This is a Phase II, open label, single arm, multicenter study of NK012 in patients with sensitive relapsed SCLC (at least 90 days since first line therapy) or refractory relapsed SCLC (< 90 days since first line therapy). NK012 will be administered by intravenous infusion over 30 minutes once every 28 days (on Day 1 of each cycle). Patients will be screened for UGT1A1 polymorphism prior to enrollment.