When people experience neuropathic pain, they often describe “pins and needles” sensations or burning, shooting, or stabbing pain that can be agonizing and difficult to bear. In some cases, neuropathic pain is so intense that the pressure of clothing or the weight of a bed sheet can cause misery. Chronic neuropathic pain can be caused by conditions such as diabetes, shingles, HIV/AIDS, chemotherapy, amputation (“phantom limb” syndrome), multiple sclerosis, or back injury. An estimated 6.9 to 10 percent of the population experiences neuropathic pain,[1] and the global market for drugs to treat neuropathic pain was $5.2 billion in 2015 and is expected to reach $8.3 billion by 2024.[2]

New approaches to treating neuropathic pain are urgently needed—particularly non-opioid agents. In this light, recent research on Enhancer of zeste homolog 2 (EZH2) has been of interest. EZH2 is a histone methyltransferase that is the catalytic subunit of the Polycomb repressive complex 2 (PRC2). This enzyme elicits transcriptional silencing by acting on histone H3 at lysine 27 (H3K27) to generate trimethylated H3K27 (H3K27me3), and also is known to be involved in Wnt/beta-catenin and Notch signaling. Two different research teams have recently published animal model and cell line data indicating that EZH2 activity is a key regulator in the development of neuropathic pain. The teams found that inhibition of EZH2 provides analgesia and reduces markers of neuroinflammation in cell lines and in rats.[3][4]

In the animal study, Yadev et al. used an established model of neuropathic pain (partial sciatic nerve ligation), confirming that both EZH2 expression and trimethylation of lysine 27 increased in the region of the spinal dorsal horn affected by the ligation procedure.³ This area of the spinal dorsal horn is involved in the genesis of neuropathic pain. Conversely, local administration of the EZH2 inhibitors 3-Deazaneplanocin (DZNep) or GSK126 attenuated pre-existing neuropathic pain (as measured by behavioral tests) and decreased expression of EZH2 and methylation of H3K27. The researchers also found that the EZH2 inhibitors blocked activation of microglia, the resident immune cells in the central nervous system, and reduced the levels of several markers of neuroinflammation including TNF-alpha, IL-1beta, and MCP-1. Altogether, the results led the team to conclude that EZH2 inhibitors pose “a novel strategy for the development of new analgesics for the treatment of neuropathic pain.”³

In the second study, Arifuzzaman et al. investigated whether inhibition of EZH2 with the selective small molecule inhibitor EPZ-6438 would protect against microglial activation in mouse microglial primary cells and a microglial cell line.⁴ EPZ-6438 is currently in Phase II human clinical trials for treatment of non-Hodgkin Lymphoma.[5].  The researchers conducted RNA-Seq analysis of the transcriptomes of resting, EPZ-6438-treated, LPS-treated and LPS + EPZ-6438-treated primary microglial cells and an immortalized murine microglial cell line (BV-2).⁴ While expected EPZ-6438 target genes and co-regulated networks were confirmed, the data also indicated unexpected effects of EPZ-6438 on inducible transcription factors and regulation of important inflammatory mediators by EZH2. Additionally, Arifuzzaman et al. found evidence that EPZ-6438 affects inflammatory gene targets via IRF1, IRF8, and STAT1.

By developing a greater understanding of the mechanism action of EPZ-6438 for anti-neuroinflammatory activity, researchers hope that it will lead to new approaches for treating a host of neurological diseases. For people living in chronic neuropathic pain, relief in the form of new therapeutic agents can’t come soon enough. But considering that microglial activation also plays a role in many other conditions including Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, rheumatoid arthritis, and some breast cancers, identifying new EZH2 inhibitors and characterizing known EZH2 inhibitors could have broad implications indeed.

– Robyn M. Perrin, PhD

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[1] van Hecke O, Austin SK, Khan RA, Smith BH, Torrance N. 2014. Neuropathic pain in the general population: a systematic review of epidemiological studies. Pain. 155(4):654-62.

[2] “Global Neuropathic Pain Market to Reach US$ 8.3 Bn by 2024 End,” November 29, 2016. http://www.persistencemarketresearch.com/mediarelease/neuropathic-pain-market.asp

[3] Yadav R, Weng HR. 2017. EZH2 regulates spinal neuroinflammation in rats with neuropathic pain. Neuroscience. 349:106-117.

[4] Arifuzzaman S, Das A, Kim SH, Yoon T, Lee YS, Jung KH, Chai YG. 2017. Selective inhibition of EZH2 by a small molecule inhibitor regulates microglial gene expression essential for inflammation. Biochem Pharmacol. 137:61-80.

[5] A Phase II, Multicenter Study of the EZH2 Inhibitor Tazemetostat in Adult Subjects With INI1-Negative Tumors or Relapsed/Refractory Synovial Sarcoma. https://clinicaltrials.gov/ct2/show/NCT02601950