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Introduction: Haploidentical peripheral blood allogeneic hematopoietic cell transplantation (PB haplo-HCT) can be complicated by graft-versus-host disease (GVHD) and cytokine release syndrome (CRS). Acute GVHD rates are higher with PB grafts compared with bone marrow, affecting 35-45% of patients and outcomes are poor in steroid refractory cases. Severe CRS occurs in 10-15% of patients receiving PB haplo-HCT and is associated with high non-relapse mortality and dismal one year overall survival between 25-30%. As interferon-γ and IL-6 are important mediators in both acute GVHD and CRS, we hypothesized that JAK1 inhibition with itacitinib could prevent these toxicities without impairing engraftment. Here we report the clinical outcomes from our pilot study of itacitinib with haplo-HCT (NCT03755414). Methods: Patients with AML, ALL, or NHL in remission undergoing PB haplo-HCT were treated with itacitinib 200 mg/day on days -3 through +100, followed by a taper. Myeloablative and reduced intensity conditioning were allowed. GVHD prophylaxis was tacrolimus, mycophenolate mofetil, and post-transplant cyclophosphamide. Primary outcomes were incidence of primary graft failure and incidence of grade III-IV acute GVHD. Secondary outcomes included incidence and severity of CRS (graded by Lee criteria). Peripheral blood and serum samples were banked prior to conditioning and on days -1, 1, 3, 7, 14, 28, 60, 100, end of treatment, and time of diagnosis of acute GVHD. Matched control samples were collected from patients undergoing haplo-HCT off clinical trials. Correlative studies include flow cytometry (FACS) with five 28-color panels for cellular subsets, mass cytometry (CyTOF) with 40-colors for intracellular signaling events, single cell RNA sequencing, and serum cytokine and chemokine measurements. Results: Twenty of a planned 20 patients completed enrollment and underwent haplo-HCT between 11/2019 and 3/2021. Median age at transplant was 49 (21-74). Diagnoses were AML (13), ALL (5) and NHL (2). Median follow up is 319 days, with 18/20 beyond 180 days. There were no cases of engraftment failure with short median times to neutrophil (14 days, range 12-20) and platelet (14 days, range 7-54) engraftment (historical 16 and 25 days, respectively). There were no cases of grade III-IV acute GVHD. The incidence of grade II acute GVHD on day 100 was 15%. Two patients developed grade I-II skin acute GVHD during itacitinib taper and responded to resumption of a higher dose. There were no cases of extensive chronic GVHD. There were no cases of severe CRS (historical rate 17%), with 90% of patients having grade 1 CRS and 10% having no CRS. Furthermore, no anti-IL6R or steroid therapy was used. Overall survival at day 180 was 90% (95% CI 75-100%) by Kaplan-Meier estimate. Incidence of relapse at 180 days was 5.5% (95% CI 0-15.6%). Refined GVHD and relapse-free survival at 180 days was 83% (95% CI 68-100%). All patients had full donor engraftment and >95% chimerism at day 100. FACS and CyTOF have been performed and analyzed for the day 28 time point from 14 patients and four controls. Flow cytometry revealed no difference in cell subset numbers between controls and patient samples. CyTOF revealed differences in intracellular signaling molecules between itacitinib and control patients - including higher Ki-67, pNF-κB, and Caspase3 in controls. Phospho-Stat1 and pStat3 were lower CD4 T subsets. FACS and CyTOF at remaining time points, single cell RNA sequencing, and serum cytokine and chemokine measurements are underway and will be presented at the ASH 2021 meeting. Conclusions: Itacitinib with PB haplo-HCT was safe with no engraftment failure and prompt engraftment. Rates of acute and chronic GVHD were low, without increased risk of relapse or transplant related mortality. Severe CRS was not seen in this trial, and no anti-IL6 or steroid therapy was used. Flow cytometry demonstrated comparable immune reconstitution in terms of cell lineage and number between treated patients and controls. Mass cytometry revealed lower Ki-67, pNF-κB, and caspase3 levels, among other markers, which suggest lower immune cell activity, proliferation, and apoptosis. An extension cohort of 20 additional patients is enrolling. Multi-platform correlative studies are underway, comparing samples from haplo-HCT patients treated with and without itacitinib. Figure 1 Figure 1. Disclosures Uy: Astellas: Honoraria, Speakers Bureau; Novartis: Consultancy; Agios: Consultancy; Jazz: Consultancy; Genentech: Consultancy; AbbVie: Consultancy; GlaxoSmithKline: Consultancy; Macrogenics: Research Funding. Ghobadi: Atara: Consultancy; Amgen: Consultancy, Research Funding; Wugen: Consultancy; Celgene: Consultancy; Kite, a Gilead Company: Consultancy, Honoraria, Research Funding. Jacoby: Abbvie: Research Funding; Jazz: Research Funding. Pusic: Syndax: Other: Advisory Board. Schroeder: Equillium Inc: Honoraria; Janssen: Honoraria; Sanofi Genzyme: Honoraria. more...