Expression patterns of gpNMB in breast cancer (BC): Retrospective evaluation of tumor tissue from the phase II EMERGE study

129 Background: Glycoprotein NMB (gpNMB) is an internalizable transmembrane protein overexpressed in ~20% of BC. gpNMB promoted BC metastases in a murine model and is a poor prognostic marker in BC patients (pts) (Rose 2010). Glembatumumab vedotin (GV, CDX-011) is a novel antibody-drug conjugate targeting gpNMB+ cancer cells with the potent cytotoxin monomethyl auristatin E. In a Phase I/II and in the Phase II EMERGE study, GV was well-tolerated (treatment-related toxicity included rash, neutropenia, and neuropathy) with promising activity in gpNMB+ BC tumors including TNBC. In EMERGE, GV vs. investigator’s choice chemotherapy, demonstrated an objective response rate of 30% (7/23) vs. 9% (1/11) in pts with tumor gpNMB overexpression (gpNMB in > 25% of tumor cells); 18% (5/28) vs. 0% (0/11) in TNBC; and 40% (4/10) vs. 0% (0/6) in gpNMB-overexpressing TNBC. Improvements in progression-free survival (hazard ratio (HR) = 0.11) and overall survival (HR = 0.14) in gpNMB-overexpressing TNBC were noted. Methods: This retrospective analysis of the EMERGE study examined frequency of gpNMB overexpression by various baseline and disease characteristics. Tumors were centrally assessed by immunohistochemistry (IHC) and included data for 328 pts. Results: Tumor gpNMB overexpression was present in 21% (69/328) of screened and 40% (38/96) of TNBC pts. gpNMB overexpression was consistent in progesterone, estrogen, or HER2+ expressing tumors at rates of 12-15%. gpNMB overexpression was not observed among 17 lobular tumors, but was present in 21% (57/270) of ductal tumors. gpNMB overexpression was seen across organ sites, including lung (43%, 3/7), lymph node (31%, 12/39), chest wall (23%, 3/13), breast (21%, 44/209), bone (13%, 1/8), and liver (13%, 3/24). There were no apparent differences in gpNMB overexpression by age, race or disease setting (early vs. advanced) at tissue collection. Analysis is ongoing to determine if prior treatments may upregulate gpNMB expression. Conclusions: gpNMB overexpression in BC, especially in TNBC, appears consistent in archival primary and/or metastatic BC, regardless of age. A randomized multicenter study of GV in gpNMB overexpressing metastatic TNBC (METRIC) is ongoing. Clinical trial information: NCT01156753.