Development of small molecules targeting the pseudokinase Her3

Article abstract of DOI:10.1016/j.bmcl.2015.04.103

Abstract Her3 is a member of the human epidermal growth factor receptor (EGFR) tyrosine kinase family, and it is often either overexpressed or deregulated in many types of human cancer. Her3 has not been the subject of small-molecule inhibitor development

Full text of DOI:10.1016/j.bmcl.2015.04.103

Bioorganic & Medicinal Chemistry Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Development of small molecules targeting the pseudokinase Her3
Sang Min Lim a,b,c, , Ting Xie a,b,c, , Kenneth D. Westover d, Scott B. Ficarro b,e, Hyun Seop Tae f,
Deepak Gurbani d, Taebo Sim g,h, Jarrod A. Marto b,e, Pasi A. Jänne i, Craig M. Crews f, Nathanael S. Gray a,b,
⇑
a Department of Cancer Biology, Dana-Farber Cancer Institute, Boston, MA 02115, USA
b Department of Biological Chemistry & Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA
c Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA
d Departments of Biochemistry and Radiation Oncology, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA
e Department of Cancer Biology and Blais Proteomics Center, Dana-Farber Cancer Institute, Boston, MA 02115, USA
f Departments of Chemistry, Pharmacology, and Molecular, Cellular and Development Biology, Yale University, New Haven, CT 06511, USA
g Chemical Kinomics Research Center, Korea Institute of Science and Technology, Seongbuk-gu, Seoul 136-791, Republic of Korea
h KU-KIST Graduate School of Converging Science and Technology, Seongbuk-gu, Seoul 136-713, Republic of Korea
i Lowe Center for Thoracic Oncology, Dana-Farber Cancer Institute, Boston, MA 02115, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Her3 is a member of the human epidermal growth factor receptor (EGFR) tyrosine kinase family, and it is
often either overexpressed or deregulated in many types of human cancer. Her3 has not been the subject
of small-molecule inhibitor development because it is a pseudokinase and does not possess appreciable
kinase activity. We recently reported on the development of the first selective irreversible Her3 ligand
(TX1-85-1) that forms a covalent bond with cysteine 721 which is unique to Her3 among all kinases.
We also developed a bi-functional compound (TX2-121-1) containing a hydrophobic adamantane moiety
and the same warhead of TX1-85-1 that is capable of inhibiting Her3-dependent signaling and growth.
Here we report on the structure-based medicinal chemistry effort that resulted in the discovery of these
two compounds.
Received 19 March 2015
Revised 22 April 2015
Accepted 30 April 2015
Available online xxxx
Keywords:
Her3
Pseudokinase
Hydrophobic tagging
Cancer
Ó 2015 Elsevier Ltd. All rights reserved.
Pyrazolopyrimidine
Her3 is a member of the human epidermal growth factor recep-
tor (EGFR) tyrosine kinase family along with Her1 (EGFR), Her2 and
Her4. EGFR family-dependent signaling frequently becomes dereg-
ulated in human cancers due to either receptor/ligand over-expres-
sion or oncogenic mutations that result in constitutive activation of
the kinase domain.1,2 Extensive efforts to target activated mutants
of EGFR and Her2 for the treatment of patients with non-small cell
lung cancers (NSCLCs) and breast cancer has resulted in develop-
ment of several FDA-approved drugs such as Gefitinib, Erlotinib,
Lapatinib, and Afatinib. In stark contrast, Her3 has not been the
intentional target of small-molecule inhibitor discovery efforts
because Her3 has been historically classified as a ‘pseudokinase’
because: (1) early biochemical assays showed that Her3 was not
capable of catalyzing auto-phosphorylation and phosphorylation
of substrates,3,4 (2) Asp 813 of EGFR, a conserved residue acting
as a catalytic base for the transfer of the phosphate group, is
replaced with the catalytically ineffective Asn 815 in Her3, and
(3) substitution of His 740 in Her3 for Glu 738 of EGFR prevents
Her3 from forming a key salt bridge that is important for maintain-
ing an active kinase conformation.5,6 A recent report suggests that
Her3 may possess very weak kinase activity,6 although it remains
unclear whether this activity is required for Her3-dependent sig-
naling. Despite questions regarding its kinase activity, Her3 is well
documented as an essential hetero-dimerization partner with
EGFR, Her2, and c-Met.1,2,7 The Her3 kinase domain serves as an
activator of a heterodimer5,8 resulting in phosphorylation of tyro-
sine residues in the C-terminus of the heterodimer followed by
eventual activation of the PI3K/Akt signaling network.1,2 This cru-
cial hetero-dimerization of Her3 has been suggested as a molecular
basis of the acquired resistance in a subset of NSCLCs and breast
cancer.7,9 These findings, in addition to the fact that Her3 is over-
expressed and deregulated in several human cancers,9–13 have
inspired the development of antibody-based antagonists such as
Pertuzimab which target the ligand binding domain of Her3.14–17
Here we describe our efforts to develop Her3-targeting small
molecules that can down-regulate Her3-dependent signaling. We
hypothesized that ATP-competitive Her3 ligands may exhibit
⇑
Corresponding author at present address: Dana Farber Cancer Institute,
Longwood Center, Room 2209, 360 Longwood Avenue, Boston, MA 02215, USA.
