A Clinical Study to Test if an Investigational Treatment Called BNT314 When Used in Combination With Another Investigational Treatment Pumitamig (BNT327) and Chemotherapy, is Beneficial and Safe for Patients With Advanced Colorectal Cancer

Participants with microsatellite stable or mismatch repair proficient (MSS/pMMR) mCRC with progressive disease to the metastatic first line (1L) setting (Parts B and C) and beyond (Part A) as well as treatment-naïve (for Part B) are eligible to participate in the study.

The main study goals are as follows:

* Part A (Phase 1, safety run-in, dose escalation): To see if BNT314 in combination with pumitamig can be given safely, without causing severe side effects in participants. * Part B (Phase 1, dose optimization): To see if BNT314 in combination with pumitamig and standard of care (SoC) chemotherapy is safe for participants and to find out the right dose of BNT314 that can be used in Part C. * Part C (Phase 2, randomization against SoC): To see how well treatment works when BNT314 and pumitamig are given in combination with the usual SoC chemotherapy treatment.

In all three parts, the study will also look whether BNT314 can shrink tumors or slow down their growth when used with pumitamig and chemotherapy.

The study consists of a period to assess eligibility, a treatment period, a safety follow-up period, and a long-term survival follow-up period.

The sponsor plans to proactively assess participant safety on a regular basis for the duration of the study according to a predefined internal review committee. In addition, an independent data monitoring committee will be developed to provide medical oversight over Part C of the study.

Participants in the study will continue to receive treatment until their disease worsens, they can no longer tolerate the treatment, the participant chooses to leave the study, or the study ends. They are expected to be on treatment for about of 6-10 months on average. Participants may stop treatment early if their disease worsens or side effects occur but can continue longer if the disease stays controlled and the therapy is well-tolerated. After that, they will be monitored for their survival and any potential long-term side effects even after they stop participating in the study. Participants will be randomized to the treatment groups in Part B and Part C, which means participants will be assigned by equal chance to a treatment group.