Results of Pivotal Phase 2 Clinical Trial of Tagraxofusp (SL-401) in Patients with Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN)

Abstract Background: Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a highly aggressive hematologic malignancy with a historical overall survival (OS) of ~8-14 months from diagnosis and no approved therapies or standard of care. Tagraxofusp (Elzonris™; SL-401) is a novel targeted therapy directed to the interleukin-3 receptor-α (CD123), a target expressed on BPDCN and other malignancies. Tagraxofusp was granted Breakthrough Therapy Designation for the treatment of patients with BPDCN, and a rolling Biologics License Application (BLA) submission to the U.S. Food and Drug Administration (FDA) was completed in June 2018. Detailed results from the pivotal trial of tagraxofusp in BPDCN will be presented. Methods: This pivotal Phase 2 clinical trial is a multicenter, open label, non-randomized, single-arm trial designed to determine safety and efficacy of tagraxofusp in patients with BPDCN. In Stage 1 (lead-in), first line (1L) and relapsed/refractory (r/r) patients with BPDCN received tagraxofusp as a daily IV infusion at 7, 9, or 12 mcg/kg/day on days 1-5 of a 21-day cycle. Patients with BPDCN enrolled in subsequent stages received tagraxofusp at the dose determined in Stage 1 (12 mcg/kg). Stage 2 (expansion) enrolled 1L and r/r patients, and Stage 3 (pivotal, confirmatory) enrolled only 1L patients. Results: 45 patients with BPDCN (Stages 1 and 2, n=32; Stage 3, n=13) were enrolled at 7 sites in the US, including 32 (71%) patients as 1L. Median age was 70 years (range, 22-84); 82% male. Median follow-up for all 1L patients treated at 12 mcg/kg (n=29) was 13.8 months (range 0.2-37.4+). The most common treatment-related adverse events (TRAEs) at 12 mcg/kg in 148 patients treated in four clinical trials with tagraxofusp were transaminitis (44%), hypoalbuminemia (44%), and thrombocytopenia (26%). Capillary leak syndrome (CLS), all grades, occurred in 17% of patients across all indications at 12 mcg/kg; 0.7% (1/148) and 1.6% (3/182) of cases resulted in death across all indications at 12 mcg/kg and all doses, respectively. The Stage 3 pivotal cohort met its primary endpoint with a 54% (7/13) rate of CR+CRc (95% CI: 25.1, 80.8). Across Stages 1, 2 and 3, in 1L patients dosed at 12 mcg/kg (n=29), ORR was 90% (26/29) with a 72% (21/29) rate of CR+CRc+CRi (ORR=overall response rate; CR=complete response; CRc=clinical CR: absence of gross disease with minimal residual skin abnormality; CRi=CR with incomplete hematologic recovery). 45% (13/29) of first-line patients treated with 12 mcg/kg were bridged to stem cell transplant (SCT) (10 allo+3 auto). In r/r patients, ORR was 69% (9/13) with a 38% (5/13) rate of CR+CRc+CRi. Additional patient follow-up will be provided. Conclusions: The pivotal trial of tagraxofusp was the largest prospectively designed, multi-center trial specifically dedicated to patients with BPDCN. This study has met its primary endpoint, and also demonstrated high response rates that were generally achieved early in the course of treatment and maintained over multiple cycles of therapy. Safety profile demonstrated most common toxicities of transaminitis, hypoalbuminemia, and thrombocytopenia; occurrence of CLS was the most serious TRAE, which was overall manageable in this population. Patients with BPDCN are being enrolled in an additional cohort, Stage 4, to ensure ongoing access. Tagraxofusp is also being evaluated in other trials including in patients with chronic myelomonocytic leukemia (CMML) and myelofibrosis (MF). Disclosures Pemmaraju: celgene: Consultancy, Honoraria; SagerStrong Foundation: Research Funding; stemline: Consultancy, Honoraria, Research Funding; cellectis: Research Funding; novartis: