The purpose of this study is to assess the safety and effectiveness of the investigational drug LNS8801 with and without pembrolizumab immunotherapy in people with advanced melanoma. LNS8801 works to reduce levels of a growth protein called MYC. By reducing these levels, LNS8801 may slow the growth of cancer, shrink the cancer, and help the immune system remove cancer from the body. Pembrolizumab boosts the ability of the immune system to find and kill cancer cells.