1 million billion billion billion billion billion billion: Number of undiscovered drugs

Jean-Louis Reymond and Mahendra Awale explain that small molecules, which are able to cross cell walls and interact with biological molecules in the body, are prime targets for scientists who develop new medicines. Most existing medications are small molecules. The authors focused on the "chemical space" inhabited by all of the small molecules that could possibly exist according to the laws of physics and chemistry. Researchers have identified millions of these compounds -- the ACS' Chemical Abstracts Service database contains almost 67 million substances. Reymond and Awale estimate that the molecules synthesized and tested as potential drugs so far represent less than 0.1 percent of chemical space. To aid researchers looking for new ways to prevent and treat disease, they set out to find the best ways to search for new small molecules.

The authors discuss several ways of getting a handle on chemical space, including by the size, shape and makeup of molecules. They show how computers can help researchers efficiently narrow a search for a new drug candidate. Computer modeling of chemical interactions can help researchers find a handful of promising molecules to synthesize and test in the lab. "Small molecule drugs are essential to the success of modern medicine," the authors note, and suggest that their methods may be particularly useful for finding new pharmaceuticals that target the central nervous system.