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Small molecule inhibitors of Hsp90 potently affect inflammatory disease pathways and exhibit activity in models of rheumatoid arthritis.

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ARTHRITIS & RHEUMATISM
Vol. 58, No. 12, December 2008, pp 3765–3775
DOI 10.1002/art.24047
© 2008, American College of Rheumatology
Small Molecule Inhibitors of Hsp90 Potently Affect
Inflammatory Disease Pathways and Exhibit Activity in
Models of Rheumatoid Arthritis
John W. Rice, James M. Veal, R. Patrick Fadden, Amy F. Barabasz, Jeffrey M. Partridge,
Thomas E. Barta, Laura G. Dubois, Kenneth H. Huang, Sarah R. Mabbett, Melanie A. Silinski,
Paul M. Steed, and Steven E. Hall
Objective. To evaluate the ability of SNX-7081, a
novel small molecule inhibitor of Hsp90, to block components of inflammation, including cytokine production, protein kinase activity, and angiogenic signaling. A
close analog was evaluated in preclinical in vivo models
of rheumatoid arthritis (RA).
Methods. SNX-7081 binding to Hsp90 was characterized in Jurkat cells and RA synovial fibroblasts
(RASFs). Inhibition of NF-␬B nuclear translocation was
evaluated in cellular systems, using lipopolysaccharide
(LPS), tumor necrosis factor ␣, or interleukin-1␤ stimulation. Suppression of cytokine production in THP-1
cells, human umbilical vein endothelial cells, and
RASFs was studied. Disruption of MAPK signaling
cascades by SNX-7081 following growth factor stimulation was assessed. SNX-7081 was tested in 2 relevant
angiogenesis assays: platelet-derived growth factor activation of fibroblasts and LPS-induced nitric oxide
(NO) release in J774 macrophages. A close analog,
SNX-4414, was evaluated in rat collagen-induced arthritis and adjuvant-induced arthritis, following oral treatment.
Results. SNX-7081 showed strong binding affinity
to Hsp90 and expected induction of Hsp70. NF-␬B
nuclear translocation was blocked by SNX-7081 at
nanomolar concentrations, and cytokine production was
potently inhibited. Growth factor activation of ERK and
JNK signaling was significantly reduced by SNX-7081.
NO production was also sharply inhibited. In animal
models, SNX-4414 fully inhibited paw swelling and
improved body weight. Scores for inflammation, pannus
formation, cartilage damage, and bone resorption returned to normal.
Conclusion. The present results demonstrate that
a small molecule Hsp90 inhibitor can impact inflammatory disease processes. The strong in vivo efficacy
observed with SNX-4414 provides preclinical validation
for consideration of Hsp90 inhibitors in the treatment of
RA.
Rheumatoid arthritis (RA) is a prevalent autoimmune inflammatory disease characterized by abnormal infiltration of a range of cell types, including macrophages, T helper cells, B cells, mast cells, and plasma
cells, into the joint synovial tissue (1–5). Accompanying
the cellular invasion is synovial hyperplasia, in which synovial fibroblasts increase in number and exhibit cancer-like
properties, including anchorage-independent growth and
proliferation. Central to disease development and progression is the role of cytokine production and release from the
localized immune cells. This progression involves synovial
fibroblast invasion of cartilage and bone destruction and is
in part osteoclast mediated, utilizing matrix metalloproteases and other matrix-degrading enzymes coupled with
an ongoing angiogenic component. Disease progression
ultimately leads to disability, and in its most severe forms,
RA is associated with increased mortality.
Current therapy for RA includes a range of
options (1). Methotrexate and other disease-modifying
antirheumatic drugs exert their effects through general
immunosuppressive properties. More recently, recogni-
John W. Rice, James M. Veal, PhD, R. Patrick Fadden, PhD,
Amy F. Barabasz, Jeffrey M. Partridge, Thomas E. Barta, PhD, Laura
G. Dubois, Kenneth H. Huang, PhD, Sarah R. Mabbett, Melanie A.
Silinski, PhD, Paul M. Steed, PhD, Steven E. Hall, PhD: Serenex Inc.,
Durham, North Carolina.
All authors own stock or stock options in Serenex Inc.
Address correspondence and reprint requests to James M.
Veal, PhD, Serenex Inc., 323 Foster Street, Durham, NC 27701.
E-mail: jveal8916@gmail.com.
Submitted for publication April 21, 2008; accepted in revised
form August 15, 2008.
3765
3766
tion of the importance of cytokine signaling in RA has
led to the discovery of cytokine-targeting agents, and
such agents are now widely prescribed (3). Drug discovery strategies targeting T cells and B cells have also
proved successful (5). However, despite this range of
options, there remains substantial unmet medical need
in the treatment of RA. For example, in a significant
percentage of patients, the disease fails to respond to
anti–tumor necrosis factor (anti-TNF) therapies, and
disease progression frequently occurs in patients whose
RA initially responded to therapy.
