Azacytidine Sensitizes AML Cells for Effective Elimination By CD123 CAR T-Cells

Chimeric antigen receptor T-cells (CAR Tc) have yielded impressive remission rates in treatment-refractory B-cell malignancies (B-ALL and B-lymphomas) by targeting CD19, resulting in the first FDA approved CAR Tc therapies, Kymriah and Yescarta. However, the translation of these results for other cancer entities remains a challenge. Pre-clinical studies using second-generation CAR Tc against the interleukin-3 receptor alpha chain (CD123) engendered strong anti-leukemic activity. CD123 CAR Tc clinical studies resulted in transient responses, or complete remission but at the expense of on-target off-tumor toxicities. Our studies employing third-generation anti-CD123 CAR Tc demonstrate strong anti-leukemic activity with no adverse effects in vivo. However, the leukemia was not completely eradicated. Combining anti-CD123 CAR Tc with DNA hypomethylating (HMA) agents may enhance the anti-leukemic effect and survival.

HMAs such as azacytidine (Aza) activate key epigenetically silenced pathways in AML cells, inhibiting cell proliferation while enhancing cell immunogenicity. We hypothesized that Aza will increase the expression of CD123 on AML cells resulting in long-term disease eradication by anti-CD123 CAR Tc. The anti-leukemic efficacy, survival advantage, safety and feasibility of the combination treatment with Aza and anti-CD123 CAR Tc were evaluated in vivo.

HL-60 (CD123^med), MLL-2 (CD123^lo), MOLM-13 (CD123^hi), primary de novo and relapsed/refractory (r/r) AML cells were cultured for 0-8 days in the presence of Aza (0µM-5µM) and analysed for their CD123 expression by flow cytometry, quantitative western blot and RNAseq. The anti-CD123 CAR was constructed with the humanized CSL362-based ScFv and the CD28-OX40-CD3ζ signaling domain, encoded in a third-generation lentiviral vector and expressed in CD3⁺ Tc from healthy donors. Rag2γc^-/- mice (n=12-16/ group) were engrafted with 1x10⁵ MOLM13/ffLuc AML cells and treated with PBS, 5x10⁶ Non-transduced (NTD) Tc orCAR Tc, 4x 2.5mg/kg Aza, or 5x10⁶ CAR Tc following 4x Aza (2.5mg/kg). Leukemic burden was assessed weekly by bioluminescence imaging. Tc activity and immunophenotyping was performed using flow cytometry at day 35 post engraftment, and survival was monitored.

HL-60, MLL-2 and MOLM-13 cells showed significant increases in HLA-DR, PD-L1, STAT1 and IRF7 expression, as well as CD123 when exposed to Aza (Fig 1A,B). Interestingly, the increased effect was seen from day one regardless of concentration. This was similarly reflected in AML patient cells. Aza treatment also arrested cell proliferation and decreased viability in both cell lines and patient cells suggesting Aza can aid in the anti-leukemic effect.

Rag2γc^-/- mice engrafted with MOLM-13 and treated with Aza and CD123 CAR Tc demonstrated suppressed growth, and eradication of MOLM-13 cells compared to mice treated with CD123 CAR Tc or Aza alone. Additionally, a significant decrease in residual CD123⁺ cells in the bone marrow (BM) of dual treated mice was seen (Fig 1C). A higher frequency of residual CD8⁺ T-cells in the BM, and CD4⁺ Tc in the peripheral blood (PB) and BM of dual treated mice was observed compared to CAR Tc only treated mice. Most prominently, we found a significantly higher mean number of stem cell-like and central memory CD8⁺ Tc in the BM of dual treated mice (232 cells/µl and 208cells/µl, respectively) compared to the CAR Tc only group (55 cells/µl and 23 cells/µl, respectively). Assessment of immune checkpoint markers on residual CAR Tc of dual treated mice revealed significantly decreased levels of CTLA-4, PD-1 and TIM-3 in the BM, and CTLA-4 in the PB compared to the CAR Tc only group. While CAR Tc treatment alone demonstrated a survival advantage compared to PBS, NTD or Aza treated mice, Aza and CAR Tc treatment had a significantly higher survival rate compared to the CAR Tc only group (92% vs. 46% at day 50, p<.01).

Our findings indicate that Aza increases immunogenicity and augments the cell surface expression of CD123 on AML cells, allowing enhanced recognition and elimination of malignant cells by CD123 CAR Tc. This is the first demonstration that HMAs and CAR Tc immunotherapy can be used synergistically to treat AML. Considering HMAs are currently under clinical investigation in AML, our data encourage further clinical evaluation of this dual treatment in r/r AML, including high-risk patients that are chemotherapy or allogeneic transplantation ineligible.

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