Background: Leucine-rich repeat-containing G protein-coupled receptor 5 (LGR5) is a cancer stem cell (CSC) marker that is abundant in solid tumors, highly expressed in colorectal cancer and involved in tumor initiation, growth, and metastasis. Its expression is linked to poor survival and poor patient response to therapy in mCRC, and presents an attractive opportunity for selectively targeting a highly specific, functional CSC receptor in this disease. CNA3103 is a first-in-class cell product consisting of autologous T cells transduced to express a CAR to target LGR5. It contains the antigen-biding domain of BNC101, a humanized monoclonal LGR5 antibody previously tested in Phase 1 in mCRC subjects and demonstrated to be safe and well tolerated with no off-target binding activity. Methods: This is a first in human, multicenter, open label, Phase 1/2a dose escalation and expansion study to determine the safety of and overall best response to CNA3103 in subjects with mCRC. Up to 44 subjects (24 in Phase 1 and 20 in Phase 2a) will be enrolled. The trial follows a BOIN design with a minimum of 3 consecutively enrolled subjects per cohort, and a 28-day DLT evaluation period. Four cell dose levels (5 × 10⁷, 1.5 × 10⁸, 4.5 × 10⁸, 1.5 × 10⁹) of CNA3103 will be evaluated. A Safety Monitoring Committee (SMC) will oversee the conduct of the study. The pharmacokinetic (PK) profile of CNA3103 in blood will be assessed. Exploratory objectives include evaluation of LGR5+ cells and CNA3103 in tumor biopsies, immune cellular profile, cytokine levels and circulating tumor DNA in blood, and immunogenicity of CNA3103. Main inclusion criteria: mCRC subjects positive for LGR5 expression and failing prior treatment with 5-FU, oxaliplatin and irinotecan-based regimens for metastatic disease; >1 radiologically measurable lesion per RECISTv1.1; >1 biopsiable lesion at baseline; ECOG 0 or 1; serum albumin >3 g/dL; and normal organ and marrow function. Main exclusion criteria: BRAF-mutated disease, clinical ascites/ pleural effusions; active autoimmune/ connective tissue disease; history of inflammatory bowel disease or active peptic ulcer disease. The trial is sponsored by Carina Biotech. Phase 1 enrolment is limited to Australian sites. Phase 2a enrolment will take place in Australia and the US. For information, contact Lina Jablonskis lina@carinabiotech.com or +61 437 891 563.