Future Directions in ISM: Long-term Outcomes, Unanswered Questions, and Pediatric Considerations

The integration of disease-targeted therapies into indolent systemic mastocytosis (ISM) care is shifting treatment goals from symptom palliation alone toward combined objectives of symptom control and disease modification. As clinicians gain comfort with prescribing KIT D816V inhibitors and more patients receive these agents, expectations increasingly encompass reduction of skin lesions, normalization or marked lowering of serum tryptase, and sustained improvement in quality of life. Long-term extension data from PIONEER, with median follow-up approaching 4 years and some patients exceeding 5 years, indicate that the symptomatic and biomarker benefits of avapritinib can be durable, with no new safety signals and a low incidence of serious adverse events over time.

Despite these advances, important knowledge gaps remain. The impact of KIT inhibition on osteoporosis, which affects approximately 30% of patients with systemic mastocytosis, is incompletely understood. Key questions include whether reducing mast cell burden and mediator release can meaningfully alter bone density trajectories and whether tyrosine kinase inhibitors can be used as standalone approaches or must be combined with standard antiosteoporotic agents such as bisphosphonates or monoclonal antibodies. Similarly, the effect of avapritinib on anaphylaxis risk—seen in roughly 40% of patients and often triggered by Hymenoptera venom or other IgE-mediated stimuli—requires dedicated study. Conceptually, lowering mast cell burden should reduce risk, but prospective, controlled data are lacking.

Longer-term considerations also include the potential for resistance, opportunities for curative or treatment-free remission strategies, and extension of targeted approaches to pediatric disease. To date, no clear cases of clinical resistance to avapritinib have been documented in ISM over follow-up periods of up to 5 years, a finding that may reflect the relatively low proliferative rate of mast cells in indolent disease. Future research may explore combination regimens pairing KIT D816V–selective inhibitors with other pathway-directed agents or monoclonal antibodies to achieve deeper clonal eradication or permit intermittent rather than continuous dosing. In this concluding segment of the video series, Akin highlights these unanswered questions and underscores the need to adapt emerging therapies and evidence-based strategies for children with cutaneous and systemic mastocytosis, a population for whom current approvals and data are limited.