This study is testing a new treatment called **AZD0901** for a type of stomach cancer known as **gastric or gastroesophageal junction adenocarcinoma**. This cancer can be advanced or have spread to other parts, known as *metastatic*. The study compares AZD0901 to other treatments chosen by doctors. Certain patients have a marker called **CLDN18.2**, which is a protein found in some cancer cells. This study aims to see if AZD0901 helps these patients more than the usual treatments.

**Key Points:**

• **Study Type:** This is a Phase III trial, meaning AZD0901 has been studied before and is now being compared to regular treatments.
• **Eligibility:** Participants must be 18 or older with specific cancer characteristics, including having the CLDN18.2 marker.
• **Duration and Compensation:** The study lasts until the researchers gather enough data on safety and efficacy. Check with the study team about visits and any compensation.

To join, participants must agree to the study rules and have had previous cancer treatments. They should not have certain other health issues like brain conditions or severe stomach problems. This study hopes to discover if AZD0901 can be a better choice for treating this cancer type.