HDAC9 inhibitor [92]. Upon coordination to the zinc, the piperazine moiety is
ejected to form a reactive quinone methide that covalently binds to a cysteine residue
in the enzyme. Finally, HDAC inhibition does not have to rely on active site
occupancy [93]. In principle, allosteric inhibitors or disruptors of protein-protein
interactions that recruit HDACs to their cellular multiprotein complexes are among
the alternative strategies that are likely to be developed in the future and offer unique
opportunities for isoform specificity.
6 Therapeutic Applications Targeting Human HDACs
The first HDAC inhibitors were identified on the basis of phenotypic effects in cell
culture such as differentiation or growth inhibition of eukaryotic cells. With this
background, cancer was an obvious therapeutic indication for HDAC inhibitors, and
the rationale is supported by the increased levels of HDAC expression observed in
many cancers. Indeed, HDAC inhibitors are potent antiproliferative agents against
human cancer cell lines, and, barring pharmacokinetic liabilities, the level of activity
correlates with that measured in mechanism-based enzyme assays. Evidence for
cellular target engagement is typically obtained through Western blotting to show
increased acetylation levels of substrate proteins, e.g., histones for nuclear HDACs
and tubulin for HDAC6, and the induction of downstream proteins such as the cyclindependent kinase inhibitor p21 (CIP1/WAF1) [94]. In addition, HDAC inhibitors
typically cause morphological changes that can be observed microscopically.
The hallmarks of proliferating cells are multiple mutations that promote division
while silencing alternative fates such as repair, differentiation, and apoptosis. Cancer
chemotherapy covers a broad spectrum of drugs ranging from blunt instruments that
are general cytotoxic agents such as cisplatin to highly specific mechanism-based
agents such as the antibody rituximab with HDAC inhibitors sitting somewhere in
between. While normal cells are relatively tolerant of HDAC inhibition, transformed
cells respond by reactivating pathways leading to cell death, cell cycle arrest,
senescence, differentiation, or tumor immunogenicity [95]. Microarray experiments
with cells treated with HDAC inhibitors indicate significant changes in expression
levels of the BCL2 family of proteins that regulate apoptosis [96]. Based on their
potent activity in cells and tumor xenograft animal models, the first candidates,
vorinostat and romidepsin, entered clinical trials in the USA, from which the most
promising results were seen in the treatment of T-cell lymphomas. Vorinostat
received FDA approval in 2006 for the treatment of cutaneous T-cell lymphoma,
and romidepsin followed suit in 2009. Romidepsin received additional approval for
the treatment of peripheral T-cell lymphoma in 2011, while belinostat and
tucidinostat were approved for the same indication in 2014 and 2015, respectively.
The T-cell lymphomas appear particularly sensitive to HDAC inhibition as this
result in the downregulation of expression of immunosuppressive cytokines such
as IL-10 [97]. Nevertheless, the rarity of these T-cell lymphomas, combined with the
fact that the HDAC inhibitors are not first-line therapies, has limited the commercial
success of these four compounds. Meanwhile, the Novartis drug panobinostat has
18
A. Ganesan
ejected to form a reactive quinone methide that covalently binds to a cysteine residue
in the enzyme. Finally, HDAC inhibition does not have to rely on active site
occupancy [93]. In principle, allosteric inhibitors or disruptors of protein-protein
interactions that recruit HDACs to their cellular multiprotein complexes are among
the alternative strategies that are likely to be developed in the future and offer unique
opportunities for isoform specificity.
6 Therapeutic Applications Targeting Human HDACs
The first HDAC inhibitors were identified on the basis of phenotypic effects in cell
culture such as differentiation or growth inhibition of eukaryotic cells. With this
background, cancer was an obvious therapeutic indication for HDAC inhibitors, and
the rationale is supported by the increased levels of HDAC expression observed in
many cancers. Indeed, HDAC inhibitors are potent antiproliferative agents against
human cancer cell lines, and, barring pharmacokinetic liabilities, the level of activity
correlates with that measured in mechanism-based enzyme assays. Evidence for
cellular target engagement is typically obtained through Western blotting to show
increased acetylation levels of substrate proteins, e.g., histones for nuclear HDACs
and tubulin for HDAC6, and the induction of downstream proteins such as the cyclindependent kinase inhibitor p21 (CIP1/WAF1) [94]. In addition, HDAC inhibitors
typically cause morphological changes that can be observed microscopically.
The hallmarks of proliferating cells are multiple mutations that promote division
while silencing alternative fates such as repair, differentiation, and apoptosis. Cancer
chemotherapy covers a broad spectrum of drugs ranging from blunt instruments that
are general cytotoxic agents such as cisplatin to highly specific mechanism-based
agents such as the antibody rituximab with HDAC inhibitors sitting somewhere in
between. While normal cells are relatively tolerant of HDAC inhibition, transformed
cells respond by reactivating pathways leading to cell death, cell cycle arrest,
senescence, differentiation, or tumor immunogenicity [95]. Microarray experiments
with cells treated with HDAC inhibitors indicate significant changes in expression
levels of the BCL2 family of proteins that regulate apoptosis [96]. Based on their
potent activity in cells and tumor xenograft animal models, the first candidates,
vorinostat and romidepsin, entered clinical trials in the USA, from which the most
promising results were seen in the treatment of T-cell lymphomas. Vorinostat
received FDA approval in 2006 for the treatment of cutaneous T-cell lymphoma,
and romidepsin followed suit in 2009. Romidepsin received additional approval for
the treatment of peripheral T-cell lymphoma in 2011, while belinostat and
tucidinostat were approved for the same indication in 2014 and 2015, respectively.
The T-cell lymphomas appear particularly sensitive to HDAC inhibition as this
result in the downregulation of expression of immunosuppressive cytokines such
as IL-10 [97]. Nevertheless, the rarity of these T-cell lymphomas, combined with the
fact that the HDAC inhibitors are not first-line therapies, has limited the commercial
success of these four compounds. Meanwhile, the Novartis drug panobinostat has
18
A. Ganesan