Pharmacology of Chemotherapy

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Pharmacology of Chemotherapy

• David W. Hedley MD
• Dept Medical Oncology and Hematology
• Division of Applied Molecular Oncology

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Chemotherapy

• Cytotoxic agents- generally given by intravenous
  injection or orally
• Most chemotherapy drugs act by damaging DNA or
  inhibiting DNA synthesis
• Important exceptions are drugs that target
  microtubules

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Evolution of Chemotherapy

• 1940s first use of successful use alkylating
  agent nitrogen mustard to treat human cancer
• 1950-1960s major alkylating agents and
  anti-metabolites currently in use synthesized.
  Effective against wide range of cancer types,
  particularly rapidly growing leukemias and
  lymphomas. Scientific principles of cancer
  chemotherapy developed.

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The Science of Chemotherapy

• Many early concepts derived from radiation
  biology- clonogenic survival- fractional cell
  kill- need to eliminate all clonogenic cells to
  achieve cure- normal host effects

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The Science of ChemotherapyL1210 Mouse
LymphomaHoward Skippers work (1960s)

• Rapidly growing tumours
• Initially used animal survival to infer cancer
  cell killing in vivo
• Based on relation between number of cells
  inoculated and survival

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The Science of Chemotherapy

• Fractional cell kill of L1210 lymphoma inoculated
  at 1x106 in vivo
• Single treatment with chemotherapy drug
• Effect on lifespan with increasing log cell kill
• Even though you get a lot of cancer cell,
  difficult to achieve cure!

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Clinical Implications of Fractional Cell Kill
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The Science of Chemotherapy- Bob Bruce Data
Showing Cell Cycle Dependence of Killing by
g-Radiation, Nitrogen Mustard, and Tritiated
Thymidine in Lymphoma vs. Normal Bone Marrow in
vivo (JNCI 196637233-245)
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Ian Tannocks PhD ProjectUsing tritiated
thymidine autoradiography, showed solid tumours
proliferate more slowly further away from blood
vessels. Cause of drug resistance
The Science of Chemotherapy
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Evolution of Chemotherapy

• 1970s - Golden Age of medical
  oncology.Development of effective combination
  chemotherapy regimens.New classes of drug
  developed- anthracyclines, platinum compounds
  Cures achieved in some forms of cancer
  (lymphomas, leukemias, testis cancer).Significant
  responses in some common types of cancer
  (breast, stomach, small cell lung
  cancer)Effective use of chemotherapy to prevent
  recurrence in high risk breast cancer patients.

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Evolution of Chemotherapy

• 1980s disillusion sets in.
• Development of increasingly complex, toxic (and
  expensive) treatment protocols
• Some improvement in response rates, but hope
  fades for curing common forms of cancer
• Intensive search for analogues of existing drugs,
  hoping for greater anticancer effect or less
  toxicity
• Introduction of remaining major types of
  chemotherapy (taxanes, topoisomerase I
  inhibitors)

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Evolution of Chemotherapy

• 1990s still hard going in the clinic, but ..
• Post-operative adjuvant chemotherapy established
  to reduce mortality in some major causes of
  cancer death (breast, colon cancer)
• Biochemical basis of drug resistance established
• Idea that development of cancers involves
  suppression of cell death pathways, and that drug
  resistance results from failure of damaged cells
  to undergo apoptosis- e.g. bcl2, p53 stories

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Evolution of Chemotherapy

• 2000s rapid development of molecular targeted
  agents as alternatives to classical chemotherapy
• Evolution of molecular oncology and rational
  cancer therapeutics - integrating basic science,
  pharmacology, pathology, and clinical oncology

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Classification of Chemotherapy(difficult to do
many drugs first identified by empirical
screening for anticancer effect, rather than
rational synthesis)

• DNA damaging agents- alkylating agents-
  platinum compounds
• Antimetabolites
• Topoisomerase inhibitors
• Anti-mitotic agents

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Nitrogen Mustard

• First chemotherapy agent used in man
• Prototype alkylating agent
• Main toxicity comes from DNA cross linkage

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DNA Cross Linkage

• Arrests DNA replication
• Can result in DNA damage and chromosome breaks
• Also mutagenic!

