What are in vivo and in vitro?
While the terms in vivo and in vitro sound very similar, their meanings are not. In vivo is Latin for “within the living.” It is a study model used for a process or procedure that is conducted “on” a living being rather than in cell samples. The in vivo method simulates biological conditions found in a living subject. In vivo studies may be conducted in animals and humans.
In vitro is Latin for “in the glass.” It is a study model in which a procedure is conducted in labs. Interventions may be done on a specific cell culture to grow a virus, bacterium, or fungus, or to test a new drug. Because we look at specific cells during an “in vitro” procedure and not the whole organism, these experiments are more suited for research and analysis.
Given that an in vivo model involves the internal environment of a living being, the results of in vivo studies are considered more reliable or more relevant than those of in vitro studies. This does not mean that an in vitro model is in any way less valuable.
For cases in which normal conception or in vivo fertilization fails, the doctor may advise the couple to try in vitro fertilization (IVF). In vitro fertilization happens when the ovum (female egg cell) and sperm (male reproductive cell) are united outside the body or in a “glass” petri dish.
In the world of research, “in vivo” and “in vitro” models are used in clinical trials, scientific studies, and up-and-coming medical procedures. Scientists use both these models to understand the way a drug (or intervention) affects the body (pharmacodynamic profile) and the way the body affects/breaks down the drug (pharmacokinetic profile). Both these studies serve to create a safer and effective drug profile before they are approved for use by the general population.
Let us look at an example. Suppose scientists want to study a new anti-epilepsy drug. This study goes through various stages. Initially, in vitro studies are conducted in specialized acute seizure models that only need slices of the brain tissue. These models create electroencephalography (EEG) signals (brain signals) that are identical to those seen in a patient having a seizure.
During initial testing of the drug, scientists prefer acute seizure models (in vitro) over time-consuming human trials that often involve high levels of individual variation and ethical concerns. Only those drug molecules that show promising results in the in vitro stage will be tested in human clinical trials (in vivo). Here, scientists will see if trends observed in preclinical data (in vitro model) still hold in human volunteers consistently and if the results are reproducible. Thus, drug development needs both in vivo and in vitro models before it is U.S. Food and Drug Administration (FDA)-approved.
Both in vitro and in vivo studies have their own set of advantages and disadvantages, which are explained in the table below.
|Category||In vivo method||In vitro model|
|Cost and preparation||
In vitro models do not put live subjects at risk. However, they cannot capture the inherent complexity of organ systems and the internal environment of the human body. For example, in vitro cell culture may not account for interactions between various body procedures and cellular biochemistry. Therefore, in vitro studies are often followed by animal studies (in vivo).
Drugs may be tested on animals such as rabbits, hamsters, mice, rats, guinea pigs, dogs, and primates (including monkeys, gibbons, and chimpanzees). Scientists can better evaluate the safety, toxicity, and efficacy of a drug candidate in a complex model (animal). The majority of animals in laboratories are purpose-bred (bred specifically to be used in experiments).
Besides ethical concerns over the use of animals for experiments, the problem of translatability persists because there are considerable physiological differences between humans and animals, and drug absorption, distribution, and excretion may differ.
Only those drugs that appear safe and effective in the in vitro stage eventually go on to the clinical trial stage or in vivo stage.
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