In part II and part I of this series you read about the lessons learnt from preclinical work with respect to NASH and NAFLD. In this last part we will look at the medical needs, possible interventions and clinical trials in this area. Enjoy reading and don't forget to register for our monthly blog mailer (to the right of this page) so you won't miss future posts.
Lifestyle interventions. Although lifestyle changes have been recognised to be a cornerstone in the prevention of diabetes, fatty liver, NASH and cardiovascular disease (Barb et al., 2016) sustainable regimes are difficult to implement and often hamper from a limited compliance of targeted people. There is a general gap between the broad availability of quality information on the impact of livestyle for health promotion and chronic disease prevention on the one hand, and the translation of into behavioural attitudes on the other.
In addition the overweight and obese population is heterogeneous with regard to both, disease risk and susceptibility to specific lifestyle interventions. Therefore the usual lifestyle interventions may be too unfocused as a specific adjustment to individual risk patterns and personal capabilities is usually missing. Advanced risk assessments should include a more accurate differentiation between metabolically healthy obese and obese at-risk, as well as the identification of healthy lean and lean but metabolically obese people. Medium-term advancements in sensor development and non- or minimally invasive continuous metabolic monitoring combined with the exploration of big data may help to tailor lifestyle interventions more to the individuals needs and capabilities. People’s benchmarking with peer groups facilitated by the wide use of social media may strengthen self-motivation and thereby also promote overall adherence thereby increasing both medical effectiveness and health economic sustainability of lifestyle interventions.
Drugs. Until today no drug has obtained marketing approval specifically for the treatment of NASH. Current pharmacological treatments of NAFLD aim at managing cardiovascular risk factors including obesity, dyslipidaemia, liver fat, hypertension and type 2 diabetes. Many established anti-diabetic drugs are currently under clinical investigation for their potential effectiveness in treating NAFLD and preventing the transition to NASH. Such drugs include metformin, dipeptidyl peptidase-4 (DPP-4) inhibitors, GLP-1 receptor agonists, SGLT-2 inhibitors, and thiazolidinediones (Barb et al., 2016). As the improvement of hepatic insulin sensitivity may be one of the key objectives in NAFLD treatment the design of insulin sensitisers should refer to a full understanding of the pathway specificity of hepatic and overall insulin resistance. In the elderly a slight insulin resistance may reflect an adaptive response protecting the ageing cells from the deleterious action of dysfunctional protein accumulation (Meijer & Codogno, 2007). Even more provocatively a cardioprotective role rather than the promotion of vascular disease has been attributed to insulin resistance (Khadori & Nguyen, 2012). Overall different semantics of insulin resistance should be addressed by the development of further advanced insulin sensitizers.
Also different antioxidants and supplements, lipid-lowering drugs, phosphodiesterase inhibitors, bile acids and farnesoid receptor pathway agents, weight loss medications, and angiotensin II receptor antagonists are currently under clinical investigation for their potential effectiveness in treating NAFLD and preventing NASH (Barb et al., 2016). In drug development an increased susceptibility of the fatty liver to potentially hepatotoxic compounds needs careful consideration.
The development of antifibrotic therapies is a current focus in experimental and translational hepatology. Today the identification of therapeutic targetes very much appreciates the dynamics in fibrogenesis and fibrolysis, i.e. the de novo formation and the removal of extracellular matrix and connective tissue (Mehal & Schuppan, 2016). There is a high diversity of fibrogenic and fibrolytic pathways whose activation depends on the pathogenic trigger and its primary target cells. Antifibrotic drug targets include the immune response, the regulation of platelet and endothelium function, and the extracellular matrix. Increased effectiveness is expected from the development of combination therapies interfering with different fibrogenic and/or fibrolytic pathways such as inflammation and extracellular matrix deposition, as well as from the consideration of the specific disease etiology. In the condition of fibrotic NASH insulin resistance and glycaemic control, lipotoxic hepatocyte death and intestinal dysbiosis provide rational targets for both anti-inflammatory and antifibrotic effectiveness (Mehal & Schuppan, 2016). To which extent even advanced liver cirrhosis could still be reversible is a current matter of discussion, as is also the cell and tissue culture and animal models to be used for the non- and preclinical characterisation of drug candidates (Trautwein et al., 2016). The ultimate goal is to increase efficacy in translating preclinical success stories into human safety, tolerability and antifibrotic effectiveness. Recently two antifibrotics for the treatment of pulmonary fibrosis (pirfenidone, nintedanib) have been approved by the American Food and Drug Administration and the European Medicines Agency (Hughes et al., 2016).
