ARRY-520 (also known as Filanesib) is a potent, selective, and reversible inhibitor of kinesin spindle protein (KSP/Eg5), a motor protein essential for the formation of the bipolar mitotic spindle during cell division. It is a synthetic small molecule belonging to the class of isoquinoline derivatives. By inhibiting KSP, ARRY-520 induces mitotic arrest and apoptosis in rapidly dividing cells, particularly cancer cells. Its primary application is as an investigational antineoplastic agent. It has been evaluated in clinical trials, both as a monotherapy and in combination with other chemotherapeutics (e.g., carfilzomib, pomalidomide), for the treatment of relapsed or refractory multiple myeloma and acute myeloid leukemia. The compound represents a targeted approach to cancer therapy aimed at overcoming resistance to conventional microtubule-targeting agents like taxanes. While not a classical PROTAC degrader, ARRY-520's mechanism involves targeted protein inhibition rather than degradation. Its development highlights the exploration of novel mitotic targets in oncology. Research into its pharmacokinetics, safety profile, and efficacy in specific patient populations has been a key focus of its clinical development program.