To the editor:

Chimeric antigen receptor (CAR)-T cell therapy has substantially improved the outcomes of patients with hematological malignancies. However, insufficient autologous T cells, an extensive manufacturing time, severe side effects and a high price have restricted the clinical use of CAR-T cells [1]. CAR-NK cell therapy could be a universal, well tolerated and affordable treatment [2]. This report summarized the latest updates at the ASH 2022 annual meeting on the methods to improve the efficacy of CAR-NK cells.

Preclinical studies

A new CAR screening platform was established to select an appropriate CAR transmembrane domain and endo-domain from 44 CAR constructs containing NK cell activating receptors, cytokine receptors and integrins to match NK cells through coculture target cell killing assays. The selected structure was tested in several cell lines, including SUP-B15, MOLT-4, and Raji, with different binding antigens and showed improved (more than 20%) antitumor efficacy in all killing assays compared with the reported NKG2D-2B4-CD3ζ CAR structure (Abstract 1983) [3]. FT555 is a GRPC5D CAR-NK cell derived from iPSCs expressing hnCD16 (> 90%) and IL15RF (> 90%) with CD38 knocked out. Combination of FT555 and anti-CD38 mAb (daratumumab) showed a prolonged persistence compared with CAR-NK cells alone in a multiple myeloma (MM) mouse model (Abstract 1992) [4]. Another study also exhibited the improved anti-tumor efficacy of BCMA/GRPC5D dual CAR-NK cells in MM model (Abstract 3283) [5]. Downregulating immune checkpoint receptor natural killer group 2A (NKG2A) with CRISPR/Cas9 to disrupt the immunosuppression mediated by the tumor microenvironment was shown to significantly improve the antileukemia efficacy of CD33 CAR NK cells in killing assays and AML mouse model (Abstract 1991) [6]. The combination of a Trispecific killer engager (TriKE) capable of binding to the CD16 Fc receptor with IL-15 stimulation and CD33-binding domains and α3 MICA/B CAR-NK cells-controlled leukemia, while progression was observed in α3 MICA/B CAR-NK cells alone under stress (effector: target ratio of 0.25:1) in vitro (Abstract 4623) [7].

In a preclinical study, CD123 CAR-NK cells (5-day OS: 100%) also showed less acute toxicity than CD123 CAR-T cells (5-day OS: 0%) in a mouse model engrafted with human hematopoietic cells, while the antileukemia efficacy was comparable in acute myeloid leukemia (AML) mouse models (Abstract 3279) [8]. Another CAR NK cell therapy for AML showed that CD33/FLT3 CAR-NK cells have exhibited promising antileukemia efficacy (> 90% killing of leukemia cells) in animal models, and the addition of endomucin-inhibiting CARs protected ~ 42% of primary healthy human HSCs and HPCs from cytotoxicity in in vitro assays (Abstract 1978) [9].

Hematopoietic stem cell-derived lymphoid progenitors with Bcl11b inhibition directly differentiate into NK cells rather than T cells, which contributes to the production of stable CAR-NK cells (Abstract 1220) [10] (Table 1).

Table 1 Preclinical studies of CAR-NK cells presented at ASH 2022
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Clinical trials

A Chinese team presented the initial results of a phase I clinical trial of human umbilical cord-derived CD33 CAR-NK cells for patients with relapsed or refractory AML (Table 2). In 10 evaluated patients, only one patient developed grade 2 cytokine release syndrome (CRS), and no higher-grade CRS occurred. There were no instances of Immune effector cell-associated neurotoxicity syndrome (ICANS) of any grade. All patients with grade 3–4 bone marrow suppression recovered within one month. Regarding antileukemia efficacy, 60% (6/10) of the patients achieved a complete response (CR) 28 days after CAR-NK cell infusion (Abstract 3317) [11]. Another phase I trial of induced pluripotent stem cell (iPSC)-derived B-cell maturation antigen (BCMA) CAR-NK cells is being conducted for MM (Table 2). Neither CRS nor ICANS was observed in 9 patients who received CAR-NK cell infusion (3 received daratumumab as a combination therapeutic agent). One patient treated with 300 million BCMA CAR-NK cells as a monotherapy achieved a very good partial response (VGPR). Two patients achieved a response after 100 million BCMA CAR-NK cells infusion and daratumumab (Abstract 2004) [12].

Table 2 Outcomes of clinical trials of CAR-NK cells presented at ASH 2022
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Expansion condition, transduction efficiency, and anti-tumor efficacy are the most significant obstacles for CAR-NK cell therapy. In the future, genetically modified methods, preconditioning regimen, cell dose, and combined immunotherapies or hematopoietic stem cell transplant need to be optimized to improve CAR-NK cell therapy, which requires more study to promote the clinical translation of CAR-NK cells.