Chapter 33 — Anti-Cancer Drugs Part I: Pharmacology — Module 4 — Topoisomerase Inhibitors and Antitumor Antibiotics
1. Topoisomerase I and topoisomerase II differ in the type of cut each enzyme makes in DNA. Which statement precisely distinguishes the two?
A) Topoisomerase I makes a double-strand break, while topoisomerase II makes a single-strand nick
B) Both enzymes make double-strand breaks, differing only in which cell-cycle phase they act
C) Topoisomerase I makes a transient single-strand break (nick), while topoisomerase II makes a transient double-strand break and passes another duplex through the gap
D) Topoisomerase I methylates DNA, while topoisomerase II cleaves it
E) Both enzymes make single-strand nicks, differing only in the strand they cut
ANSWER: C
Rationale:
The defining mechanistic distinction is the cut: topoisomerase I (Topo I) relieves supercoiling by making a transient single-strand break, allowing controlled rotation before resealing, whereas topoisomerase II (Topo II) makes a transient double-strand break in one duplex and passes a second intact duplex through the gap before religation. This difference explains why camptothecins (Topo I inhibitors) are S-phase specific while Topo II poisons act more broadly and can decatenate chromosomes.
Option A: Option A is incorrect because it reverses the two enzymes; Topo I is the single-strand enzyme and Topo II is the double-strand enzyme.
Option B: Option B is incorrect because Topo I does not make a double-strand break; only Topo II does, so the two do not differ merely by cell-cycle phase.
Option D: Option D is incorrect because neither topoisomerase methylates DNA; both cleave and reseal DNA, and methylation is an unrelated epigenetic process.
Option E: Option E is incorrect because Topo II makes a double-strand break, not a single-strand nick, so the claim that both make single-strand nicks is wrong.
2. Camptothecins such as topotecan and irinotecan are described as S-phase specific (most lethal to cells synthesizing DNA). Which statement correctly explains why?
A) The stabilized single-strand cleavable complex becomes a lethal double-strand break only when a replication fork collides with it, which occurs during S-phase DNA synthesis
B) Camptothecins are only able to enter cells during S phase because the membrane transporter is expressed only then
C) Topoisomerase I is present only during S phase and absent at all other times
D) Camptothecins chemically degrade in cells that are not in S phase, so only S-phase cells are exposed
E) The drug binds DNA only after mitosis is complete, restricting activity to a single phase
ANSWER: A
Rationale:
Camptothecins stabilize the Topo I single-strand cleavable complex, which by itself is reversible and not necessarily lethal. The lethal event is the collision of an advancing replication fork with the trapped complex, converting the nick into a double-strand break; because that collision requires active DNA replication, the cytotoxicity is concentrated in S phase.
Option B: Option B is incorrect because camptothecin uptake is not restricted to an S-phase-only transporter; the phase specificity arises from the replication-collision mechanism, not from phase-gated drug entry.
Option C: Option C is incorrect because topoisomerase I is expressed throughout the cell cycle, not only in S phase.
Option D: Option D is incorrect because the phase specificity is not due to selective drug degradation outside S phase; it is due to when the lethal collision happens.
Option E: Option E is incorrect because camptothecins bind the Topo I-DNA complex during the catalytic cycle, not specifically after mitosis, so the explanation is the replication-fork collision.
3. Irinotecan is a prodrug. Which enzyme performs the activation step that converts irinotecan into its active metabolite SN-38?
A) UGT1A1, the enzyme that adds a glucuronic acid group to the drug
B) Cytochrome P450 3A4, acting as the sole activating enzyme
C) Beta-glucuronidase in the intestinal lumen
D) Carboxylesterase, which hydrolyzes irinotecan to release the active metabolite SN-38
E) Thymidylate synthase, which activates the drug during DNA synthesis
ANSWER: D
Rationale:
Irinotecan is activated by carboxylesterase enzymes (in liver, intestinal wall, and tumor) that hydrolyze the bulky side chain to release SN-38, the potent topoisomerase I-inhibiting metabolite. Distinguishing the activating step (carboxylesterase) from the inactivating step (UGT1A1 glucuronidation) is central to understanding irinotecan pharmacology.
Option A: Option A is incorrect because UGT1A1 inactivates SN-38 by glucuronidation; it is the detoxifying enzyme, not the activator.
Option B: Option B is incorrect because while CYP3A4 contributes to oxidative metabolism of irinotecan, it is not the activating enzyme that generates SN-38; carboxylesterase performs activation.
Option C: Option C is incorrect because intestinal beta-glucuronidase deconjugates SN-38G back to SN-38 in the gut (driving late diarrhea); it does not perform the initial prodrug activation.
Option E: Option E is incorrect because thymidylate synthase is a target of fluorouracil and plays no role in activating irinotecan.
