LY309887 is a potent and selective inhibitor of the reduced folate carrier (RFC), a key transmembrane transporter responsible for the cellular uptake of folates and antifolate chemotherapeutic agents. It is primarily used in preclinical research to study folate transport mechanisms, cellular folate homeostasis, and the development of drug resistance in cancer cells. By blocking RFC function, LY309887 enables researchers to investigate the efficacy of antifolate drugs like methotrexate and explore novel strategies for overcoming resistance in oncology. Its applications are confined to experimental models, serving as a critical tool in advancing understanding of targeted molecular pathways in folate-dependent diseases.