Phase 1 Cohort Expansion of Flotetuzumab, a CD123×CD3 Bispecific Dart® Protein in Patients with Relapsed/Refractory Acute Myeloid Leukemia (AML)

Acute myeloid leukemia (AML) blasts and leukemia stem cells highly express the α chain of the IL-3 receptor (CD123), compared to normal hematopoietic stem cells. CD123 expression is associated with high-risk features, increased risk of induction failure and poor prognosis (Vergez F, et al. Haematologica 2011; 96: 1792-8). Flotetuzumab (FLZ), a CD123 × CD3 bispecific DART molecule, is being tested in a phase 1/2 study in patients with relapsed/refractory (R/R) AML. The recommended Phase 2 dose (RP2D) of FLZ is 500 ng/kg/day (d) administered as a 7-day/week continuous infusion. Patients receive a lead-in dose (30 ng/kg/d x 3d then 100 ng/kg/d x 4d) during week (W) 1, followed by 500 ng/kg/d during W2-4 of cycle 1, and a 4d on/3 d off schedule for cycle 2 and beyond. Disease status was assessed by modified IWG criteria; samples were collected to investigate candidate biomarkers, including CD123 receptor density/cell (RD), and gene expression profiling using the NanoString® PanCancer IO 360™ assay. This platform was used to assess the expression of 770 genes, including 14 immune cell types and 32 immuno-oncology biological signatures in bone marrow (BM) samples from patients treated with FLZ. Thirty patients with R/R AML, median age 64.5 years, received FLZ at the RP2D. Most patients enrolled had primary refractory disease (60% [18/30; 14 to cytotoxic chemotherapy (refractory to ≥ 2 induction attempts, or first CR with initial CR + BM blasts was 85% (range 1.4100%), and the median CD123 receptor density on BM blasts was 4,084 (range 357-28,818 RD). Infusion-related reaction/cytokine release syndrome (IRR/CRS), the most common AE, occurred in all patients, including grade ≥3 in 4/30 (13.3%) of patients. IRR/CRS was managed with conventional supportive care and a management paradigm based on early intervention with tocilizumab (anti-IL-6R) to ameliorate the progression of CRS. Most IRR/CRS events were of short duration and reversible with protocol-specified supportive care. Antileukemic activity was reported in 18/27 (67%) response-evaluable patients, with an overall response rate (ORR) of 22% (6/27) and a CR/CRi rate of 19% (5/27). Interestingly, responsiveness to FLZ appeared quite distinct in patients with relapsed disease versus those who were refractory to standard induction chemotherapy. Estimates from the literature indicate that up to 45% of AML patients are refractory to standard front-line therapy. In patients treated with FLZ, the CR/CRi rate was 31% (4/13) among patients with primary chemotherapy refractory AML, while no responses (0/11) were observed in relapsed patients. A prior report suggests that AML patients with an immune-enriched tumor microenvironment (TME) as evidenced by increased expression of genes associated with CD8 T cells, Th1 cells, B cells, cytotoxicity, CXCL9 and CXCL10, are less likely to respond to anthracycline-based cytotoxic chemotherapy and experience significantly shorter relapse-free survival (Vadakekolathu J, et al. Blood 2017; 130: 3942A). We hypothesized that the presence of an immune-enriched gene signature in the BM TME of patients with AML could help to identify patients more likely to respond to FLZ immunotherapy. Initial exploratory studies suggest that the expression of inflammatory genes may be increased in patients with refractory AML, including patients that respond to FLZ. In conclusion, among this initial cohort of patients treated at the RP2D (500 ng/kg/day), FLZ demonstrates antileukemic activity with an acceptable safety profile, in particular, among refractory patients, a difficult-to-treat patient population that represents a significant area of unmet medical need. Enrollment to the current study has now been expanded to further define the antileukemic activity of FLZ in patients with refractory AML, and investigate candidate biomarkers to enable identification of patients more likely to respond to FLZ. In addition, studies are now being initiated to investigate opportunities to expand the antileukemic activity of FLZ via combined administration with either concurrent or sequenced anti-PD-1 checkpoint blockade. Disclosures Uy: GlycoMimetics: Consultancy; Curis: Consultancy. Rettig: Novimmune: Research Funding; Amphivena Therapeutics: Research Funding. Foster: