Many aspects of drug development focus on satisfying the regulatory requirements of drug licensing authorities . These generally constitute a number of tests designed to determine the major toxicities of a novel compound prior to first use in humans. It is a legal requirement that an assessment of major organ toxicity be performed (effects on the heart and lungs, brain, kidney, liver and digestive system), as well as effects on other parts of the body that might be affected by the drug (e.g., the skin if the new drug is to be delivered  through the skin). Increasingly, these tests are made using in vitro methods (e.g., with isolated cells), but many tests can only be made by using experimental animals to demonstrate the complex interplay of metabolism and drug exposure on toxicity. Madorran E, Stožer A, Bevc S, Maver U. In vitro toxicity model: Upgrades to bridge the gap between preclinical and clinical research. Bosn J of Basic Med Sci. 2020;20(2):157-68. DOI: URL
Many aspects of drug development focus on satisfying the regulatory requirements for a new drug application . These generally constitute a number of tests designed to determine the major toxicities of a novel compound prior to first use in humans. It is a legal requirement that an assessment of major organ toxicity be performed (effects on the heart and lungs, brain, kidney, liver and digestive system), as well as effects on other parts of the body that might be affected by the drug (e.g., the skin if the new drug is to be delivered on or through the skin). Such preliminary tests are made using in vitro methods (e.g., with isolated cells), but many tests can only use experimental animals to demonstrate the complex interplay of metabolism and drug exposure on toxicity. 
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Madorran E, Stožer A, Bevc S, Maver U. In vitro toxicity model: Upgrades to bridge the gap between preclinical and clinical research. Bosn J of Basic Med Sci. 2020;20(2):157-68. DOI: URL