The evolving landscape of metastatic HER2-positive, hormone receptor-positive Breast Cancer.

Therapeutic agents targeting Human Epidermal Growth Factor Receptor 2 (HER2) demonstrated to positively impact the prognosis of HER2-positive breast cancer. HER2-positive breast cancer can present either as hormone receptor-negative or positive, defining Triple-positive breast cancer (TPBC). TPBC demonstrate unique gene expression profiles, showing reduced HER2-driven gene expression, as recapitulated by a higher proportion of Luminal-type intrinsic subtypes. The different molecular landscape of TPBC dictates distinctive clinical features, including reduced chemotherapy sensitivity, different patterns of recurrence, and better overall prognosis. Cross-talk between HER2 and hormone receptor signaling seems to be critical to determine resistance to HER2-directed agents. Accordingly, superior outcomes have been achieved with the use of endocrine therapy, representing the first subtype-specific pharmacological intervention unique to this subgroup. Additional targeted agents capable to tackle resistance mechanisms to anti-HER2, hormone agents, or both might further improve the efficacy of treatments, such as PI3K/AKT/mTOR inhibitors, particularly in a biomarker-enriched setting, and CDK4/6-inhibitors, with preliminary data suggesting a role of PAM50 subtyping to predict higher benefits in luminal tumors. Finally, the distinct biology of triple-positive tumors may yield the rationale for considering combinations within antibody-drug conjugate regimens. Accordingly, in this review, we summarized the current evidence and rationale for considering TPBC as a different entity, in which distinct therapeutical approaches leveraging on the different biological profile of TPBC may result in superior anticancer regimens and improved patient-centric outcomes.
Competing Interests: Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: EN was supported by an American-Italian Cancer Foundation Post-Doctoral Research Fellowship. MR received travel expenses reimbursement from Sanofi. AP reports advisory and consulting fees from Roche, Pfizer, Novartis, Amgen, BMS, Puma, Oncolytics Biotech, MSD, Guardant Health, Peptomyc and Lilly, lecture fees from Roche, Pfizer, Novartis, Amgen, BMS, Nanostring Technologies and Daiichi Sankyo, institutional financial interests from Boehringer, Novartis, Roche, Nanostring, Sysmex Europa GmbH, Medica Scientia Innovation Research, SL, Celgene, Astellas and Pfizer; stockholder and consultant of Reveal Genomics, SL; AP is also listed as an inventor on patent applications for the HER2DX assay. NF has received honoraria for consulting, advisory role, speaker bureau, travel, and/or research grants from Merck Sharp & Dohme (MSD), Novartis, AstraZeneca, Roche, Menarini, Daiichi Sankyo, GlaxoSmithKline (GSK), Gilead, Diaceutics, Adicet Bio Sermonix, Reply, and Leica Biosystems. GC received honoraria for speaker's engagement: Roche, Seattle Genetics, Novartis, Lilly, Pfizer, Foundation Medicine, NanoString, Samsung, Celltrion, BMS, MSD; Honoraria for providing consultancy: Roche, Seattle Genetics, NanoString; Honoraria for participating in Advisory Board: Roche, Lilly, Pfizer, Foundation Medicine, Samsung, Celltrion, Mylan; Honoraria for writing engagement: Novartis, BMS; Honoraria for participation in Ellipsis Scientific Affairs Group; Institutional research funding for conducting phase I and II clinical trials: Pfizer, Roche, Novartis, Sanofi, Celgene, Servier, Orion, AstraZeneca, Seattle Genetics, AbbVie, Tesaro, BMS, Merck Serono, Merck Sharp Dome, Janssen-Cilag, Philogen, Bayer, Medivation, Medimmune. The remaining authors have no conflicts of interest to declare.
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