AZD-9291 mesylate is primarily used in the treatment of non-small cell lung cancer (NSCLC), particularly in patients with specific mutations in the epidermal growth factor receptor (EGFR), such as the T790M mutation. It acts as a tyrosine kinase inhibitor, targeting and blocking the activity of mutated EGFR proteins that drive cancer cell growth. This helps to slow or stop the progression of the disease. It is often administered when earlier EGFR inhibitor treatments have become ineffective due to resistance development. Additionally, it is being studied for its potential in treating other cancers with similar genetic mutations.