In search of the magic bullet: can liver inflammation and fibrosis be reversed with medications?

Abstract

Recent clinical studies comprising patients successfully treated for viral hepatitis have shown that liver fibrogenesis may be reverted, even at later stages including during bridging fibrosis and cirrhosis. Intensive research has identified numerous potential novel targets in liver disease. Multiple innovative compounds have now entered clinical trials, mostly in non-alcoholic steatohepatitis (NASH) and NASH-associated cirrhosis due to their outstanding epidemiological relevance. In general, regression from liver fibrosis follows four major mechanistic principles: termination of chronic damage, shifting the cellular bias from inflammation to resolution, deactivation of myofibroblasts and direct matrix degradation. Obeying these principles, several promising approaches are currently evaluated, for example, targeting inflammatory macrophages via inhibition of chemokine CCL2, its receptor CCR2 or galectin-3, bone marrow-derived cell transfer, or antibodies against matrix-stabilizing lysyl oxidase-like-2. The ongoing trials will reveal which of the many potential targets prove to have clinical efficacy, bearing in mind that fibrosis reversibility is less likely to be achieved in humans than in animal models.

Keywords:

• chemokine
• hepatic stellate cell
• liver fibrosis
• macrophages
• matrix
• regression

Perpetuating liver damage that does not subside eventually leads to fibrosis and cirrhosis. Clinical hepatologists earlier apprehended cirrhosis as an irreversible sequela to chronic liver injury, which may result in decompensation and development of hepatocellular carcinoma. However, recent clinical observations and trials have fueled the hope that liver fibrogenesis by far is not a unidirectional process but may be reverted even at later stages including bridging fibrosis (i.e., cirrhosis), provided that the underlying cause can be removed [1]. Most compelling evidence that successful medical treatment of a liver disease helps in amelioration of architectural disturbances originates from chronic hepatitis B virus infection. In hepatitis B virus-infected patients with cirrhosis at baseline, administration of the nucleotide analog tenofovir led to reversal of cirrhosis in almost 75% of cases [2]. However, there is still an unmet need to combat established cirrhosis due to the lack of specific therapeutic tools in many conditions. Non-alcoholic fatty liver disease, for instance, encompasses non-alcoholic steatohepatitis (NASH) as its most severe form, which in turn can result in end-stage liver disease. Despite a growing prevalence of this disease, no specific pharmacological therapies are currently at hand. Similarly, cirrhosis in the course of alcoholic liver disease, which still represents one of the major causes for liver transplantation in western countries, cannot be reverted in most cases in spite of cessation of alcohol consumption [1]. In addition, autoimmune liver diseases such as autoimmune hepatitis or primary biliary cirrhosis are often only diagnosed at very late stages, in which immunosuppressive drugs or ursodeoxycholic acid fail to re-establish the integrity of liver histology, respectively [1]. Thus, whenever the goal of curing the primary disease cannot be achieved, we need approaches that interfere with generic mechanisms of liver scar formation. Intensive research in recent years has identified a multitude of potential novel targets in liver disease and a considerable number of innovative compounds have now entered clinical trials [3]. The outstanding epidemiological relevance of NASH and NASH-associated cirrhosis entails that the vast majority of clinical trials involving antifibrotic drugs in liver disease are conducted in this patient cohort.

In general, regression may follow four major mechanistic principles: termination of chronic damage, shifting the cellular bias from inflammation to resolution, deactivation of myofibroblasts and direct matrix degradation [4,5]. Macrophages are key players in the fibrotic intercellular network and exert dual functions orchestrating either fibrosis progression or regression depending on their phenotype, origin and functional state [6,7]. Monocyte-derived macrophages, expressing Ly6C in mice, are massively recruited from blood stream to the injured liver, drive inflammation as vigorous secretors of inflammatory cytokines including TNF-α, activate hepatic stellate cells and, hence, trigger a cascade of events leading to fibrosis [7]. Furthermore, inflammatory macrophages foster angiogenesis, a process that is closely linked to fibrosis progression and hepatocellular carcinoma formation [8]. However, angiogenesis driven by myeloid-derived vascular endothelial growth factor is also critical for fibrosis resolution, as vascular endothelial growth factor-stimulated sinusoidal endothelium secretes matrix degrading metalloproteinases [9].

During the further course, Ly6C^hi macrophages can transdifferentiate into restorative Ly6C^lo macrophages that promote scar resolution through phagocytosing cellular debris and secreting metalloproteinases [10]. This phenomenon could be recapitulated by transplantation of bone marrow–derived macrophages that deliver collagen-degrading proteinases into the scar and imprint a rather anti-inflammatory environment in experimental liver fibrosis [11]. Similar to the latter study, transfer of autologous mixed bone marrow–derived cells corroborated fibrosis resolution mediated by matrix degrading metalloproteinase 9 and matrix degrading metalloproteinase 13 in mice [12]. Delivering immune cells to the fibrotic nodules seems to be a rational approach considering the fact that acellular scars are those that lack the potential to resolve [13]. Of note, there is already evidence that infusion of bone marrow–derived cells might be beneficial in end-stage liver disease, and a large controlled trial (REALISTIC trial) investigating the effect of G-CSF–mobilized stem cells on cirrhosis outcome is under way (EuDRACT number 2009-010335-41). Given the postphagocytic nature of restorative macrophages, either in vivo or ex vivo modification of macrophages using, for example, liposomes could even enhance their beneficial capacities [14].

