and a covalent binding mode, molecular modelling studies performed on the 11dp300 interaction show that this inhibitor occupies the same binding pocket as 11c,
with a reversible and non-competitive mode of action. Interestingly, upon test in
human leukaemia cells (U937), 11d reduces the levels of H4K5 and H3K9 acetylation and induces the arrest of the cell cycle in the G0/G1 phase [96].
The two fungal metabolites of Penicillium species 12a (NK13650A) and 12b
(NK13650B) are peptidic compounds that display a very potent and selective p300
inhibition (IC 50 s ¼ 11 and 22 nM, respectively). Moreover, they are able to repress
the transcriptional activation mediated by the oestrogen and androgen receptors and
to reduce the cell viability in various tumour cell lines. Their main disadvantage is
the low metabolic stability and cell permeability due to the peptidic nature. Anyway,
since through proper chemical modifications it is possible to decrease the peptide
character, the very high inhibitory potency of both 12a and 12b suggests a great
potential for their possible future applications in cancer management [97].
8 Synthetic Inhibitors
Over the years, a wide range of synthetic HATi have been reported exploiting
strategies such as in silico library screening, high-throughput screening and structure- or ligand-based drug design approaches. Structure-activity relationship studies
have provided a good understanding of the structural requirements for a strong HAT
inhibition, but only very recently has been reported the first example of a fully
synthetic, extremely potent, selective and safe HATi (Fig. 5).
Identified in 2005 by the means of a high-throughput screening, the
isothiazolones of general formula 13 are PCAF and p300 inhibitors, able to reduce
histone acetylation and block cell proliferation in different cellular settings. Their
proposed mechanism of inhibition relies on a covalent binding between the
isothiazolone sulphur and a key cysteine residue within the enzyme active site,
forming a disulphide bridge. This assumption has been confirmed by the complete
loss of HAT inhibition that can be observed after co-incubation with dithiothreitol
(DTT) that likely competes for the formation of the disulphide bridge [98]. As for
other HATi, the thiol reactivity of this series of inhibitors is their major weakness,
since they can also cross-react with many other proteins [99]. A SAR study has
highlighted the pivotal role of the substituent on the nitrogen atom in modulating the
inhibitory activity of this series of HATi [100]. In fact, N-aryl-substituted derivatives
are able to inhibit all enzymes tested in the study (p300/CBP, PCAF, Gcn5 and
MOF), while N-alkyl or N-benzyl substituents lead to different patterns of isoformselective inhibition. In particular, N-alkyl derivatives are weak inhibitors of both
PCAF and Gcn5, while are still effective at inhibiting p300, CBP and MOF, with a
small preference for p300/CBP [100]. An interesting series of related inhibitors is
represented by the pyridoisothiazolones 14a and 14b. First reported as a low
micromolar PCAF inhibitor, the para-fluorophenyl derivative PU139 (14a) [101],
when tested in the above-cited SAR study, revealed to be a non-selective HATi with
110
D. Trisciuoglio and D. Rotili
with a reversible and non-competitive mode of action. Interestingly, upon test in
human leukaemia cells (U937), 11d reduces the levels of H4K5 and H3K9 acetylation and induces the arrest of the cell cycle in the G0/G1 phase [96].
The two fungal metabolites of Penicillium species 12a (NK13650A) and 12b
(NK13650B) are peptidic compounds that display a very potent and selective p300
inhibition (IC 50 s ¼ 11 and 22 nM, respectively). Moreover, they are able to repress
the transcriptional activation mediated by the oestrogen and androgen receptors and
to reduce the cell viability in various tumour cell lines. Their main disadvantage is
the low metabolic stability and cell permeability due to the peptidic nature. Anyway,
since through proper chemical modifications it is possible to decrease the peptide
character, the very high inhibitory potency of both 12a and 12b suggests a great
potential for their possible future applications in cancer management [97].
8 Synthetic Inhibitors
Over the years, a wide range of synthetic HATi have been reported exploiting
strategies such as in silico library screening, high-throughput screening and structure- or ligand-based drug design approaches. Structure-activity relationship studies
have provided a good understanding of the structural requirements for a strong HAT
inhibition, but only very recently has been reported the first example of a fully
synthetic, extremely potent, selective and safe HATi (Fig. 5).
Identified in 2005 by the means of a high-throughput screening, the
isothiazolones of general formula 13 are PCAF and p300 inhibitors, able to reduce
histone acetylation and block cell proliferation in different cellular settings. Their
proposed mechanism of inhibition relies on a covalent binding between the
isothiazolone sulphur and a key cysteine residue within the enzyme active site,
forming a disulphide bridge. This assumption has been confirmed by the complete
loss of HAT inhibition that can be observed after co-incubation with dithiothreitol
(DTT) that likely competes for the formation of the disulphide bridge [98]. As for
other HATi, the thiol reactivity of this series of inhibitors is their major weakness,
since they can also cross-react with many other proteins [99]. A SAR study has
highlighted the pivotal role of the substituent on the nitrogen atom in modulating the
inhibitory activity of this series of HATi [100]. In fact, N-aryl-substituted derivatives
are able to inhibit all enzymes tested in the study (p300/CBP, PCAF, Gcn5 and
MOF), while N-alkyl or N-benzyl substituents lead to different patterns of isoformselective inhibition. In particular, N-alkyl derivatives are weak inhibitors of both
PCAF and Gcn5, while are still effective at inhibiting p300, CBP and MOF, with a
small preference for p300/CBP [100]. An interesting series of related inhibitors is
represented by the pyridoisothiazolones 14a and 14b. First reported as a low
micromolar PCAF inhibitor, the para-fluorophenyl derivative PU139 (14a) [101],
when tested in the above-cited SAR study, revealed to be a non-selective HATi with
110
D. Trisciuoglio and D. Rotili