Abstract OT1-22-01: Liquid biopsy and Ado-trastuzumab emtansine (T-DM1): drug-resistance traits in the blood of HER2-positive metastatic breast cancer patients

Abstract Background: A previous study on 22 metastatic breast cancer (mBC) patients (Allegretti et al. Mol Cancer 2021) has associated drug resistance to Ado-trastuzumab emtansine (T-DM1) with two sets of genomic events: a) reversal of HER2 amplification, b) ‘oncogenic replacement’ of HER2 by alternative cancer drivers. To expand on this, we designed GIM21 (Gruppo Italiano Mammella) study. Materials and Methods: GIM21 is a multicentre, prospective study; tumor and circulating total nucleic acids (tTNA and ctTNA) were obtained from primary and metastatic lesions (n=36 altogether), as well as plasma samples (n=501), the latter serially collected coincident with medical imaging re-evaluations. tCNAs/ctTNAs were sequenced by ultra-deep, largely overlapping 50-gene panels on Ion Torrent Gene Studio S5. Ortogonal dPCR validation was by dPCR (Quant Studio 3D). ctTNAs were correlated with clinical readouts and patient outcomes. Results: from September 2018 to January 2022, 50 HER2+ mBC patients receiving T-DM1 as second-line treatment. Median time to progression was 6.5 months (range 2.0-27.2). As previously shown, only a minority of patients (9/50 – 18%) retained residual HER2 amplification in blood at baseline, likely due to HER2 counterselection during previous therapy lines; all of them (9/9) underwent further neutralization during T-DM1 treatment, but an HER2-neutral blood status was reached in only 5 patients. In contrast, 2 HER2-neutral patients acquired HER2 amplification, suggesting either insufficient T-DM1 pressure or an unprecedented clonal escape mechanism. Overall, the circulating HER2 status did not correlate with progression free survival (PFS), further highlighting a loss of clinically relevant HER2 oncogenic dependence. At baseline, 24 circulating mutations were detected in 29/50 (58%) patients, 9 of which did not overlap with tissue mutations. Of note, carrying a given aberration in either blood or tissue resulted in outcome trends (p=0.16), but no clear association with therapeutic response. Rather, it was serial monitoring (appearance of any aberration in the blood) that predicted a dismal clinical outcome in 16/41 (39%) patients, with a median time to progression of 2.8 months (2.2-3.9). Unfortunately, liquid biopsy (LB) was confirmed to miss most patients developing brain (3/3 patients) or skin (2/3 patients) metastases. Most circulating alterations present at progression (40/60) were actionable (OncoKB level < 3B) in 34/41 (82.9%) patients, suggesting that LB may guide therapeutic strategies in post-T-DM1 settings. Conclusions: GIM21 trial showed that LB accurately predicts clinical outcome and reveals actionable drivers of T-DM1 escape. The final analysis are ongoing. Citation Format: Alessandra Fabi, Matteo Allegretti, Elena Giordani, Gianluigi Ferretti, Grazia Arpino, Alberto Zambelli, Claudia Omarini, Ida Paris, Andrea Botticelli, Emilio Bria, Antonella Palazzo, Stefania Gori, Francesco Cognetti, Patrizio Giacomini. Liquid biopsy and Ado-trastuzumab emtansine (T-DM1): drug-resistance traits in the blood of HER2-positive metastatic breast cancer patients [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-22-01.