Drug research encompasses several diverse disciplines united by a common goal, namely the development of novel therapeutic agents. The search for new drugs can be divided functionally into two stages: discovery and development. The former consists of setting up a working hypothesis of the target enzyme or receptor for a particular disease, establishing suitable models (or surrogate markers) to test biological activities, and screening the new drug molecules for in vitro and/or in vivo biological activities. In the development stage, efforts are focused on evaluation of the toxicity and efficacy of new drug candidates.Recent surveys indicate that the average new chemical entity taken to market in the United States requires 10 to 15 years of research and costs more than $300 million. Once the target enzyme or receptor is identified, medicinal chemists use a variety of empirical and semiempirical structure-activity relationships to modify the chemical structure of a compound to maximize its in vitro activity.