Calming the (Cytokine) Storm: Dimethyl Fumarate as a Therapeutic Candidate for COVID-19

COVID-19 has rapidly spread worldwide and incidences of hospitalisation from respiratory distress are significant. While a vaccine is in the pipeline, there is urgency for therapeutic options to address the immune dysregulation, hyperinflammation and oxidative stress that can lead to death. Given the shared pathogenesis of severe cases of COVID-19 with aspects of multiple sclerosis and psoriasis, we propose dimethyl fumarate as a viable treatment option. Currently approved for multiple sclerosis and psoriasis, dimethyl fumarate is an immunomodulatory, anti-inflammatory and anti-oxidative drug that could be rapidly implemented into the clinic to calm the cytokine storm which drives severe COVID-19.


Introduction
The severe acute respiratory syndrome coronavirus type-2 (SARS-CoV-2) is responsible for the COVID-19 pandemic. Transmission, infection and mortality rates are significant (particularly compared to seasonal influenza) indicating the immediate need for a vaccine. Since it is currently unclear when an effective vaccine will be widely available and there are no standard-of-care treatment options for COVID-19, there is high unmet clinical need for therapeutics that can rapidly translate to improve patient care and reduce mortality rates.
Clinical presentation of COVID-19 varies but typical symptomology includes fever, cough and fatigue. In most cases hospitalisation is not required; however, for the elderly and those with co-morbidities (i.e., diabetes, cardiovascular disease, obesity, respiratory disease), the likelihood of hospitalisation (and mortality) increases significantly [1,2]. Severe cases usually present with acute respiratory distress syndrome (ARDS), the result of dysregulated host immune response to the virus [3]. The outcome of this immunodysregulation, which may be driven by underlying inflammation associated with age and co-morbidities, is a cytokine storm, i.e., elevated interferon (IFN), interleukins (IL), tumour necrosis factor-α (TNF-α), amongst others. The cytokine storm potentiates hyperinflammation, oxidative stress and haematological changes including lymphopenia, thrombocytopenia and macrophage activation syndrome. Collectively, these pathologies exacerbate the dysregulated host response and cause significant tissue injury to lung (and other) tissues resulting in respiratory (and often multi-organ) failure ( Figure 1) [4]. Given the widespread organ/system assault in severe cases of COVID-19, there is clinical need for a therapeutic which addresses the multifactorial pathogenesis to induce systemic cytoprotection and re-establish host responsivity. Dimethyl fumarate (DMF), an approved drug with immunomodulatory, anti-oxidative and anti-inflammatory properties in all tissues, is one potential treatment that could be rapidly implemented into the clinic. Here, we provide perspectives on the potential re-purposement of DMF to treat the cytokine storm caused by severe COVID-19. We have searched the National Library of Medicine Pubmed ® database from 20 August 2020 to 14 November 2020 using the key search terms "dimethyl fumarate", "severe inflammation", "cytokines", "immunomodulation" and "COVID-19/SARS-CoV-2" by severe COVID-19. We have searched the National Library of Medicine Pubmed ® database from 20 August 2020 to 14 November 2020 using the key search terms "dimethyl fumarate", "severe inflammation", "cytokines", "immunomodulation" and "COVID-19/SARS-CoV-2" and included in our discussion, those papers which match the mode of action of DMF with the symptomology of the cytokine storm induce by SARS-CoV-2. Infection with the SARS-CoV-2 virus can lead to a dysregulated immune response in which pro-inflammatory cells dominate the immune cell population. These pro-inflammatory cells intensify cytokine production and release resulting in hyperinflammation. This hyperinflammatory state promotes lung (and systemic) pathology, which correlates with poorer prognosis. It is well documented that dimethyl fumarate (DMF) can modulate immune cell populations to shift the ratio of anti-inflammatory to pro-inflammatory cytokine production and release, which in turn reduces hyperinflammation and subsequent tissue injury.

