Blueprint Medicines announced updated results for the Phase 1 EXPLORER clinical trial of avapritinib, a potent and highly selective inhibitor of D816V mutant KIT, the common disease driver in nearly all patients with systemic mastocytosis (SM). The updated EXPLORER trial data in patients with advanced SM showed durable clinical responses that deepened over time, regardless of disease subtype, prior therapy or starting dose. Avapritinib was generally well-tolerated, and most adverse events (AEs) reported by investigators were Grade 1 or 2. These results will be presented today in an oral presentation at the 60th American Society of Hematology Annual Meeting and Exposition in San Diego, California.

As of the data cutoff date of September 30, 2018, the updated results from the ongoing EXPLORER trial showed an overall response rate (ORR) of 83 percent. Twenty-four percent of patients had a complete response with a full or partial recovery of peripheral blood counts (CR/CRh). Responses deepened over time, with a median time to initial response of two months and a median time to CR/CRh of nine months. The median duration of response (DoR) was not reached, and the 12-month DoR rate was 76 percent.

In addition, statistically significant improvements in patient-reported disease symptoms were observed. A 41 percent mean reduction (p=0.043) in patient-reported disease symptoms was demonstrated on the Advanced SM Symptom Assessment Form (AdvSM-SAF), the first patient-reported outcomes tool designed specifically to assess advanced SM.

"Systemic mastocytosis is a complex rare disorder that causes debilitating symptoms across all forms of the disease and reduced survival in advanced patients," said Jason Gotlib, M.D., professor of Medicine, Hematology, at the Stanford University Medical Center and an investigator on the EXPLORER trial. "The updated EXPLORER study results show that selectively targeting D816V mutant KIT with avapritinib led to profound and durable clinical activity in patients with advanced systemic mastocytosis, including significant improvements in patient-reported disease symptoms and quality of life. Combined with encouraging preliminary data from an initial cohort of indolent systemic mastocytosis patients from the EXPLORER study, these results highlight the potential of avapritinib to improve objective and subjective measures of disease burden across the spectrum of mastocytosis subtypes."

"The data further validate Blueprint Medicines' precision therapy approach, where we target genetic drivers of disease with potent and highly selective inhibitors," said Andy Boral, M.D., Ph.D., Chief Medical Officer of Blueprint Medicines. "With robust clinical data from the EXPLORER trial, favorable FDA feedback on potential registration pathways and Breakthrough Therapy Designation for advanced systemic mastocytosis, we believe avapritinib has a strong foundation for expedited development across all forms of the disease. In particular, the new data showing a significant decrease in symptom burden in patients enrolled in the EXPLORER trial increase our confidence in avapritinib's potential in indolent systemic mastocytosis."

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