Inhibitors of EZH2 methyltransferase activity
The first EZH2 inhibitor was 3-deazaneplanocin A (DZNep). DZNep, a known S-adenosyl-L-homocysteine (SAH) hydrolase inhibitor, indirectly inhibits EZH2 through the increase of SAH, which directly represses S-adenosyl-L-methionine-dependent histone methyltransferase activity. Thus, DZNep globally inhibits histone methylation and is not specific to EZH2 [
58].
Since 2012, several potent and highly selective S-adenosyl-methionine-competitive inhibitors of EZH2 methyltransferase activity have been developed. Most of them have a common structural feature, a 2-pyridone core. The pyridone can occupy partially the site for the co-substrate S-adenosyl-methionine (SAM) in the binding pocket of EZH2 while SAM is the methyl donor [
59].
GSK126 (GSK2816126) can inhibit wild type and Y641 mutant EZH2 with similar potency and is highly selective compared to EZH1 (150-fold increased potency) or 20 other methyltransferases (> 1000-fold selective for EZH2) [
60]. A multicenter phase 1 clinical trial to evaluate the safety, maximum-tolerated dose (MTD), pharmacokinetics, and pharmacodynamics of GSK126 (NCT02082977) in patients with advanced hematologic and solid tumors was terminated. Forty-one participants (21 solid tumors, 20 lymphoma) received escalating doses of GSK2816126 ranged from 50 to 3000 mg twice weekly as an intravenous solution for 28 days (3 weeks on/1 week off) in the study. The results showed insufficient evidence of clinical activity, and did not justify further clinical investigation while the dosing method and relatively short half-life limited effective exposure [
61]. In an experiment administering GSK126 treatment on tumor model mice, the results turned out that GSK126 had no effect on tumors in immunocompetent hosts, unlike that observed in immunodeficient hosts. The difference in the presence of myeloid-derived suppressor cells (MDSCs) between these two kinds of hosts might be the reason. MDSCs play an immunosuppressive role in the tumor microenvironment as it produces IL-6 and NO. GSK126 could promote hematopoietic progenitor cells differentiation to MDSCs, leading to increased MDSCs and immunosuppression, which might offset its antitumor effect on immune-competent hosts. Neutralizing antibodies against the myeloid differentiation antigen GR-1 or gemcitabine/5-fluorouracil can deplete MDSCs; thus, they can enhance the activity of GSK126 [
62]. EPZ005687 [
63], an EZH2 inhibitor concurrently developed with GSK126, with high affinity and selectivity for EZH2, has substandard pharmacokinetic properties that limit its clinical application. EI1 [
64], another highly selective SAM-competitive inhibitor of EZH2, can inhibit the growth of DLBCL cells carrying Y641 mutations.
Later on, several other SAM-competitive inhibitors of EZH2 were developed including GSK343 [
65], GSK926 [
65], and tazemetostat (E7438/EPZ6438) [
66]. GSK926 and GSK343 can suppress histone H3K27me3 level and inhibit EZH2 activity in breast and prostate cancer cells, while GSK343 can only be used in vitro due to the high clearance in rat PK studies [
65]. Tazemetostat has improved potency and pharmacokinetic properties compared to EPZ005687 and can be taken orally in animals [
67]. This compound is under evaluation in a series of clinical trials (Table
1). In a phase 2, multicenter, open-label, single arm, 2-stage study of tazemetostat 800 mg BID administered orally in continuous 28 day cycles (NCT02601950), patients were enrolled into seven cohorts based on tumor type after screening through 21 days of the first planned dose of tazemetostat [
68]. In cohort 5, there were 62 patients (59 adults and 3 pediatric ≥ 16 years) with a documented diagnosis of metastatic or unresectable, locally advanced epithelioid sarcoma (ES). The objective response rate (ORR) for patients with ES was 15%, and the median duration of response (DOR) was 16 months. Sixty-eight percent of patients had a reduction in tumor burden. Median overall survival (mOS) was 19 months [
69]. As for the clinical safety, tazemetostat shows a favorable safety and tolerability profile while most adverse events (AEs) are mild to moderate and resolved without the need for discontinuation or dose reductions [
70]. Inspiringly, on January 23, 2020, tazemetostat (TAZVERIK, Epizyme, Inc.) was approved by FDA for adults and pediatric patients aged 16 years and older with metastatic or locally advanced epithelioid sarcoma not eligible for complete resection. Now, a phase 2, open-label, multi-center clinical trial for relapsed/refractory follicular lymphoma (FL) is ongoing [
71]. As of the cutoff date, after screening and archival tissue analysis for EZH2 hot spot activating mutations, patients were enrolled into two cohorts (FL, EZH2 MT (cohort 1,
n = 45) and FL, EZH2 WT (cohort 2,
n = 54)). Patients were treated with tazemetostat 800 mg BID until progressive disease or withdrawal, and responses were assessed every 8 weeks. Treatment with tazemetostat was generally well tolerated and no treatment-related deaths were observed. The ORR for patients in cohort 1 was 77% and ORR in cohort 2 was 34%. Median DOR was 8.3 months in cohort 1 and 13 months in cohort 2. Median PFS was 11.1 months in cohort 1 and 5.7 months in cohort 2 (median DOR and PFS were not mature for the MT cohort). The results showed tazemetostat is a promising therapeutic drug for patients with relapsed/refractory follicular lymphoma.