In line with the two hit model of NAFLD the design of trials depends on the expected action mode of the drug candidate under investigation. NAFLD trials could focus on fatty liver (first hit) resolution mainly aiming at reducing liver fat and NASH progression by preventing the generation of and reducing hepatic susceptibility to the second hit. Alternatively NAFLD trials could focus on the resolution of existing liver fibrosis or cirrhosis aiming at the regeneration of functional liver tissue.
Proof-of concept (phase I/IIa) trials with a focus on fatty liver resolution should be designed to assess a potential reduction in liver fat (primary endpoint) by magnetic resonance imaging (MRI)- or magnetic resonance spectroscopy (MRS). In order to capture a more comprehensive activity profile and to widen the base for decision making on further development of a drug candidate it is recommended to also monitor changes in insulin sensitivity of the liver and adipose tissue by using 2-step hyperinsulinaemic euglycaemic clamp combined with the infusion of stable isotope-labeled glucose and glycerol. Also a potential re-distribution of body fat (MRI/MRS), changes in body composition (e.g. air displacement plethysmography), and cardiovascular risk factors (e.g. assessment of the intima media thickness and flow-mediated dilatation by high-resolution vascular ultrasound, telemetric ECG and 24-hour blood pressure monitoring, lipid profiling) also deserve consideration. Depending on the non- and pre-clinical data set it could be meaningful to address hepatic ATP synthesis (MRS), energy expenditure (indirect calorimetry for assessment of resting energy expenditure and diet-induced thermogenesis, spiroergometry for the assessment of activity-induced energy expenditure, doubly labeled water method for the assessment of total energy expenditure), as well as adipose tissue inflammation and metabolic activity (histological and gene expression analyses in adipose tissue biopsies).
To economise trial conduct participants should be pre-selected by an abdominal ultrasound investigation or by using surrogates for the presence of a fatty liver like the fatty liver index or signs of the metabolic syndrome before performing MRI or MRS. In case the candidate drug is expected to improve early stages of NASH e.g. by ameliorating liver inflammation, preventing progression of liver fibrosis or even reversing liver fibrosis a pre-selection of trial participants for the presence of NASH by using non-invasive methods is recommended before sampling a liver biopsy for confirmation. Transient elastrography with controlled attenuation parameter would be appropriate for simultaneously addressing fatty liver and liver fibrosis in patients with NAFLD (Mikolasevic et al., 2016). A combination of transient elastrography with a panel of serum markers for hepatocyte apoptosis (e.g. cytokeratin-18 alone or combined with FGF-21) and/or liver fibrosis (e.g. SteatoTest) and/or with calculation of the NAFLD fibrosis score could increase the accuracy of NASH detection (Festi et al., 2015).
Although liver fibrosis has multiple etiologies NASH is considered to be the dominant disease indication for the development of antifibrotic drugs (Trautwein et al., 2016). Valuable insight about the human potential for fibrosis regression comes from the treatment of viral hepatitis. In patients with chronic hepatitis B a long-term antiviral treatment resulted in a reversal of liver fibrosis and cirrhosis with remarkable reductions in inflammation and necrosis after 1 year and improvements in fibrosis and even cirrhosis after approximately 5 years (Chang et al., 2010). In morbididly obese patients with NASH bariatric surgery led to a clinically relevant reduction in the NASH activity score after 1 year. The antifibrotic effect of bariatric surgery was less pronounced during this observation interval (Lassailly et al., 2015), suggesting the requirement for a longer-term follow-up in order to capture a continued fibrosis regression. For the design of antifibrotic drug trials this implies that a significant improvement in necroinflammation often occurs after 1 year whereas longer treatments are needed to establish antifibrotic effectiveness at the level of histologic classification.
