Iadademstat + SBRT With Atezo in ES-SCLC

Inclusion Criteria: 1. Signed informed consent form. 2. Ability to comply with protocol, in the treating investigator's judgment. 3. Age 18 years or older. 4. Participant must have a body weight of at least 50 Kg. 5. Histologically documented ES-SCLC previously treated with at least one regimen, including a platinum containing chemotherapy regimen, with progression of disease on or after their most recent therapy or achieving only SD after completion of one line of a platinum containing chemotherapy-based regimen. a. Participants previously diagnosed with limited stage SCLC treated with concurrent chemoradiation with a platinum doublet now diagnosed with recurrent extensive disease in the relapsed setting are eligible. 6. Prior treatment with immune checkpoint blockade therapies, including anti-CTLA-4, antiPD-1, and anti-PD-L1 therapeutic antibodies is allowed. 7. ECOG performance status of 0 to 2. 8. At least one identified tumor lesion to be treated with SBRT. 9. At least one additional tumor lesion, separate from the site of SBRT, preferably outside of the 5Gy isodose line, amenable to incisional, excisional, core or forceps (transbronchial) biopsy. Participants must be willing to undergo tumor biopsies before starting trial therapy and 2 weeks (+/- 1 week) after the completion of SBRT. 1. If the initial biopsy is excisional, the excised tumor cannot be counted as a biopsy target lesion and there must be another lesion amenable to incisional, excisional, core or forceps biopsy. In this scenario, the second biopsy can only be excisional if the lesion to be excised is not a target lesion. 2. Cytology tumor specimens (e.g., from fine-needle biopsies, or drainage of pleural/ pericardial or ascites fluid) are not acceptable. Biopsies of bone lesions that do not have a soft tissue component are also not acceptable (i.e., decalcified tumor samples are not acceptable). Non-lymph node sites will be biopsied preferentially. 3. If enrolled with stable, but not progressive, disease at the completion of first line platinum-based therapy regimen, participants may undergo biopsy of the same site that is to be irradiated. If a separate site is identified for biopsy, it should be preferentially biopsied over the site that is to be irradiated. 10. Participants must agree to submit blood samples for exploratory biomarker research at the times identified in the study calendar. 11. Measurable disease, as defined by RECIST v1.1. Previously irradiated lesions can be counted as target lesions if clearly progressing after radiation. 12. Participants may have asymptomatic CNS metastases or treated CNS metastases if they are not currently receiving corticosteroids equal to or greater than prednisone 10 mg daily or equivalent for seven days prior to the first dose of study drug. Participants should have completed stereotactic radiation at least 7 days or whole brain radiation at least 2 weeks prior to Cycle 1, Day 1. 1. Participants with new asymptomatic CNS metastases detected on their screening scan will undergo repeat CNS imaging at four weeks to ensure safety. 2. Participants must have no history of intracranial hemorrhage or spinal cord hemorrhage from CNS disease. 3. Metastases are limited to the cerebellum and supratentorial region (i.e. no metastases to the midbrain, pons, medulla or spinal cord). 13. Prior palliative radiotherapy must have been completed at least 1 week before the first dose of the study drug. 14. Anticonvulsants are allowed. 15. Participants must adhere to the following reproductive and contraceptive requirements while on study treatment and for at least five months after the last dose of the study drugs. Exclusion Criteria: 1. History of leptomeningeal disease. 2. Untreated symptomatic CNS metastases. Participants with asymptomatic CNS metastases are eligible. Participant with treated symptomatic brain metastases is eligible provided they meet all of the following criteria: 1. Completed stereotactic radiosurgery at least seven days prior to Cycle 1, Day 1 or completed whole brain radiation at least two weeks prior to Cycle 1, Day 1. 2. No evidence of interim progression between the completion of CNS-directed therapy and the start of trial therapy. 3. Ongoing steroid requirement of \<10 mg of prednisone per day (or equivalent) as therapy for CNS disease; anticonvulsants are allowed. 4. The participant has no history of intracranial hemorrhage or spinal cord hemorrhage. 3. Spinal cord compression not definitively treated with surgery and/or radiation or previously diagnosed and treated spinal cord compression without evidence that disease has been clinically stable for more than or equal to 1 week prior to enrollment. 4. Uncontrolled tumor-related pain. 5. Symptomatic lesions (e.g., bone metastases or metastases causing nerve impingement) amenable to palliative radiotherapy should be treated prior to enrollment. Participants should be recovered from the effects of radiation. There is no required minimum recovery period. 6. Asymptomatic metastatic lesions that would likely cause functional deficits or intractable pain with further growth (e.g., epidural metastasis that is not currently associated with spinal cord compression) should be considered for loco-regional therapy if appropriate prior to enrollment. 7. Uncontrolled pleural effusion, pericardial effusion, or ascites requiring recurrent drainage procedures (once monthly or more frequently) a. Participants with indwelling catheters (e.g., PleurX) are allowed. 8. Uncontrolled or symptomatic hypercalcemia (ionized calcium greater than 1.5 mmol/L, calcium greater than 12 mg/dL or corrected serum calcium greater than ULN) 9. Active or history of autoimmune disease or immune deficiency, including, but not limited to, myasthenia gravis, myositis, autoimmune hepatitis, systemic lupus erythematosus, rheumatoid arthritis, inflammatory bowel disease, antiphospholipid antibody syndrome, Wegener gr