Background: Relapsed small cell lung cancer (SCLC) is incurable with limited therapeutic options. This phase II study evaluated efficacy and tolerability of carfilzomib + irinotecan in SCLC pts who progressed after prior platinum-based therapy, based on expected synergy of proteosome inhibitor carfilzomib and topisomerase 1 inhibitor irinotecan. Methods: SCLC pts who progressed after one platinum-containing regimen (no maintenance therapy allowed) for recurrent/metastatic disease were eligible. Pts were stratified by response to platinum-based therapy: sensitive (progressive disease (PD) > 90 days after chemo) versus refractory (PD 30 to 90 days after chemo). Pts were treated with up to 6 cycles of carfilzomib (20/36 mg.m2 D1, 2, 8, 9, 15, 16 q28D) and irinotecan (125 mg/m2 D1, 8, 15 q28D), imaging was performed every 2 cycles. The primary efficacy endpoint was 6-month overall survival (OS). Results: 62 pts enrolled and were evaluable for efficacy and adverse events. The 6-month OS was 59% in the platinum sensitive stratum and 54% in the platinum refractory stratum. Overall response rate: sensitive stratum 21.6% (1.6% CR + 16.4% PR) and refractory stratum 12.5% (all PR). Disease control (SD+PR+CR) was 68% in platinum sensitive and 56% in refractory patients. Progression free survival and OS were 3.6 months (95% CI 2.6 - 4.6) and 6.9 months (95% CI 4.3 - 12.3) in the sensitive stratum, and 3.3 months (95% CI 1.8 – 3.9) and 6.8 months (95% CI 4.1-11) in the refractory stratum. Twenty-nine pts (47%) experienced at least one grade 3 AE and 8 subjects had grade 4 toxicities: decreased neutrophils, leukocytes, and lymphocytes, diarrhea, vomiting, sepsis, hypokalemia, hypocalcemia, and dehydration. There were three treatment related deaths: myocardial infarction (possible), lung infection (possible), sepsis (probable). Conclusions: In previously treated pts with relapsed SCLC, irinotecan and carfilzomib was effective in platinum-sensitive and, notably, platinum-refractory pts with similar toxicity profile. This combination is a viable option in relapsed SCLC, can be considered following progression on immunotherapy (IO) or in subjects who cannot receive IO, and should be further explored in a randomized phase III trial. Clinical trial information: NCT01941316