This study is researching an experimental drug called dupilumab. The study is focused on participants with active eosinophilic gastritis (EoG) with or without eosinophilic duodenitis (EoD). Participants with EoD only are not eligible for enrollment. EoG and EoD are uncommon, persistent, allergic/immune diseases in which eosinophils (a type of white blood cell) gather in large numbers in the stomach and small intestine and cause inflammation and damage.

The aim of the study is to evaluate the effect of dupilumab on relieving EoG (with or without EoD) symptoms and reducing inflammation in the stomach and, if applicable, small intestine in adults and adolescents aged 12 years and older after at least 24 weeks (about 6 months) and up to 52 weeks (1 year) of treatment.

The study is looking at several other research questions, including:

• What side effects may happen from taking the study drug
• How much study drug is in the blood at different times
• Whether the body makes antibodies against the study drug (which could make the drug less effective or could lead to side effects)

This study has 2 parts and a 12-week (about 3 months) Follow-up Period for all participants

• Part A is an open-label treatment period lasting up to 24 weeks (up to 6 months). "Open-label" means that you and the study doctors and the staff staff will know that you are taking the study drug
• Part C is an open-label extended treatment period lasting up to 28 weeks (about 7 months). "Extended Treatment Period" means that you will take the study drug for an additional 28 weeks after finishing Part A if you are eligible to take part in this part of the study

Key Inclusion Criteria:

1. Adolescent participants will only be enrolled at study sites in countries/regions as permitted by local regulatory authorities and ethic committees (ECs)
2. Documented endoscopic biopsy supporting a pathologic diagnosis of Eosinophilic gastritis (EoG) at least 3 months prior to screening
3. Screening endoscopic biopsies with a demonstration of eosinophilic infiltration for a diagnosis of EoG, as defined in the protocol
4. Completed at least 11 of 14 days of EoG/EoD-SQ eDiary data entry in the 2 weeks prior to the baseline visit
5. History (by participant report) of at least 2 episodes of EoG (with or without EoD) symptoms per week in 8 weeks before screening, as defined in the protocol
6. An average TSS of ≥ 20 calculated using data collected via the EoG/EoD-SQ eDiary per week for the 2 weeks prior to baseline. An average severity score of ≥4 (on a scale of 0-10) per week for the 2 weeks prior to baseline for at least 2 of the 6 symptoms, as defined in the protocol.

Key Exclusion Criteria:

1. Body weight less than 40 kg at screening
2. Prior participation in a dupilumab clinical trial, or past or current treatment with dupilumab
3. Helicobacter pylori infection
4. Any esophageal stricture unable to be passed with a standard, diagnostic, upper endoscope or any critical esophageal stricture that requires dilation at screening
5. History of achalasia, Crohn's disease, eosinophilic colitis, ulcerative colitis, celiac disease, and prior gastric or duodenal surgery, as defined in the protocol
6. Other causes of gastric and, if applicable, duodenal eosinophilia or the following conditions: eosinophilic granulomatosis with polyangiitis (Churg-Strauss syndrome) or hyper-eosinophilic syndrome
7. History of bleeding disorders, esophageal or gastric varices that, in the opinion of the investigator, would put the participant at undue risk for significant complications from an endoscopy procedure
8. Initiation or change of a food-elimination diet regimen or re-introduction of a previously eliminated food group in the 4 weeks prior to the screening visit. Participants on a food-elimination diet must remain on the same diet throughout the study
9. Planned or anticipated use of any prohibited medications and procedures during the study
10. Planned or anticipated major surgical procedure during the study
11. Receiving tube feeding or parenteral nutritional at screening

NOTE: Other Protocol Defined Inclusion / Exclusion Criteria Apply

All Individual Patient Data (IPD) that underlie publicly available results will be considered for sharing.

When Regeneron has:

• received marketing authorization from major health authorities (e.g., FDA, European Medicines Agency (EMA), Pharmaceuticals and Medical Devices Agency (PMDA), etc.) for the product and indication or has globally discontinued development of the product for all indications on or after April 2020 and has no plans for future development
• made the study results publicly available (e.g., scientific publication, scientific conference, clinical trial registry)
• the legal authority to share the data, and
• ensured the ability to protect participant privacy

Qualified researchers can submit a proposal for access to individual patient or aggregate level data from a Regeneron-sponsored clinical trial through Vivli. Regeneron's Independent Research Request Evaluation Criteria can be found at: https://www.regeneron.com/sites/default/files/Regeneron-External-Data-Sharing-Policy-and-Independent-Research-Request-Evaluation-Criteria.pdf