A Study of Rovalpituzumab Tesirine as Maintenance Therapy Following First- Line Platinum-Based Chemotherapy in Participants With Extensive Stage Small Cell Lung Cancer (MERU)

* Histologically or cytologically confirmed extensive-stage disease small cell lung cancer (ED SCLC) at initial diagnosis with ongoing clinical benefit (stable disease \[SD\], partial response \[PR\], or complete response \[CR\]) following completion of 4 cycles of first-line platinum-based therapy * Participant is eligible to be randomized at least 3 but no more than 9 weeks from Day 1 of the fourth cycle of first-line platinum-based chemotherapy. * Participants with a history of central nervous system (CNS) metastases prior to the initiation of first-line platinum-based chemotherapy must have received definitive local treatment and have documentation of stable or improved CNS disease status * Eastern Cooperative Oncology Group (ECOG) performance score of 0 or 1 * Participants must have adequate bone marrow, renal and hepatic function * Availability of archived or representative tumor material for assessment of DLL3 expression

* Any prior systemic chemotherapy, small molecule inhibitors, immune checkpoint inhibitors, other monoclonal antibodies, antibody-drug conjugates, radioimmunoconjugates, T-cell or other cell-based or biologic therapies, or any other anti-cancer therapy than that described in inclusion criteria for SCLC. * Any disease-directed radiotherapy (except prophylactic cranial irradiation, palliative radiotherapy to a radiographically documented non-progressing lesion for symptom control, or pre-planned radiotherapy for CNS metastases present prior to start of first-line therapy and non-progressing) after last dose of first-line chemotherapy. * Prior exposure to a pyrrolobenzodiazepine (PBD-based) or indolinobenzodiazepine-based drug, prior participation in a rovalpituzumab tesirine clinical trial, or known hypersensitivity or other contraindications to rovalpituzumab tesirine or excipient contained in the drug formulation.