Flotetuzumab (FLZ), an Investigational CD123 x CD3 Bispecific Dart® Protein-Induced Clustering of CD3+ T Cells and CD123+ AML Cells in Bone Marrow Biopsies Is Associated with Response to Treatment in Primary Refractory AML Patients

Background: Acute myeloid leukemia (AML) is a molecularly and clinically heterogeneous hematological malignancy. Despite recent advancements in the treatment of AML, approximately 50% of patients with acute myeloid leukemia (AML) do not respond to induction therapy (primary induction failure, PIF) or relapse after < 6-month remission (early relapse, ER). Immunotherapy is an attractive modality to exploit in patients (pts) with AML, however, the ability to predict the groups of pts that will respond to immune targeting remains limited. Immune infiltration into tumors has been associated with therapeutic effects in preclinical models and patients with cancer. We have recently shown an association between an immune-enriched tumor microenvironment (TME) and resistance to cytarabine-based chemotherapy. Flotetuzumab (MGD006) is a bispecific DART® antibody-based molecule to CD3ε and CD123 being tested in adults with relapsed/refractory AML. Herein, we report the results of a multicenter, open-label, phase 1/2 study of flotetuzumab and the correlative data used to determine the target AML population of PIF/ER AML.

Methods: The recommended Phase 2 dose (RP2D) of flotetuzumab has been identified as 500 ng/kg/day administered as a 7 -day/week continuous infusion following multi-step step-up dosing during week 1 of cycle 1. Disease status was assessed by modified IWG criteria. Bone marrow (BM) aspirates were collected to investigate biomarkers, including CD123 receptor density (RD), and gene expression profiling using the NanoString PanCancer IO 360™ panel. FFPE BM biopsies were evaluated using multi-plex IHC staining panels. Slides were stained using a Leica BondRx autostainer and fluorescence imaged using a Polaris Vectra 3 and analyzed using inForm software. Lastly, BM biopsies were also profiled using the GeoMx DSP platform.

Results: Total of 88 AML patients were enrolled, 42 in dose-finding and 46 at the RP2D. Consistent with flotetuzumab’s mode of action, the most frequent adverse events were infusion-related reactions (IRR)/cytokine release syndrome (CRS). Stepwise dosing, pre-treatment dexamethasone, prompt use of tocilizumab and temporary dose reductions/interruptions successfully managed IRR/CRS. Flotetuzumab showed acceptable tolerability. Based on previous finding of immune-enriched TME and association with chemotherapy resistance in AML, we hypothesized that higher expression of IFNγ inducible genes might identify AML pts who derive benefit from immunotherapy with flotetuzumab. BM samples from pts with evidence of FLZ anti-leukemic activity had an immune infiltrated TME relative to non-responders. Interestingly, the IFNγ-signaling score (from IO360) was significantly higher in pts with chemotherapy-refractory AML compared with relapsed AML at time of flotetuzumab treatment, and in individuals with evidence of anti-leukemic activity compared to non-responders. Additionally, interferon-related gene modules strongly predicted flotetuzumab’s activity. In line with these findings, clinical benefit accrued exclusively to PIF/ER AML patients, who had a predominantly immune-infiltrated TME at baseline. Flotetuzumab modified the TME; on-treatment BM samples displayed increased expression of antigen presentation and immune activation genes relative to baseline. BM biopsies from six pts with PIF/ER AML were analyzed by IHC. In baseline BM samples, CD3 and CD8 cell infiltrates were higher in responder vs non-responders. Interestingly, T-cell clusters surrounding CD123+ cells were identified in on-treatment BM biopsies in responders. GeoMx DSP analysis identified increased T-cell activation markers within the T-cell clusters.

Conclusions: In conclusion, we show that FLZ elicited clinical activity in heavily pre-treated patients with poor response rates to primary therapy. We also show that increased IFNγ signaling gene expression scores in baseline BM appeared to associate with response to FLZ therapy. Flotetuzumab represents an innovative experimental approach associated with acceptable safety and encouraging evidence of activity in AML patients with PIF/ER. Trial registration number: NCT02152956.