Prithviraj Bose, MD, The University of Texas MD Anderson Cancer Center, Houston, TX, discusses updated data from the Phase I EXPLORER study (NCT02561988) of avapritinib, a highly potent and selective inhibitor of mutant KIT, in advanced systemic mastocytosis (SM) patients. The study sought to evaluate the use of pure pathologic response (PPR) criteria in advanced SM patients, including increased clearance of the bone marrow mast cells, reduction of tryptase levels and reduction of mutational burden of KIT. The results indicated that PPR was associated with overall survival in advanced SM patients receiving avapritinib and applying these new criteria increased the complete response rate and increased the evaluable population, indicating its potential as a primary endpoint in future trials. This interview took place during the 62nd American Society of Hematology (ASH) Annual Meeting and Exposition, 2020.