Main
DMF is a methyl ester of fumaric acid (chemical formula C6H8O4) that is hydrolysed in the small intestine to the active metabolite monomethyl fumarate [5][6][7][8]. DMF is a potent activator of the nuclear factor erythroid 2-related factor 2 (Nrf2) pathway which modulates inflammation and oxidative stress by upregulating cellular defence mechanisms (i.e., cytoprotection through Phase II antioxidant expression, chiefly superoxide dismutase Infection with the SARS-CoV-2 virus can lead to a dysregulated immune response in which proinflammatory cells dominate the immune cell population. These pro-inflammatory cells intensify cytokine production and release resulting in hyperinflammation. This hyperinflammatory state promotes lung (and systemic) pathology, which correlates with poorer prognosis. It is well documented that dimethyl fumarate (DMF) can modulate immune cell populations to shift the ratio of anti-inflammatory to pro-inflammatory cytokine production and release, which in turn reduces hyperinflammation and subsequent tissue injury.
(2) DMF can directly inhibit nuclear factor kappa B (NF-κB), which prevents the translocation of NF-κB into the nucleus, binding to the κB site, release of pro-inflammatory cytokines and subsequent inflammation and damage. DMF can also indirectly inhibit NF-κB through HO-1 expression and immune cell modulation through hydroxycarboxylic acid receptor 2 (HCAR2) activation. (3) DMF's immunomodulatory effects are predominantly mediated through HCAR2 activation which modulates immune cell populations (pro-inflammatory to anti-inflammatory shift) and inhibits NF-κB. Adapted from [7]. Created with BioRender.com.
(2) DMF can directly inhibit nuclear factor kappa B (NF-κB), which prevents the translocation of NF-κB into the nucleus, binding to the κB site, release of pro-inflammatory cytokines and subsequent inflammation and damage. DMF can also indirectly inhibit NF-κB through HO-1 expression and immune cell modulation through hydroxycarboxylic acid receptor 2 (HCAR2) activation. (3) DMF's immunomodulatory effects are predominantly mediated through HCAR2 activation which modulates immune cell populations (pro-inflammatory to anti-inflammatory shift) and inhibits NF-κB. Adapted from [7]. Created with BioRender.com.  The SARS-CoV-2 virus has been demonstrated to affect a significant number of immune cell populations including T cells, B cells, natural killer cells, monocytes, eosinophils and basophils [38][39][40][41][42]. These populations typically decrease in number (the severity of the disease dictates the magnitude of reduction), albeit pro-inflammatory phenotypes dominate the remaining immune cell population. Characteristic of the pro-inflammatory immune response is the increase in neutrophil-to-lymphocyte ratio (NLR). Neutrophilia and lymphopenia, which increase the NLR, are associated with severe viral infection and correlate with a poorer prognosis [43][44][45][46][47]. Moreover, in severe cases, the elevated neutrophil count is correlated with the formation of neutrophil extracellular traps (NETs). NETs are an important innate immunity defense mechanism as they trap and kill pathogens; however, their dysregulation induces oxidative stress (through reactive oxygen species (ROS) production), inflammation, damage, thrombosis and fibrosis to the surrounding tissues. It has been documented that neutrophils infiltrate the lungs [48][49][50] and induce elevated NET formation [48][49][50][51][52][53][54] in severe cases of COVID-19. DMF has been shown to modulate neutrophil counts [12,55] and NET formation [56,57]. Importantly, DMF reduces neutrophil adhesion, migration and infiltration [12,55,56,58] and neutrophil-induced ROS production [56] indicating that DMF can moderate the pro-inflammatory (and oxidative) effects of a dysregulated neutrophil response. In addition to DMF's modulatory effects on various immune cell populations to shift from pro-inflammation to anti-inflammation, DMF also modifies an extensive cytokine profile [16,20,22,23,59], which is consistent with that observed in the cytokine storm characteristic of COVID-19 (i.e., granulocyte-colony stimulating factor (CSF), granulocyte-macrophage-CSF, IFN-γ, interferon-γ-inducible protein-10 (IP-10), IL-1β, IL-6, macrophage inflammatory protein (MIP)-1α, MIP-1β, monocyte chemoattractant protein-1 (MCP-1), TNF-α; see Table 2) [3,40,46,[60][61][62]. Given the cytokine storm is strongly correlated with poorer prognosis [40,61,62], calming it is a logical approach. In this regard, the corticosteroid, dexamethasone, which shares some comparable immunosuppressive properties as DMF, has been used successfully in the clinical treatment of COVID-19 [63,64]. However, where corticosteroids elicit immunosuppression mainly through sequestration of CD4+ T-lymphocytes in the mononuclear phagocyte system and by inhibiting cytokine and lymphokine transcription (especially IL-1 and IL-6) [65], DMF modulates a more extensive cytokine profile as well as potent anti-oxidation activity. Of note, there is evidence that DMF induces lymphopenia in some MS patients [66] and, as such, diligent monitoring would be pertinent to ensure DMF does not exacerbate the lymphopenia documented in severe COVID-19 cases [43]. Despite this, DMF is generally well tolerated and is clinically approved indicating scope for rapid clinical translation. Importantly, the overall antiinflammatory and anti-oxidative phenotype induced by DMF in MS and psoriasis patients would be beneficial for COVID-19 patients given the similar pathological mechanisms which advance disease severity and progression.
SARS-CoV-2 uses angiotensin converting enzyme II (ACE2) as its cellular entry receptor [67,68]. The binding of SARS-CoV-2 to ACE2 receptors is likely to reduce ACE2 receptor binding ability and attenuate downstream signalling from anti-inflammatory to pro-inflammatory pathways [69][70][71][72][73][74][75]. In the lungs, pro-inflammatory ACE2 signalling mediates immune cell infiltration, inflammation, injury and fibrosis [70][71][72][73][74][75]. While there is little research into the effect of DMF on ACE2 signalling, there is evidence that DMF can promote anti-inflammatory ACE2 signalling since it reduces inflammatory mediators (NF-κB-derived) and cytokine production in a murine model of acute lung injury [76]. In support of Nrf2 playing a role in ACE2 signalling, pharmacological Nrf2 inhibition decreases ACE2 mRNA [77]. Recent modelling also demonstrates that SARS-CoV-2 interacts with nicotinic acetylcholine receptors which may inhibit the cholinergic anti-inflammatory pathway and mediate COVID-19 pathology [78]. Consistent with this, the incidence of hospitalisation in smokers with COVID-19 is lower than predicted (albeit hospitalised smokers have poorer prognosis [79,80]) indicating that binding of nicotine to the receptor may competitively obstruct SARS-CoV-2 virulence [81]. DMF has been shown to facilitate cholinergic stimulation in MS patients [82] suggesting that, in the presence of SARS-CoV-2, DMF may competitively bind nicotinic acetylcholine receptors to reduce COVID-19 pathogenesis in the first instance. This is consistent with only several reported cases of COVID-19 in DMF-treated MS patients and with none of them having significant symptoms that required hospitalization [83].

Conclusions
Marked by immune dysregulation, hyperinflammation and oxidative stress, severe cases of COVID-19 may benefit from the immunomodulatory, anti-inflammatory and antioxidative properties of DMF. However, caution must be taken-the immunosuppressive effect of DMF may be counterproductive to mounting the host anti-viral immune response in the early stages of COVID-19 and, therefore, may expediate virulence. Thus, DMF may only be suitable for severe, progressed cases of COVID-19.