Receptor tyrosine kinases as therapeutic targets for cancer

1
Receptor tyrosine kinases as therapeutic targets
for cancer

• John Heymach, M.D., Ph.D.
• Depts. of Thoracic/Head and Neck Medical Oncology
  and Cancer Biology
• University of Texas M.D. Anderson Cancer Center
• March 13, 2009

2
Key points

• Even though cancer cells are complex, they may be
  addicted to one or a few key pathways (oncogene
  addiction), often driven by an RTK.
• 2. Some RTKs are validated or very promising as
  therapeutic targets. Particularly for tumors with
  oncogene addiction.
• 3. We are testing RTK inhibitors in the clinic.
  Some work well for some diseases.
• 4. Resistance to RTK inhibitor may arise through
  several different mechanisms. Understanding these
  mechanisms of resistance is important for
  designing better combination therapies.

3
Cancer cells are very complex.

• Genetically unstable.
• Large number of mutations
• Multiple deregulated pathways.
• ? Conventional wisdom was Targeting any one
  pathway is not likely to be successful.

4
Hahn and Weinberg 2002
5
Efficacy of imatinib, an inhibitor of BCR-ABL, in
CML(Druker et al, NEJM, 2001)

• Out of 54 patients treated with imatinib who had
  failed interferon alfa, at 300 mg or higher bid
• 53 complete hematological responses
• 29 cytological responses

6
Efficacy of imatinib, an inhibitor of BCR-ABL, in
CML(Druker et al, NEJM, 2001)

• Out of 54 patients treated with imatinib who had
  failed interferon alfa, at 300 mg or higher bid
• 53 complete hematological responses
• 29 cytological responses
• ?Conventional wisdom Perhaps hematological
  malignancies can be treated by targeting a single
  pathway. But solid tumors are different.

7
18FDG-PET of patient with GIST treated initially
with SU5416 and later with imatinib mesylate.
Pre- and post- treatment with SU5416
Pre- and post- treatment with imatinib
Heymach et al, CCR, 2004
8
Efficacy of imatinib, an inhibitor of c-KIT, in
gastrointestinal stromal tumors(Demetri et al,
NEJM, 2002)

• Chemotherapy ineffective
• Families with inherited GIST found to have
  activating KIT mutations
• Imatinib tested in 147 patients
• 81 with partial response or prolonged stable
  disease
• ? Selected solid tumors can be treated by
  targeting a single pathway

9
The Normal Cell
Growth factors
DNA
10
The Cancer Cell
Growth factors
DNA
11
The Hallmarks of Cancer

• Self-sufficiency in growth signals
• Insensitivity to anti-growth signals
• Evading apoptosis
• Limitless replicative potential
• Sustained angiogenesis
• Tissue invasion and metastasis
• (Genetic instability)
• ? RTKs may be involved in all, or almost all, of
  these traits.

Hanahan and Weinberg, Cell 2000
12
Hallmarks of cancer
Luo et al, Cell 2009
13
Oncogene addiction

• Despite the multitude of genetic and epigenetic
  alterations in a cancer, a given tumor is likely
  to be driven by only a few select changes- i.e.
  gain of an oncogene or loss of a tumor
  suppressor.
• Maintenance often depends on the continued
  activity of certain oncogenes.
• Myc
• K-RAS, HRAS
• BCR-ABL
• EGFR
• Her2
• Others being studied BRAF, MDM2, PI3K pathway

Luo et al, Cell 2009
14
RTK inhibitors

• Small molecule inhibitors bind to ATP pocket and
  prevent receptor phosphorylation (ATP donates
  phosphate)
• Examples
• EGFR inhibitors gefitinib and erlotinib
• Kit and BCR inhibitor imatinib
• VEGFR inhibitors SU5416, SU11248
• Because of structural similarity of different
  tyrosine kinase domains, small molecule
  inhibitors typically inhibit several different
  RTKs
• Antibodies typically bind to extracellular
  domain of receptor or ligand and prevent receptor
  activation
• Example
• EGFR inhibitor cetuximab (Erbitux)
• VEGF bevacizumab (Avastin)

15
EGFR complexed with Erlotinib
Stamos and Eigenbrot, 2002 http//www.ncbi.nlm.nih
.gov
16
Structure of wild-type and mutated EGFR tyrosine
kinase domain

• Key structures
• C and N lobes (ATP cleft in between)
• P lobe phosphate binding
• A loop activation

