This study tests a new medicine, **BNT327**, with chemotherapy to treat people with a type of lung cancer called extensive-stage small-cell lung cancer (**ES-SCLC**). In this study, some people will get BNT327 with chemotherapy, while others will get a different treatment called **atezolizumab** with chemotherapy. The study is **randomized** (participants are chosen randomly) and **double-blinded** (neither participants nor doctors know who gets which treatment). It's conducted at several locations (multisite).

There are two stages. Stage 1 has three groups: two with different treatments and one control group. Stage 2 has two groups: one treatment group and one control group. To join, participants must be newly diagnosed with ES-SCLC and not have had any prior treatments for it. The study includes a screening period, treatment period, and follow-up visits. Participants can continue in the study until their cancer worsens, they can't tolerate the treatment, they choose to leave, or for up to two years.

• The study lasts up to 2 years with regular follow-up visits.
• Participants will be randomly assigned to different treatment groups.
• Eligibility includes having ES-SCLC and not having prior treatment for it.