Tel.: +1 617 582 8590; fax: +1 617 582 8615.
E-mail address: Nathanael_Gray@dfci.harvard.edu (N.S. Gray).
These authors equally contributed to this work.
http://dx.doi.org/10.1016/j.bmcl.2015.04.103
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Lim, S. M.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.04.103

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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
pharmacology either because they can induce a conformation of
Her3 that results in non-productive heterodimerization or because
they block the low but requisite kinase activity of Her3.6 Our strat-
egy to target Her3 was to develop ligands that would be capable of
forming a covalent bond with Cys721 which is uniquely present in
Her3 relative to all other kinases in the kinome. Cysteine 721 of
Her3 is situated on the ‘roof’ of the ATP-binding pocket, located
approximately 3.4 Å above the adenine ring of ATP. Kinome-wide
sequence alignments demonstrate that this cysteine residue is
unique to Her3.18 This suggests that a suitably designed covalent
Her3 inhibitor could be exceptionally selective through forming a
new covalent bond. One of the common strategies to develop irre-
versible kinase inhibitors is to employ structure-based design to
modify reversible inhibitors with a reactive electrophile.18 When
we initiated this work there were no reported Her3 ligands so
we developed an ATP-competitive ligand binding assay employing
the fluorescence resonance energy transfer (FRET) based
LanthaScreen™ Eu methodology19 and screened our in-house
library composed of approximately 1500 known ATP-competitive
kinase inhibitors. The most potent Her3 binders identified from
the screening assay were KIN001-111,20 dasatinib,21 bosutinib,22
and KIN001-51,23,24 all of which possessed IC50 of below 100 nM
against Her3 in the binding assay (Fig. 1). We selected KIN001-
51 as our starting point to develop covalent Her3 binders because
of its molecular simplicity and potential for facile modification to
synthesize analogs. Molecular docking studies using the Her3 crys-
tal structure (PDB ID: 3KEX) indicated that the phenyl ring imme-
diately adjacent to the pyrrolopyrimidine ring of KIN001-51 is
located close to Cys 721. Modeling results suggested that installa-
tion of an electrophile at the meta position of the phenyl ring
might provide the correct trajectory for forming a covalent bond
with Cys 721 (Fig. 2). Based on these docking study results,
SML4-82-1 (1a) possessing a meta-acryl amide, was synthesized
as our first covalent Her3 binder. SML4-82-1 was prepared by
iodination of 4-amino-7H-pyrrolo[2,3-d]pyrimidine followed by
N-alkylation with bromocyclopentane (Scheme 1). The
Figure 1. The four most potent Her3 binders obtained from high-throughput screening of our in-house kinase inhibitor library employing a FRET-based protein binding
assay.
Figure 2. A docking study of KIN001-51 against a Her3 crystal structure (PDB ID: 3KEX) suggested that SML4-82-1, the meta-acryl amide, might be capable of forming a
covalent bond with Cys 721 of Her3.