Gazdar et al, Trends Mol Med. 2004
Oct10(10)481-6.
17
Epidermal Growth Factor Receptors
Ligand
R
R
K
K
Signal transduction cascade
18
EGFR Inhibitors
Antibodies Erbitux (cetuximab), ABX-EGF,
ICR62, Omnitarg(pertuzumab)
Ligand
R
R
Plasma membrane
TK inhibitors Iressa (gefitinib), Tarceva
(erlotinib), EKB-569, CI-1033, PKI166
K
K
Signal transduction cascade
19
EGFR Inhibitor Specificity
erbB1
erbB2
erbB3
erbB4
R
R
R
R
R
R
R
R
Plasma membrane
K
K
K
K
K
K
K
K
Gefitinib (Iressa) Erlotinib (Tarceva)
EKB-569
CI-4033
20
EGFR and Tumor Specificity
Prostate
Breast B1,B2,B4
Gastric, Ovarian
HN
NSCLC
erbB1
erbB2
erbB3
erbB4
R
R
R
R
R
R
R
R
Plasma membrane
K
K
K
K
K
K
K
K
21
Kinase dendrogram for selected RTKIs
Adapted from Fabian, M, Nature Biotechnology VOL
23, March 2005
22
Mechanisms of resistance to RTK inhibitors

• 1. Target not critical for a given tumor (primary
  resistance)
• 2. Incomplete receptor inhibition
• Inhibitor not sufficiently potent
• Secondary mutations that prevent binding of
  inhibitor (i.e. T790M mutation in lung cancer
  prevents gefitnib or erlotinib from inhibiting
  EGFR)
• 3. Target bypass the tumor circumvents the
  inhibited pathway by activation of other pathways
  (i.e. MET)

23
Potential mechanisms of resistance to targeted
agents
Effective target inhibition
1. Incomplete target inhibition
Ligand
Ligand
R
R
R
R
X
(partial) inhibitor
Pathway X
Pathway X
Effect (I.e. tumor cell survival)
Effect (I.e. tumor cell survival)
24
Potential mechanisms of resistance to targeted
agents
Effective target inhibition
2. Target bypass
1. Incomplete target inhibition
Ligand 2
Ligand
Ligand
Ligand
R2
R2
R
R
R
R
R
R
A
X
X
Pathway Y
Pathway X
Pathway X
Pathway X
B
Effect (I.e. tumor cell survival)
Effect (I.e. tumor cell survival)
Effect (I.e. tumor cell survival)
25
Approaches to combating resistance
2. Target bypass
1. Incomplete target inhibition
Ligand 2
Ligand
Ligand
R2
R2
R
R
R
R
A
X
Add additional inhibitor or try more potent
inhibitor for same target
Pathway Y
Pathway X
Pathway X
B
Effect (I.e. tumor cell survival)
Effect (I.e. tumor cell survival)
26
Approaches to combating resistance
2. Target bypass
1. Incomplete target inhibition
Ligand 2
Ligand
Ligand
R2
R2
R
R
R
R
A
X
Pathway Y
Pathway X
Pathway X
B
Inhibit downstream or bypass pathways
Effect (I.e. tumor cell survival)
Effect (I.e. tumor cell survival)
27

• EGFR Tyrosine Kinase Inhibitor Therapy in NSCLC

28
Leading causes of cancer death in the U.S.
Jemal et al Ca Can J Clin 2003, 53 5-26
29
Median survival in advanced NSCLC (SEER data)
1970s-1990s
7.3 months
6.9 months
Breathnach et al (2001) JCO 19(6) 1734
30
Are we approaching the ceiling for improved
benefit from combination chemotherapy regimens?
1.0
ECOG 1594

• Probably yes.
• All recent randomized studies have similar
  results
• No clear efficacy benefit for nonplatinum
  combinations or triplet combinations

Cisplatin/Paclitaxel Cisplatin/Gemcitabine Cisplat
in/Docetaxel Carboplatin/Paclitaxel
0.8
Stage IIIB/IVPatientSurvival,
0.6
0.4
0.2
0.0
0
5
10
15
20
25
30
Months
Schiller JH et al. N Engl J Med. 200234692-98.
31
NCI Canada BR.21 Advanced NSCLCTarceva vs
Placebo Phase III Trial
Previously Treated Advanced NSCLC N
638 Stratified by Center PS, 0/1 vs
2/3 Response to prior Rx (CR/PRSDPD) Prior
regimens, (1 vs 2) Prior platinum,
(Yes vs no)
R A NDOMIZE D
Erlotinib 150 mg PO QD
21 randomization
Placebo PO QD
Shepherd, et al. PASCO 2004
32
Overall Survival
1.00
TARCEVA


Placebo

(n488)

(n243)

Median survival (mo)



6.7
4.7
0.75
1
-
year survival ()



31.2
21.5
0.50
Plt0.001 HR0.73 (95 CI, 0.61-0.86)
Survival distribution function
0.25
TARCEVA Placebo
0
0 5 10 15 20 25 30
Months
Adjusted for stratification factors at
randomization, and HER1/EGFR status. HR hazard
ratio. TARCEVA (erlotinib) PI.
33
Analysis of tumor from patients with major
response to EGFR inhibitors

