GEN1042 Safety Trial and Anti-tumor Activity in Participants With Malignant Solid Tumors

This is an open-label, multicenter phase 1/2 study designed to assess the safety, pharmacokinetics, pharmacodynamics and anti-tumor activity of GEN1042 administered as a monotherapy or in combination in participants with metastatic or locally advanced solid tumors. Participants will receive either GEN1042 alone, GEN1042 with pembrolizumab, or GEN1042 with pembrolizumab and chemotherapy. All participants will receive active drug; no one will receive placebo. This trial has 2 parts. The purpose of the first part is to find out if GEN1042 is safe and to find out the best doses of GEN1042 to use. The purpose of the second part is to give GEN1042 to more participants to see how well the dose(s) of GEN1042 selected in the first part work against cancer with GEN1042 when given alone or in combination with pembrolizumab or in combination with pembrolizumab and chemotherapy. Trial details include: * The average trial duration will be about 2 years and 4 months. * The treatment duration will be up to 2 years (when GEN1042 is combined with pembrolizumab). * The visit frequency will be weekly at first and lessening over time until visits are only once every 3 weeks. A long-term extension phase (LTEP) has been added to allow continued access to trial treatment for up to 2 years from the individual participant's Cycle 1 Day 1 visit.