Study Purpose: This clinical trial is testing if using an investigational medication with chemoradiotherapy can treat severe rectal cancer, a type of colon cancer. The treatment involves several phases with medications and radiation, followed by surgery.

Participation Details: The study will last several months, with participants going through different treatment phases including medication cycles, radiation therapy, and surgery. Participants will need to attend multiple visits over this time.

• **Duration:** Participation includes multiple phases over several months.
• **Visits:** Expect multiple visits for medication, radiation, and surgery.
• **Criteria:** Ages 18-75 with specific types of rectal cancer; must not have had prior treatments like chemotherapy or surgery.

Eligibility: You can join if you're between 18 and 75 years old and have a confirmed type of rectal cancer. You should be able to swallow pills and not have had any cancer treatments before. Women who can have children need to take a pregnancy test before starting and use birth control during and after the study.

Important Notes: If you're allergic to certain medications, have a history of autoimmune diseases, or have had certain infections, you may not be eligible. The study involves some risks, such as side effects from medications and treatments, but aims to find effective ways to treat rectal cancer.