ERBBal Remedies: Combination Therapy for EGFR-mutant Lung Cancers

In this issue of Clinical Cancer Research, Romaniello and colleagues (1) describe a combination of clinically approved therapies that target two members of the ERBB/HER family of receptor tyrosine kinases, EGFR and HER2 (ERBB2), and can overcome resistance to EGFR inhibition in an animal model of EGFR-mutant lung cancers.

Initial treatment with an EGFR inhibitor is the standard of care for non–small cell lung cancers (NSCLC) harboring activating mutations in EGFR. Four small-molecule tyrosine kinase inhibitors (TKI) spanning three generations of drug development have been approved by the FDA for first-line therapy of EGFR-mutant lung cancers. Despite high initial response rates to erlotinib (first-generation), gefitinib (first), and afatinib (second), the development of acquired resistance typically ensues, and has emerged in response to treatment with osimertinib (third) as well. The mechanisms underlying acquired resistance frequently result in reactivation of the kinase activity of EGFR or activation of bypass signaling tracts that promote tumor growth and survival (2). Reactivation of EGFR kinase activity occurs through secondary alterations in EGFR that alter drug binding, such as the T790M mutation that confers resistance to first- and second-generation EGFR TKIs and the C797S mutation that emerges upon treatment with third-generation EGFR inhibitors. Commonly activated bypass signaling tracts include HER2 and the receptor tyrosine kinase MET, both most often driven mechanistically by amplification of their respective genes. In resistance that is associated with MET amplification, downstream activation of the PI3K pathway is critically dependent on HER3 (ERBB3), yet another member of the ERBB/HER family.

Given the prominent roles of EGFR, HER2, and HER3 signaling in promoting resistance to EGFR inhibitors, simultaneous targeting of all three of these ERBB/HER family members is an attractive therapeutic strategy to investigate. The authors previously reported that a combination of three mAbs against EGFR, HER2, and HER3 suppressed compensatory feedback loops triggered by targeting EGFR alone, and markedly reduced the growth of xenografts of erlotinib-resistant PC9ER cells that harbor both an EGFR exon 19 deletion and EGFR T790M (3). They subsequently demonstrated that a similar triplet of mAbs promoted degradation of all three receptors, induced cellular senescence, and overcame resistance to osimertinib in cells with an established EGFR C797S mutation (4). Relapsed outgrowth of PC9ER xenograft tumors was seen after discontinuation of osimertinib administered at doses high enough to eradicate measurable tumors but lower than levels corresponding to therapeutic doses in human patients, but no outgrowth was observed when the triplet of antibodies was added to the same subtherapeutic doses of osimertinib. The authors have now simplified the treatment regimen to include only FDA-approved therapies targeting EGFR and HER2, and demonstrate that continuous or intermittent therapy with a combination of subtherapeutic doses of osimertinib with the mAbs, cetuximab (anti-EGFR) and trastuzumab (anti-HER2), durably prevented relapses in PC9ER xenografts (1). The addition of the two antibodies to low-dose osimertinib also effectively eliminated relapses that emerged after treatment with low-dose osimertinib alone.

Combination therapy against EGFR and HER2 has been investigated before using afatinib (a potent tyrosine kinase inhibitor of both EGFR and HER2) together with cetuximab to treat acquired resistance to first-generation EGFR TKIs. Preclinical studies using transgenic mouse models and cell line xenografts showed dramatic shrinkage of tumors harboring both the activating EGFR L858R mutation and the T790M mutation with the combination of afatinib and cetuximab, not with either drug alone (5). In a phase Ib clinical trial of afatinib and cetuximab that included 126 patients with EGFR-mutant lung cancers resistant to either erlotinib or gefitinib, the overall objective response rate was 29% and median progression-free survival was 4.7 months, and the objective response rates were comparable in T790M-positive and negative tumors (6).

Would the combination of low-dose osimertinib, cetuximab, and trastuzumab be expected to exhibit more robust clinical activity in this setting? The most obvious difference between the two regimens is the mode of inhibition of HER2. Although afatinib inhibits the kinase activity of HER2, administration of trastuzumab leads to increased endocytosis and degradation of the receptor. As expected, treatment with cetuximab and trastuzumab together results in degradation of both EGFR and HER2 in preestablished PC9ER xenografts, but Romaniello and colleagues have also shown that this combination dramatically lowers the levels of HER3, MET, and AXL, another receptor tyrosine kinase implicated in acquired resistance to EGFR inhibition. The mechanism underlying the reduced levels of these receptors remains unclear, but the finding provides an intriguing preclinical rationale for why the combination of low-dose osimertinib, cetuximab, and trastuzimab might exhibit greater clinical activity than the pairing of afatinib and cetuximab in the treatment of acquired resistance to EGFR TKIs.

The striking response of PC9ER xenografts to the novel three-drug regimen both warrants and requires further validation. Although the authors have previously shown that the C797S mutation can be identified in PC9ER tumors that have relapsed on subtherapeutic osimertinib, a single xenograft model cannot faithfully replicate the full spectrum of mechanisms that have emerged in clinical cases of resistance to osimertinib. In the setting of osimertinib resistance, there have been reports of more off-target resistance mechanisms such as RET and ALK fusions (7) and BRAF mutations (8) that may be less amenable to multidrug ERBB/HER family inhibition. Testing the three-drug combination in additional patient-derived and genetically engineered models will determine which molecular subsets of patients may respond to this treatment.

Other combinations of kinase inhibitors and antibodies that target the ERBB/HER family are actively being explored in the setting of acquired resistance to EGFR TKIs. In this context, it is important to note that the decision to utilize an anti-HER2 antibody instead of an anti-HER3 antibody in the three-drug regimen was based on the FDA-approved status of the trastuzumab, and not on any observed differences in preclinical activity. There are robust preclinical data highlighting the involvement of HER3 activation in resistance to EGFR inhibitors that have provided the basis for an ongoing clinical trial of a HER3 antibody–drug conjugate, U3-1402, in patients with EGFR-mutant lung cancers with acquired resistance to EGFR TKIs (NCT03260491). In addition, Corte and colleagues recently demonstrated that osimertinib at therapeutic doses combined with cetuximab can reverse osimertinib resistance in multiple xenograft models (9). A clinical trial is currently investigating the use of osimertinib and another EGFR antibody, necitumumab, in patients with EGFR-mutant lung cancers (NCT02496663; Fig. 1).

In summary, the authors report efficacy of the novel combination of low-dose osimertinib, cetuximab, and trastuzumab in in vitro and animal studies that suggests a role for the combination in the acquired resistance setting. The regimen appears to effectively block downstream EGFR signaling as well as multiple relevant bypass signaling pathways including MET, AXL, and HER3. We look forward to seeing these works corroborated in additional preclinical studies. Ongoing studies assessing HER3 antibodies and osimertinib/EGFR–antibody combinations in the acquired resistance setting may inform whether related combinations also have merit.

H.A. Yu reports receiving commercial research grants from AstraZeneca, Daiichi, Lilly, and Novartis, and is a consultant/advisory board member for AstraZeneca. No potential conflicts of interest were disclosed by the other author.

Conception and design: P.-D. Fan, H.A. Yu

Development of methodology: H.A. Yu

Acquisition of data (provided animals, acquired and managed patients, provided facilities, etc.): H.A. Yu

Analysis and interpretation of data (e.g., statistical analysis, biostatistics, computational analysis): P.-D. Fan, H.A. Yu

Writing, review, and/or revision of the manuscript: P.-D. Fan, H.A. Yu

Study supervision: H.A. Yu