Lecture 4: Topoisomerase inhibitors and Microtubule inhibitors

Question 1

Dna topoisomerases

Answer 1

Important in DNA replication and RNA transcription
Type 1 topoisomerases change the degree of supercoiling of DNA by causing SS breaks and re-ligation
Type 2 topoisomerases cause a ds break
Subtype top2A and top2B
Topoisomerase are generally present at elevated levels in tumors
The current target for cancer chemotherapeutic agents are topoisomerase 1, 2A and 2B.

Question 2

Topoisomerase inhibitors types

Answer 2

Topoisomerase poisons-inhibits re-ligation step and locks enzyme into a cleavage complex enhancing rate of cleavage
MAIN TYPE
Competitive inhibitor of the ATP binding site-only TYPE 2 topo-prevents ATP-hydrolysis drive enzymatic action
Inhibitor of DNA-topoisomerase-prevent DNA binding
Inhibits ATP hydrolysis and DNA release-last step-only TYPE 2

Question 3

Topo 1B inhibitors

Answer 3

Camptothecin:
Forms a stable ternary complex with the topo 1-DNA complex
Prevent DNA re-ligation via stabilized binding interaction btwn -1 and +1 DNA base pairs(1)
Cause DNA strand breaks= apoptosis
Remarkable activity in preclinical trial
Low solubility
Adverse drug reaction

Question 4

Substitution of the A ring increase solubility while retaining cytotoxicity

Answer 4

Irinotecan and topotecan: semisynthetic analogues of camptothecin-prodrug
Irinotecan = colon cancer- FOLFIRI(5-fluorouracil, leucovorin, and irinotecan)
Topotecan= ovarian and lung cancer

Question 5

Irinotecan(CPT-11) metabolism

Answer 5

See diagram…

Converted to active metabolite SN-38 by carboxylesterase
SN-38 is 1000 times more active than irinotecan itself
Down-regulation of carboxylesterase-> resistance
SN38 inactivated through glucuronidation by UGT1A1
Less UGT1A1 activity = toxic drug, Hi UGT1A1 expression=resistance
Also inactivated by CYP3A4-not cytotoxic metabolites

Increased expression of either CYP3A4 or UGT1A1 can lead to resistance

Question 6

Anthracyclines: antitumor antibiotics

Answer 6

Doxorubicin, Daunorubicine: solid tumors, leukemia, lymphoma
Idarubicin: AML(acute myeloid leukemia)
Epirubicin: breast cancer
Valrubicin: bladder cancer

Question 7

Anthracyclines mechanism of action

Answer 7

Inhibition of topoisomerase 2: Subtype-independent=causing DNA strand breakage
Intracalative binding to DNA:
Partial undwind of DNA
Much of the DNA is organized and folded into chromatin and maybe protected form this type of damage
Free radical formation: SFX-generation of OD and O2, DNA strand breaks, cell membrane damage

Question 8

Toxicity of Anthracycline

Answer 8

Myelosuppression
Mucositis(Gi irritation)
Cardiac toxicity: acute arrhythmias and delayed cardiomyopathies
Delayed cardiotoxicity is related to a patient’s cumulative lifetime dose
Mechanism not entirely known, may be superoxide radicals, calcium and iron levels in cells

Question 9

Preventing/limiting anthracyclins cardiotoxicity

Answer 9

Dexrazoxane(topo 2 inhibitor): indicates a reduce adverse cardiac effect in women on doxorubicine who have already received a total dose of over 300mg/m2-iron chelating activity
Co-admin of Carvedilol(adrenoceptor blocker)- also antioxidant in cardiac
Used of anthraquinones(mitoxantrone, pixantrone)- act like anthracycline drugs but with fewer adverse effects/toxicities

Question 10

Main cause of resistance to anthracyclines

Answer 10

DNA repair
Efflux
Apoptosis suppression
Increase antioxidants

Question 11

Podophyllotoxin derivatives

Answer 11

Semi-synthetic glycoside derivatives of podophyllotoxins

Ex. Etoposide, teniposide-better cell accumulation

Question 12

Etoposide (VP-16)

Answer 12

Non-intercalating inhibitor of topo 2
Formation of a stabilized ternary complex
etoposide -topo 2-DNA
Single and DS breaks
Cell cycle phase specific for late S and early G2
Testicular, lung and acute myeloid leukemia
SFX: myelosuppression

Question 13

Etoposide resistance

Answer 13

Enhanced efflux
Decreased binding to topoisomerase 2
Increased glutathione conjugation-pumped out

Question 14

Antimicrotubule agents

Answer 14

Vinca alkaloids
Vinblastine
Vincristine
Taxanes
Paclitaxel
Docetaxel
Anti-microtubule agents interfere with formation and function of MT
Stabilizers: build up
Destabilizers: fall a part

Question 15

Vinca alkaloids: catharanthus roseus

Answer 15

Vincristine(VCR)=CHO, Vinblastine(VLB)=CH3
TARGET G1 and S phase
Bind to the tubulin end of MT and to tubulin dimers
Mechanism: blocks assembly of MT=DESTABILIZER
Cause dissolution of mitotic spindle, inhibit organelle transport=especially mitochondria in neuronal cells

Question 16

Clinical Activity

of Vinca alkaloids

Answer 16

Vincristine(VCR): acute lymphoblastic leukemia, hodgkin’s lymphoma, pediatric solid tumors, peripheral neuropathy(numb/tingle in periph)
Vinblastine(VBL): hodgkin’s lymphoma, non-small cell lung cancer, MYELOSUPPRESSION

Question 17

VCR and VBL resistance:

Answer 17

ABC efflux primary

Question 18

Paclitaxel(Taxol)

Answer 18

Targets primarily G2/M phase
Ovarian, breast and lung cancer
BINDS to tubulin and enhances polymerization(STABILIZER)
Discrete bundles of stable MT, inhibition of cell replication

Question 19

Docetaxel(taxotere)

Answer 19

Higher affinity for tubulin than taxol
Ovarian and breast cancer
MYELOSUPPRESSION
Hypersensitivity reaction

Question 20

Noscapine

Answer 20

Isoquinoline alkaloid in opium latex
Sigma opioid receptor agonist
Used as non-sedating antitussive
Prostate cancer treatment? At 100X conc for antitussive
Fewer SFX than other MT disruptors
Less resistance since not as good of a substrate for efflux transporters