Cem Akin, MD 

Future directions in ISM, including evolving treatment goals, unanswered questions about osteoporosis, anaphylaxis, resistance, curative strategies, and the need to extend advances to pediatric populations.

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.

Videos

Use of symptom and quality-of-life tools alongside biomarkers in PIONEER, principles of dose titration from 25 mg to 50 mg, and practical recommendations for longitudinal safety monitoring.

Measuring Response in ISM: PRO Tools, Biomarkers, and Dose Optimization With Avapritinib

Assessment of therapeutic response in indolent systemic mastocytosis (ISM) requires integration of patient-centered outcomes with objective disease markers. In PIONEER, investigators used an ISM-specific symptom assessment form and a mastocytosis quality-of-life questionnaire (eg, MC-QoL) in conjunction with traditional biomarkers. These instruments captured dimensions such as daily functioning, emotional well-being, and the global impact of disease, thereby extending beyond what can be inferred from serum tryptase, bone marrow mast cell percentages, or KIT D816V variant allele frequency. Clinical experience confirms that symptom burden and mast cell burden may be discordant across individuals, underscoring the need to evaluate both domains when gauging treatment success.

Dose flexibility represents another tool for individualizing therapy. During the extension phase of PIONEER, patients who derived suboptimal benefit at 25 mg daily—judged by persistent symptoms or limited biomarker change—were eligible for escalation to 50 mg at the investigator’s discretion and with patient agreement. Early dose-finding work demonstrated that 25‑, 50‑, and 100‑mg doses all produced substantial symptom reductions in a small cohort, without clear dose-limiting toxicities, prompting selection of 25 mg as the minimal effective dose for broader study. Subsequent experience indicated that some patients who did not achieve satisfactory control at 25 mg improved after titration to 50 mg, without emergence of new safety signals.

From a monitoring standpoint, avapritinib is generally well tolerated in ISM, and current product labeling does not mandate intensive laboratory surveillance. Nonetheless, many experts advocate periodic complete blood counts with differential—particularly to follow platelet trends—as part of good clinical practice. Serial measurement of serum tryptase is commonly used to track mast cell burden, and centers with access to KIT D816V allele burden testing may incorporate this as an additional longitudinal marker. In this video portion, Akin outlines a pragmatic approach to long-term follow-up for patients receiving avapritinib, emphasizing judicious laboratory monitoring, reproductive counseling, and ongoing assessment of clinical response and tolerability.

Shared decision-making around initiating targeted therapy, key safety considerations for avapritinib, and core efficacy and safety findings from the 6‑month randomized phase of the PIONEER trial.

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.


Role of patient-reported outcomes in trials and practice, criteria for escalation beyond supportive care, and the paradigm shift introduced by selective KIT D816V inhibition, including avapritinib.

When Supportive Care Is Not Enough: Patient-Reported Outcomes and Escalation to Targeted Therapy

Symptom assessment tools play distinct but complementary roles in the clinical trial and routine practice settings for indolent systemic mastocytosis (ISM). In drug development, rigorously validated PRO instruments serve as primary endpoints, linking targeted biologic effects to patient-experienced benefit. For example, in pivotal trials of KIT D816V–selective inhibitors, sponsors developed disease-specific symptom scores incorporating multiple items rated on numerical scales and administered at each visit to quantify change over time. Similar frameworks have been applied in parallel ISM programs, enabling comparison of treatment arms and supporting regulatory decisions on the basis of clinically meaningful symptom improvement.

In everyday practice, formal PROs are less commonly used, but their value is increasingly recognized alongside tools such as the Mastocytosis Control Test. These instruments complement laboratory and histologic markers—serum tryptase levels, bone marrow mast cell burden, and KIT D816V allele fraction—by providing standardized insight into domains that may not correlate directly with mast cell burden. Importantly, eligibility criteria for clinical trials of targeted therapies have helped crystallize a practical definition of “treatment-refractory” ISM: patients receiving 2 or more optimized anti–mast cell mediator regimens (eg, H1 and H2 antihistamines, leukotriene antagonists, cromolyn sodium, omalizumab, or corticosteroids) who nonetheless exhibit persistently high symptom scores.

The emergence of selective KIT D816V inhibition has altered treatment paradigms for both indolent and advanced systemic mastocytosis. Avapritinib, the first such agent approved by the US Food and Drug Administration (FDA) for ISM, directly reduces the clone of KIT-mutated mast cells, distinguishing it from earlier multikinase inhibitors that primarily targeted wild-type KIT and had limited applicability in D816V-positive disease. In this segment of the video series, Akin explains how avapritinib’s demonstration of robust symptom reduction and biomarker improvement in clinical trials has validated KIT D816V as a driver mutation in ISM and opened a new therapeutic avenue for patients whose disease remains inadequately controlled with supportive care alone.

Cem Akin, MD

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