Replacement of the saturated propoxy group in compound
Replacement of the saturated -propoxy group in compound with the unsaturated allyloxy group to obtain compound resulted in 10-fold decrease in ET receptor antagonism and more than 20-fold decrease in ET-selectivity. Analogous unsaturated allyloxy compound in the tetrazole series was found to have diminished ET receptor antagonist activity by 60-fold and ET-selectivity by 50-fold compared to -propoxy compound . This suggests that a saturated alkyl chain at position 6 of 4-oxo-1,4-dihydro-quinoline-2-carboxylic NMS-E973 mg is beneficial for ET receptor antagonist activity. Also, when the -propoxy chain in is replaced by isosteric methoxymethyloxy chain in the carboxylic acid series to obtain analogue , there is 6-fold loss of ET receptor antagonism and 15-fold loss of ET-selectivity. A similar reduction in ET-selectivity for methoxymethoxy analogue in the tetrazole series was observed although the ET receptor antagonist activity reduced by nearly 20-fold. Replacement of the terminal methyl group of the -propoxy side chain of in the carboxylic acid series with an amino group led to compound which exhibited 85-fold reduction in ET receptor antagonism. ET-selectivity also declined drastically from 683 to 4 compared to . The 2-aminoethoxy analogue in the tetrazole series resulted in more than 450-fold reduction in ET receptor antagonist activity and a virtual loss of ET-selectivity compared to . This suggests that substitution of an amino group in place of the terminal methyl group of the -propoxy side chain is detrimental for ET receptor antagonism and selectivity. A series of analogues with alicyclic side chains were synthesized to determine tolerance of steric bulk at position 6 of the 4-oxo-1,4-dihydro-quinoline-2-carboxylic acid. Cyclopropyl-methoxy analogue in the carboxylic acid series showed more than 10 times lower ET receptor antagonist activity and selectivity compared to the corresponding open chain -butyl analogue . Reduction in ET receptor antagonist activity and selectivity of cyclopropylmethoxy analogue (IC=78.4nM) of the tetrazole series compared to the -butyl compound were found to parallel the effects of the carboxylic acid derivatives and . This considerable reduction in antagonist activity and receptor selectivity could be due to the fact that C–C bonds in cyclopropane have more π character than σ character, resulting in cyclopropylmethoxy substituent acting like an unsaturated allyloxy substituent at position 6 of the quinolone ring. This behavior resembles the ET receptor binding nature of the unsaturated allyloxy derivatives and compared to the corresponding saturated -propoxy derivatives and . ET receptor antagonist activity was found to decrease by 3-fold with the increment of each methylene group to increase the ring size of the alicyclic analogue to obtain cyclobutylmethoxy analogue and cyclopentylmethoxy in the carboxylic acid series. Compounds and also exhibited poor ET-selectivity. A similar reduction of 3-fold in ET receptor antagonist activity and poor ET-selectivity was observed with an increase in the ring size of cyclopropylmethoxy analogue in the tetrazole series to yield cyclobutylmethoxy analogue and cyclopentylmethoxy . In conclusion, a series of endothelin receptor antagonists was synthesized and evaluated for their ET and ET receptor antagonist activity and selectivity using GeneBLAzer® FRET assay technology. The structure activity relationship deduced for the carboxylic acid series was found to parallel that of the tetrazole series. The -propoxy analogue in the tetrazole series was found to be the most potent and selective ET receptor antagonist with an IC value of 0.8nM and an ET-selectivity of 1004. Target compounds with ethoxy, -propoxy, -butoxy, -butoxy substituents in both series were found to have similar or better activity than the positive control BQ-123. These results suggest that short chain saturated alkoxy groups at position 6 of the 4-oxo-1,4-dihydro-quinoline-2-carboxylic acid impart good ET receptor antagonism and selectivity over ET receptor. Tetrazole exhibits similar or better ET receptor antagonist activity as compared to the clinically approved endothelin receptor antagonists for PAH () with ET-selectivity profile similar to that of the most recently approved drug, macitentan.