[41–46]. Interestingly, a recent study revealed that NAM could stimulate SIRT1
activity in cells even though it acted as an inhibitor in vitro [47].
Several NAM analogs were developed as sirtuin inhibitors [48–51]. A series of
2-anilinobenzamide derivatives showed SIRT1 inhibition with IC 50 values between
52 and 300 μM [48]. Some of these derivatives were even more potent toward
SIRT2. The most potent derivative displayed an IC 50 value of 0.57 μM (compound
4 in Fig. 2) for SIRT2 [50]. Recently, a 2-anilinobenzamide scaffold was combined
with a peptide substrate mimicking inhibitor, and a novel type of inhibitor for SIRT2
was identified. This compound, 3
0 -phenethyloxy-2-anilinobenzamide, was an
isoform-selective inhibitor, with an IC 50 value of 28 μM for SIRT2 [52].
In addition, other NAM derivatives, (5-benzamidonaphthalen-1/2-yloxy)nicotinamides, have been reported to be potent sirtuin inhibitors for several sirtuin isoforms:
The IC 50 values of the derivatives were 0.80–100 μM for SIRT1, 48 nM–1 μM for
SIRT2, and 4.4–232 μM for SIRT3. Interestingly, the most potent of these derivatives (compound 5 in Fig. 2) showed excellent selectivity toward SIRT2 with an
IC 50 value of 48 nM, whereas its ability to inhibit the activity of SIRT1 and SIRT3
was at the micromolar level. Based on the kinetic studies, it was postulated that
compound 5 acted as a competitive inhibitor against the peptide substrate and in a
noncompetitive manner against NAD
+
. Compound 5 exhibited also moderate anticancer activity in breast cancer (MCF7), prostate cancer (DU 145), and chronic
myelogenous leukemia cell lines [41].
2.2 Substrate-Based Inhibitors
The first substrate-based inhibitor was developed from human p53 which is a SIRT1
substrate by taking the part of its C-terminal (residues 372–389). This peptide-type
inhibitor had an IC 50 value of 2 μM for SIRT1. The inhibition of its truncated
analogs such as N
α -Fmoc-N
ε -thioacetyl-lysine and N
α
-acetyl-N
ε -thioacetyl-lysine
was also tested on SIRT1, but they displayed virtually no inhibition at all [53]. However, the inhibition potential of various acetylated and thioacetylated peptides was
studied with SIRT1, SIRT2, and SIRT3, and several low micromolar inhibitors were
found [54]. The mechanism of thioacetylated peptidic inhibitors was examined, and
it was observed that the replacement of acetyl-group with thioacetyl-group formed a
covalent and stable 1
0 -S-alkylimidate intermediate in the deacetylation reaction
instead of O-alkylamidate. This stalled intermediate produced very slowly a
deacetylated peptide and 1
0 -SH-2
0 -O-acetyl-ADP-ribose [55].
The acetyl-lysine in peptide substrate can be replaced with various groups such as
thioacetyl- or trifluoroacetyl-group (examples in Fig. 3). Huhtiniemi and coworkers
devised a set of acetyl-lysine analogs to evaluate their inhibitory activity on SIRT1
and SIRT2 [56]. N
ε -Thioacetyl-lysine (compound 7 in Fig. 3) displayed the best
inhibitory activity toward SIRT1, whereas the selenoacetyl moiety (compound 8 in
Fig. 3) had the best inhibition toward SIRT2. The study revealed that the substrate
binding site can accommodate moieties larger than an acetyl-group such as
60
M. Rahnasto-Rilla et al.
activity in cells even though it acted as an inhibitor in vitro [47].
Several NAM analogs were developed as sirtuin inhibitors [48–51]. A series of
2-anilinobenzamide derivatives showed SIRT1 inhibition with IC 50 values between
52 and 300 μM [48]. Some of these derivatives were even more potent toward
SIRT2. The most potent derivative displayed an IC 50 value of 0.57 μM (compound
4 in Fig. 2) for SIRT2 [50]. Recently, a 2-anilinobenzamide scaffold was combined
with a peptide substrate mimicking inhibitor, and a novel type of inhibitor for SIRT2
was identified. This compound, 3
0 -phenethyloxy-2-anilinobenzamide, was an
isoform-selective inhibitor, with an IC 50 value of 28 μM for SIRT2 [52].
In addition, other NAM derivatives, (5-benzamidonaphthalen-1/2-yloxy)nicotinamides, have been reported to be potent sirtuin inhibitors for several sirtuin isoforms:
The IC 50 values of the derivatives were 0.80–100 μM for SIRT1, 48 nM–1 μM for
SIRT2, and 4.4–232 μM for SIRT3. Interestingly, the most potent of these derivatives (compound 5 in Fig. 2) showed excellent selectivity toward SIRT2 with an
IC 50 value of 48 nM, whereas its ability to inhibit the activity of SIRT1 and SIRT3
was at the micromolar level. Based on the kinetic studies, it was postulated that
compound 5 acted as a competitive inhibitor against the peptide substrate and in a
noncompetitive manner against NAD
+
. Compound 5 exhibited also moderate anticancer activity in breast cancer (MCF7), prostate cancer (DU 145), and chronic
myelogenous leukemia cell lines [41].
2.2 Substrate-Based Inhibitors
The first substrate-based inhibitor was developed from human p53 which is a SIRT1
substrate by taking the part of its C-terminal (residues 372–389). This peptide-type
inhibitor had an IC 50 value of 2 μM for SIRT1. The inhibition of its truncated
analogs such as N
α -Fmoc-N
ε -thioacetyl-lysine and N
α
-acetyl-N
ε -thioacetyl-lysine
was also tested on SIRT1, but they displayed virtually no inhibition at all [53]. However, the inhibition potential of various acetylated and thioacetylated peptides was
studied with SIRT1, SIRT2, and SIRT3, and several low micromolar inhibitors were
found [54]. The mechanism of thioacetylated peptidic inhibitors was examined, and
it was observed that the replacement of acetyl-group with thioacetyl-group formed a
covalent and stable 1
0 -S-alkylimidate intermediate in the deacetylation reaction
instead of O-alkylamidate. This stalled intermediate produced very slowly a
deacetylated peptide and 1
0 -SH-2
0 -O-acetyl-ADP-ribose [55].
The acetyl-lysine in peptide substrate can be replaced with various groups such as
thioacetyl- or trifluoroacetyl-group (examples in Fig. 3). Huhtiniemi and coworkers
devised a set of acetyl-lysine analogs to evaluate their inhibitory activity on SIRT1
and SIRT2 [56]. N
ε -Thioacetyl-lysine (compound 7 in Fig. 3) displayed the best
inhibitory activity toward SIRT1, whereas the selenoacetyl moiety (compound 8 in
Fig. 3) had the best inhibition toward SIRT2. The study revealed that the substrate
binding site can accommodate moieties larger than an acetyl-group such as
60
M. Rahnasto-Rilla et al.