A Trial to Learn if Dupilumab is Safe for and Helps Adult and Adolescent Participants With Eosinophilic Gastritis With or Without Eosinophilic Duodenitis

The 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, compared to placebo.

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 your 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 trial will have 3 parts plus screening and follow-up parts:

* Parts A and B: Participants will either be included in part A or B. Each is a 24-week double-blind (this means none of the participants, doctors, or other trial staff will know what treatment each participant took) part where participants will receive either dupilumab or a placebo (a placebo looks like a trial drug but does not have any medicine in it).
* Part C: 28-week extension part that will include participants from parts A and B and all participants will receive dupilumab

Eligibility Criteria:
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. Baseline 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 patient report) of at least 2 episodes of EoG (with or without EoD) symptoms per week in 8 weeks before screening
6. For the 2 weeks prior to baseline visit, an average total symptom score (TSS) of at least of 20 calculated using data from the EoG/EoD-SQ eDiary and an average severity score of at least 4 (on a scale of 0-10) per week for at least 2 of the 6 symptoms, as defined in the protocol.

Key Exclusion Criteria:

1. Body weight less than 40 kg
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
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 (Part A and B).

NOTE: Other Protocol Defined Inclusion / Exclusion Criteria Apply