TNF-α Inhibition Decreases Risk of Alzheimer’s Disease
On June 4, 2019, the Washington Post published a story describing how researchers at Pfizer made an interesting observation about the arthritis drug Enbrel® in 2015 showing that the drug reduced the risk of developing Alzheimer’s disease (AD) by 64%. Enbrel® is a soluble tumor necrosis factor (TNF)-alpha receptor that is designed to bind TNF-α and block its interaction with cell surface TNF receptors. It is currently approved for the treatment of rheumatoid arthritis, psoriatic arthritis, polyarticular juvenile idiopathic arthritis, ankylosing spondylitis, and plaque psoriasis. It 2018 it generated worldwide revenues of $7.4 billion (EvaluatePharma), however its patents outside of the U.S. expired in Aug. 2015. Its patent protection in the U.S. is scheduled to expire in the U.S. in Apr. 2029, but that is subject to change pending the outcome of ongoing litigation.
The results showing a decreased risk of AD in patients taking Enbrel® was based on an analysis by Pfizer statisticians of 254,000 insurance claims involving individuals with rheumatoid arthritis (RA) and other inflammatory diseases. The cohort was divided into two groups: those who received an Alzheimer’s diagnosis and those that did not. There were 302 individuals treated with Enbrel® in the group that did not receive an Alzheimer’s diagnosis compared to only 110 treated with Enbrel® in the group with an Alzheimer’s diagnosis. Interestingly, the same effect was seen in another insurance database when examined by Pfizer and similar results were seen with Humira®, an anti-TNF-α monoclonal antibody.
Pfizer’s results are also similar to what was seen in a 2016 analysis of 8.5 million insured adults in the Verisk Health claims database (Chou et al., 2016). The results from that study showed that AD was more prevalent among RA patients (0.79%) than those without RA (0.11%) and exposure to anti-TNF agents as a class, but not other immunosuppressive drugs, was associated with a lower risk of AD among RA patients (P=0.02). However, a subgroup analysis showed that only Enbrel®, but not other anti-TNF agents, was associated with a decreased risk of AD in RA patients (P=0.02).
The association between inflammation and AD has been known for a number of years (Akiyama et al., 2000), however it has been overlooked as a potential means of treating and preventing AD due to the focus on amyloid beta. A multitude of inflammatory markers are found in AD patients’ brains and a number of studies have shown a link between chronic inflammation and an increased risk of developing AD (Walker et al., 2017; Tao et al., 2018). Neuroinflammation is mediated by microglia cells, resident phagocytes of the central nervous system (CNS), which are the major source of cytokines in AD, particularly TNF.
A number of preclinical studies have shown an association between inflammation and AD pathology in several animal models of AD, including APPV717F mice (Qiao et al., 2001), APPSwe Tg2576 mice (Sheng et al., 2003), and 3xTgAD mice (Kitazawa et al., 2005). An elevation in TNF mRNA is seen in 3xTgAD mice prior to the appearance of AD pathology (Janelsins et al., 2005) and is correlated with cognitive deficits (Billings et al., 2005). Those results led to a number of experiments that examined the inhibition of TNF in preventing AD pathology. In 3xTgAD mice, inhibition of sTNF with XPro1595 (INmune Bio’s TNF-α inhibitor, discussed below) prevented inflammation-induced accumulation of C-terminal amyloid beta protein (McAlpine et al., 2009). In TgCRND3 amyloid transgenic mice, treatment with XPro1595 for four weeks starting at age 1 month prevented synaptic deficits at age 6 months (Cavanagh et al., 2016).
An association between TNF and AD is also seen in the clinic. Elevated levels of TNF are found in the serum of AD patients (Fillit et al., 1991), a TNF promoter polymorphism was found to be associated with AD (Ma et al., 2004), elevated TNF serum levels are associated with an increased risk of conversion from mild cognitive impairment (MCI) to AD (Tarkowski et al., 2003), and a small, open label pilot study of Enbrel® in patients with mild-to-severe AD showed improvement in a number of AD outcomes (Tobinick et al., 2006). Additional studies involving Enbrel® in AD patients includes a small study showing an improvement in cognition in AD patients with RA (Chen et al., 2010) and a more recent study that in 41 patients with mild to moderate AD that showed some interesting trends, however no statistically significant changes in cognition, behavior, or global function (Butchart et al., 2015). In total, we believe there is ample rationale for investigating therapeutics that target TNF for the treatment of AD.
