ABSTRACT
It has been demonstrated that drugs act at receptors and that these receptors are specific entities on the surface of cells. There is a large number of different receptors, each with its own individual family of related drugs. In general, there is little or no cross-reactivity between drugs in these separate families, e.g. tubocurarine has an action at the nicotinic acetylcholine receptor and diazepam at the benzodiazepine receptor. Neither of these agents has any effect on the other receptor. Tubocurarine and diazepam differ considerably in their chemical structure, and it might therefore be expected that the characteristics and shape of the molecule are relevant to activity at a receptor. A molecule of a certain shape and/or size is required in order for an effect to appear on a particular receptor system. However, the whole molecule is not necessarily involved. For example, pancuronium, vecuronium, and atracurium have the same effect as tubocurarine on the nicotinic acetylcholine receptor, whereas midazolam, temazepam, and lorazepam all affect the benzodiazepine receptor. The actual chemical structure of each individual component of each series differs, but there are similarities within the molecules. It would thus appear logical to infer that there must be some component part of each molecule which is common to a series and which possesses the ability to combine with the receptor. That moiety on benzodiazepines must be different from that on the cholinergic agonists. Therefore, a particular structure has an activity at certain receptors, and this is the basis of the study of the relation between structure and activity.