4. Regarding the handling of the active metabolite SN-38, which statement is correct?
A) UGT1A1 activates SN-38 by removing a glucuronic acid group, increasing its potency
B) UGT1A1 inactivates SN-38 by adding a glucuronic acid group (glucuronidation), and the UGT1A1*28 variant reduces this enzyme's activity, raising toxicity risk
C) SN-38 is inactivated primarily by renal filtration with no enzymatic step
D) The UGT1A1*28 variant increases UGT1A1 activity, lowering SN-38 levels
E) SN-38 is inactivated by carboxylesterase, the same enzyme that activates irinotecan
ANSWER: B
Rationale:
SN-38 is inactivated by UGT1A1-mediated glucuronidation, which attaches a glucuronic acid group to form the inactive, excretable SN-38G. The UGT1A1*28 promoter variant lowers UGT1A1 expression and activity, so homozygous carriers clear SN-38 more slowly and are at higher risk of severe neutropenia and diarrhea.
Option A: Option A is incorrect because glucuronidation adds (does not remove) a glucuronic acid group and inactivates rather than potentiates SN-38.
Option C: Option C is incorrect because SN-38 inactivation is an enzymatic glucuronidation step, not simple renal filtration.
Option D: Option D is incorrect because UGT1A1*28 reduces, not increases, enzyme activity, so SN-38 accumulates rather than falling.
Option E: Option E is incorrect because carboxylesterase activates irinotecan to SN-38; it does not inactivate SN-38, which is the role of UGT1A1.
5. Early irinotecan-associated diarrhea (occurring within 24 hours, often during the infusion) differs in mechanism and treatment from late diarrhea. Which pairing is correct for the early syndrome?
A) Mechanism: direct mucosal toxicity from SN-38; treatment: high-dose loperamide
B) Mechanism: bacterial overgrowth; treatment: oral vancomycin
C) Mechanism: osmotic load from the drug vehicle; treatment: fluid restriction
E) Mechanism: cholinergic excess from acetylcholinesterase inhibition; treatment: atropine
ANSWER: E
Rationale:
Early irinotecan diarrhea is cholinergic: irinotecan inhibits acetylcholinesterase, producing a parasympathetic excess syndrome (cramping, diaphoresis, lacrimation, rhinorrhea, diarrhea) within hours of the infusion, treated with atropine.
Option A: Option A is incorrect because direct mucosal toxicity from SN-38 treated with loperamide describes the late diarrhea syndrome, not the early one.
Option B: Option B is incorrect because the early syndrome is a predictable cholinergic drug effect, not bacterial overgrowth, so vancomycin is inappropriate.
Option C: Option C is incorrect because the early diarrhea is cholinergic rather than osmotic, and fluid restriction is not the treatment.
Option D: Option D is incorrect because the mechanism is acetylcholinesterase inhibition, not histamine release, so antihistamines are not the treatment.
6. The late diarrhea of irinotecan (onset more than 24 hours after infusion) is driven by SN-38 in the intestinal lumen. Which statement about its origin and treatment is correct?
A) It is caused by acetylcholinesterase inhibition and is treated with atropine
B) It results from the drug vehicle and resolves only with cycle delay, not medication
C) Intestinal bacterial beta-glucuronidase deconjugates SN-38G back to active SN-38, which damages the mucosa; treatment is high-dose loperamide started at the first loose stool
D) It is an immune-mediated colitis requiring corticosteroids as first-line therapy
E) It is caused by renal accumulation of irinotecan and is treated by forced diuresis
ANSWER: C
Rationale:
Late diarrhea arises because intestinal bacterial beta-glucuronidase cleaves the inactive glucuronide SN-38G back to active SN-38 in the gut lumen, where it directly injures the mucosa; the correct treatment is high-dose loperamide begun at the first loose stool and continued until the patient is diarrhea-free for a sustained interval.
Option A: Option A is incorrect because acetylcholinesterase inhibition with atropine treatment describes the early cholinergic syndrome, not late diarrhea.
Option B: Option B is incorrect because late diarrhea has a defined SN-38 mucosal mechanism and is actively treated with loperamide, not merely managed by cycle delay.
Option D: Option D is incorrect because late irinotecan diarrhea is a direct cytotoxic mucosal injury, not an immune colitis, so corticosteroids are not first-line.
Option E: Option E is incorrect because the mechanism is intraluminal SN-38 toxicity, not renal accumulation, so forced diuresis is not the treatment.
7. Topotecan and irinotecan are both topoisomerase I inhibitors, but they differ pharmacologically. Which statement about topotecan is correct?
A) Topotecan is active as the parent compound (no prodrug activation needed), is eliminated mainly by the kidney, and is a substrate of the BCRP efflux transporter (breast cancer resistance protein, a pump that expels drug from cells)
B) Topotecan is an inactive prodrug requiring carboxylesterase activation, like irinotecan
C) Topotecan is eliminated almost entirely by hepatic metabolism, so renal function does not affect dosing
D) Topotecan's dose-limiting toxicity is cardiomyopathy
E) Topotecan requires UGT1A1 genotyping before dosing because of an activation polymorphism
ANSWER: A
Rationale:
Topotecan is active as administered (it does not require prodrug activation), is primarily renally eliminated so dose reduction is needed in renal impairment, and is a substrate of BCRP, an efflux transporter that limits oral absorption and contributes to resistance.