Celgene: Research Funding; Macrogenics: Research Funding; Pfizer: Research Funding; Shire: Honoraria. Rizzieri: Arog: Consultancy, Membership on an entity9s Board of Directors or advisory committees; Pfizer: Consultancy, Membership on an entity9s Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity9s Board of Directors or advisory committees; Incyte: Consultancy, Membership on an entity9s Board of Directors or advisory committees, Speakers Bureau; Novartis: Consultancy; Teva: Consultancy, Membership on an entity9s Board of Directors or advisory committees, Speakers Bureau; Gilead: Consultancy, Membership on an entity9s Board of Directors or advisory committees, Speakers Bureau; Jazz: Consultancy, Membership on an entity9s Board of Directors or advisory committees. Arellano: Cephalon: Research Funding. Topp: Amgen: Honoraria, Membership on an entity9s Board of Directors or advisory committees, Other: Travel, Research Funding; Boehringer Ingelheim: Research Funding; Regeneron Pharmaceuticals, Inc.: Honoraria, Research Funding; F. Hoffmann-La Roche Ltd: Membership on an entity9s Board of Directors or advisory committees, Research Funding. Sweet: Phizer: Consultancy; BMS: Honoraria; BMS: Honoraria; Agios: Consultancy; Jazz: Speakers Bureau; Novartis: Consultancy, Honoraria, Speakers Bureau; Astellas: Consultancy; Astellas: Consultancy; Jazz: Speakers Bureau; Celgene: Honoraria, Speakers Bureau; Agios: Consultancy; Celgene: Honoraria, Speakers Bureau; Phizer: Consultancy; Novartis: Consultancy, Honoraria, Speakers Bureau. Ravandi: Abbvie: Research Funding; Xencor: Research Funding; Astellas Pharmaceuticals: Consultancy, Honoraria; Seattle Genetics: Research Funding; Bristol-Myers Squibb: Research Funding; Jazz: Honoraria; Jazz: Honoraria; Macrogenix: Honoraria, Research Funding; Sunesis: Honoraria; Bristol-Myers Squibb: Research Funding; Seattle Genetics: Research Funding; Amgen: Honoraria, Research Funding, Speakers Bureau; Orsenix: Honoraria; Macrogenix: Honoraria, Research Funding; Amgen: Honoraria, Research Funding, Speakers Bureau; Xencor: Research Funding; Orsenix: Honoraria; Astellas Pharmaceuticals: Consultancy, Honoraria; Sunesis: Honoraria; Abbvie: Research Funding. Church: NanoString Technologies: Employment. Rutella: NanoString Technologies: Research Funding. Sun: MacroGenics: Employment. Yang: MacroGenics: Employment. Baughman: MacroGenics: Employment. Curtis: MacroGenics: Employment. Timmeny: MacroGenics: Employment. Cali: MacroGenics: Employment. Tran: MacroGenics: Employment. Muth: MacroGenics: Employment. La Motte-Mohs: MacroGenics: Employment, Equity Ownership. Poirot: Servier: Employment. Pallis: Servier: Employment. Cesano: NanoString Technologies: Employment. Bonvini: MacroGenics: Employment, Equity Ownership. Wigginton: MacroGenics: Employment. Lowenberg: Astex: Consultancy; Agios Pharmaceuticals: Membership on an entity9s Board of Directors or advisory committees; Clear Creek Bio Ltd: Consultancy, Honoraria; Celgene: Membership on an entity9s Board of Directors or advisory committees; Supervisory Board, National Comprehensive Cancer Center (IKNL), Netherlands: Membership on an entity9s Board of Directors or advisory committees; Royal Academy of Sciences and Arts, The Netherlands: Membership on an entity9s Board of Directors or advisory committees; international Scientific Advisory Board, Institute Gustave Roussy, Paris: Membership on an entity9s Board of Directors or advisory committees; Editorial Board "International Journal of Hematology": Membership on an entity9s Board of Directors or advisory committees; Elected member, Royal Academy of Sciences and Arts, The Netherlands: Membership on an entity9s Board of Directors or advisory committees; Chairman, Leukemia Cooperative Trial Group HOVON (Netherlands): Membership on an entity9s Board of Directors or advisory committees; Chairman Scientific Committee and Member Executive Committee, European School of Hematology (ESH, Paris, France): Membership on an entity9s Board of Directors or advisory committees; Editorial Board "The Netherlands Journal of Medicine": Membership on an entity9s Board of Directors or advisory committees; "Up-to-Date", section editor leukemia: Membership on an entity9s Board of Directors or advisory committees; Editorial Board "European Oncology & Haematology": Membership on an entity9s Board of Directors or advisory committees. Davidson-Moncada: MacroGenics: Employment.