Inclusion Criteria:

          -  Consenting adults of Eastern Cooperative Oncology Group (ECOG) Performance Status 0 or
             1

          -  Histologically confirmed locally advanced or metastatic disease that has progressed
             under or after, failed to respond to, or are intolerant or have a contraindication to
             available Standard of Care (SoC) treatment options as per the assessment of the
             investigator; initially, subjects with the disease below may be considered:

               1. Subjects with Extensive Disease (ED-SCLC) that have progressed on or after
                  platinum-based chemotherapy or Limited Disease (LD-SCLC) that have progressed and
                  for which subject has exhausted standard treatment options; subjects may have
                  received prior immunotherapy.

               2. Subjects with Human Epidermal Growth Factor Receptor (HR+)/(HER2-) metastatic BC
                  after a failure of prior standard-of-care treatments and who have received, if
                  indicated, at least one line of hormonal therapy, Cyclin-dependent kinase
                  (CDK4/6) inhibitor for advanced or metastatic disease and at least one line of
                  chemotherapy for metastatic disease; subjects with a Breast Cancer (BRCA)-mutated
                  metastatic disease who may have received a Poly adenosine diphosphate ribose
                  polymerase (PARP) inhibitor, if available, are eligible; prior adjuvant hormonal
                  treatment and prior adjuvant chemotherapy are allowed.

          -  Documented progressive disease (radiological, based on RECIST v1.1) within 3 months
             prior to first study drug administration. All images should be sent to the Imaging
             core laboratory (ICL).

          -  Adequate organ function determined within 21 days prior to 177Lu-OPS201
             administration, defined as follows:

               -  Haematological: white blood cells (WBC) ≥3000/μL, with absolute neutrophil count
                  ≥1000/μL, platelet ≥100,000/μL and haemoglobin ≥9 g/dL (without a need for
                  hematopoietic growth factor or transfusion support).

               -  Renal: Estimated glomerular filtration rate (eGFR) ≥55 mL/minute

               -  Hepatic: total serum bilirubin ≤2×ULN; aspartate aminotransferase/ alanine
                  aminotransferase ≤2.5×ULN (≤5×ULN if a subject has liver metastases)

          -  Formalin fixed paraffin embedded tumour sample (archival tumour sample obtained within
             21 days prior to the start of study treatment) from the primary or metastatic lesion
             OR is willing to undergo newly obtained biopsy prior to the first dose of study
             treatment. Subjects who are unable to provide acceptable tissue may not be enrolled
             unless there has been prior agreement with the sponsor.

          -  68Ga OPS202 uptake in the target tissue (a primary tumour, lymph nodes longest
             diameter on PET/CT as confirmed by a central reader.

          -  Radiologically, ≥50% matching between the lesions detected on 68Ga OPS202-PET/CT and
             on 18F-fluorodeoxyglucose (18F-FDG)-PET/CT as confirmed by central reader

        Exclusion Criteria:

          -  Male subjects with BC.

          -  Unstable central nervous system metastasis

          -  Centrally located tumours with radiographic evidence (CT or MRI) of cavitary or
             necrotic lesions or focal invasion of major blood vessels.

          -  Previously received external beam irradiation to a field that includes more than 30%
             of the Bone marrow (BM).

          -  Previous chemotherapy within a cycle interval, curative radiotherapy within 4 weeks or
             palliative radiotherapy within 7 days prior to Investigational radiopharmaceutical
             product (IRPP) administration.

          -  Prior treatment with any other investigational medicinal product (IMP) within five
             half-lives of the previous IMP or within 2 weeks, if the previous compound is a
             mechanism-based molecularly targeted agent whose half-life is not well characterized
             and toxicities have not resolved from Grade 2 or higher prior to IRPP administration.

          -  Any unresolved NCI-CTCAE Grade 2 or higher toxicity (except alopecia and Grade 2
             platinum-therapy related neuropathy) from previous antitumour treatment and/or
             medical/surgical procedures/interventions.

          -  Nephrectomy, renal transplant or concomitant nephrotoxic therapy putting the subject
             at high risk of renal toxicity during the study as assessed by the investigator.

          -  Any significant medical or surgical condition that would affect safety or the
             assessment of efficacy or the ability of a person to comply with the protocol.

          -  Subjects weighing greater than 130 kg (287 lb).

          -  Pregnant or lactating female. Female subject of childbearing potential who is
             unwilling to use acceptable method(s) of effective contraception during study
             treatment and through 6 months after the last dose of 177Lu-OPS201.

          -  Male subject who is unwilling to use acceptable method of effective contraception
             during treatment and through 6 months after the last dose of 177Lu-OPS201.