Targeted Therapy With CDK4/6 Inhibitors in Chemo-Refractory, Rb Wild-Type Extensive SCLC

Inclusion Criteria: * Subjects must have histologically confirmed extensive stage small cell lung cancer, large cell neuroendocrine lung cancer, extrapulmonary small cell cancer or other high grade neuroendocrine cancer of the lung. * Pathology confirmed Retinoblastoma wild type tested by NGS or ctDNA. * Subjects must have: * Platinum refractory disease: defined as no response after 1-2 cycles of chemotherapy, or * Relapse: defined as initial response but relapse after completing platinum-based chemotherapy. * Subjects must have measurable disease per the Response Evaluation Criteria in Solid Tumors (RECIST), version 1.1. * Subjects shall have archival tumor material for correlative studies if available. If tissue is not available they still may be eligible for the trial * Performance status: ECOG Performance status ≤ 2 * Patients who received chemotherapy must have recovered CTCAE Grade ≤1) from the acute effects of chemotherapy except for residual alopecia or Grade 2 peripheral neuropathy prior to enrollment. A washout period of at least 21 days is required between last chemotherapy dose and enrollment (provided the patient did not receive radiotherapy). Please refer to eligibility criteria for specific laboratory requirements. * Patients who received radiotherapy must have completed and fully recovered from the acute effects of radiotherapy. A washout period of at least 14 days is required between end of radiotherapy and enrollment. * Patients with treated brain metastase…