A First-in-Patient Study to Evaluate the Safety and Tolerability of HB-2121 as a Diagnostic for Celiac Disease

The goal of this clinical trial is to learn about the safety of a single dose of HB-2121 in adults with suspected celiac disease. It will also look at how the drug affects the small intestine. The main questions it aims to answer are: * What side effects do participants have after receiving HB-2121? * How does the drug interact with the small intestine in people with suspected celiac disease? Researchers will follow participants for 30 days after receiving HB-2121 to understand how the drug behaves in the body and how safe it is. Participants will: * Receive one oral dose of HB-2121 four hours before their standard-of-care esophagogastroduodenoscopy * Attend 4 in-person clinic visits for checkups, lab tests, and monitoring * Complete 2 remote visits that include safety lab assessments * Fill out a short daily questionnaire for 7 days about symptoms and health status

Inclusion Criteria: * Age 18 to 75 years * Undergoing esophagogastroduodenoscopy (EGD) for suspected celiac disease * Body Mass Index (BMI) between 18 and 45 kg/m2 * Creatinine \< 1.5 x Upper Limit of the Normal (ULN) * Total bilirubin ≤ 1.5 mg/dL x (ULN) * Aspartate aminotransferase (AST)/Serum glutamic oxaloacetic transaminase (SGOT) \& Alanine Aminotransferase (ALT)/Serum glutamic pyruvic transaminase (SGPT) ≤ 1.5 x ULN * Overall good health, as determined by medical history and a physical exam * No use of an investigational drug within 12 weeks * Able and willing to follow study procedures and provide written informed consent * If of childbearing potential, participants must agree to use highly effective birth control during the study period. The same applies to male participants with partners of childbearing potential Exclusion Criteria: * Pregnant, breastfeeding, planning to become pregnant, or intending to donate eggs during the study period * History of cancer or malignancy * History of chemotherapy and/or pelvic radiation * History of congenital long QT syndrome or prolonged QTcF interval * Prisoners, institutionalized individuals, or individuals who are unable to consent for themselves * Known HIV infection, or positive test for hepatitis B or C, or other clinically significant chronic liver disease * Current use of immunosuppressant medications * Known allergy or sensitivity to any ingredients in the study drug * History of eosinophilic enteritis, Crohn's disease, or ulcerative colitis