Mastocytosis is associated with an activating mutation in the KIT oncoprotein (KIT D816V ) that results in autophosphorylation of the KIT receptor in a ligandindependent manner. This mutation is inherently resistant to imatinib and, to date, there remains no effective curative therapy for systemic mastocytosis associated with KIT D816V . Dasatinib (BMS-354825) is a novel orally bioavailable SRC/ ABL inhibitor that has activity against multiple imatinib-resistant BCR-ABL isoforms in vitro that is presently showing considerable promise in early-phase clinical trials of chronic myeloid leukemia (CML). Pharmacokinetic analysis suggests that high nanomolar concentrations of dasatinib can be achieved safely in humans. In this study, we demonstrate significant inhibitory activity of dasatinib against both wild-type KIT and the KIT D816V mutation in the nanomolar range in in vitro and cell-based kinase assays. Additionally, dasatinib leads to growth inhibition of a KIT D816V -harboring human mastocytosis cell line. Significantly, dasatinib selectively kills primary neoplastic bone marrow mast cells from patients with systemic mastocytosis while sparing other hematopoietic cells. Computer modeling suggests that the KIT D816V mutation destabilizes the inactive conformation of the KIT activation loop to which imatinib binds, but it is not predicted to impair binding of KIT IntroductionSystemic mastocytosis and mast-cell leukemia are relatively rare conditions that are characterized by a pathologic accumulation of mast cells in various tissues. 1 Previous work has demonstrated the clonal nature of the disorder as evidenced by an activating point mutation in the KIT receptor tyrosine kinase. 2,3 This mutation, KIT D816V , occurs near the activation loop of the kinase and can be identified in more than 80% of systemic mastocytosis cases and thus represents an attractive target for this disorder, for which few effective therapeutic options exist. 1 Although a small molecule tyrosine kinase inhibitor of KIT, imatinib mesylate, has been available to patients recently, both preclinical and clinical studies have demonstrated convincingly that the KIT D816V mutation is inherently resistant to imatinib. [4][5][6] The crystal structure of the wild-type KIT kinase domain has been solved in an autoinhibited state, with the activation loop of the kinase in a closed conformation, 7 similar to the cocrystal structure of imatinib bound to the ABL kinase domain. 7,8 These structural studies suggest that substitution of valine for aspartic acid at position 816 in KIT may alter the conformation of the activation loop and thus prevent imatinib from efficiently binding to KIT D816V .Inhibition of KIT D816V has thus become the focus of recent studies aiming to find a curative therapy for mast-cell disease, and several potential drug candidates with an ability to inhibit KIT D816V have been identified. [9][10][11][12][13][14][15] Dasatinib (BMS-354825) is a novel, potent, oral, multitargeted kinase inhibitor that targets ABL, SRC, KIT, PDGFR, ...

Disruption of the intestinal microbiota occurs frequently in allogeneic hematopoietic cell transplantation (allo-HCT) recipients and predisposes them to development of graft-versus-host disease (GvHD). In a prospective, single-center, single-arm study, we investigated the effect of donor fecal microbiota transplantation (FMT) on symptoms of steroid-refractory or steroid-dependent, acute or late-onset acute intestinal GvHD in 15 individuals who had undergone allo-HCT. Study participants received a fecal suspension from an unrelated healthy donor via nasoduodenal infusion. Donor FMT was well tolerated, and infection-related adverse events did not seem to be related to the FMT procedure. In 10 of 15 study participants, a complete clinical response was observed within 1 month after FMT, without additional interventions to alleviate GvHD symptoms. This response was accompanied by an increase in gut microbial α-diversity, a partial engraftment of donor bacterial species, and increased abundance of butyrate-producing bacteria, including Clostridiales and Blautia species. In 6 of the 10 responding donor FMT recipients, immunosuppressant drug therapy was successfully tapered. Durable remission of steroid-refractory or steroid-dependent GvHD after donor FMT was associated with improved survival at 24 weeks after donor FMT. This study highlights the potential of donor FMT as a treatment for steroid-refractory or steroid-dependent GvHD, but larger clinical trials are needed to confirm the safety and efficacy of this procedure.