Talazoparib and Thoracic RT for ES-SCLC

Inclusion Criteria: * Histological documented diagnosis of SCLC confirmed by a UHN pathologist. * Documented extensive disease * Completion of induction chemotherapy, 4-6 cycles of a platinum agent and etoposide. * No disease progression (i.e.SD or better response by RECIST 1.1) at the completion of chemotherapy. * Eastern Cooperative Oncology Group (ECOG) performance status 0-2 (Karnosfsky Performance Score (KPS) ≥50; see Appendix B). * Adequate organ and marrow function, * Postmenopausal or evidence of non-childbearing status for women of childbearing potential negative urine or serum pregnancy test within 28 days of study treatment and confirmed prior to treatment on day 1. Exclusion Criteria: * Untreated brain metastases. * Previous radiotherapy to thorax (prior breast RT is permitted). * Patients receiving any systemic chemotherapy, radiotherapy or immunotherapy (except for standard of care treatments or palliative reasons) within 3 weeks prior to study treatment. * Exposure to an investigational product within 30 days or 5 half-lives (whichever is longer) prior to start of the current study drug. * Any previous treatment with PARP inhibitor, including talazoparib. * Concomitant use of strong P-gp inhibitors * Concomitant use of other known P-gp inhibitors, P-gp inducers, or BCRP inhibitors * Persistent toxicities (\>Common Terminology Criteria for Adverse Event (CTCAE) grade 2) caused by previous cancer therapy, excluding alopecia. * Patients with myelodysplastic syndrome/acute leukaemia or with features suggestive thereof. * Major surgery within 2 weeks of study treatment initiation and patients must have recovered from any effects of any major surgery. * Patients considered a poor medical risk due to a serious, uncontrolled medical disorder, non-malignant systemic disease or active/uncontrolled infection. Examples include, but are not limited to, uncontrolled ventricular arrhythmia, recent (within 3 months) myocardial infarction, uncontrolled major seizure disorder, unstable spinal cord compression, superior vena cava syndrome, extensive interstitial bilateral lung disease on High Resolution Computed Tomography (HRCT) scan or any psychiatric disorder that prohibits obtaining informed consent * Patients unable to swallow orally administered medication and patients with gastrointestinal disorders likely to interfere with absorption of the study medication. * Immunocompromised patients, * Previous allogenic bone marrow transplant or double umbilical cord blood transplantation (dUCBT). * Whole blood transfusions in the last 120 days prior to entry to the study * Other malignancy within the last 5 years * Patients with spinal cord compression