Flotetuzumab, an Investigational CD123 x CD3 Bispecific Dart® Protein, in Salvage Therapy for Primary Refractory and Early Relapsed Acute Myeloid Leukemia (AML) Patients (2019)

Approximately 40% of patients (pts) with newly diagnosed (AML) either fail to achieve complete remission with intensive induction therapy or experience disease recurrence after a short remission duration (&amp;lt; 6 months). These pts, herein considered to have primary refractory disease, are an extremely challenging population to treat, with only 14% achieving remission following conventional chemotherapy and with subsequent salvage attempts being nearly universally ineffective (1). Increased immune infiltration of the tumor microenvironment (TME) and high CD123 expression on AML blasts have been associated with primary induction failure and poor prognosis (2, 3). Flotetuzumab (FLZ), a CD123 x CD3 bispecific DART molecule, is currently being tested in a phase 1/2 study in pts with either relapsed or refractory (R/R) AML. We have previously reported FLZ activity in primary refractory AML (4); herein, we provide additional scientific rationale supporting the investigation of FLZ in this patient population. The recommended Phase 2 dose (RP2D) of FLZ identified in an ongoing Phase 1/2 study is 500 ng/kg/day administered as a 7 -day/week continuous infusion. Pts receive a lead-in dose during week (W) 1, followed by 500 ng/kg/day during W2-4 of Cycle 1, and a 4-day on/3-day off schedule for Cycle 2 and beyond. Disease status was assessed by modified IWG criteria; bone marrow (BM) samples were collected to investigate biomarkers, including CD123 receptor density (RD), and gene expression profiling using the NanoString PanCancer IO 360™ panel, which measures the expression of 770 genes, including 14 immune cell types and 32 immuno-oncology biological signatures. Gene expression comparisons are presented at fold change (FC) and a t-test was used for statistical analysis. Fifty pts with R/R AML received FLZ at the RP2D. Thirty (60%) pts had primary refractory AML: 24 failed ≥2 induction attempts and 6 recurred after remission of &amp;lt;6 months (median duration of remission 32 [range: 29-45] days). This population was heavily pretreated (median 4 previous lines of therapy [range 2-9]), with 40% (12/30) having secondary AML and most having non-favorable cytogenetic risk (60% adverse and 23% intermediate by ELN 2017 risk category). Compared to relapsed pts, those with primary refractory disease had greater CD123 expression on AML blasts (11277±3246 vs 6254±1779 sites/cell), with baseline BM samples showing higher inflammatory chemokine signature scores (1.7x increase, p=0.018), and an enhanced interferon gamma (IFNγ) signaling gene expression score (1.85x increase, p=0.014), a signature associated with resistance to cytotoxic chemotherapy (2). Among 28 primary refractory pts evaluable for disease assessment, FLZ complete remission (CR) rate was 32.1% (3 CR, 3 CRh, 3 CRi). Four pts (1 CR, 1CRh, and 2 CRi) subsequently underwent allogeneic hematopoietic stem cell transplantation. For comparison, the expected CR rate to conventional salvage therapy in this population was calculated as &amp;lt;10%, estimated based on historical response rates of pts subjected to similar numbers and types of salvage therapies (1, 5). Pts with CR showed higher CD123 RD compared to pts with no response (15186.5 vs 10836.8 sites/cell). FLZ anti-leukemic activity (&amp;gt;30% decrease in BM blasts) was associated with increased baseline immune gene signatures, including significantly higher IFNγ scores (1.8 FC, p=0.0212; AUROC=0.74), and an increased tumor inflammation signature (TIS) score (1.658 FC, p=0.00827, AUROC=0.847 compared to non-responders. FLZ was well tolerated, with no increased cytokine release syndrome events in primary refractory pts (30% G1, 67% G2, 3% G3) compared to relapse pts (26% G1, 58% G2, 16% G3), notwithstanding the increased CD123 RD in the former. In conclusion, we show that FLZ elicits clinical response in heavily treated patients with poor response rates to primary therapy. We also show that increased IFNγ signaling gene expression scores in baseline BM appears to associate with response to FLZ therapy. Enrollment has been expanded to further define FLZ's activity specifically in pts with primary refractory AML, and candidate biomarkers to enable identification of pts more likely to respond to FLZ. Estey E, et al. Blood (1996), 