124O Patritumab deruxtecan (HER3-DXd) in hormonal receptor-positive/HER2-negative (HR+/HER2-) and triple-negative breast cancer (TNBC): Results of part B of SOLTI TOT-HER3 window of opportunity trial

We previously reported the biologic and clinical activity of HER3-DXd in Part A of SOLTI TOT-HER3 in patients (pts) with HR+/HER2- early breast cancer (BC) after a single dose of 6.4 mg/kg (Prat A. et al, ESMO BREAST 2022). Here, we present the main efficacy and safety results of TOT-HER3 Part B. In Part B of this window of opportunity trial, 2 cohorts of pts with treatment-naïve HER2- primary operable BC, ≥1 cm by US or MRI were recruited. Pts received a single dose of HER3-DXd 5.6 mg/kg. Primary objective was the variation in tumor cellularity and tumor-infiltrating lymphocytes (CelTIL) score between pre- and post-treatment tumors. Associations of baseline clinical and biological features with CelTIL changes and overall response rate (ORR) by US were explored. Thirty-seven pts with HER2- BC were recruited (HR+ n=20; TNBC n=17). Mean age was 53 (30-81); 20 (54%) were pre-menopausal; mean tumor size was 21 mm (range 10-81 mm); and median Ki67 was 30% (12-95%). In response to treatment with HER3-DXd CelTIL score increased and decreased in 21 (57%) and 16 (43%) pts, respectively. Overall, a statistically significant change in CelTIL was observed between paired samples overall (p=0.046) and in TNBC (p=0.016), but not in HR+ (p=0.793). ORR by US in all patients was 32% (35% in TNBC and 30% in HR+). The absolute change in CelTIL was associated with ORR (AUC=0.693; p=0.049). Baseline ERBB3 levels were not associated with CelTIL change or ORR. HER3-DXd induced high expression of immune-related genes (e.g., PD1, CD8 and CD19), and suppressed proliferation-related genes. Thirty-one (84%) patients reported AEs at any grade. Most common AEs were nausea, fatigue, alopecia, diarrhea, constipation and vomiting. Grade 3 treatment-related nausea occurred in 1 patient. No ILD events were reported. A single dose of HER3-DXd was associated with clinical response and significant biological changes in HER2- tumors, irrespective of baseline ERBB3 levels. The safety profile was consistent with that previously reported. Additional correlative analysis will be presented on site.