Ghosh A, Comerota MM, Wan D, Chen F, Propson NE, Hwang SH, Hammock BD, Zheng H. An epoxide hydrolase inhibitor reduces neuroinflammation in a mouse model of Alzheimer's disease. Sci Transl Med. 2020 Dec 9;12(573) PubMed.

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  1. The study by Ghosh et al. follows a line of research that started in the early ’90s. It showed that astrocytes are a major source of production of epoxyeicosatrienoates (EETs) in the brain (Davis et al., 2017). EETs are metabolites of arachidonic acid (AA) produced by cytochrome P450 epoxygenase. In acute brain injury such as that which occurs in stroke and traumatic brain injury (TBI), EET’s precursor AA is released from plasma membranes, where it resides as part of membrane phospholipids. AA is subsequently metabolized by multiple enzymes into various eicosanoids that could be detrimental or protective. In chronic neurodegenerative diseases such as Alzheimer's disease and vascular cognitive impairment (VCI), neuroinflammatory signals alter activities of eicosanoid synthetic and metabolizing enzymes, disrupting the balance between neuroprotective and neurotoxic eicosanoids (Zhang et al., 2007). 

    EETs have been shown to have neuroprotective properties via multiple mechanisms, including vasodilation and anti-inflammation. They have been shown to be protective in stroke (Chen et al., 2020), AD (Liu et al., 2018), and VCI (Pardeshi et al., 2019; Nelson et al., 2014). The levels of EETs are regulated by synthesis and metabolism, and a key enzyme involved in EET’s metabolism is soluble epoxide hydrolase (sEH). In VCI, the expression and activity of sEH have been reported to be increased in cerebrovascular endothelium in postmortem human brain tissue, and genetic polymorphisms have been linked to white-matter hyperintensity (WMH) burden, an early marker and predictor of VCI (Liang et al., 2019). 

    Inhibition and deletion of sEH have been explored as a therapeutic strategy to increase endogenous EETs bioavailability in multiple brain diseases, especially stroke (Liang et al., 2019). More recently, several studies have demonstrated potential beneficial effects of sEH inhibition in cell-based (Lee et al., 2019) and animal models (Lee et al., 2019; Griñán-Ferré et al., 2020; Tu et al., 2018) of AD.

    There have been some clinical trials using at least two different sEH inhibitors: AR9281 was tested in hypertension, but it did not show efficacy (NCT00847899), and GSK2256294 is currently being tested in subarachnoid hemorrhage (NCT03486223) and insulin resistance (NCT03486223). The sEH inhibitor used in the study by Ghosh et al. (TPPU) is not approved for clinical use, but it has been used in experimental models of stroke (Tu et al., 2018), VCI (Griñán-Ferré et al., 2020), and AD (Lee et al., 2019). A third compound, EC5026, also used in the study by Ghosh et al., has completed a Phase Ia safety trial (NCT04228302).

    The current study shows that sEH is elevated in postmortem brain tissue from patients with AD and in mouse models of AD, and that long-term sEH inhibition reduces neuroinflammation and Aβ pathology, and improves synaptic integrity and cognitive function in the 5xFAD mouse model of AD. Although confirmatory to previous studies, the study is well-designed and -conducted. Another interesting aspect of the study is its focus on neuroinflammatory mechanisms, showing that astrocytes are the cellular source of sEH and EETs, which then act in autocrine fashion to suppress microglial activation.

    Some aspects of the study were not fully explored. For example, in addition to sEH upregulation, the authors found increased expression of PLA2 in postmortem AD human brains, but the mechanisms of upregulation of sEH and PLA2 remain unclear. Since sEH upregulation in this study starts at 2 months, which is around the same time astrogliosis and microgliosis begin in this animal model, it would be important to determine which comes first; i.e., whether inflammatory signals induce sEH expression, or whether sEH upregulation leads to neuroinflammation, and how.

    PLA2 upregulation suggests a more generalized alteration in eicosanoid signaling than just EETs/sEH signaling. This is further suggested by the authors’ observation regarding expression of both COX2 and CYP4F8 genes, which produce prostaglandins and hydroxyeicosanoids (such as 20-hydroxyeicosatetraenoate, 20-HETE), respectively. Prostaglandins and 20-HETE are important vasoactive and inflammatory signals, but their role in this study and their potential interactions with EETs/sEH signaling have not been investigated.

