From Academic Kids
In vivo is a delineation which differentiates whole organism research from in vitro research, which is performed on organs, tissues, cells, cellular components, proteins, or biomolecules. Clinical trials are a form of in vivo research, albeit on humans.
In vivo research is more suited to observe an overall effect than in vitro research, which is better suited to deduce mechanisms of action. In vitro research aims to describe and understand the effect of an experimental variable on a subset of an organism's components. In vitro research has the advantage over in vivo research that there are fewer variables which can confound an experiment, and that if an experimental effect is subtle the result will be more clearly visible.
In vivo research has the advantage, over in vitro research, that the experimental system is a more complex biological system. This means that in vivo research will likely give a better indication of what will happen in a population when a compound is administered to or a procedure is performed on an animal model of disease. This is why all new drugs must first undergo animal testing, followed by clinical trials, before they are released to the general population.
Christopher Lipinski's rationale for this observation is:
Whether the aim is to discover drugs or to gain knowledge of biological systems, the nature and properties of a chemical tool cannot be considered independently of the system it is to be tested in. Compounds that bind to isolated recombinant proteins are one thing; chemical tools that can perturb cell function another; and pharmacological agents that can be tolerated by a live organism and perturb its systems are yet another. If it were simple to ascertain the properties required to develop a lead discovered in vitro to one that is active in vivo, drug discovery would be as reliable as drug manufacturing. (Lipinski 2004)
The massive adoption of low-cost, in vitro, molecular biology techniques has caused a move away from in vivo research, which is considered too idiosyncratic and, above all, expensive compared to its molecular counterpart. Currently, in vitro models and experiments are a vital and highly productive research tool.
The guinea pig was previously such a commonly used in vivo experimental model that they became part of idiomatic English: 'being a guinea-pig for someone/something'. Their use in research has been substantially replaced by the smaller, cheaper and faster breeding rats and mice.
As the term is in Latin, it is written in italics.
Lipinski, C. & Hopkins, A. Navigating chemical space for biology and medicine. Nature. 2004. 432: 855-861.