These studies investigate what happens to the drug after delivery. How long does the drug stay in the tissues? How often do you have to give it for the concentration in that tissue to stay above the level that is thought to be effective? How much drug, and how often do you need to give it to get a level in the tissue that works, and what dose causes side effects?
Essentially, these studies determine how much drug should be given and how often. These studies are typically done in animal models, and there are calculations that predict how to convert the amount given to an animal to the amount that should be given to humans.
Different drugs that act in similar ways (such as the exon-skipping drugs developed by Sarepta Therapeutics and by GSK/Prosensa) may have different results, and these can really affect how much drug is given, and if there is the potential for toxicity. These studies are usually done in more than one type of animal to get the best possible estimations, but the results often still do not directly correlate to human doses.
Clinical trials still often include multiple-dose groups (starting with low doses for safety), but the animal studies suggest an approximate range of doses that are hoped to be effective.
At the same time, the developer studies what is happening to the drug inside the animal — where does it go, and how much active drug is there in the places that matter? Do toxic levels accumulate anywhere? At the end of this stage, the developer knows approximately what doses of the drug to test in humans.
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