A chemical compound that contains an asymmetrical center and is capable of occurring in two nonsuperimposable mirror images. This phenomenon was first described by Louis Pasteur. “Chiral” is a word that is derived from the Greek cheir (meaning “hand”). For example, human hands may be used to illustrate chirality in that when the left and right hands are held one on top of the other, one thumb sticks out on one side while the other thumb sticks out on the other side. The point is that the same number and type of fingers and thumbs exist in both hands, but their arrangement in space may be different. So it is with the arrangement of a given molecule’s (e.g., a drug’s) atoms in three-dimensional space.
Approximately 40 percent of drugs on the market today consist of chiral compounds. In many chiral drugs, only one type of the molecule is beneficially biologically active (i.e., acts beneficially to control disease, reduce pain, etc.), while the other type of the drug molecule is either inactive or else causes undesired impacts (called **side effects” of the drug mixture). For example, one enantioner of the drug thalidomide is a potent angiogenesis inhibitor, but the other enantiomer causes birth defects in babies of pregnant women taking it.