BTN1A1 expression as a biomarker for patient selection: Phase 1b IHC analysis and clinical outcomes supporting a biomarker-guided phase 2 trial in colorectal cancer.

Abstract

TPS281 Background: Metastatic colorectal cancer (CRC) has poor prognosis in patients resistant or intolerant to oxaliplatin- and irinotecan-based chemotherapy, and durable responses to salvage regimens such as trifluridine/tipiracil plus bevacizumab remain limited. BTN1A1, a novel immune checkpoint implicated in tumor progression and immune evasion, is under clinical evaluation as a therapeutic target. Nelmastobart, a first-in-class anti-BTN1A1 monoclonal antibody, is being tested in the STCUBE-003 Phase Ib/II trial (NCT06873763) in combination with trifluridine/tipiracil and bevacizumab. This study aimed to validate BTN1A1 as a patient selection biomarker in CRC clinical trials and to explore its relationship with other immune and tumor biomarkers in the tumor microenvironment using ongoing Phase 1b patient samples. BTN1A1 expression was assessed by single-marker IHC for prevalence and multiplex IHC (Opal platform) for spatial profiling with immune markers. Methods: Archived tumor samples from Phase 1b CRC patients in the STCUBE-003 trial were analyzed. BTN1A1 expression was assessed by single-plex IHC using Tumor Proportion Score (TPS) and H-score (0–300). Multiplex IHC was performed to evaluate BTN1A1 with PD-L1, CD8, Ki-67, SLFN11, cytokeratin (CK), and YAP1, enabling assessment of co-expression, spatial distribution, and immune–tumor interactions. Results: The safety run-in portion of STCUBE-003 was conducted from June 17 to August 7, 2025. Six Phase 1b CRC tumor samples were evaluated by IHC. BTN1A1 expression was detected in all samples: one patient (16.7%) had low expression (TPS 20, H-score 20), while five (83.3%) had high expression (TPS ≥50). H-scores ranged from 20 to 300, strongly correlating with TPS (r = 0.84). These results were consistent with prior findings in CRC TMAs and previous BTN1A1 studies. At first response evaluation, two high-TPS patients achieved partial responses (52% and 31% reductions), while four experienced decreasing stable disease. No progressive disease was observed. Multiplex IHC revealed BTN1A1 spatial association with PD-L1, Ki-67, SLFN11, CK, and YAP1. Conclusions: BTN1A1 was consistently expressed across all tumors, with high TPS associated with favorable outcomes. These findings support BTN1A1 as a robust biomarker for patient selection and provide rationale for the ongoing BTN1A1-guided Phase 2 trial. Clinical trial information: NCT06873763 .