Seatbelts in CAR therapy: How Safe Are CARS?

Abstract

T-cells genetically redirected with a chimeric antigen receptor (CAR) to recognize tumor antigens and kill tumor cells have been infused in several phase 1 clinical trials with success. Due to safety concerns related to on-target/off-tumor effects or cytokine release syndrome, however, strategies to prevent or abate serious adverse events are required. Pharmacologic therapies; suicide genes; or novel strategies to limit the cytotoxic effect only to malignant cells are under active investigations. In this review, we summarize results and toxicities of investigations employing CAR redirected T-cells, with a focus on published strategies to grant safety of this promising cellular application.

Keywords: adoptive immunotherapy; cell therapy; chimeric antigen receptor; safety switch; suicide gene.

Figure 1

CAR constructs and dual targeting pre-clinical approaches to reduce toxicity. (A) CAR extracellular domain includes leader sequence, single chain variable fragment (scFv) (H (heavy) and L (light) chain), connected by a linker, (e.g., SG..GS). A spacer, (e.g., hinge region of human immunoglobulin D molecule) grants flexibility and joins to the transmembrane domain (TM). TM includes for example the constant region of the human G immunoglobulin, whereas the intracellular domain includes CD3 zeta endo-domain (1st generation CARs), plus minus a CD28 domain (2nd generation), or plus minus an additional co-stimulatory domain (3rd generation CARs) [28]. (B) “Dual targeting” approaches to reduce toxicities: (i) Trans-signaling CAR divides killing domain and co-stimulatory domain in separate molecules targeting two distinct tumor antigens, limiting CAR activation capacity when only a single antigen is engaged [29,30,31]. (ii) Inhibitory CAR replaces the zeta chain with an inhibitory domain, overcoming the activatory signal from a coexpressed activatory CAR [6]. (iii) Tandem CAR is composed by two different antigen binding fragments allowing targeting of two different antigens by a single construct, with synergistic effect when both are engaged simultaneously [32]. CAR-1 and CAR-2 used as example, recognized antigen 1 and antigen 2, respectively.