cancer chemotherapy

Cancer Chemotherapy

Chapter 42

Normal cells…•Differentiate, grow, mature, divide

–Regulated, balanced; cell birth=cell death

•Regulation: intracell signaling

–Hyperplasia: new cells prod’d w/ growth stimulus via hormones, endogenous signals

–Ex: hyperplasia of endometrial tissue during menstrual cycle is normal and necessary

BUT if intense, prolonged demand …• May cell structural, functional

abnormalities– Metaplasia: replacement of one cell type by

another• Thicker cell layer better accommodates

irritation– Ex: bronchial epithelium chronically

irritated ciliated columnar epithelial cells replaced by sev layers cuboidal epithelium

»Note: Replacement cells normal, just different

»Reversible

– Dysplasia: replacement cells disordered in size, shape• Incr’d mitosis rate• Somewhat reversible, often precancerous

– Neoplasia: abnormal growth/invasion of cells• “New growth”• Neoplasm = tumor• Irreversible• Cells replicate, grow w/out control

Neoplasms

• = Tumors = groups of neoplastic cells• Two major types: benign, malignant• Benign – “noncancerous”

– Local; cells cohesive, well-defined borders

– Push adjacent tissue away– Doesn’t spread beyond original site– Often has capsule of fibrous

connective tissue

• Malignant – grow more rapidly; often called “cancer”– Not cohesive; seldom have capsule– Irregular shape; disrupted

architecture– Invade surrounding cells– Can break away to form second

tumor•“Metastasis” from 1o to 2o site

Cancer (Neoplastic) Cells

• May be:– Well-differentiated = retain normal

cell function • Mimic normal tissue • Often benign

– Poorly differentiated = disorganized• Can’t tell tissue of origin• “Anaplastic”

Oncogenesis = Process of Tumor Development

• Probably multi-step process Decr’d ability to differentiate

and control replication and growth

Steps to Cancer

• Initation = impt change introduced into cell– Probably through DNA alteration – >1 event probably needed for tumor prod’n– Reversible unless and until:

• Promotion = biochem event encourages tumor form’n

• Gen’ly need both initiation and promotion– Initiators, promoters may be toxins OR

radiation OR viruses)

Genetics vs. Environment• Most tumors arise “spontaneously” w/out

known carcinogen exposure, AND• Proto-oncogenes can be inherited (ex:

“breast cancer gene”)• BUT environmental agents are known to

cause DNA mutations, AND• Risk factors known (Ex:

– Cigarette smoking lung cancer– UV light exposure skin cancer)

• Theory: “Genetics loads the gun; the environment pulls the trigger”

Cell Cycle = Growth, Division

Synth DNA precursors,proteins, etc.

Premitotic synth ofstructures, mol’s

Cell Cycle Phases

Cycle Checkpoints

Cdk’s, Cyclins Implement Cycle Decisions

Brody 42.1 – G0

G0• Quiescent phase outside cell cycle• Most adult cells• Cyclin D in low concent• Rb prot hypophosph’d

– Inhib’s expression prot’s impt to cycle progression

– Binds E2F transcr’n factors• Controls genes impt to DNA repl’n

• Growth factor binding act’n to G1

Apoptosis Review

• In healthy cells, survival factors signal act’n anti-apoptotic mech’s– Cytokines, hormones, cell contact factors

• Programmed cell death• Cascade of proteases initiate process

– Initiator caspases that act on effector caspases

• Effector caspase act’n may be through Tumor Necrosis Factor Receptor

• Second pathway act’d by intracell signals, e.g. DNA damage– Players are p53 gene & prot;

mitochondrial cytochrome c; Apaf-1 (prot); caspase 9

• Effector caspases initiate pathway cleavage cell constituents cluster membr-bound “entities” (used to be cell) that are phagocytosed

