Background: hSTC810 is a humanized anti-BTN1A1 monoclonal antibody of the IgG4 isotype that binds to human BTN1A1 and blocks the interaction between BTN1A1 and its ligands. Butyrophilin 1A1 (BTN1A1) is a novel immune checkpoint protein that exhibits an expression pattern mutually exclusive to that of PD-1/PD-L1. BTN1A1 has been shown to inhibit anti-CD3-induced cytokine production and T-cell proliferation and also inhibit anti-CD3-induced p38 mitogen-activated protein kinase (MAPK), cAMP response element-binding protein (CREB) and target of rapamycin (TOR) phosphorylation in human T-cells. Mouse studies demonstrated that an anti-BTN1A1 antibody exhibited antitumor activity both as a single agent and in combination with anti-PD-1/PD-L1 or radiation therapy. In the first in human phase 1 study, investigating the safety and efficacy of hSTC810 in solid tumors, hSTC810 showed acceptable safety profile and the disease control rate (DCR) was 39.3%, which indicates its potential efficacy. Methods: This multi-center, open label phase 1b/2 study evaluates the safety, tolerability, efficacy, pharmacokinetics (PK), and pharmacodynamics (PD) of hSTC810 combined with capecitabine in patients with metastasis/recurrent colorectal cancer. The trial is enrolling adults who are (1) histologically or cytologically confirmed colorectal adenocarcinoma (2) resistant or intolerant to oxaliplatin and irinotecan-based chemotherapy (3) adequate hematologic, hepatic, renal function (4) measurable or non-measurable disease by RECIST 1.1 (5) ECOG PS 0~2 (6) resolved acute effects of any prior therapy. Treatment will continue until disease progression or unacceptable toxicity, whichever occurs first. The primary endpoint of phase 1b is to determine dose-limiting toxicity (DLT), and recommended phase 2 dose (RP2D) of hSTC810 combined with capecitabine, each in 21-day cycles. Phase 1 comprises 3 escalation arms, and if the initial dose is deemed unsafe, 2 deescalation arms will be initiated. Once the RP2D is determined, the Phase 2 will enroll up to 39 patients. The primary endpoint of phase 2 is progression free survival (PFS); secondary endpoints include objective response rate, overall survival, disease control rate, duration of response and quality of life. The study began in Q4 2023 and is recruiting patients. AEs are assessed according to CTCAE v5. Tumor response is determined according to RECIST 1.1 criteria. An exploratory study is underway to investigate the impact of hSTC810 on the tumor microenvironment and cytokines, utilizing both tissue biopsies and blood sampling. Clinical trial information: NCT05990543.