Hsp90 is a member of the chaperone family and
is responsible for guiding late-stage tertiary folding for
multiple protein clients (6,7). Beyond initial folding,
Hsp90 function continues to be needed for maintaining
the conformational integrity of many of its clients. The
chaperoning process is complex, involves multiple cochaperone proteins, and requires an ATP binding and
hydrolysis cycle on the part of Hsp90 for proper functioning. If ATPase activity is disrupted, e.g., by inhibitors
of ATP binding, client proteins lose their proper folding,
which in turn activates degradation-related cellular processes. In particular, client proteins are ubiquitinated via
E3 ligases, leading to proteasome-mediated degradation.
Inhibition of Hsp90 as a treatment strategy has
been primarily evaluated in the context of cancer treatment (7,8). However, there are substantive data indicating that an Hsp90-targeted agent could be useful in the
treatment of inflammatory diseases, including RA. Aberrant cytokine and receptor signaling, angiogenesis, and
cellular invasion are common to both cancer and inflammation. Natural product inhibitors of Hsp90 block activation of the NF-␬B pathway, leading to loss of cytokine
production in macrophages and other cell types (9–13).
Receptor-interacting protein (RIP) and IKK, members
of the NF-␬B signaling pathway, have been shown to be
Hsp90 clients that are degraded following Hsp90 inhibition (14–17). Similarly, activation of MAP kinases
(ERK-1/2, p38, JNK) can be blocked by Hsp90 inhibition (11,13,18). Interleukin-1 receptor–associated kinase
is also a client of Hsp90, and consequently, inhibition of
Hsp90 also has the ability to diminish innate immunity
responses via Toll-like receptor signaling (18). CD14 cell
surface expression and activity are regulated by Hsp90
(19,20). SGT1, a component of the inflammasome, has
also recently been reported to be degraded following
Hsp90 inhibition (21). The receptor tyrosine kinase KIT,
which regulates mast cell actions, appears to be a
sensitive client for Hsp90 with relevance to multiple
diseases (22). In vivo efficacy of Hsp90 inhibitors in
RICE ET AL
preclinical models of arthritis, sepsis, and uveitis supports the relevance of Hsp90 inhibition observed in vitro
(23,24).
Hsp90 chaperone functions are also central to
angiogenesis and invasion. When Hsp90 is inhibited,
hypoxia-inducible factor 1␣ (HIF-1␣) is degraded under
both normoxic and hypoxic conditions, and vascular
endothelial growth factor (VEGF) signaling and production are reduced (25–29). VEGF receptor 1 (VEGFR-1)
and VEGFR-2 are observed to be sensitive clients (30).
Additionally, Hsp90 both regulates Akt activation
(31,32) and forms a critical complex with endothelial
nitric oxide synthase (eNOS) and activated Akt, and
Hsp90 inhibition leads to loss of NO production (33–38).
Hsp90 inhibition is effective in angiogenesis models such
as the Matrigel assay (39). Finally, the transforming
activities of platelet-derived growth factor receptor ␣
(PDGFR␣) are sensitive to Hsp90 inhibition (40).
We have recently reported on a novel series of
small molecule inhibitors of Hsp90 (41). The chemical
scaffold binds competitively to the ATP-binding site of
Hsp90 and is composed of a benzamide moiety coupled
to a tetrahydroindolone. X-ray data demonstrate that
the benzamide functionality acts as a structural isostere
to the adenine ring of ATP. The compounds are of low
molecular weight and demonstrate good pharmacokinetic properties, including bioavailability on oral dosing.
In the present study we evaluated one of these inhibitors, SNX-7081, and confirmed that it is a potent and
specific Hsp90 inhibitor that is able to affect multiple
inflammation-related signaling processes in vitro. Additionally, we found that a close analog of SNX-7081 was
highly active in preclinical in vivo models of RA.
MATERIALS AND METHODS
Reagents. The NIH3T3, THP-1, and J774 mouse macrophage cell lines were obtained from American Type Culture
Collection (Manassas, VA) and grown in Dulbecco’s modified
Eagle’s medium (DMEM)/Ham’s F-12 (Hyclone, Logan, UT)
or RPMI 1640 medium supplemented with 10% fetal bovine
serum (FBS) and 20 mM HEPES buffer. Primary cultures of
human umbilical vein endothelial cells (HUVECs) and
EGM-2 medium (insulin-like growth factor 1 [IGF-1], epidermal growth factor [EGF], fibroblast growth factor [FGF],
VEGF, and 2% FBS) were from Lonza (Rockland, ME).
Cultures of synovial fibroblasts from RA patients (RASFs)
were from Asterand (Detroit, MI) and were also maintained in
DMEM/Ham’s F-12 supplemented with 10% FBS. Primary
antibodies used for high-content analysis of Hsp70 were from
Assay Design (Ann Arbor MI), phospho–ERK-1/2, phospho–
c-Jun, and phospho–tyrosine 100 were from Cell Signaling
(Boston, MA), and NF-␬B was from Santa Cruz Biotechnology
SMALL MOLECULE INHIBITORS OF Hsp90 IN INFLAMMATORY DISEASE
(Santa Cruz, CA). Conjugated secondary antibodies were from
Invitrogen (Carlsbad, CA).
