Background: In the I-SPY 2 TRIAL, HER2- patients were randomized to receive standard chemotherapy or chemotherapy plus the oral PARP inhibitor veliparib in combination with carboplatin (V+C), which graduated in the HR-/HER2- arm. Exploratory analysis of protein signaling was performed to identify biomarker candidates that correlated with pCR in the HER2- population. We evaluated 110 key signaling proteins using reverse phase protein microarray (RPPA) data from pre-treatment LCM purified tumor epithelium.

Methods: Of 115 patients, 97 (V+C: 61 controls: 36) had RPPA and pCR data. RPPA data was correlated to pCR in both the treated and control patients using parametric (t-test) or non-parametric (Wilcoxon) statistical analysis, depending on data distribution. Only analytes whose pre-treatment levels were associated with response in the V+C but not the control arm were identified (P<0.05). Markers are analyzed individually; p-values are descriptive and were not corrected for multiple comparisons.

Results: 11 protein/phosphoprotein markers were significantly associated with pCR in the V+C arm but not in controls. Two were positive predictors of response: YAP S127 p= 0.03 and LC3B p=0.04. Negative predictors of response included Cyclin D1 p=0.001, and a number of phosphorylated RTKs: ROS Y2274 p=0.03, IGF1R Y1135/Y1136-IR Y1150/Y1151 p=0.03, ERBB4 Y1284 p=0.002, total HER2 p=0.04, and total IGF1R p=0.01. Moreover, a number of AKT-mTOR pathway proteins were found to be negative predictors of V+C response: ACC S79 p=0.005, p70S6K S371 p=0.01, and B-RAF S445 p=0.01.

Conclusion: Our sample size is too small to draw definitive conclusions and the results are exploratory. Coordinated RTK-mTOR pathway activation appears to be a hallmark signature of lack of response to veliparib in HER2- tumors. We also found that HER2 levels were correlated paradoxically with lack of response in this HER2- population, suggesting potential added clinical value of quantitative HER2 measurement techniques. Such exploratory results merit evaluation in larger trials with HER2- breast cancer patients.