Applying the same deconstruction and regrowth strategy, the Novartis
researchers also developed compound 32 (Fig. 3) from the HTS hit 25 [64]. In the
same period, also AbbVie Inc. reported the identification of a novel potent and
selective EED binder, compound A-395 (33, Fig. 3), able to bind EED in the
H3K27me3 binding pocket and to inhibit H3K27 methylation in vitro via inhibition
of PRC2 activation. When tested in cell-based assays, compound 33 strongly
reduced the H3K27 methylation and inhibited cancer cell growth and proved to be
effective also in cell lines with acquired resistance to SAM-competitive EZH2
inhibitors. Finally, the novel AbbVie compound was efficacious in a diffuse large
B-cell lymphoma (DLBCL) xenograft mouse model (using cell lines containing an
activating heterozygous EZH2 A677G mutation) [66].
Lastly, Barnash et al. optimized a low-affinity methylated Jarid2 peptide to give
the first peptidomimetic EED ligands UNC5114 (34) and UNC5115 (35) (Fig. 3),
using a coupled combinatorial and structure-based design approach. The binding
mode and the exact mechanism of PRC2 inhibition are not yet fully understood.
However, it is confirmed that the novel peptidomimetic binder is an allosteric inhibitor, targeting the methyl-lysine reader function [68].
Recently, EZH2 inhibitors have been used in combination with known or novel
epi-drugs or standard drugs, for example, DZNep was combined with HDACi and/or
DNMTi resulting in an effective and synergistic treatment in AML than each single
agent alone [71], but also the pyrimidine-based inhibitors with HDACi displayed
antitumor activity in glioblastoma [72].
1.5 The H3K79 Histone Methyltransferase DOT1L
DOT1L (disruptor of telomeric silencing 1-like), also known as KMT4, is a histone
methyltransferase responsible for mono-, di-, and trimethylation of H3K79 [73],
being associated with active gene transcription. H3K79 methylation is involved in
the expression of genes involved in cell cycle progression, DNA damage response, as
well as embryonic development, hematopoiesis, and cardiac function [74]. DOT1L
was believed to be the sole H3K79 methyltransferase, but recent findings support the
idea that MMSET isoform RE-IIBP (interleukin-5 response element II-binding
protein) methylates this mark as well [75]. DOT1L possesses no SET domain,
different from the other identified human PKMTs [76]; instead it exhibits its enzymatic activity via a non-SET catalytic domain, similar to those observed in PRMTs
and DNMTs [77, 78]. The turnover of the modifications catalyzed by DOT1L is
rather slow, and no demethylase capable to remove this mark has been described to
date [79]. DOT1L has been studied mainly for its hematopoietic regulation properties
[74], being described as an attractive epigenetic target in AML with mixed-lineage
leukemia (MLL) gene translocations. In vitro studies in such cancer cell lines
confirmed that DOT1L pharmacological inhibition or its genetic ablation led to
differentiation and apoptosis [80]. Furthermore, it has been discovered as a potential
promising target also in breast cancer [81].
Lysine Methyltransferases and Their Inhibitors
135
researchers also developed compound 32 (Fig. 3) from the HTS hit 25 [64]. In the
same period, also AbbVie Inc. reported the identification of a novel potent and
selective EED binder, compound A-395 (33, Fig. 3), able to bind EED in the
H3K27me3 binding pocket and to inhibit H3K27 methylation in vitro via inhibition
of PRC2 activation. When tested in cell-based assays, compound 33 strongly
reduced the H3K27 methylation and inhibited cancer cell growth and proved to be
effective also in cell lines with acquired resistance to SAM-competitive EZH2
inhibitors. Finally, the novel AbbVie compound was efficacious in a diffuse large
B-cell lymphoma (DLBCL) xenograft mouse model (using cell lines containing an
activating heterozygous EZH2 A677G mutation) [66].
Lastly, Barnash et al. optimized a low-affinity methylated Jarid2 peptide to give
the first peptidomimetic EED ligands UNC5114 (34) and UNC5115 (35) (Fig. 3),
using a coupled combinatorial and structure-based design approach. The binding
mode and the exact mechanism of PRC2 inhibition are not yet fully understood.
However, it is confirmed that the novel peptidomimetic binder is an allosteric inhibitor, targeting the methyl-lysine reader function [68].
Recently, EZH2 inhibitors have been used in combination with known or novel
epi-drugs or standard drugs, for example, DZNep was combined with HDACi and/or
DNMTi resulting in an effective and synergistic treatment in AML than each single
agent alone [71], but also the pyrimidine-based inhibitors with HDACi displayed
antitumor activity in glioblastoma [72].
1.5 The H3K79 Histone Methyltransferase DOT1L
DOT1L (disruptor of telomeric silencing 1-like), also known as KMT4, is a histone
methyltransferase responsible for mono-, di-, and trimethylation of H3K79 [73],
being associated with active gene transcription. H3K79 methylation is involved in
the expression of genes involved in cell cycle progression, DNA damage response, as
well as embryonic development, hematopoiesis, and cardiac function [74]. DOT1L
was believed to be the sole H3K79 methyltransferase, but recent findings support the
idea that MMSET isoform RE-IIBP (interleukin-5 response element II-binding
protein) methylates this mark as well [75]. DOT1L possesses no SET domain,
different from the other identified human PKMTs [76]; instead it exhibits its enzymatic activity via a non-SET catalytic domain, similar to those observed in PRMTs
and DNMTs [77, 78]. The turnover of the modifications catalyzed by DOT1L is
rather slow, and no demethylase capable to remove this mark has been described to
date [79]. DOT1L has been studied mainly for its hematopoietic regulation properties
[74], being described as an attractive epigenetic target in AML with mixed-lineage
leukemia (MLL) gene translocations. In vitro studies in such cancer cell lines
confirmed that DOT1L pharmacological inhibition or its genetic ablation led to
differentiation and apoptosis [80]. Furthermore, it has been discovered as a potential
promising target also in breast cancer [81].
Lysine Methyltransferases and Their Inhibitors
135