Targeting the Fibroblast Growth Factor Axis for NASH Therapy
Nonalcoholic Fatty Liver Disease (NAFLD)
NAFLD is characterized by fat accumulation in the liver due to increased de novo lipogenesis and impaired fatty acid oxidation and/or export. NAFLD can establish a state of chronic liver inflammation that advances to nonalcoholic steatohepatitis (NASH): scarring of liver tissue through progressive hepatic fibrosis. NASH can then advance to cirrhosis and liver cancer. NAFLD and NASH global prevalences are estimated at 25% and 3 to 5%, respectively.
The two lead contenders in the race for our first FDA approved NASH drug have undergone recent Phase 3 stumbles. The Agency determined that Intercept Pharmaceuticals’ obeticholic acid trial will require long-term trial analysis due to lipid abnormalities documented during Phase 2, and due to deaths in patients using it under approved use for a separate indication. Genfit’s elafibranor hit the burgeoning NASH rejects bin after failing to meet its early trial endpoints.
There is now renewed hope for later stage aspirants. In this post, I’ll take a look at a mechanistic axis targeted by several high profile Phase 2 candidates: endocrine regulation of liver metabolic functions through the FGF19 subclass of FGF hormones.
Fibroblast Growth Factors
FGFs are a superfamily of structurally similar proteins with autocrine, paracrine, and/or endocrine activity. Despite the name, not all can stimulate fibroblast activity. They play diverse roles in development, organogenesis, and metabolism. The 22 FGFs are distributed across seven subclasses by their phylogenetic similarity.
The FGF19 subclass, comprising FGF23, FGF19, and FGF21, are systemically circulating endocrine hormones. Like most FGFs, their hormonal activity is dependent on interaction with FGF receptor (FGFR) proteins on target cell surfaces; these are tyrosine kinase enzymes that initiate downstream signaling cascades.
There are four FGFR family members, but FGFR1-3 have splice variants, leading to many different sub-types. The FGF19 subclass has the additional requirement of complexing with transmembrane Klotho proteins to engage with FGFRs: βKlotho for FGF19/21 and αKlotho for FGF23. A tissue’s distribution of FGFR subtypes and Klotho proteins is what determines which signaling pathways FGFs might activate in that tissue.
Preclinical Identification of FGF19 and FGF21 and Their Effects on Liver Metabolism
The endocrine characterizations and biomedical adaptations of FGF19 and FGF21 have occurred over relatively recent timelines, as these factors were only discovered in 1999 and 2000, respectively. Preclinical studies clearly confirmed both play key roles in maintaining body weight and regulating carbohydrate and lipid homeostasis in adult mammals.
Mice knocked out for Fgf15 (the syntenic ortholog of human FGF19) are glucose intolerant and defective in hepatic glucose storage. Administration of human FGF19 to diet-induced obese and leptin deficient mice can reverse their diabetes. FGF19 transgenic mice undergo increased energy expenditure without decreased food uptake, leading to reduced adiposity, circulating lipids and cholesterol, and hepatic triglycerides.
The metabolic activity of FGF21 was discovered in a cell-based assay where it stimulated glucose uptake by adipocytes. The same study showed it reduced plasma glucose and triglycerides to near normal levels in obese, leptin-deficient mice. Other rodent studies showed FGF21 is induced directly by peroxisome proliferator-activated receptor α (PPARα) in the liver in response to fasting, and induces hepatic lipid oxidation, triglyceride clearance, glycolysis, and ketogenesis. Exogenous application of FGF21 can induce similar activity in rodent models of fatty liver disease.
Interestingly, rodent models also suggest FGF21 may not directly activate hepatic metabolism. It penetrates the blood brain barrier and triggers corticosterone release in the hypothalamus which in turn drives hepatic gluconeogenesis. This route of action also been shown to increase andrenergic signaling to promote browning of white adipose tissue.
Current Understanding of Roles of FGF19 and FGF21 in Liver Metabolism
Further research into FGF19 revealed that this hormone is postprandially produced by ileal epithelial cells. Bile acids travel from the gallbladder to the intestine to aid in lipid digestion, and also activate the farnesoid X receptor, a key regulator of intestinal FGF19. Secreted FGF19 travels back to the liver to engage FGFR4, leading to repression of the cholesterol 7α-hydrolase gene CYP7A1. CYP7A1 is the first rate-limiting step in bile acid synthesis. FGF19 also promotes relaxation of the constricted gallbladder, inducing refilling. Therefore, FGF19 acts as a feedback regulator of bile acid synthesis. This is a crucial facet of liver health maintenance, as bile acids have inherent toxicity.
The antihepatoxic action of FGF19 is the primary mechanistic rationale for designing NASH therapeutics around its biology, but it presents additional benefits related to obesity and diabetes. Through FGFR4, FGF19 promotes hepatic glucose uptake and glycogen synthesis independently of insulin, while repressing hepatic gluconeogenesis. There is also evidence that FGF19 acts through FGFR1 in adipose tissue to mediate effects similar to FGF21, as discussed below.
The therapeutic potential of FGF19-derived treatments may be offset by its effects on hepatocyte proliferation. FGF19 transgenic mice develop hepatocellular carcinomas. This may be overcome by replacing two short regions of its sequence to abolish its proliferative effects while retaining its metabolism regulating abilities.
FGF21 is more broadly expressed than FGF19, and has more complex metabolic effects. It acts through FGFR1 to mediate direct autocrine effects on adipose tissue metabolism, and indirect effects on liver metabolism and adipose tissue physiology via the central nervous system. It is predominantly synthesized by liver in response to fasting, ketogenic diet, diabetes, and obesity. It is also made by the pancreas, skeletal muscle, adipose tissues, and cardiac endothelium.