The American Association for the Study of Liver Diseases (AASLD) recently sponsored an emerging trends conference on strategies and endpoints of antifibrotic drug trials (Torok et al., 2015). Regulatory authorities require demonstration of impact at the level of hard clinical endpoints which in anti-fibrotic drug trials would be the prevention of liver cirrhosis with their inherent risk of decompensation, hepatocellular carcinoma, or death. Optionally , the use of surrogate endpoints can be considered provided they are reasonably likely to reflect clinically relevant outcomes. Due to the diversity of pathogenetic traits individual forecasts about the progression of fatty liver to NASH with liver fibrosis and cirrhosis are difficult to provide. Therefore, in order to reduce sample size, proof-of concept clinical trials in NASH require optimal patient selection and stratification e.g. based on presence of a fatty liver, signs of the metabolic syndrome, and surrogates of hepatic inflammation. For the assessment of liver fibrosis progression or reversal patients with an intermediate stage of fibrosis should be pre-selected by using a non-invasive measure before taking a biopsy for confirmation which is still considered the gold standard for defining inclusion criteria and pharmacodynamic efficiency in antifibrotic drug trials (Mehal & Schuppan, 2015). On the other hand the limitations on the use of liver biopsies related to invasiveness of sampling, high sampling variability, and not indicating early changes in fibrogenic and fibrolytic activity are increasingly made a subject of debate (Trautwein et al., 2016).
Liver first? – Conclusion
Undoubtedly the liver plays a pivotal role in the pathogenesis of diabetes and its comorbid conditions – within and much beyond the glucocentric framework traditionally engaged when investigating the disease complex and evaluating anti-diabetic drugs and devices.
As discussed in part I of this blog the liver is a main contributor to the development of hyperglycaemia in (pre-)diabetic states. On the other hand homeostatic functions of the liver contribute to the maintenance of cell and tissue function in the presence of insulin resistance. In the 1970s increases of hepatic glucose production mounted by an increase in hepatic insulin resistance have been interpreted as an adaptive response stimulating beta cells to secrete additional insulin for overwriting overall insulin resistance thereby maintaining normal cell growth and tissue function at the expense of normoglycaemia. Most recently the existance of a liver-pancreas axis has been corroborated at the cutting edge of molecular research by the demonstration that proteins secreted by the insulin resistant liver account for the adaptation of beta cell mass and function to insulin resistant states. Even a total insulin resistance of the liver as artificially induced in the LIRKO mouse model does not lead to overt diabetes as this is prevented by a compensatory beta-cell hyperplasia triggered by proteins released from the insulin resistant liver. Only an impairment of the liver-pancreas axis due to a genetically reduced beta cell viability and/or proliferative capacity on top of total hepatic insulin resistance is required for the precipitation of overt diabetes in LIRKO mice.
In the pathogenesis of human type 2 diabetes a selective insulin resistance of the liver with increased production of glucose, free fatty acids and triglycerides mounts a fatty liver which is a potential source of systemic dyslipidaemia, inflammation and oxidative stress, which again may impair beta cell and cardiovascular function. Bidirectional interactions between fatty liver and the insulin-resistant, inflamed adipose tissue may further aggravate pathogenetic circles. In NASH pathways highly redundant to those triggering type 2 diabetes and cardiovascular disease account for a loss of liver function due to a progessive re-modelling of liver tissue eventually resulting in liver cirrhosis with an inherent risk of hepatocellular carcinoma and other comorbid conditions.
An in depth understanding of both the liver’s homeostatic role in healthy physiology and its prominent involvement in the pathogenesis of type 2 diabetes and cardiovascular disease is required for a rational development of anti-diabetic drugs effectively preventing NASH and cardiovascular disease. The increased susceptibility of the fatty liver to agents which are potentially hepatotoxic (second hit) requires special attention when establishing the safety and tolerability profile. The development of anti-fibrotic drugs will be greatly facilitated by the further validation of non invasive assessments of hepatic fibrogenesis and fibrolysis as well as the availability of biomarkers stratifying for people with a fatty liver and being at high risk for hepatic fibrogenesis and liver cirrhosis.
For more information, have a look at our free on-demand online seminar about NASH.