ATP displacement assay and mass spectrometry identification of Hsp90. Ground porcine spleen tissue and Jurkat
cell lysates were prepared by flash freezing followed by homogenization in saline solution at 4°C. These lysates were then
incubated with a Sepharose affinity resin column in order to
reversibly bind a range of proteins containing purine-binding
sites, as described previously (41). SNX-7081 at various concentrations (0.8–500 ␮M) was then incubated with the resin for
5 minutes (spleen lysates) or 90 minutes (Jurkat cell lysates).
Column elution, gel electrophoresis, trypsin digestion of isolated proteins, and mass spectrometry (MS)–based protein
identification were all performed as previously described (41).
High-content analysis assays. A Cellomics (Pittsburgh,
PA) Arrayscan 4.5 was used to perform all high-content
analysis assays and to obtain all images, except for the use of a
BD 435 imager to collect the 20⫻ images of HUVEC NF-␬B
translocation. Cells were seeded onto 96-well plates (Greiner,
Frickenhausen, Germany) and allowed to adhere overnight.
The following day, test compound was added. For PDGF and
growth factor activation assays, cells were treated with medium
containing 0.1% FBS. For NF-␬B and phospho–c-Jun assays in
HUVECs, cells were pretreated for 24 hours before activation
for 30 minutes with 1 ng/ml (final concentration) cytokine.
J774 cells were plated onto collagen and pretreated for 6 hours
before activation for 30 minutes with 100 ng/ml (final concentration) lipopolysaccharide (LPS). NIH3T3 cells were pretreated for 24 hours and then activated for 15 minutes with a
mixture of recombinant human PDGF-BB and PDGF-DD
(100 ng/ml final concentration; R&D Systems, Minneapolis,
MN). HUVECs were pretreated for 24 hours in 0.1% FBS
medium, followed by removal of the treatment medium, which
was replaced for 15 minutes with EGM-2 medium.
Following treatment and activation, cells were fixed
and permeabilized (15 minutes each step) with 4% phosphate
buffered formalin and phosphate buffered saline (PBS) plus
0.1% Triton X-100, respectively. This was followed by 1 hour
of incubation with primary antibody (diluted 1:100 in PBS–1%
bovine serum albumin) and finally, 1 hour of incubation with
fluorescence-conjugated secondary antibody (diluted 1:1,000)
and Hoechst DNA binding dye (diluted 1:50,000). Average
staining intensity for each end point was determined using
either the target activation or compartmental analysis algorithms (Cellomics). For each well of the plate, a minimum of
250 (compartmental analysis) or 500 (target activation) individual cells per well were identified by nuclear staining intensity. Corresponding antibody staining intensity for each individual cell was then determined. Translocation inhibition was
measured as the ratio of staining intensity in activated cells to
that in nonactivated cells.
Cytokine release assays. Cells were pretreated for
either 6 hours (THP-1) or 24 hours (HUVECs and RASFs),
followed by activation for 18–24 hours with interleukin-1␤
(IL-1␤) or LPS. After activation, the supernatants were removed and analyzed using commercially available Luminex
reagents (Bio-Rad, Los Angeles, CA).
Pharmacokinetics. In vivo studies were conducted at
Washington Biotech (Columbia, MD). SNX-4414 was administered by oral gavage at 5 mg/kg to each of 3 female
Sprague-Dawley rats weighing 160–170 gm. Blood samples
3767
were collected into prechilled EDTA-treated tubes by retroorbital sinus bleeding at 5, 15, and 30 minutes and 1, 2, 4, 8, 12,
24, and 48 hours posttreatment and converted to plasma by
centrifuging at 4°C. The samples were stored frozen until
liquid chromatography–tandem MS (LC-MS/MS) analysis.
The concentration of SNX-4414 active species in rat
plasma was determined by LC-MS/MS following protein precipitation with acetonitrile. Samples were thawed on ice and
were not allowed to reach room temperature. To 50 ␮l of
sample, 10 ␮l of 160 mM ammonium acetate (pH 7.4) was
added, followed by 180 ␮l of acetonitrile that contained internal standard (1 ␮g/ml). The LC-MS/MS system consisted of
a Prominence high-performance liquid chromatograph (Shimadzu, Columbia, MD) coupled to a hybrid triple quadrupole/
linear ion trap mass spectrometer with a Turbospray source
(Q-Trap; Applied Biosystems, Foster City, CA). Ten microliters of sample was injected onto an Eclipse XDB-C18 column
(2.1 ⫻ 30 mm, 3.5 mm; Agilent, Santa Clara, CA), and elution
of analytes and internal standard was achieved using a water–
methanol gradient in the presence of 0.08–0.1% formic acid
and 4 mM ammonium formate. The electrospray ionization
mass spectrometer was operated in positive ion mode, using
multiple reaction monitoring. Fragmentation was achieved by
collision-activated dissociation, with collision energy of 31 eV.
Quantitation was performed using analyte-to-standard peak
area ratios, with calibration curves of 2–4,000 ng/ml.
