Poster session at the 23rd Congress of the European Hematology Association (EHA)

STOCKHOLM—The KIT/PDGFRA inhibitor avapritinib has produced durable responses in patients with systemic mastocytosis (SM).

In the phase 1 EXPLORER trial, avapritinib produced an overall response rate of 83%.

Responses have lasted up to 22 months, and 79% of responders remained on avapritinib as of the data cutoff.

The most common treatment-related adverse events (AEs) were periorbital edema, anemia, nausea, and fatigue.

These data were presented in a poster (abstract PF612) at the 23rd Congress of the European Hematology Association (EHA).

The trial was sponsored by Blueprint Medicines Corporation.

As of the data cutoff (April 30, 2018), 52 patients had been treated with avapritinib in the dose-escalation and expansion portions of the EXPLORER trial.

This included 25 patients with aggressive SM (ASM), 15 with advanced SM and an associated hematologic neoplasm (SM-AHN), 5 with mast cell leukemia (MCL), 5 pending central pathology diagnosis, and 2 with smoldering SM.

Thirty-five patients (67%) were previously treated, including 10 (19%) who previously received midostaurin. The patients’ median age was 63 (range, 34-83), and 52% were male.

Treatment

Thirty-two patients were treated in the dose-escalation portion of the study and received avapritinib at doses ranging from 30 mg to 400 mg daily. The 35 patients in the expansion portion received avapritinib at 300 mg daily.

Among all 52 enrolled patients, 42 remained on treatment as of the data cutoff date. Four patients discontinued treatment with avapritinib due to AEs. Three of these were treatment-related, and 1 was unrelated.

Three patients discontinued treatment due to clinical progression as determined by the investigator. None of the patients had documented disease progression by IWG-MRT-ECNM criteria.

Two patients discontinued due to investigator decision, and 1 withdrew consent.

Safety

All 52 patients were evaluable for safety.

Treatment-related AEs included periorbital edema (62%), anemia (33%), nausea (33%), fatigue (31%), peripheral edema (27%), diarrhea (25%), hair color changes (23%), thrombocytopenia (19%), cognitive effects (19%), vomiting (19%), and dizziness (12%).

Grade 3 or higher AEs, regardless of drug relationship, included thrombocytopenia (17%), anemia (15%), fatigue (6%), vomiting (6%), periorbital edema (4%), nausea (4%), diarrhea (2%), hair color changes (2%), and cognitive effects (2%).

Efficacy

As of the data cutoff, 23 patients were evaluable for response by IWG-MRT-ECNM criteria. This included 8 patients with ASM, 10 with SM-AHN, and 5 with MCL.

The overall response rate was 83% (n=19). All responses observed in the dose-escalation portion of the trial have been confirmed, and all responses in the dose-expansion portion of the trial are pending confirmation.

Four patients (17%) had a confirmed complete response with a full (n=1) or partial (n=3) recovery of peripheral blood counts. All of these responses occurred in patients with ASM.

Twelve patients (52%) had a partial response (7 confirmed, 5 pending confirmation). This included 6 patients with SM-AHN, 4 with MCL, and 2 with ASM.

Three patients (13%) had clinical improvement (2 confirmed, 1 pending confirmation), and 4 had stable disease. None of the patients progressed.

The duration of response ranged from 8 months to 22 months, and 79% of responders (15/19) remained on treatment at the data cutoff.

“As a clinician treating patients with this devastating and sometimes fatal rare disease, I’m excited to see that most patients with advanced systemic mastocytosis respond to treatment with avapritinib, and these responses deepen over time and are durable,” said study investigator Michael W. Deininger, MD, PhD, of Huntsman Cancer Institute at the University of Utah in Salt Lake City.

“These data further support avapritinib’s unique approach of selectively targeting D816V mutant KIT, the disease driver in most patients with systemic mastocytosis. If these results are confirmed in the planned phase 2 trial, avapritinib has the potential to become a new standard of care for patients with advanced forms of the disease.”

Avapritinib produces durable responses in SM