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Chlorambucil

• Derivative of nitrogen mustard
• Much less reactive
• Well absorbed by mouth
• Remains major drug for treating low grade
  lymphomas

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Cyclophosphamide

• More complex activation than nitrogen mustards
  requires cytochrome p450 in liver
• Can be given orally or intravenously
• Main side effects are bone marrow suppression

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Other DNA Damaging Agents

• Platinum compounds- prototype cisplatin- main
  effect interstrand cross links- many analogues
  produced with broader spectrum
• Nitrosoureas- transfer chloroethyl group to
  guanine at O6 position

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Antimetabolite Drugs

• Designed to block DNA synthesisBased on idea
  that cancer cells divide more rapidly than normal
  cells, so more vulnerable
• Originally considered to be cytostatic rather
  than cytotoxic, but now recognized that many
  produce cell death by triggering apoptosis
• Unlike most conventional chemotherapy drugs,
  development by rational synthesis rather than
  empirical screening for anticancer effects
• Most are either nucleoside analogues that
  interfere with DNA synthesis, or block
  methylation of uracil to thymidylate

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Antimetabolite Basics
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Nucleoside Analogues- biochemical pharmacology
mirrors the uptake and metabolism of normal
nucleosides
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Cytosine arabinoside (cytarabine Ara-C)- major
chemotherapy drug to treat acute leukemias
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• Ara-CTP competes with dCTP
• inhibits DNA polymerases
• incorporation into DNA produces strand breaks
• triggers apoptosis

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Ara-C in treatment of acute leukemia

• High doses to overcome transport resistance
• Because ara-C targets cells in S-phase, given
  over 5-7 days to account for slow cycling
  populations of leukemia cells
• Major toxicity is suppression of blood counts

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Inhibition of Thymidylate Synthesis

• Pyrimidine base 5-Fluorouracil (5FU) inhibits
  thymidylate synthase
• Methotrexate inhibits dihydrofolate reductase,
  reducing flow of methyl group carried by reduced
  folate

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Effects of TS Inhibition

• Decrease in dTTP associated with build up of dUTP
• Mis-incorporation of dUTP into DNA
• This is removed by DNA repair pathways
• However, DNA repair synthesis also
  mis-incorporates dUTP futile repair cycle
  results in extensive DNA damage
• 5-fluorouracil and methotrexate both clinically
  important drugs

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Natural Products

• Discovered by empirically screening compounds for
  anticancer effects in vitro (similar to
  antibiotic discovery)
• Mechanisms of action subsequently identified
• Important compounds showing effect in lymphoma
  (and other cancers) are- topoisomerase
  inhibitors- microtubule inhibitors

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Topoisomerase II Inhibitors

• Topoisomerase II allows replicated DNA strands to
  separate by making breaks, then re-ligating
• Main class of topo II inhibitors are the
  anthracyclines, originally from Streptomyces
• Intercalate into DNA and prevent re-ligation step
• Daunorubicin is the classical anthracycline used
  to treat acute leukemia

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Daunorubicin toxicity

• Bone marrow suppression is most important early
  toxicity
• Also causes gastro-intestinal toxicity- nausea,
  vomiting, diarrhea
• Hepatobiliary excretion is major route for drug
  elimination toxicity greater in presence of
  jaundice
• Cardiac toxicity is most important late effect-
  risk increases with accumulated dose- can result
  in fatal cardiac failure

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Mechanism of Anthracycline Cardiac Toxicity

• As well as intercalating into DNA, daunorubicin
  avidly binds mitochondrial inner membrane of
  cardiac muscle
• Daunorubicin chelates iron, which catalyzes
  formation of the free radical semiquinone
• Redox cycling transfers high energy electron to
  oxygen, generating oxygen free radicals
• Produce lipid peroxidation damage to
  mitochondrial membranes

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Microtubule Inhibitors

• Vinca alkaloids (from periwinkle plant)-
  destabilize microtubules- vincristine commonly
  used to treat acute lymphoblastic leukemias
• Taxanes (from Pacific yew tree bark)- stabilize
  microtubules- taxotere most active in current
  use
• Main effect of these drugs is to cause metaphase
  arrest and chromosomal damage- probably have
  additional effects due to microtubule disruption
  in interphase cells

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Effects of Vincristine on T-cell Leukemia Cell
Line
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Chemical Structure of TaxolMany natural products
are very complex organic molecules. Complexity
can make them useful starting points for drug
development
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Why Dont We Cure Cancer With Chemotherapy?