Research Funding; abbvie: Research Funding; samus: Research Funding; daiichi sankyo: Research Funding; plexxikon: Research Funding; Affymetrix: Research Funding. Lane:N-of-one: Consultancy; Stemline Therapeutics: Research Funding. Sweet:Agios: Consultancy; Jazz: Speakers Bureau; BMS: Honoraria; Novartis: Consultancy, Honoraria, Speakers Bureau; Novartis: Consultancy, Honoraria, Speakers Bureau; Astellas: Consultancy; Celgene: Honoraria, Speakers Bureau; Jazz: Speakers Bureau; Astellas: Consultancy; BMS: Honoraria; Phizer: Consultancy; Agios: Consultancy; Celgene: Honoraria, Speakers Bureau; Phizer: Consultancy. Stein:Amgen Inc.: Speakers Bureau; Celgene: Speakers Bureau. Vasu:Boehringer Ingelheim Inc: Membership on an entity's Board of Directors or advisory committees. Blum:Tolero: Research Funding; Forma: Research Funding; Astellas: Consultancy; Xencor: Research Funding; Boehringer Ingelheim: Research Funding; Pfizer: Consultancy. Rizzieri:Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Arog: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy; Teva: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Incyte: Consultancy, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees. Wang:Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Jazz: Speakers Bureau; Novartis: Speakers Bureau; Novartis: Speakers Bureau; Jazz: Speakers Bureau; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees. Duvic:Guidepoint Global: Consultancy; Eisai: Research Funding; Allos: Research Funding; Clinical Care Options: Consultancy; Array Biopharma: Consultancy, Honoraria; Spatz Foundation: Research Funding; Defined Health: Consultancy; Medivir AB: Membership on an entity's Board of Directors or advisory committees; MiRagen Therapeutics: Consultancy; MEDACorp: Consultancy; Taiwan Liposome Company LTD: Consultancy; Medscape: Other: Speaker/Preceptor; Concert Pharmaceuticals, Inc.: Consultancy; Kyowa Hakko Kirin, Co: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Huya Bioscience Int'l: Consultancy; Shape: Research Funding; Kiniksa Pharmaceuticals: Consultancy; Soligenix, Inc.: Membership on an entity's Board of Directors or advisory committees, Research Funding; Forty Seven, Inc.: Membership on an entity's Board of Directors or advisory committees; Celgene Corp: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Cell Medica Inc.: Consultancy, Honoraria; Dr. Reddy's Laboratories (A.K.A. Promius Pharma): Consultancy; Huron Consulting Group: Consultancy; Aclaris Therapeutics Int'l Ltd.: Honoraria, Membership on an entity's Board of Directors or advisory committees; UT MD Anderson Cancer Center: Employment; The Lynx Group: Consultancy; Evidera, Inc.: Consultancy; Mallinckrddt Pharmaceuticals (formerly Therakos): Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; American Council on Extracorporeal Photopheresis (ACE): Membership on an entity's Board of Directors or advisory committees; Millennium Pharmaceuticals, Inc.: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Tetralogics: Research Funding; Precision Oncology, LLC: Membership on an entity's Board of Directors or advisory committees; Oncoceuticals: Research Funding; Jonathan Wood & Associates: Other: Speaker; Seattle Genetics: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Rhizen Pharma: Research Funding. Spence:Stemline Therapeutics: Consultancy. Shemesh:Stemline Therapeutics: Employment, Equity Ownership. Brooks:Stemline Therapeutics: Employment, Equity Ownership. Bergstein:Stemline Therapeutics: Employment, Equity Ownership. Chen:Stemline Therapeutics: Employment, Equity Ownership. Dunn:Stemline Therapeutics: Employment, Equity Ownership. McDonald:Stemline Therapeutics: Employment, Equity Ownership. Sloan:Stemline Therapeutics: Consultancy. Konopleva:Stemline Therapeutics: Research Funding.