Current Topics in Medicinal Chemistry - Volume 14, Issue 19, 2014

The modulation of FXR receptor remains an attractive area in drug discovery to develop novel therapeutic opportunities for liver and metabolic disorders. Despite the large variety of FXR ligands reported so far, only a very restricted number of agonists have entered in clinical settings. In this review article we provide the reader with an overview on the different classes of natural and synthetic ligands that have been developed by academic groups and pharmaceutical companies to target FXR. We discuss their structure-activity relationships, analyzing the binding modes that some of these compounds adopt to interact with the receptor.

The Farnesoid X Receptor (FXR) was recently validated in clinical studies using the bile acid analogue Obeticholic Acid (OCA) as an attractive drug target for liver diseases such as Primary Biliary Cirrhosis (PBC) or Non-alcoholic Steatohepatitis (NASH). OCA, however, turned out to induce cholesterol- related side effects upon prolonged treatment and it shows bile acid like pharmacokinetics. The quest for synthetic non-steroidal FXR agonists with general drug likeliness and improved pharmacokinetic and – dynamic properties has started more than a decade ago: The first non-steroidal and selective FXR agonist with decent submicromolar potency, GW4064, was patented in 1998 and published in 2000. Since then, many pharmaceutical companies have taken GW4064 as a structural template for their efforts in identifying novel patentable FXR agonists with the GW-derived trisubstituted isoxazole general structure. However, so far only one compound out of these different series has made it into the early stages of clinical development: The Px-102/Px-104 from Phenex is currently tested in a phase IIa study in patients with Non-Alcoholic Fatty Liver Disease (NAFLD). In this review we try to summarize from the patent and scientific literature the attempts to improve the GW4064 structure into different directions. Furthermore, we suggest directions for further improvements of this special class of synthetic FXR agonists which all display the typical “hammerhead”-conformation in the FXR ligand binding pocket that provides the basis for their impressive in vitro and in vivo potencies.

Bile acids are a peculiar class of steroidal compounds that never cease to amaze. From being simple detergents with a primary role in aiding the absorption of fats and fat-soluble vitamins, bile acids are now widely considered as crucial hormones endowed with genomic and non-genomic functions that are mediated by their interaction with several proteins including the nuclear receptor Farnesoid X Receptor (FXR). Taking advantages of the peculiar properties of bile acids in interacting with the FXR receptor, several biliary derivatives have been synthesized and tested as FXR ligands. The availability of these compounds has contributed to characterize the receptor from a structural, patho-physiological and therapeutic standpoint. Among these, obeticholic acid is a first-in-class FXR agonist that is demonstrating hepatoprotective effects upon FXR activation in patients with liver diseases such as primary biliary cirrhosis and nonalcoholic steatohepatitis. This review provides an historical overview of the rationale behind the discovery of obeticholic acid and chemical tools generated to depict the molecular features and bio-pharmacological relevance of the FXR receptor, as well as to summarize structure-activity relationships of bile acid-based FXR ligands so far reported.

Farnesoid X receptor (FXR, NR1H4), a nuclear receptor (NR) highly expressed in the liver, intestine, kidney, adrenal glands and other cholesterol-rich tissues, functions as the master regulator for bile acid homeostasis. FXR, which regulates the expression of genes encoding proteins involved in cholesterol homeostasis, plays an essential role in regulating cholesterol, lipid, and glucose metabolism. Recently, some FXR agonists are reported to have low selectivity on NRs, which forces the researchers to move their eyes onto the development of FXR antagonists with high selectivity. The development of non-steroidal FXR antagonists with different scaffolds including AGN34, tuberatolides, atractylenolides, andrographolides, GW4064 derivatives and 1,3,4-trisubstitutedpyrazolones, provides us a prospect for the therapy of in ammation, metabolic syndrome, diabetes, cholesterol gallstones, and cancer.

The nuclear bile acid sensor farnesoid X receptor (FXR) constitutes a rising target for the treatment of a variety of diseases including metabolic disorders, inflammation and certain forms of cancer. While the research on FXR agonists has yielded many compounds and first clinical candidates, only few FXR antagonists have been discovered so far and the knowledge about their in vivo effects is quite narrow. We have evaluated available in vitro and in vivo studies with FXR antagonists as well as FXR knockout models to elucidate a potential pharmacological use of FXR antagonism. To date, the in vitro and in vivo data suggests that FXR inhibition by knockout or the use of antagonists causes beneficial effects on cholesterol metabolism, ameliorates liver toxicity in cholestasis and can reduce the proliferation and migration of some cancer cell lines. Unfortunately, also many disadvantageous effects are connected with FXR antagonists.