Checkpoint Blockade in Combination with CD33 Chimeric Antigen Receptor T Cell Therapy and Hypomethylating Agent Against Acute Myeloid Leukemia

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-IL2Rg^null (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.