A Phase 1-1b Study to Evaluate the Safety, Efficacy and Dosimetry of Study Drug A9-3408 in Subjects With Metastatic Melanoma

Inclusion Criteria: * Able to provide written informed consent * \[68Ga\]Ga-A9T-3202 uptake in at least one measurable lesion (per RECIST v1.1) on PET scan * Histologically or cytologically confirmed unresectable or metastatic melanoma with disease progression on prior standard of care therapy * Adequate ECOG performance status * Adequate baseline organ function within 14 days of first dose of investigational product * Recovered from side effects of prior anticancer therapy * Women of childbearing potential (WOCBP) must have a negative pregnancy test and follow adequate birth control method(s) during the treatment period and for at least 6 months after last dose of \[225Ac\]Ac-A9-3408. Sexually active males with partners who are WOCBP must agree to adequate birth control method(s) during the treatment period and for at least 3 months after last dose of \[225Ac\]Ac-A9-3408 Exclusion Criteria: * Previous treatment with radioactive nuclides except radioactive imaging tracers * Treatment with another investigational product shortly prior to first dose of \[225Ac\]Ac-A9-3408 with exception of anti-PD-1/PD-L1 agents. * Concurrent anticancer therapy * Major surgery within 4 weeks of first dose of investigational product * Second malignancy within 2 years * Active, clinically serious infection * Known infusion reactions to components of the investigational product * Other clinically serious health conditions including cardiovascular and or severe infectious diseases * Significant central nervous system metastatic disease * Pregnant, breastfeeding or unwilling to practice adequate birth control method(s) * Any condition per the opinion of the investigator that would impact the safety of the subject, protocol adherence or ability to provide informed consent