Before a drug candidate can be tested in humans, its safety and efficacy must be explored in preclinical studies. These studies can either be ‘in vitro’, Latin for “within the glass” and referring to studies using cell cultures studied outside of the body, or ‘in vivo’, Latin for “within the living”, referring to studies that take place in the body.
In preclinical studies, only animals are used for in vivo tests as this stage comes prior to human testing to ensure its safety.
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For in vitro studies, cell lines are derived from either humans or non-human animals and are introduced to the new pharmaceutical under development within a Petri dish or test tube. In vitro studies have many benefits. The first, and most obvious benefit is that they do not cause harm to the animal or person that the cell cultures have been derived from, they are free of the drawbacks of animal testing. Other benefits of in vitro models include their relative cheapness in set up and running; they are also reliable and efficient, and produce robust results.
However, while in vitro studies offer numerous significant benefits, they have one major drawback. In vitro studies are limited as they cannot model how a pharmaceutical compound may interact with all the molecules and cell types that exist within a complex organ. They can only be tested in isolation, whereas the human body is a dynamic environment where numerous pathways and cells are in constant communication. This can make it difficult for in vitro studies to predict the complexities of potential interactions.
Despite this limitation, in vitro studies remain a key part of preclinical research. Here, we will look at the example of in vitro testing in preclinical studies for new cancer therapeutics to show how in vitro studies are moving forward and how technology is developing to help overcome the limitations of in vitro studies.
Historically, preclinical in vitro cancer models have faced the drawback of being ineffective at reliably predicting patient outcomes. While in vitro testing in cell-based models has always been a fundamental part of cancer research, these preclinical studies have not always been successful at producing data that reflect the exact behavior of malignancies in response to candidate drugs.
To address this issue, scientists are working on developing the technology used in these trials. In particular, technology is now emerging that is based on more complex co-culture technology using live cells in vitro.
Recent decades have seen a shift from 2-dimensional cell culture toward 3-dimensional cell growth, an emerging technique that is more effective at capturing the physiologic environment and better reflects the growth patterns of tumor tissue. This new technology is able to provide three-dimensional architecture and preserved heterogeneity of tumor cells observed in vitro. In comparison to 2-D cell cultures, it is more accurate at demonstrating the complex microenvironments and surrounding stromal components.
In vivo vs. in vitro drug development
In vivo studies, in comparison to in vitro, take place within a living organism. In preclinical trials, this happens within animal subjects. In clinical trials, in vivo studies can use either humans or animals as subjects,
In vivo studies are able to address the major limitation of in vitro studies, they are able to demonstrate the impact of a pharmaceutical on the body as a whole, rather than how it impacted isolated cells. This allows in vivo studies to better visualize potential interactions, which can improve its predictions of safety, toxicity, and efficacy. This helps scientists predict the impact of candidate drugs on human disease.
However, while in vivo studies address the drawback of in vitro studies, they have their own major setback. In vivo studies face significant ethical concerns, particularly for preclinical studies where just animal models are permitted. The debate over the ethics of animal testing has raged for decades.
Currently, the regulations and laws governing animal testing are tightening, and preclinical in vivo studies scientists wishing to conduct preclinical studies with animals are required to demonstrate that no other alternative methodology can be used to conduct the experiment. They are also required to demonstrate a sense of balance, that is, the benefits of the study (gain in knowledge) outweigh the drawbacks (suffering caused to the animals).
Like in vitro studies, in vivo studies are also undergoing a technological transformation. Emerging technologies such as CRISPR will make complex animal models increasingly simple to conduct, cheaper, and faster.
While in vivo studies face significant ethical considerations, it is likely that they will remain a fundamental part of preclinical studies. The future is predicted to bring significant advances in preclinical technologies, both in vitro and in vivo, that should facilitate the gathering of more accurate data, as well as faster and simpler methodologies. It is expected that these advances will improve the quality of preclinical data, as well as reduce the reliance on traditional animal models.
- Gargiulo, G., 2018. Next-Generation in vivo Modeling of Human Cancers. Frontiers in Oncology, 8. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6192385/
- Højelse, F., 2000. Preclinical Safety Assessment: In vitro - in vivo Testing. Pharmacology & Toxicology, 86, pp.6-7. https://pubmed.ncbi.nlm.nih.gov/10905745/
- Lorian, V., 1988. Differences between in vitro and in vivo studies. Antimicrobial Agents and Chemotherapy, 32(10), pp.1600-1601. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC175930/