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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
3
4-phenoxyphenyl group was installed by a Suzuki coupling using
3-nitro-4-fluorophenylboronic acid and subsequent nucleophilic
aryl substitution reaction with phenol. The protection of the exo-
amine with di-Boc group, reduction of the nitro group, selective
formation of an acryl amide followed by the final deprotection of
the di-Boc group provided the target compound. The IC50 of
SML4-82-1 was 286 nM in the fixed time-point binding assay. In
order to determine whether SML4-82-1 could covalently modify
Her3, we incubated purified Her3 with SML4-82-1 at a molar ratio
of 1:10 (Her3/SML4-82-1) for 3 h at ambient temperature.
Electrospray ionization mass spectrometry detected single
a
adduct between Her3 and SML4-82-1, although complete labeling
was not achieved under the conditions examined (Fig. S1A).
Several other incubation conditions were tested to improve the
labeling efficiency without success. We attributed this, in part, to
the intrinsic instability of purified Her3 protein which prevented
incubations longer than 3 h or elevation of the temperature above
25 °C. Despite the incomplete labeling, proteolytic digestion and
Scheme 1. Synthesis of SML4-82-1. Reagents and Conditions: (i) bromocyclopentane, K2CO3, DMF, 100 °C, 28%; (ii) 3-nitro-4-fluorophenylboronic acid, PdCl2(PPh3)2, 1 M
Na2CO3, dioxane, 100 °C, 42%; (iii) phenol, K2CO3, DMF, 80 °C, 95%; (iv) (Boc)2O, DMAP, DCM, rt, 70%; (v) Raney Ni, H2, MeOH, rt; (vi) acryloyl chloride, DIPEA, DCM, 0 °C; (vii)
TFA, DCM, rt, 61% for 3 steps.
Table 1
Effects of electrophiles and heterocyclic cores on potency of compounds
O
E
NH2
N
X
N
N
1
Compounds
X
E
IC50 (nM)
286
Compounds
X
E
IC50 (nM)
>10,000
H
N
MeO2S
1a (SML4-82-1)
CH
1e
N
O
H
N
MeO2C
O
Cl
1b
1c
1d
CH
CH
N
878
1f
N
N
>10,000
1020
O
O
6960
130
1g
N
H
H
N
O
Please cite this article in press as: Lim, S. M.; et al. Bioorg. Med. Chem. Lett. (2015), http://dx.doi.org/10.1016/j.bmcl.2015.04.103

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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
Figure 3. Design of TX1-85-1 by structure-guided hybridization between 1d and KIN001-111. TX1-85-1 is composed of the covalent bond forming moiety of 1d (square) and
the solubilizing tail of KIN001-111 (oval).
Scheme 2. Synthesis of TX1-85-1. Reagents and Conditions: (i) 1,4-dioxaspiro[4.5]decan-8-ol, Ph3P, DEAD, THF, rt; (ii) acetone, 5 M HCl, 45% for 2 steps; (iii) 1-
acetylpiperazine, Na(OAc)3BH, ClCH2CH2Cl, rt, 62%; (iv) 3-nitro-4-fluorophenylboronic acid, PdCl2(PPh3)2, K2CO3, dioxane/H2O (3:1), 100 °C, 75%; (v) phenol, K2CO3, DMF,
80 °C, 90%; (vi) Raney Ni, H2, MeOH, rt; (vii) acryloyl chloride, THF, NaHCO3, 0 °C, 35% over 2 steps.
analysis of the resulting peptides by LC/MS demonstrated the
exclusive covalent labeling of Cys 721 (Fig. S1B). These combined
results demonstrate that SML4-82-1 is a potent, covalent Her3
binder.
