Results from a Phase 1/2 Clinical Trial of Tagraxofusp (SL-401) in Patients with Intermediate, or High Risk, Relapsed/Refractory Myelofibrosis

Background Patients with myelofibrosis (MF) who fail or are intolerant to JAK inhibitors (JAKi) have limited treatment options. Tagraxofusp is a targeted therapy directed to CD123 that was approved by the FDA for the treatment of patients with blastic plasmacytoid dendritic cell neoplasm (BPDCN). CD123 is expressed on a variety of malignancies including BPDCN, acute myeloid leukemia (AML), and certain myeloproliferative neoplasms (MPN), including MF. High-expressing CD123+ plasmacytoid dendritic cells (pDCs), the cell of origin of BPDCN, have been detected in the microenvironment of certain myeloid neoplasms, including MF, where they may play a tumor-promoting role. Notably, pDCs share a common precursor cell with monocytes, and monocytosis has been reported as a poor prognostic factor in patients with MF, associated with shortened overall survival. We hypothesized that tagraxofusp may offer a novel and rational therapeutic approach in patients with relapsed/refractory MF, and in particular in patients with monocytosis. Aims To assess safety and evaluate efficacy of tagraxofusp in patients with MF who were relapsed, refractory, or unable to tolerate JAKi. Methods This multicenter, open-label Phase 1/2 trial is enrolling patients with MF. In the Stage 1 (dose escalation), tagraxofusp was administered as a daily IV infusion at 7, 9, or 12 mcg/kg on days 1-3 every 21 days (cycle 1-4), 28 days (cycles 5-7), and 42 days (cycles 8+). In Stage 2 (expansion), patients are receiving the recommended Phase 2 dose (12 mcg/kg). Results 27 patients with MF have been treated, including 14 patients who had previously received ≥ 3 lines of therapy. Median age was 69 years (range 54-81), 52% were female, and 26% had baseline monocytosis (≥ 1x109/L). Baseline risk assessment based on the DIPSS Plus risk group assessment showed 1 patient (4%) with intermediate-1, 16 patients (59%) with intermediate-2, and 9 patients (33%) with high-risk. At study entry, median platelet count was 59 K/uL; 67% of patients had baseline platelets <100 K/uL, of which 10 patients had platelets <50 K/uL. 74% of patients had baseline splenomegaly (spleen palpable ≥ 5 cm below the left costal margin). In terms of molecular mutations: JAK2 (n=18); CALR (n=3); and MPL (n=1), and additional molecular analysis showed ASXL1 (n=5). In terms of cytogenetics, 30% (8/27) of patients had an abnormal karyotype, mainly 20q- (n=3), -5/5q-, -7/7q-, and 12p- (each n=2). Most common treatment-related adverse events (TRAEs, incidence ≥ 15%) include headache, hypoalbuminemia, increased levels of alanine aminotransferase, thrombocytopenia, and anemia. Most common ≥ grade 3 TRAE's were thrombocytopenia (8%) and anemia (15%). Capillary leak syndrome was reported in 1 patient (grade 3). Among the 17 evaluable patients with baseline splenomegaly, 53% had spleen size reductions, of which 4 patients had reductions of > 45%. In 5 patients with baseline splenomegaly and monocytosis (>1x109/L monocytes), 80% (4/5) had spleen size reductions, of which 2 had reductions of >45%. Five patients had treatment duration of 12 months or more, with two of those patients ongoing (13+,and 24+ months, respectively). Six patients with platelets <100 K/uL had treatment duration of 6 months or more, including 3 patients with baseline monocytosis who had treatment duration of 12 months or more. 7 patients had objective responses (IWG-MRT 2013 responses), including Clinical improvement (CI) (n=5) and CI (Symptom) (n=2). 