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.