Introduction
Because cell therapies are mostly human in origin, they are challenging to study in animals (due to tissue rejection). But, these studies are required before starting human trials. Cells are also unlike chemically identical molecules in a standard drug product. That is, they are always a mixture and are always changing in nature, so product characterization is a moving target. While designing a successful preclinical toxicology program can be challenging, you can still apply classical drug toxicology experience, and as long as you take all of the steps needed to ensure cells act in animals the way they would in humans, the program has a good chance of predicting human risks. This article shows you how to start programs off on the right foot.

Cell Product Characterization
Cell source and immunologic character can translate into different levels of safety concern. Risks are assigned to how foreign cells are and how many steps were involved in manipulating them in culture. Any significant manipulation means that the sponsor will have to file an IND with the FDA and provide detailed plans for their chemistry, manufacturing stages, pharmacology, toxicology and clinical trials. The FDA is strict about these products because they are hard to characterize. Identity and potency attributes are linked to toxicology. Even slight production changes can affect their character and risks. For these and related reasons, biomarkers, assay methods, process controls, and cell product specifications must be carefully identified before initiating preclinical studies.

Proof of Concept
Cell function is often studied in animal models of disease. Reversing or slowing disease progression is a key point in establishing the cells’ potential activity in disease, route, dosage and regimen. At this early stage, many key attributes can be learned–and these affect the duration and nature of toxicology work. These features include: cell engraftment, local migration, acute and chronic effects, and need for re-dosing. These features also help guide and justify the clinical regimen. From these studies, you can learn the pharmacologically sensitive species that permit the cell to act or engraft–this model may also be the best one to use as a toxicology model. While performing toxicology in an animal model of disease is uncommon in normal “drug circles,” the disease process may be needed to support the viability and action of the cells. But there are some good rules to apply to avoid downsides. The animal should be robust, the animal disease should be well-understood, the disease should be expressed similarly in most animals, and the disease course should be predictable. That way, toxicity can be defined as changes from usual. Immunocompromised rodents are often used for both pharmacology and toxicology studies. Large animal use is reserved for ensuring safety of a full human dose (for embryonic-origin cells, for instance) or ensuring a device or surgical implant method, used with the cells, works well and safely in the long term. These models are made complicated because large, normal animals will reject human cells easily, so these models must be given high doses of immunosuppressant drugs. The bottom line is that there are just no easy choices for animal models of cell therapies.

Distribution
Understanding of the cells’ fate in the host body (including survival, proliferation, distribution, differentiation and integration) is a key safety concern. Tracking them is like finding a needle in a haystack, so highly advanced and sensitive methods are needed to find markers that are unique to the implanted cells. Usually, Polymerase Chain Reaction (PCR) or immunohistochemistry are used to track cell therapies in the host body. Early biodistribution studies can help guide the selection of the animal model and justify the duration of toxicology studies.

Toxicology
Toxicology/safety concerns for a cell product are identical to ordinary drugs. Like for all biologics programs, pharmacodynamics, typical toxicology measurements, and immunogenicity are all concerns. Some cells don’t live long in the body, just after that is demonstrated, long chronic studies are not needed. But if cells endure past (about) a month, then chronic (12-18 month) studies in rodents may be needed before human trials can start. Cell therapy programs do not need (routine) testing in two species, genotoxicity batteries, safety pharmacology batteries and carcinogenicity assessments. Special end points may be added that focus on the therapy’s properties and potential risks.

Tumorigenicity
The phenotypic stability (e.g. tumorigenic potential or ectopic tissue formation) of a cellular product is a serious concern for most cell therapies. Tumorigenicity studies include a positive control cell line where the cell-based product is injected subcutaneously, or instead, by the clinical route of administration. Usually, such studies are done in one species (e.g. immunocompromised rodent). The study is like other carcinogenicity studies from there on out, but any masses or ectoptic tissues which appear must be assessed to learn if the mass might have come from the host or the implanted cells.

Conclusion
A successful preclinical program for a cell therapy requires careful customization around the fact that the animals and the cell product are both alive and interact. There are few, if any, appropriate guidances for cell therapy toxicology, so sponsors are advised to work closely with their funding partners, contract research organization and consultants, and also reviewers at CBER to design a custom approach to every product and trial. Despite all of the complexities, many cell therapies are tested successfully in animals and have entered human trials. They may be the next major therapeutic advance that will cure, rather than just treat, some serious human diseases.