Cancer chemotherapy

how are chemotherapy drugs classified

biochemical activities or origins

what is doxorubicin and where does it come from

one of the most potent chemo drugs it comes from daunorubicin which is a natural product made by bacteria

classes of chemo drugs

1. alkylating agents

2. platinating agents

3. antimetabolites

4. topoisomerase inhibitors

5. anti-microtubular agents

6. miscellaneous

7. molecular targeted agents

How do alkylating agents work

chemically diverse drugs with alkyl groups that form covalen bonds with macromolecules (DNA) through interactions of their positively charged intermediates with nucleophiles. This attacks DNA and can cause single-strand breaks or mutations

examples of alkylating agents

1. nitrogen mustards

2. nitrosoureas

what are nitrogen mustards and give an example

alkylating aents derived from the prototype alkylating agent mechlorethamine, they alkylate the N7 position of guanine. e.g cyclophosphoamide

How is cyclophosphamide activated

it is metabolized in the liver to alkylating intermediate phosphoramide mustard

what are nitrosoureas and give an example

lipid soluble drugs that can penetrate the CNS e.g BCNU with 2-chloro-ethyl groups

how do platinating agents work

leaving group on drug anf then N-7 of guanine attacks the drug

what is cisplatin

platinating agent with substantial toxicity. one of the most useful anti-cancer agents

what are antimetabolites

drugs that interfere with normal cellular function

examples of antimetabolites

1. antifolates

2. pyrimidine analogues

3. purine analogues

what do antifolates do, give an example

prevent the formation of reduced folates required for DNA synthesis, methotrexate

what do purine/pyrimidine analogues do, give two pyrimidine examples and one purine example

inhibit formation of normal nucleotides

pyrimidine - 5-fluorouracil, cytosine arabinoside

purine - 6-thioguanine

what is methotrexate

analog of folic acid, competitive inhibitor of DHFR preventing the formation of reduced folate which leads to the depletion of dTMP and purines

what does DHFR do

converts FH₂ to FH₄

what is FH₄ required for

1. transder -CH₃ in purine biosynthesis

2. convert dUMP to dTMP

how does 5-fluorouracil work

resembles uracil and thymine and thus gets metabolized to a nucleoside that is phosphorylated to 5-FUTP and 5-FdUMP. 5-FUTP is incorporated into RNA thus inhibiting RNA processing. 5-FdUMP inhibits thymidylate synthase thus depleting dTMP

what is cytosine arabinoside

a cytodine analog, it gets converted to ara-CTP which is a competitive inhibitor of DNA polymerase

what is 6-thioguanine

a purine antimetabolite that resembles guanine with a sulphur in the place of the O in position 6

how does 6-thioguanine work

gets metabolized to deoxynucleotides and incorporated into DNA to interrupt double helix formation. metabolites of the drugs inhibit purine and RNA synthesis

where are topoisomerase inhibitors isolated from and what is an example

plants, fungi or bacteria

how do topoisomerase inhibitors work

bind to topoisomerase leading to permanent DNA damage

what is camptothecin

topoisomerase I inhibitor, isolated from wood of a tree, clinical trials terminated due to high toxicity

current topoisomerase I inhibitors

topotecan and irinotecan (CPT-11)

what is an etoposide

a synthetic glycoside that acts as a topoisomerase II inhibitor

what is daunorubicin

an anthracycline that is a product of streptomyces and is highly effective against leukemia. it is chemically modified to doxorubicin

mechanism of action of doxorubicin

interaction with topoisomerase II leads to double-strand breaks. the formation of free radicals leads to oxidative damage. free radicals bind directly to cell membranes makin them leaky and causing cell death

what are vinca alkaloids and give examples

anti-microtubular agents that bind to tubulin and inhibit polymerization that form microtubules e.g vinblastine and vincristine

what are taxanes and give an example

anti-microtubular agents that bind to tubulin and inhibits microtubular dissassemble by stabilizing the polymer and preventing monomer formation e.g paclitaxel (taxol)