• Approximately 10-20 of patients had dramatic
  tumor shrinkage after treatment (major response)
• In majority of tumors from patients with major
  response, mutations in EGFR tyrosine kinase
  domain observed (exon 19 deletion, point
  mutation)
• Mutation led to constitutive activation of EGFR
  and increased sensitivity to EGFR inhibition.
• Tumors with amplification of EGFR may also have
  increased sensitivity
• Patients who never smoked, or were of Asian
  origin, had a high frequency of EGFR mutations.
• (Paez et al, Science, 2003 Lynch et al, NEJM,
  2003)

34
Never smokers overall survival- erlotinib vs
Placebo
1.0 0.8 0.6 0.4 0.2 0
Erlotinib Placebo
Survival rate
0 5 10 15 20 25
Months on study
Herbst et al., ASCO 2004
35
Secondary mutations in EGFR (T790M) lead to
acquired resistance to EGFR TKIs
Kobayashi et al, NEJM 2005
36
Secondary mutations in EGFR (T790M) lead to
acquired resistance to EGFR TKIs
T790M mutation
Kobayashi et al, NEJM 2005
37
Amplifications of MET receptor lead to resistance
in previously EGFR-addicted NSCLC cells.
Engelman et al, Science, 2007
38
Pivotal role of VEGF in tumor angiogenesis
1. Tumor secretes VEGF
4.VEGF induces mobilization and differentiation
of BM- derived CEPs
2. VEGF increases endothelial protease
expression
Bone marrow
3. Endothelial cell migrationproliferation, and
capillary tube formation promoted by VEGF
39
Different approaches to inhibitionof VEGF
signaling
Ligand sequestration MAbs, soluble
receptors (i.e. bevacizumab)
Receptor blockingMAbs
p85
ras
GRB2
SOS
PLCg
Inhibition of tyrosine phosphorylation and
downstream signaling inhibition
Tyrosine kinase inhibition TKIs
Transcription factor inhibition
TKI tyrosine kinase inhibitor
40
SU11248 Multitargeted Receptor Tyrosine Kinase
Inhibitor
O
N
N
H
F
N
Split Kinase Domain RTKs
H
OH
O
COOH
N
HOOC
H
H
Enzymatic Ki (µM)
VEGFR-2 0.009
PDGFR? 0.008
FGFR1 0.83
EGFR gt10
VEGFR-3 0.017
PDGFR? PDGFR? CSF1R KIT FLT3
VEGFR-1VEGFR-2 VEGFR-3 Fms
Cellular IC50 (µM)
FLT3 (WT) 0.25
KIT 0.01
VEGFR-2 0.009
PDGFR? 0.008
EGFR 8.9
MET 12.0
Receptor phosphorylation
Mendel DB, et al. Clin Cancer Res 2003932737
41
LSC analysis of PDGFR-beta and VEGFR-2
phosphorylation after treatment of GIST patients
with SU11248 for 10-14 days
-b
change in phosphorylation post-treatment
PD
SD
PR
Patients (n20)
PD progressive disease SD stable disease PR
partial response
Davis et al, ECCO 2005
42
Lessons from tumor-based biomarkers of activity

• For SU5416/SU6668, lack of clinical activity
  likely due, at least in part, to incomplete
  target inhibition.
• It is critical to not only identify important
  targets, but also to determine the duration and
  degree of target inhibition needed for anti-tumor
  activity.
• 2. SU11248-treated GIST patients with clinical
  benefit had a
• greater post-treatment induction in TEC apoptosis
  (9.55 vs 1.78) and TC apoptosis than patients
  with progressive disease
• greater degree (but not complete) inhibition of
  PDGFR-ß and VEGFR phosphorylation.
• There is likely to be room for improvement with
  more potent inhibitors or combinations.

43
Key points (again)

• Even though cancer cells are complex, they may be
  addicted to one or a few key pathways (oncogene
  addiction), often driven by an RTK.
• CML addicted to BCR-ABL
• Gastointestinal stromal cancer Addicted to C-KIT
• Some NSCLC EGFR mutations.
• 2. Some RTKs are validated or very promising as
  therapeutic targets. Particularly for tumors with
  oncogene addiction.
• BCR-ABL (CML)
• KIT (GIST)
• PDGFR (GIST)
• EGFR (lung cancer)
• VEGFR (multiple diseases)
• RET (medullary thyroid)
• 3. We are testing RTK inhibitors in the clinic.
  Some work well for some diseases.
• Imatinib GIST, CML
• Erlotinib NSCLC
• Sunitinib GIST, RCC
• 4. Resistance to RTK inhibitor may arise through
  several different mechanisms. Understanding these
  mechanisms of resistance is important for
  designing better combination therapies.
• Secondary mutations or other changes that prevent
  the inhibitor from binding.
• Activation of a bypass pathway.