The following blog pertains to the 2017 SOT Annual Meeting Scientific Session "Organs-on-a-Chip, Tissue Bioprinting, and 3D Cultures: Next Generation Models for Toxicology in the 21st Century."
Over the past few years, a number of new and exciting technologies have been developed for toxicity testing without necessitating the use of experimental animals. In my opinion, the general sentiment among most toxicologists at this year’s SOT Annual Meeting is that we need to do a better job in predicting chemical-induced toxicity in humans.
Historically, toxicity testing has relied upon the use of experimental animals, an approach that has a number of benefits. First and foremost, in vivo toxicity data are required for most regulatory submissions. Another obvious advantage is that an agent can be tested in the context of a whole organism. This allows us to understand how a complex biological system will react to exposure to our compound of interest. Further, by taking advantage of a standardized battery of tests that have been used for decades, a researcher can assess pharmacokinetics, acute toxicity, carcinogenic potential, and developmental or reproductive toxicity using an established and accepted paradigm.
However, there are a number of drawbacks to conducting experimental animal tests. It is very evident that our animal models are not doing a great job at predicting drug efficacy or chemical-induced toxicity. Animal testing also consumes a great deal of time and resources and is not of sufficient throughput to meet the needs of many scientists. Further, the increasing regulatory and ethical burdens of testing chemicals in animals are pressuring us to find alternative models. While the short-comings of experimental animal models are a major concern of toxicologists in all employment sectors, I think that our colleagues in the pharmaceutical industry are especially interested in alternative models, as drug attrition continues to be a major issue that is preventing novel therapeutics from reaching patients.
To address this challenge, scientists like Kristin Fabre, PhD, AstraZeneca, and her colleagues are working on developing and testing new technologies to improve the chances that the drugs in their pipelines will complete the trek from bench to bedside. In her presentation during the 2017 SOT Scientific Session “Organs-on-a-Chip, Tissue Bioprinting, and 3D Cultures: Next Generation Models for Toxicology in the 21st Century,” Dr. Fabre highlighted the use of “in vitro models incorporating multi-cellular tissues, scaffolding, and mechanical factors that can recapitulate a dynamic physiological environment” and how this approach can be used for multiple stages of drug development, including target selection, lead identification, and teasing apart molecular mechanisms of toxicity. These in vitro models, which contain human cells in an environment that is similar to the tissue state in vivo, are an attractive alternative to static 2D cell culture models.
One interesting point that Dr. Fabre made was that they also are developing these microphysiological systems with experimental animal cells. To me, this is a useful approach, as it will allow one to directly compare this in vitro system to in vivo toxicity in a non-clinical setting. Another really great point that Dr. Fabre made was that no institution can develop, test, and validate these models alone; it requires collaboration between multiple sectors, including industry, regulatory bodies, and the engineers who are actually building the microphysiological systems. Dr. Fabre is planning for success by forming partnerships, including with the IQ Consortium, and collaborations with government agencies and engineers. Dr. Fabre and colleagues also are setting realistic expectations to appreciate the time, cost, and complexity or simplicity required to best suit their needs. As Peter Loskill, PhD, Fraunhofer Institute for Interfacial Engineering and Biotechnology, would later point out during his talk, the use of these microphysiological systems are best-suited to complement experimental animal testing, rather than fully replace it.
While developing a tissue chip is a step in the right direction, perhaps a more physiologically-relevant approach would be to have multiple organs connected in series as they are in our bodies. Dr. Loskill and colleagues are developing these types of “plug-and-play” devices that can mimic organ-organ interactions. For example, if there is a chemical that is metabolized in the liver to a toxic metabolite, that metabolite is allowed to travel through the “circulation” to extrahepatic organs, as it would in vivo.
Another exciting technology that is different from a tissue chip is a 3D bioprinted organ. Rhiannon N. Hardwick, PhD, Organovo, and colleagues are developing a technology where different cell types can be printed onto a matrix and allowed to differentiate into an organ. Dr. Hardwick’s talk focused on the utility of this approach for investigating drug-induced liver injury (DILI). These bioprinted livers display key characteristics, such as albumin production, xenobiotic metabolizing enzyme activity, and bile canaliculi formation. Dr. Hardwick presented compelling evidence that drug-induced steatosis, hepatocellular necrosis, and fibrosis can be assessed biochemically and histopathologically using Organovo’s technology.
In addition to drugs, these types of microphysiological systems also can be useful for studying the effects of other agents, such as tobacco products. Kambez Hajipouran Benam, DPhil, Hansjorg Wyss Institute for Biologically-Inspired Engineering, Harvard University, is utilizing a microengineered “lung-on-a-chip” seeded with airway cells that can be exposed to air pollutants at the air-liquid interface. Dr. Benam and colleagues can then expose the cells to combustible or electronic cigarette smoke by coupling a cigarette smoke generator to the microphysiological device. Dr. Benam showed that this system was able to recapitulate the human in vivo response to cigarette smoke in primary cells isolated from healthy volunteers or individuals with chronic obstructive pulmonary disease (COPD).
The last speaker of the session was Thomas Hartung, MD, PhD, Center for Alternatives to Animal Testing, Johns Hopkins Bloomberg School of Public Health. Dr. Hartung began his talk by shining light on the issues of culture artifacts, including genetic instability, mycoplasma infection, and misidentification of cell lines, which can lead to “irreprodu-CELL-bility” in traditional in vitro systems. To combat this, Dr. Hartung and colleagues are building 3D “mini-brains” which are stable in size, have a stable composition of cell types (oligodendrocytes, astrocytes, and neurons), contain myelinated axons, and are electrophysiologically-active. These mini-brains are useful for studying cell-type specific neurotoxicity, such as 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced dopaminergic cell death. Further, these mini-brains can also be used to studying viral infections, such as HIV.
In summary, the session’s speakers highlighted the state-of-the-science of these technologies and their potential use for studying chemical-induced toxicity. There is certainly a great deal of promise that the efforts being exhibited by these individuals and their colleagues will eventually produce in vitro models which can be applied to toxicity testing. This is not to say that there will not be substantial obstacles that need to be faced head-on. A few that come to mind are:
- Being able to produce enough supply to meet the potential demand;
- Controlling the cost per assay;
- Establishing protocols that are sufficient for the user to follow;
- Being able to quantify the delivered dose of test material to the cell type of interest; and
- Use a commercially-available cell type, such as induced pluripotent stem cells or other metabolically-competent cells, so that users can obtain consistent results across studies, facilities, and time.
I am personally very excited to see how these technologies will continue to progress over time. It also is important that engineers, biologists, toxicologists, users, and federal regulatory agencies continue to have an open dialogue about these emerging in vitro models so that we can reduce, refine, and potentially replace the use of experimental animals in toxicity testing. As a community, we should be supportive of these types of novel technologies, as it is very clear that our traditional animal and cell culture models are not independently predictive of the adverse effects of chemicals in humans.
Disclaimer: This blog post serves as a summary of the aforementioned session. The views and opinions expressed in this article are of mine alone and do not necessarily reflect the views and opinions of any particular institution, agency, or company. This is in no way an endorsement of any particular product or service.