Cancer Genes and Targets for Therapy

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Cancer Genes and Targets for Therapy
Helen C Hurst

• Molecular Oncology Unit
• Charterhouse Square

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Cancer Treatment
Surgery
Chemotherapy Radiotherapy
Apoptosis
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Cells in multicellular organisms are continually
receiving signals from each other and their
environment
This leads to proliferation, differentiation or
even cell death (apoptosis) as appropriate to the
needs of the organism as a whole In cancer, this
normal balance goes awry ? Cancer Genes
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Cancer progression in ductal carcinoma of the
pancreas
.progressive mutation/activation of cancer
genes
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What is a Cancer Gene?

• Proliferation Oncogenes and Tumour suppressor
  genes
• Cell survival Apoptosis vs DNA repair
• Epithelial-stromal interactions Angiogenesis,
  Invasion and Metastasis
• Cell surface markers Immune Evasion
• Membrane pumps Drug resistance and response to
  therapy
• Metabolism allow more rapid growth (e.g.
  ribogenesis)

• ? virtually any gene product may be a target for
  therapy as long as
• Its expression level/structure/activity is
  sufficiently different between normal and tumour
  cells
• It is required for continued growth/survival of
  the tumour cells
• Many are involved in cellular signalling pathways

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Signalling Pathways
Growth/survival
GFs
2nd messenger cascade
Small Molecules
Arrest/apoptosis
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Examples of Targeted Therapies in Clinical Use

• Anti-endocrine therapies
• Tamoxifen (anti-ER therapy) - breast cancer
• Anti-androgen therapy - prostate cancer
• Anti-ErbB therapies
• Herceptin - immunotherapy against HER2/ ERBB2 in
  breast cancer
• Iressa - small molecule tyrosine kinase inhibitor
  against EGFR for solid tumours
• Glivec - small molecule tyrosine kinase inhibitor
  against Bcr-abl for CML

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Oestrogen Receptor in Breast Cancer
Normal - only a few cells express ER
ER ve tumour
65 of breast tumours are ER ve ? show
proliferative response to oestrogens (ovaries) ?
benefit from anti-oestrogen therapy
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The ER is a ligand dependent transcription factor
Proteins that ? Growth/survival

ERE (oestrogen response element)
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Use of anti-oestrogens in treating breast cancer

• Anti-oestrogens block the binding of oestrogen to
  the ER ? proliferative gene expression and
  signalling are blocked
• Giving early stage, ERve patients Tamoxifen for
  5 years immediately after surgery has ?
  mortality by 28
• Tamoxifen use in early stage disease ? UK annual
  breast cancer mortality rate fell from 16,000 to
  12,800 in 12 years (1988-2000)

But...
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. there are problems

• Tamoxifen is associated with a 2-fold ? risk of
  blood clot formation (thromboembolism)
• Tamoxifen is linked to a 2.5-fold ? risk of
  endometrial cancer
• Significant numbers of ER ve patients never
  respond to Tamoxifen (de novo resistance)
• Those that do respond initially, can relapse with
  resistant disease (acquired resistance)

because oestrogen has a bad and a good side.
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Pluses and minuses

• Anti-oestrogens like Tamoxifen and Raloxifene are
  partial agonists ? block oestrogen action in
  breast allow some signalling in other organs
• This has consequences that are both positive and
  negative
• Tamoxifen and Raloxifene are both agonists in
  bone ? protect against osteoporosis
• In the endometrium Tamoxifen (but not Raloxifene)
  is an agonist, hence ? endometrial cancer

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Alternative strategies

• Use total oestrogen agonists like Faslodex that
  block all oestrogenic activity and result in
  down-regulation of the ER
• Remove oestrogens altogether using aromatase
  inhibitors which prevent synthesis of oestrogens

Clinical trials have shown aromatse inhibitors to
be effective and well-tolerated and resistance is
slower to develop.
however, resistance to these agents is an issue
and develops for largely similar reasons as
Tamoxifen resistance
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Signal pathway cross-talk ? oestrogen-independence
? target 1 or more of these pathways in addition
(combination therapy)
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• AR in Prostate Cancer
• All PC initially respond
• to anti-androgen therapy
• After 2-5 years tumours
• become resistant
• Various mechanisms e.g.
• mutation of AR and/or
• gene amplification
• Increased signalling via
• other pathways (as in
• breast cancer) also
• important

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What are ErbB proteins?

• ErbB family of trans-membrane glycoproteins
  with an extracellular ligand binding domain and
  an intracellular tyrosine kinase domain
• Referred to as receptor tyrosine kinases
• Ligand binding ? receptor dimerisation, kinase
  activation, auto-phosphorylation (on Y) ?
  signalling cascade initiation
• Normal function ? mediate cell-cell interactions
  in organogenesis and during adulthood

Docking sites for signalling proteins
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The ErbB Network
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IHC
FISH
ERBB2 overexpressed in many solid tumours e.g.
25 breast carcinomas ? correlates with
ER negativity and poor prognosis
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The development of Herceptin(Trastuzumab)

• Researchers at Genentech raised mouse monoclonal
  antibodies against the extra-cellular domain of
  ErbB2
• One of these, 4D5, potently inhibited growth of
  ErbB2 overexpressing cultured human breast tumour
  cells
• Murine antibodies are limited clinically due to
  being immunogenic
• ? Recombinant, humanised antibody created
• Herceptin has a higher affinity for ErbB2 than
  4D5 and has a cytostatic growth inhibitory effect
  against ErbB2ve breast cancer lines

