Lecture 6 - Topo-microtubule Inhibitors

Question 1

Topoisomerase mechanisms

Answer 1

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

Question 2

Topo1 inhibitors

Answer 2

irinotecan

Question 3

Topo2 intercalator

Answer 3

doxorubicin

Question 4

Topo2 inhibitors

Answer 4

etoposide
bleomycin

Question 5

Topoisomerase 1

Answer 5

type I topoisomerase cut one strand of double stranded DNA, relax remaining strand and reanneal

Question 6

Topoisomerase I inhibitor mechanism

Answer 6

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

Question 7

Topoisomerase I inhibitors

Answer 7

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

Question 8

Topoisomerase I inhibitor drug resistance

Answer 8

PGP overexpression, multidrug resistant protein overexpression, topoisomerase downregulation or mutation to prevent inhibitor binding

Question 9

Topoisomerase I inhibitors - camptothecins

Answer 9

topotecan and irinotecan

Question 10

Irinotecan

Answer 10

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

Question 11

Topoisomerase II relieves

Answer 11

torsional strain and untangles DNA by catalyzing double-stranded DNA breaks

Question 12

Topoisomerase II inhibitors

Answer 12

doxorubicin
etoposide
non-cell cycle dependent, although activity is greater in G2/M

Question 13

Doxorubicin

Answer 13

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

Question 14

Dexrazoxane

Answer 14

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

Question 15

Etoposide

Answer 15

inhibits religation of double stranded breaks induced by topoII but does not intercalate
G2 block-cell cycle specific
produces G2/M block

Question 16

Resistance to topo II inhibitors

Answer 16

PGP overexpression, MRP overexpression, glutathione S-transferase overexpression (doxorubicin only), topoisomerase II downregulation or mutation, increased DNA damage repair

Question 17

Bleomycin

Answer 17

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

Question 18

Bleomycin toxicity

Answer 18

pulmonary toxicity is dose-limiting and cumulative - pulmonary inflammation progressing to pulmonary fibrosis
myelosuppression is minimal

Question 19

Bleomycin inactivated by

Answer 19

bleomycin aminohydrolase, which is in high concentrations everywhere but skin and lung
increased levels of aminohydrolase in resistant cancers

Question 20

Dynamic instability

Answer 20

proteins cap tubulins, build up, then fall apart
growing and shrinking microtubules

Question 21

Microtubules during cell division

Answer 21

responsible for moving chromosomal material into daughter cells during mitosis

Question 22

Microtubule inhibitors

Answer 22

vincristine
paclitaxel

Question 23

Spindle assembly checkpoint

Answer 23

kinetochores need to be attached to spindle microtubules; needs to be kinetochore tension

Question 24

Spindle assembly checkpoint with inhibitors

Answer 24

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

Question 25

Vinca alkaloids

Answer 25

prevent microtubule assembly - prevent tubular monomer from forming microtubules

Question 26

Taxanes

Answer 26

prevent microtubule disassembly these stabilize microtubules

Question 27

Microtubule destabilizers and stabilizers bind

Answer 27

different sites on tubulin this is how they have their different functions

Question 28

Microtubule destabilizers

Answer 28

vinca alkaloids (vincristine and analogs) erubulin

Question 29

Vinca alkaloids

Answer 29

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

Question 30

Vinca alkaloids bind to

Answer 30

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

Question 31

Vinca alkaloids - vincristine

Answer 31

toxicity - extravasation causes severe local inflammation neurotoxicity

Question 32

Vinca alkaloids - vinblastine and vinorelbine

Answer 32

neurotoxicity less severe than with vincristine

Question 33

Eribulin

Answer 33

microtubule polymerization inhibitor - binds at microtubule ends and prevents elongation lower rate of neurotoxicity

Question 34

Microtubule stabilizers

Answer 34

taxanes (paclitaxel) epothilones (ixabepilone)

Question 35

Taxanes

Answer 35

paclitaxel, docetaxel, cabazitaxel

Question 36

Taxanes bind to

Answer 36

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

Question 37

Paclitaxel

Answer 37

linked to albumin to increase solubility and circulation time of drug toxicity: myelosuppression and neurotoxicity

Question 38

Epothilones

Answer 38

binds to tubulin and promotes tubulin polymerization and microtubule stabilization not cross-resistant with taxanes, poor PGP substrate toxicity: neurotoxicity