Gene therapy aims to deliver a correct form of a missing or mutant gene with the hope of alleviating disease symptoms. This should lead to the development of improved drugs with fewer side effects. Understanding the pharmacology of these new forms of treatment is a vital part of the drug development process. Every potential drug must undergo extensive preclinical safety and toxicity testing that includes evaluations of toxicity and lethal doses, reproductive effects, and the potential to cause cancer or DNA mutations. Preclinical testing is expensive and time consuming taking from 2-5 years to complete.
Animal testing is conducted to provide the necessary preclinical data.
Even then, the extrapolation of animal data to humans is not completely reliable due to genetic and metabolic differences. Also, rare adverse effects are unlikely to be detected because of the nature of the statistics. If a drug has not been proven to be safe and effective for a specific use, then it cannot be marketed for that use. After preclinical testing has proven a compound worthy of study in humans, clinical testing can begin. Phase 1 of clinical trials consists of tests conducted in humans and animals to determine the probable limits of the clinical dose range.