Abstract P5-02-31: Genetic determinants of response to patritumab deruxtecan (HER3-DXd) in hormone receptor-positive/HER2-negative (HR+/HER2-) breast cancer: a correlative analysis from SOLTI TOT-HER3 trial
Authors: Brasó-Maristany Fara , Falato Claudette , Martínez-Sáez Olga , Cejalvo Juan Miguel , Margelí Mireia , Tolosa Pablo , Bofill Francisco Javier Salvador , Cruz Josefina , González-Farré Blanca , Sanfeliu Esther , Arumí Míriam , Villacampa Guillermo , Ciruelos Eva , Espinosa-Bravo Martín , Izarzuzaga Yann , Galván Patricia , Matito Judit , Pernas Sonia , Santhanagopal Anu , Esker Stephen , Patel Parul , Fan Pang-Dian , Ferrero-Cafiero Juan Manuel , Vivancos Ana , Pascual Tomás , Prat Aleix , Oliveira Mafalda 
Published: March 2023

Background: Baseline HER3 protein or ERBB3 mRNA levels do not seem to predict efficacy from HER3-DXd in early-stage and advanced HR+/HER2- breast cancer (Prat et al. ESMO Breast 2022; Krop et al. ASCO 2022). Here, we evaluated potential baseline pre-treatment genetic determinants of efficacy to HER3-DXd. Methods: SOLTI TOT-HER3 (NCT04610528) is a window of opportunity, multicenter, pre-operative trial which enrolled, in part A, 77 evaluable patients with untreated HR+/HER2- operable (≥1 cm) breast cancer. Patients received a single dose of HER3-DXd (6.4 mg/kg). The primary objective was to evaluate the CelTIL score variation between pre- and post-treatment (day 21) samples. CelTIL combines % of tumor cellularity and % of tumor-infiltrating lymphocytes into a single score. DNA and RNA were purified from pre-treatment baseline FFPE tumor samples. Gene expression was evaluated using a custom 67-gene panel on the nCounter. NGS-based DNA-seq was performed using the VHIO-300 panel, which estimates tumor mutational burden (TMB), identifies copy-number aberrations (CNAs) across the entire genome and calls mutational status of &gt;300 genes. From CNA data, 150 previously defined DNA-based signatures (Xia et al. Nat Comm 2019) trained to capture RNA- and protein-based phenotypes such as the PAM50-related biology were evaluated. Associations of each variable with efficacy (i.e., CelTIL relative changes, and tumor cellularity relative changes) were adjusted for multiple-testing (false discovery rate [FDR] &lt; 5%). The area under the ROC Curve (AUC) was used to estimate the discrimination performance of each variable. Results: RNA and DNA data were obtained from 45 (58%) patients. Baseline characteristics in this subset of patients were generally similar to the original TOT-HER3 population. Among 228 variables (single mutation status, single gene expression, PAM50 signatures, TMB, and DNA CNA-based signatures), 139 (61%) were found significantly associated (FDR&lt; 5%) with CelTIL changes at day 21. Among them, TP53 mutations (n=7) were found associated with higher CelTIL response compared to TP53 wild type (71% [95% CI=-5.4-17.8] vs. 24% [95% CI=15.9-55.4], FDR=2.1%). In addition, RNA-based genes tracking Basal-related biology (e.g., CCNE1, AUC=0.71) or immune expression (e.g., PDCD1, AUC=0.73, or CD68, AUC=0.62), together with RNA/DNA-based signatures tracking proliferation and/or basal-related biology (e.g., retinoblastoma loss-of-heterozygozity [RB-LOH], AUC=0.76), were associated with high CelTIL response. Conversely, RNA/DNA-based signatures tracking endocrine sensitivity/Luminal A-related biology (e.g., Scorr_IE_Correlation, AUC=0.76) were associated with low/lack of CelTIL response. PIK3CA somatic mutations (n=14, 31% of cases), and TMB (range 2.2-12.7) were not found associated with CelTIL response. Similar overall results were obtained when relative changes in tumor cellularity (instead of CelTIL) was evaluated as the efficacy endpoint. Conclusions: TP53 mutations, immune-related genes, and DNA/RNA-based phenotypic signatures tracking Basal- or Luminal A-related biology such as the DNA-based RB-LOH score or the endocrine sensitivity score (Scorr_IE_Correlation) are associated with CelTIL changes in response to HER3-DXd in HR+/HER2- breast cancer. Further RNA- and DNA-based analyses will be evaluated.

Citation Format: Fara Brasó-Maristany, Claudette Falato, Olga Martínez-Sáez, Juan Miguel Cejalvo, Mireia Margelí, Pablo Tolosa, Francisco Javier Salvador Bofill, Josefina Cruz, Blanca González-Farré, Esther Sanfeliu, Míriam Arumí, Guillermo Villacampa, Eva Ciruelos, Martín Espinosa-Bravo, Yann Izarzuzaga, Patricia Galván, Judit Matito, Sonia Pernas, Anu Santhanagopal, Stephen Esker, Parul Patel, Pang-Dian Fan, Juan Manuel Ferrero-Cafiero, Ana Vivancos, Tomás Pascual, Aleix Prat, Mafalda Oliveira. Genetic determinants of response to patritumab deruxtecan (HER3-DXd) in hormone receptor-positive/HER2-negative (HR+/HER2-) breast cancer: a correlative analysis from SOLTI TOT-HER3 trial [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 P5-02-31.