This study is testing a combination of two drugs, tarlatamab and AB248, to see if they are safe and how they work together in people with a type of lung cancer known as Extensive Stage Small Cell Lung Cancer (ES-SCLC). *Tarlatamab* is a medicine that targets cancer cells, and *AB248* is a drug being tested to see how it works with tarlatamab. The study will also figure out the best dose for these drugs when used together.

To join, participants must be 18 years or older and have ES-SCLC that has come back or gotten worse after at least one treatment. They need to have a measurable tumor and good organ function. A fresh tumor sample is needed unless it’s not possible, then an older sample after the last treatment can be used.

Participants can't join if they have certain brain conditions, have needed oxygen at rest recently, or have had similar treatments before.

• The study's main goal is to check safety and how patients handle the drug combination.
• Participants must provide consent and meet specific criteria to join.
• Fresh tumor samples are needed unless safely not possible.