Improving the affinity of SL0101 for RSK using structure-based design

Article abstract of DOI:10.1021/ml300298v

Enhanced activity of the Ser/Thr protein kinase, RSK, is associated with transformation and metastasis, which suggests that RSK is an attractive drug target. The natural product SL0101 (kaempferol 3-O-(3″,4″-di-O- acetyl-α-L-rhamnopyranoside)) has been shown to be an RSK selective inhibitor. However, the K_i for SL0101 is 1 μM with a half-life of less than 30 min in vivo. To identify analogues with improved efficacy we designed a set of analogues based on the crystallographic model of SL0101 in complex with the RSK2 N-terminal kinase domain. We identified an analogue with a 5″-n-propyl group on the rhamnose that has >40-fold improved affinity for RSK relative to SL0101 in an in vitro kinase assay. This analogue preferentially inhibited the proliferation of the human breast cancer line, MCF-7, versus the normal untransformed breast line, MCF-10A, which is consistent with results using SL0101. However, the efficacy of the 5″-n-propyl analogue to inhibit MCF-7 proliferation was only 2-fold better than for SL0101, which we hypothesize is due to limited membrane permeability. The improved affinity of the 5″-n-propyl analogue for RSK will aid in the design of future compounds for in vivo use.

Full text of DOI:10.1021/ml300298v

Letter
pubs.acs.org/acsmedchemlett
Improving the Affinity of SL0101 for RSK Using Structure-Based
Design
Roman M. Mrozowski,†,‡ Rajender Vemula,‡,§ Bulan Wu,‡,∥ Qi Zhang,‡,§ Benjamin R. Schroeder,⊥
Michael K. Hilinski,# David E. Clark,# Sidney M. Hecht,¶ George A. O’Doherty,*,§
and Deborah A. Lannigan*,†
†Department of Pathology, Microbiology & Immunology, Vanderbilt University, Nashville, Tennessee, United States
§Department of Chemistry, Northeastern University, Boston, Massachusetts, United States
∥Department of Chemistry, West Virginia University, Morgantown, West Virginia, United States
⊥Oratory Preparatory School, Summit, New Jersey, United States
#Department of Microbiology, Immunology & Cancer Biology, University of Virginia, Charlottesville, Virginia, United States
¶Center for BioEnergetics, Biodesign Institute, Arizona State University, Tempe, Arizona, United States
S
* Supporting Information
ABSTRACT: Enhanced activity of the Ser/Thr protein kinase, RSK, is associated with
transformation and metastasis, which suggests that RSK is an attractive drug target. The natural
product SL0101 (kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside)) has been shown
to be an RSK selective inhibitor. However, the Ki for SL0101 is 1 μM with a half-life of less than
30 min in vivo. To identify analogues with improved efficacy we designed a set of analogues
based on the crystallographic model of SL0101 in complex with the RSK2 N-terminal kinase
domain. We identified an analogue with a 5″-n-propyl group on the rhamnose that has >40-fold
improved affinity for RSK relative to SL0101 in an in vitro kinase assay. This analogue
preferentially inhibited the proliferation of the human breast cancer line, MCF-7, versus the
normal untransformed breast line, MCF-10A, which is consistent with results using SL0101.
However, the efficacy of the 5″-n-propyl analogue to inhibit MCF-7 proliferation was only 2-
fold better than for SL0101, which we hypothesize is due to limited membrane permeability.
The improved affinity of the 5″-n-propyl analogue for RSK will aid in the design of future compounds for in vivo use.
KEYWORDS: Kaempferol 3-O-(3″,4″-di-O-acetyl-α-L-rhamnopyranoside), SL0101, RSK, p90 ribosomal S6 kinase, kinase inhibitor
he family of Ser/Thr protein kinases, p90 ribosomal S6
Tkinase (RSK), has been implicated in numerous different
cancers including breast, lung and prostate cancer.1−3 There are
four RSK family members, and of these RSK1 and RSK2
promote metastasis. RSK is an unusual kinase in that it contains
two nonidentical functional kinase domains, an N-terminal
(NTKD) and a C-terminal (CTKD) kinase domain.4 The
NTKD, which has a high sequence homology between family
members, belongs to the AGC kinase family and is responsible
for phosphorylation of target substrates. We previously found
that SL0101 (1) (Figure 1), a kaempferol-α-L-(3″,4″)-
diacetylrhamnoside, was a relatively specific inhibitor of the
Figure 1. The RSK inhibitor, SL0101 (1), and analogues examined in
NTKD of RSK and did not inhibit the two most closely related
this study.
kinases, p70 S6 kinase and mitogen- and stress-activated
kinase.5 In in vitro kinase assays with ∼70 kinases, SL0101 (1)
was found to partially inhibit Aurora B and PIM 3.6,7 However,
An effective inhibitor of RSK in vivo would be invaluable in
the study of RSK function in homeostasis and in disease states.
