This study is for adults aged 18 to 75 with severe rectal cancer, known as rectal adenocarcinoma. The research is testing if an investigational medication, combined with chemoradiotherapy, is effective in treatment. Chemoradiotherapy is a mix of chemotherapy (using drugs to kill cancer cells) and radiation therapy (using high-energy rays to kill cancer cells).

The study has three parts:

• Induction: Participants receive the investigational medication and a chemotherapy regimen called CAPOX for one cycle (21 days).
• Radiotherapy: Participants undergo short-course radiotherapy over five consecutive days.
• Consolidation: Participants continue with the investigational medication and CAPOX for five more cycles (each cycle is 21 days) before having surgery.

Key eligibility criteria include not having previous cancer treatments and being able to swallow pills. Surgery follows within 4 to 8 weeks after the final cycle. If you have allergies to study drugs, certain heart or immune conditions, or are pregnant, this study may not be suitable.