• Anti-apoptotic genetic lesions nec for dev’t cancer – Apoptosis resistance characteristic of

cancer cells

Genes Impt to Oncogenesis

• Code for prot’s that regulate cell div/prolif’n when turned on/off– Malfunctions, mutations may

oncogenesis– Changes w/ viruses, chem’s: point mutations,

gene amplifications, chromosome translocations

• Two impt routes:– Proto-Oncogenes – code for prot’s turning cell div

ON• Mutations overexpression cancer

– Tumor suppressor genes – code for prot’s turning cell div OFF• Mutations repression cancer

50.2 Rang

Uncontrolled Proliferation

• Result of act’n proto-oncogenes or inact’n tumor suppressor genes – Change in growth factors, receptors

• Incr’d growth factors prod’d

– Change in growth factor pathways• 2nd messenger cascades (esp tyr-kinase

receptor cascades)

– Change in cell cycle transducers• Cyclins, Cdk’s, Cdk inhibitors

– Change in apoptotic mech’s– Change in telomerase expression– Change in local blood vessels

angiogenesis

• Note: Genes controlling any of these prot’s/mech’s can be considered proto-oncogenes or tumor suppressor genes

• Note: Dev’t malignant cancer depends on sev transform’ns

Anticancer Drugs are Antiproliferative

• Affect cell division– Active on rapidly dividing cells

• Most effective during S phase of cell cycle– Many cause DNA damage

• Damage DNA init’n apoptosis

• Side effects greatest in other rapidly-dividing cells– Bone marrow toxicity – Impaired wound healing– Hair follicle damage – Gi epith damage – Growth in children– Gametes– Fetus

• May themselves be carcinogenic

Difficulties in Chemotherapy Effectiveness

• Solid tumors – Growth rate decr’s as neoplasm size incr’s

• Outgrows ability to maintain blood supply AND• Not all cells proliferate continuously

– Compartments• Dividing cells (may be ~5% tumor volume)

– Only pop’n susceptible to most anticancer drugs

• Resting cells (in G0); can be stim’d G1– Not sensitive to chemotherapy, but act’d when

therapy ends

• Cells unable to divide but add to tumor bulk

• Suspended cancer cells (leukemias)– Killing 99.99% of 1011 cancer cell

burden, 107 neoplastic cells remain– Can’t rely on host immunological

defense to kill remaining cancer cells• Diagnosis, treatment difficult if

rapidly growing– Ex: Burkitt’s lymphoma doubles ~24 h– Approx 30 doublings tumor mass of 2

cm (109 cells)• May be detected, if not in deep organ

– Approx 10 add’l doublings 20 cm mass (1012 cells) – lethal

– Therefore, “silent” for first ¾ existence

Drugs Used in Cancer Chemotherapy

• Cytotoxic Agents– Alkylating Agents– Antimetabolites– Cytotoxic antibiotics– Plant derivatives

• Hormones– Suppress nat’l hormone secr’n or

antagonize hormone action

• Misc (mostly target oncogene products)

Rand 50.3

Alkylating Agents

• Contain chem grps that covalently bind cell nucleophiles

• Impt properties of drugs– Can form carbonium ions

• C w/ 6 electrons highly reactive

• React w/ -NH2, -OH, -SH

– Bifunctional (2 reactive grps)• Allow cross-linking

• Impt targets– G N7 – strongly nucleophilic

• A N1, A N3, C N3 also targets

• DNA becomes cross-linked w/ agent– Intra- or inter-strand Decr’d transcr’n, repl’n Chain scission, so strand breaks Inappropriate base pairing