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Humanising an antibody
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Herceptin in the clinic

• Shown to be well-tolerated have anti-tumour
  activity
• In randomised trials - improved survival in
  patients with amplification of the ERBB2 gene
• Approved for use in metastatic ErbB2ve breast
  tumours (1998)
• Largely used in combination with chemotherapy
  drugs (taxol, cisplatin cardiac side-effects
  with dox)
• Mode of action ErbB2 downregulation prevents
  cleavage of extracellular domain (causes
  activation) activates patients own immune
  response

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Future improvements

• Herceptin has no activity on tumours that express
  moderate levels of ErbB2 ? limits its use
• 2C4 binds a different epitope ? blocks ErbB2
  dimerisation with other ErbB receptors ? prevents
  signalling in low- and high-expressing lines
• Anti-tumour effects in xenografts of breast and
  prostatic tumours
• Shown to be safe (Phase I) now in Phase II
  (efficacy) trials
• May be useful in a wide range of ErbB2 ve solid
  tumours.

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No signalling
Proliferation/Survival
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Iressa (Gefitinib ZD1839)

• Selective and reversible small molecule inhibitor
  of EGFR tyrosine kinase activity (from
  AstraZeneca)
• Also inhibits signalling via EGFR dimerisation
  with other ErbB family members
• Preclinical studies - inhibited growth of various
  tumour lines and xenografts
• Synergised with cytotoxic chemotherapy agents
  (e.g. paclitaxel) and radiation therapy in
  sensitive lines
• Paradox senisitive lines could not be predicted
  from their level of EGFR expression

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Mode of Action of Iressa
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Iressa in the clinic

• Good oral bio-availability and well-tolerated ?
  can be taken once daily (Phase I)
• Good anti-tumour responses in mono- and
  combination therapy in a variety of solid
  tumours NSCLC, colorectal, breast, head neck
  (Phase II/III)
• Approved for use in patients with advanced,
  chemo-resistant NSCLC
• Assays to determine which patients (NSCLC and
  other) will benefit most being developed
• Combination therapies being optimised

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Chronic Myeloid Leukaemia (CML)

• Characterised by a massive clonal proliferation
  of myeloid cells
• Accounts for 15-20 of all leukaemia cases
• Has 3 phases chronic (or stable) accelerated
  blast
• Chronic phase excess numbers of myeloid cells
  that still differentiate (i.e. cease dividing) as
  normal
• In 4-6 years disease progresses to blast crisis ?
  accumulated mutations ? ability to differentiate
  is lost
• Transplantation can cure (but problematic) ? less
  than 20 of cases can be cured

What mutations cause this?
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Chromosome 1
Gene A
Gene B
Chromosome 2
Fusion Gene
Primary transcript
Fusion mRNA
Unique Properties
Altered Pattern of gene expression
Chimaeric protein
Acts as an oncogene
Differentiation Blocked
65 of leukaemias are characterised by particular
somatically acquired chromosome translocations
Continued self-renewal
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Bcr-abl constitutively active tyrosine kinase
(The protein product from this fusion gene only
found in 70 of patients)
Chronic myeloid leukaemia (CML) is characterised
by the t(922)(q34q11) reciprocal translocation
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Bcr-abl inhibitor, Glivec(Gleevec Imatinib
ST1571)

• Rationally designed small molecule that binds to
  an inactive form of Bcr-abl and prevents ATP
  recruitment ? tyrosine kinase activation is
  blocked
• Pre-clinical studies ? growth inhibition and
  induction of apoptosis specifically in Bcr-abl
  expressing cells
• Shown to be orally active and well tolerated
• Effective therapy for all stages of CML inducing
  remission in 80 of patients
• Approved in May 2001 lt 3yrs after first Phase I
  study
• gt95 patients with chronic phase (stable) disease
  ? durable response

but
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The downside

• all patients with advanced disease will relapse
  ? develop resistance to Glivec
• Main mechanism reactivation of Bcr-abl kinase
  via point mutations that ? drug sensitivity 3- to
  gt100-fold

The solution

• Combination therapy using Glivec with cytotoxic
  agents and/or interferon
• Use rational drug design to make similar drug
  that binds more avidly AMN107 with gt20-fold
  higher affinity for wt and mutant Bcr-abl,
  published Feb 2005

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Summary

• Targeted therapies can be more selective and show
  improved efficacy with minimal toxicity
• Almost invariably, initial response and latency
  followed by disease resistance
• ? inherent weakness of monotherapy
• Combination therapy with cytotoxic drugs is being
  assessed but the mutagenic nature of these may
  accelerate the development of resistance
• Simultaneous use of multiple targeted agents may
  ? faster responses and more durable remissions
• Need yet more detailed knowledge of the molecular
  changes during cancer progression ? TARGETS

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Suggested Reading

• Tamoxifen a most unlikely pioneering medicine
• Jordan VC (2003) Nat. Rev. Cancer 2, 205-13
• Aromatase Inhibitors for breast cancer lessons
  from the laboratory Johnston SRD Dowsett M
  (2003) Nat. Rev. Cancer 3, 821-31
• Untangling the ErbB signalling network Yarden Y
  Sliwkowski MX (2001) Nat. Rev. Cancer 2,
  127-37
• STI571 (Gleevec) as a paradigm for cancer
  therapy BJ Drucker (2002) Trends Mol. Medicine
  8, S14-18