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CAR-T (Chimeric antigen receptor – Wikipedia) Toxicities Reported From 2006 to 2016

As more CAR-T clinical trials get done for a variety of tumors, different types of side effects are emerging. These include not only cerebral edema and other neurotoxicity but also Cytokine release syndrome – Wikipedia (CRS) which can range from mild flu-like to the more serious, life threatening Systemic inflammatory response syndrome – Wikipedia (SIRS), Anaphylaxis – Wikipedia, and other immunopathologies (See below figure from 1, and tables from 2, 3).

Poor antigen choice, & Variability in CAR-T purity & quality: Main Reasons For CAR-T Toxicities

Poor antigen choice and CAR-T purity and quality are two major, not mutually exclusive, sources of CAR-T toxicity that explain serious adverse effects in many if not most cases.

Poor antigen choice. Preferably on the cell surface, an optimal antigen would be expressed by tumor, not normal, cells, i.e., tumor-specific antigens or tumor neoantigens. After all, CAR-Ts are only tumor-specific if they target antigen(s) expressed by tumors and not by normal cells, and the CAR on CAR-Ts only binds cell-surface expressed antigens. CAR-T attack of normal tissues implies inadequate validation of chosen antigen(s) since they were clearly expressed on normal cells as well. Such instances are called on-target, off-tumor responses. However, not surprising considering vast majority of CAR targets tested thus far have been not tumor-specific but rather tumor-associated antigens (TAA), i.e., antigens over-, but not solely, expressed on tumor cells (see tables above from 2, 3).

For example, commonly seen in CAR-T trials targeting CD19 – Wikipedia positive B cell – Wikipedia tumors (4, 5, 6) as well as with Blinatumomab – Wikipedia, a mAb (Monoclonal antibody – Wikipedia) that also targets the CD19 molecule (7, 8), neurotoxicity, ranging from confusion to hallucinations to more rarely, cerebral edema, is rarely seen with other CAR-T constructs. Could be Cross-reactivity – Wikipedia with an as-yet unidentified nervous system antigen (3).

Outcome of antigenic cross-reactivity is however not unavoidable (for example, toxic outcome of anti-HER2 CAR could be altered by using a different CAR at a lower dose, see above tables from 2, 3) since it depends on a variety of factors, namely, CAR affinity for the cross-reactive antigen, how much of it is expressed on healthy tissue, and potency, i.e., effector function, of CAR-T once it binds antigen.

Variability in CAR-T purity & quality. There is as yet no consensus on a single closed-system manufacturing protocol for CAR-T generation. Rather, different groups use different multi-step protocols (see generalized schema below from 9), many of which still entail manual, open process steps and reagents such as human serum (10, 11, 12) that may contain allogeneic (genetically different) or xenogeneic (cross-species) components. Such protocols increase chances of lot-to-lot variability and even microbial contamination.

Given such profound protocol differences, not surprising that final CAR-T product, even one with the same antigenic target, can be profoundly different both phenotypically and functionally (13; See below from 14).

OTOH, fully defined culture medium would minimize lot-to-lot variability and yield a more consistent product.

However, switching to closed-system protocols comes with its own set of problems, one of the major ones being reduction in CAR transduction efficiency when T cells are cultured in cell culture bags (15). That’s to say, standardizing CAR-T generation is far from straightforward and reaching a consensus will be rocky and time-consuming. As we’ve seen with gene therapy, its 1990s heyday, sharp 2000s crash and slow mid-2010s comeback, making CAR-T ready for prime-time will depend on the efforts of those who hang in there through the t(r)ough times.

