Tumor expression and microenvironment in HER2-positive breast cancer before and on HER2-targeted therapy: Analysis of microarray expression data from the TRIO-US B07 trial

Background: Neoadjuvant HER2-targeted therapy in combination with chemotherapy is a standard treatment approach for early-stage HER2-positive breast cancer. Proposed biomarkers to predict pathologic complete response (pCR), and thereby inform which patients may benefit from de-escalation of therapy, include expression-based subtyping and immune enrichment scores. Little is known about how tumors and their microenvironment may change with HER2-targeted therapy alone, and whether these changes may predict outcome. Methods: The TRIO-US B07 phase II trial randomized 128 participants with stage I-III HER2-positive breast cancer to trastuzumab (N=34), lapatinib (N=36), or the combination (N=58) for three weeks, followed by six cycles of docetaxel and carboplatin with continued HER2-targeted therapy. Fresh-frozen core biopsies of the tumor prior to therapy (N=110) and after 14-21 days of HER2-targeted therapy alone (N=89) were collected, and RNA was extracted and subjected to Agilent Whole Human Genome 44K 2-color chip. The pre-treatment tumor RNA was normalized against a mixed breast tumor reference, and the on-treatment tumor RNA against the matched pre-treatment sample. Absolute intrinsic molecular subtyping was used to determine intrinsic subtype, the iC10 expression-based classifier to determine integrative subtype, gene set enrichment analysis (GSEA) to assess signature changes across treatment, single-sample GSEA to compare individual gene signature scores between tumors, and CIBERSORT to quantify immune cell populations before and on treatment. Results: Primary trial results have been reported previously and showed a pCR rate of 47% with trastuzumab, 25% with lapatinib, and 52% with the combination. Prior to treatment, 56% of tumors classified as the HER2-enriched intrinsic subtype and 78% as the iC5 integrative subtype. HER2-enriched tumors trended toward a higher rate of pCR relative to other intrinsic subtypes (50% vs 33%, P=0.12), as did iC5 tumors relative to other integrative subtypes (48% vs 25%, P=0.08). However, in multivariate analysis, HER2 FISH ratio (P=0.04) and hormone receptor status (P=0.02), each associated themselves with intrinsic and integrative subtype, proved the most valuable in predicting pCR, with little information added by expression-based subtyping. Immune cell signatures correlated with pCR in the trastuzumab-containing arms only. Of 65 gene signatures tested, 47 changed across HER2-targeted therapy with false discovery rate Conclusions: In the TRIO-US B07 study, the biomarkers most predictive of response to neoadjuvant HER2-targeted therapy were hormone receptor status in combination with HER2 FISH ratio. Multiple changes in the tumor and its microenvironment occurred with HER2-targeted therapy, but these changes did not predict pCR. Tumors treated with lapatinib tended to decrease proliferation more than tumors treated with trastuzumab, despite trastuzumab being more effective in preventing recurrence, an observation with implications for window of opportunity studies. Citation Format: Jennifer L. Caswell-Jin, Katherine L. McNamara, Judy Dering, Hsiao-Wang Chen, Robert Dichmann, Alejandra Perez, Ravindranath Patel, Eran Kotler, Jason J. Zoeller, Joan S. Brugge, Michael F. Press, Dennis J. Slamon, Christina Curtis, Sara A. Hurvitz. Tumor expression and microenvironment in HER2-positive breast cancer before and on HER2-targeted therapy: Analysis of microarray expression data from the TRIO-US B07 trial [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-07-01.