Treatment of indolent systemic mastocytosis with sarilumab is not supported in a randomized trial.

Background: Indolent systemic mastocytosis is a clonal mast cell disease that results in an increase in mast cells in the skin, bone marrow, and other organ systems. IL-6 has been shown to promote mast cell maturation, proliferation, and reactivity . Serum levels of IL-6 correlate with severity of disease and risk of progression of systemic disease.

Objective: We conducted a double-blind placebo-controlled study to assess safety and efficacy for the use of sarilumab in improving the quality of life for those with indolent systemic mastocytosis. ClinicalTrial.gov registration NCT03770273.

Methods: A double-blind trial randomized 16 participants. The primary analysis compared the arms on the Mastocytosis Quality of Life Questionnaire (MC-QoL) index at 16 weeks, adjusting for the baseline MC-QoL. Mean baseline MC-QoL was 47.8.

Results: The difference between the arms in the primary analysis was not statistically significant, with the results favoring the placebo arm; mean absolute MC-QoL improvement in the placebo arm was 14.7 relative to 8.9 in the sarilumab arm. The estimated treatment effect from regression analysis was a 6.0-unit advantage of MC-QoL in the placebo arm ( = .40; 95% confidence interval, -8.9, 20.8), limiting a possible sarilumab advantage to at most 9 units of MC-QoL. Similar conclusions were observed for other quality-of-life indices.

Conclusions: Results in this small trial did not support using sarilumab to treat this population and highlights the importance of double-blind randomized studies.