DS 73001-003

This is a multicenter, randomized, phase II study conducted worldwide in patients with histologically or cytologically confirmed extensive stage small cell lung cancer (ES-SCLC) who received at least one prior line of platinum-based chemotherapy and a maximum of three prior lines of treatment.;In the dose optimization part, approximately 80 patients will be randomized in a 1:1 ratio to an 8 mg/kg treatment arm (arm 1) or a 12 mg/kg treatment arm (arm 2). and treated with DS-7300a on day 1 of each 21-day cycle until unacceptable impairment, disease progression (PD), or withdrawal of consent occurs.;The Primary Objective: To study the antitumor activity of DS-7300a in patients with previously treated ES-SCLC.