CLASSIFICATION OF DRUGS

Drugs can be classified in many different ways, ranging from their chemical structure to the principal effect they produce, or the disease that they treat. Which method of classification used is usually dependent upon one’s point of view. For example, the drug amphetamine could be classified in at least five different ways (Table ) depending upon who was doing the classifying:

   o   Physician: Appetite-suppressing agent (anorexigenic)

   o   Pharmacologist: Sympathomimetic

   o   Chemist: 2-amino-1-phenylpropane

   o   Lawyer: Drug of abuse falling in schedule II of the 1970 federal drug law

                      o   Psychologist: Stimulant.

By analyzing the method of classification imposed upon a drug we can gain some insight into which of its characteristics is being emphasized by the classifier. However, there is an alternative classification system to those just described that can also be instructive. This system seeks to put drugs into four functionally distinct categories that divulge important distinctions about therapeutic and non therapeutic principles.The four categories are listed below.

Drugs used to combat infection:

Drugs in this category are based on the concepts of selective toxicity and chemotherapy developed by Paul Ehrlich in the late nineteenth and early twentieth centuries. Ehrlich made the observation that the dye methylene blue specifically stained neural tissue but not any other. From this specific observation he generalized that some molecular characteristic of neural tissue conferred selectivity on the dye and that a similar situation might exist in foreign organisms, which could form the basis for selective chemotherapy. Unfortunately, there are few pure examples of true selective toxicity. Perhaps the best is penicillin. The therapeutic specificity of this antibiotic is based upon the qualitative difference between bacterial cell wall synthesis and mammalian cell membrane synthesis. Synthesis of the former can be inhibited by penicillin while the latter is unaffected. Thus, penicillin is one of the few examples of a drug that can actually “cure” an illness. A similar example involves the sulfa drugs, which interfere with the synthesis of folic acid, used in nucleic acid formation, in bacteria. While bacteria must synthesize their own folic acid, mammalian cells utilize dietary, preformed folic acid and are not susceptible to interference with its formation.

Drugs used to replace inadequacies of naturally occurring substances:

In an ideal sense this class of drugs represents the “purest” form of drug use in that they are not “foreign” to the body. Examples include the use of hormones, such as insulin, in replacement therapy. Insulin is obviously an endogenous hormone and, if the human preparation is used, is exactly the same in all of us. The therapeutic goal in treating diabetes mellitus is to replace normal, physiological levels of insulin. The neurogenic chemical l-dopa can also be thought of in a similar manner since it is used to treat inadequate brain levels of dopamine in certain cases of parkinsonism. It must be understood, however, that if hormones are given in supraphysiological amounts they have the capacity to produce undesirable effects just as any xenobiotic does.

Drugs that change regulation:

This group contains the largest total number of drugs used because they deal with the treatment of symptoms. Drugs used in this category do not cure, or replace, but can effectively manage acute or chronic disorders, often involving regulatory changes in the cardiovascular or nervous system, for example. Drugs in this category include antihypertensives, antianginals, diuretics, anticoagulants, analgesic and antipyretics, sedatives, anticonvulsants, and birth control pills.

Drugs to alter mood or behavior:

This class includes relatively widely used licit, as well as illicit, drugs such as tranquilizers, alcohol, and tetrahydrocannabinol (THC, the active ingredient in marijuana). In addition, “hard” drugs such as cocaine, opiates, and hallucinogens are also included. This class of drugs is usually taken to change our perceptions of our environment and ourselves. They are often taken to relieve anxiety or to facilitate our involvement in certain social or “recreational” settings.

In addition to the variety of drug classification systems just described, a similar diversity, and somewhat bewildering array, of systems is used to name drugs during their development. This is because in the course of a drug’s development, it usually acquires more than one identifying name. An example is the common drug aspirin:

Chemical name—A systematized and standardized nomenclature that encodes within the name descriptive information about the molecular constitution of the drug (e.g., 2-acetoxybenzoic acid).

Trivial name—A coined name in general use. It is a common name by which the drug is identified although it may not be intrinsically descriptive. There may be more than one trivial name (e.g., acetylsalicylic acid).

Generic or established name—A similar or contrived or coined name in general use. It usually refers to the U.S. name adopted by nomenclature groups known as the USAN and USP Committees. The generic or established names are trivial names but they have a somewhat more official status (e.g., aspirin).

Trade name—A brand or proprietary name; a legally registered trademark of a drug or dosage form of a drug. This name is the property of the registrant. There may be more than one trade name for a drug (e.g., Empirin™). Before considering the pharmacology of any particular class of drugs, it is important to understand the basic underlying principles of drug action.