In vivo rat arthritis models. In vivo studies and
followup histopathologic analysis were conducted at Bolder
BioPath (Boulder, CO) (42). Animal care including room,
cage, and equipment sanitation conformed to the guidelines
cited in the Guide for the Care and Use of Laboratory Animals
and the applicable standard operating procedures of the
University of Colorado vivarium.
Collagen-induced arthritis (CIA). Female Lewis rats
(140–170 gm; 10 per group with arthritis, 4 per normal control
group) were anesthetized with isoflurane and injected at the
base of the tail and 2 sites on the back with 300 ␮l Freund’s
incomplete adjuvant (Difco, Detroit, MI) containing 2 mg/ml
bovine type II collagen (Elastin Products, Owensville, MO), on
days 0 and 6. Treatment by oral gavage of a 5 ml/kg solution of
SNX-4414 (at 48- or 72-hour intervals) was initiated on day 9
of the study and continued through day 16, with animals killed
on day 17. Rats were weighed on day 0 and days 9–17 of the
study, and caliper measurements of the ankles were obtained
daily beginning on day 9. Histopathologic features (inflammation, pannus formation, cartilage damage, and bone resorption) in ankle and knee joint specimens were scored on a 0–5
scale (0 ⫽ normal; 5 ⫽ severe disease).
Adjuvant-induced arthritis (AIA). Male Lewis rats (165–
185 gm; 8 per group with arthritis, 8 per normal control group)
were anesthetized with isoflurane and injected at the base of
the tail with 100 ␮l Freund’s complete adjuvant and lipoidal
amine, on day 0. Dosing was the same as in the CIA study and
was initiated on day 7 (at the time of semi-established disease).
Animals were weighed on days 0, 4, 7, and daily from day 8
through day 14, at which time, dose volumes were adjusted.
Starting on day 7 (prior to onset of swelling but after establishment of systemic disease), caliper measurements of the
ankles were obtained daily through day 14. Final body weight
was recorded on day 14. Animals were killed on day 14.
Histopathologic analysis was as described for the CIA study.
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RICE ET AL
Statistical analysis. The significance of differences in
mean paw weights and histopathologic scores between treatment groups was analyzed by Student’s t-test or other tests as
appropriate. P values less than or equal to 0.05 were considered significant.
RESULTS
SNX-7081 binding to Hsp90 and induction of
Hsp70. SNX-7081 is a member of a novel class of small
molecule binders to Hsp90 (Figure 1A). SNX-7081
binding to Hsp90 was evaluated using a previously
described affinity chromatography method (41), that
allows nonenzymatic evaluation of binding of ligands to
proteins that contain ATP or other purine ligand–
binding sites. SNX-7081 bound strongly to Hsp90 from
both tissue and cell lysates (Figure 1B). A 10-point
concentration evaluation in human Jurkat cells yielded
an apparent Ki of 26 nM for binding of SNX-7081 to
Hsp90 (data not shown). No apparent binding to other
proteins captured on the resin was detected. Hsp70
induction was evaluated to assess the ability of SNX7081 to inhibit Hsp90 in RASF cells. Consistent with
inhibition of Hsp90 as a mechanism of action, SNX-7081
potently induced Hsp70, with a 50% inhibition concentration (IC50) of 44 ⫾ 9 nM (mean ⫾ SD) (Figures 1C
and D).
Inhibition of NF-␬B nuclear translocation by
SNX-7081. Inflammatory response due to bacteria (e.g.,
LPS), cytokine signaling (e.g., IL-1␤ or TNF␣), or other
agents involves activation of NF-␬B transcription factor
and its translocation to the nucleus, where it activates
the transcription of multiple genes (43). Stimulation of
HUVECs with 1 ng/ml IL-1␤ or TNF␣ for 30 minutes
caused the expected migration of NF-␬B into the nucleus (Figure 2A). Pretreatment with SNX-7081 for 24
hours fully blocked IL-1␤–induced NF-␬B nuclear translocation, with a mean ⫾ SD IC50 of 114 ⫾ 29 nM. For
comparison, the natural product Hsp90 inhibitor, 17AAG, was also evaluated. Both potency and maximal
inhibition levels were similar to those observed with
SNX-7081 (data not shown). NF-␬B translocation in
J774 macrophage cells was also examined (Figure 2B),
and SNX-7081 inhibited translocation, with an IC50 of
241 ⫾ 57 nM. Additionally, in RASF cells, both IL-1␤–
initiated and TNF␣-initiated NF-␬B nuclear translocation were inhibited by SNX-7081 (Figure 2C), with IC50
values of 44 ⫾ 19 nM and 88 ⫾ 11 nM, respectively. We
also noted that in RASF cells, the level of overall NF-␬B
translocation was reduced only ⬃50–75% as compared
Figure 1. SNX-7081 binding to Hsp90 and induction of Hsp70. A,
Chemical structure of SNX-7081. B, Elution of Hsp90 from a porcine
spleen lysate bound reversibly to a Sepharose capture resin. The resin
was developed to allow reversible binding of a wide range of proteins
that utilize ATP or other purines as ligands. Compound concentrations
increase from left to right (20, 100, and 500 ␮M). C, Representative
images of vehicle control– and SNX-7081–treated rheumatoid arthritis
synovial fibroblast (RASF) cells (magnification ⫻ 10). D, Induction of
Hsp70 in RASFs by SNX-7081. Values are the mean ⫾ SD from
triplicate wells. IC50 ⫽ 50% inhibition concentration.
with levels in control cells that were not activated with
cytokine (Figure 2D).
