Recent Advances in the Development of Anti-FLT3 CAR T-Cell Therapies for Treatment of AML

Abstract

Following the success of the anti-CD19 chimeric antigen receptor (CAR) T-cell therapies against B-cell malignancies, the CAR T-cell approach is being developed towards other malignancies like acute myeloid leukemia (AML). Treatment options for relapsed AML patients are limited, and the upregulation of the FMS-like tyrosine kinase 3 (FLT3) in malignant T-cells is currently not only being investigated as a prognostic factor, but also as a target for new treatment options. In this review, we provide an overview and discuss different approaches of current anti-FLT3 CAR T-cells under development. In general, these therapies are effective both in vitro and in vivo, however the safety profile still needs to be further investigated. The first clinical trials have been initiated, and the community now awaits clinical evaluation of the approach of targeting FLT3 with CAR T-cells.

Keywords: AML; CAR; CAR T; FLT3; FLT3L; FMS-like tyrosine kinase-3; chimeric antigen receptor; leukemia.

Figure 1

An overview of different anti-FLT3 CAR T-cells developed to date. (A) A second-generation CAR with CD28 as co-stimulatory domain [30,31,32]. (B) A second-generation CAR with 4-1BB as co-stimulatory domain [30,33]. (C) A second-generation CAR with 4-1BB as co-stimulatory domain and the FLT3 ligand (FLT3L) instead of an anti-FLT3 scFv [34]. (D) Two CARs with scFvs targeting FLT3 and NKG2DS, respectively. The CARs were encoded on a bicistronic lentiviral vector using the same promoter and separated by a self-cleaving 2A peptide to ensure equal expression on the cell surface [35]. (E) A third generation anti-FLT3 CAR with two co-stimulatory domains, 4-1BB and inducible T-cell co-stimulator (ICOS) [36].

Figure 2

The structure of the FLT3 receptor and the FLT3 ligand (FL). FLT3 consists of an extracellular segment organized in five Ig-like domains (D1-D5), a transmembrane domain with a single helix, a juxtamembrane domain (JM), and two tyrosine kinase domains (TKDs) [55,56]. The FLT3 receptor binds the FLT3 ligand (FL) through interactions with D3 [55]. The FLT3 receptor exists as a monomer until binding of FL, and dimerization of two receptors promotes phosphorylation of the tyrosine kinase, which in turn activates downstream signaling involved in cell proliferation and activation. Phosphorylation is proposed to be controlled by the autoinhibitory effect of the JM [56]. The internal tandem duplications (ITDs) are in-frame duplication or insertion of 3–1236 nucleotides typically situated in the JM [57]. The TKD mutations are mostly caused by point mutations or small deletions and typically situated in the second tyrosine kinase domain (TKD2) resulting in a single amino acid change or deletion [54]. Among the preclinical anti-FLT3 CAR T-cell therapies reviewed here, Chen et al. [31] and Jetani et al. [30] derived their scFv from the anti-FLT3 antibody clone 4G8, reported by Rappold et al. and Hofmann et al. and it binds Domain 4 on FLT3 [58,59]. Li et al. derived their anti-FLT3 scFV from a monoclonal antibody clone EB10, which binds Domain 4 [60]. Maiorova et al. [36] and Wang et al. [34] used the FLT3 ligand instead of a scFv, which binds to Domain 3. Sommer et al. [33] developed nine anti-FLT3 CARs targeting domain 1–5, and the superior constructs, P3A1 and P3E10, bind to Domains 2 and 4 respectively. Karbowski et al. [32] did not detail the binding location of their scFv.