New Paradigms for Testing CAR T Drugs
How drug discovery and safety assessment are working in tandem to bring cellular therapies to market faster
In October, the US Food and Drug Administration approved the second in a new class of treatments that genetically reboot a patient’s own immune cells to kill cancer, in this instance a particular type of blood cancer.
Unlike other gene therapy strategies that replace defective genes with healthy ones, this approach collects T cells from patients, re-engineers them to recognize a particular target antigen expressed on the surface of the cancer cell and then re-infuses the modified T cells into the patient equipped with the ability to recognize and kill cancer cells harboring that same antigen.
Chimeric antigen receptor (CAR) T cells, the latest chapter in the blockbuster story of cancer immunotherapy, have the potential to transform how we attack cancer, but clearing all of the regulatory hurdles needed to get these CAR T drugs to market can be exceptionally difficult. Unlike small molecule drugs and biologics, where the safety testing procedures are well established and a large historical database of pharmacological safety information exists to guide drug makers and regulators, safety testing for CAR T products is not standardized and the level of testing required varies from product to product depending upon the specificity of the chimeric antigen receptor and other features unique to the CAR T being developed.
CAR T’s are not the only cellular therapies coming into play. Hematopoietic stem cell transplants, cancer vaccines, and other cell-based products also use intact, living cells to treat disease. Because all of these products have the capacity to fill tremendous unmet medical needs, they are moving into the clinic much faster than a new drug for diabetes or arthritis. This expedited timetable means less time to conduct the rigorous safety tests that are needed to move a drug into humans.
Yet it also presents a dilemma for the makers of these products. How can we perform the due diligence in a compressed timeline on therapies as complex as cell-based products? To meet these challenges, drug developers have been departing from the usual step-wise fashion of discovery first, safety assessment later, and begun to incorporate safety and efficacy endpoints when candidate therapies are still in the early stages of preclinical testing.
While safety assessment studies have not traditionally been performed during the discovery process, it is possible to work collaboratively with teams of pathologists and toxicologists to include critical safety endpoints—such as blood pressure, body temperature and proper dosage—into a preclinical efficacy study. The reasons for this approach are three-fold. Earlier safety assessment reduces development costs, accelerates approval timelines and mitigates risk when the product is finally tested in humans. For cell- and gene-based therapies, such as the CAR-T cells that several companies are pursuing, it’s wise to incorporate safety endpoints into the drug discovery process to identify or rule out any potentially fatal side effects or unanticipated toxicity that could negatively impact success in clinical testing.
But adopting this multi-tasking strategy is not as simple as it may sound. Not every laboratory has the diversity of expertise needed to make this happen. CAR T is a great case in point as the development of a successful preclinical program requires expert knowledge of gene editing and transduction, in vitro and in vivo immuno-oncology, and importantly, an understanding of the safety and regulatory process required to obtain approval for cellular therapies. Here are some important factors to consider when designing studies which integrate analysis of functional activity and safety assessment:
- Dueling expressions. Has the CAR T cell been engineered to express additional molecules that could affect the safety profile or modulate the activity of the CAR T cell?
- Safety switches, suicide genes, dual recognition systems. Are the CAR T's engineered in such a way that they can be turned off, killed or recognize more than one target on the tumor cell? These features are important since the ability to control CAR T activity or induce suicide, enhances the safety profile of the product.
- Dodging endogenous protein. Has the CAR T cell been engineered to eliminate expression of the endogenous T-cell receptor (TcR) and/or MHC? This is important since CAR T’s lacking expression of a TcR would be less likely to induce graft vs. host disease. CARs that lack MHC would not be subject to rejection. These modifications would be expected to enhance CAR safety and have been proposed as a way to develop “off-the-shelf” CAR’s that could be used in any patient, irrespective of HLA status.
- Know your CAR T's composition and phenotype. Is the product comprised of CD8, CD4 or a mixture of both cell types? What is the optimal ratio? And do we know whether the CAR T’s have been cultured in vitro under defined conditions to induce differentiation to a specific phenotype, such as central memory or effector memory function? This is critical basic information about the therapy which impacts efficacy and safety.
- How does your CAR T make the kill? What in vitro functional characterization has been performed with the CAR T’s prior to testing in animal studies? Do we know anything about the cytotoxic function and cytokine profile of the activated CAR T product?
If you decide to incorporate safety and efficacy endpoints into your in vivo Car T study you will need to establish an optimal safety dose and proper controls. When to implant CAR T cells is going to vary depending on the growth characteristics of the tumor model, and monitoring changes in blood pressure and body temperature will need to be considered as potential indicators of neurotoxicity. Other design elements include:
- Persistence and expansion. How long do the CAR T cells persist in the animal and can they be tracked easily in blood? Many CAR T cells are engineered to express a tag or marker that allows for their detection by flow cytometry. Multiple longitudinal time points should be identified to allow for sampling and detection of CAR T cells.
- Cytokines and biomarkers. Cytokine storm is one of the most serious clinical side effects associated with CAR T therapy, and even led to the deaths of several patients enrolled in clinical trials. Regulating the release of cytokines is key, and although mouse models may not reflect the full spectrum of cytokine-mediated events seen in humans, it is important to analyze the cytokine levels in serum in any preclinical CAR T study.
Lastly, choosing the right model to engraft the CAR T cells is critical. The most useful models are immune-compromised mice, humanized mice and implanting murine CAR T’s in an immune competent host.
New tools and technologies present an unprecedented opportunity to change the course of incurable diseases. Researchers need to strive to get these products into the clinic as fast as possible, yet without compromising safety. Performing some of the safety assessment tests early in the development process is a cost-effective and rationale way to achieve this objective.