Topoisomerase Inhibitors Flashcards
What is the general function of topoisomerases? What 2 processes do they allow?
topoisomerases are nuclear enzymes that relax supercoiled ds DNA to allow DNA replication and RNA transcription
What is the function of topoisomerase I? topoII?
topoisomerase I – > releases torsional strain via SINGLE strand nick
topoisomerase II – > releases torsional strain via DOUBLE strand nick
After nicks, swiveling of supercoiled DNA occurs at the nicks followed by re-ligation to relieve torsional strain
Topo inhibitors bind and stabilize DNA/topo cleavable complex – > preventing what?
Topo inhibitors bind and stabilize DNA/topo cleavable complex – > preventing RE-LIGATION of DNA STRANDS
Irreversible damage results when advancing DNA replication fork encounters drug-stabilized cleavable complex – > lethal DS DNA breaks and cell death
Name the 2 topoisomerase I inhibitors, what group are they in?
Topo I
Camtothecins (Topotecan & Irinotecan)
Name the topo II inhibitors, what groups are they in?
Anthracyclines:
- Daunorubicin
- Doxorubicin
- Epirubicin (doxorubicin analog)
- Idarubicin (daunorubicin analog)
Anthracenediones:
- Mitoxantrone
Epipodophyllotoxins:
- Etoposide, Teniposide
What are 2 types of resistance for the topoisomerase inhibitors (both I and II) ?
- Topo II (NOT topo I) are substrates for p-glycoprotein and have MDR phenotype
- For both topo I/II inhibitors, can see resistance w reduced topoisomerase levels or changes in the enzymes dt mutation
Describe metabolism/elimination of topotecan
How administered?
Topotecan - NOT metabolized. Excreted unchanged by kidenys, so dose reduce if renal disease
PO or IV
What is the active metabolite of irinotecan?
Irinotecan – > SN-38 (active)
Irinotecan requires esterification by serum/tissue ________ to become activated to SN-38
Irinotecan requires esterification by serum/tissue ___carboxylesterases__ to become activated to SN-38
Are topoisomerase I inhibitors cell phase specific?
NO but Cells in S-phase are very sensitive bc DNA replication requires topo I activity; topo associated ss breaks are converted to ds breaks
What is most interesting side effect of irinotecan?
early onset diarrhea and other atropine-responsive cholinergic signs
Name the doxorubicin analog and the daunorubicin analog
Epirubicin is the doxorubicin analog.
Idarubicin is the daunorubicin analog.
Teniposide and etoposide are semisynthetic derivatives of podophyllotoxin, a _______ agent derived from the _______ plant
Teniposide and etoposide are semisynthetic derivatives of podophyllotoxin, a __antimitotic_ agent derived from the _mandrake__ plant
DLT of etoposide at high doses: _____
What is another concern? ________
etoposide
- mucositis at high dose
- secondary leukemia
Do etoposide and teniposide act against tubulin?
In contrast to the parent podophyllotoxin, the epipodophyllotoxins are INACTIVE against tubulin and are PURE inhibitors of DNA topoisomerase II
Which is more potent, etoposide or teniposide?
teniposide is 10x more potent than etoposide
Is etoposide or teniposide a radiosensitizer?
Which requires dose reduction in proportion to creatinine?
ETOPOSIDE for both
Which topoisomerase II inhibitors also target free radicals?
Anthracyclines, Mitoxantrone
Which topo II inhibitors also target DNA intercalation?
Anthracyclines, Mitoxantrone, Ellipticines
Which topo II are considered PURE top II poisons?
epipodophyllotoxins (teniposide, etoposide)
**they are NON intercalators!
What organs metabolize the epipodophyllotoxins?
Etoposide -hepatic metabolism and 35-40% renal excretion (unchanged)
Teniposide -probable hepatic metabolism
Name 2 interesting toxicities of mitoxantrone
Cardiac dysfunction, especially after doxorubicin
BLUE tint to nails, sclerae, urine
Name 2 mechanisms of resistance of mitoxantrone
- Cross resistance w many other natural products (vincas, adria) - may be mediated by amplification of P170-glycoprotein (MDR1); others likely involved
- Altered cellular pH in tumor cells
Name 4 modes of cytotoxicity of mitoxantrone
Mitoxantrone:
- Topo II inhibition
- Avid binding to nucleic acids, inhibiting DNA/RNA synthesis (mode of binding includes intercalation bt opposing DNA strands, w preference for GC pairs)
- One electron reduction (more than doxorubicin) but reduced potential for free radical generation
- Readily induces apoptosis, like doxorubicin
Describe the structure of mitoxantrone (anthracenedione)
*lacks the sugar and tetracyclic ring
Is synthetic drug w 3 planar rings
Mitoxantrone intercalates into DNA w a preference for which types of pairs?
