This is a multicenter, two-part trial in participants with FAP.
Male or female and ≥ 18 years of age
Have provided written informed consent to participate in the study
Diagnosis of phenotypic classical FAP with disease involvement of the duodenum or the residual colon/rectum/pouch as the primary disease site.
Genetic diagnosis of FAP with APC gene mutation (Part 2 only).
Has any clinically significant laboratory abnormality, medical or psychiatric illness which, in the opinion of the Investigator, could interfere with the conduct or interpretation of the study or put the participant at risk.
Has had prior pelvic irradiation.
Has gastrointestinal disease or recent gastrointestinal procedure that could interfere with oral absorption of REC-4881, including difficulty swallowing capsules.
Has received treatment with other investigational agents within the 4 weeks prior to Study Day 1 or a period during which the investigational agent has not been cleared from the body (that is, at least a period of 5 half-lives, if known), whichever is longer.