An introduction to principles of veterinary clinical pharmacology: General principles

Abstract

Extract

A drug is a chemical substance that acts in a biological system to preferentially give a beneficial pharmacological or chemotherapeutic effect. The effects of drugs are well documented and drugs are sometimes classified on the basis of their principal effect. With certain exceptions most drugs are thought to produce their effects by combining with enzymes, cell membranes, or other specialized functional components of cells. The effects of a drug in a biological system must be regarded as ultimate consequences of physico-chemical interaction between the drug and functionally important molecules in the living organism, that is, receptor sites (Goldstein et al, 1968). Morphine and its surrogates, for example, produce their major effects on the central nervous system and the bowel. Structurally, the narcotic analgesics may be thought of as N-methyl piperidine compounds with bulky ring substituents. Receptor attachment (Beckett and Casy, 1954) and analgesic activity (Gero, 1954) are related to the γ-phenyl-N-methyl piperidine moiety of morphine, methadone, pethidine and their derivatives. The attachment of drug to receptors usually involves ionic and other relatively weak, reversible bonds. Occasionally, firm covalent bonds are involved and the, drug effect is only slowly reversible. An example of drug-receptor interaction through formation of a covalent bond is the long-lasting inhibition of the Cholinesterase enzymes by organic phosphates and carbamates.