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Molecular Targets in Cancer
NHS Trust
Learning Objectives
By the end of this lecture you should:
 Appreciate the principles involved in chemotherapy
 Recognise the diversity of molecular targets
 Understand the mechanisms of action of a range of
antineoplastic agents
 Be able to discuss mechanisms contributing to drug
resistance in chemotherapy
Case
 J.L., 23 yr graduate student in previous good health noted hard lump in left
testis while showering
 J.L.’s physician ordered an ultrasound examination – solid mass suggestive
of cancer
 Testis removed surgically, pathology confirmed diagnosis of testicular cancer
 Chest X-ray revealed several lung nodules – thought to represent metastatic
spread of cancer
 J.L. Tx with several cycles of combined chemotherapy: bleomycin, etoposide
and cisplatin
 Lung nodules disappeared completely
 1 yr later J.L. resumes studies & no sign of cancer recurrence
 Nonetheless, on subsequent follow-up visits Physician asks if he is
developing shortness of breath
Questions
 What is the molecular target of each of the drugs in
J.L.’s combination chemotherapy regimen?
 By what mechanism could etoposide, bleomycin &
cisplatin act synergistically against J.L.’s testicular
cancer?
 Why does the Physician inquire about shortness of
breath at each follow-up visit?
Some Principles of Cancer Therapy
 Goal of antineoplastic drug therapy is selective toxicity
– Genetic/biochemical pathway, structure (isoform) of a protein,
metabolic requirement
 Curative chemotherapy must reduce tumour cells to nil
/ v. low numbers so that body defenses kill the rest
 Aim allow more rapid recovery of normal cells whilst
killing cancer cells by pulsed therapy
 Adjuvant therapy to eradicate seeding metastases
– eg. Cytotoxic drugs post primary treatment by surgery or
radiotherapy (breast cancer)
 Treatment is a balance between toxicity (particularly on
bone marrow) and efficacy
Antineoplastic Drug Targets & Classes
“Log-kill Model” of Tumour Progression
 Cell destruction follows first-order kinetics: each dose of drug kills a constant
fraction of cells
 Rapidly growing cells most sensitive to drugs, in particular drugs that
interfere with cell growth and division (mitotoxicity hypothesis)
Cell-Cycle & Antineoplastic Drug Class
 Cell-cycle specific (Phase)
 Cell-cycle nonspecific
 Non cell-cycle specific
Implications of Non-cycle /cycle and
phase specific killing
Non cycle Specific
Cycle Specific
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% Live cells
% Live cells
% Live cells
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Phase Specific
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Normal cells
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Neoplastic cells
Dose
Dose
Dose
Global Target: Nucleotide Synthesis
 Folate metabolism
– methotrexate
 Purine metabolism
– 6-Mercaptopurine &
Azathioprine
 Pyrimidine metabolism
– 5-Fluorouracil
 Nucleotide incorporation
– Thioguanine, cytarabine
Specific Target: Folate Metabolism
 Methotrexate – structural
analogue of folic acid
 Reversibly inhibits
dihydrofolate reductase
 ↓ intracellular THF levels
– cessation of de novo
synthesis of purines and
thymidine
– stops DNA / RNA synthesis
 cells arrested in S phase
– Also induces apoptosis
 Se: GI mucosa and bone
marrow also targeted
Specific Target: Purine Metabolism
 6-Mercaptopurine & Azathioprine (prodrug: non enzymatic)
 Inosine analogues that inhibit interconversion of purines
 6MP → T-IMP by hypoxantine-guanine phosphoribosyl transferase (HGPRT)
– Inhibits enzymes that convert IMP to AMP/GMP eg. IMP dehydrogenase
– Feedback inhibition on 1st committed step in purine nucleotide synthesis
 ↓cellular AMP & GMP levels which affects DNA & RNA synthesis, energy
stores and cell signaling
 Toxicity & efficacy potentiated by allopurinol
Specific Target: Pyrimidine Metabolism
 5-Fluorouracil: irreversible
inhibitor
 5-FU converted to FdUMP by
pathway that converts uracil to
dUMP
 FdUMP inhibits thymidylate
synthase by forming a stable
enzyme-substrate-cofactor
complex with MTHF & TS
 ↓ dTMP levels inhibit DNA
synthesis: “thymineless death”
Specific Target: Nucleotide Incorporation
 Thioguanine: guanine analogue
• Converted by HGPRT to 6-thioGMP
• 6-thioGMP converted by guanylyl kinase to 6-thioGTP for incorporation
into DNA
– Interferes with RNA transcription and DNA replication leading
to cell death
– 6-thioGMP inhibits inosine monophosphate dehydrogenase
depleting cellular pools of GMP
 Cytarabine: cytidine analogue
• Arabinose sugar replaces ribose sugar (differ by position of OH group)
– Metabolised to araCTP
– Competes with CTP for DNA polymerase
