Safety and Efficacy Study of GEN1046 as a Single Agent or in Combination With Pembrolizumab for Treatment of Recurrent (Non-small Cell) Lung Cancer

The goal of this clinical trial is to compare the safety and efficacy (how well the drug works) of acasunlimab (also known as GEN1046) when it is used alone (monotherapy) versus when it is combined with a cancer drug (pembrolizumab) for participants with relapsed/refractory (disease has returned after treatment or did not respond to treatment) non-small cell lung cancer (NSCLC; the most common type of lung cancer). This trial has 2 parts. The purpose of the first part is to find out if the combination of acasunlimab and pembrolizumab is safe and to find out the best doses to use. The purpose of the second part is to give acasunlimab and pembrolizumab to more participants to evaluate efficacy. In the second part of the trial, participants will be randomized to participate in 1 of the 3 arms of the trial. Randomized means that the participant will be randomly assigned to a treatment arm based on chance; no one chooses their treatment arm. Participants will receive either acasunlimab alone (100 followed by 500 mg into the vein) or acasunlimab with pembrolizumab (200 or 400 mg into the vein) once every 3 or 6 weeks, depending on which arm the participant is randomized into. All participants will receive active drug; no one will receive placebo. Trial details include: * The average trial duration for an individual participant will be about 10 months. * The average treatment duration for an individual participant will be about 6 months. * The visit frequency will be weekly at first and lessening over time until visits are only once every 3 weeks.