    Despite the promising results, chronic administration of sEH inhibitors in humans should proceed with caution, especially as a treatment for AD. In a recent study of global cerebral ischemia induced by cardiac arrest in piglets, the same sEH inhibitor TPPU led to neuronal abnormality in sham-treated animals, with neuronal cell body attrition and nuclear condensation. Furthermore, TPPU was ineffective in protecting neurons and reducing neurologic deficit after global cerebral ischemia (O'Brien et al., 2020). 

    This is reminiscent of another study of cardiac arrest in mice (Hutchens et al., 2008), where sEH knockout mice had greater mortality compared to wild-type mice, raising the possibility that sEH inhibition may interfere with other organ functions (e.g., cardiovascular or pulmonary) required for recovery after cardiac arrest. Although sEH has been extensively investigated as a therapeutic target, its physiological function remains unknown, and therefore, chronic inhibition of sEH may interfere with its function and cause unwanted side effects. Of particular concern in chronic administration of sEH inhibitors is tumorigenesis, given the growth-promoting potential of EETs (Panigrahy et al., 2011; Panigrahy et al., 2012). 

    References:

    Alkayed NJ, Narayanan J, Gebremedhin D, Medhora M, Roman RJ, Harder DR. Molecular characterization of an arachidonic acid epoxygenase in rat brain astrocytes. Stroke. 1996 May;27(5):971-9. PubMed.

    Davis CM, Liu X, Alkayed NJ. Cytochrome P450 eicosanoids in cerebrovascular function and disease. Pharmacol Ther. 2017 Nov;179:31-46. Epub 2017 May 18 PubMed.

    Zhang W, Koerner IP, Noppens R, Grafe M, Tsai HJ, Morisseau C, Luria A, Hammock BD, Falck JR, Alkayed NJ. Soluble epoxide hydrolase: a novel therapeutic target in stroke. J Cereb Blood Flow Metab. 2007 Dec;27(12):1931-40. Epub 2007 Apr 18 PubMed.

    Chen W, Wang M, Zhu M, Xiong W, Qin X, Zhu X. 14,15-Epoxyeicosatrienoic Acid Alleviates Pathology in a Mouse Model of Alzheimer's Disease. J Neurosci. 2020 Oct 14;40(42):8188-8203. Epub 2020 Sep 24 PubMed.

    Liu X, Davis CM, Alkayed NJ. P450 Eicosanoids and Reactive Oxygen Species Interplay in Brain Injury and Neuroprotection. Antioxid Redox Signal. 2018 Apr 1;28(10):987-1007. Epub 2017 Apr 20 PubMed.

    Pardeshi R, Bolshette N, Gadhave K, Arfeen M, Ahmed S, Jamwal R, Hammock BD, Lahkar M, Goswami SK. Docosahexaenoic Acid Increases the Potency of Soluble Epoxide Hydrolase Inhibitor in Alleviating Streptozotocin-Induced Alzheimer's Disease-Like Complications of Diabetes. Front Pharmacol. 2019;10:288. Epub 2019 Apr 24 PubMed.

    Nelson JW, Young JM, Borkar RN, Woltjer RL, Quinn JF, Silbert LC, Grafe MR, Alkayed NJ. Role of soluble epoxide hydrolase in age-related vascular cognitive decline. Prostaglandins Other Lipid Mediat. 2014 Oct;113-115:30-7. Epub 2014 Sep 30 PubMed.

    Liang Z, Zhang B, Xu M, Morisseau C, Hwang SH, Hammock BD, Li QX. 1-Trifluoromethoxyphenyl-3-(1-propionylpiperidin-4-yl) Urea, a Selective and Potent Dual Inhibitor of Soluble Epoxide Hydrolase and p38 Kinase Intervenes in Alzheimer's Signaling in Human Nerve Cells. ACS Chem Neurosci. 2019 Sep 18;10(9):4018-4030. Epub 2019 Aug 19 PubMed.

    Lee HT, Lee KI, Chen CH, Lee TS. Genetic deletion of soluble epoxide hydrolase delays the progression of Alzheimer's disease. J Neuroinflammation. 2019 Dec 17;16(1):267. PubMed.

