Brief‐exposure to preoperative bevacizumab reveals a TGF‐β signature predictive of response in HER2‐negative breast cancers

To best define biomarkers of response, and to shed insight on mechanism of action of certain clinically important agents for early breast cancer, we used a brief‐exposure paradigm in the preoperative setting to study transcriptional changes in patient tumors that occur with one dose of therapy prior to combination chemotherapy. Tumor biopsies from breast cancer patients enrolled in two preoperative clinical trials were obtained at baseline and after one dose of bevacizumab (HER2‐negative), trastuzumab (HER2‐positive) or nab‐paclitaxel, followed by treatment with combination chemo‐biologic therapy. RNA‐Sequencing based PAM50 subtyping at baseline of 46 HER2‐negative patients revealed a strong association between the basal‐like subtype and pathologic complete response (pCR) to chemotherapy plus bevacizumab (p ≤ 0.0027), but did not provide sufficient specificity to predict response. However, a single dose of bevacizumab resulted in down‐regulation of a well‐characterized TGF‐β activity signature in every single breast tumor that achieved pCR (p ≤ 0.004). The TGF‐β signature was confirmed to be a tumor‐specific read‐out of the canonical TGF‐β pathway using pSMAD2 (p ≤ 0.04), with predictive power unique to brief‐exposure to bevacizumab (p ≤ 0.016), but not trastuzumab or nab‐paclitaxel. Down‐regulation of TGF‐β activity was associated with reduction in tumor hypoxia by transcription and protein levels, suggesting therapy‐induced disruption of an autocrine‐loop between tumor stroma and malignant cells. Modulation of the TGF‐β pathway upon brief‐exposure to bevacizumab may provide an early functional readout of pCR to preoperative anti‐angiogenic therapy in HER2‐negative breast cancer, thus providing additional avenues for exploration in both preclinical and clinical settings with these agents.