177Lu-BetaBart in Patients With Relapsed/Refractory, Locally Advanced Inoperable, or Metastatic Solid Tumors

Inclusion Criteria: 1. Willing and able to provide informed consent prior to start of any study procedures and assessments and must be willing to comply with all study procedures. 2. Participants ≥ 18 years of age. 3. Participants with a documented history of histopathologically confirmed CRPC\*, CRC, NSCLC, SCLC, HNSCC, ovarian cancer, cervical cancer, endometrial cancer, TNBC, or ESCC. (Note: inclusion or exclusion criteria below marked with \* refer to CRPC only, criteria without \* refer to all tumor indications including CRPC) a. \*Progressive CRPC defined as castrate levels of testosterone and progressing by at least one of the following criteria: i. Serum PSA progression consisting of two consecutive increases in PSA measured at least 1 week apart. The minimal baseline value is 2.0 ng/mL. ii. Soft tissue progression defined as a ≥20% increase in the sum of the diameter (short axis for nodal lesions and long axis for non-nodal lesions) of all target lesions based on the smallest sum of the diameter since the previous treatment was started or the appearance of one or more new lesions by computed tomography (CT)/magnetic resonance imaging (MRI). iii. Progression of bone disease defined by Prostate Cancer Working Group 3 (PCWG3) as evaluable disease or new bone lesions by bone scan. iv. Identification of new soft tissue or bone lesions on prostate-specific membrane antigen (PSMA) positron emission tomography (PET) imaging. b. \*Metastatic disease defined as either or both of the following: i. Documented M1 disease on conventional imaging (CT/MRI of the chest/abdomen/pelvis and/or Technetium 99m \[99mTc\] whole-body bone scan) ii. Identification of bone lesion(s), extra-pelvic soft tissue lesion(s), or visceral metastases on PSMA PET imaging with an FDA-approved imaging agent (e.g., 68Ga-PSMA-11, 18F-DCFPyL, or 18F-rhPSMA-7.3) c. \*Progression following treatment with ADT and at least one ARSI (e.g., enzalutamide, apalutamide, darolutamide, and/or abiraterone acetate). If a participant is currently on ADT, they should continue ADT for the duration of their participation in the study but will not be permitted to start a new therapy or ADT regimen. If a participant has progressed on an ARSI, they will have the option to remain on the same ARSI or discontinue therapy. If they discontinue the ARSI, a 28-day washout period will be required prior to initiating study intervention. d. Prior definitive and palliative external beam radiation therapy and stereotactic body radiation therapy is allowed. Note: Participants with extended external beam radiation therapy to the axial skeleton, which in the opinion of the Investigator may pose a risk for increased myelotoxicity, will be discussed with the Sponsor to determine eligibility. e. Participants with liver metastases are eligible if they meet the following criteria: i. ≤3 lesions i. All lesions must be ≤2 cm in the short axis ii. SUVmean ≥2 x that of liver parenchyma f. \*Prior treatment with one taxane-based chemotherapy is allowed but not required. A taxane-based chemotherapy is defined as a minimum exposure of two cycles of a taxane chemotherapy. 4. Participants must have documented disease progression during or after their most recent line of anticancer therapy. Participants must be refractory to or intolerant of standard of care therapy or have no standard of care therapy available that is likely to provide clinical benefit. Any number of prior treatment lines are allowed. 5. Must have at least 1 measurable target lesion according to RECIST v1.1. (Note: this does not apply for CRPC) 6. Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 7. Participants must have a life expectancy of ≥ 4 months in the opinion of the Investigator. 8. Participants of child-bearing potential (CBP) must have a negative β-hCG test and must not be breastfeeding. Participants of CBP are defined as those who are not surgically sterile or post-menopausal. Participants will be considered post-menopausal if they have been amenorrheic for 12 months without an alternative medical cause. Participants \< 50 years of age who meet the criteria for post-menopausal status without previous surgical sterilization should be considered for further investigation with luteinizing hormone (LH) and follicle stimulating hormone (FSH) levels to confirm serological post-menopausal status. 9. Participants of CBP must agree to use a highly effective method of contraception during the study and for 6 months after the last dose of 177Lu-BetaBart, as described in Appendix 4. 10. Male participants who are able to father a child must agree to avoid impregnating a partner and to adhere to a highly effective method of contraception during the study and for 6 months after the last dose of 177Lu-BetaBart, as described in Appendix 4. All male participants must agree to not donate sperm during the study and for 6 months after the last dose of 177Lu-BetaBart. 11. Participants who have received prior radiation therapy \>28 days before the first dose of 177Lu-BetaBart are permitted. Documentation of the dates the radiotherapy was received, the cumulative dose, and the absorbed dose to critical organs, if available, should be provided. 12. Participants with previously treated brain metastases are eligible to participate if: * they are neurologically and radiologically stable (no evidence of progression by imaging; same imaging modality \[MRI or CT scan\] must be used for each assessment) for at least 28 days prior to the first dose of 177Lu-BetaBart; and * do not require corticosteroids to treat associated neurological symptoms or if required, are on a stable dose of corticosteroids not exceeding 10 mg/day of prednisone (or equivalent), and * have no history of leptomeningeal disease or spinal cord compression. Exclusion Criteria: 1. History of prior organ transplant. 2. Any other known, active malignancy, except for treated cervical intraepithelial neoplasia, or non-melanoma skin cancer. Participants with a history of malignancies of low recurrence potential who have received curative-intent therapy may be approved on a case-by-case basis in discussion with the Sponsor, if it is determined not to put the participant at an increased risk of adverse drug effects and/or interfere with the integrity of the study outcome. 3. Have any medical condition that would, in the Investigator's judgment, prevent the participant's full participation in the clinical study due to safety concerns or compliance with clinical study procedures such as participants with severe claustrophobia who are unresponsive to oral anxiolytics, participants with low back pain who cannot lie comfortably on an imaging table, participants who are hyperactive or hyperkinetic such that they cannot tolerate lying still for multiple time point imaging procedures, etc. 4. Residual toxicity ≥ Grade 2 from prior anti-cancer therapy (except alopecia and peripheral sensory neuropathy). 5. History of uncontrolled allergic reactions and/or known or expected hypersensitivity to protein therapeutics, 177Lu-BetaBart, or any of its excipients. 6. Inadequate organ functions as reflected in laboratory parameters: * Estimated glomerular filtration rate (eGFR) \< 50 mL/min adjusted for participant's body surface area using the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI 2021) formula * Platelet count of \< 100 x 109/L * Absolute neutrophil count (ANC) \< 1.5 x 109/L * Hemoglobin \< 9 g/dL * Alanine aminotransferase (ALT) or aspartate aminotransferase (AST) \> 3 x upper limit of normal (ULN), or \> 5 x ULN for participants with known liver metastases * Total bilirubin \> 1.5 x ULN, except for participants with documented Gilbert's syndrome who are eligible if total bilirubin ≤ 3 x ULN * For participants not taking warfarin or other anticoagulants: INR ≤1.5 or PT ≤1.5 x ULN; and either PTT or aPTT ≤1.5 x ULN. Participants ta