EPZ011989 [
72], another selective and orally bioavailable EZH2 inhibitor reported in 2015, was able to inhibit tumor growth significantly in a mouse xenograft model of human B cell lymphoma. Then, CPI-1205 [
73], an orally bioavailable, indole-based, small-molecule inhibitor of EZH2 optimized from CPI-169 [
74] was reported. CPI-169, a previously disclosed indole based EZH2 inhibitor, shows significant antitumor activity and pharmacodynamic (PD) target engagement in a mouse xenograft model of a KARPAS-422 lymphoma while accompanied by limited oral bioavailability [
74]. CPI-1205 was evaluated in a completed phase 1 clinical trial for B cell lymphoma (NCT02395601). Furthermore, CPI-1205 is currently being evaluated in a phase 1/2 clinical trial for advanced solid tumors (NCT03525795) and a phase 1/2 clinical trial for metastatic castration-resistant prostate cancer (NCT03480646). ZLD1039 is a highly selective, and orally bioavailable inhibitor of EZH2, which inhibits breast tumor growth and metastasis in mice [
75]. PF-06821497 [
76] reported in 2018 is currently under evaluation in a phase 1 clinical trial in patients with relapsed/refractory small cell lung cancer (SCLC), castration-resistant prostate cancer (CRPC), FL and diffuse large B-cell lymphoma (DLBCL) (NCT03460977).
Given the fact that EZH1, a homolog of EZH2 physically presented in a non-canonical PRC2 complex, complements EZH2 in mediating H3K27 methylation and also has histone methyltransferase activity [
77], dual EZH1/EZH2 inhibition may have greater antitumor efficacy. UNC1999 is the first oral SAM-competitive inhibitor of wild-type and Y641 mutant EZH2 as well as EZH1 [
78]. UNC1999 effectively inhibited the growth of MLL-rearranged leukemia in mice instead of GSK126 in a study [
79]. A more recent study introduced (R)-OR-S1 and (R)-OR-S2, two orally bioavailable EZH1/2 dual inhibitors produced by Daiichi Sankyo [
80]. It was found that (R)-OR-S1 and (R)-OR-S2 suppressed H3K27me3 in HCT116 colorectal cancer cells more highly than OR-S0, an EZH2 selective inhibitor. Besides, (R)-OR-S1 and (R)-OR-S2 showed greater antitumor efficacy than OR-S0 in DLBCL cells harboring Y641N mutation of EZH2 both in vitro and in vivo. Despite of importance of EZH1 in hematopoietic stem cell maintenance [
81], long-term EZH1/2 dual inhibition in vivo does not cause serious lympho-hematopoietic toxicity according to this study. Daiichi Sankyo soon put DS-3201b, an EZH1/2 inhibitor, into several clinical trials for patients with leukemia, lymphoma, or small cell lung cancer (NCT04276662, NCT03110354, NCT04102150, NCT02732275, NCT03879798).
Inhibitors that break PRC2’s structure
In addition to targeting the enzyme catalytic domain of EZH2, disrupting the protein-protein interactions among the PRC2 subunits is a novel strategy to inhibit PRC2-dependent functions of EZH2.
