Although small cell lung cancer (SCLC) responds dramatically to initial platinum-based chemotherapy, recurrences are nearly universal. The addition of atezolizumab, an immune checkpoint inhibitor, to front-line chemotherapy has recently demonstrated an improvement in overall survival (OS) in extensive stage SCLC (ES-SCLC). Subsequent lines of therapies are associated with modest efficacy in patients with relapsed disease, and the median overall survival is still 12 to 13 months at best. Cirtuvivint is a small molecule inhibitor of the CDC2-like kinases (CLKs) and dual-specificity tyrosine-regulated kinases (DYRKs); inhibiting CLKs and DYRKs has been shown in preclinical models to cause tumor growth inhibition and sensitize cancer cells to cytotoxic chemotherapy. This study is testing the hypothesis that adding cirtuvivint to chemotherapy in patients with relapsed SCLC will be well tolerated and improve the response rate and progression-free survival (PFS).