(alkylated G w/ T)• Most impt: S phase repl’n (strands

unwound, more susceptible) G2 block, apoptosis

Rang 50.4

42-5 structures

Nitrogen Mustards

•Loss Cl intramolec cyclization of side chain

Reactive ethylene immonium derivative

Cyclophosphamide

• Most common• Prodrug – liver metab by CYP P450

MFO’s• Effects lymphocytes

– Also immunosuppressant• Oral or IV usually• SE’s: n/v, bone marrow dpression,

hemorrhagic cystitis– Latter due to acrolein toxicity;

ameliorated w/ SH-donors

42.6 cyclophosph

42.7 nitrosourea

Nitrosoureas

•Also activated in vivo

•Alkylate DNA BUT alk’n prot’s toxicity

Temozolomide•Methylates G, A improper G-T base pairing

Cisplatin• Cl- dissoc’s reactive complex that

reacts w/ H2O and interacts w/ DNA intrastrand cross-link (G N7 w/ adjacent G O6) denaturation DNA– Nephrotoxic– Severe n/v ameliorated w/ 5-HT3

antagonists (decr gastric motility)• Carboplatin – fewer above SE’s, but

more myelotoxic

Antimetabolites

• Mimic structures of normal metabolic mol’s– Inhibit enz’s competitively OR– Inc’d into macromol’s

inappropriate structures• Kill cells in S phase• Three main groups

– Folate antagonists– Pyr analogs– Pur analogs

Folic Acid Analogs• Folic acid essential for synth purines,

and thymidylate• Folate: pteridine ring + PABA +

glutamate– In cells, converted to polyglutamates then

tetrahydrofolate (FH4)

• Folate FH4 cat’d by dihydrofolate reductase in 2 steps:– Folate FH2– FH2 FH4

• FH4 serves as methyl grp donor (1-C unit) to deoxyuridine (dUMP dTMP), also regenerating FH2

Methotrexate

• Higher affinity for enz than does FH2– Add’l H or ionic bond forms

Depletion FH4 in cell depl’n dTMP “thymine-less death”

Inhib’n DNA synth • Uptake through folate transport

system– Resistance through decr’d uptake

• Metabolites (polyglutamate deriv’s) retained for weeks, months

50.8 Rand

Pemetrexed

45.2 Rand

FYI…

Pyrimidine Analogs

• 5-Fluorouracil – dUMP analog also works through dTMP synthesis pathway– Converted “fraudulent” nucleotide

FdUMP – Competitive inhibitor for thymidylate

synthetase active site, but can’t be converted to dTMP

– Covalently binds thymidylate synthetase– Mech action uses all 3routes decr’d

DNA synthesis, also transcr’n/transl’n inhib’n

• Gemcitabine– Phosph’d tri-PO4’s

• “Fraudulent nucleotide”

– Also inhib’s ribonucleotide reductase decr’d nucleotide synth

• Capecitabine is prodrug– Converted to 5FU in liver, tumor

• Enz impt to conversion overexpressed in cancer cells (?)

• Cytosine arabinoside– Analog of 2’dC– Phosph’d in vivo cytosine

arabinoside triphosphate– Inhibits DNA polymerase

• Gemcitabine – araC analog– Fewer SE’s

http://www.pfeist.net/ALL/arac/images/spongo2.gif

42-11

Gemcitabine

Purine Analogs• 6-Mercaptopurine, 6-Thioguanine

– Converted to “fraudulent nucleotides”– Inhibit enz’s nec for purine synth

• Fludarabine– Converted to triphosphate– Mech action sim to ara-C

• Pentostatin– Inhibits adenosine deaminase

• Catalyzes adenosine inosine

– Interferes w/ purinemetab, cell prolif’n

42-10

Fludarabine Pentostatin

Cytotoxic Antibiotics

• Substances of microbial origin that prevent mammalian cell division

• Anthracyclines– Doxorubicin

• Intercalates in DNA• Inhibits repl’n via action at topoisomerase II

– Topoisomerase II catalyzes nick in DNA strands– Intercalated strand/topoisomerase complex

stabilized permanently cleaved helix

– Epirubicin, mitozantrone structurally related

– SE’s: cardiotoxicity (due to free radical prod’n), bone marrow suppression

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Mitozantrone

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– Dactinomycin• Intercalates in DNA minor groove between

