuximab we found that 1) EGFR ubiquitination and degradation is altered in cetuximab-resistant cells resulting in increased steady-state expression of the EGFR and 2) enhanced signaling from the EGFR to HER3 and the PI(3)K/Akt pathway indicating that activation of HER3 may lead to escape from cetuximab therapy.

To investigate the role of HER3 in acquired resistance to cetuximab we used both RNAi and targeted approaches using the anit-HER3 antibody U3-1287/AMG888. siRNA directed against HER3 indicated that resistant cells had a strong dependency on HER3. Further, targeting HER3 using U3-1287/AMG888 showed a dose-dependent anti-proliferative response in resistant cells. In studies using siRNA against the EGFR and HER3 resulted in robust inhibition of cell proliferation than did silencing of either receptor alone in resistant cells. Comparable effects on cell proliferation were observed after combinatorial treatment of cells by the EGFR antibody cetuximab and the HER3 antibody U3-1287/AMG888. In addition, combined EGFR and HER3 inhibition by either siRNAs or antibodies decreased the phosphorylation of downstream signaling pathways AKT and MAPK. Annexin-V binding assays showed a significant increase in apoptosis in U3-1287/AMG888 plus cetuximab treated cells as compared to either agent alone. To determine if U3-1287/AMG888 could have therapeutic benefit as a combination treatment with cetuximab in vivo, mice harboring NCI-H226 xenografts will be treated with cetuximab for approximately 70-100 days until cetuximab-resistant tumors develop. At this time mice with resistant tumors will be divided into two groups that will be treated with both cetuximab and U3-1287/AMG888 or cetuximab alone.

The work presented herein suggests that cells with acquired resistance to cetuximab maintain their dependence on EGFR and HER3 signaling via the AKT and MAPK pathways.

Citation Format: Mari Iida, Toni M. Brand, Megan M. Starr, Neha Luthar, Matthew J. Wleklinski, Deric L. Wheeler. Overcoming acquired resistance to cetuximab by dual targeting of HER family members using antibody based therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5646. doi:10.1158/1538-7445.AM2013-5646