The goal of this clinical trial is to learn about the safety of a single dose of HB-2121 in adults. It will also look at how the body processes the drug. The main questions it aims to answer are:

• What side effects do participants have after receiving HB-2121?
• How much HB-2121 is in the blood over time?

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
• 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

General Inclusion Criteria (applies to all participants):

• Aged 18 years and older
• Body Mass Index (BMI) between 18 and 45 kg/m2
• Overall good health, as determined by medical history and a physical exam
• No use of an investigational drug in the past 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

Additional Inclusion Criteria for Healthy Volunteers:

• No diagnosis of celiac disease
• No first-degree relatives (parent, sibling, child) with celiac disease
• Able to eat gluten-containing foods without adverse effects
• No history of autoimmune gastrointestinal disorders such as Crohn's disease, ulcerative colitis, or eosinophilic enteritis

Additional Inclusion Criteria for Participants with Celiac Disease:

• Biopsy-confirmed diagnosis of celiac disease
• No history of eosinophilic enteritis, autoimmune gastrointestinal disorders such as Crohn's disease or inflammatory bowel disease

Exclusion Criteria (applies to all participants):

• Pregnant, breastfeeding, or planning to become pregnant during the study period
• History of chemotherapy or pelvic radiation
• History of congenital long QT syndrome or prolonged QTcF interval
• 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
• Active cancer or history of cancer