To evaluate the suitability of SL0101 (1) for in vivo use we
these results are not straightforward to interpret because the
relative kinase inhibition is dependent on the [ATP] in the
assay. In both screens the [ATP] was higher in the RSK assay
than with Aurora B and PIM 3, which would result in SL0101
(1) being less effective against RSK than in the assay conditions
used for Aurora B and PIM 3.
Received: September 24, 2012
Accepted: December 26, 2012
Published: December 26, 2012
© 2012 American Chemical Society
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Letter
analyzed its pharmacokinetic behavior by intravenous (iv) and
intraperitoneal (ip) administration of a single dose into male
CD-1 mice. Because of the limited solubility of SL0101 (1) a
carrier of 1:1:15 Cremophor:EtOH:phosphate-buffered saline
was required. Regardless of the dosing method, the half-life
(t1/2) of SL0101 (1) was <30 min (Table 1). More importantly,
Table 1. Pharmacokinetic Analysis of SL0101 (1) in Male
a
CD-1 Mice
iv dose
(mg/kg)
AUC/D (ng·h·kg/mL/
C0 (ng/
mL)
Cmax (ng/
mL)
t1/2
(h)
mg)
1 (iv)
38.7
291
0.15
0.40
2.5 (ip)
1023
1851
a
AUC/D: area under the curve, extrapolated to infinity and
normalized to the dose in mg/kg. C0 (ng/mL): maximum plasma
concentration extrapolated to t = 0. Cmax (ng/mL): maximum plasma
concentration. t1/2 (h): half-life.
Figure 2. A hydrophobic pocket within the RSK2 NTKD allows for an
extended 5″ aliphatic chain on the rhamnose. Molecular docking of
analogues 3″,4″-diacetyl versions of 3 and 4 (inset) were performed
using the X-ray crystallographic structure of SL0101 (1) in complex
with RSK2 NTKD.8
the maximum concentration achieved was ∼10-fold below that
required to inhibit proliferation of the breast cancer cell line,
MCF-7, in culture.5 Thus, SL0101 (1) is not suitable for in vivo
testing and a medicinal chemistry effort is required to identify
analogues with improved pharmacokinetic properties, as well as
potency and stability.
and positioning of the acetyl groups were varied in combination
with the 5″-n-propyl group. Efficacy of SL0101 analogues in
cell-based assays can be limited by their solubility10 so we
selected the 5″-n-propyl analogue 3 as the parent compound
based on its lower LogP compared to 4 (Table 2).
The crystal structure of SL0101 (1) in complex with the
RSK2 NTKD has been reported.8 The SL0101 (1) binding
pocket overlaps with the ATP binding site of the NTKD but is
distinct from it, as it is formed by a substantial structural
rearrangement of the N-lobe of the kinase domain. The
interaction of SL0101 (1) with RSK is partially stabilized by
hydrogen bonds between the protein and phenolic hydroxyls
on the kaempferol backbone in which the hydroxyl groups
serve as hydrogen bond donors. Previously, we determined that
loss of any of these hydroxyl groups substantially decreased the
affinity of RSK for SL0101 (1).9 Furthermore, an analogue in
which the hydroxyl groups were O-methylated and therefore
could not donate hydrogen bonds did not inhibit RSK.9 Thus
our previous experience with the SAR of SL0101 (1) analogues
is in good agreement with the crystallographic model, which
supports its use in designing future analogues.