Down-regulation of cytokine signaling. The ability of SNX-7081 to reduce cytokine levels was evaluated
in several cell types following application of distinct
stimuli. Production of cytokines, including TNF␣, IL-6,
SMALL MOLECULE INHIBITORS OF Hsp90 IN INFLAMMATORY DISEASE
3769
Figure 2. Effects of SNX-7081 (SNX) on NF-␬B translocation. A, Images of human umbilical vein endothelial cells (HUVECs), showing NF-␬B
nuclear localization after stimulation with 1 ng/ml interleukin-1␤ (IL-1␤) and reduced levels of translocation with 24-hour pretreatment with 10
␮M SNX-7081. B, Images of J774 macrophage cells, showing NF-␬B nuclear localization after stimulation with 100 ng/ml lipopolysaccharide
(LPS) and reduced levels of translocation with 6-hour pretreatment with 10 ␮M SNX-7081. C, Images of rheumatoid arthritis synovial fibroblast
(RASF) cells, showing NF-␬B nuclear localization after stimulation with 1 ng/ml tumor necrosis factor ␣ (TNF␣) and reduced levels of
translocation with 24-hour pretreatment with 10 ␮M SNX-7081. (Magnification ⫻ 10.) D, Levels of NF-␬B translocation inhibition achieved with
each cell line and agonist. The degree of translocation inhibition by SNX-7081 was measured by scaling to the ratio of translocation in activated
versus nonactivated cells (mean ⫾ SD of quadruplicate determinations).
IL-1␤, granulocyte–macrophage colony-stimulating factor, and RANTES, was inhibited by SNX-7081, with IC50
values of ⬍100 nM observed in several cases (Table 1).
In LPS-stimulated THP-1 monocytes, potent inhibition
of IL-1␤ and TNF␣ production (IC50 33 nM and 61 nM,
respectively) occurred after only 6 hours of SNX-7081
pretreatment. IL-8 inhibition was relatively weaker. IL-6
production in both HUVECs and RASFs was markedly
inhibited. In contrast to the partial abrogation of NF-␬B
nuclear translocation, if a particular cytokine appeared
sensitive to Hsp90 inhibition, SNX-7081 treatment at
higher concentrations led to complete abrogation of its
production. In addition, overall proliferation of activated HUVECs was potently inhibited by SNX-7081
(IC50 3 nM), but RASF proliferation showed minimal
sensitivity (IC50 7.8 ␮M).
Effects of SNX-7081 on MAP kinase and angiogenic signaling. Activation of MAP kinase signaling
pathways (ERK-1/2, p38, JNK) is implicated in aberrant
inflammatory responses (44). The ability of SNX-7081
to inhibit two of these pathways, i.e., ERK-1/2 signaling
as measured by phospho-ERK levels after growth factor
activation and JNK signaling as measured by phospho–
c-Jun levels after IL-1␤ activation, was evaluated in
stimulated HUVECs (Figure 3). Growth factor–starved
HUVECs showed increased levels of ERK-1/2 signaling
upon activation with a mixture of EGF, IGF-1, VEGF,
FGF, and a low percentage of FBS (2% final in medium); pretreatment with SNX-7081 fully blocked this
effect, with a mean ⫾ SD IC50 of 11 ⫾ 6 nM. In contrast
to ERK-1/2 signaling, a higher baseline level of JNK
signaling was observed as measured by phospho–c-Jun
levels, which increased ⬃2.5-fold upon IL-1␤ stimulation. Nonetheless, SNX-7081 also demonstrated the
ability to markedly reduce phospho–c-Jun levels, with an
IC50 of 94 ⫾ 19 nM. At maximum inhibition, levels of
phospho–c-Jun were ⬃25% below levels measured in
growth factor–starved HUVECs (Figure 3C).
