Cell-based treatments such as Chimeric Antigen Receptor T-cell (CAR-T) therapy are a form of immunotherapy that uses specially altered T-cells to aid the immune system to fight cancer cells. CAR-T therapy holds great potential in the treatment of both solid and liquid tumours. Treatment is carried out by genetically altering human-derived immune cells in a laboratory setting, before reinfusing them into the patient. The genetic alterations allow them to specifically target and kill cancer cells. Though effective, a range of side effects present a major hurdle to the wide-spread use of this therapy. In pre-clinical studies Giordano-Attianese, G. et al offer a potential solution to this problem using a new form of CAR-T cell that can be deactivated to minimalize side effects using a clinician administered drug . To fully understand this new technology we must first look at CAR-T therapy in more detail.
CAR-T therapy involves the harvesting of either patient (autologous) or donor (allogeneic) T-cells, which are then genetically altered to express a Chimeric antigen receptor (CAR). These are made up of an extracellular (ligand) binding domain linked to an intracellular signalling domain (fig 1). The extracellular domain is designed to have a high binding affinity for an antigen protein expressed on the membrane of cancerous cells; the intracellular domain, is programmed to aid T-cell proliferation, cause release of cytokines (toxic to tumour cells) or kill the targeted cell upon the binding of the extracellular domain to the tumour cell. Once altered and multiplied in vitro, the engineered CAR-T cells are then injected into the patient, commonly as a single dose, to treat the cancer.
Effectiveness of CAR-T therapies
To-date two CAR-T therapies have proven to be effective treatments for certain types of B- cell Lymphomas (liquid tumours): Kymriah (Tisagenlecleucel, Novartis, Aug 2017) and Yescarta (axicabtagene ciloleucel, Gilead, Oct 2017), both approved in third line or greater (3L+) treatment settings. The objective response rates (ORR) of the trials gaining these products FDA approval were 83%  and72% respectively, far surpassing the previous comparable chemotherapeutic treatments, which had ORRs ranging from 38%-53.3%. These response rates though quite remarkable carry a significant burden, with regard to the severity and frequency of adverse events (AEs) ranging from mild to life threatening. This means that regardless of efficacy, CAR-T remains a last resort.
Adverse Responses to CAR-T use in B-cell type Lymphomas have been categorised into six major groups, these have been visualised in the graphic below (fig 2) including:
- Off-target toxicity - the CAR portion of the engineered T-cell may interact with innate immune cells causing antigen independent activation
- On-target off-tumour toxicity – the CAR-targeted antigen is found in healthy somatic cells which are attacked causing adverse events; identification of target antigens that are uniquely expressed by tumour cells remains the most substantial barrier to CAR-T use in solid tumours
- On-target on-tumour toxicity – Cytokine production exceeds tolerable limits causing damage to healthy and cancerous tissue (cytokine release syndrome (CRS)) 
- Neurotoxicity – Brain & CNS cells damaged by CAR-T therapy 
- Genotoxicity – the viral vector used to engineer the T-cell to express CARs may pose a threat to causing oncogenic insertional mutagenesis (in-vitro, not in-vivo) 
- Immunogenicity – immune response against infused CAR-T cells
Of these adverse responses two have been particularly problematic, the first being on-target on-tumour toxicity. This is a by-product of efficacious CAR-T therapy as tumour cell lysis syndrome and CRS (cytokine release syndrome) are both results of the desired mechanism of action T-cells use to attack tumour cells. CRS is often a delayed response to CAR-T therapy, which can occur at any point after treatment whilst CAR-T cells are actively attacking tumour cells. Symptoms range from mild to life-threatening, dependent on the extent of cytokine overproduction, and the length of time a patient is exposed to elevated cytokine levels . The second major adverse response is on-target off-tumour toxicity, which has a positive correlation with the efficacy of the therapy. For example Kymriah used to treat DBCLC patients targets the CD19 antigen found on the surface of cancer cells, but also on the surface of healthy B-cells (immune cells), therefore the more efficacious the targeting mechanism against the tumour cell, the greater the impact of the therapy on healthy B-cells. This can lead to B-cell depletion and hypogammaglobulinaemia (lowered immunoglobulin (antibodies) levels in the blood) which impairs immune function, increasing risk of serious infection .
Currently multiple avenues of development exist in a bid to mediate adverse responses to CAR-T therapy. These include suicide switches activated by small molecule drugs (non-dynamic control), split signalling CAR-T cells that require more than one binding ligand (increases target specificity) and mediation using adaptor proteins (for CAR to link to tumour antigen an additional protein is required) . To date these methods are limited by cost and the short half-life of small molecules used as mediators. Safety of use is a major concern for CAR-T therapy indications in a wider patient base higher up the treatment pathways, as well as across a multitude of other oncologic disease areas.
Giordano-Attianese, G. et al have proposed a novel dynamic solution to mediating CAR-T activation using computationally designed “STOP-CAR-T cells”, which have a chemically disruptable heterodimer (CDH) incorporated into the CAR signalling cascade . This CDH, when exposed to a specific small molecule will experience a conformational change, temporarily inactivating the STOP-CAR-T cell. This will allow for the modulation of CAR-T therapy in response to the adverse events as well as its reactivation in the absence of the small molecule, once symptoms have subsided. The inserted CDH was found to be effective both in-vitro and in-vivo as a CAR-T control, with cells regaining full anti-tumour activity in the absence of the inhibitory small molecule. Furthermore, T-cell expansion was found to be as efficient with the CDH component in the CAR structure, and efficacy largely equivalent to that of second-generation CAT-T therapies .
Giordano-Attianese, G. et al’s pre-clinical study has demonstrated proof of concept, for the use of computationally designed CAR-T therapies, which can be dynamically modulated, to alleviate adverse events. This has the potential to widen indications for future CAR-T therapy allowing them to address a greater proportion of patients across more lines of therapy in current and future indications, as well as create a safer and more time efficient clinical development process for first-in-human testing of CAR-T therapies, in which safety profiles are not known. Great efficacy and a greatly reduced side effect profile mean the only true barrier to the broad adoption of CAR-T, would be the high price point relative to other more affordable treatment options.
Writen by Jai Bains, Oncology Market Forecast Analyst
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