The crystallographic model of the RSK2 NTKD in complex
with SL0101 (1) indicates that the 5″-methyl of the rhamnose
only partially fills a hydrophobic pocket.8 We modeled a set of
analogues bearing longer aliphatic chains at the 5″ position
using the docking function of the Molecular Operating
Environment (MOE) program. The RSK2 NTKD in complex
with SL0101 (1) was used as a starting point for the
calculations. The kinase was processed and the analogues
constructed in the binding pocket from the crystallized
inhibitor SL0101 (1) using the build function. We performed
a docking calculation using the “rigid receptor” presets on both
SL0101 (1) and the analogues, and the highest-scoring binding
pose as determined by the calculated binding free energy in
each case was consistent with the crystal structure of the
complex. The results clearly show that the longer aliphatic
chains occupy a hydrophobic area in the binding pocket
unoccupied by any fragment of SL0101 (1) (Figure 2). Thus,
we envisioned a series of C-5″ substituted analogues of SL0101
(2−4, Figure 1). The acetyl groups on the rhamnose contribute
substantially to the IC50 for RSK inhibition as deacetylated
SL0101 is ∼10-fold less potent than SL0101 (1).9 Therefore,
we also envisioned a series of analogues in which the number
a
Table 2. In Vitro Potency of SL0101 (1) and Analogues
RSK2 IC50 (μM)
LogP
p (1)
p (3)
SL0101(1)
0.99 (0.74−1.32)
1.51 (0.45−5.09)
0.71 (0.47−1.09)
0.48 (0.17−1.33)
1.50 (0.90−2.48)
0.62 (0.18−2.15)
0.02 (0.01−0.04)
1.77 (1.24−2.53)
2.11
1.64
2.20
2.41
2.90
3.39
3.17
3.87
2
3
4
5
6
7
8
0.917
0.211
0.073
0.753
0.232
0.007
0.008
0.183
0.804
0.005
0.008
a
IC50: concentration needed for 50% inhibition of RSK2; the 95%
confidence interval (CI) is shown in parentheses; n > 2 in
quadruplicate. p(1): Student’s t test compared to SL0101 (1). p(3):
Student’s t test compared to analogue 3.
Previously, we reported both a traditional carbohydrate11 and
a de novo asymmetric approach12,13 to SL0101 (1)14 and several
carbohydrate analogues (1, 1a−d, Scheme 1).15 While the
carbohydrate approach has some real advantages in terms of
convergency, the de novo approach has the advantage of being
amenable to the divergent late stage substitution of the
Scheme 1. De Novo Approach to SL0101 (1)
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pyranone ring.16−20 Because of its inherent ability to
incorporate C-5″ substitution, we decided to further develop
the de novo approach to pursue the synthesis and evaluation of
the rhamno-sugar of SL0101 analogues 2−8.
Scheme 3. Synthesis of SL0101 Analogues 2−8
Based on our previous experience with SL0101 (1), we
envisioned the desired target molecules 2−8 as being derived
from the appropriately substituted pyranones 12a−d (Scheme
2). Key to the overall efficiency of this approach is that the
Scheme 2. Retrosynthetic Analysis of SL0101 (1) Analogues
enone served as precursor for the installation of the desired
triol functionality in 2−8 for further acylation. Similarly,
pyranones 12a−d could be prepared from a palladium-
catalyzed glycosylation of aglycon 10 with Boc-pyranones
11a−d serving as the glycosyl donor.21,22 Finally, pyranones
11a−d could be derived from furans 13a−d.23,24 To
accomplish this divergent synthetic effort we planned on
preparing all the desired target molecules 2−8 by executing
multistep parallel reactions on three key intermediates (14b−d,
vide infra).
With the desired coupling partners in hand (10 and 11b−d),
we first pursued their coupling and diastereoselective trans-
formation into three structurally divergent allylic alcohols 14b−
d. Exposure of 11b−d and 10 to our typical Pd-catalyzed
glycosylation procedure (2.5 mol % Pd2(DBA)3·CHCl3 and 10
mol % of PPh3) gave the coupling products 12b−d with
complete transfer of anomeric stereochemistry (Scheme 3). A
subsequent Luche reduction25−27 (NaBH4 at −78 °C)
diastereoselectively converted the pyranones 12b−d into the
desired allylic alcohols 14b−d (dr >20:1).
We next pursued the direct conversion of allylic alcohols
14b−d into the desired triols 2−4 with rhamno-stereo-
chemistry. This was accomplished in parallel by exposing
14b−d to typical Upjohn dihydroxylation conditions (NMO/
OsO4 in t-BuOH/acetone).28 The crude products from the
dihydroxylation reactions were filtered through a pad of silica
gel (to remove osmium, N-methylmorpholine and its N-oxide
(NMO)) and subjected to typical hydrogenolysis conditions
(10% Pd/C, 1 atm of H2). To our delight, the three desired
SL0101 (1) analogues 2−4 could be isolated in pure form after
careful silica gel chromatography (43−63% yields).
