Abstract PS4-03-12: Quantifyher (trial in progress): quantitative immunofluorescence and/or messenger rna to measure her2 expression in patients with metastatic breast cancer treated with trastuzumab deruxtecan (tbcrc066)

Abstract Antibody drug conjugates (ADCs) can be effective therapeutic options for some patients (pts) with metastatic breast cancer (MBC). However, given serious potential toxicities, identifying pts most likely to benefit from ADCs is critical to optimizing quantity and quality of life. Trastuzumab-deruxtecan (T-DXd), a HER2-targeted ADC, is approved for pts with MBC expressing low/ultra-low levels of HER2 by immunohistochemistry (IHC 1+/2+, FISH-neg, or IHC 0 with faint membrane staining in 10% tumor cells). However, significant inter-reader variability exists in the assessment of HER2 IHC 1+ vs IHC 0 (especially “ultra-low”), with a recent study demonstrating only 26% concordance in IHC 0 across pathologists. We therefore have an incomplete understanding of how HER2 expression informs who will benefit from T-DXd (and for how long), raising concern that IHC is an inadequate biomarker for T-DXd pt selection. A quantitative immunofluorescence (QIF) assay measuring HER2 and TROP-2 (TROPLEX), developed in the Rimm lab, was designed to address the need for accurate quantification of low-level tumor protein expression, detected at amol/mm2 levels. In the TROPLEX analytic validation cohort of 400 retrospective samples, ∼50% of HER2 IHC 0 cases had HER2 QIF levels above the limit of quantification, with a significant relationship demonstrated between HER2 QIF and time-to-next-treatment in a retrospective cohort of pts treated with T-DXd. Retrospective data using Cepheid’s Xpert Breast Cancer STRAT4 mRNA assay have also demonstrated high ERBB2 concordance with HER2 IHC, with potentially greater range. Both assays hold promise to improve measurement of surface protein expression at lower limits of detection than traditional IHC and may better select pts likely to respond to T-DXd. The primary objective of QuantifyHER is to determine the relationship between HER2 QIF and/or ERBB2 mRNA levels and real-world objective response (rwOR) to T-DXd. Secondary objectives include evaluating the relationships between TROP2 and rwOR, HER2 QIF/mRNA and real-world progression-free survival, determining response thresholds and the relationship with ER status, and evaluating a joint QIF/mRNA approach. QuantifyHER (TBCRC066/NCT06551116) is a multi-site prospective cohort study. Key eligibility criteria include biopsy-proven HER2 IHC2+ MBC, available tissue, measurable disease, and initiation of T-DXd within 30 days of registration. Pts receive T-DXd per standard of care and are followed for real-world endpoints. Up to 10 unstained FFPE/1 H2+ archival specimen (most recent metastatic biopsy preferred) are sent to the Quantitative Diagnostics in Anatomic Pathology (QDAP) CLIA lab at Yale for analysis with both TROPLEX (returning HER2 and TROP2 QIF levels in amol/mm2) and the Cepheid Xpert STRAT4 Research Use Only (RUO) assay (returning ERBB2 mRNA delta Cycle threshold (dCt) levels). Patient data (demographics, clinical features, prior treatment, follow-up) are abstracted from the medical record, with clinical endpoints assessed by local investigators using real-world response categories and toxicities collected only as related to T-DXd discontinuation or modification. The planned sample size is 200 evaluable pts, providing 80% power (with a 2.5% 1-sided alpha), to detect a difference in proportions of 9-10 % per standard deviation QIF/mRNA increase, corresponding to an odds ratio of 1.6. A non-binding interim review at n=70 will assess preliminary biomarker distribution for any necessary sample size adjustment. QuantifyHER is open within the Translational Breast Cancer Research Consortium (TBCRC). As of 7/1/25, 10/12 planned sites are open and 23 pts have enrolled. Citation Format: E. P. Taranto, W. Fawn, C. Desirae, E. Walinsky, S. Deluca, L. Bayne, E. P. Wileyto, A. Shirali, C. J. Robbins, Y. Abdou, J. Anampa, M. Balic, J. V. Canzoniero, N. C. Chen, F. Howard, L. Huppert, A. Kahn, C. Mainor, J. Mouabbi, V. Nair, T. Degeer, J. Weidler, T. McNicholas, C. Serway, J. Reischl, M. Bates, R. Monroe, D. L. Rimm, A. DeMichele. Quantifyher (trial in progress): quantitative immunofluorescence and/or messenger rna to measure her2 expression in patients with metastatic breast cancer treated with trastuzumab deruxtecan (tbcrc066) 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 PS4-03-12.