This study is for people with solid tumors (a type of cancer). The study tests a new drug called XMT-1660 to see how safe it is and what side effects it may cause. Side effects are things a drug does other than treat the disease. The study has two parts: Dose Escalation (finding how much XMT-1660 to give) and Dose Expansion (seeing how safe and effective it is).

Participants should have a solid tumor that has returned or spread. They must meet specific health criteria like having a good performance status (ability to do daily activities) and provide tumor tissue for testing. Certain conditions, like recent major surgery or untreated brain cancer, may prevent participation.

• The study has two parts, and the first part will determine the best dose to use.
• Participants will need to provide tumor tissue for testing.
• Recent major surgery or untreated brain cancer may exclude participation.

Consider if you meet the health criteria and can commit to the study requirements before deciding to participate.