• Provide written consent on an informed consent form (ICF), approved by an Institutional Review Board (IRB)/Independent Ethics Committee (IEC), prior to any study-specific evaluation. Patients should have the ability to read and understand the ICF, ask for any clarifications from the study staff, and be able to comply with all planned study procedures.

• 18 years of age or older at the time of informed consent.

• Female patients must be at least 2 years postmenopausal (defined as 2 years without menses), surgically sterile (at least 6 months prior to dosing; must be documented) or patients of childbearing potential under the following conditions:

‣ Must be nonlactating and have a negative serum (preferred) or urine pregnancy test results within 72 hours prior to the first dose of MBRC-201.

⁃ Must agree not to try to become pregnant during the study and for at least 6 months after the final dose of MBRC-201

⁃ Must agree to practice effective contraception (must agree to use 2 forms of contraception, 1 of which must be a barrier method) and willing to continue to use effective contraception for the duration of study participation and for 6 months after the final dose of study drug.

• Male patients whose partners are of childbearing potential must agree to use effective contraception (must agree to use 2 forms of contraception, 1 of which must be a barrier method) (Section 10.4) for the duration of study participation and for 6 months after the final dose of study drug.

• Have a histologic or cytologic diagnosis of malignant solid tumor for which there are no standard-of-care treatment options known to confer a clinical benefit or for which the patient is ineligible or declines (except for Phase 1b-Cohort A).

• A. For Phase 1a dose escalation: Patients must have one of the following tumor types:

• i. mCRPC, breast cancer (TNBC, HR+/HER2-negative or HER2-low, HR-/HER2+), CRC, NSCLC, or PDAC

• B. For Phase 1b: Patients must have one of the following tumor types:

• i. Cohort A: Histologic or cytologic diagnosis of mCRPC (with confirmed adenocarcinoma histology) refractory to standard treatment.

• Patients must have had prior exposure to at least one novel AR-targeted therapy (e.g., abiraterone acetate, enzalutamide, apalutamide, darolutamide). Prior taxane or lutetium Lu 177 vipivotide tetraxetan is acceptable but not required.

• ii. Cohort B: Histologic or cytologic diagnosis of advanced metastatic NSCLC refractory to standard treatment.

• iii. Cohort C: Histologic or cytologic diagnosis of advanced metastatic breast cancer (TNBC, HR+/HER2-negative or HER2-low, HR-/HER2+) refractory to standard treatment.

• iv. Cohort D: Histologic or cytologic diagnosis of advanced metastatic CRC, PDAC refractory to standard treatment. The Sponsor may add or remove specific tumor indications based on emerging, real-time study results.

• Availability of a tumor tissue sample (formalin-fixed paraffin-embedded \[FFPE\]) must be confirmed if feasible. Patients without tumor sample may be eligible with medical monitor approval. Tumor biopsies are not required and should not be performed to assess eligibility.

• For Dose Escalation (Phase 1a), patients may have evaluable disease or measurable disease according to Response Evaluation Criteria in Solid Tumor (RECIST) v1.1. For both Dose Expansion (Phase 1b) and Phase 2, patients must have measurable disease according to RECIST v1.1

• Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2.

• Life expectancy ≥ 3 months

⁃ Patient must have adequate organ and marrow function as defined below.

∙ Absolute neutrophil count (ANC) ≥ 1500/uL

‣ Hemoglobin (Hgb) ≥ 9 g/dL

‣ Platelet count ≥ 100,000/uL

‣ International normalized ratio (INR) \< 1.5 (or ≤ 3.0 if on therapeutic anticoagulation)

‣ Estimated glomerular filtration rate (eGFR) ≥ 60 mL/min by the CKD-EPI or similar equation or as measured by 24-hour urine collection

‣ Total bilirubin ≤ 1.5 × ULN \[or ≤ 3-times ULN for patients with Gilbert's disease or documented hepatic tumor involvement\]

‣ ALT and AST ≤ 3 × ULN \[or ≤ 5-times ULN for patients with documented hepatic tumor involvement\]