Buoyed by the success, we next pursued the direct
conversion of 14c into 5−8, by incorporating selective acylation
steps to the previous 2-step sequence. For instance, the allylic
alcohol 14c, with an n-Pr-group, was cleanly converted to the
C-4 acetylated rhamno-diol 5 (i.e., acylation/dihydroxylation/
hydrogenolysis) in 15% yield after silica gel chromatography.
By simply switching the order of acylation and dihydroxylation,
the triacetate 8 was similarly prepared in a 60% overall yield. In
contrast, the synthesis of diacetates 6 and 7 required a selective
diacylation using the orthoester/acylation/hydrolysis/hydro-
genolysis protocol to give 6 (57% overall yield). Finally, by
incorporating an isomerization step into the sequence (i.e.,
orthoester/acylation/isomerization/hydrogenolysis) C-3/C-4
diacetate 7 could be prepared in a 25% overall yield.
The affinities of the C-5″ substituted analogues for RSK were
determined by their ability to inhibit the activity of purified,
recombinant RSK2 in an in vitro kinase assay.5 The data were fit
using nonlinear regression analysis. There was a trend toward
improving the in vitro potency by increasing the chain length,
which is consistent with the modeling results indicating that
longer chains would be preferred, although the differences did
not obtain statistical significance (Table 2). We have reported a
lower IC50 for the synthesized SL0101 (1).29 However, the IC50
is relative and dependent on numerous variables including the
batch of purified, recombinant RSK2. Therefore, to accurately
evaluate the relative potencies of the various analogues each
assay was performed in parallel with SL0101 (1).
In the in vitro kinase assay, introduction of a single acetyl
group as in analogue 5 increased the IC50, but this difference
was not statistically significant (Table 2). Acetyl groups on the
2″, 3″ and 4″ positions (analogue 8) doubled the IC50 in
comparison to the 5″-n-propyl (analogue 3) (Table 2). Acetyl
groups on the 2″ and 4″ positions (analogue 6) did not alter
the IC50. Surprisingly, analogue 7 with acetyl groups on the 3″
and 4″ positions has an IC50 >40-fold lower than SL0101 (1)
(Table 2). These results were unexpected because, although
previously we had found acetylation to be important, the
number or positioning of the acetyl groups was not.9 In the
crystal structure of SL0101 (1) in complex with the RSK2
NTKD the acetyl groups are unresolved.8 However, we
speculate that the favorable van der Waals interactions between
the 5″-n-propyl group and hydrophobic residues in the binding
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pocket restrict the binding orientation of this set of analogues
in such a way that only acetyl groups in the 3″ and 4″ positions
can be accommodated without causing unfavorable interactions
in the binding pocket.
Analogue 7 was further evaluated for its ability to inhibit the
proliferation of the breast cancer cell line MCF-7, in
comparison to SL0101 (1). The aqueous solubility of 7 limited
the concentrations that could be tested to ≤25 μM. At 25 μM
compound 7 was ∼50% more potent at inhibiting proliferation
than SL0101(1) (Figure 3). To determine whether analogue 7
phosphorylates eEF2.30 MCF-7 cells were pretreated with
inhibitor or vehicle and then stimulated with the mitogen,
phorbol myristate acetate (PMA). In the presence of PMA,
peEF2 levels decreased as expected as RSK is active and inhibits
EF2 kinase. Treatment with SL0101 (1) and compound 7 both
increased peEF2 levels compared to the PMA control (Figure
5), which is consistent with inhibition of RSK. We have also
Figure 3. Efficacy of SL0101 (1) and 7 in MCF-7 cells. Various
concentrations of inhibitors were added at time 0, and ATP content
was measured after 48 h of treatment. Values are the fold proliferation
as a percentage of that obtained with vehicle-treated cells (n = 3 in
quadruplicate; bars = SD; *p < 0.05 in a Student’s t test compared to
Figure 5. Comparison of compound 7 and SL0101 (1) on altering
RSK biomarkers in intact cells. Lysates of MCF-7 cells that were
pretreated with inhibitor (SL0101 (100 μM); 7 (25 μM)) and then
treated with vehicle (DMSO) or PMA were analyzed by
immunoblotting. The arrow indicates a band whose intensity decreases
upon treatment of cells with SL0101 (1) and analogue 7.