Hampering macrophage influx into the inflamed liver can be achieved by aiming at the chemokine axis CCL2/CCR2. We could previously demonstrate that an RNA-aptamer–based inhibitor of CCL2, termed mNOX-E36, reduces the constant infiltration of Ly6C^hi macrophage subsets in fibrotic mouse models, thereby favoring the net accumulation of their restorative Ly6C^lo counterparts. This resulted in accelerated fibrosis regression [15]. Apart from fibrogenesis, this compound also proved effective in amelioration of liver steatosis [16]. A clinical trial (CENTAUR) investigating the efficacy and safety of the oral combined CCR2/CCR5 inhibitor cenicriviroc in NASH patients has recently been initiated (NCT02217475). Another chemokine-based approach that seems feasible in combating liver fibrosis is the compound Met-RANTES that counteracts CCL5. It interferes with stellate cell activation and migration, and thereby directly limits excessive matrix synthesis [17].

Galectin-3 is an interesting molecule emerging as a possible target to revert liver fibrosis. It acts as a pleiotropic lectin that is released by macrophages and other immune cells and supports fibrogenesis in a multifaceted fashion [18]. In a rodent toxic model of cirrhosis, the Galectin-3 inhibitors GR-MD-02 and GM-CT-01 both led to sustained amelioration of liver scarring and inflammation [19]. Given the putative distinguished role of Galectin-3 in cirrhosis and fibrotic diseases of other organs, a company was founded that launched a Phase-I trial to explore the therapeutic use of GR-MD-02 in NASH patients (NCT01899859). The trial is now completed and reports of preliminary results heralded excellent tolerability and improved surrogate markers of cell death, inflammation and fibrosis.

Besides macrophage recruitment, differentiation or functionality, the complex inflammatory micro-milieu in chronic liver disease provides a substantial number of additional potential targets [3]. The cytokine osteopontin is highly upregulated in fibrotic tissues and functionally linked to progenitor cell responses as well as wound healing [20]. In mouse models, neutralization of osteopontin effectively abrogates fibrogenesis [20]. Other fibrosis-restricting mechanisms are the gamma–delta T cell–mediated and NK cell–mediated apoptosis of hepatic stellate cells [21], as well as the IL-22 driven senescence of stellate cells [22]. Recent data from patients with chronic liver infections suggest that IL-22 protects against liver fibrosis and its competitor IL-22 binding protein aggravates liver fibrosis, suggesting that pharmacological modulation of IL-22 binding protein may be a promising strategy to limit cirrhosis [23].

The endocannabinoid system is profoundly involved in modulating the inflammatory and fibrotic response upon liver damage. Interestingly, the endocannabinoid receptors CB1 and CB2 display opposing functions. Whereas CB1 downstream signaling is detrimental by promoting matrix deposition and its steatogenic properties, CB2 mediates hepatoprotection [24]. Counteracting CB1 signaling is potentially harmful, as observed with the CB1 antagonist rimonabant that was previously licensed for treating morbid obesity and had to be withdrawn from the market owing to severe mood disorders. Albeit currently available CB1 antagonists preclude application in man, non blood–brain barrier penetrating second-generation CB1 antagonist drugs might be a future option given the encouraging findings in small animal models [25]. In contrast, stimulating CB2 by the drug JWH-133 efficiently reduced fibrosis directly through promoting stellate cell apoptosis/quiescence and indirectly via containing immune cell infiltration and various other beneficial impacts on parenchymal and non-parencyhmal cells [24]. Though these findings render CB2 agonists as interesting novel compounds for liver fibrosis, no testing in humans has been performed so far.

Another budding approach is the cell-independent targeting of scar formation. Polymerization of collagen is a prerequisite during organ fibrosis and is catalyzed by the matrix enzymes, lysyl oxidases. In a seminal work, lysyl oxidase-like-2 was identified as critical for liver fibrosis [26]. A humanized monoclonal antibody against lysyl oxidase-like-2, Simtuzumab (GS-6624), is currently being investigated in diverse ongoing Phase II trials involving HCV (NCT01707472), primary sclerosing cholangitis (NCT01672853) and NASH-related fibrosis (NCT01672866)/cirrhosis (NCT01672879).

Taken together, there is a substantial body of evidence that liver inflammation and fibrosis can be reverted with medications. However, many hurdles have to be overcome in search of the ‘magic bullet’, including appropriate patient selection and timing of drug application. Whenever possible, therapeutic efforts should aim at the primary disease. Successful exertion of pharmacological means to treat inflammation and fibrosis has to accommodate a myriad of individual parameters such as stage of fibrosis and etiology of the underlying liver disease. Results of the ongoing trials have to be awaited to clearly dissect which of the many potential targets prove clinical efficacy, bearing in mind that fibrosis reversibility is less likely achieved in humans than in animal models.

Financial & competing interests disclosure

The authors were supported by grants from the German Research Foundation (grant numbers: DFG; SFB/TRR57, TA434/3-1). The work in the laboratory of F Tacke is supported by funding from Noxxon Inc. and Tobira Therapeutics Inc. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.