6-Methylergoline-8-carboxylic acid esters as serotonin antagonists: N1-substituent effects on 5HT2 receptor affinity

Three series of 6-methylergoline-8-carboxylic acid esters with various alkyl substituents in the N1-position were prepared and their 5HT2 receptor affinities measured. Some overlap occurred in the 5HT2 receptor affinities of the different ester series, indicating that both the ester side chain and the indole substituent influenced 5HT2 receptor affinity. While 5HT2 receptor affinity was affected by the structure of the ester side chain, the N1-substituent played a more crucial role in determining 5HT2 receptor affinity. When the ester side chain was held constant, maximal 5HT2 receptor affinity for that series of esters was obtained when the N1-substituent was isopropyl. Smaller substituents in the N1-position resulted in reduced 5HT2 receptor affinity. Groups C4 or larger in the N1-position resulted in a further decline in 5HT2 receptor affinity. The importance of the N1-substituent in determining 5HT2 receptor affinity was further substantiated when several 2-methyl-3-ethyl-5-(dimethylamino)indoles with various N1-substituents were tested. Again, maximal 5HT2 receptor affinity was obtained when the N1-substituent was isopropyl.