Abstract PS2-04-30: Enrolment of Patients with Metastatic Lobular Breast Cancer in Clinical Trials in the Multicenter ESME Cohort

Abstract Background: Invasive lobular carcinoma (ILC) represents about 15% of breast cancers and is the second most common histological subtype after invasive carcinoma of no special type (NST). ILC differs from NST in clinical presentation, histological features, and molecular profile. Nevertheless, ILC-specific characteristics are often overlooked when making treatment decisions in clinical practice. Available evidence suggests that patients (pts) with ILC are underrepresented in clinical trials, possibly due to features such as predominant bone involvement, which are often non-measurable by RECIST 1.1 criteria, leading to exclusion from trials. This study aimed to evaluate clinical trial enrolment among pts with metastatic breast cancer (MBC) and ILC subtype. Methods: The ESME MBC (Epidemiological Strategy and Medical Economics - Metastatic Breast Cancer) cohort is a national registry collecting individual-level patient data from 18 French Comprehensive Cancer Centers (NCT03275311). This analysis included all pts treated between 2008 and 2023 who had a histological classification of ILC, NST, or mixed ILC+NST. We assessed clinical trial enrolment rates according to histological subtype, considering trials initiated at any treatment line and specifically at first line. Trial characteristics and enrolment distribution across treatment periods (<2012, 2012-2018, >2018) were described by histological subtype. A subpopulation of pts with ILC considered potentially eligible for clinical trials (female, ECOG 0-1, aged less than 70 years, no CNS metastases, no recent other malignancy) was defined to study clinical characteristics, treatments, and predictors of trial participation using multivariable logistic regression. Results: Out of 38,285 pts in the ESME database, 32,722 were eligible: 27,493 (84.0%) NST, 4,897 (15.0%) ILC, and 332 (1.0%) mixed ILC+NST. A total of 5,124 pts (15.7%) were enrolled in at least one clinical trial. Trial inclusion rates at any treatment line were 12.6% for ILC and 16.2% for NST (p<0.0001). It was 13.0% for mixed subtypes. Overall, trial participation at first line was low (9.1%). ILC pts had lower enrolment in first-line trials (7.4%) compared to NST (9.5%) and mixed (7.5%). Enrolment peaked between 2012 and 2018 whatever the subtype: 59.1% for ILC, 54.6% for NST and 48.0 for mixed. Pts with ILC were more often enrolled in phase III trials (58.1%) than pts with NST (50.0%), but less than those with mixed subtype (63.6%). Conversely, pts with ILC and mixed were underrepresented in phase I trials (9.0% and 4, 5% respectively) compared to 14.5% for pts with NST. Considering first-line trials, pts with ILC were more likely to receive endocrine therapy based regimen (61.7%) compared to pts with NST (33.6%) or mixed (48.0%). Conversely, they were less likely to receive chemotherapy (27.5%, 50.0% and 44.0% for pts with ILC, NST and mixed respectively) and immunotherapy (1.9%, 7.2% and 4.0% respectively). Among the 4,897 pts with ILC, 1,168 (23.9%) met eligibility criteria for clinical trials and 179 (15.3%) of these were enrolled in first line trials. Grade I tumors were less frequent among trial participants (p = 0.030), and enrolment peaked during 2012-2018 but declined thereafter (p < 0.0001). No other significant differences were observed. Multivariable logistic regression identified only histological grade II as significantly associated with clinical trial enrolment (OR 2.07(95% CI [1.13; 3.77]), p = 0.018). Conclusion: In this large real-life series, pts with metastatic ILC had lower enrolment in clinical trials than those with NST, suggesting access disparities. However, multivariable analysis did not clearly identify factors influencing first line trial enrolment, highlighting the need for further investigation. Additional analyses will be shared later this year. Citation Format: E. De Maio, L. Chaltiel, V. Massard, A. Mailliez, T. Grinda, J. Frenel, E. Brain, T. Bachelot, A. Gonçalves, B. Sauterey, D. Pasquier, T. Petit, L. Bosquet, S. Guiu, F. Dalenc. Enrolment of Patients with Metastatic Lobular Breast Cancer in Clinical Trials in the Multicenter ESME Cohort [abstract]. In: Proceedings of the San Antonio Breast Cancer Symposium 2025; 2025 Dec 9-12; San Antonio, TX. Philadelphia (PA): AACR; Clin Cancer Res 2026;32(4 Suppl):Abstract nr PS2-04-30.