Targeting FXR in cholestasis: hype or hope

Bile acids, the end product of cholesterol metabolism, are signaling molecules. The farnesoid X receptor (FXR) is a bile acid sensor and is part of a network of nuclear receptors that regulate bile acid homeostasis. In addition to FXR, bile acids activate other nuclear receptors (CAR, PXR and VDR), cell surface receptors including the G protein-coupled bile acid receptor 1 (GP-BAR1/TGR5), muscarinic receptor and calcium-gated potassium channels.

The semisynthetic bile acid derivative 6-ethyl chenodeoxycholic acid (6-ECDCA, INT-747 later christened obeticholic acid) is a dual FXR/GP-BAR1 ligand that attenuates bile flow impairment in cholestasis induced by 17β-estradiol; a model of pregnancy-induced cholestasis. Phase II trials with this agent in early stage primary biliary cirrhosis have shown beneficial effects on surrogate markers of damage progression, specifically alkaline phosphatase, with a dose-dependent itching being the most severe and common side effect (up to 70% of patients) leading to therapy discontinuation in 38% of patients. GP-BAR1 activation in the skin triggers itching, thus providing a molecular explanation for this side effect.

While the role of FXR activation in treating severe cholestasis needs confirmation, the activation of GP-BAR1 is likely involved in pruritus development that associates with clinical use of dual FXR/GP-BAR1 ligands. FXR antagonist could be an interesting opportunity for treatment of severe/obstructive cholestasis.