Structure-activity relationship studies on natural and synthetic bile acid analogs

The objective of our research was to develop ursodiol analogs that are structurally modified to modulate hepatic side-chain amidation and prevent 7-dehydroxylation by intestinal bacteria while at the same time maintaining the critical micellar concentration (CMC) and hydrophilicity of ursodiol. More than 20 naturally occurring bile acids were screened for physicochemical properties. Then, two generations of analogs were studied, and those with physicochemical properties similar to ursodiol's were analyzed for physiologic properties. The first generation of analogs included molecules with steric and/or electronic hindrance on the side chain; the second group consisted of the same molecules conjugated with glycine or taurine and also "pseudoconjugated" analogs (23-hydroxylated, esterified, and amidated with other amino acids). Of the first-generation analogs, only cyclopropane D derivative and trans-olefin were useful to our purposes, being conjugated by the liver and almost completely recovered in bile. These two analogs were deconjugated and 7-dehydroxylated but with slower kinetics. The hydrophilicity of the molecules could be augmented by increasing the polarity of the steroid ring. Among the pseudoconjugated analogs, the CMC values were similar to those of the natural analogs, although hydrophobicity differed among the group. The analogs that were not deconjugated were not 7-dehydroxylated either. All of the pseudoconjugated bile acids were efficiently taken up by the liver, and their recovery in bile was similar to that of glycine and taurine ursodiol. From these studies we now know that side chain configuration and conformation are important in the conjugation and deconjugation processes. Mild modification of the side chain can prevent 7-dehydroxylation and thus yield a bile acid more resistant to intestinal bacteria and more bioavailable. Prevention of hepatic conjugation improves biliary secretion and recovery of many analogs.