What is an Enantiomeric Medication? Chiral Drugs Overview

Many "new" drugs on the market today are nothing more than variants of old drugs. They are sometimes called "purified" drugs, yet the original drugs are certainly not "impure." The new drugs' names are variations on the old drugs' names, often with a prefixes like "es," "ar," "lev," or "dextro" added (for example, omeprazole and esomeprazole; amphetamine and dextroamphetamine). What do these mysterious distinctions mean, and why are the "new" drugs considered better than their predecessors?

In organic chemistry, many molecules can exist in one of several forms, or arrangements. These related molecules are called isomers. Some have completely different structures and are related only by having the same chemical formula. Others have the same basic structure, but have different arrangements in space. These variations are called stereoisomers.

Most types of stereoisomers have different chemical properties from each other because the arrangement of the atoms in a molecule affects its properties. Some stereoisomers, however, are mirror images of each other. These variant molecules are called enantiomers, and they have identical physical properties. Molecules that can exist as two enantiomers (there are always exactly two) are called chiral molecules, and the specific "form" an enantiomer takes is called its chirality.

Enantiomers are like hands. The left hand and right hand are identical, except that neither can be "superimposed" on the other; they are mirror images, and no matter how you rotate one hand, it will never look like the other hand. In fact, chirality is sometimes referred to as the "handedness" of a molecule.

A quantity of a compound that contains a mixture of two enantiomers is called a racemic mixture of that molecule. Many of the medications developed during the twentieth century are racemic mixtures. But even though the chemical properties of both "mirror images" are the same, sometimes the body does not recognize them as being the same. One enantiomer might be more active than the other; one may even be completely inactive.

Because their chemical properties are identical, separating the enantiomers of a racemic mixture requires special techniques. There are two main strategies.

One is to start with a racemic mixture and separate the enantiomers. Louis Pasteur was the first to use this approach with tartaric acid. Because their structures are mirror images, some chiral molecules like tartaric acid form crystals that are mirror images, and the two types of crystals can be identified and separated.

The other approach is to synthesize the molecule as a chiral molecule, not using a racemic mixture at all. This kind of synthesis can be accomplished by starting with chiral molecules, by using enzymes that produce chiral molecules, or by using other chiral molecules to "shape" the substance being synthesized.

Enantiomers are being used more and more often in pharmacology. In part two, find out more about the importance of pure enantiomers in drug design.