A Clinical Trial to Test if the Investigational Drug BNT329 is Safe and Potentially Beneficial for People With Advanced Solid Tumors Known to Express the Tumor Marker CA19-9

The main goal of this study is to evaluate the safety of BNT329 and to identify the best dose of BNT329. This will be done by measuring the number of side effects that participants experience and how severe they are. The second goal of this study is to evaluate how well BNT329 works. This will be done by measuring the number of participants who respond to the treatment. The length of time where the tumor does not grow or spread will also be measured. The study will also evaluate how BNT329 moves into, through, and out of the body and how the treatment affects the body.