Targeted Therapy in ISM: Shared Decision-making, Safety, and Early PIONEER Data

Initiation of targeted therapy in indolent systemic mastocytosis (ISM) requires nuanced shared decision-making that incorporates symptom burden, prior treatment history, comorbidities, and patient preferences. The first step is to optimize conventional mediator-directed therapy; only when patients remain significantly symptomatic despite well-structured regimens should a tyrosine kinase inhibitor be considered. Because avapritinib does not eradicate the disease clone but substantially lowers mast cell burden and improves symptoms, it is generally anticipated to be a chronic therapy. Patients should therefore be counseled regarding the likelihood of long-term use and the potential for symptom and lesion recurrence upon discontinuation.

Safety is a critical component of these discussions. Avapritinib carries a contraindication in patients with severe thrombocytopenia (platelet count <50 × 10^9/L) owing to a risk of intracranial hemorrhage observed in earlier, higher-dose studies in advanced systemic mastocytosis. Notably, such events were not seen in the indolent population treated at lower doses in ISM trials, where baseline platelet counts are, by definition, within normal limits. Cognitive adverse events, including memory and concentration difficulties, were likewise reported at higher doses in advanced disease but have not emerged as a significant signal in ISM at the approved dose. These historical data nonetheless inform risk–benefit conversations and underscore the importance of individualized monitoring strategies.

The phase 2 PIONEER study, reported in a randomized, placebo-controlled design over 24 weeks, established the efficacy and safety of avapritinib 25 mg once daily in patients with ISM. Participants receiving avapritinib experienced significant reductions in validated symptom scores—the primary endpoint—as well as meaningful decreases in mast cell–related biomarkers, including serum tryptase, bone marrow mast cell burden, and KIT D816V allele fraction. In this video segment, Akin reviews these findings and highlights how the PIONEER trial’s short‑term results laid the foundation for longer-term extension studies that further characterize the durability of benefit and safety profile of avapritinib in chronic ISM management.