Change In Mast Cell Bone Marrow Burden and In Serum Tryptase Level Are Not Accurate Markers of Response In Patients with Systemic Mastocytosis

Bone marrow involvement, with or without cutaneous or visceral involvement, is almost universal in patients with systemic mastocytosis (SM). The KIT^D816V mutation is present in most patients with SM, thus confirming its clonal nature. Patients with ASM are usually managed with cytoreductive agents such as hydroxyurea (HU), cladribine (2CDA), or interferon-alpha (IFN-α), although the activity of these therapies is limited as they do not target specifically the malignant clone. Response assessment in SM relies on symptom improvement and reduction in serum tryptase levels and visceral and/or bone marrow mast cell burden (percent mast cell involvement). We contend that the later two relatively objective metrics may not be appropriate markers of response because serum tryptase levels may vary significantly at different time-points in the same patient in the absence of intervention, do not correlate accurately with mast cell burden, and bone marrow mast cell burden determination is subject to sampling bias given the patchy infiltration observed in many cases of SM.

To assess the utility of bone marrow mast cell burden reduction and serum tryptase level reduction as criteria for response in patients with SM.

We studied a cohort of 50 patients with SM for whom at least 2 sequential bone marrow biopsies and 2 serum tryptase level determinations were available at our center. The KIT^D816V mutation was present in 20 (59%) of 34 assessable patients. No patient carried the JAK2^V617F mutation or the FIP1L1-PDGFRA rearrangement. Patients had a diagnosis of indolent SM (ISM, n=25), aggressive SM (ASM, n=16), or SM-AHNMD (n=9). All but 1 patient received SM-directed therapy (median number of therapies 2, range 1–5), including: imatinib (n=16), dasatinib (n=23), RAD001 (n=8), denileukin diftitox (n=7). The median number of bone marrow biopsies available per patient was 4 (range, 2–14) and the median number of tryptase measurements was 6 (range, 2–18), which were obtained both on and off SM-directed therapies.

Four patients had a bone marrow complete response: 1 with imatinib, 2 with dasatinib, and 1 with decitabine (with SM-MDS). However none of the responders normalized their tryptase levels. We used the coefficient of variation (CV) as a normalized measure of dispersion of a probability distribution for the percentage of mast cells in bone marrow biopsies and serum tryptase levels. In this manner, the CV summarizes/describes the variation in tryptase levels and bone marrow mast cell percentage from the baseline (first recorded value) in the patients evaluated. We found that among the 49 treated patients, the percentage of bone marrow mast cells varied significantly with a CV ranging from 6 – 173% and an average of 65%. Forty-four percent of patients had a CV equal or higher to the average. Similar results were observed regarding tryptase levels, with an average CV of 19% that ranged from 0 to 96%. Thirty-six percent of patients had a CV higher than average.

While most patients fail to respond to currently available SM-directed therapies, sequential bone marrow biopsies and tryptase level determinations exhibit remarkable variation both during and in the absence of SM-directed therapy. Therefore, it seems that single time point measurements of these values do not represent proper tools to assess accurately response to therapy.

No relevant conflicts of interest to declare.