The Effect of Ortho Substitution on the Pharmacology of Benzoylcholine

Abstract

The effects of mono-and di-ortho substitution of benzoylcholine on actions on the rat phrenic diaphragm and frog rectus abdominis preparations have been examined. Ortho substitution of benzoylcholine does not necessarily lead to a reduction in activity and may increase potency. It appears that mono-ortho-iodo substitution provides most stability combined with pharmacological activity, The enzymatic cleavage of esters by the esterase group of enzymes is one of the more important routes of drug metabolism (Brodie, Gillette andLa Du, 1958, Brodie, Mackel andJondorf, 1958) and consequently methods of changing the rate of ester hydrolysis are of potential importance in drug design. The stability of an ester towards hydroxide ion-catalysed hydrolysis is dependent on the steric and electronic characteristics of the component acid and alcohol (Hammett, 1940). Levine and Clarke (1955), Glick (1938Glick ( , 1939Glick ( , 1941Glick ( , 1942 and Fu, Birnbaum and Greenstein (1954), have shown that the same factors are also important in in vitro and presumably in in vivo enzyme catalysed hydrolysis of esters.It is possible to modify the stability of benzoic acid esters towards hydroxide ion-catalysed hydrolysis by substitution in the aromatic nucleus (Kindler, 1928 ;Evans, Gordon and Watson, 1937). Substituents in the ortho position accelerate less or retard more the rate of hydrolysis of the ester group than the same substituents in the meta and para positions (Ingold, 1953). This is an example of the ortho effect. (For a full discussion of the ortho effect see Stoker, 1959.) To investigate the possibility of using ortho substitution as a general method of stabilising ester groups in drug molecules Thomas and Stoker (1961) synthesised a series of mono-and di-ortho substituted benzoylcholine derivatives. The hydrolysis of benzoylcholine is catalysed both by hydroxide ions and cholinesterase (Mendel, Mundell and Rudney, 1943) and was, therefore, a suitable molecule to use to compare the effect of ortho substitution on the relative rates of chemical and enzymic catalysed hydrolysis. It was found that ortho substitution in benzoylcholine produced compounds which were either more rapidly hydrolysed by cholinesterase than the unsubstituted ester, more slowly hydrolysed or completely stable towards the esterase. The stability of the esters towards both hydroxide ion-and cholinesterase catalysed hydrolysis followed roughly that predicted from theoretical considerations of the "ortho effect" and the mechanism of hydrolysis. The "stable" substituted benzoylcholines were shown to be inhibitors of cholinesterase. All the ortho substituted benzoylcholines were more powerful inhibitors of acetylcholinesterase than benzoylcholine itself. The inhibition studies 225