Despite SML4-82-1 being a covalent binder, based on our fixed
3 hour binding assay, its IC50 is approximately 3-fold less potent
than the reversible, non-acrylamide modified KIN001-51
(IC50 = 85 nM). One possible interpretation is that Her3 labeling is
incomplete after 3 h in agreement with the labeling efficiency
determined by mass spectrometry. We therefore hypothesized that
optimization of the electrophile or the trajectory of the acrylamide
moiety would improve the potency of this compound. Therefore,
we screened several different electrophiles varying the length
and the reactivity to identify ones that could more rapidly modify
Her3. The
a-chloroacetamide 1b and the methylene acrylamide 1c
were synthesized analogously to SML4-82-1. The
a,b-unsaturated
sulfone 1e, methyl ester 1f, and ketone 1g were synthesized by a
Suzuki coupling reaction with 4-fluoro-3-formylphenylboronic
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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
5
acid followed by Horner–Wadsworth–Emmons reaction with cor-
responding phosphonate esters, respectively. Unfortunately, none
of these alternative electrophiles resulted in more efficient labeling
of Her3 relative to the acryl amide (Table 1). We next sought to
alter the dihedral angle between the pyrrole core and phenyl sub-
stituent by introducing ortho nitrogen. We observed that switching
from the pyrrolopyrimidine core to the pyrazolopyrimidine (1a vs
1d) increased the potency by approximately 2 fold (Table 1).
Considering the most potent hit KIN001-111 also contains the
pyrazolopyrimidine core, we decided to synthesize acryl amides
of the pyrazolopyrimidine for the further development of potent,
covalent Her3 binders.
To further improve the labeling efficiency of the compounds, we
employed structure-guided hybridization of our lead compounds
with alternative Her3 scaffolds as this has previously proven to
be a productive approach.25,26 We envisioned that incorporation
of the solubility enhancing tail of the most potent hit KIN001-
111 to 1d might provide more potent covalent Her3 binders, which
led us to synthesize TX1-85-1 (Fig. 3). The synthesis of TX1-85-1
commenced with installation of the solubilizing tail to 3-iodo-4-
KiNativ approach27–29 demonstrated that TX1-85-1 potently binds
to Her3 as well as to Lyn, Her2 and several other Src family kinases
at 2 lM (Table S1). We next evaluated the ability of TX1-85-1 to
inhibit Her3-dependent signaling and cell proliferation. However,
as reported previously,30 TX1-85-1 showed an EC50 of greater than
10 lM in cancer cell lines validated as Her3 dependent for growth
by siRNA-mediated depletion of Her3.30 Moreover, there was no
inhibition of Akt phosphorylation, an important downstream effec-
tor of Her3 at a concentration of 5 lM. These results indicate that
despite successful intracellular target engagement of Her3 by TX1-
85-1,30 the compound is not capable of inhibiting Her3-dependent
growth and signaling in the lines we tested. Collectively, although
we cannot exclude the possibility of residual Her3 activity follow-
ing inhibitor treatment, these data suggest that inhibition of low
kinase activity of Her3 with ATP-competitive, covalent Her3 bin-
ders may not be an effective strategy to inhibit Her3-dependent
functions.
Her3 requires hetero-dimerization with either EGFR, Her2, and
c-Met to initiate Her3 cellular signaling. Despite its low kinase
activity, the Her3 kinase domain has proven to serve as an ‘‘activa-
tor’’ within the asymmetric heterodimer resulting in Her3-depen-
dent signaling.5,8 We hypothesized that perturbation of Her3
dimer may impair the activating function of Her3 and eventually
inhibit Her3-dependent functions. One approach we envisioned
was targeted, selective Her3 degradation by using our selective,
covalent Her3 binders as a foothold. Recent reports have shown
that covalent modification of target proteins with hydrophobic
aminopyrazolo[3,4-d]pyrimidine through
a 3-step sequence:
Mitsunobu reaction with 1,4-dioxaspiro[4.5]decan-8-ol,
deprotection of the ketal group, and reductive amination with
1-acetylpiperazine (Scheme 2). The synthesis of TX1-85-1 was
completed using the same route as we described in the synthesis
of SML4-82-1. TX1-85-1 possessed a substantially improved IC50
of 23 nM in our FRET-based binding assay, approximately 6 times
more potent than 1d. Electrospray ionization mass spectrometry
unambiguously revealed covalent modification of Her3 by TX1-
85-1, although the labeling was still not complete after the 3 hour
incubation (Fig. S1C).
