Additional Study Information: The purpose of this study is to assess whether an investigational drug, called XmAb808, given in combination with pembrolizumab, has any effects on your cancer, to determine the best dose of XmAb808 to treat your cancer, and if it is safe and well-tolerated. XmAb808 is an investigational drug being developed for the treatment of advanced solid tumors. An investigational drug is a drug that has not been approved by the U.S. Food and Drug Administration (FDA), the health authority that approves new medicines to be prescribed in the United States. Cancers include: bladder cancer, breast cancer, colon and rectal cancer, esophageal cancer, gynecologic cancers, head and neck/oral cancer, kidney cancer, liver cancer, lung cancer, melanoma, pancreatic cancer, prostate cancer, stomach cancer, testicular cancer, and thyroid cancer.