dynamic regulation is critical for proper development and differentiation [38]. The
wide biological role of EZH2 and, specially, its deep involvement in the regulation of
cell-cycle progression, as well as its pivotal role in several cellular pathways, well
explain why its dysfunction is associated with several solid or hematological cancers
as well as its involvement in stem cell maintenance and tumor development
[36]. Activating or inactivating somatic EZH2 mutations and deletions and missense,
nonsense, and frameshift EZH2 heterozygous or homozygous mutations have been
found in various cancers [36]. Considering that both gain- and loss-of-function EZH2
mutations have been reported in cancers, we could conclude that EZH2 behaves in
turn as an oncogene or as a tumor suppressor, based on the context. A balanced EZH2
activity is required to keep homeostasis [39]. In the light of these findings, EZH2 has
been considered an attractive target for cancer therapy.
1.4.1 EZH2 Catalytic Inhibitors
The carbocyclic adenosine analogue 3-deazaneplanocin (DZNep, 11, Fig. 2), a derivative of the natural antibiotic neplanocin A, has been one of the first small molecules to
be tested as EZH2 inhibitor [40]. By mechanism, compound 11 is an S-adenosyl-Lhomocysteine hydrolase inhibitor, affecting all SAM-dependent processes; hence it is
an indirect and unselective inhibitor [41]. The poor PK and toxicological profile of
compound 11 [42] encouraged the development of novel, potent, and selective inhibitors of EZH2. High-throughput biochemical screenings led to the development of
SAM-competitive catalytic EZH2 inhibitors, many of them containing a
dimethylpyridone moiety. In 2012, Epizyme Inc. reported EPZ005687 (12, Fig. 2) as
a potent, selective, and SAM-competitive small-molecule inhibitor of EZH2 with a K i
of 24 nM [43]. Treatment with compound 12 in EZH2-WT and Y641- or A677-mutant
lymphoma cells, as well as in other cancer cell lines, including breast and prostate
cancers, resulted in dose-dependent ablation of H3K27 methylation. Simultaneously,
GlaxoSmithKline (GSK), via a high-throughput biochemical screening, followed by an
extensive medicinal chemistry optimization, disclosed GSK126 (13, Fig. 2), able to
potently (K i
app
: 0.5–3 nM) and selectively inhibit WT and mutant EZH2. Compound
13 markedly inhibits the growth of EZH2-mutant diffuse large B-cell lymphoma in
xenograft mice [44] and is currently being under evaluation in phase I clinical trials
against various types of lymphoma [45]. GSK343 (14, Fig. 2) is another indazole-based
potent EZH2 inhibitor (IC 50 : 4 nM) [46]. EI1 (15, Fig. 2), a SAM-competitive inhibitor, is effective against WT and mutant EZH2 and displays >10,000-fold selectivity
for EZH2 over other methyltransferases and 90-fold selectivity over EZH1. In cellbased studies, EI1 reduced H3K27 methylation levels reactivating PRC2 target genes.
Moreover, it was able to decrease proliferation and to induce cell-cycle arrest and
apoptosis in Y641 mutant large B-cell lymphoma [47]. Reported as the first orally
bioavailable inhibitor in mice, UNC1999 (16, Fig. 2) was a dual and highly selective
inhibitor of WT and Y641 mutant EZH2, as well as of EZH1. Also compound 16 is a
Lysine Methyltransferases and Their Inhibitors
131
wide biological role of EZH2 and, specially, its deep involvement in the regulation of
cell-cycle progression, as well as its pivotal role in several cellular pathways, well
explain why its dysfunction is associated with several solid or hematological cancers
as well as its involvement in stem cell maintenance and tumor development
[36]. Activating or inactivating somatic EZH2 mutations and deletions and missense,
nonsense, and frameshift EZH2 heterozygous or homozygous mutations have been
found in various cancers [36]. Considering that both gain- and loss-of-function EZH2
mutations have been reported in cancers, we could conclude that EZH2 behaves in
turn as an oncogene or as a tumor suppressor, based on the context. A balanced EZH2
activity is required to keep homeostasis [39]. In the light of these findings, EZH2 has
been considered an attractive target for cancer therapy.
1.4.1 EZH2 Catalytic Inhibitors
The carbocyclic adenosine analogue 3-deazaneplanocin (DZNep, 11, Fig. 2), a derivative of the natural antibiotic neplanocin A, has been one of the first small molecules to
be tested as EZH2 inhibitor [40]. By mechanism, compound 11 is an S-adenosyl-Lhomocysteine hydrolase inhibitor, affecting all SAM-dependent processes; hence it is
an indirect and unselective inhibitor [41]. The poor PK and toxicological profile of
compound 11 [42] encouraged the development of novel, potent, and selective inhibitors of EZH2. High-throughput biochemical screenings led to the development of
SAM-competitive catalytic EZH2 inhibitors, many of them containing a
dimethylpyridone moiety. In 2012, Epizyme Inc. reported EPZ005687 (12, Fig. 2) as
a potent, selective, and SAM-competitive small-molecule inhibitor of EZH2 with a K i
of 24 nM [43]. Treatment with compound 12 in EZH2-WT and Y641- or A677-mutant
lymphoma cells, as well as in other cancer cell lines, including breast and prostate
cancers, resulted in dose-dependent ablation of H3K27 methylation. Simultaneously,
GlaxoSmithKline (GSK), via a high-throughput biochemical screening, followed by an
extensive medicinal chemistry optimization, disclosed GSK126 (13, Fig. 2), able to
potently (K i
app
: 0.5–3 nM) and selectively inhibit WT and mutant EZH2. Compound
13 markedly inhibits the growth of EZH2-mutant diffuse large B-cell lymphoma in
xenograft mice [44] and is currently being under evaluation in phase I clinical trials
against various types of lymphoma [45]. GSK343 (14, Fig. 2) is another indazole-based
potent EZH2 inhibitor (IC 50 : 4 nM) [46]. EI1 (15, Fig. 2), a SAM-competitive inhibitor, is effective against WT and mutant EZH2 and displays >10,000-fold selectivity
for EZH2 over other methyltransferases and 90-fold selectivity over EZH1. In cellbased studies, EI1 reduced H3K27 methylation levels reactivating PRC2 target genes.
Moreover, it was able to decrease proliferation and to induce cell-cycle arrest and
apoptosis in Y641 mutant large B-cell lymphoma [47]. Reported as the first orally
bioavailable inhibitor in mice, UNC1999 (16, Fig. 2) was a dual and highly selective
inhibitor of WT and Y641 mutant EZH2, as well as of EZH1. Also compound 16 is a
Lysine Methyltransferases and Their Inhibitors
131