groups can result in proteasome-mediated degradation.31–33
A
detailed understanding of this phenomenon has not been fully
achieved but presumably relies on mimicry of a partially unfolded
state for the target protein by adding a hydrophobic tag which
turns on the protein homeostasis machinery degrading the target
protein by the proteasome.31 We modified our covalent Her3
We investigated the specificity of TX1-85-1 against the entire
kinome. A live cell chemical proteomics experiment using the
Scheme 3. Synthesis of TX2-121-1. Reagents and conditions: (i) tert-butyl 4-bromopiperidine-1-carboxylate, K2CO3, DMF, 64%; (ii) 3-nitro-4-fluorophenylboronic acid,
PdCl2(PPh3)2, K2CO3, dioxane/H2O (3:1), 100 °C, 75%; (iii) phenol, K2CO3, DMF, 80 °C, 90%; (vi) 1 M HCl, THF, rt, 95%; (v) N-boc-2-bromoethylamine, K2CO3, DMF, 80 °C, 81%;
(vi) 1 M HCl, THF, rt, 98%; (vii) (R)-4-((3R,5R,7R)-adamantan-1-yl)-2-methylbutanoic acid, EDCI, DMAP, DIPEA, DCM, 43%; (viii) (Boc)2O, DMAP, DCM, 95%; (ix) Raney Ni, H2,
MeOH, rt; (x) acryloyl chloride, THF, NaHCO3, 0 °C; (xi) TFA, DCM, rt, 32% over 3 steps.
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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
binders with dense hydrophobic adamantine tags, which have
been used previously for induction of proteasomal degradation
with a minimal increase of molecular weight.32 One of these biva-
lent conjugates TX2-121-1 (2e) was prepared by N-alkylation of
3-iodo-1H-pyrazolo[3,4-d]pyrimidin-4-amine
with
tert-butyl
4-bromopiperidine-1-carboxylate followed by deprotection of
Boc group to furnish a free piperidine (Scheme 3). Another N-alky-
lation with N-boc-2-bromoethylamine, subsequent installation of
the 4-phenoxyphenol group, and deprotection of Boc group
provided a free primary amine. The adamantane moiety was intro-
duced by an EDC-mediated amide coupling with (R)-4-((3R,5R,7R)-
adamantan-1-yl)-2-methylbutanoic acid,32 and the final formation
of an acryl amide completed the synthesis of TX2-121-1. The other
four compounds were prepared in an analogous fashion. The FRET-
based binding assay indicated that these adamantane conjugates
still retain the ability of binding to Her3, showing IC50s of most
conjugates were close to 50 nM (Table 2). We also performed a live
cell chemical proteomics experiment using the KiNativ approach
for TX2-121-1, which proved again that TX2-121-1 is a selective
Figure 4. After treatment of PC9 GR4 cell lines with bi-functional conjugates for
12 h at concentrations of 1 and 5 M, the degradation of Her3 was most evident
with TX2-121-1 at both concentrations.
l
binder of Her3 at 2 lM (Table S1). Next, we tested the feasibility
of the degradation of Her3 with 4 adamantane conjugates and
TX2-120-1 (2a) (a negative control possessing no adamantane
group) that possessed an IC50 of approximately 50 nM. Treatment
of PC9 GR4 cells with 4 conjugates at concentrations of 1 and
and signaling. To examine the requirement of the covalent bond
formation, we replaced the reactive acryl amide group with the
non-reactive propyl amide to produce a negative control (TX2-
135-2 (2f)) that maintained the binding ability to Her3 showing
an IC50 of 326 nM (Table 2).
With TX2-121-1 and its two different negative controls (TX2-
135-2 and TX2-120-1) in hand, we investigated and compared the
effect of these three compounds on downstream signaling of Her3
5 lM for 12 h induced partial degradation of Her3 (Fig. 4). The
degree of Her3 degradation was dependent on both the type and
length of the linker, and the best potency was seen with TX2-
121-1 which we advanced for further investigation of the effects
of the adamantane conjugates on Her3-dependent cell growth
Table 2
Bivalent TX1-85-1-adamantane conjugates to induce the degradation of Her3
H
N
O
O
NH2
N
N
N
2
N
N
R
Compounds
R
IC50 (nM)
Compounds
R
IC50 (nM)
O
2a (TX2-120-1)
CH3
56
2d
49
O
HN
O
O
HN
HN
HN
HN
2b
2c
53
51
2e (TX2-121-1)
2f (TX2-135-2)a
49
O
O
326
a
A propyl amide was installed instead of an acryl amide to prevent the formation of a covalent bond.