Peptides known as stabilized alpha-helix of EZH2 (SAH-EZH2) were reported in 2013. SAH-EZH2, derived from the domain of EZH2 that interacts with EED, can disrupt the EZH2-EED interaction through targeting EED leading to an increased level of H3K27me3, reduced EZH2 protein and growth arrest, and differentiation of MLL-AF9 leukemic cells [
82]. Furthermore, SAH-EZH2 impaired viability while GSK126 had no effect in MDA-MB231 (breast cancer) and DU145 (prostate cancer) cell lines which have been reported to be driven by non-enzymatic functions for EZH2 [
82].
Then, several other inhibitors of the EZH2-EED interaction of PRC2 were identified. Astemizole, an FDA-approved H1 histamine receptor antagonist, was reported to arrest the proliferation of PRC2-driven lymphoma cells by disrupting the EZH2-EED complex [
83]. Wedelolactone which has a high affinity for EED was screened out in natural compounds [
84]. Four other FDA-approved drugs (apomorphine hydrochloride, oxyphenbutazone, nifedipine and ergonovine maleate) were discovered as potential EZH2-EED interaction inhibitors through a high-throughput fluorescence polarization assay [
85]. A recent study found that AZD9291 (Osimertinib, TAGRISSO), a EGFR inhibitor approved by FDA for the treatment of patients with metastatic EGFR T790M mutation-positive NSCLC, can break the structure of EZH2-EED [
85]. Besides, AZD9291 can also suppress the expression of EZH2 through upregulating miR-34a which can bind to EZH2 mRNA [
86].
An EED inhibitor developed by Novartis is being evaluated in a phase 1/2 clinical trial for advanced malignancies including DLBCL, nasopharyngeal carcinoma, gastric cancer, ovarian cancer, prostate cancer, and sarcoma (NCT02900651). This inhibitor, MAK683/EED226, can make PRC2 allosteric through directly binding to the H3K27me3 pocket of EED [
87].
Apart from targeting EZH2-EED interaction, disrupting the interaction between EZH2 and SUZ12 was reported in a study [
88]. In this study, they demonstrated that AMP-activated protein kinase (AMPK) can disrupt the EZH2-SUZ12 interaction through directly phosphorylating EZH2 at Thr311 and decrease the level of H3K27me3 in ovarian and breast cancer cells [
88].
Suppressing EZH2 through triggering EZH2 degradation
Given the fact that EZH2 plays a PRC2- and methylation-independent role in cancer and many cancers do not respond to EZH2 enzymatic inhibitors, triggering EZH2 degradation may be a novel method to inhibit EZH2.
In addition to the abovementioned SAH-EZH2 reducing EZH2 protein through disrupting the PRC2 complex, post-translational modifications of EZH2 can aslo induce the degradation of EZH2. Wang et al. reported a gambogenic acid (GNA) derivative, GNA022, directly covalently bound to Cys668 within the EZH2-SET domain, decreasing the stability of PRC2 complex as well as H3K27 trimethylation, triggering EZH2 degradation through COOH terminus of Hsp70-interacting protein (CHIP)-mediated ubiquitination [
89]. Another group found that long non-coding RNA (lncRNA) ANCR can facilitate the CDK1-EZH2 interaction, then phosphorylate EZH2 at Thr345 and Thr487, hence, result in EZH2 ubiquitination and its degradation in breast cancer cells in vitro, and ANCR represses tumor growth and distant metastasis in mice in breast cancer [
90]. F-box and WD repeat domain-containing 7 (FBW7), a novel E3 ligase of EZH2, can mediate the phosphorylation, ubiquitination, and degradation of EZH2 with the involvement of the activated CDK5 kinase in pancreatic cancer [
91].
Lu et al. revealed a new signaling network of SKP2-TRAF6-EZH2/ H3K27me3 and found knockout of SKP2 can upregulate TRAF6-mediated and lysine (K) 63-linked ubiquitination of EZH2 for degradation in prostate cancer (PCa) and CRPC cells in vitro and in vivo [
92]. In a study in 2018, ZRANB1 was identified as the EZH2 deubiquitinase and stabilizes EZH2 through interacting with EZH2 via its OTU domain in breast cancer cells. Thus, ZRANB1 small interfering RNA (siRNA) and other ZRANB1 inhibitors have anticancer effects in vitro and in vivo [
93].