    Griñán-Ferré C, Codony S, Pujol E, Yang J, Leiva R, Escolano C, Puigoriol-Illamola D, Companys-Alemany J, Corpas R, Sanfeliu C, Pérez B, Loza MI, Brea J, Morisseau C, Hammock BD, Vázquez S, Pallàs M, Galdeano C. Pharmacological Inhibition of Soluble Epoxide Hydrolase as a New Therapy for Alzheimer's Disease. Neurotherapeutics. 2020 Jun 2; PubMed.

    Tu R, Armstrong J, Lee KS, Hammock BD, Sapirstein A, Koehler RC. Soluble epoxide hydrolase inhibition decreases reperfusion injury after focal cerebral ischemia. Sci Rep. 2018 Mar 27;8(1):5279. PubMed.

    O'Brien CE, Santos PT, Kulikowicz E, Lee JK, Koehler RC, Martin LJ. Neurologic effects of short-term treatment with a soluble epoxide hydrolase inhibitor after cardiac arrest in pediatric swine. BMC Neurosci. 2020 Oct 31;21(1):43. PubMed.

    Hutchens MP, Nakano T, Dunlap J, Traystman RJ, Hurn PD, Alkayed NJ. Soluble epoxide hydrolase gene deletion reduces survival after cardiac arrest and cardiopulmonary resuscitation. Resuscitation. 2008 Jan;76(1):89-94. Epub 2007 Aug 28 PubMed.

    Panigrahy D, Greene ER, Pozzi A, Wang DW, Zeldin DC. EET signaling in cancer. Cancer Metastasis Rev. 2011 Dec;30(3-4):525-40. PubMed.

    Panigrahy D, Edin ML, Lee CR, Huang S, Bielenberg DR, Butterfield CE, Barnés CM, Mammoto A, Mammoto T, Luria A, Benny O, Chaponis DM, Dudley AC, Greene ER, Vergilio JA, Pietramaggiori G, Scherer-Pietramaggiori SS, Short SM, Seth M, Lih FB, Tomer KB, Yang J, Schwendener RA, Hammock BD, Falck JR, Manthati VL, Ingber DE, Kaipainen A, D'Amore PA, Kieran MW, Zeldin DC. Epoxyeicosanoids stimulate multiorgan metastasis and tumor dormancy escape in mice. J Clin Invest. 2012 Jan;122(1):178-91. Epub 2011 Dec 19 PubMed.

    View all comments by Nabil Alkayed

  2. Some sEH inhibitors are in clinical development. Arete Therapeutics took a compound from my laboratory through Phase IIa trials targeting hypertension and diabetes. We learned that this was a poor choice of clinical path and a poor clinical candidate. The sEH inhibitors can work on these targets, but there are competing compounds on the market for these indications that are quite effective. In addition to very rapid metabolism, this compound was reported to not get into the brain.

    GSK developed an excellent sEH inhibitor with good pharmacokinetics and efficacy. They targeted COPD, but the compound was dropped for several reasons. Among them was a regulatory decision of wanting to see proof of life span extension—a high bar for a clinical trial. Oregon Health Sciences University has gotten excellent results in a small human clinical trial on stroke with the GSK compound. These data were just announced last month. I would expect that brain penetration would be poor. Possibly if the compound is potent enough it will preserve the epoxy fatty acids in the periphery sufficiently to have a positive CNS effect.  

    EicOsis Human Health, a company I founded, is developing EC5026 on a clinical path to treat chronic diabetic neuropathy and arthritic pain. One goal is to provide an effective alternative to nonsteroidal and opioid analgesics. EicOsis is developing a related compound for treating arthritis in dogs, pain in cats, and laminitis (neuropathic pain) in horses. As a small company, EicOsis must be very focused on pain.

    My academic laboratory, and numerous friends and collaborators around the world, are looking at sEH inhibitors as probes to understand many diseases and as tools to potentially treat diseases. A key action of the compounds is to shift the endoplasmic reticulum stress pathway away from cell senescence and severe inflammation and toward cell survival and resolution of inflammation. Hence the sEH inhibitors act on many chronic diseases ranging from heart failure and fibrosis to diseases of aging and, in particular, those involving chronic neuroinflammation such as Parkinson's, autism, depression, schizophrenia, etc. Certainly neuroinflammation and ER stress contribute to Alzheimer's disease.  