Bibliography

1. Bonifant, Challice L., et al. “Toxicity and management in CAR T-cell therapy.” Molecular Therapy-Oncolytics 3 (2016): 16011. https://pdfs.semanticscholar.org…

2. Sharpe, Michaela, and Natalie Mount. “Genetically modified T cells in cancer therapy: opportunities and challenges.” Disease models & mechanisms 8.4 (2015): 337-350. http://dmm.biologists.org/conten…

3. Bedoya, Felipe, Matthew J. Frigault, and Marcela V. Maus. “The Flipside of the Power of Engineered T Cells: Observed and Potential Toxicities of Genetically Modified T Cells as Therapy.” Molecular Therapy 25.2 (2017). http://www.cell.com/molecular-th…

4. Maude, Shannon L., et al. “Chimeric antigen receptor T cells for sustained remissions in leukemia.” New England Journal of Medicine 371.16 (2014): 1507-1517. http://www.nejm.org/doi/pdf/10.1…

5. Davila, Marco L., et al. “Efficacy and toxicity management of 19-28z CAR T cell therapy in B cell acute lymphoblastic leukemia.” Science translational medicine 6.224 (2014): 224ra25-224ra25. https://www.researchgate.net/pro…

6. Lee, Daniel W., et al. “T cells expressing CD19 chimeric antigen receptors for acute lymphoblastic leukaemia in children and young adults: a phase 1 dose-escalation trial.” The Lancet 385.9967 (2015): 517-528. https://www.researchgate.net/pro…

7. Buie, Larry W., et al. “Blinatumomab A First-in-Class Bispecific T-Cell Engager for Precursor B-Cell Acute Lymphoblastic Leukemia.” Annals of Pharmacotherapy 49.9 (2015): 1057-1067. http://citeseerx.ist.psu.edu/vie…

8. Topp, Max S., et al. “Safety and activity of blinatumomab for adult patients with relapsed or refractory B-precursor acute lymphoblastic leukaemia: a multicentre, single-arm, phase 2 study.” The Lancet Oncology 16.1 (2015): 57-66.

9. Themeli, Maria, Isabelle Rivière, and Michel Sadelain. “New cell sources for T cell engineering and adoptive immunotherapy.” Cell Stem Cell 16.4 (2015): 357-366. http://ac.els-cdn.com/S193459091…

10. Kochenderfer, James N., et al. “Eradication of B-lineage cells and regression of lymphoma in a patient treated with autologous T cells genetically engineered to recognize CD19.” Blood 116.20 (2010): 4099-4102. http://www.bloodjournal.org/cont…

11. Kochenderfer, James N., et al. “B-cell depletion and remissions of malignancy along with cytokine-associated toxicity in a clinical trial of anti-CD19 chimeric-antigen-receptor–transduced T cells.” Blood 119.12 (2012): 2709-2720. http://www.bloodjournal.org/cont…

12. Kochenderfer, James N., et al. “Chemotherapy-refractory diffuse large B-cell lymphoma and indolent B-cell malignancies can be effectively treated with autologous T cells expressing an anti-CD19 chimeric antigen receptor.” Journal of Clinical Oncology 33.6 (2014): 540-549. Chemotherapy-Refractory Diffuse Large B-Cell Lymphoma and Indolent B-Cell Malignancies Can Be Effectively Treated With Autologous T Cells Expressing an Anti-CD19 Chimeric Antigen Receptor: Journal of Clinical Oncology: Vol 33, No 6

13. Gargett, Tessa, and Michael P. Brown. “Different cytokine and stimulation conditions influence the expansion and immune phenotype of third-generation chimeric antigen receptor T cells specific for tumor antigen GD2.” Cytotherapy 17.4 (2015): 487-495. https://www.researchgate.net/pro…

14. Smith, Michelle J., Brittni M. Peterson, and Barbara A. Nelsen. “Unlocking the therapeutic and commercial potential of CAR-T technology.” http://www.nelsenbiomedical.com/…

15. Lu, Tangying Lily, et al. “A Rapid Cell Expansion Process for Production of Engineered Autologous CAR-T Cell Therapies.” Human Gene Therapy Methods 27.6 (2016): 209-218.

https://www.quora.com/What-caused-cerebral-edema-and-other-neurotoxicity-in-CAR-T-therapy/answer/Tirumalai-Kamala

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