This study has a screening period, a treatment period, and a follow up period. During screening, participants will have tests such as physical exams, blood and urine tests, heart tests, and scans to see if they can join the study.

Participants will receive a study medicine called risvutatug rezetecan through a vein every three weeks. People with osteosarcoma will be placed into one of two groups by chance. One group will receive the study medicine alone, and the other group will receive a higher dose of the study medicine along with another medicine that helps the body make more white blood cells.

People with soft tissue sarcoma will receive the study medicine every three weeks. During the study, participants will have regular visits with blood tests, questionnaires, and scans such as CT or MRI to check their health and see how the cancer is responding. Treatment may continue as long as it is helping and side effects remain manageable.

This study is for people who are 12 years old or older and have a type of cancer called osteosarcoma or soft tissue sarcoma. The cancer must have come back after treatment, spread to other parts of the body, or be unable to be removed with surgery. The study includes one group for people with osteosarcoma and another group for people with certain types of soft tissue sarcoma, such as liposarcoma and leiomyosarcoma. Researchers are studying new treatment options for these cancers to help improve care for patients.

People may be able to join this study if they are 12 years old or older and have osteosarcoma or certain types of soft tissue sarcoma that have already been treated but have come back, spread to other parts of the body, or cannot be removed with surgery. They must have at least one tumor that can be measured on scans.

People cannot join if they have untreated cancer that has spread to the brain or central nervous system, or if cancer in those areas is getting worse. They also cannot join if they have serious uncontrolled heart problems or severe lung disease.

This study is testing a new medicine called risvutatug rezetecan for people who have osteosarcoma or soft tissue sarcoma that has come back, spread to other parts of the body, or stopped responding to treatment. Researchers want to find the best dose of the medicine and learn how safe it is. They also want to see if the medicine can slow the growth of cancer or help shrink tumors. The goal is to learn if this new treatment could become a helpful option for people with these cancers in the future.