20 patients were evaluable for symptomatic assessment (MPN-SAF TSS). Symptom response rate was 45% (defined as ³50% reduction in TSS score). Reductions in both symptom score and spleen size was noted in 6/6 patients who had splenomegaly at baseline. Conclusions Tagraxofusp demonstrated single agent clinical activity, with a predictable and manageable safety profile, in patients with relapsed/refractory MF, including in patients with monocytosis, a poor prognostic factor and unmet medical need. Tagraxofusp may offer MF patients, and MF patients with monocytosis in particular, a novel treatment option. Enrollment continues, and updated trial data will be presented. This clinical trial is registered at clinicaltrials.gov: NCT02268253. Disclosures Pemmaraju: affymetrix: Research Funding; sagerstrong: Research Funding; Daiichi-Sankyo: Research Funding; plexxikon: Research Funding; novartis: Consultancy, Research Funding; Stemline Therapeutics: Consultancy, Honoraria, Research Funding; cellectis: Research Funding; celgene: Consultancy, Honoraria; samus: Research Funding; mustangbio: Consultancy, Research Funding; abbvie: Consultancy, Honoraria, Research Funding; incyte: Consultancy, Research Funding. Gupta:Sierra Oncology: Honoraria, Membership on an entity's Board of Directors or advisory committees; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Incyte: Honoraria, Research Funding. Yacoub:Dynavax: Equity Ownership; Cara: Equity Ownership; Ardelyx: Equity Ownership; Incyte: Consultancy, Honoraria, Speakers Bureau; Seattle Genetics: Honoraria, Speakers Bureau; Novartis: Consultancy, Speakers Bureau; Agios: Speakers Bureau; Hylapharm: Equity Ownership. Wang:Agios: Other: Advisory role; Amgen: Other: Advisory role; Daiichi: Other: Advisory role; Stemline: Other: Advisory role, Speakers Bureau; Pfizer: Other: Advisory role, Speakers Bureau; celyad: Other: Advisory role; Abbvie: Other: Advisory role; Kite: Other: Advisory role; Jazz: Other: Advisory role; Astellas: Other: Advisory role, Speakers Bureau. Lee:AstraZeneca Pharmaceuticals: Consultancy; Ai Therapeutics: Research Funding; Karyopharm Therapeutics: Consultancy; Roche Molecular Systems: Consultancy; Jazz Pharmaceuticals, Inc: Consultancy; Helsinn: Consultancy. Schiller:Agios: Research Funding, Speakers Bureau; Karyopharm: Research Funding; Onconova: Research Funding; Novartis: Research Funding; Pfizer Pharmaceuticals: Equity Ownership, Research Funding; Sangamo Therapeutics: Research Funding; Amgen: Other, Research Funding; Astellas: Research Funding; Biomed Valley Discoveries: Research Funding; Bristol Myer Squibb: Research Funding; Celgene: Research Funding, Speakers Bureau; Constellation Pharmaceutical: Research Funding; Daiichi Sankyo: Research Funding; Eli Lilly and Company: Research Funding; FujiFilm: Research Funding; Genzyme: Research Funding; Gilead: Research Funding; Incyte: Research Funding; J&J: Research Funding; Jazz Pharmaceuticals: Honoraria, Research Funding. Sardone:Stemline Therapeutics: Employment, Equity Ownership. Wysowskyj:Stemline Therapeutics: Employment, Equity Ownership. Chen:Stemline Therapeutics: Employment, Equity Ownership. Brooks:Stemline Therapeutics: Employment, Equity Ownership, Patents & Royalties. Poradosu:Stemline Therapeutics: Employment, Equity Ownership, Patents & Royalties. McDonald:Stemline Therapeutics: Employment, Equity Ownership. Rupprecht:Stemline Therapeutics: Employment, Equity Ownership. Talpaz:Imago BioSciences: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; CTI BioPharma: Research Funding; Constellation: Research Funding; Incyte: Research Funding; Novartis: Research Funding; Samus Therapeutics: Research Funding. Taparia:Novartis: Consultancy. Verstovsek:Protaganist Therapeutics: Research Funding; Ital Pharma: Research Funding; Astrazeneca: Research Funding; Pharma Essentia: Research Funding; Sierra Oncology: Research Funding; Novartis: Consultancy, Research Funding; Blueprint Medicines Corp: Research Funding; Incyte: Research Funding; Roche: Research Funding; NS Pharma: Research Funding; Celgene: Consultancy, Research Funding; Gilead: Research Funding; Promedior: Research Funding; CTI BioPharma Corp: Research Funding; Genetech: Research Funding; Constellation: Consultancy; Pragmatist: Consultancy. Khoury:Kiromic: Research Funding; Angle: Research Funding; Stemline Therapeutics: Research Funding. Patnaik:Stem Line Pharmaceuticals.: Membership on an entity's Board of Directors or advisory committees.

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