Evaluate REC-4881 in Patients With FAP

1. Male or female and ≥ 18 years of age'

2. Have provided written informed consent to participate in the study

3. Diagnosis of phenotypic classical FAP with disease involvement of the duodenum or the residual colon/rectum/pouch as the primary disease site.

4. Genetic diagnosis of FAP with APC gene mutation (Part 2 only).

5. Has undergone colectomy or subtotal colectomy

6. No significant cardiovascular abnormalities

7. No significant hematopoietic abnormalities

8. No significant hepatic abnormalities

9. No significant renal abnormalities

10. Female participants must have a negative serum pregnancy test prior to Study Day 1

11. All participants must be willing to follow the contraceptive guidance in the protocol

12. Absence of gross blood in stool at Screening

13. Participant must be willing to discontinue use of non-steroidal anti-inflammatory agents (NSAIDs) prior to Study Day 1

1. No clinically significant laboratory abnormality, medical or psychiatric illness

2. Has had prior pelvic irradiation.

3. Has gastrointestinal disease or recent gastrointestinal procedure that could interfere with oral absorption of REC-4881

4. Has received treatment with other investigational agents prior to Study Day 1

5. Treatment with other FAP-directed drug therapy within 8 weeks of screening endoscopy (Part 2 only).

6. Is currently under treatment for desmoid tumors.

7. Use of omega-3 fatty acids or oral corticosteroids prior to Study Day 1

8. Use of strong CYP3A inhibitors or inducers prior to Study Day 1

9. History of an ongoing or newly diagnosed eye abnormality.

10. Cancer on biopsy at screening endoscopy in GI tract (including stomach, duodenum, and colon/rectum/pouch) (Part 2 only).

11. Has a large polyp (>1 cm) not amenable to complete removal

12. Has active pancreatitis secondary to pancreatic duct obstruction

13. Has active gall bladder disease

14. Is pregnant, lactating or is planning to attempt to become pregnant during the study

15. Has had major surgery prior to Study Day 1

16. Has an active infection requiring systemic therapy.

17. Has known hypersensitivity to the study drug or its excipients.

18. History of alcohol or substance abuse.

19. Received treatment with another MEK inhibitor prior to Screening

20. Active or known HIV, hepatitis B or hepatitis C infections

21. Has a severe or uncontrolled medical condition

22. Use of strong BCRP or MRP2 inhibitors prior to Study Day 1