CAR-T immunotherapies: Biotechnological strategies to improve safety, efficacy and clinical outcome through CAR engineering

Abstract

T cells engineered to express a chimeric antigen receptor (CAR) have re-shaped the way hematological malignancies are treated. Despite the overwhelming early clinical success, CAR-T therapies are associated with severe side-effects, disease relapse and often exhibit limited efficacy. In this Review article we summarize the most recent biotechnological advances that have been developed to enhance the efficacy and specificity of CAR-T therapies, as well as to address the key challenges associated with them. We place particular emphasis on the most recent clinical data that indicate which CAR-T populations are the most relevant to clinical success, and indicate how the molecular structure of the CAR receptor can affect clinical outcome. Finally, we outline what we believe is the next generation of immunotherapies.

Introduction

Two autologous chimeric antigen receptor (CAR)-T cell therapies (Kymriah and Yescarta) were recently licensed by American and European agencies (FDA and EMA respectively). CAR-T cell therapies are a type of cancer immunotherapy in which a patient’s T cells are genetically engineered in the laboratory so they can attack cancer cells. Both licensed products target CD19, an antigen expressed on B cells and leukemic cells. Kymriah (tisagenlecleucel) brought to market by Novartis and Yescarta (axicabtagene ciloleucel) brought to market by Kite Pharma/ Gilead, are indicated for the treatment of pediatric and relapse/refractory (R/R) B cell acute lymphoblastic leukemia (B-ALL) and certain lymphoma subtypes and adult refractory diffuse large B cell lymphoma (R/DLBCL) respectively. Remarkably, the overall response rate in the short term was 83% based on a single dose of Kymriah with patients entering into remission within 3 months of being treated (Grupp, 2018; Maude et al., 2018; Maude et al., 2014) (ELIANA, NCT02435849) (Table 2), extending to 76% and 70% at 12 months and 18 months respectively (Grupp, 2018)_. Similarly, the recipients of Yescarta had an overall response rate of 71% within 3 months (Locke et al., 2018; Locke et al., 2015; Long et al., 2015; Neelapu et al., 2017) (ZUMA, NCT02348216) and 39% of patients remained in remission for 27 months (Locke et al., 2018; Neelapu, 2018).

Although CAR-T therapies have a remarkable life-saving potential they are nevertheless associated with severe toxicities (most notably cytokine release syndrome and neurotoxicities), high cost (> £200,000) and limited efficacy on cancers other than blood malignancies. Furthermore, despite the high short-term response rates a proportion of patients relapse after CAR-T treatment. As a result, further understanding of T and CAR-T cell biology is required in order to improve these immunotherapies.

The immune system is comprised of the innate and adaptive systems. While the innate system is the first line of defense and is poised to act rapidly, the adaptive system displays immunological memory and contains cells such as T and B cells that are highly specific for any pathogenic threat. T cells are defined by the expression of the T cell receptor (TCR) which interacts with the Major Histocompatibility Complex (MHC) (also termed human leukocyte-associated [HLA] antigens), found on the surface of antigen presenting cells (APCs) such as dendritic cells (DC). This interaction is necessary for the recognition of the pathogenic threat by the T cell and its subsequent activation (Fig. 1). The immunogenic response elicited by the T cell is dependent on the type of MHC that is presented by the APC (class I MHC or class II MHC), which subsequently defines whether the T cell will differentiate into one of the two major T cell subsets, CD8+ (T cytotoxic) or CD4+ (T helper). While most T cells disappear after the threat has been eliminated, others will survive and form memory cells that can survive for years. Memory T cells are classified according to the expression of certain surface markers (Table 1) (Gattinoni et al., 2011; Gattinoni et al., 2009; Gebhardt et al., 2009; Hofmann and Pircher, 2011; Lugli et al., 2013; Masopust et al., 2010; Sallusto et al., 1999).

This review outlines the challenges associated with CAR-T immunotherapies and focuses on the most recent biotechnological and genomic engineering advancements that have been developed in order to address such challenges. Furthermore the review summarizes the recent CAR-T clinical data that provide an indication as to which subsets of CAR-T cells have the most potential to expand within the patient and therefore provide sustainable remissions. Finally the review discusses alternative CAR immunotherapies by utilizing other cells of the immune system. The review is therefore intended for individuals with a biochemical engineering, biotechnology or similar background who wish to further understand CAR-T immunotherapies from a biological or biotechnological perspective.