ARV-6723

This is a study to evaluate the safety and potential anti-tumor activity of an investigational drug called ARV-6723, in participants with advanced solid tumors. This is an open-label study which means that participants and study staff will know that all participants will receive ARV-6723. The investigational drug ARV-6723 will be given as an oral tablet on its own and also in combination with other drugs. Researchers think that ARV-6723 can help the body's immune system to better recognize, attack, and destroy cancer cells in adults with advanced solid tumors. ARV-6723 is an investigational drug, and this is the first time ARV-6723 will be used in humans. Depending on the treatment assignment, the investigational drug, ARV-6723, will be given as an oral tablet either on its own (monotherapy) or in combination with pembrolizumab, either through a vein (intravenously or IV) or through an injection beneath the skin (subcutaneously or SQ). This study will include multiple parts: In Part A1 (Phase 1a), different small groups of participants will receive lower to higher doses of ARV-6723 as monotherapy or in combination with standard-dose of pembrolizumab. In Part A2 (Phase 1b), two groups of participants with specific tumor indication(s) (TBD) will receive one of two doses selected based on information from Part A1. Details of Part B will be determined based on information generated from Part A.