SACI-IO HR+: A Randomized Phase II Trial of Sacituzumab Govitecan with or Without Pembrolizumab in Patients with Metastatic Hormone Receptor-Positive/her2-negative Breast Cancer.

LBA1004 Background: Sacituzumab govitecan (SG) is a TROP2 antibody drug conjugate (ADC) with a topoisomerase I inhibitor payload (SN-38) approved for previously treated triple negative and hormone receptor-positive/HER2-negative (HR+/HER2-) metastatic breast cancer (MBC). Double-strand DNA breaks induced by SN-38 activate cGAS-STING, stimulating type I IFN production and T cell recruitment. SN-38 can upregulate MHC class-I and PD-L1 expression, enhance cytotoxic T cell effector functions and deplete regulatory T cells. To evaluate if SG synergizes with pembrolizumab (PD-1 inhibitor) we conducted a randomized, open-label phase 2 study comparing SG with or without pembrolizumab in HR+/HER2- MBC (NCT04448886). Methods: Eligible patients (pts) had unresectable locally advanced or metastatic HR+ (ER≥1% and/or PR≥1%), HER2- breast cancer treated with ≥1 prior endocrine therapy and 0-1 chemotherapy (CT) for MBC. Pts with brain metastases were eligible if locoregional therapy was completed and steroids discontinued ≥7 days before study therapy. Pts who received prior topoisomerase I inhibitor ADC, irinotecan or PD-1/L1 inhibitors were excluded. Pts were randomized 1:1 to Arm A [SG 10 mg/kg IV (D1, D8) + pembrolizumab 200 mg IV (D1), 21-day cycle] or Arm B (SG alone). The primary endpoint was progression-free survival (PFS); secondary endpoints included PFS in PD-L1+ pts (22C3 CPS ≥1), overall survival (OS), objective response rate (ORR) and toxicity (NCI CTCAE v5.0). Baseline and on-treatment biopsies were performed for correlative analyses. For this preliminary analysis, data were locked 1/12/24. Results: Between 03/2021-01/2024, 110 pts enrolled; 104 pts (52 Arm A; 52 Arm B) started study therapy and were included in the analysis. Median age was 57 yrs (range: 27-81); 102 pts (98.1%) were female. 80 pts (76.9%) received prior CDK4/6 inhibitor for MBC; 58 (55.8%) had no prior CT, 46 (44.2%) had 1 prior line of CT for MBC. At a median follow-up of 9.2 months (mo), median PFS was 8.4 mo in Arm A vs 6.2 mo in Arm B (HR 0.76, 95% CI 0.47-1.23, log-rank p=0.26); ORR 21.2% and 17.3%, respectively. OS data are immature with only 26 events to date; OS was 16.9 mo vs 17.1 mo (HR 0.65, 95% CI 0.30-1.41, log-rank p=0.28), respectively. The most frequent treatment-related toxicities (≥G2) in Arm A were neutropenia (67.3%), fatigue (36.5%), alopecia (36.5%), anemia (32.7%), leukopenia (26.9%), diarrhea (21.2%) and nausea (21.2%); in Arm B, neutropenia (59.6%), alopecia (38.5%), diarrhea (34.6%), nausea (32.7%), fatigue (32.7%) and anemia (21.2%). Conclusions: Addition of pembrolizumab to SG showed a non-significant trend toward improved PFS in unselected HR+/HER2- MBC at this preliminary time point. Final PFS and updated OS with further follow-up will be presented at the meeting. Exploratory outcome analyses by TROP2 and PD-L1 expression will be reported. Clinical trial information: NCT04448886 .