Background: Immunotherapy has shown efficacy in the treatment of recurrent, extensive-stage small cell lung cancer (ES-SCLC). In the Checkmate 032 trial, ipilimumab and nivolumab combination therapy resulted in a 21% objective response rate in relapsed SCLC. At present, there are no biomarkers used in clinical practice to predict treatment responsiveness in SCLC. Ipilimumab and nivolumab act by blocking key co-inhibitory immune pathways of CTLA-4 and PD-1/PD-L1, respectively, leading to reinvigoration of anti-tumor cytotoxic T cell responses and a decrease in immune suppressive tumor infiltrating leukocytes. The ratio of intratumor Teff (CD8+) cells to Treg (CD4+/Foxp3+) cells (Teff/Treg) could be a more reliable biomarker than effector cell infiltration alone. Methods: In this open-label, single arm trial, we enrolled patients with ES-SCLC who previously received platinum-based chemotherapy; prior anti-PD-1 /PD-L1 therapy was allowed. Patients were treated with nivolumab 1 mg/kg and ipilimumab 3 mg/kg, for a total of 4 doses each and received nivolumab 480 mg beginning with cycle 5, every 4 weeks until progression, unacceptable toxicity or study discontinuation. On-study biopsies were performed prior to initiation of therapy and during week 4 for the biomarker primary objective—to correlate disease response with intratumor Teff/Treg changes. Secondary objectives include determining ORR, DOR, PFS, and OS. Results: Twenty-two patients (median age 63.5 [range 54-80] years, ECOG 0/1/2 [41%/50%/9%], sex M/F [45%/55%]) were enrolled and received treatment. Fourteen (64%) had paired biopsies while on treatment. Fifteen patients were evaluable per RECIST with an ORR of 13% (2/15, 2 partial responses [13%]) and DCR was 40% (6/15, 4 stable disease [27%]). Grade 3 treatment-related adverse events (TRAEs) occurred in 9/22 [40%]. Grade 4 TRAEs occurred in 2/22 [9%] (elevated lipase and elevated bilirubin) and Grade 5 TRAEs occurred in 1/22 patients (hepatic failure). Out of the 9 patients previously treated with anti- PD-1/PD-L1 therapy, 1 had a partial response and 2 had stable disease. Multiplexed quantitative immunofluorescence analysis revealed changes of both CD8+ effector T cells and Tregs in the tumor micro-environment associated with clinical benefit to immunotherapy. Conclusions: Combination immunotherapy with ipilimumab and nivolumab shows clinical efficacy in relapsed extensive-stage SCLC, including those previously treated with anti-PD-1/PDL-1 therapy. Obtaining paired biopsies was shown to be successful in this prospective trial to study the tumor microenvironment in SCLC tumors treated with checkpoint inhibitors. Early biomarker evaluation during week 4 shows local immunomodulatory effect of treatment and supports exploration as predictive biomarker in this population. Clinical trial information: 03670056.