Abstract P1-11-04: Assessing the clinico-pathological characteristics of HER2 positive metastatic breast cancer patients experiencing radiologic complete response in a nationwide cohort

Abstract Background: Up to 6% of patients (pts) with HER2 positive (pos) metastatic breast cancer (MBC) experience a radiologic complete response (rCR) to a first line of therapy, but these results mostly derive from dated and/or limited cohorts. Aim of this study was to define the clinico-pathological characteristics of HER2 positive (pos) MBC pts experiencing a rCR. Methods: Pts were selected from the database of the GIM14 study (NCT02284581) and classified according to the best radiologic response obtained to the first line chemotherapy (CT) and upon time-to-treatment-failure (TTF). rCR was defined as complete response (CR) with a TTF > 3 months. The association across variables was tested through logistic regression and their prognostic impact in terms of overall survival (OS) was estimated using the Kaplan-Meier method and compared using the log-rank test. Results: Of the 3,423 pts included in the GIM14 study, 814 had HER2 pos MBC. After exclusion of pts treated with first line endocrine therapy and/or with TTF < 3 months, 656 pts were included in the present analysis, of which 96 (14.6%) experienced a rCR. Instead, the best response was a partial response for 295 pts (45.0%), stable disease for 221 pts (33.7%), and progression for 44 pts (6.7%). Most pts (59.8%) presented de novo MBC; 379 pts (57.8%) had visceral metastases (mets), 609 pts (92.8%) did not have central nervous system (CNS) involvement and 318 pts (48.5%) had only 1 site of distant mets. Also, 445 pts (67.9%) had hormone receptor (HR) pos disease, a HER2 3+ score at immunohistochemistry (IHC) was present in 59.8% of cases versus 40.2% with HER2 2+ at IHC and in situ hybridization (ISH) + disease. Taxanes were the main CT backbone (489 pts, 74.5%), 341 pts (52.0%) had received a Trastuzumab-Pertuzumab doublet. At multivariable analysis, higher odds of experiencing a rCR were reported for presence of non-visceral mets (OR 1.87, 95%CI 1.10-3.17), low number of metastatic sites (OR 2.42, 95%CI 0.80-7.33 for 1 site only) and HER2 3+ score at IHC (OR 1.80, 95%CI 1.09-2.98). Disease-free interval (DFI) was associated to rCR at univariable but not at multivariable analysis. HR status, CT backbone and type of anti-HER2 regimen were not associated with rCR neither at univariable nor at multivariable analysis. Median follow-up was 76.2 months. Amongst pts with TTF>12 months, those with rCR had a significantly higher OS compared to those not experiencing a rCR (median OS 133 and 90 months, respectively; p=0.0191). OS rates in pts with TTF ≥ 12 months were 97.8% at 2-year follow-up and 59.4% at 5-year follow-up. Instead, in pts with TTF ≥ 60 months, OS rates were 76.7% at 10-year follow-up. Amongst the 96 pts experiencing a CR, 38 had a rCR with TTF between 12 and 60 months, while 22 pts had a rCR with a TTF ≥ 60 months. The remaining pts had a CR with a TTF < 12 months. Pts with HR negative (neg) disease were found to be more likely to experience a rCR with a with TTF between 12 and 60 months, whilst pts with HR pos disease had a higher probability to experience a rCR with a TTF ≥ 60 months (p=0.0074). Pts with HER2 3+ score at IHC had a higher probability to achieve a rCR with a TTF ≥ 12 months compared to pts with HER2 2+ score at IHC and ISH + (p=0.0216). Age at diagnosis, menopausal status, DFI, number and site of mets, CT backbone and anti-HER2 therapy did not influence the duration of the rCR obtained. Conclusions: This study characterized a real-world cohort of HER2 positive MBC patients experiencing radiologic complete response to a first line treatment. Based on these results a clinical trial focused on liquid biopsy-based minimal residual disease is being designed. Novel anti-HER2 agents are gaining momentum as ever increasingly effective treatments and future de-escalation strategies after complete response will represent a growing need. Citation Format: Linda Cucciniello, Eva Blondeaux, Claudia Bighin, Simona Gasparro, Stefania Russo, Arianna Dri, Palma Pugliese, Andrea Fontana, Giuseppe Naso, Antonella Ferzi, Ferdinando Riccardi, Valentina Sini, Luca Boni, Alessandra Fabi, Filippo Montemurro, Michelino De Laurentiis, Grazia Arpino, Lucia Del Mastro, Lorenzo Gerratana, Fabio Puglisi. Assessing the clinico-pathological characteristics of HER2 positive metastatic breast cancer patients experiencing radiologic complete response in a nationwide cohort [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 P1-11-04.