Elevated serum FGF21 levels are found in overweight/obese individiuals and in patients with NAFLD, suggesting it plays a role in the adaptive response to excess caloric burden. The strategies driving FGF21 research for NASH therapeutics are more holistic than for FGF19. It relies on coordinating whole-body fat use and energy expenditure to redirect calories away from the liver, as well as directly conversion of existing stores of liver fat.
For example, FGF21 can drive beiging and thermogenesis in adipose tissue as a mechanism to dispose of disproportionately high levels of carbohydrate in a protein restricted diet. FGF21 can also improve blood sugar levels by driving uptake of glucose by adipocytes and hepatocytes through the GLUT1 transporter. Then, through the hypothalamic-pituitary-adrenal axis, FGF21 of hepatic origin loops back to induce gluconeogenesis, ketogenesis, and fatty acid oxidation in the liver.
FGF21 has not demonstrated mitogenic activity and is not considered carcinogenic. However, native FGF21 has a tendency to aggregate and is also subject to rapid proteolytic degradation, giving it an undesirable pharmacokinetic profile. Recombinant versions or conjugates have overcome this limitation, and are discussed below.
Translating the Effects of FGF19 and FGF21: Drugs in Clinical Trials
There are numerous candidates in clinical development for NASH which exert their therapeutic effects by stimulation of FGF21 signaling responses to improve glucose and lipid homeostasis, induce weight loss, enhance insulin sensitivity, and/or resolve NAFLD/NASH.
- Akero Therapeutics’ AKR-100 is a FGF21 protein fusion to human immunoglobulin 1 Fc region for improved pharmacology. Further, the FGF21 Cterminal region has two amino acid substitutions that reduce proteolytic degradation and increase affinity for βKlotho, enhancing potency and duration of action. Akero recently reported 12 week efficacy data in a Phase 2a NASH study evaluating weekly doses of 28 to 70mg (BALANCED). These included achieving the key endpoints of reductions in liver fat as measured by noninvasive MRI (12.3 to 14.1%; 0.3% for placebo) and highly significant decreases in alanine aminotransferase (ALT), a surrogate serum marker of liver damage. Biopsy data were expected in Q2 but may be inaccessible due to the COVID19 pandemic. AKR-100 also improved numerous pharmacodynamic (PD) endpoints in a multidose Phase 1 diabetes trial, including fasting glucose, insulin sensitivity, and cholesterol and triglyceride levels.
- Bristol-Myers Squibb has met or will meet 24 week primary completion dates for pegbelfermin, a pegylated FGF21 analog, in a Phase 2b NASH/Stage 3 fibrosis trial (FALCON 1) and a Phase 2b NASH/cirrhosis trial (FALCON 2). In an earlier Phase 2a NASH trial, 16 weekly doses of 20mg pegbelfermin achieved significant reduction in absolute hepatic fat fraction. Daily dosing, however, was required to improve serum cholesterol and triglycerides in an earlier Phase 2 diabetes trial.
- 89bio recently completed enrollment for a Phase 1b/2a, 12 week, escalating single dose trial in NAFLD/NASH subjects. Reduction of hepatic fat will be a key 7 week endpoint. Their candidate, BIO89-100, is a glycopegylated FGF21 analog. 89bio has presented Phase 1 data for healthy subjects, showing lasting improvements (8 to 15 days) for serum triglycerides and cholesterol at a dose as low as 9.1mg.
- Roche has completed a Phase 1b trial for NAFLD patients to evaluate BFKB8488A. This bispecific antibody binds to both FGFR1c and βKlotho to mimic FGF21 stimulation, with favorable effects on cholesterol, triglycerides, ALT, and hepatic fat fraction. The antibody is also being evaluated in a Phase 1b trial for patients with type 2 diabetes.
- Merck has exercised its option to license MK-3655 from NGM Pharmaceuticals. This a monoclonal antibody that activates the βKlotho/FGFR1c complex. NGM had completed evaluation in a Phase 1b obesity trial where a single dose of 240mg with concomitant use of daily pioglitazone showed favorable effects on insulin sensitivity, blood glucose, serum liver markers, serum lipid profile, and liver fat, over pioglitazone alone.
- Having cashed out early from the FGF21 clique, NGM Biopharmaceuticals is moving forward with aldafermin, a FGF19 analog. This is an engineered version that retains bile acid regulatory activity but is nontumorigenic, and even antagonizes the oncogenic activity of native FGF19. Aldafermin does however increase serum cholesterol by inhibiting CYP7A1, a side effect that can be mitigated with coadministration of a statin drug. Aldafermin improved fat content in 12 weeks and liver fibrosis and histology in 24 weeks in a Phase 2 NASH trial. NASH resolution was particularly impressive as all parameters in the NAFLD activity score (NAS) where improved; 22% of patients exhibited signs of both NAS and fibrosis improvement. Another Phase 2b NASH trial is in progress with an estimated completion date in Q2 2021.
Research into fibroblast growth factors 19 and 21 has uncovered appealing mechanistic axes for ameliorating steatosis and fibrosis in NAFLD and NASH. These may provide broader benefits for obesity and diabetes, including reduction of whole body fat and improved glucose homeostasis.
The field of Phase 3 aspirants is becoming increasingly crowded, but none have fully graduated from Phase 2 yet, and COVID19 is complicating enrollment for future clinical trials. This should give all contenders an opportunity to improve their preclinical data packages.
The FGF21 mechanistic route seems to be the overwhelmingly popular approach. But don’t rule out aldafermin, our lone candidate traveling the FGF19 route: it is as advanced as any of its rivals, and may steal the show if Phase 3 uncovers pervasive hurdles for FGF21 dependent efficacy.