Latest progress in the development of cyclooxygenase-2 pathway inhibitors targeting microsomal prostaglandin E₂ synthase-1

mPGES-1-produced prostaglandin E₂ (PGE₂) plays a critical role in pain, inflammation and tumor growth [1,2]. By isoform-1 or -2 of cyclooxygenase (COX), arachidonic acid (AA) is converted into PGH₂, an unstable metabolite. PGH₂ is then isomerized by mPGES-1 into inflammatory PGE₂. Thus, PGE₂ produced by mPGES-1 can be blocked by inhibiting COX-1 and COX-2 using nonsteroidal anti-inflammatory drugs (NSAIDs), including COX-2 inhibitors. However, these medications can lead to serious side effects, such as gastrointestinal bleeding and prothrombotic events. Recent advances in the study of inhibitors targeting mPGES-1 show promising results and new potential for the development of the next generation of NSAIDs without the side effects of the current NSAIDs and COX-2 inhibitors on the market [3]. However, PGE₂, synthesized by mPGES-1, is complex in its relation to the production and resolution of inflammation in disease states. In this editorial, the principles and implications of inhibiting mPGES-1 and the inhibitors and systems that set up the framework for the future of animal experiments and possible clinical use of mPGES-1 inhibitors are reviewed.

Structure & function relationship

The crystal structures of mPGES-1 have given insight into how potential inhibitors bind to the active site through the study of the enzyme and inhibitors' structure–activity relationship [4]. mPGES-1 is a homotrimer with each monomer consisting of four helixes where the N and C termini reside in the lumen of the endoplasmic reticulum [5]. The active site of mPEGS-1 is formed by the interphase of each monomer's membrane-spanning helixes that form three active site cavities. Proline residues that lie within helix II and IV of each monomer allow for the formation of a cytoplasmic cavity resembling a cone that allows for the entrance of substrates to the active site [5]. Many studies have suggested that ligand binding occurs within the pocket above the glutathione (GSH) and blocks the helix IV of the first monomer [5]. A critical aspect of inhibitor design is the conservation of sequences of the active sites of mPGES-1 between human and animal models [3,4].

mPGES-1 as an anti-inflammatory drug target

A number of recent advances in terms of the inducible mPGES-1-synthesized PGE₂‘s involvement in heart disease, chronic inflammation, fibrosis, angiogenesis, multiple sclerosis, infection and cancer have been identified. While these topics remain broad, greater discussion and research are required to evaluate the possible implications of mPGE-1 as a promising drug target. In a study of inflammation in equine models, results showed that mPGES-1 inhibitors have a better ability to maintain basal levels of PGE₂ over COX inhibitors and the authors postulated that mPGES-1 inhibitors are a safer and more effective alternative in clinical settings to reduce side effects of known COX inhibitors [6]. In inflammation, T cells can sense and initiate PGE₂ production, and a recent study sought to determine how PGE₂ production can affect T-cell phenotyping in colitis [7]. Maseda et al. found that PGE₂ production from different cell types and intestinal locations can impact T-cell function due to the leukocyte's reliant nature on the microenvironment that produced it. Interestingly, Luo and Wang et al. found that mPGES-1-deficient mice were not able to produce FoxP3+ T cells, which play a role in immune tolerance and correlated with severe colitis in mice. Continuing with the immune system, mPGES-1-derived PGE₂ plays a part in the activation of the NLRP3 inflammasome, which is part of the innate immune system that regulates inflammatory mediators [8,9]. PGE₂ production correlates with fibrosis, the formation of scar tissue, in people receiving peritoneal dialysis [8]. One study focused on the spleen to see if the enzyme and the cholinergic anti-inflammatory pathway (CAP), which were the parasympathetic and sympathetic nervous systems, work jointly to control inflammation [10]. In the study, Revathikumar et al. found that mPGES-1 plays an important role in the synthesis of acetylcholine and on molecular events for vagal nerve stimulation (VNS) in mice. Activation of CAP by VNS has paved way for novel therapeutic strategies in treating inflammatory diseases. In an animal model of multiple sclerosis, mPGES-1 was found to be upregulated in vascular epithelial cells and contribute to the aggravation of symptoms such as increased vascularity that can cause inflammation, demyelination and paralysis in multiple sclerosis [11].

