A Phase 1 Open-Label, First-in-human, Dose Escalation and Expansion Study to Determine the Safety, Tolerability, Dosimetry, Pharmacokinetics, and Preliminary Efficacy of 212Pb-DOTAM-GRPR1 in Adult Participants With Recurrent or Metastatic GRPR-expressing Tumors

1. Adult participants (age ≥ 18 years old) with any of the following advanced or metastatic solid tumors (documented history of histologically confirmed diagnosis): 1. Metastatic castration-resistant prostate cancer (mCRPC) including neuroendocrine prostate cancer (NEPC) (enrolled only in SAD and MAD Q6W) 2. HR+/HER2- breast cancer (estrogen receptor/ER expression \>10% of tumor cell nuclei stain, regardless of progesterone receptor/PgR expression); HER2-negative including HER2-low (as per relevant ASCO/CAP guidelines) 3. Colorectal cancer 4. Cervical cancer 5. Non-small-cell lung cancer (NSCLC) 6. Recurrent glioblastoma (only enrolled in MAD Q4W cohorts) with evidence of recurrent disease (RD) demonstrated by disease progression using modified Response Assessment in Neuro-Oncology (RANO 2.0) criteria. Note: If surgery is performed for GBM recurrence, pre-surgery MRI will be used for confirmation of RD and residual and measurable disease post-surgery is not required but surgery must have confirmed the recurrence diagnosis by MRI. 2. Capable of giving signed informed consent 3. All participants must have progressed on at least 2 prior systemic therapies, except for recurrent GB 4. 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) 5. Presence of at least 1 measurable lesion per RECIST 1.1 as assessed by the Investigator (not applicable for GBM). At least 1 identified measurable lesion must show GRPR uptake in 203Pb-DOTAM-GRPR1 SPECT/CT (uptake greater than that of the background) as assessed by the Investigator. 6. For participants with prostate cancer that do not have measurable soft tissue disease, 203Pb-DOTAM-GRPR1 uptake in bone lesions \> uptake in background is acceptable for eligibility. 7. Eastern Cooperative Oncology Group (ECOG) status 0-1. Participants with ECOG status of 2 may be approved on a case-by-case basis in discussion with the Sponsor. 8. Adequate bone marrow, hepatic, and renal function, as assessed by the following laboratory requirements: 1. White blood cell (WBC) ≥3000/ mm3 (≥ 3 x 109/L) 2. Absolute neutrophil count (ANC) ≥1500/mm3 (≥1.5 x 109/L) 3. Platelets ≥100,000/mm3 (≥ 100 x 109/L) 4. Hemoglobin (Hb) ≥9.0 g/dL 5. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤3x upper limit of normal (ULN) or ≤ 5 x ULN in the presence of liver metastases 6. Total bilirubin: ≤1.5 x ULN, except if documented history of Gilbert's disease who are eligible if total bilirubin ≤ 3 x ULN 7. 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 8. Serum amylase and/or lipase ≤1.5 x ULN 9. For women of childbearing potential (WOCBP) and men with partners of childbearing potential: be willing to use highly effective methods of contraception or sexual abstinence, if part of participant's lifestyle, throughout the study and for 7 months for WOCBP, 4 months for men after the last \[212Pb\]Pb-DOTAM-GRPR1 administration or for 10 days following \[203Pb\]Pb-DOTAM-GRPR1 administration and participant is not proceeding to 212Pb-DOTAM-GRPR1 treatment, as outlined in protocol. Participants with Recurrent Glioblastoma: 10. Having first or second glioblastoma recurrence, after standard therapy that includes prior radiation therapy (RT) and at least 12 weeks from completion of RT prior to first administration of 212Pb-DOTAM-GRPR1. In case surgery has been performed for GBM recurrence, the surgery has to be completed at least 4 weeks prior to 212Pb-DOTAM-GRPR1 treatment start, with post-surgery recovery without any complications related to surgical procedure. 11. Presence of 203Pb-DOTAM-GRPR1 uptake by SPECT/CT scan in the tumor lesion(s). 12. Presence of Gadolinium enhancement in the MRI in the tumor lesion(s) shown at the time of diagnosis of tumor recurrence.