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S. M. Lim et al. / Bioorg. Med. Chem. Lett. xxx (2015) xxx–xxx
7
by western blot.30 We observed that treatment of TX2-121-1 with
serum-starved PC9 GR4 cells for 12 h at concentrations of 0.5 and
2 lM followed by stimulation with Neuregulin (NRG) attenuated
creative/challenging research program of National Research
Foundation of Korea NRF-2011-0028676, and NIH Grant P01
CA154303.
phosphorylation of downstream Her3 effectors (Erk and Akt), while
the non-covalent analogs, TX2-135-2 and TX2-120-1, lacking the
adamantane group were less effective at inducing Her3 degradation
or blocking Her3-dependent signaling.30 Furthermore, an anti-
proliferation assay with PC9 GR4 cell lines also demonstrated that
TX2-121-1 was approximately 7-fold more potent than TX2-135-
2, and TX2-120-1 apparently showed little anti-proliferation
effects.30 These combined results emphasize the importance of biva-
lent conjugates in that both covalent modification of Her3 and the
hydrophobic adamantane moiety are required to inhibit Her3-de-
pendent signaling and cell proliferation. Finally, we examined the
effect of the partial degradation of Her3 on the hetero-dimerization
of Her3, especially with Her2 and c-Met by treating PC9 GR4 cells
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bmcl.2015.04.
103.
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To summarize, we have developed the first ATP-competitive,
covalent Her3 binders targeting Cys 721 using high-throughput
screening followed by structure-based drug design. The most
potent covalent Her3 binder, TX1-85-1 was obtained by struc-
ture-guided hybridization between the most potent screening hit
KIN001-111 and 1d which was evolved from the very first covalent
Her3 binder SML4-82-1 by adapting the pyrazolopyrimidine core.
The covalent modification of Her3 by TX1-85-1 was unambigu-
ously confirmed by mass spectrometry, and the KiNativ profiling
demonstrated that TX1-85-1 is a highly selective Her3 binder.
Despite complete intracellular target engagement of Her3 with
TX1-85-1, however, TX1-85-1 did not effectively inhibit either
Her3-dependent signaling or cell proliferation, suggesting that
inhibition of the low kinase activity of Her3 may not be effective
for blocking Her3-dependent biological functions. It is well estab-
lished that hetero-dimerization of Her3 is required to activate
Her3-dependent signaling, and perturbation of the hetero-
dimerization of Her3 has emerged as an alternative approach to
inhibit Her3-dependent signaling. For this purpose, we pursued
the hydrophobic tagging approach for the selective degradation
of Her3, resulting in the synthesis of the bivalent TX1-85-1-
admantane conjugate, TX2-121-1. This bivalent conjugate indeed
induced the partial degradation of Her3, attenuated phosphoryla-
tion of downstream effectors of Her3, and suppressed the growth
of Her3-dependent cell lines. Furthermore, TX2-121-1 perturbed
the hetero-dimerization of Her3 with either Her2 or c-Met. Both
the covalent bond formation and the adamantane moiety were
required for these effects. Although we need to further increase
potency of these bivalent conjugates in order to make them clini-
cally meaningful, detailed mechanistic investigation of the protein
degradation process will likely enable the optimization of linkers
and hydrophobic tags to produce more potent bi-functional conju-
gates. Her3 has been classified as a ‘pseudokinase’, thus it has been
believed ‘undruggable’ by small molecules. Our results provide one
of the first successful demonstrations of employing small mole-
cules to target Her3-dependent functions, and we hope to stimu-
late subsequent research efforts to inhibit Her3-dependent
signaling with small molecules.
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This work is supported by the Dana Farber Cancer Institute
Lander Fellowship, Claudia Adams Barr Program Award, US
National Institutes of Health (NIH) Grant AI084140, Cancer
Prevention Research Institute of Texas Grant R1207,
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