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