Lecture 6 - Topo-microtubule Inhibitors Flashcards
Topoisomerase mechanisms
transcription and translation induce supercoiling
topoisomerases provide mechanism to reduce localized supercoiling and provide access to double stranded DNA by enzymes responsible for replication, transcription and repair
Topo1 inhibitors
irinotecan
Topo2 intercalator
doxorubicin
Topo2 inhibitors
etoposide
bleomycin
Topoisomerase 1
type I topoisomerase cut one strand of double stranded DNA, relax remaining strand and reanneal
Topoisomerase I inhibitor mechanism
inhibitor covalently attached to enzyme that is covalently attached to DNA, blocks religation from occurring
provides physical barrier to replication and transcription, prevent DNA from being functional
Topoisomerase I inhibitors
S phase specific! - cells in S phase are most sensitive to Topo I induced cleavage
bind to and form a ternary drug-enzyme-DNA complex, glues topoisomerase to DNA
inhibitor binding stabilizes Topo-DNA complex and blocks DNA religation
Topoisomerase I inhibitor drug resistance
PGP overexpression, multidrug resistant protein overexpression, topoisomerase downregulation or mutation to prevent inhibitor binding
Topoisomerase I inhibitors - camptothecins
topotecan and irinotecan
Irinotecan
is a prodrug, irinotecan converted to SN-38 (active metabolite) by carboxylesterases
SN-38 is metabolized by uridine diphospate glucosyltransferase (UGT1A1)
~10% of the pop has polymorphisms predicting low expression of UGT1A1, leading to increased toxicity of irinotecan
Topoisomerase II relieves
torsional strain and untangles DNA by catalyzing double-stranded DNA breaks
Topoisomerase II inhibitors
doxorubicin
etoposide
non-cell cycle dependent, although activity is greater in G2/M
Doxorubicin
toxicity: cardiotoxicity - damage to cardiac muscle dependent on cumulative dose; severe local tissue damage if extravastated
free radical damage causes cardiotoxicity since heart tissue has low levels of enzymes that neutralize free radicals
Dexrazoxane
drug to mediate toxicity, helps to manage cardiac damage; protects against anthracycline-induced cardiotoxicity
cyclic analog of metal chelating agent EDTA
enters cell and converts to ring-opened chelating agent - binds to iron, blocks iron-oxygen induced toxicities; cardiotoxicity of doxorubicin believed to be caused by iron-catalyzed free radical formation
Etoposide
inhibits religation of double stranded breaks induced by topoII but does not intercalate
G2 block-cell cycle specific
produces G2/M block
Resistance to topo II inhibitors
PGP overexpression, MRP overexpression, glutathione S-transferase overexpression (doxorubicin only), topoisomerase II downregulation or mutation, increased DNA damage repair
Bleomycin
charged side chain, intercalates into DNA, generates free radicals from imidazole, causes Fe++ iron oxygen species to generate DNA free radical
radical intermediate leads to DNA single strand and double strand breaks
greatest effect on cells in G2 and M phases of cell cycle
Bleomycin toxicity
pulmonary toxicity is dose-limiting and cumulative - pulmonary inflammation progressing to pulmonary fibrosis
myelosuppression is minimal
Bleomycin inactivated by
bleomycin aminohydrolase, which is in high concentrations everywhere but skin and lung
increased levels of aminohydrolase in resistant cancers
Dynamic instability
proteins cap tubulins, build up, then fall apart
growing and shrinking microtubules
Microtubules during cell division
responsible for moving chromosomal material into daughter cells during mitosis
Microtubule inhibitors
vincristine
paclitaxel
Spindle assembly checkpoint
kinetochores need to be attached to spindle microtubules; needs to be kinetochore tension
Spindle assembly checkpoint with inhibitors
microtubule assembly inhibitors –> in cancer cells, you never get to checkpoint, if it can’t get to checkpoint –> apoptosis
microtubule de-assembly inhibitors lead to sustained checkpoint activation –> cell death
defective spindle assembly checkpoint –> chromosomes skrewed up
Vinca alkaloids
prevent microtubule assembly - prevent tubular monomer from forming microtubules
Taxanes
prevent microtubule disassembly
these stabilize microtubules
Microtubule destabilizers and stabilizers bind
different sites on tubulin
this is how they have their different functions
Microtubule destabilizers
vinca alkaloids (vincristine and analogs)
erubulin
Vinca alkaloids
large molecules require a specific transporter to get into cells
excellent substrates for PGP transporter: drugs rapidly pumped out of resistant cells, cross-resistant with other large molecule antitumor agents
Vinca alkaloids bind to
tubulin
binding leads to inhibition of microtubule assembly (polymerization) and inhibition of microtubule shortening
no attachment of microtubules to mitotic spindle, leads mitotic arrest because no spindle checkpoint activation
SE: peripheral neuropathy
Vinca alkaloids - vincristine
toxicity - extravasation causes severe local inflammation
neurotoxicity
Vinca alkaloids - vinblastine and vinorelbine
neurotoxicity less severe than with vincristine
Eribulin
microtubule polymerization inhibitor - binds at microtubule ends and prevents elongation
lower rate of neurotoxicity
Microtubule stabilizers
taxanes (paclitaxel)
epothilones (ixabepilone)
Taxanes
paclitaxel, docetaxel, cabazitaxel
Taxanes bind to
tubulin with two consequences: promotion of microtubule assembly into stable (non-functional) bundles decreases free tubulin and prevents microtubule formation at spindle; stabilization of existing microtubules blocks depolymerization (shortening) and segregation of sister chromatids to daughter cells
leads to mitotic arrest
excellent substrates for PGP transporter - drugs rapidly pumped out of resistant cells, cross-resistant with other large molecule antitumor agents
Paclitaxel
linked to albumin to increase solubility and circulation time of drug
toxicity: myelosuppression and neurotoxicity
Epothilones
binds to tubulin and promotes tubulin polymerization and microtubule stabilization
not cross-resistant with taxanes, poor PGP substrate
toxicity: neurotoxicity
