Additional Study Information: This study will test the investigational drug, DS-7300a, for the first time in humans. DS-7300a is an antibody-drug conjugate (ADC) being developed as an anti-cancer agent. ADCs are made up of a monoclonal antibody chemically linked to a drug. The monoclonal antibody binds to specific proteins or receptors found on certain types of cells, including cancer cells. The linked drug enters these cells and kills them without harming other cells. Some antibody-drug conjugates are used to treat cancer. Also called ADC. This study is in one single group of participants with advanced solid tumors who have not been cured by other treatments. The study will be conducted in two parts. Cancers may include bladder cancer, breast cancer, esophageal cancer, uterine cancer, head and neck/oral cancers, lung cancer, skin cancers, pancreatic cancer, prostate cancer, and bone and soft tissue cancer.