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Molecular entities essential in various types of communication; intra- and intercellular, ecological and anthropogenic are discussed with an emphasis on endogenic and exogenic messengers and on the congeniality of receptors and enzymes.

For decades the receptor concept has been indispensable for discussing and understanding the mode of action of pharmaca. With the mass-action law as a basis, various receptor theories and models have been worked out to interpret the dose—response and dose—binding curves observed. With the possibility of receptor localization, identification and isolation, the use of receptor-binding studies as a screening method in the development of new bioactive agents and the development of receptor pathology, the receptor concept has been converted to hard reality.

Enantiomers, and stereoisomers in general, must be regarded as different chemical compounds, particularly for biological purposes. Recemic drugs, pesticides etc. usually contain 50% or more of an ‘inactive' isomer that is presumed harmless. E. J. Ariens describes various actions of such ‘inactive products' and the resultant therapeutic and toxicological implications. The role of enantioselectivity in ‘hybrid' drugs is discussed. Neglect of stereochemistry in the study and evaluation of racemic products results in highly misleading data, the waste of researchers' time on ‘sophisticated nonsense' and the acceptance of inferior biological products. For pharmacological — particularly pharmacokinetic-studies, the following maxim holds true: it may be impossible to do it the wrong way! right

On the basis of recent experimental evidence the classical concept of bronchospasmogens and bronchospasmolytics is challenged, switching the emphasis from bronchial smooth muscle hyperreactivity to tracheal and bronchial mucosa hypersensitivity or irritability. Against the background of the revised views some possibilities for development of new anti-asthmatic agents are discussed.

Publisher Summary This chapter discusses the design of safer chemicals. There are good arguments that 60% to possibly even 90% of cancers are preventable. The same might hold true for chemicogenic diseases in general. This implies that the proper analysis of structure–action relationships in the field of carcinogenicity and the related fields of mutagenicity, allergic sensitization etc. and the application of the insights gained into the design of safer chemicals, be it drugs, pesticides, household chemicals, or whatever, would contribute to that prevention. Asking for 100% safety is as unwise as neglecting the potential risks of chemistry and its products is unwarrantable. The practice will have to be a compromise, a sound balance, between higher safety and technological and economic feasibility. Although there is still a long way to go, there are perspectives for the development of safer chemicals. This is essential because life without chemicals is impossible, taking into account the large world population, its urbanization, and high standards of living, with the inherent necessity of mass production and conservation of food, massive substitution of scarce natural products, and progressive technology.

Binding of the anticholinergic agent 3 H-dexetimide, the active isomer of (±)-benzetimide, to subcellular fractions of the caudate nucleus of the male Wistar rat was studied by equilibrium dialysis. The binding is stereospecific, reversible, appears to involve the muscarinic type of acetylcholine receptor (AchR) and correlates well with the physiological response of the AchR as can be concluded from displacement experiments using cholinergic and non-cholinergic ligands. The binding sites are located on membrane fragments probably originating from the post-synaptic membranes.