    Regarding the question of the safety profile of EC5026 and other sEH inhibitors in clinical trials so far, one can never prove a drug safe so we are working quite hard to prove sEH inhibitors to be unsafe. So far we have failed in this. Our studies needed to obtain investigational new drug status from the FDA showed a high safety margin in animals. In human phase 1a single ascending dose studies, there were no adverse effects reported at doses far above those expected to be therapeutic. Although we have FDA "fast track status," there are many more steps needed to adequately test safety in humans.  

    Regarding inhibiting soluble epoxide hydrolase in Alzheimer’s disease patients, to date, none of our preclinical data suggest downsides to the use of the drug. Worries are, of course, that the drug is stabilizing fundamental biology and resolving inflammation. Inflammation in moderation is a beneficial and critical biological process. The sEH inhibitors appear to resolve deleterious inflammation rather than being anti-inflammatory. Thus we hope that they will be beneficial in diseases involving neuroinflammation. Another concern is that treatment for diseases such as AD will have to be chronic, and some of the patients will be fragile due to age and comorbidities. That said, based on the work from Kenji Hashimoto's laboratory at Chiba University on ER stress in inflammation in the central nervous system, and now the effort of many other investigators, I am optimistic about the value of the sEH inhibitors alone or in combination with other drugs in AD therapy and other chronic diseases associated with aging.

    Hashimoto is an expert on chronic CNS diseases; some of his students work on AD. He has shown that sEH inhibitors can reverse and prevent a number of chronic CNS disease in rodent models including autism, Parkinson's, Lewy body disorder, schizophrenia, bipolar, and others. In each case, he showed neuroinflammation and ER stress were critical to the disease. He showed the expected biomarkers, including the sEH protein, went up in inflammation and the natural inflammation-resolving chemical mediators went down in the appropriate brain regions for the disease, and in animals and, in some cases, in human pluripotent stem cells. He also showed that the inflammation-resolving epoxyfatty acids were down in cadaveric human samples of chronic CNS disease. We hope to take the same approach with AD (Hashimoto et al., 2019; Atone et al., 2020).

    References:

    Hashimoto K. Role of Soluble Epoxide Hydrolase in Metabolism of PUFAs in Psychiatric and Neurological Disorders. Front Pharmacol. 2019;10:36. Epub 2019 Jan 30 PubMed.

    Atone J, Wagner K, Hashimoto K, Hammock BD. Cytochrome P450 derived epoxidized fatty acids as a therapeutic tool against neuroinflammatory diseases. Prostaglandins Other Lipid Mediat. 2020 Apr;147:106385. Epub 2019 Nov 5 PubMed.

    View all comments by Bruce Hammock

  3. Drugs developed for the treatment of Alzheimer’s’ disease have had very limited success, to say the least. This study examining a soluble epoxide hydrolase (sEH) inhibitor is a novel and welcome approach for the field.

    The brain is abundant in arachidonic acid, which regulates many functions including inflammation, which, in turn, is thought to contribute to AD. While most famous for its pro-inflammatory products, arachidonic acid can also be converted to epoxy fatty acids. They play important anti-inflammatory roles, keeping balance with the classical products of arachidonic acid.  

    However, the enzyme sEH rapidly degrades the anti-inflammatory epoxy fatty acids, limiting their ability to counteract inflammation.  By adding an sEH inhibitor drug called TTPU orally in mice that have elevated β-amyloid, the team demonstrated that, as expected, epoxy fatty acids are increased in the brain, but also that β-amyloid was reduced as well as markers and neuroinflammation leading to improved synaptic integrity and cognitive behavior scores in the mice. 

    A surprising and interesting finding is that while the study largely focused on mice, the authors also demonstrated that sEH was elevated in postmortem Alzheimer brains while epoxy fatty acids were lower. Thus, it appears that sEH might be altered in Alzheimer’s and could be a new target for drugs. Clearly, and especially given that other drugs that worked in animal models subsequently failed in human trials, we should not over-extrapolate the current results. However, the study identifies novel testable hypotheses for people with Alzheimer’s. 

    This is particularly exciting as several sEH inhibitors have produced promising results in Phase 1 human studies. They could be tested in similar animal models to see if they can enter the brain and recapitulate the results observed in the current study with TPPU as we wait for TPPU to enter Phase 1 trials.

    View all comments by Richard Bazinet

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