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.