TNF is an inflammatory cytokine that is known to play an important role in inflammation and immunity (Aggarwal, 2003). It is a 26 kDa transmembrane protein that is expressed on the cell surface (known as transmembrane TNF, tmTNF) and can be cleaved by TNFα-converting enzyme to produce a 17 kDa trimeric soluble TNF (sTNF), which is found circulating in the bloodstream.
Both tmTNF and sTNF bind to two transmembrane receptor molecules, TNFR1 and TNFR2 (Tartaglia et al., 1991). While most cells express TNFR1, TNFR2 is predominantly expressed on cells of the immune system. TNF exists as a trimer in its active form, in which it is capable of binding to its receptors and exerting a number of broad-spectrum effects including cell proliferation, differentiation, and apoptosis (Baud et al., 2001). sTNF has also been implicated in a number of indications including inflammation, infections, and cancer. Studies with knockout mice have shown that the pro-inflammatory actions of sTNF are mostly mediated through binding of TNFR1 (Bradley, 2008). In contrast, tmTNF exerts most of its biological activities through binding to TNFR2, including T cell activation, proliferation, growth factor production, and the host defense to infections (Grell et al., 1995). In summary, sTNF is predominantly responsible for inflammatory responses while tmTNF is predominantly responsible for immune-mediated responses.
XPro1595 is designed to inhibit sTNF signaling while not impacting tmTNF signaling. It is an engineered protein that is nearly identical to sTNF except for two point mutations in the coding sequence and the addition of polyethylene glycol (PEG) to increase its half-life in circulation (Steed et al., 2003). The following figure shows the crystal structure of XPro1595, the location of the two altered amino acids, and the PEG site.
The mechanism of action for XPro1595 is predicated on the fact that sTNF forms homotrimers in order to bind to its receptors. XPro1595 is capable of forming heterotrimers with sTNF (a combination of 1 sTNF + 2 XPro1595 or 2 sTNF + 1 XPro1595), and the heterotrimers that form are incapable of binding to TNFR1 or TNFR2. This is depicted in the cartoon below. The end result is that sTNF is sequestered and prevented from binding to TNFR, thus decreasing pro-inflammatory signals.
Importantly, studies show that XPro1595 inhibits sTNF signaling without interfering with tmTNF signaling (Zalevsky et al., 2007). The following figures show the effect of increasing concentration of anti-TNF molecules on TNF-induced caspase activity for both sTNF (lower left) and tmTNF (lower right). XPro1595 failed to inhibit tmTNF even at >100 μg/ml, which is approximately 10,000-fold higher than the dose that inhibits sTNF.
As discussed previously, the use of anti-TNF therapy can lead to immunosuppression and an increased risk of infection. The following graph shows that in contrast to Enbrel® (etanercept), XPro1595 (labeled as XENP1595) did not increase susceptibility to Listeria in a mouse model of infection. All mice treated with Enbrel® succumbed to Listeria within eight days, while no mice treated with XPro1595 died. Thus, XPro1595 affects sTNF signaling but does not affect tmTNF signaling, and thus does not alter the TNF-induced response to infection.
$1 Million Grant to Support Phase 1 Trial
In order to advance XPro1595 in the treatment of AD, INmune (NASDAQ:INMB) was recently awarded a $1 million grant from the Alzheimer’s Association. The award derives from the Part the Cloud grant, which is intended to accelerate novel preclinical research findings into clinical testing. The grant will support a biomarker-directed Phase 1 clinical trial in 18 patients with mild-to-moderate AD. XPro1595 will be administered subcutaneously once a week for three months and biomarkers of inflammation will be assessed at 0, 6, and 12 weeks. The primary endpoints for the trial are safety and a decrease in inflammation as measured in the blood, cerebrospinal fluid (CSF), the brain and the breath. Secondary endpoints include measures of cognition, psychiatric symptoms, and quality of life, however since the trial is only 12 weeks it is doubtful cognitive benefits will be seen. We anticipate this trial initiating in the summer of 2019.
While the Washington Post article tried to paint the executives at Pfizer as irresponsible for not following up on the potential for Enbrel® to be used in AD, we believe the more salient point of the article is that another research group saw an association between inhibiting TNF-α and a decreased risk of AD, adding to what we believe is already ample evidence to support investigating the effect of inhibiting TNF in treating and/or preventing the disease. While current TNF-α inhibitors are likely not the best candidates for treating AD, given the increased risk of infection and other potentially serious adverse side effects, XPro1595 could potentially mitigate the inflammatory effects of TNF-α while leaving the response to infection intact. We look forward to the upcoming Phase 1 clinical trial of XPro1595 in patients with mild-to-moderate AD and the potential for a new paradigm in AD research. Our current valuation for INmune remains at $19 per share.
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