Congenital hyperinsulinism (HI) is a genetic disorder in which pancreatic β-cell insulin secretion is excessive and results in hypoglycemia that, without treatment, can cause brain damage or death.Most patients with loss-of-function mutations in ABCC8 and KCNJ11, the genes encoding the β-cell ATP-sensitive potassium channel (KATP), are unresponsive to diazoxide, the only U.S.Food and Drug Administration-approved medical therapy and require pancreatectomy.The glucagon-like peptide 1 receptor (GLP-1R) antagonist exendin-(9-39) is an effective therapeutic agent that inhibits insulin secretion in both HI and acquired hyperinsulinism.Previously, we identified a highly potent antagonist antibody, TB-001-003, which was derived from our synthetic antibody libraries that were designed to target G protein-coupled receptors.Here, we designed a combinatorial variant antibody library to optimize the activity of TB-001-003 against GLP-1R and performed phage display on cells overexpressing GLP-1R.One antagonist, TB-222-023, is more potent than exendin-(9-39), also known as avexitide.TB-222-023 effectively decreased insulin secretion in primary isolated pancreatic islets from a mouse model of hyperinsulinism, Sur1-/- mice, and in islets from an infant with HI, and increased plasma glucose levels and decreased the insulin to glucose ratio in Sur1-/- mice.These findings demonstrate that targeting GLP-1R with an antibody antagonist is an effective and innovative strategy for treatment of hyperinsulinism.