Liver Fibrosis: From Pathogenesis to Novel Therapies

Chronic liver injury is accompanied by a dysbalanced scarring process, termed fibrosis. This process is mainly driven by chronic inflammation and an altered activity of a multitude of different chemokines and cytokines, resulting in the infiltration by immune cells (especially macrophages) and increase of matrix-expressing cell types. These processes might lead to cirrhosis representing the end-stage of fibrosis. Recent clinical studies comprising patients successfully treated for viral hepatitis showed that liver fibrogenesis and even cirrhosis may be reverted. The hepatic capacity to remodel scar tissue and to revert into a normal liver follows specific mechanistic principles that include the termination of chronic tissue damage, shifting the cellular bias from inflammation to resolution, initiation of myofibroblast apoptosis or senescence and, finally, fibrinolysis of excess scar tissue. The plurality of molecular and cellular triggers involved in initiation, progression and resolution of hepatic fibrogenesis offers an infinite number of therapeutic possibilities. For instance, inflammatory macrophages can be targeted via inhibition of chemokine CCL2 or its receptor CCR2 (e.g., by cenicriviroc) as well as by transfer of restorative macrophage subsets. Another target is galectin-3 that acts at various stages along the continuum from acute to chronic inflammation. Profibrogenic cytokines (e.g., transforming growth factor-β), matricellular proteins (e.g., CCN1/CYR61) or signaling pathways involved in fibrogenesis offer further possible targets. Other options are the application of therapeutic antibodies directed against components involved in biogenesis or remodeling of connective tissue such as lysyl oxidase-like-2 or synthetic bile acids like obeticholic acid that activate the farnesoid X receptor and was antifibrotic in a phase 2 study (FLINT trial). Factors affecting the gut barrier function or the intestinal microbiome further expanded the repertoire of drug targets. In this review, we discuss novel concepts in resolution of hepatic fibrosis and focus on drug targets that might be suitable to trigger resolution of fibrosis.