Importance of Stereochemistry in Pharmacy with Special attention to DrugMolecules Introduction The importance of stereochemistry in drug action is gaining greater attention in pharmaceuticalsciemces, and a basic knowledge of the subject will be necessary for clinicians to make informeddecisions regarding the use of single-enantiomer drugs. Many of the drugs currently used in psychiatric practice are mixtures of enantiomers. For some therapeutics, single-enantiomer formulations can provide greater selectivities for their biological targets, improved therapeuticindices, and/or better pharmacokinetics than a mixture of enantiomers.
This assignment reviewsthe importance of stereochemistry with special attention to different drug molecules, thenomenclature for describing stereochemistry and enantiomers, emphasizes the potential biologicaland pharmacologic differences between the 2 enantiomers of a drug, and highlights the clinicalexperience with single enantiomers. In some cases, both a mixture of enantiomers and a single-enantiomer formulation of a drug will be available simultaneously. In these cases, familiarity withstereochemistry and its pharmacologic implications will aid the practicing physician to provideoptimal pharmacotherapy to his or her patients.
Some Basic Concepts Before going into the depth of this discussion let us have some concepts about the basic aspects of stereochemistry which eill ultimately help us to understand the importance of stereochemistry later on. 1 This section contains the basics needed to understand chiral drugs. The most important point is thatchiral drugs have 2 structurally similar forms that can behave very differently in biological systemsdue to their different shapes in 3-dimensional space. These 2 possible forms are termed enantiomers , and the 2 enantiomers of a given chiral drug should be considered 2 different drugs. This topic is discussed further in the next section. Chirality is formally defined as the geometric property of a rigid object (like a molecule or drug) of not being superimposable with its mirror image. Molecules that can be superimposed on their mirror images are achiral (not chiral).
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Chirality is a property of matter found throughout biological systems, from the basic building blocks of life such as amino acids, carbohydrates, and lipids to the layout of the human body.
Chirality is often illustrated with the idea of left- and right-handedness: a left hand and right handare mirror images of each other but are not superimposable. The 2 mirror images of a chiral molecule are termed enantiomers . Like hands, enantiomers comein pairs. Both molecules of an enantiomer pair have the same chemical composition and can bedrawn the same way in 2 dimensions (e. g. , a drug structure on a package insert), but in chiralenvironments such as the receptors and enzymes in the body, they can behave differently. A racemate (often called a racemic mixture) is a mixture of equal amounts of both enantiomers of achiral drug. Chirality in drugs most often arises from a carbon atom attached to 4 different groups, but there can be other sources of chirality as well. Single enantiomers are sometimes referred to as singleisomers or stereoisomers . These terms can also apply to achiral drugs and molecules and donot indicate that a single enantiomer is present. For example, molecules that are isomers of eachother share the same stoichiometric molecular formula but may have very different structures.
2 However, many discussions of chiral drugs use the terms enantiomer , single isomer , and/or single stereoisomer interchangeably. The 2 enantiomers of a chiral drug are best identified on the basis of their absolute configuration or their optical rotation. Other designations such as D and L (note the upper case) are used for sugarsand amino acids but are specific to these molecules and are not generally applicable to other compounds. The terms d , or dextro, and l , or levo , are considered obsolete and should
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