EZH2 inhibitors combined with other therapy methods
Combining EZH2 inhibitors with other therapy methods such as immune therapy, conventional chemotherapy, and target therapy might improve the treatment efficacy and overcome the limitation of monotherapy.
As mentioned before, the levels of EZH2 negatively correlated with intratumoral CD8+ T cells in ovarian cancer, so EZH2 inhibitor could increase effector T cell tumor infiltration, slowed down tumor progression, and synergistically improved the efficacy of adoptive T cell therapy [
45]. Furthermore, EZH2 inhibition can increase the cytotoxicity of human effector T cells in vitro and improve the efficacy of anti-CTLA-4 therapy in murine bladder cancer and melanoma as anti-CTLA-4 increases the expression of EZH2 in peripheral T cells [
94]. This study provided basis for the treatment of combining CPI-1205 with ipilimumab (anti-CTLA-4). In a phase 1/2, multi-center, open-label study (NCT03525795), the strategy of CPI-1205 plus ipilimumab is under evaluation in patients with advanced solid tumors. A recent study revealed the inhibition of EZH2 could enhance cancer cell antigen presentation in head and neck squamous cell carcinoma (HNSCC) and avoid anti-PD-1 resistance [
95].
Synergy between EZH2 inhibitors and conventional chemotherapy has been demonstrated in a series of preclinical studies. Synergistic effects of tazemetostat with cyclophosphamide, doxorubicin, oncovin, and prednisone were observed in EZH2 mutant DLBCL [
96]. In a multicenter, double-blind, placebo-controlled, randomized phase 3 clinical trial (NCT04204941), the combination of tazemetostat with doxorubicin for patients with advanced ES is being evaluated. Part 1 is designed to evaluate the safety of the combination of tazemetostat plus doxorubicin, as well as to establish the MTD and the recommended dose for part 2. Part 2 is planned to compare the effects of tazemetostat plus doxorubicin with doxorubicin plus placebo, when used as first-line treatment in locally advanced unresectable or metastatic ES [
97]. Besides, tazemetostat plus R-CHOP combination (rituximab, cyclophosphamide, vincristine, doxorubicin, prednisolone) in patients with newly diagnosed DLBCL with poor prognosis features is under evaluation in a phase 1/2 clinical trial (NCT02889523). Phase 1 is designed to determine the recommended phase 2 dose for tazemetostat in patients treated with R-CHOP 21. Phase 2 is designed to determine the safety of tazemetostat in patients treated with 8 cycles of R-CHOP 21 and to determine the complete response rate after 8 cycles of Epi-RCHOP (tazemetostat plus RCHOP) 21. Up to present, 17 patients were enrolled and further evaluation in phase 2 is warranted while this therapeutic method showed preliminary efficacy in phase 1 [
98].
Several studies demonstrated co-inhibition of both EZH2 and EGFR could show a synergic effect on tumor growth inhibition through inducing autophagy and increasing apoptosis in colon cancer cells [
99], gastric cancer cells [
100], and lung cancer cells [
101]. Besides, this combination therapy can reverse EGFR-tyrosine kinase inhibitor (EGFR-TKIs) resistance in lung cancer [
99]. Futhermore, Hirukawa et al. in 2019 found EZH2 inhibitors could enhance the efficacy of anti-HER2 monoclonal antibodies such as trastuzumab through promoting interferon-driven immune responses in trastuzumab-resistant breast cancer models of mice [
102]. As previously mentioned, phosphorylated EZH2 activates AR and the activation of AR supports CRPC growth, so combination of EZH2 inhibition and AR-targeted therapies may be effective in CRPR [
6]. There are two ongoing clinical trials to evaluate the effects of EZH2 inhibitors plus enzalutamide (an AR antagonist) in mCRPR (NCT04179864, NCT03480646).
To overcome the limitation that EZH2 inhibitors can only benefit certain hematological malignancies, Huang et al. in 2018 found BRD4 inhibitors can decrease the resistance of EZH2 inhibitors caused by H3K27ac upregulation and restore the sensitivity of the insensitive cell lines to EZH2 inhibitors [
103]. Besides, as EZH2-BRD4 inhibitor combo differentially activates multiple pathways such as MAPK pathway, a triple combination plus MAPK pathway inhibitors may expand the treatment scope of cancers [
103]. However, the therapeutic effects of multi-drug combination require further evaluation in clinical trials, and the side effects and patients’ tolerance may be a big problem.