SumMET: Study of [225Ac]Ac-FL-261 in Participants With Solid Tumors Overexpressing c-MET

Inclusion Criteria: * They have signed the consent form before any study-related tests or procedures are done and are willing to follow all study requirements. * They are 18 years of age or older. * They have a confirmed diagnosis of an advanced or metastatic cancer that cannot be cured with surgery, radiation, or other treatments. * Phase 1: Non-squamous non-small cell lung cancer (ns-NSCLC). * Phase 2: Non-squamous non-small cell lung cancer (ns-NSCLC), colorectal cancer (CRC), head and neck squamous cell carcinoma (HNSCC), or cholangiocarcinoma (bile duct cancer). * Their cancer has worsened after previous treatments, or they cannot receive further standard treatments because of reasons such as side effects or medical conditions. * For lung cancer (ns-NSCLC): Previously treated with chemotherapy, immunotherapy, and targeted therapies if applicable. * For colorectal cancer (CRC): Previously treated with standard chemotherapy and other suitable targeted or immune therapies if specific genetic changes were present. * For head and neck cancer (HNSCC): Previously treated with platinum-based chemotherapy and immunotherapy. * For cholangiocarcinoma: Previously treated with at least one chemotherapy regimen. * The cancer must be c-MET positive, meaning laboratory testing of the tumor shows a moderate to high level of c-MET protein in at least half of the cancer cells. * At least one tumor can be measured on scans according to standard cancer assessment criteria. * The participant is generally well enough for daily activities, with an ECOG Performance Status of 0 or 1 (fully active or only restricted in strenuous activities). * Important organs are functioning adequately, based on blood and laboratory test results, including: * Normal heart rhythm measurement (QTc interval). * Women who can become pregnant must have a negative pregnancy test during screening and again shortly before starting study treatment. * Women who can become pregnant must use an effective birth control method from the first dose of study treatment until 6 months after the last dose. * Sexually active men must use a barrier method of contraception (such as condoms) during the study and for about 3 months after the last dose. They must also not donate sperm during this period. Exclusion Criteria: * Cancer has spread to the brain or spinal cord and has not been treated. * People with previously treated brain metastases may still be eligible if they are stable, have no symptoms, and are not taking steroids for neurological problems. * They have a serious medical or mental health condition that could make participation unsafe or affect the study results. * They have previously received radioactive cancer treatments, such as radioembolization (for example, Y-90 treatment). * They received anti-cancer treatment within the last 4 weeks before starting the study drug. They received an experimental (investigational) anti-cancer treatment within the last 4 weeks or within 5 half-lives of that treatment (whichever period is longer). * They received radiation therapy too recently, including: * Radiation to a large area of bone marrow within 6 weeks before study treatment, or * Radiation to a specific area within 2 weeks before study treatment. * They had major surgery within the last 6 weeks or have not completely recovered from the surgery and its side effects. * They are allergic to the study drug or any of its ingredients. * They are currently participating in another clinical trial that involves treatment or intervention. * They still have significant side effects from previous cancer treatments. * People may still be eligible if they have: * Hair loss of any severity, * Mild to moderate nerve damage (Grade 2), * Moderate fatigue (Grade 2), * Stable diabetes, * Well-controlled underactive or overactive thyroid disease. * They have a history of interstitial lung disease (a serious lung condition that causes scarring and inflammation of the lungs). * They have serious heart disease, including: * Heart attack within the last 6 months, * Unstable chest pain (angina), * Moderate to severe heart failure, * Uncontrolled high blood pressure, * Uncontrolled abnormal heart rhythms, * Severe narrowing of the aortic heart valve. * They have an active infection that requires treatment, including: * HIV, * Hepatitis B, * Hepatitis C, * COVID-19 (SARS-CoV-2). However, some people with chronic HIV, hepatitis B, or hepatitis C may be allowed to participate if their infection is well controlled and approved by the study doctor. * They have had another type of active cancer within the last 3 years, apart from the cancer being studied. * Some exceptions may be allowed if the other cancer is well controlled and does not require treatment during the study. * They are pregnant or breastfeeding, or plan to become pregnant or donate eggs during the study or within 6 months after the last dose of the study drug.