It also functions as a _________
- GC pairs
2. Topo II inhibitor
What are the mechanisms of toxicity for the doxorubicin-related drugs: (4)
Cytotoxic Mechanisms for Doxorubicin-Related Drugs
- Topo II inhibition - directly binds, preventing resealing of topo II-induced DNA cleavage
- DNA intercalation - intercalates bt base pairs perpendicular to long axis of double helix – > partial unwinding of helix
- Free radical formation - Adria may undergo metabolism of its quinone ring to a SEMIQUINONE RADICAL that reacts w oxygen to yield a SUPEROXIDE RADICAL – > oxidative dmg of cell membranes/DNA – > death (=cardiotoxicity)
- Affects cell membrane = bind cell membranes which can lead to death
How does doxorubicin form a superoxide radical?
Doxorubicin can undergo metabolism of quinone ring, forming SEMIQUINONE RADICAL – > which reacts w oxygen – > superoxide radical.
Leads to oxidative damage of cell membranes and DNA – > death
Anthracyclines:
What are the products of one-electron reduction?
Two-electron reduction?
- One-electron reduction to semiquinone free radical – > aerobic production of (1) superoxide anion, (2) hydrogen peroxide (3) hydroxyl radical
- Two-electron reduction: (1) aglycone species which can be conjugated for export in bile
- What drug will increase doxorubicin cardiotoxicity?
- What will bind doxorubicin, causing aggregation, and increase doxorubicin clearance?
- What types of drugs will sensitize liver to anthracycline toxicity?
- Paclitaxel - dt delayed adria clearance
- Heparin
- Drugs that decrease hepatic reduced glutathione pools (acetominophen, CCNU)
What happens to Adria if….
- Intracellular acidosis
- Acidification of extracellular fluid
- Intracellular acidosis – > enhanced drug accumulation bc un-ionized drug enters, gets protonated, then can’t diffuse out of cell
- Acidification of ECF – > drug shift out of cell
NOTE: Tumor masses 1cm + can have acidic ECF, so better to treat microscopic disease!
Doxorubicin can directly inhibit which enzyme, which can produce alterations in vascular tone, w/in the heart (and tumors)
doxo can directly inhibit NO synthase
One-electron reduction (adria) – >
- leads to formation of __A____ in presence of ___B____ , An electron is then donated to form superoxide anion, which yields ___C____.
- Both A and __D__ cleave H2O2 to produce the ___E____ — > this initiates apoptosis or necrosis secondary to….
- Oxidative damage to cell memb – > can incite Fas/FasL induced apoptosis
- Mitochondrial membrane injury – > leads to release of cytochrome c
- DNA base oxidation
- Altered calcium sequestration
- Energy loss
One-electron reduction (adria) – >
- leads to formation of SEMIQUINONE RADICAL in presence of OXYGEN , An electron is then donated to form superoxide anion, which yields HYDROGEN PEROXIDE.
- Both SEMIQUINONE RADICAL and IRON cleave H2O2 to produce the _HYDROXYL RADICAL__ — > this initiates apoptosis or necrosis secondary to….
- Oxidative damage to cell memb – > can incite Fas/FasL induced apoptosis
- Mitochondrial membrane injury – > leads to release of cytochrome c
- DNA base oxidation
- Altered calcium sequestration
- Energy loss
These 3 molecules act as intracellular protectants to reduce superoxide, H2O2, and lipid hydorperoxides, to WATER w/o formation of hydroxyl/peroxyl radicals
(ADRIA)
- Superoxide dismutase
- Catalase
- Glutathione peroxidase
Glutathione also reacts w radicals and helps glutathione peroxidase
Cardiotoxicity of doxorubicin results from these 4 things:
- Low levels of cardiac CATALASE
- High levels of mitochondria/myoglobin that enhance drug activation
- Cardiac glutathione peroxidases are very sensitive to free radical attack – > free radicals destroy this enzyme while also stimulating cardiac H2O2 formation
- Doxorubicinol = alcohol metabolite of doxorubicin that is produced by TWO-electron reduction of C-13 side chain carbonyl group – > causes toxicity
Doxorubicin can directly inhibit which enzyme, which can produce alterations in vascular tone, w/in the heart (and tumors)
doxo can directly inhibit NO synthase
One-electron reduction (adria) – >
- leads to formation of __A____ in presence of ___B____ , An electron is then donated to form superoxide anion, which yields ___C____.
- Both A and __D__ cleave H2O2 to produce the ___E____ — > this initiates apoptosis or necrosis secondary to….
- Oxidative damage to cell memb – > can incite Fas/FasL induced apoptosis
- Mitochondrial membrane injury – > leads to release of cytochrome c
- DNA base oxidation
- Altered calcium sequestration
- Energy loss
One-electron reduction (adria) – >
- leads to formation of SEMIQUINONE RADICAL in presence of OXYGEN , An electron is then donated to form superoxide anion, which yields HYDROGEN PEROXIDE.
- Both SEMIQUINONE RADICAL and IRON cleave H2O2 to produce the _HYDROXYL RADICAL__ — > this initiates apoptosis or necrosis secondary to….
