Nal-IRI (irinotecan liposome injection, MM-398, ONIVYDE®), in combination with 5-fluorouracil and leucovorin, is currently approved by the FDA for treatment of patients with advanced pancreatic cancer who have progressed on gemcitabine-based therapies. Nal-IRI is designed for extended circulation relative to non-liposomal irinotecan and to exploit leaky tumor vasculature for enhanced drug delivery to tumors. Following tumor deposition, nal-IRI is taken up by phagocytic cells followed by irinotecan release and conversion to its active metabolite SN-38 in the tumors. Sustained inhibition of topoisomerase 1 (TOP1) by extended SN-38 delivery is hypothesized to enable superior anti-tumor activity compared to traditional TOP1 inhibitors. Topotecan, a TOP1 inhibitor, is currently a standard of care for second-line treatment of small cell lung cancer (SCLC). Here, we evaluate nal-IRI compared to topotecan in preclinical models of SCLC. Anti-tumor activity of nal-IRI as a monotherapy was evaluated in DMS-53 and NCI-H1048 xenograft models. Cells were implanted subcutaneously into right flanks of NOD-SCID mice; treatments were initiated when tumors had reached approximately 280 mm3. Nal-IRI was dosed at 16 mg/kg salt, q1w, which is equivalent to a proposed clinical dose of 90 mg/m2 free base, q2w. Topotecan was dosed at 0.83 mg/kg/week, Day 1-2 every 7 days, which approximates a clinical dose intensity of 1.5 mg/m2 (Day 1-5 every 21 days). Tumor metabolite levels were compared in mice treated with nal-IRI or non-liposomal irinotecan at 24 hours post-injection, using previously established high performance liquid chromatography methods. Carboxylesterase activity and sensitivity to SN-38 in mouse SCLC models were comparable to those measured in tumor types where either nal-IRI or irinotecan HCl has proven to be efficacious clinically (e.g. pancreatic cancer, colorectal cancer). Nal-IRI delivered irinotecan to SCLC tumors to a similar or greater extent than other tumor types including pancreatic tumors. The tumor irinotecan and SN-38 levels of nal-IRI (16 mg/kg salt) were 12 to 57-fold and 5 to 20-fold higher than non-liposomal irinotecan (30 mg/kg salt), respectively. Nal-IRI demonstrated anti-tumor activity in both xenograft models of SCLC at clinically relevant dose levels, and resulted in complete or partial responses after 4 cycles in NCI-H1048 or DMS-53 models, respectively, compared with topotecan which had limited tumor growth control. This study demonstrated that nal-IRI is more active than topotecan at clinically relevant doses in SCLC preclinical models, and thus support further clinical development of nal-IRI versus topotecan in patients with SCLC that have progressed on prior platinum-based therapy.