Preclinical Evaluation of Chimeric Antigen Receptor-Modified Natural Killer Cells Targeting Membrane-Proximal CD33 in Acute Myelogenous Leukemia

Augmenting the cytotoxic potential of natural killer (NK) cells through the expression of a chimeric antigen receptor (CAR) promises to overcome many of the limitations of CAR-modified T cells, notably T cell-associated toxicities and difficulties with off-the-shelf allogeneic use. However, to date, the clinical efficacy of CAR-modified NK cells has been limited to CD19-expressing B cell malignancies. Toward translating this success to myeloid malignancies, we report the generation and optimization of membrane-proximal CD33-targeted CAR-modified NK cells produced by gammaretroviral transduction of peripheral blood-derived primary NK cells. The CD33-targeted CD28/CD3ζ-based CAR is derived from a high-affinity binder obtained through membrane-proximal fragment immunization, and it has demonstrated superior preclinical functionality compared to the current best-in-class CAR T cell binders that target membrane-distal CD33 epitopes with high affinity or membrane-proximal epitopes with low affinity.CD33-targeted CAR-modified NK cells displayed stable CAR expression (>40%), excellent proliferation (>1000-fold), and augmented short- and long-term cytotoxicity, serial killing following repeated antigen-positive target challenges, and NK cell activation (CD107a degranulation and interferon-γ response) against CD33-positive AML cell lines compared to non-transduced or mock-CAR-modified NK cells in vitro. Their short- and long-term cytotoxicity, serial killing, and proliferation were improved in vitro by the addition of cytokine transgenes (IL-15 and/or IL-33) to the CAR construct. While excessive NK cell proliferation by CAR-IL-15 was associated with significant systemic toxicity in vivo, this was attenuated by the addition of IL-33 (CAR-IL-33-IL-15). In fact, mice treated with CAR-IL-33-IL-15 had the best tumor control and overall survival.In conclusion, we have developed a novel immunotherapy approach using engineered NK cells, which are easy to produce and exhibit promising efficacy. Given these data, anti-CD33 CAR-IL-33-IL-15-modified NK cells are a candidate for further development toward clinical translation for the treatment of acute myelogenous leukemia (AML).