KEY INCLUSION CRITERIA:

1. Diagnosed with unresectable Stage III or Stage IV metastatic melanoma
2. Previously progressed (clinical or radiological progression) on at least one approved first-line therapy for metastatic melanoma
3. Subjects on prior intravenous therapy (e.g., chemotherapy or checkpoint inhibitors), or prior oral therapy (e.g.,proto-oncogene B-RAF or mitogen-activated extracellular signal-regulated kinase inhibitors) who demonstrate MC1R positivity during screening are eligible for enrollment, provided that they undergo a wash-out period of 21 days, or 7 days, respectively, prior to Cycle 1 Day 1 treatment with study drug.
4. Presence of measurable disease by RECIST v1.1 assessed within 30 days prior to the first dose of study drug on Cycle 1 Day 1 (C1D1)
5. Eastern Cooperative Oncology Group (ECOG) performance score of < 2 at Screening
6. Satisfactory organ function determined by laboratory testing

KEY EXCLUSION CRITERIA:

1. Active secondary malignancy
2. Prior systematic treatment with radioactive nuclides. Subjects who had localized treatment with radioactive nuclides or imaging using radioactive imaging agents may be enrolled
3. Any serious/active/uncontrolled infection requiring parenteral antibiotics within 2 weeks before the first administration of study drug
4. Treatment with another investigational drug product (therapeutic IND agents) within the last 45 days before the first dose of study drug on C1D1
5. Existence of any medical or social issues likely to interfere with study conductor that may cause increased risk to the subject or to others, e.g., lack of ability to follow radiation safety precautions
6. Concurrent malignancy requiring treatment or history of prior malignancy active within 2 years prior to the first dose of study drug
7. Subjects with an active, known, or suspected autoimmune disease
8. Acute or chronic hepatitis B (e.g., Hepatitis B surface antigen reactive), hepatitis C (e.g., HCV RNA [qualitative] is detected) or known history of Human Immunodeficiency Virus