∧
the vehicle. p < 0.05 in a Student’s t test compared to SL0101 (1)).
was specific for RSK we compared the ability of the compound
to inhibit proliferation of MCF-7 versus the immortalized,
normal breast line, MCF-10A. We have previously found that a
preferential ability to inhibit MCF-7 compared to MCF-10A
proliferation indicates specific inhibition of RSK.5,9,10,29 We
observed that compound 7 inhibits MCF-7 but not MCF-10A
proliferation (Figure 4). These results suggest that compound 7
and SL0101 (1) have similar specificities.
To further examine the specificity of analogue 7 we
compared the ability of SL0101 (1) and compound 7 to alter
the phosphorylation of eukaryotic elongation factor 2 (eEF2).
Inhibition of RSK is known to activate EF2 kinase, which
found that the levels of the oncogene, cyclin D1, are dependent
on RSK activity in MCF-7 cells.31 Consistent with these data
SL0101 (1) and compound 7 decreased cyclin D1 levels. As a
further comparison of the inhibitory profiles of SL0101 (1) and
analogue 7, we immunoblotted the lysates with an antibody
that recognizes the (Lys/Arg)X(Lys/Arg)XX(pSer/pThr)
motif, where X is any amino acid.4 This motif is recognized
by a number of kinases including RSK. Treatment with the
inhibitors resulted in a decrease in the intensity of a band at
∼27 kDa. It is not surprising that we only observed a decrease
in a single band as PMA is a potent mitogen that will activate
many kinases. The intensities of other bands in the immunoblot
were similar between SL0101 (1) and analogue 7. Taken
together, these results indicate that SL0101 (1) and compound
7 have similar specificities.
In summary, we used structure-based design to identify new
analogues that improve on the in vitro potency of SL0101 (1).
A 5″-n-propyl substituent in combination with 3″- and 4″-acetyl
groups (7) on the rhamnose improved the in vitro affinity for
RSK by >40-fold compared to SL0101 (1). Analogue 7
specifically inhibits RSK, but its ability to inhibit the
proliferation of the breast cancer cell line, MCF-7, is only 2-
fold better compared to SL0101 (1), which we hypothesize is
due to limited membrane permeability. These studies will
provide further guidance in designing a potent SL0101 (1)
analogue that can be used in vivo.
Figure 4. Specificity of analogue 7 for inhibition of RSK activity.
Various concentrations of 7 were added to MCF-7 or MCF-10A cells,
and the assay was performed as described in Figure 3 (n = 3 in
quadruplicate; bars = SD; *p < 0.05 in a Student’s t test compared to
the vehicle).
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glycosylated methymycin analogues with structural and stereochemical
diversity. Org. Lett. 2010, 12 (22), 5150−5153.
(14) Shan, M.; O’Doherty, G. A. De novo asymmetric syntheses of
SL0101 and its analogues via a palladium-catalyzed glycosylation. Org.
Lett. 2006, 8 (22), 5149−5152.
ASSOCIATED CONTENT
* Supporting Information
Experimental details. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
S
(15) Shan, M.; O’Doherty, G. A. Synthesis of SL0101 carbasugar
analogues: carbasugars via Pd-catalyzed cyclitolization and post-
cyclitolization transformations. Org. Lett. 2010, 12 (13), 2986−2989.
(16) Wang, H. Y.; Wu, B.; Qi, Z.; Kang, S. W.; Rojanasakul, Y.;
O’Doherty, G. A. C5′-alkyl substitution effects on digitoxigenin α-L-
glycoside cancer cytotoxicity. ACS Med. Chem. Lett. 2011, 2 (4), 259−
263.
(17) Wang, H. Y.; Rojanasakul, Y.; O’Doherty, G. A. Synthesis and
evaluation of the α-D-/α-L-rhamnosyl and amicetosyl digitoxigenin
oligomers as anti-tumor agents. ACS Med. Chem. Lett. 2011, 2 (4),
264−269.
(18) Wang, H. Y.; Xin, W.; Zhou, M.; Stueckle, T. A.; Rojanasakul,
Y.; O’Doherty, G. A. Stereochemical survey of digitoxin mono-
saccharides: new anticancer analogues with enhanced apoptotic
activity and growth inhibitory effect on human non-small cell lung
cancer cell. ACS Med. Chem. Lett. 2011, 2 (1), 73−78.
