 |
By Jacqueline Carrico, BS, MD Candidate
Given the rapid advances in CAR-T therapy, there have been major efforts to improve the specificity and safety of these therapies. Very few targets exist that are only expressed in the malignant cell population, resulting in on-target/off-tumor toxicities. Most have been minor and controllable; however, several trials have been terminated due to severe and life-threatening toxicities. There is increasing concern about this problem with dually targeted therapies, such as dual-CAR or tandem CAR.
One approach to increasing on-tumor specificity is the development of CAR-T cells that express synthetic Notch receptors. These receptors permit conditional expression of a second chimeric antigen-receptor, only upon binding of the antigen specific to the primary CAR. This may help mitigate toxicities resulting from having two highly targeted CAR molecules active at once. The major concern with this approach is the potential for immunogenicity, as well as the potential for cytokine release syndrome. A major challenge for any dually targeted CAR therapy is the difficulty in identifying two unique tumor-associated antigens which independently confer anti-tumor activity.
Inhibitory-CAR is an approach in which the CAR-T cells express an inhibitory form of the receptor which is specific to the antigen present in healthy tissues. This differential binding occurs via single chain variable fragment (scFv) modification, allowing differential affinity for the antigen in unique tissues. Theoretically, this should reduce the cytotoxic effect of the CAR-T cells in healthy tissues. Another scFv modification that increases on-tumor specificity is the expression of protease-activated antibodies in the chimeric antigen receptor. These pro-antibodies remain inactive until they are cleaved by proteases within the tumor microenvironment.
eBook: Foundations of Immuno-Oncology Advancing Basic Science & Translational Research
Another approach to mitigate CAR-T toxicity is to limit CAR expression and activity. Unlike traditional retroviral and lentiviral systems, novel gene transfer techniques have allowed for temporary expression of the CAR molecules. One approach is electroporation of the CAR mRNA, which permits only temporary expression. However, efficacy of this approach necessitates multiple infusions, which increases IgE levels and the risk of anaphylaxis.
A very interesting development is switchable CAR-T molecules. These molecules have a bridge of dimerizing small molecules in between the intracellular and extracellular domains of the normal CAR structure. Activation of the CAR-T cells occurs only upon dimerization of the small molecules, which can be administered locally into the tumor microenvironment and titrated to induce a therapeutic effect. Lastly, the expression of suicide genes within the CAR-T cells may also limit the persistence and toxicity of the therapy. One example is inducible Caspase-9 (iCasp9), which causes rapid depletion of the infused CAR-T cells after administration of an iCasp9 agonist. This therapy could potentially curtail life-threatening toxicities, though this is yet to be clinically proven. Additionally, auto-dimerization of the molecules in the absence of agonist could result in limited efficacy of the therapy.
Each of these techniques is designed to improve the specificity of CAR-T therapy, which in turn may improve the tumor cytotoxicity while decreasing the off-tumor toxicities. As these techniques become more sophisticated, it will be important to rigorously study the potential side effects in vitro and in vivo before utilizing them in humans. Many of these are in early stages of development and will be interesting to follow going forward. Check back for updates!
Jacqueline Carrico, MD Candidate
University of Colorado, School of Medicine
Jackie is completing her medical training in Anesthesiology and has a diverse background in basic science, translational, and clinical research.
References
Hartmann J, Schüßler-lenz M, Bondanza A, Buchholz CJ. Clinical development of CAR T cells-challenges and opportunities in translating innovative treatment concepts. EMBO Mol Med. 2017 Sep;9(9):1183. PMID: 28765140.
Wang Z, Wu Z, Liu Y, Han W. New development in CAR-T cell therapy. J Hematol Oncol. 2017;10(1):53. PMID: 28222796.
