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