SNX-7081 was further investigated for its ability
to block PDGF-initiated signaling. Using a mixture of
100 ng/ml each of PDGF-BB and PDGF-DD to activate
NIH3T3 fibroblasts, global tyrosine signaling was assessed by measuring phospho–tyrosine 100 staining (Figures 3B and C). This activation was fully reversible by
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RICE ET AL
Table 1. Inhibition of cytokine release following treatment with SNX-7081*
Cells, treatment
Cytokine
THP-1 cells, 18-hour
activation
with LPS
HUVECs, 24-hour
activation
with IL-1␤
RASFs, 24-hour
activation
with IL-1␤
IL-1␤
IL-6
IL-8
IL-15
G-CSF
GM-CSF
MCP-1
MIP-1␣
MIP-1␤
RANTES
TNF␣
IFN␥
IP-10
33 ⫾ 13
NE
791 ⫾ 300
NE
NE
NE
76 ⫾ 5
132 ⫾ 66
NE
250 ⫾ 59
61 ⫾ 24
NE
NE
ND
50 ⫾ 19
161 ⫾ 81
88 ⫾ 10
57 ⫾ 10
43 ⫾ 22
94 ⫾ 9
NE
60 ⫾ 12
53 ⫾ 22
937 ⫾ 37
241 ⫾ 91
56 ⫾ 16
ND
27 ⫾ 16
44 ⫾ 9
56 ⫾ 15
13 ⫾ 4
95 ⫾ 41
NE
NE
NE
31 ⫾ 23
NE
189 ⫾ 129
NE
* Values are the mean ⫾ SEM 50% inhibition concentration (IC50; nM). THP-1 cells were pretreated with
SNX-7081 for 6 hours; human umbilical vein endothelial cells (HUVECs) and rheumatoid arthritis synovial
fibroblasts (RASFs) were pretreated with SNX-7081 for 24 hours. LPS ⫽ lipopolysaccharide; IL-1␤ ⫽
interleukin-1␤; ND ⫽ not determined (since the cytokine was used for stimulation); NE ⫽ no observed effect
(IC50 ⬎ 10 ␮M); G-CSF ⫽ granulocyte colony-stimulating factor; GM-CSF ⫽ granulocyte–macrophage
colony-stimulating factor; MCP-1 ⫽ monocyte chemotactic protein 1; MIP-1␣ ⫽ macrophage inflammatory
protein 1␣; TNF␣ ⫽ tumor necrosis factor ␣; IFN␥ ⫽ interferon-␥; IP-10 ⫽ IFN␥-inducible 10-kd protein.
Figure 3. Effects of SNX-7081 on cellular signaling events. A, Images depicting levels of phospho-ERK in control growth factor–starved HUVECs,
growth factor–stimulated HUVECs, and HUVECs pretreated for 24 hours with SNX-7081 prior to stimulation with growth factor. SNX-7081 prevented
both activation of and increased levels of phospho-ERK. B, Images depicting levels of phospho–tyrosine 100 (p-tyr[100]) and phospho-ERK in control
serum-starved NIH3T3 cells, serum-starved NIH3T3 cells activated with platelet-derived growth factor (PDGF), and serum-starved NIH3T3 cells
pretreated for 24 hours with SNX-7081 prior to activation with PDGF. Levels of phospho-tyrosine and phospho-ERK were quantified by high-content
analysis methods. SNX-7081 prevented both activation of and increased levels of phospho–tyrosine and phospho-ERK. (Magnification ⫻ 10.) C, Signal
transduction inhibition by SNX-7081. The degree of inhibition was measured by scaling to the ratio of signal transduction in activated versus nonactivated
cells (mean ⫾ SD of triplicate determinations). D, Levels of nitric oxide (NO) production in J774 macrophage cells. Activation of J774 cells with 100 ng/ml
LPS caused an increase in NO production, which was blocked by 6-hour pretreatment with SNX-7081. The degree of inhibition of NO release by SNX-7081
was measured by scaling to the ratio of NO release in activated versus nonactivated cells (mean ⫾ SD of quadruplicate determinations). See Figure 2 for
other definitions.
SMALL MOLECULE INHIBITORS OF Hsp90 IN INFLAMMATORY DISEASE
treatment with SNX-7081, and concentration response
evaluation demonstrated an IC50 of 56 ⫾ 7 nM. Additionally, staining for phospho–ERK-1/2 following PDGF
activation demonstrated that phospho-ERK signaling
was inhibited by SNX-7081, with an IC50 of 14 ⫾ 4 nM.
Nitric oxide production. Stimulation of angiogenesis occurs in part through increased production of NO,
and the increase is accomplished by activation of a
complex involving Akt, Hsp90, and eNOS (33–38). SNX7081 was evaluated to determine whether its inhibition
of Hsp90 would disrupt NO production. J774 macrophages were activated with LPS for 18 hours, either in
the absence of SNX-7081or following 6-hour pretreatment with SNX-7081. SNX-7081 pretreatment caused
potent disruption of NO production (Figure 3D), with
an IC50 of 6 ⫾ 2 nM.
In vivo evaluation of an orally active analog of
SNX-7081 in preclinical models of arthritis. Pharmacokinetic evaluation of SNX-7081 indicated that its exposure levels following oral dosing were inadequate to
support evaluation of the compound in vivo. However,
SNX-4414, a closely related analog containing a simple
solubilizing prodrug functionality, was identified and
found to have favorable pharmacokinetic properties
following oral dosing (Figure 4A). SNX-4414 is rapidly
converted from prodrug to active species in vivo, and the
parent compound showed a well-behaved absorption
profile with a half-life of 8.7 hours and a plasma
exposure of 23 ␮g/ml/hour following administration of a
5 mg/kg dose.
SNX-4414 was evaluated in both the rat CIA
model and the rat AIA model (42). Consistent with the
in vitro profile observed with this class of compounds,
significant inhibition of inflammation was demonstrated
in vivo (Figures 4B and C). A clear dose-response and
full suppression of ankle swelling were attained with
both every-2-day and every-3-day treatment schedules.