what does bleomycin do

causes DNA double-strand breaks

what does mitomycin C need

metabolism to an alkylating agent

what are the advantages to molecular targeted agents

they have the potential for greater specificity and thus less toxicity

what is Gleevec

a protein kinase inhibitor that inhibits tyrosine kinase if the BCR-ABL fusion protein

is there a relationship between tyrosine kinases and Ser/Thr kinases

all 318 have structural relatedness

what is the consequence of the similar structure of kinases

its difficult to produce a specific kinase inhibitor

what tyrosine kinases does gleevec target

c-kit receptor

what does gleevec associate with

the ATP-binding domain, it binds and stabilizes a catalytically inactive conformation of the enzyme

what tumours is gleevec used to treat and why

gastrointestinal stromal tumours because it inhibits the Tyr kinase c-kit receptor that is over-expressed in gastrointestinal tumours

how is resistance to gleevec in CML cells developed

after reseistance gleevec doses go from 0.1 to 8 uM as the levels of the fused BCR-ABL gene increase there is an overexpression of the bcr-abl oncoprotein thus exceeding the ability of normal therapeutic concentration of Gleevec

what are the two mechanisms the CML uses to develop resistance to Gleevec

1. mutations in Bcr-Abl gene results in direct interference with gleevec binding or stereochemical shift that prevents gleevec binding. the Thr315 was replaced by Ile315 which interferes with insertion of Gleevec into the cavity

2. gene amplification of Bcr-abl

what is tarceva

a selective inhibitor of the EGFR tyrosine kinase that blocks the ATP-binding site of the EGF-receptor thus depriving cells of EGFR signalling and leading to cell death

what is iressa

a selective inhibitor of the EGFR tyrosine kinase that blocks the ATP-binding site of the EGF-receptor

what causes Iressa resistance

mutations in the cytoplasmic domain of EGFR which deregulate and activate Tyr kinase function and stimulate the down-stream signalling pathways. often a Thr-to-Met substitution that blocks Iressa binding

what is velcade

a competitve inhibitor of the proteasome that is effectve against multiple myeloma

how does velcade work

it prevents NF-kB from activating expression of anti-apoptotic genes, IL-2 and IL-4 genes, VEGF and cell adhesion molecules. strong preference for inhibiting beta5 chymotryptic activity

effect of Velcade on multiple myeloma

single species IgG decreases and a heterogenous polyclonal pattern of IgG is exhibited

how does velcade work woth alkylating agents

mepahlan in the prescence of velcade induces apoptosis at low concentrations

how do cells act without velcade treaatment

IkB is phosphorylated causing it to be degraded. this means that NF-kB is free to move into the nucleus activating the expression of anti-apoptotic and prolieration genes

how do cells act with velcade treatment

IkB is phosphorylated and ubiquitylated bu cannot be degreaded becuase the proteasomes have been engorged with unprocessed polypeptides. this leads to accumulation of IkB in the cytoplasm and the continued sequestration of NF-kB by the IkB. NF-kB is prevented from moving into the nucleus leading to apoptosis and no proliferation

how is velcade activity assessed

electophoretic mobility shift assays, if velcade blocks NF-kB binding theres no shift in the EMSA

what are the six assays for assessing anti-tumour activity

1. trypan blue exclusion assay

2. measuringg uptake of ³H-thymidine into DNA, ³H-uridine into RNA or ³H-amino acids into proteins

3. assays of apoptosis

4. MTT assay

5. clonogenic assay

6. invasion assay

what is the trypan blue exclusion assay

cells with damaged plasma membrane will fail to exclude the blue dye and are stained blue

why is labelled H measured

cells with affected enzymes will have impairement of macromolecular synthesis

how is apoptoses assayed

DNA laddering

what is the MTT assay

MTT stains mitochondria, yellow tetrazolium is reduced to purple formazan be mitochondrial dehydrogenases

what is the clonogenic assay

assesses the formation of colonies of semi-solid mediums such as agar or methyl cellulose. looking for dependence on anchorage

what is the invasion assay

uses matrigel to assess invasiveness