ASP1012 is a special virus made to fight cancer. It attacks cancer cells but leaves healthy cells alone and helps the immune system see and fight cancer. Scientists need to understand how ASP1012 works in the body before it can be used as a treatment. This study is the first time ASP1012 is being tested on humans. Participants will be adults with advanced solid tumors, meaning their cancer has spread or grown beyond its original spot. Some participants will also receive pembrolizumab, a cancer medicine already in use.

There are three parts to the study, each testing different doses of ASP1012. Participants will get ASP1012 through an infusion (a slow injection through a vein) every 21 days. The study doctors will check health signs like temperature, pulse, and blood pressure during visits. They will also do blood tests and scans to see how well the treatment works. The study will last for at least three cycles, and participants can stop early if they have problems or choose to.

• Each treatment cycle lasts 21 days; you may receive more cycles if responding well.
• Some visits can be virtual or by phone; clinic visits are required.
• The study includes checks for medical problems and vital signs.