adjacent GC pairs• Interferes w/ RNA polymerase movement

decr’d transcr’n• Also may work through topoisomerase II

– Bleomycin• Glycopeptide• Chelates Fe, which interacts w/ O2 Gen’n superoxide and/or hydroxyl radicals• Radicals degrade DNA fragmentation, release

of free bases• Most effective in G2, also active against cells in

G0• Little myelosuppression BUT pulmonary fibrosis

Dactinomycin

Bleomycin

Plant Alkaloids

• Work at mitosis• Effect tubulin, therefore microtubule

activity Prevention spindle form’n OR– Stabilize (“freeze”) polymerized

microtubules Arrest of mitosis• Other effects due to tubulin defects

– Phagocytosis/chemotaxis– Axonal transport in neurons

http://biotech.icmb.utexas.edu/botany/gifs/vdes.gif

Vinca Alkaloids

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Taxanes: Paclitaxel, Docetaxel

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• Etoposide, teniposide– From mandrake root– Inhibit mitoch function, nucleoside

transport, topoisomerase II• Campothecins: irinotecan, topotecan

– Irinotecan requires hydrolysis active form

– Bind, inhibit topoisomerase II– Repair is difficult

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Ironotecan

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Topotecan

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Hormones

• Tumors der’d from tissues responding to hormones may be hormone-dependent– Growth inhib’d by hormone antagonists

OR other hormones w/ opposing actions OR inhibitors of relevant hormone

• Glucocorticoids– Inhibitory on lymphocyte prolif’n– Used against leukemias, lymphomas

• Estrogens– Block androgen effects (ex:

fosfestrol)– Used to recruit cells in G0 G1, so

better targets for cytotoxic drugs

• Progestogens (ex: megestrol, medroxyprogesterone)– Used in endometrial, renal tumors

• GnRH analogs (ex: goserelin)– Inhibit gonadotropin release decr’d

circulating estrogens

• Hormone antagonists– Tamoxifen impt in breast cancer

treatment• Competes w/ endogenous estrogens for

receptor• Inhibits transcr’n estrogen-responsive

genes

– Flutamide, cyproterone impt in prostate tumors• Androgen antagonists

– Trilostane, aminoglutethimide inhibit sex hormone synth at adrenal gland

– Formestane inhibits aromatase at adrenal gland

http://www.wellesley.edu/Chemistry/chem227/nucleicfunction/cancer/tamoxifen.gifhttp://www.neurosci.pharm.utoledo.edu/MBC3320/images/Flutamide.gif

Formestane

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Trilostane

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Rang 50.1

Antitumor Agents Working through Cell Signalling

Binding Epidermal Growth Factor Receptors Cell

Prolif’n• EGFR present on many solid tumors• Tyr-kinase type receptors• Ligand binding kinase cascade

transcription factor synth incr’d cell prolif’n metastasis decr’d apoptosis

• Cells expressing EGFR resistant to cytotoxins; poor clinical outcome predicted

• Cetuximab– Monoclonal Ab directed against EGFR

• Erbitux – Famous anti-EGFR Ab

Drugs Targeting Growth Factor Receptors

• Trastuzumab– “Humanized” mouse

monoclonal Ab– Binds HER2

• Membr prot structurally similar to EGFR

• Has integral tyr kinase activity

• Impt in breast cancer cells

– May also induce p21 and p27 • Cell cycle inhibitors

http://www.gene.com/gene/products/information/oncology/herceptin/images/moa.jpg

• Imatinib (Gleevec, Glivec)– Small inhibitor of kinases– Inhibits PDGF activity via its tyr kinase

receptor– Inhibits Bcr/Abl kinase

• Cytoplasmic kinase impt in signal transduction

• Unique to chronic myeloid leukemia

– Also used against non-small cell lung cancer

• Gefitinib – Similar to Imatinib

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Imatinib

Gefitinib

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