88 (2) 756Vadakekolathu J, et al. Blood (2017) 130:3942Vergez F, et al. Haematologica (2011), 96(12):1792-8Uy GL, et al. Blood (2018) 132:764Duong V, et al. Leukemia (2013),13(6):711-5 Disclosures Uy: Astellas: Consultancy; Pfizer: Consultancy; Curis: Consultancy; GlycoMimetics: Consultancy. Aldoss:Agios: Consultancy, Honoraria; AUTO1: Consultancy; Jazz Pharmaceuticals: Honoraria, Other: travel/accommodation/expenses, Speakers Bureau; Helocyte: Consultancy, Honoraria, Other: travel/accommodation/expenses. Foster:Bellicum Pharmaceuticals, Inc: Research Funding; Daiichi Sankyo: Consultancy; MacroGenics: Research Funding; Celgene: Research Funding. Sallman:Celgene: Research Funding, Speakers Bureau; Celyad: Membership on an entity's Board of Directors or advisory committees; Incyte: Speakers Bureau; Jazz: Research Funding; Novartis: Speakers Bureau; Abbvie: Speakers Bureau. Sweet:Abbvie: Membership on an entity's Board of Directors or advisory committees; Agios: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Celgene: Speakers Bureau; Stemline: Consultancy; Pfizer: Consultancy; Jazz: Speakers Bureau; Incyte: Research Funding. Rizzieri:AbbVie, Agios, AROG, Bayer, Celgene, Gilead, Jazz, Novartis, Pfizer, Sanofi, Seattle Genetics, Stemline, Teva: Other: Advisory Board; AROG, Bayer, Celgene, Celltron, Mustang, Pfizer, Seattle Genetics, Stemline: Consultancy; Celgene, Gilead, Seattle Genetics, Stemline: Other: Speaker; Stemline: Research Funding. Advani:Amgen: Research Funding; Macrogenics: Research Funding; Abbvie: Research Funding; Pfizer: Honoraria, Research Funding; Glycomimetics: Consultancy, Research Funding; Kite Pharmaceuticals: Consultancy. Emadi:Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; NewLink Genetics: Research Funding; Genentech: Consultancy, Honoraria; KinaRx: Membership on an entity's Board of Directors or advisory committees, Other: Co-Founder and Scientific Advisor, Patents &amp; Royalties; Jazz Pharmaceuticals: Research Funding. Wieduwilt:Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen, Leadiant, Merck, Servier: Research Funding; Reata Pharmaceuticals: Equity Ownership. Vey:Novartis: Consultancy, Honoraria; Janssen: Honoraria. Church:NanoString Technologies, Inc.: Employment, Equity Ownership. Rettig:WashU: Patents &amp; Royalties: Patent Application 16/401,950. Arellano:Gilead: Consultancy. Löwenberg:Up-to-Date", section editor leukemia: Membership on an entity's Board of Directors or advisory committees; Frame Pharmaceuticals: Equity Ownership; Elected member, Royal Academy of Sciences and Arts, The Netherlands: Membership on an entity's Board of Directors or advisory committees; Editorial Board "European Oncology &amp; Haematology": Membership on an entity's Board of Directors or advisory committees; Clear Creek Bio Ltd: Consultancy, Honoraria; Abbvie: Membership on an entity's Board of Directors or advisory committees; Agios Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Astex: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; CELYAD: Membership on an entity's Board of Directors or advisory committees; Chairman Scientific Committee and Member Executive Committee, European School of Hematology (ESH, Paris, France): Membership on an entity's Board of Directors or advisory committees; Chairman, Leukemia Cooperative Trial Group HOVON (Netherlands: Membership on an entity's Board of Directors or advisory committees; Supervisory Board, National Comprehensive Cancer Center (IKNL), Netherland: Membership on an entity's Board of Directors or advisory committees; Hoffman-La Roche Ltd: Membership on an entity's Board of Directors or advisory committees; Royal Academy of Sciences and Arts, The Netherlands: Membership on an entity's Board of Directors or advisory committees. Ravandi:Selvita: Research Funding; Xencor: Consultancy, Research Funding; Menarini Ricerche: Research Funding; Macrogenix: Consultancy, Research Funding; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Cyclacel LTD: Research Funding. Muth:MacroGenics, Inc.: Employment, Equity Ownership. Tran:MacroGenics: Employment. Timmeny:MacroGenics, Inc.: Employment, Other: Stock Ownership. Topp:Boehringer Ingelheim: Membership on an entity's Board of Directors or advisory committees, Research Funding; KITE: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Roche: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Regeneron Pharmaceuticals, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committ