Avapritinib for the treatment of KIT mutation-negative systemic mastocytosis.

Systemic mastocytosis results from the spread of abnormal mast cells in different parts of the body, with variable clinical presentation. It is difficult to diagnose and to determine the appropriate therapy regimen. We present a case of a 53-year-old man diagnosed with KIT-negative advanced systemic mastocytosis based on the 2016 World Health Organization criteria. The patient presented with widespread symptoms that continued to worsen despite supportive therapy and traditional tyrosine kinase inhibitors. He was ultimately started on avapritinib, which reduced his tryptase level and provided symptomatic relief many years after his diagnosis.