Genes contain genetic code which tell the body which proteins to make. Many types of cancer are caused by changes, or mutations, in a gene called KRAS. Researchers are looking for ways to stop the actions of abnormal proteins made from the mutated KRAS gene. The so-called G12D mutation in the KRAS gene is common in people with some solid tumors. ASP3082 is a potential new treatment for certain solid tumors in people who have the G12D mutation in their KRAS gene. Before ASP3082 is available as a treatment, the researchers need to understand how it is processed by and acts upon the body. This information will help find a suitable dose and to check for potential medical problems from the treatment. People in this study will be adults with locally advanced, unresectable or metastatic solid tumors with the G12D mutation in their KRAS gene (G12D mutation). Locally advanced means the cancer has spread to nearby tissue. Unresectable means the cancer cannot be removed by surgery. Metastatic means the cancer has spread to other parts of the body. They will have been previously treated with standard therapies or refused to receive those treatments. In the European Union (EU) and South Korea, people who have refused to receive treatment with standard therapies cannot take part. The main aims of the study are: to check the safety of ASP3082 by itself and together with cetuximab (a common cancer medicine), how well it is tolerated, and to find a suitable dose of ASP3082 by itself and together with cetuximab. This is an open-label study. This means that people in this study and clinic staff will know that they will receive ASP3082. This study will be in 2 parts. In Part 1, different small groups of people will receive lower to higher doses of ASP3082, by itself, or together with cetuximab. Only people with colorectal cancer will receive ASP3082 together with cetuximab. Any medical problems will be recorded at each dose. This is done to find suitable doses of ASP3082 by itself or together with cetuximab to use in Part 2 of the study. The first group will receive the lowest dose of ASP3082. A medical expert panel will check the results from this group and decide if the next group can receive a higher dose of ASP3082. The panel will do this for each group until all groups have received ASP3082 (by itself or together with cetuximab) or until suitable doses have been selected for Part 2. In Part 2, other different small groups of people will receive ASP3082 by itself or together with cetuximab, with the most suitable doses worked out from Part 1. This will help find a more accurate dose of ASP3082 to use in future studies. ASP3082, and cetuximab (if used), will be given through a vein. This is called an infusion. Each treatment cycle is 21 days long. They will continue treatment until: they have medical problems from the treatment they can't tolerate; their cancer gets worse; they start other cancer treatment; they ask to stop treatment; they do not come back for treatment. People will visit the clinic on certain days during their treatment, with extra visits during the first 2 cycles of treatment. During these visits, the study doctors will check for any medical problems from ASP3082 by itself or together with cetuximab. At some visits, other checks will include a medical examination, echocardiogram (ECHO) or multigated acquisition (MUGA) scan, blood and urine tests and vital signs. Vital signs include temperature, pulse, breathing rate, and blood pressure. (Blood oxygen levels will also be checked for people treated with ASP3082 together with cetuximab.) Tumor samples will be taken during certain visits during treatment and when treatment has finished. People will visit the clinic within 7 days after stopping treatment. The study doctors will check for any medical problems from ASP3082 by itself or together with cetuximab. Other checks will include a medical examination, echocardiogram (ECHO) or multigated acquisition (MUGA) scan, urine and blood tests and vital signs. After this, people will continue to visit the clinic every 9 weeks. This is to check the condition of their cancer. They will do this until 45 weeks after treatment stopped, or if their cancer is worse, they start other cancer treatment, they ask to stop treatment, or they do not come back for treatment. Also, people may visit the clinic at 30 days and 90 days after stopping treatment. At the 30-day visit, the study doctors will check for any medical problems from ASP3082 by itself or together with cetuximab. People will have their vital signs checked and have some bloo