Measurement of body weights showed improved weight
relative to both untreated and dexamethasone-treated
arthritic rats (Figure 5B), and splenomegaly was reversed. Histopathologic evaluation of ankle and knee
joint specimens for inflammation, pannus formation,
cartilage damage, and bone erosion showed marked
inhibition of these parameters (Figure 5A). Joint sections from both the ankle and the lateral knee showed
extensive pannus formation and severe synovitis in
vehicle-treated rats (Figure 5C). In contrast, joint sections from rats treated with 8 mg/kg oral SNX-4414 did
not exhibit histopathologic abnormalities and were characterized by normal synovium and absence of cartilage
damage or bone erosion.
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Figure 4. In vivo evaluation of the effects of SNX-4414 treatment. A,
Pharmacokinetics following oral (PO) dosing. Values are the mean
from 3 rats treated with a single 5 mg/kg dose of SNX-4414. B, Ankle
diameter in rats with collagen-induced arthritis treated with SNX-4414
administered once every 2 days (Q2D) or once every 3 days (Q3D).
Values are the mean ⫾ SEM in 8 rats per group; values in rats treated
with 8 mg/kg SNX-4414 every 2 days and in positive control rats
treated with 0.6 mg/kg dexamethasone (Dex) per day (QD) overlie the
values in normal control rats. C, Ankle diameter in rats with adjuvantinduced arthritis treated with SNX-4414 administered once every 2
days. Values are the mean ⫾ SEM in 8 rats per group; values in rats
treated with 8 mg/kg, 10 mg/kg, and 12 mg/kg SNX-4414 overlie the
values in normal control rats.
DISCUSSION
Previous clinical evaluations of the therapeutic
potential of Hsp90 inhibition have focused on oncology.
3772
RICE ET AL
Figure 5. Effects of SNX-4414 in rat arthritis models. A, Histopathologic scores in the collagen-induced arthritis (CIA) model.
Histopathologic parameters in rats treated with vehicle or with 8 mg/kg SNX-4414 once every 2 days were scored on a scale of
0–5. B, Weight effects in the adjuvant-induced arthritis model. Rats were treated with 10 mg/kg SNX-4414 once every 2 days
(Q2D), with vehicle, or with 0.1 mg/kg dexamethasone (Dex) per day (QD). Values in A and B are the mean ⫾ SEM in 8 rats
per group. C, Photomicrographs of the lateral knee and ankle in the CIA model. Specimens from a normal rat and a rat treated
with 8 mg/kg SNX-4414 once every 2 days exhibit normal synovium, no cartilage damage (large arrows), and no marginal zone
pannus or bone resorption (small arrows). The specimen from a vehicle-treated arthritic rat exhibits severe synovitis, marked
cartilage damage (large arrow), and moderate pannus and bone resorption (small arrows). F ⫽ fibula; T ⫽tibia (original
magnification ⫻ 25).
However, research has indicated that Hsp90 is also
critically involved in multiple processes related to inflammation, and that inhibition of Hsp90 might therefore also be of benefit in autoimmunity-related diseases.
The commonalities of RA and cancer have been frequently noted (1,2,4). At least 2 compounds, methotrexate and rituximab, are approved for both cancer and RA.
Recently, there was an anecdotal report of RA responding to the cancer drug imatinib in patients who had both
diseases (45). In the present study, we evaluated the
ability of a novel small molecule chemical scaffold,
focusing on the compound SNX-7081, to impact inflammation generally and preclinical models of RA in particular. The objective was to assess the ability of SNX7081 to affect specific cellular processes such as NF-␬B
and PDGF pathway signaling, and also to consider
downstream effects relevant to inflammation, such as
cytokine and NO production.
SNX-7081 was observed to inhibit NF-␬B nuclear
translocation, consistent with reports that proteins involved in NF-␬B pathway signaling, IKK and RIP, are
Hsp90 clients (14–17). The induction of Hsp70, as a
result of Hsp90 inhibition (46), may also contribute to
the observed effects. Hsp70 induction has antiapoptotic
effects (7,47) and, unlike in cancer cells, in which Hsp70
levels are typically strongly up-regulated, this effect may
be beneficial in the setting of inflammatory disease. We
also observed that the maximal level of inhibition attainable with SNX-7081 did not completely abolish NF-␬B
translocation. However, full blockade of cytokine production was observed with multiple cytokines following
SNX-7081 treatment.
One possible explanation for these observations
is that, even if some limited transcriptional activation
remains due to residual NF-␬B translocation, posttranscriptional processes are also affected by Hsp90 inhibition. Supporting this rationale is the observation by
others that activation of p38 can be blocked by inhibiting
Hsp90 (11,13,18). Activated p38 is implicated in maintaining messenger RNA (mRNA) stability through its
actions on 3⬘ AU-rich elements involved in transcriptional stability (9,11). The effect of JNK signaling knockdown, as shown here, on mRNA stability is less clear.
