MAPPING THE MELATONIN RECEPTOR .4. COMPARISON OF THE BINDING AFFINITIES OF A SERIES OF SUBSTITUTED PHENYLALKYL AMIDES

A series of 2-, 3-, and 4-substituted phenylalkyl amides were prepared as potential melatonin analogs in order to investigate the nature of the binding site of the melatonin receptor in chicken brain. The lengt h of the alkyl chain was systematically varied from n = 1 to 4, and me thoxyl substituents were incorporated into the phenyl ring at the 2-, 3-, and 4-positions. The maximum binding affinity was found to occur w hen n = 3 and when the methoxyl substituent was in the 3-position, the direct analog of the carbon framework of melatonin in which the 1,2-a toms of the indole ring have been removed. Whereas there was only a re latively small decrease in binding affinity for the corresponding 2-me thoxy derivatives, 4-methoxyl substitution led to a large decrease in binding affinity, suggesting that the binding sites for the side chain and methoxyl group could not now be occupied at the same time. As in the indole analogs of melatonin, replacement of the methyl group of th e amide by a longer alkyl chain led to an increase in binding affinity for ethyl and propyl with a subsequent decrease in binding affinity f or butyl chains. Thus N-propanoyl-3-(3-methoxyphenyl)propanamine (6f) has a binding affinity of 5.6 nM, a remarkably high affinity for so si mple a compound. Substitution of halogen for 3-methoxyl in the propana mide series gave a series of compounds with lower, but still substanti al, binding affinities, the 3-chloro derivative 7e showing the highest affinity, 113 nM. In the case of the 3-fluoro propanamides, a maximum in the binding affinity was not observed in the series synthesized, a nd these derivatives will merit further exploration. These results dem onstrate the utility of simple, readily modified phenylalkylamines as a ''framework'' for studying the effect of changes in the nature and p osition of substituents on the melatonin receptor binding affinity.