Geukens T, De Schepper M, Richard F, Maetens M, Van Baelen K, Mahdami A, Nguyen HL, Isnaldi E, Leduc S, Pabba A, Zels G, Mertens F, Vander Borght S, Smeets A, Nevelsteen I, Punie K, Neven P, Wildiers H, Van Den Bogaert W, Floris G, and Desmedt C
Introduction: Anti-HER2 antibody-drug conjugates (ADCs) have shown important efficacy in HER2-low metastatic breast cancer (mBC). Criteria for receiving ADCs are based on a single assay on the primary tumour or a small metastatic biopsy. We assessed the intra-patient inter-metastasis heterogeneity of HER2-low status in HER2-negative mBC., Patients and Methods: We included samples of 10 patients (7 ER-positive and 3 ER-negative) donated in the context of our post-mortem tissue donation program UPTIDER. Excisional post-mortem biopsies of 257 metastases and 8 breast tumours underwent central HER2 immunohistochemistry (IHC), alongside 41 pre-mortem primary or metastatic samples. They were classified as HER2-zero, HER2-low (HER2-1+ or HER2-2+, in situ hybridisation [ISH] negative) or HER2-positive (HER2-3+ or HER2-2+, ISH-positive) following ASCO/CAP guidelines 2018. HER2-zero was further subdivided into HER2-undetected (no staining) and HER2-ultralow (faint staining in ≤10% of tumour cells)., Results: Median post-mortem interval was 2.5 h. In 8/10 patients, HER2-low and HER2-zero metastases co-existed, with the proportion of HER2-low lesions ranging from 5% to 89%. A total of 32% of metastases currently classified as HER2-zero were HER2-ultralow. Intra-organ inter-metastasis heterogeneity of HER2-scores was observed in the liver in 3/6 patients. Patients with primary ER-positive disease had a higher proportion of HER2-low metastases as compared to ER-negative disease (46% versus 8%, respectively). At the metastasis level, higher percentages of ER-expressing cells were observed in HER2-low or -ultralow as compared to HER2-undetected metastases., Conclusions: Important intra-patient inter-metastasis heterogeneity of HER2-low status exists. This questions the validity of HER2-low in its current form as a theranostic marker., Competing Interests: Declaration of Competing Interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: TG, MDS, FR, KVB, MM, AM, H-LN, EI, SL, AP, GZ, FM, SVB, AS, IN, PN, WVDB, GF, CD: no conflicts to declare. KP: research grants paid to institution: MSD and Sanofi, speaker fees and honoraria for consultancy and advisory board functions: Astra Zeneca, Eli Lilly, Exact Sciences, Focus Patient, Gilead, MSD, Novartis, Pfizer, Roche, Seagen, speaker fees and honoraria for consultancy and advisory board functions paid to institution: Astra Zeneca, Eli Lilly, Exact Sciences, Gilead, MSD, Novartis, Pfizer, Roche, Seagen, stock options: Need Inc, travel grants: Astra Zeneca, Novartis, Pfizer, PharmaMar, Roche. HW: his institution received financial compensation on his behalf for advisory boards, lecture fees and/or consultancy fees from Immutep Pty, MSD, Astrazenca, Daiichi, AbbVie, Lilly, Roche, EISAI, Pfizer, Sirtex, Gilead. He received travel support from Pfizer and Roche., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)