• Toxic side effects limit dose
• Cancer cells show drug resistance- innate drug
  resistance, or acquired resistance during
  treatment

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Toxic Effects of Chemotherapy

• Generic side effects are damage to rapidly
  dividing normal cells- bone marrow, gut mucosa,
  hair follicles
• Nausea due to triggering CNS vomiting centres
• Drug-specific side effects- myocardium
  (anthracylines)- kidney (platinum)- nervous
  system (microtubule agents)
• Toxicity increases with dose of drug used

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Combination Chemotherapy

• Drugs selected for combinations based on-
  differences in side effects allows each to be
  used at full dose- different mechanisms of
  action cancer less likely to be cross-resistant
• Prototype curative combination is MOPP (nitrogen
  mustard, vincristine (Oncovin), procarbazine and
  prednisone (glucocorticoid steroid) - 1970
• Typically given as outpatient every 3-4 weeks, to
  allow recovery of normal tissue side effects

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How Far Can You Push Chemotherapy?

• Generally, the more you give the greater the
  anticancer effect
• Bone marrow suppression is main lethal side
  effect of chemotherapy
• Bone marrow transplantation (either patients or
  normal donor) bypasses this dose-limiting
  toxicity, allows more chemo. Can be curative in
  some situations, including acute leukemia
• Although higher response rates, other side
  effects may prevent curative doses from being
  achieved

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Drug Resistance

• Main factor determining if a cancer will be cured
  with chemotherapy
• Complex and multifactorial, but main causes of
  drug resistance are probably now understood

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Development of Drug Resistance in a Leukemia
Patient

• Antique chart (from my residency days!) of newly
  diagnosed AML patient treated with ara-C plus
  daunorubicin, followed by ara-C plus
  6-thioguanine maintenance chemotherapy
• represents circulating leukemic
  blasts
• Shows initial clearance of leukemia with
  treatment, and reappearance of normal
  granulocytes and platelets

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Development of Drug Resistance in a Leukemia
Patient

• Blood counts remaining fairly normal during
  maintenance chemotherapy

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Development of Drug Resistance in a Leukemia
Patient

• Eventually leukemic blasts reappear in
  circulation
• Initially respond to intensified chemotherapy
• Then rapid accumulation of drug resistant
  population

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• High dose Ara-C can overcome transport resistance
  because transporter is non-saturable
• Activity of deoxycytidine kinase can be increased
  by inhibiting endogenous deoxycytidine using
  ribonucleotide reductase inhibitors- but these
  can also enhance normal tissue toxicity

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Overcoming Drug Resistance

• Some cellular mechanisms of multidrug resistance
  (P-glycoprotein-mediated drug efflux, glutathione
  conjugation) can be reversed pharmacologically
• Able to enhance anticancer effects in model
  systems
• Results in clinical trials disappointing-
  probably because of multifactorial nature of drug
  resistance

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P-glycoprotein

• First multidrug resistance mechanism to be
  characterized (Vic Ling, OCI, 1975)
• P-glycoprotein is transmembrane ATP-dependent
  efflux pump
• Actively transports many types of chemotherapy
  from cells (anthracyclines, vinca alkaloids,
  taxanes)
• Overexpression in cancers causes drug resistance
• P-glycoprotein inhibitors tested in clinical
  trials

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P-glycoprotein in Acute Leukemia

• P-glycoprotein overexpressed in some AML patients
• Higher levels associated with drug resistance and
  worse prognosis
• But clinical trials of P-glycoprotein inhibitors
  fail to show significant improvement in
  chemotherapy response- other resistance
  mechanisms also operating- high Pgp levels might
  be linked to aggressive biology, rather than
  directly to drug resistance

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Targeting Cell Survival Pathways

• Recent evidence that failure of DNA damaged cells
  to undergo apoptosis is major cause of multidrug
  resistance
• Suppression of apoptosis often occurs due to
  oncogenic mutations i.e. common feature of
  cancers
• Potential to reverse this mechanism by molecular
  therapies e.g. p53 gene therapy or small
  molecule inhibitors of PI3-kinase pathway

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Where is Chemotherapy Going?

• Incremental improvements in patient outcome
  continue, using newer drugs and combinations
• Unlikely that this will result in major
  improvements in cure rates for common forms of
  cancer
• Over past 2-3 years drug development programs
  refocused on molecular targeted therapeutics
• Potential for major advances based on new biology
• Molecular oncology revolution will need close
  interactions between clinical oncology,
  pathology, pharmacology, and basic science