S3I-201 is primarily used in biochemical and pharmaceutical research as a STAT3 (Signal Transducer and Activator of Transcription 3) inhibitor. It blocks STAT3 activation by preventing its phosphorylation and dimerization, processes that are often dysregulated in cancer cells. Due to this mechanism, S3I-201 is employed in studies targeting tumor growth, especially in cancers where STAT3 signaling contributes to cell proliferation, survival, and immune evasion—such as breast cancer, leukemia, and multiple myeloma. It is also utilized to investigate inflammation and autoimmune diseases, as STAT3 plays a role in cytokine signaling and immune cell function. Researchers use S3I-201 in cell-based assays and animal models to validate STAT3 as a therapeutic target and to explore downstream effects of its inhibition, including apoptosis induction and suppression of angiogenesis. Its application remains mainly confined to preclinical research due to limitations in potency and pharmacokinetic properties.