Checkpoint Blockade in Combination with CD33 Chimeric Antigen Receptor T Cell Therapy and Hypomethylating Agent Against Acute Myeloid Leukemia
Authors: Xue Tongyuan , Del Real Marissa , Marcucci Emanuela , Toribio Candida , Setayesh Sonia Maryam , Forman Stephen J , Horne David A , Budde Lihua E 
Published: November 2019

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. The cure rate for primary AML patients is only 35% and decreases with age. Novel and effective immunotherapies for patients with relapsed and/or refractory (r/r) AML remain an urgent unmet need. CD33 is an attractive immunotherapeutic target for myeloid malignancies given its expression on more than 85% of AML patient samples. We therefore set out to design and test CD33 chimeric antigen receptor (CD33CAR) T cells preclinically as a single agent and in combinational therapy.
To assess antileukemic responses of CD33CAR T cells in vitro and in vivo, we enriched CD4/CD8 T cells from peripheral blood mononuclear cells (PBMCs) and genetically modified them to express a second-generation CD33CAR. CD33CAR T cells exhibited potent antigen dependent CD107a degranulation, IFN-γ production and killing activities against AML cells in vitro. Using a NOD-SCID-IL2Rgnull (NSG) xenograft model engrafted with MOLM-14-ffluc, a CD33 expressing AML cell line transduced with lentivirus carrying firefly luciferase (ffluc) and enhanced green fluorescent protein (eGFP), 3 million CD33CAR or mock T cells were introduced intravenously. CD33 CAR T cell-treated group displayed 98.2% leukemic regression 4 days post CAR T infusion, and 99.6% reduction on day 31. Bioluminescent imaging (BLI) and Kaplan-Meier analysis demonstrated that CD33CAR T cells significantly decreased leukemic burden and prolonged overall survival compared to mock T cells in vivo.
Decitabine, a DNA hypomethylating agent (HMA), is a main therapeutic agent for treating AML. We observed HMA treatment led to increased CD33 expression on MOLM-14 cells in vitro. We hypothesized that decitabine can potentiate CD33CAR T cell-mediated AML killing by increasing CD33 expression. MOLM-14 cells were treated with either decitabine alone, CD33CAR T cells alone, or sequential treatment using various concentrations of decitabine or DMSO followed by CD33CAR or mock T cells in an E:T ratio of 1:100. We determined the target specific killing activities in each group using flow cytometric based analysis 48 and 96 hours later. The decitabine followed by CD33CAR T cells treatment reproducibly resulted in the most robust antileukemic activity with 80.6% MOLM-14 cells killed. In comparison, CD33CAR T cells or decitabine monotherapy resulted in 11.5% and 50.9% killing, respectively. In vivo testing of the combinational effects of decitabine and CD33CAR T cells are underway and will be updated at the meeting.
Finally, checkpoint blockade targeting programmed death-1 (PD-1)/programmed death-ligand 1 (PD-L1) has shown survival benefits, particularly in combination with HMA, for patients with r/r AML (Daver et al. 2019). We observed elevated PD-L1 expression on residual AML blasts that survived the treatment with decitabine in combination with CD33CAR T cells. Therefore, we hypothesized that blockade of PD-1/PD-L1 interaction might further improve the antileukemic effect of CD33CAR T cells against AML cells post antigen induction by decitabine. MOLM-14 cells were treated with decitabine for 2 days and CD33CAR T cells were added in an E:T ratio of 1:75. Anti-PD-1 or IgG4 antibody was added to the culture at various concentrations. The most robust CD33 specific killing was seen in the culture with anti-PD-1 antibody added. Further characterization are underway and will be presented.
Taken together, our preclinical findings have demonstrated the potency of the CD33CAR T cell therapy and ways to optimize its efficacy. Our results support clinical translation of CD33CAR T cells for patients with AML.

Disclosures
Budde: F. Hoffmann-La Roche Ltd: Consultancy.