Culturing cells is the most widely used in-vitro method in pharmacology and toxicology, with cells being used either as permanent cell lines or as freshly obtained "primary cultures". Co-cultures of two or more cell types express organ-specific functions even better, for example, human keratinocytes and fibroblasts in bioengineered human skin models. Today, human skin models are commercially available and have been used successfully to investigate the pharmacology and toxicology ofnew drugs and cosmetics. This example illustrates that human tissues and tissue models are the most promising tools in drug development. However, it must be borne in mind that each cell and tissue culture model has its inherent limitations, which are usually identified in validation studies.
In primary cultures the organ-specific properties are well conserved, and primary liver cells both from humans and test animals are widely used in drug metabolism studies. Permanent cell lines are easy to handle and can be obtained from cell banks in high quality.
Molecular genetics allow the development of permanent cell lines expressing molecules of interest, thus opening a new perspective for cell culture methods in pharmacology and toxicology. Transgenic cells can express genes coding for receptor molecules on their surface or genes coding for drug-metabolizing enzymes within the cytosol. The genes introduced may even be of human origin.
The advantages of in-vitro systems include the following:
• controlled testing conditions;
• reduction of systemic effects;
• reduction of variability between experiments;
• the same dose range can be tested in a variety of test systems (cells and tissues);
• time-dependent studies can be performed and samples taken;
• testing can be conducted rapidly and at low cost;
• very small amounts of test material are required
• a limited amount of toxic waste is produced;
• human cells and tissues can be used;
• transgenic cells carrying human genes can be used; and
• there is a reduction of testing in animals.
The limitations of in-vitro systems include the following:
• systemic effects and side effects cannot be evaluated;
• interactions between tissues and organs cannot be tested;
• metabolism is limited;
• pharmacokinetic effects cannot be evaluated;
• specific organ sensitivity cannot be assessed; and
• chronic effects cannot be tested.
Today, in vitro systems serve as the basis for studying the effects of drugs and other chemicals (Fig. 10.1), whereby they are used initially for screening purposes
Human clinical studies
Fig. 10.1 The role of in-vitro studies in pharmacology and toxicology. Today, both in drug development and in toxicology, experimental investigations are starting with in-vitro studies. Studies in animals are conducted as the next step. If the results are still promising, clinical studies on a limited number of patients must be conducted before a new drug can be released onto the market. Epidemiological studies on a representative population will provide the final proof of the efficacy and effects of a new drug.
and in mechanistic studies. Although animal studies take considerably longer to conduct and are more expensive, until today only studies in animals have allowed the investigation of the systemic effects of exposure to chemicals. It must also be taken into account that studies in animals are used as surrogates for studies in humans. Thus, only clinical studies in humans can provide the final proof that a new drug is effective in man. It must also be considered that clinical studies in patients are even more expensive to conduct than studies in animals, and are more time-consuming.
At present, the assessment of adverse effects of chemicals can only be monitored by using in-vitro systems to a limited extent. Although some in-vitro systems are available that allow prediction of the local effects of chemicals when applied to the skin (e.g., irritation and phototoxicity), even the most sophisticated tests or test batteries cannot yet be used to predict systemic effects.
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