The study involves receiving an infusion of either pembrolizumab or ivonescimab every three weeks for up to 2 years. A computer will assign you to receive pembrolizumab or ivonescimab in a process called 'randomization'. Neither you nor the study doctors can choose your group or know which group you are in. Only the pharmacist or nurse who prepare the drug knows which of the two drugs you get. You will meet with a member of the study team in addition to your oncology care team during each of your clinic visits. In addition to normal treatment activities, your blood and urine will be collected for analysis. Between 2 and 11 tablespoons of blood will be collected on treatment cycles 1-7, 9, 11, 13, 15, 17, 19, 21 and at the end of treatment to monitor how your body breaks down the study drug or pembrolizumab, and to see if your body has produced specific antibodies to the study drug or pembrolizumab. You will also be required to fill out 3 questionnaires about your well-being on treatment visits 1-18, and every 3 treatment cycles thereafter.