Another interesting consideration is whether heat-shock
SMALL MOLECULE INHIBITORS OF Hsp90 IN INFLAMMATORY DISEASE
factor 1 (HSF-1) may participate in the observed effects
of SNX-7081 on cytokine production. HSF-1 is known to
block transcription of certain cytokine genes (48). This
function could be relevant to the results obtained with
SNX-7081, since HSF-1 is normally maintained in a
repressed monomeric state through binding to Hsp90,
but is released when Hsp90 is inhibited (49).
Hsp90 inhibition by a compound related to SNX7081 has been shown previously to impact p-ERK signaling in a cancer cell line (41). We have demonstrated
in this study that SNX-7081 is capable of abrogating
ERK-1/2 and JNK signaling in HUVECs and 3T3 fibroblasts following growth factor stimulation. It is interesting to note that, while p38 has been a well-established
therapeutic target for inflammation and RA, the RAF/
MEK/ERK pathway has been traditionally thought to be
more relevant to oncology. However, MEK inhibition
has also recently shown promise as a strategy for RA
treatment (50). An Hsp90 inhibitor may be attractive in
that different activated cell types in the RA phenotype
may either preferentially use certain MAP kinase signaling pathways or make redundant use of them (2,44), so
a general blockade of activated signaling would be
preferred.
Angiogenesis is viewed as a key element of the
invasive and destructive phenotype of RA (1,4), and the
importance of Hsp90 for VEGF and HIF-1␣–related
signaling has been established (25–30). The findings
described herein focus on additional aspects of angiogenesis and the role of Hsp90. First, we evaluated the
effects of Hsp90 inhibition on PDGF signaling, given the
increasing understanding of its importance in stromal
angiogenesis and the recent observation that PDGFR␣
is also a client of Hsp90 (40). Treatment with SNX-7081
potently disrupted PDGF-BB activation of protein kinase signaling in HUVECs, RASF cells, and NIH3T3
fibroblasts. SNX-7081 also abrogated NO production in
stimulated macrophages. Multiple mechanisms may account for this strong inhibition. More specifically, maximal eNOS activity requires a complex between eNOS
and Hsp90 with functional ATPase activity, as well as
recruitment of activated Akt (32–38). Disruption of
either the Hsp90 ATPase activity or Akt phosphorylation down-regulates eNOS activity. SNX-7081 potently
inhibited Hsp90 ATPase activity. Akt phosphorylation
was not assessed in this study, but an SNX-7081–related
compound has been shown to block Akt activation in
proliferating cells (41).
A key feature of the chemical series described
herein is that many of its members are potent Hsp90
inhibitors that can be administered orally with good
3773
pharmacokinetics. The in vitro effects shown for SNX7081 were validated in vivo by the efficacy of the analog
SNX-4414 in preclinical models of RA. Ankle and knee
swelling were eliminated in both CIA and AIA. Efficacy
at well-tolerated doses was equal to or superior to that of
dexamethasone, and in contrast to arthritic rats treated
with dexamethasone, continued weight loss was not
observed in those treated with SNX-4414. Histopathologic evaluation of joint sections revealed normal synovium, bone, and cartilage.
Given the broad spectrum of cellular roles played
by Hsp90, the central question arises as to whether an
Hsp90 inhibitor would have a sufficient therapeutic
index to be efficacious in the treatment of RA or other
inflammatory diseases. Obviously, although the animal
data presented here are encouraging, this question can
ultimately be addressed only by clinical evaluation.
However, several observations may suggest that Hsp90
inhibition would be effective in treating inflammation,
possibly even more than has been found in cancer. In
cancer, one treatment goal is to disrupt oncogenic
signaling via Hsp90 client degradation, but the more
difficult and stringent goal, perhaps achieved in combination with other agents, is to cause cancer cell apoptosis. For inflammation, simple disruption of Hsp90
client–based signaling may well be sufficient to have
meaningful impact on disease. Encouragingly, 6-houronly treatments with SNX-7081 were able to cause
substantial cellular response. Additionally, the rebound
of elements such as cytokine and phosphorylation signaling, following Hsp90 client recovery, may be more
muted in RA or other inflammatory diseases relative to
proliferation in cancer, such that less frequent dosing
with an Hsp90 inhibitor would be needed for achievement of a prolonged effect.
RA is a multifaceted disease for which additional
treatments are still needed. The pleiotropic abilities of
the Hsp90 inhibitors described herein to affect cytokine
production, signaling cascades, and angiogenesis support their further evaluation as therapies for RA and
other inflammatory diseases.
AUTHOR CONTRIBUTIONS
Dr. Veal had full access to all of the data in the study and
takes responsibility for the integrity of the data and the accuracy of the
data analysis.
Study design. Rice, Veal, Fadden, Barta, Steed, Hall.
Acquisition of data. Rice, Barabasz, Partridge, Dubois, Mabbett,
Silinski, Steed.
Analysis and interpretation of data. Rice, Veal, Barabasz, Partridge,
Dubois, Mabbett, Silinski, Steed, Hall.
Manuscript preparation. Rice, Veal, Hall.
3774
RICE ET AL
Statistical analysis. Rice.
Design and synthesis of compounds. Barta, Huang.
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