• Male or female ≥18 years old with the following histologically confirmed metastatic or recurrent GRPR-expressing tumors:

∙ Metastatic castrate resistant prostate cancer (mCRPC);

‣ HR+/HER2- breast cancer;

‣ Colorectal cancer;

‣ Cervical cancer;

‣ Cutaneous melanoma;

‣ Non-small-cell lung cancer (NSCLC).

• Subjects with recurrent disease must have progressed on at least 2 prior systemic therapies.

‣ For participants with mCRPC: Prior orchiectomy and/or ongoing androgen deprivation therapy and a castrate level of serum testosterone (\<50 ng/dL or \<1.7 nmol/L).

⁃ Presence of at least 1 site of measurable disease per RECIST 1.1. At least 1 identified measurable lesion must show GRPR expression in 203Pb-DOTAM-GRPR1 imaging (uptake greater than that of the liver).

• Eastern Cooperative Oncology Group (ECOG) status 0-1.

• Sufficient bone marrow, hepatic and renal function, as assessed by the following laboratory requirements:

∙ White blood cell (WBC) ≥2,500/ mm³

‣ Absolute neutrophil count (ANC) ≥1500/mm³

‣ Platelets ≥75,000/mm³

‣ Hemoglobin (HgB) ≥9.0 g/dL;

‣ Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) \<2.5 x upper limit of normal (ULN) or ≤ 5 x ULN in the presence of liver metastases.

‣ Total bilirubin: ≤1.5 x ULN, except if history of Gilbert's disease.

‣ Adequate renal function defined by creatinine clearance (CLCR) ≥ 60 mL/min calculated as follows: CLCR = eGFR in ml/min/1.73 m2 calculated by the Modified Diet in Renal Disease (MDRD) x participant body surface area (BSA) in m2 ÷ 1.73.

‣ Serum amylase and/or lipase ≤1.5 x ULN.

• For women of childbearing potential (WOCBP) and men with partners of childbearing potential: be willing to use highly effective methods of contraception throughout the study and for 7 months (for WOCBP, 4 months for men) after discontinuation of study intervention, as outlined in Appendix 4: Contraceptive Guidance and Collection of Pregnancy Information.