(19) Iyer, A. K.; Zhou, M.; Azad, N.; Elbaz, H.; Wang, L.; Rogalsky,
D. K.; Rojanasakul, Y.; O’Doherty, G. A.; Langenhan, J. M. A direct
comparison of the anticancer activities of digitoxin MeON-neoglyco-
sides and O-glycosides: oligosaccharide chain length-dependent
induction of caspase-9-mediated apoptosis. ACS Med. Chem. Lett.
2010, 1 (7), 326−330.
(20) Zhou, M.; O’Doherty, G. The de novo synthesis of
oligosaccharides: application to the medicinal chemistry SAR-study
of digitoxin. Curr. Top. Med. Chem. 2008, 8 (2), 114−125.
(21) Babu, R. S.; O’Doherty, G. A. Palladium-catalyzed glycosylation
reaction: de-novo synthesis of trehalose analogues. J. Carbohydr. Chem.
2005, 24 (2), 169−177.
AUTHOR INFORMATION
Corresponding Author
*G.A.O.: e-mail, g.odoherty@neu.edu. D.A.L.: e-mail, deborah.
lannigan@vanderbilt.edu.
■
Author Contributions
‡These authors contributed equally.
Funding
This work was supported by the Department of Defense
(#W81XWH-11-1-0068 to M.K.H.), NIH (GM084386 to
D.A.L. and GM090259 to G.A.O.), and NSF (CHE-1213596).
Notes
The authors declare no competing financial interest.
REFERENCES
■
(1) Eisinger-Mathason, T. S.; Andrade, J.; Lannigan, D. A. RSK in
tumorigenesis: connections to steroid signaling. Steroids 2010, 75 (3),
191−202.
(2) Clark, D. E.; Errington, T. M.; Smith, J. A.; Frierson, H. F., Jr.;
Weber, M. J.; Lannigan, D. A. The serine/threonine protein kinase,
p90 ribosomal S6 kinase, is an important regulator of prostate cancer
cell proliferation. Cancer Res. 2005, 65 (8), 3108−3116.
(3) Lara, R.; Mauri, F. A.; Taylor, H.; Derua, R.; Shia, A.; Gray, C.;
Nicols, A.; Shiner, R. J.; Schofield, E.; Bates, P. A.; Waelkens, E.;
Dallman, M.; Lamb, J.; Zicha, D.; Downward, J.; Seckl, M. J.; Pardo, O.
E. An siRNA screen identifies RSK1 as a key modulator of lung cancer
metastasis. Oncogene 2011, 30 (32), 3513−3521.
(4) Anjum, R.; Blenis, J. The RSK family of kinases: emerging roles in
cellular signalling. Nat. Rev. Mol. Cell Biol. 2008, 9 (10), 747−758.
(5) Smith, J. A.; Poteet-Smith, C. E.; Xu, Y.; Errington, T. M.; Hecht,
S. M.; Lannigan, D. A. Identification of the first specific inhibitor of
p90 ribosomal S6 kinase (RSK) reveals an unexpected role for RSK in
cancer cell proliferation. Cancer Res. 2005, 65 (3), 1027−1034.
(6) Doehn, U.; Hauge, C.; Frank, S. R.; Jensen, C. J.; Duda, K.;
Nielsen, J. V.; Cohen, M. S.; Johansen, J. V.; Winther, B. R.; Lund, L.
R.; Winther, O.; Taunton, J.; Hansen, S. H.; Frodin, M. RSK is a
principal effector of the RAS-ERK pathway for eliciting a coordinate
promotile/invasive gene program and phenotype in epithelial cells.
Mol. Cell 2009, 35 (4), 511−522.
(7) Bain, J.; Plater, L.; Elliott, M.; Shpiro, N.; Hastie, C. J.;
McLauchlan, H.; Klevernic, I.; Arthur, J. S.; Alessi, D. R.; Cohen, P.
The selectivity of protein kinase inhibitors: a further update. Biochem.
J. 2007, 408 (3), 297−315.
(8) Utepbergenov, D.; Derewenda, U.; Olekhnovich, N.; Szukalska,
G.; Banerjee, B.; Hilinski, M. K.; Lannigan, D. A.; Stukenberg, P. T.;
Derewenda, Z. S. Insights into the inhibition of the p90 ribosomal S6
kinase (RSK) by the flavonol glycoside SL0101 from the 1.5 Å crystal
structure of the N-terminal domain of RSK2 with bound inhibitor.
