This is a Phase II, multi-site, open-label, parallel group study in participants with untreated extended-stage small-cell lung cancer (ES-SCLC) (Cohort 1) or small-cell lung cancer (SCLC) which has progressed on first- or second-line treatment (Cohort 2 and Cohort 3). This study will assess the safety, efficacy, and pharmacokinetics (PK) of BNT327.