Picking the Right Patient for Human Epidermal Growth Factor Receptor 3–Targeted Therapy in Platinum-Resistant Ovarian Cancer

Surgical cytoreduction followed by platinum-based chemotherapy has been the standard first-line treatment of patients with high-risk early-stage and advanced epithelial ovarian cancer for nearly two decades. Although the majority of women with advanced disease will respond to combined platinum/taxane therapy, most will ultimately experience recurrence and eventually die as a result of their ovarian cancer. Platinum-refractory and -resistant disease is defined as progression on first-line chemotherapy or within 6 months of platinum completion, respectively. These patients are treated with nonplatinum chemotherapy; however, anticipated response rates are low. Despite the urgent need for more effective treatments, few new agents have demonstrated sufficient efficacy to warrant approval by the Food and Drug Administration (FDA) in the last 10 years. In 2014, the Avastin Use in Platinum-Resistant Epithelial Ovarian Cancer (AURELIA) trial established that the addition of bevacizumab to nonplatinum chemotherapy increases progression-free survival (PFS) from 3.4 to 6.7 months, leading to FDA approval. In 2015, Kaufman et al showed that olaparib achieved a 31% response rate in heavily pretreated women with germline BRCA1/2mutations, also leading to FDA approval in germline BRCA1/2 carriers. Despite this recent progress, there remains a significant unmet need for improved therapies in platinum-resistant and -refractory ovarian cancer. In the article that accompanies this editorial, Liu et al report the results of a 233-patient open-label, randomized phase II trial of once-weekly paclitaxel with or without the human epidermal growth factor receptor 3 (HER3) antibody, seribantumab, in platinumresistant/refractory epithelial ovarian cancer. Patients were randomly assigned 2:1 in favor of seribantumab and enrolled without prospective biomarker selection or stratification. Unfortunately, the study did not reach its primary end point, showing no difference in PFS between the two arms (3.8 months with the combination compared with 3.7 months with paclitaxel alone). Despite this disappointing result, the study fortunately mandated the collection of both archival and fresh tumor biopsies, and these were used to conduct an extensive retrospective biomarker analysis to determine if a subset of patients may have benefited from the addition of seribantumab. By evaluating multiple biomarkers relevant to the mechanism of action of seribantumab, Liu et al identified a tumor-based bivariate signature defined by high heregulin (HRG, also named neuregulin 1), the ligand of HER3, and low human epidermal growth factor receptor 2 (HER2) that was predictive of seribantumab benefit. Specifically, in the 38% of evaluable patients who were biomarker positive, the median PFS was 5.7 months with the combination compared with 3.5 months with paclitaxel alone. In addition to being predictive of benefit to seribantumab, this signature seemed to be prognostic in the paclitaxel monotherapy arm, with biomarker-positive patients experiencing more rapid disease progression (PFS, 3.5 months v 5.4 months). Together, these observations suggest that seribantumab overcomes the negative prognosis associatedwith highHRG and lowHER2 levels in platinum-resistant/ refractory ovarian cancer. Distressingly, the biomarker-negative patients did worse when exposed to seribantumab, although the mechanism underlying this apparent harm is not understood. HER3, the protein encoded by ErbB3, is a member of the human epidermal growth factor (EGFR) family and the only one that lacks catalytic kinase function. Instead, HER3 mediates its effects on signaling through heterodimerizationwith, and allosteric activation of, other EGFR family members, leading to downstream activation of the phosphatidylinositol 3-kinase/AKT pathway. Pertuzumab, an anti-HER2 monoclonal antibody, is believed to act by preventing dimerization with HER3 and has been approved for treatment of HER2-positive breast cancer, further credentialing HER3 as a therapeutic target in cancer. In ovarian cancer, HER3 is highly expressed in a subset of patients and is associated with a worse prognosis. Autocrine signaling loops between HER3 and its ligand, HRG, promote growth in patient-derived ovarian cancer models and cell lines. Treatment of ovarian cancer cell lines with certain chemotherapies increases activation of HER3, suggesting HER3 may be one of several mechanisms responsible for chemotherapy resistance in ovarian cancer. Seribantumab is a fully humanized monoclonal antibody that blocks binding of HRG to HER3 and has been shown to cause tumor growth arrest in ovarian cancer xenograph models. In tumors with high HRG expression, seribantumab is believed to block ligand-dependent activation of HER2/HER3 dimers. Conversely, high levels of HER2, leading to a greater presence of HER2/HER3 dimers, may mitigate therapeutic benefit by promoting ligand-independent signaling. Thus, the findings by Liu et al that the combination of high HRG and low HER2 levels were associated with benefit to seribantumab is consistent