analogues was evaluated against p300 and PCAF, and some compounds resulted
more potent than 15b and 15c at inhibiting p300, with the most active ones, 15d and
15e, that retain the 4-(para-chlorophenyl)-2-thiazolylhydrazone core of the prototypes and display IC 50 values of 77.6 and 78.3 μM, respectively [107].
Identified through a structure-based virtual screening carried out on a library of
commercially available small molecules that were docked into the binding pocket of
Lys-CoA within p300 [108], the pyrazolone C646 (16a) is a selective (sub)micromolar inhibitor (K i ¼ 460 nM and IC 50 ¼ 1.6 μM) of p300. Various site-directed
mutagenesis, molecular docking and SAR studies have allowed to clarify the binding
mode and to identify the structural elements required for its strong inhibition. In this
regard, the carboxylic acid seems essential for binding, but also crucial is a hydrogen
bond acceptor in the position of the nitro group, while the oxidation to carbonyl or
reduction of the C646 exo-methylene leads to a drop of inhibitory potency,
suggesting that the electronic properties and planarity of the conjugated system are
critical [108]. Despite reported as a non-covalent and competitive p300 inhibitor, the
presence of an α,β-unsaturated system makes 16a susceptible to Michael addition. In
fact, subsequent studies have demonstrated that C646 reacts with numerous
cysteine-containing proteins that could be, at least in part, responsible for its cellular
effects [99, 109]. Another weakness of 16a is its great intrinsic fluorescence which
prevents the use in fluorescence-based assays. In order to overcome this problem, the
furan ring of the prototype has been replaced with a phenyl moiety, leading to the
non-fluorescent p300 inhibitor C107 (16b, IC 50 ¼ 9 μM) [110]. Able to reduce
histone H3 and H4 acetylation and block cellular proliferation in lung and melanoma
cancer cells [108], when assayed in AML1-ETO-positive leukaemia cells, 16a
inhibits cell growth and colony formation, with a partial cell cycle arrest in G1
phase and a significant apoptosis, associated with reduction of histone H3 acetylation and bcl-2 and c-kit levels [111]. Interestingly, 16a is also able to inhibit HDACs
with K i values ranging from 7 to 25 μM depending on the specific HDAC subtype, so
confirming that it is not selective and that its cellular effects could also arise from the
inhibition of enzymes other than p300 [112].
Very recently has been reported the first drug-like catalytic inhibitor of p300/CBP
[113]. This spiro-oxazolidinedione compound, indicated as A-485 (17), has been
identified through a virtual ligand screening performed on open conformation of the
docked p300 binding pocket of Lys-CoA, followed by a medicinal chemistry optimization study performed on the first hits [113, 114]. With single-digit nanomolar
inhibitory potencies against p300 (IC 50 ¼ 9.8 nM and K d ¼ 15 nM) and CBP
(IC 50 ¼ 2.6 nM), 17 is particularly selective over various HAT enzymes (GCN5,
PCAF, HAT1, TIP60, MYST3 and MYST4) and more than 150 epigenetic and
non-epigenetic targets. Indeed, the co-crystal structure of A-485 bound to the p300
active site, in addition to show the competition with acetyl-CoA for the binding to the
same pocket, by highlighting key interactions within the catalytic site, clearly
explains the elevated HAT isoform specificity of 17 (Fig. 6). Moreover, A-485
reduces in a dose-dependent way the acetylation of H3K18 and H3K27 in various
cell lines so confirming the selective inactivation of p300/CBP, displays selective
antiproliferative effects in multiple prostate and haematological tumour cell lines,
112
D. Trisciuoglio and D. Rotili
more potent than 15b and 15c at inhibiting p300, with the most active ones, 15d and
15e, that retain the 4-(para-chlorophenyl)-2-thiazolylhydrazone core of the prototypes and display IC 50 values of 77.6 and 78.3 μM, respectively [107].
Identified through a structure-based virtual screening carried out on a library of
commercially available small molecules that were docked into the binding pocket of
Lys-CoA within p300 [108], the pyrazolone C646 (16a) is a selective (sub)micromolar inhibitor (K i ¼ 460 nM and IC 50 ¼ 1.6 μM) of p300. Various site-directed
mutagenesis, molecular docking and SAR studies have allowed to clarify the binding
mode and to identify the structural elements required for its strong inhibition. In this
regard, the carboxylic acid seems essential for binding, but also crucial is a hydrogen
bond acceptor in the position of the nitro group, while the oxidation to carbonyl or
reduction of the C646 exo-methylene leads to a drop of inhibitory potency,
suggesting that the electronic properties and planarity of the conjugated system are
critical [108]. Despite reported as a non-covalent and competitive p300 inhibitor, the
presence of an α,β-unsaturated system makes 16a susceptible to Michael addition. In
fact, subsequent studies have demonstrated that C646 reacts with numerous
cysteine-containing proteins that could be, at least in part, responsible for its cellular
effects [99, 109]. Another weakness of 16a is its great intrinsic fluorescence which
prevents the use in fluorescence-based assays. In order to overcome this problem, the
furan ring of the prototype has been replaced with a phenyl moiety, leading to the
non-fluorescent p300 inhibitor C107 (16b, IC 50 ¼ 9 μM) [110]. Able to reduce
histone H3 and H4 acetylation and block cellular proliferation in lung and melanoma
cancer cells [108], when assayed in AML1-ETO-positive leukaemia cells, 16a
inhibits cell growth and colony formation, with a partial cell cycle arrest in G1
phase and a significant apoptosis, associated with reduction of histone H3 acetylation and bcl-2 and c-kit levels [111]. Interestingly, 16a is also able to inhibit HDACs
with K i values ranging from 7 to 25 μM depending on the specific HDAC subtype, so
confirming that it is not selective and that its cellular effects could also arise from the
inhibition of enzymes other than p300 [112].
Very recently has been reported the first drug-like catalytic inhibitor of p300/CBP
[113]. This spiro-oxazolidinedione compound, indicated as A-485 (17), has been
identified through a virtual ligand screening performed on open conformation of the
docked p300 binding pocket of Lys-CoA, followed by a medicinal chemistry optimization study performed on the first hits [113, 114]. With single-digit nanomolar
inhibitory potencies against p300 (IC 50 ¼ 9.8 nM and K d ¼ 15 nM) and CBP
(IC 50 ¼ 2.6 nM), 17 is particularly selective over various HAT enzymes (GCN5,
PCAF, HAT1, TIP60, MYST3 and MYST4) and more than 150 epigenetic and
non-epigenetic targets. Indeed, the co-crystal structure of A-485 bound to the p300
active site, in addition to show the competition with acetyl-CoA for the binding to the
same pocket, by highlighting key interactions within the catalytic site, clearly
explains the elevated HAT isoform specificity of 17 (Fig. 6). Moreover, A-485
reduces in a dose-dependent way the acetylation of H3K18 and H3K27 in various
cell lines so confirming the selective inactivation of p300/CBP, displays selective
antiproliferative effects in multiple prostate and haematological tumour cell lines,
112
D. Trisciuoglio and D. Rotili