– Incorporation into DNA results in chain termination and cell
death
Global Target: DNA replication & Mitosis
 Agents that directly modify DNA structure
– Alkylating agents (cyclophosphamide, carmustin)
– Platinum compounds (cisplatin, carboplatin)
– Bleomycin
 Topoisomerase inhibitors
– Epipodophylotoxins (etoposide)
– Antitumour antibiotics (doxorubicin – intercalating agent)
 Microtubule inhibitors
– Vinca alkaloids (vinblastine & vincristine)
– Taxanes
Direct Modification of DNA: Alkylating Agents
 A group of cell cycle-nonspecific compounds that transfer an alkyl group
usually to the N7 of guanine in one or both strands of DNA
 Mode of action: Prevent strand separation inhibiting DNA replication and
transcription, also alkylates proteins and enzymes to produce cellular
dysfunction
Direct Modification of DNA: Platinum Agents
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Targets nucleophilic centres in guanine, adenine and cytosine
Crosslinks adjacent residues on the same DNA strand
Inhibits DNA synthesis
Dose limiting toxicity is nephrotoxicity
Direct Modification of DNA: Bleomycin
 Bleomycins: family of glycopeptides from a species of
Streptomyces
 A mixture of 2 peptides is used clinically
 Binds to DNA and combines with Fe2+ to form a haem
like ring complex
 Bleomycin/Fe complex reacts with oxygen to produce
free radicals which cause single and double stranded
DNA breaks
– cytotoxic
 Dose limiting toxicity: pulmonary fibrosis (due to
reactivity with air)
Topoisomerase II: MOA
Topoisomerase Inhibitors: Doxorubicin
 Antitumour antibiotic
 Intercalation into the DNA structure prevents strand passage & religation
step of the catalytic cycle of type II topoisomerase
 Formation of free radicals (contain quinone/hydroquinone moieties that
enable compound to accept/donate electrons promoting FR generation)
 Strand scission and cell death
 Dose limiting toxicity: cardiotoxicity (FR mediated damage of cell membrane)
 Epipodophyllotoxins: etoposide – inhibit Topo II mediated religation  get
strand breakages – SE: bone marrow suppression
Microtubule Inhibitors: Plant Alkaloids
Vinblastin/vincristin
– Periwinkle plant
 Bind to tubulin preventing
polymerisation and formation
of mitotic spindle
 Cell arrests at metaphase
– VC: peripheral neuropathy
– VB: myelosuppression
Taxanes (Paclitaxel)
 Bind to inside of microtubule
stabilising tubulin polymer
preventing depolymerisation
– peripheral neuropathy
Hormones used in Chemotherapy
May produce remission in some cancers but do not
eradicate disease
 Oestrogen
– used in cancers which are partially hormone dependent
– Prostatic carcinoma
 Anti-oestrogens for breast cancer (oestrogen receptor
positive)
– eg. Tamoxifen
– compete with oestradiol for cytoplasmic oestrogen receptor
 Anti-androgens
– inhibit translocation of the androgen receptor to the nucleus
– Treatment of prostate cancer
Toxicity Associated with Chemotherapy
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Major problems due to inability of drug action to differentiate
normal from neoplastic cells
Bone marrow: leucopenia; thrombocytopenia; rarely anemia or
total aplasia. Causes infection and bleeding
GI tract: ulceration of mouth & intestine, diarrhoea
Testis: azoospermia and infertility
Ovary: infertility; premature menopause
Hair follicles: alopecia
Local irritation: some cause ulceration if extravasated during
injection
Vomiting: major problem with some drugs
Drug Resistance in Chemotherapy
 Neoplastic cells can defend themselves in several
ways
 Primary resistance:
– tumour is insensitive to the drug from first exposure
– genetic property of an individual cell
 Acquired resistance:
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Increased DNA repair
Changes in target enzymes (multiplication)
Drug inactivation
Decreased drug accumulation
• Multi drug resistance - resistant to drugs of differing structure following
exposure to a single agent, often associated with increased
expression of the MDR-1 gene (P-glycoprotein)
– Alternative metabolic pathways
Acquired Tumour Resistance to
Cytotoxic Agents
Mechanism
Reduced drug uptake
Example
Increased detoxication of drug
Methotrexate
Daunorubicin
Cytosine arabinoside
5-fluorouracil
6 mercaptopurine
Increased concentration of target enzyme
Methotrexate
Deletion of enzymes to activate drug
Decreased requirement for specific metabolic Asparaginase
product
Increased utilisation of alternative metabolic Antimetabolites
pathways
Rapid repair of drug-induced lesion
Alkylating agents
Decreased number of receptors for drug
Hormones
Alteration in proliferation rate.
?? underlying mechamism
Myeloma, chronic myeloid leukaemia