Chimeric Antigen Receptor T Cells, In Vitro and In Vivo Preclinical Models, plus regulatory and safety considerations
In a recent webinar, we presented on and discussed the adoptive cell therapy preclinical program central to an IND/IMPD submission. The following is a brief summary, plus insightful Q&As from that event.
Cell therapy is one of the fastest-growing sectors within oncology therapeutic development. With the growth of adoptive cell therapies clinically, a wide array of preclinical/nonclinical strategies are available to transition these novel therapeutic candidates through the development continuum. One of the most promising approaches involves “designer” T cells that are engineered to attack a chosen disease target. Cells are harvested from the patient’s own white blood cells, including T cells, and a viral vector delivers a gene encoding a chimeric antigen receptor (CAR) into the T cells. The engineered T cells are then expanded in a bioreactor, while the patient receives chemotherapy to lower his or her white blood cell numbers to accommodate the incoming CAR-T cells. These CAR-T cells are infused into the patient’s blood, where they proliferate, then detect and destroy cancer cells.
A robust set of in vitro and in vivo preclinical pharmacology assays and models are required for these adoptive cell therapies. Preclinical processes include in vitro functional verification such as transduction efficiency, vector copy number, T cell phenotyping, in vitro cytotoxicity assessment and cytokine secretion. In vivo models for adoptive cell therapies include industry-standard human or mouse cell line derived (CDX) models, human patient derived (PDX) models and humanized mouse models. These in vivo models are complemented by the capability to track and monitor disease burden and response to therapy by longitudinal small animal imaging and ex vivo analysis of T cell persistence by flow cytometry.
While the industry has been dominated by CAR and T cell receptor (TCR) T cell therapies, emerging modalities include CAR natural killer (NK) cells, which may be used in combination with Bispecific Killer cell Engagers (BiKEs) and Trispecific Killer cell Engagers (TriKEs)1.
Safety assessment strategies for adoptive cell therapies share similarities to the assessment of conventional therapeutics, but the differences require careful consideration in the overall safety plan. For example, the clustering of common CAR-T cell side effects may be considered adverse by traditional toxicology standards, but many of those standards have toxicologists questioning how to apply the conventional rules to this relatively new modality. These adverse effects depend on routes of exposure and dose level, with tests using surrogate animal models to assess the safety and efficacy of the personalized cellular medicine.
Special regulatory designations, including the FDA ”Fast Track” and “Breakthrough” programs, may help to expedite advanced therapies (including CAR-T cells) for serious conditions. Fast Track therapies involve the collection of safety and efficacy data demonstrating the potential to address an unmet medical need. The Breakthrough Designation is for therapies where preliminary clinical evidence indicates that the drug demonstrates a substantial improvement over available marketed therapies.
Q: Can we run in vivo hematological NSG models in a GLP format, and where? Which facility conducts tumor model studies?
A: Yes, we run tumor-bearing GLP studies within our laboratories in Greenfield, IN, and we conduct non-tumor-bearing studies in Madison, WI. The Ann Arbor, MI facility conducts non-GLP tumor-bearing studies.
Q: For imaging the cell therapy’s migration to the solid tumor, can you tag cells with luciferase or a fluorescent probe?
A: In theory, yes. There are some published studies where luciferase-expressing T cells have been used with optical imaging. The process can be challenging and requires deeper discussion and consideration; levels of instrument sensitivity must be overcome to accurately assess the process with respect to imaging true cell based therapies.
Q: What are the factors in selecting tumor cell lines for a targeted antigen in an in vivo model?
A: Ensure sufficient expression of the target antigen on the cell line of interest prior to in vivo use. For an external antigen on the cell surface, we can use flow cytometry, Western blot or other methodologies for evaluation.
Q: Does the team understand the FDA’s expectations for GLP safety studies to enable INDs for ACTs?
A: Yes. We have been involved in the CAR-T cell therapeutics being marketed today, as well as IND approvals for other adoptive cell therapies, with an extensive track record.
Q: Do GLP safety studies include tumorigenicity or leukemic transformation?
A: Yes, we can perform these studies – both in vivo and in vitro. However, one should first consider whether applying these tests is scientifically useful for this type of therapeutic, since most adoptive cell therapies tend not to persist for sufficient time and in sufficient amounts to elicit a tumorigenic condition or leukemic transformation event.
Q: Does the company have GLP biodistribution available?
A: Yes. We can assess biodistribution by qPCR or distribution (and persistence) by flow cytometry, depending on the target or biomarker.
Q: How should the starting dose for a clinical study be determined, based on NOAEL in toxicology studies?
A: The clinical dose is established from a combination of the safety and the efficacy data. One needs to consider how to scale the dose from an animal model to humans, and take into account tumor volume, targets and malignancy.
Q: Which is faster – CD34 engraftment or PBMCs – and why?
A: The human CD34 (stem cell) engraftment process takes longer, with stable engraftment requiring 12-16 weeks. This is because the stem cell must home in on the bone marrow, co-establish with endothelial cells in the perivascular niche, begin symmetric division, and produce differentiated progeny (pre-cells followed by the definitive cells). Human PBMCs involve injecting purified human peripheral blood cells directly into a mouse. The lifespan of the human PBMC model is shorter, but given that differentiated progeny are already present in PBMCs, the route to a mature immune cell is expedited. Thus, you have a model generally established in the appropriate manner from the start.
Authors: Maryland Franklin, PhD, Executive Director, Scientific Development, Preclinical Oncology and Brian McIntosh, PhD, Study Director and Cell and Gene Therapy Lead
- Szun Szun Tay, et al. (2016) TriKEs and BiKEs join CARs on the cancer immunotherapy highway. HUMAN VACCINES & IMMUNOTHERAPEUTICS. 12(11): 2790–2796.
This webinar is part of a scientific series we are offering focused on trends, opportunities and challenges associated with current and future applications in oncology. Learn more and register for upcoming webinars on topics such as adoptive cell therapies, biomarker technologies for the detection of cancer driver mutations, novel targeted therapies and more. You may also learn more from our Cell & Gene Therapy Education center here.