CAR-T () 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(CRS) which can range from mild flu-like to the more serious, life threatening (SIRS), , and other immunopathologies (See below figure from , and tables from , ).
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, ).
For example, commonly seen in CAR-T trials targetingpositive tumors ( , , ) as well as with , a mAb ( ) that also targets the CD19 molecule ( , 8), neurotoxicity, ranging from confusion to hallucinations to more rarely, cerebral edema, is rarely seen with other CAR-T constructs. Could be with an as-yet unidentified nervous system antigen ( ).
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, ) 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), many of which still entail manual, open process steps and reagents such as human serum ( , , ) 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 (; See below from ).
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.
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