Biochemistry 2012, 51 (33), 6499−6510.
(9) Smith, J. A.; Maloney, D. J.; Hecht, S. M.; Lannigan, D. A.
Structural basis for the activity of the RSK-specific inhibitor, SL0101.
Bioorg. Med. Chem. 2007, 15 (14), 5018−5034.
(22) Wu, B.; Li, M.; O’Doherty, G. A. Synthesis of several
cleistrioside and cleistetroside natural products via a divergent de
novo asymmetric approach. Org. Lett. 2010, 12 (23), 5466−5469.
(23) Harris, J. M.; Keranen, M. D.; Nguyen, H.; Young, V. G.;
O’Doherty, G. A. Syntheses of four D- and L-hexoses via
diastereoselective and enantioselective dihydroxylation reactions.
Carbohydr. Res. 2000, 328 (1), 17−36.
(24) Li, M.; Scott, J. G.; O’Doherty, G. A. Synthesis of 7-oxa-
phomopsolide E and its C-4 epimer. Tetrahedron Lett. 2004, 45 (5),
1005−1009.
(25) Luche, J. L. Lanthanides in organic chemistry. 1. Selective 1,2
reductions of conjugated ketones. J. Am. Chem. Soc. 1978, 100 (7),
2226−2227.
(26) Luche, J.-L.; Rodriguez-Hahn, L.; Crabbe, P. Reduction of
natural enones in the presence of cerium trichloride. J. Chem. Soc.,
Chem. Commun. 1978, 14, 601−602.
(27) Gemal, A. L.; Luche, J. L. Lanthanoids in organic synthesis. 6.
Reduction of α-enones by sodium borohydride in the presence of
lanthanoid chlorides: synthetic and mechanistic aspects. J. Am. Chem.
Soc. 1981, 103 (18), 5454−5459.
(28) VanRheenen, V.; Kelly, R. C.; Cha, D. Y. An improved catalytic
OsO4 oxidation of olefins to cis-1,2-glycols using tertiary amine oxides
as the oxidant. Tetrahedron Lett. 1976, 17 (23), 1973−1976.
(29) Smith, J. A.; Maloney, D. J.; Clark, D. E.; Xu, Y.; Hecht, S. M.;
Lannigan, D. A. Influence of rhamnose substituents on the potency of
SL0101, an inhibitor of the Ser/Thr kinase, RSK. Bioorg. Med. Chem.
2006, 14 (17), 6034−6042.
(30) Wang, X.; Li, W.; Williams, M.; Terada, N.; Alessi, D. R.; Proud,
C. G. Regulation of elongation factor 2 kinase by p90(RSK1) and p70
S6 kinase. EMBO J. 2001, 20 (16), 4370−4379.
(31) Eisinger-Mathason, T. S.; Andrade, J.; Groehler, A. L.; Clark, D.
E.; Muratore-Schroeder, T. L.; Pasic, L.; Smith, J. A.; Shabanowitz, J.;
Hunt, D. F.; Macara, I. G.; Lannigan, D. A. Codependent functions of
RSK2 and the apoptosis-promoting factor TIA-1 in stress granule
assembly and cell survival. Mol. Cell 2008, 31 (5), 722−736.
(10) Hilinski, M. K.; Mrozowski, R. M.; Clark, D. E.; Lannigan, D. A.
Analogs of the RSK inhibitor SL0101: optimization of in vitro
biological stability. Bioorg. Med. Chem. Lett. 2012, 22 (9), 3244−3247.
(11) Maloney, D. J.; Hecht, S. M. Synthesis of a potent and selective
inhibitor of p90 RSK. Org. Lett. 2005, 7 (6), 1097−1099.
(12) Shan, M.; Sharif, E. U.; O’Doherty, G. A. Total synthesis of
jadomycin A and a carbasugar analogue of jadomycin B. Angew. Chem.,
Int. Ed. 2010, 49 (49), 9492−9495.
(13) Borisova, S. A.; Guppi, S. R.; Kim, H. J.; Wu, B.; Penn, J. H.; Liu,
H. W.; O’Doherty, G. A. A de novo approach to the synthesis of
179
dx.doi.org/10.1021/ml300298v | ACS Med. Chem. Lett. 2013, 4, 175−179