- Oxidative damage to cell memb – > can incite Fas/FasL induced apoptosis
- Mitochondrial membrane injury – > leads to release of cytochrome c
- DNA base oxidation
- Altered calcium sequestration
- Energy loss
These 3 molecules act as intracellular protectants to reduce superoxide, H2O2, and lipid hydorperoxides, to WATER w/o formation of hydroxyl/peroxyl radicals
(ADRIA)
- Superoxide dismutase
- Catalase
- Glutathione peroxidase
Glutathione also reacts w radicals and helps glutathione peroxidase
Cardiotoxicity of doxorubicin results from these 4 things:
- Low levels of cardiac CATALASE
- High levels of mitochondria/myoglobin that enhance drug activation
- Cardiac glutathione peroxidases are very sensitive to free radical attack – > free radicals destroy this enzyme while also stimulating cardiac H2O2 formation
- Doxorubicinol = alcohol metabolite of doxorubicin that is produced by TWO-electron reduction of C-13 side chain carbonyl group – > causes toxicity
Doxorubicin increases intracellular iron by:
- ____
- _____
- Releasing it from ferritin and microsomes
2. Increasing iron uptake into cells by transferrin-mediated process (enhancing intracellular iron availability)
The hydroquinone structure of doxo/dauno binds what form of iron?
This leads to what 3 effects:
FERRIC IRON (doxo is v potent chelator of other metal ions too)
- Oxidative destruction of MEMBRANES
- Oxidation of SULFHYDRYL groups
- Binding to DNA (differently than intercalation mechanism)
Dexrazoxane is an iron chelator that prevents doxorubicin-induced lipid peroxidation and cardiac toxicity.
It is a PRODRUG that undergoes _____ to become active. This is markedly enhanced after uptake into cardiac myocytes, with rapid conversion to active drug.
hydrolysis.
Two electron reduction leads to formation of ____. These are far less toxic than parent drug. Two electron reduction mainly leads to ______ of anthracyclines.
Two electron reduction leads to formation of deoxyaglycone metabolites. These are far less toxic than parent drug. Two electron reduction mainly leads to __inactivation_ of anthracyclines.
Cell membrane damage from anthracyclines leads to activation of which pathway? (a phospholipid in the cell membrane)
This leads to formation of _____ (a bioactive lipid) — > activates PKC and leads to ______ membrane permeability — > apoptosis
Cell membrane damage from anthracyclines leads to activation of which pathway? SPHINGOMYELIN PATHWAY
This leads to formation of CERAMIDE (a bioactive lipid) — > activates PKC and leads to MITOCHONDRIAL membrane permeability — > apoptosis
Cell membrane damage from anthracyclines leads to alterations in which pathway? ______
In the heart, it decreases ____ phosphorylation (critical for cell survival); this is blocked by pretreatment with dexraz
Cell membrane damage from anthracyclines leads to alterations in which pathway? PI3K/AKT PATHWAY
In the heart, it decreases AKT/ERK phosphorylation (critical for cell survival); this is blocked by pretreatment with dexraz
What is the major pharmacokinetic risk for adria-induced cardiotoxicity? How do you lower the risk?
PEAK DRUG CONCENTRATION is major risk.
Prolonged infusion will significantly reduce risk.
Are the topo II drugs given IV or PO?
All given IV except Idarubicin, which is given PO
Anthracyclines induce apoptosis by:
- stimulating interaction between ______ & ______ .
- This leads to _______ generation (secondary to cell membrane damage) – > increased mitochondrial membrane permeability and activation of proapoptotic _________ and ___________ protein phosphatases. (note: altered mitochondrial membr permeability leads to cytochrome c release which can cause apoptosis AND necrosis)
- Causing ______ damage (of cell membrane, mitochondrial membrane, DNA)
- Leading to elevated ______.
Anthracyclines induce apoptosis by:
- stimulating interaction between FAS & FASL .
- This leads to CERAMIDE generation (secondary to cell membrane damage) – > increased mitochondrial membrane permeability and activation of proapoptotic _CASPASES__ and __SERINE-THREONINE__ protein phosphatases. (note: altered mitochondrial membr permeability leads to cytochrome c release which can cause apoptosis AND necrosis)
- Causing __FREE RADICAL_ damage (of cell membrane, mitochondrial membrane, DNA)
- Leading to elevated __p53_.
Doxorubicin is metabolized in liver to _____
doxorubicinol (which retains some activity) and other metabolites…all excreted in bile
5 modes of resistance for anthracyclines
- P53 mutations (diminish apoptosis, increase doxorubicin resistance)
- Enhanced drug efflux (P170 glycoprotein-mediated anthracycline efflux) - also, vincas, actino D, epipodophyllotoxins
- Altered topo II activity
- Altered free radical biochemistry, sensitivity to apoptosis (via overexpression of bcl-2, p53 mutations…)
- Coadministration of heparin (increased doxorubicin clearance)
P170-glycoprotein-mediated anthracycline efflux reversed with which 4 drugs?
verapamil
cyclosporine A
calmodulin inhibitors
tamoxifen