 Regarding cancer, the production of mPGES-1 and PGE₂ are known to promote tumor vascular growth and stimulate the secretion of VEGF via paracrine release [12]. Terzuoli et al. found that tumor-derived PGE₂ decreases the expression of miRNAs, which have multiple roles in cellular functioning, through repressing Dicer by an autocrine mechanism. Li et al. suggested MAPK and mPGES-2 signaling pathways may cross-talk, but this issue remains complex and requires further investigation in T-cell acute lymphoblastic leukemia, a cancer that has a declining prognosis with age [13]. In T-cell acute lymphoblastic leukemia there is a possibility of generating a therapy to treat this disease by regulating both pathways and researchers found that more apoptosis and cell cycle arrest occurred in this cancer by disrupting mPGES-1 function consistently. Silva et al. found that mPGES-1 is involved in the formation and bursting of intracranial aneurysms and vascular mPGES-1 was found to play a protective role in reducing cranial aneurysms [14]. In contrast with vascular mPGES-1, mPGES-1 knock-out mice show increasing cerebral aneurysm ruptures and higher mortality, but these effects were mitigated by aspirin. In zymosan-induced peritonitis, researchers sought to verify the role of mPGES-1 in mediating and resolving inflammation [15]. Blocking mPGES-1 and CX3CL1, a chemokine, failed to increase myeloid cells number in the abdomen of mice. Using asymmetric dimethylarginine (ADMA) as a biomarker of cardiovascular toxicity in murine animal models, mPGES-1 inhibition was examined to determine if ADMA was also produced as it is for COX-2 inhibition [16]. ADMA was not produced in the kidneys when mPGES-1 was inhibited, suggesting that mPGES-1 inhibition is a probable approach to reducing inflammation, along with fewer side effects, with COX-2 inhibition. Chen et al. reported that by knocking out macrophage mPGES-1, the mouse survival rate after myocardial infarction increased and did not alter the deleterious effect of cardiac remodeling [17]. Macrophages generously produce mPGES-1 and play an important role in maintaining heart homeostasis, but can promote proinflammatory effects after myocardial injury [17,18]. In another study, Chen et al. found an effective and efficient way to measure mPGES-1 inhibition for the screening of new inhibitors using reporter cells in a fluorescence imaging strategy through a CRISPR/Cas9 knock-in system, which allows the technology to promote discovery of novel anti-inflammatory and tumor therapies [19].

mPGES-1 inhibitors

There have been many recent discoveries of new inhibitors, which also give us insight into the dynamic nature of mPGES-1 and its metabolite, PGE₂. PGE₂ produces its response by activating its four receptors, EP1–4, to exert different downstream effects based on its affinity in nonconserved regions [20]. Regarding mPGES-1, the development of new technologies to discover inhibitors was impeded by the finding that many of these human mPGES-1 inhibitors were not able to inhibit mice mPGES-1 [3]. However, in a study using scaffolding technology to compare the conserved regions of mouse and human mPGES-1, researchers found that an inhibitor known as 4b blocked both human and mouse mPGES-1, and is a promising inhibitor that is also orally bioavailable [3]. Another study demonstrated five new mPGES-1 inhibitors that are bioavailable and potent as a class to reduce inflammation and the potential side effect of inhibition of mPGES-1 in the cardiovascular system and poses them as tools to evaluate mPGES-1 in various diseases [21]. Inhibition of mPGES-1 is more favorable to producing less cardiovascular toxic effects than COX-2 inhibition [16]. With the findings regarding new inhibitors and the development of technology vastly improving each year, the need for further development of the pharmacodynamic profiles of these inhibitors to increase the success rates in preclinical and clinical testing to propel them in use in clinical settings remains [22]. In previous years, studies were conducted using COX-2 and 5-LOX, another enzyme well regarded in its role in inflammation and cancer, were stopped before clinical trials due to the side effects of long-term treatment [23]. Zhou et al. demonstrated how inhibiting both mPGES-1 and 5-LOX by LFA-9 promoted anti-inflammatory effects in an acute inflammation model and prevented stemness and spheroid genesis in colon tumors. Another approach to inhibit mPGES-1 is using Drug Repurposing Effort Applying Integrated Modeling-in vitro/vivo-Clinical Data Mining (DREAM-in-CDM) technology, which has already predicted 15 FDA-approved drugs that may inhibit mPGES-1 [24]. This technique provides a possible way to treat inflammatory diseases without the discovery of new drugs. In studying inflammation in cell and animal models, NSAID screening may become more efficient with the development of a novel hybrid enzyme bthat connects COX-2 to mPGES-1 via a 10 amino acid biolinker. This novel enzyme has shown a new orientation, leading to more effective catalytic activity, more PGE₂ production and fewer side products [25]. The technology that linked the two enzymes together, called Enzymelink, was used in a previous study by the authors where its ability to screen for potent inhibitors more rapidly and convincingly than in previous efforts was noted [26]. In the future, Enzymelink could be used to screen newly found inhibitors to set the stage for preclinical testing to further review the pharmacodynamic profiles of such drugs [3,21,24–26].