Background: In the second-line setting of patients with advanced SqCLC, docetaxel is a standard of care, but the benefit provided is only modest in terms of progression free survival (PFS) while adding significant toxicity. Recent evidence from patients with advanced SqCLC who had progressed with platinum-based chemotherapy showed that use of immunotherapy was associated with longer overall survival (OS) than docetaxel. Abemaciclib is a potent and selective small molecule inhibitor of CDK 4 and 6 that appears to have single-agent activity in multiple tumor types and a manageable safety profile that is amenable to chronic administration. Patients are currently enrolling in a Phase 2 study designed to evaluate the effectiveness of abemaciclib vs docetaxel in patients with Stage IV squamous SqCLC who have relapsed after prior platinum-based therapy. Methods: Study JPBX (NCT02450539) is an ongoing randomized, open-label study evaluating treatment with abemaciclib (200 mg orally every 12 hours on Days 1 to 21 of a 21-day cycle) vs docetaxel (75 mg/m² intravenous infusion on Day 1 of a 21-day cycle). Patients with Stage IV SqCLC patients who have progressed after platinum-based therapy are stratified at randomization according to: ECOG performance status (0 vs 1); number of prior therapies (platinum-based therapy only vs platinum-based therapy plus immune checkpoint inhibitor); and time since initiation of first-line therapy ( ≤ 9 vs > 9 months).The primary endpoint of the study is PFS; key secondary endpoints include OS and overall response rate. The study will enroll ~150 patients in 2:1 randomization (abemaciclib, 100 patients; docetaxel, 50 patients). The primary PFS analysis will be performed after 120 PFS events. By incorporating historical docetaxel data, the study has > 90% power to detect a hazard ratio (HR) of 0.64 in PFS. If the true median PFS for the docetaxel arm is 3 months, the HR of 0.64 corresponds to an approximately 1.7-month improvement in median PFS for the abemaciclib arm under an additional assumption of exponential survival distribution.The study began in August 2015; and as of 1/21/16, 48 patients have been randomized. Clinical trial information: NCT02450539