Option B: Option B is incorrect because topotecan is not a prodrug; the carboxylesterase activation step applies to irinotecan.
Option C: Option C is incorrect because topotecan is mainly renally cleared, so renal function does affect dosing.
Option D: Option D is incorrect because topotecan's dose-limiting toxicity is myelosuppression (especially neutropenia), not cardiomyopathy.
Option E: Option E is incorrect because the UGT1A1*28 consideration applies to irinotecan (affecting SN-38 clearance), not to topotecan dosing.
8. What is the precise molecular mechanism of etoposide?
A) It intercalates DNA and generates reactive oxygen species that cleave strands
B) It inhibits topoisomerase I by trapping the single-strand cleavable complex
C) It cross-links guanine bases on opposite DNA strands
D) It stabilizes the topoisomerase II-DNA cleavable complex, preventing religation and leaving persistent double-strand breaks
E) It blocks RNA polymerase by intercalating at GC base pairs
ANSWER: D
Rationale:
Etoposide is a topoisomerase II poison: it binds and stabilizes the Topo II-DNA cleavable complex after the enzyme has cut both strands, preventing religation and leaving persistent double-strand breaks that trigger the DNA damage response and apoptosis.
Option A: Option A is incorrect because reactive oxygen species generation with intercalation describes the anthracyclines and bleomycin, not the primary etoposide mechanism.
Option B: Option B is incorrect because trapping the single-strand cleavable complex describes the camptothecins acting on topoisomerase I, not etoposide.
Option C: Option C is incorrect because interstrand guanine cross-linking describes alkylating agents and platinum compounds, not etoposide.
Option E: Option E is incorrect because blocking RNA polymerase by GC intercalation describes actinomycin D, not etoposide.
9. Etoposide is associated with a characteristic form of treatment-related acute myeloid leukemia (AML). Which description correctly captures its distinguishing features?
A) Translocations involving chromosome 22 (BCR-ABL), latency 10 years, always preceded by a myelodysplastic phase
B) Rearrangements of the MLL gene at chromosome 11q23, short latency of 1 to 3 years, typically without a preceding myelodysplastic phase
C) Monosomy 7 with a latency of 5 to 7 years, always preceded by a myelodysplastic phase, characteristic of alkylating agents
D) Trisomy 21 acquired after therapy, latency under 6 months
E) No specific cytogenetic signature; indistinguishable from de novo AML in all respects
ANSWER: B
Rationale:
Etoposide-related (topoisomerase II inhibitor-related) secondary AML characteristically involves balanced rearrangements of the MLL gene at 11q23, has a relatively short latency of 1 to 3 years, and typically arises de novo without the preceding myelodysplastic phase seen with alkylating-agent leukemias.
Option A: Option A is incorrect because BCR-ABL at chromosome 22 defines chronic myeloid leukemia, not the topoisomerase II-related AML pattern.
Option C: Option C is incorrect because monosomy 7 with long latency and a preceding myelodysplastic phase describes alkylating-agent-related AML, the contrasting pattern, not the etoposide signature.
Option D: Option D is incorrect because acquired trisomy 21 with sub-6-month latency is not the recognized etoposide-related AML profile.
Option E: Option E is incorrect because etoposide-related AML does have a distinguishing cytogenetic signature (11q23/MLL) rather than being indistinguishable from de novo AML.
10. Doxorubicin is distinguished from the single-mechanism topoisomerase II poisons because it acts through several mechanisms at once. Which option correctly lists three established mechanisms of doxorubicin?
A) Microtubule stabilization, thymidylate synthase inhibition, and DNA cross-linking
B) Folate antagonism, ribonucleotide reductase inhibition, and spindle poisoning
C) RNA polymerase blockade, purine analog incorporation, and topoisomerase I trapping
D) Beta-tubulin binding, alkylation of guanine, and acetylcholinesterase inhibition
E) Intercalation with stabilization of the topoisomerase II cleavable complex, generation of reactive oxygen species, and binding to the mitochondrial lipid cardiolipin
ANSWER: E
Rationale:
Doxorubicin exerts antitumor and toxic effects through at least three simultaneous mechanisms: intercalation into DNA with stabilization of the Topo II cleavable complex producing double-strand breaks; one-electron redox cycling generating reactive oxygen species; and binding to cardiolipin in the mitochondrial inner membrane, disrupting electron transport and promoting apoptosis.
Option A: Option A is incorrect because microtubule stabilization (taxanes), thymidylate synthase inhibition (fluorouracil), and DNA cross-linking (alkylators/platinums) are mechanisms of other drug classes, not doxorubicin.
Option B: Option B is incorrect because folate antagonism (methotrexate), ribonucleotide reductase inhibition (hydroxyurea), and spindle poisoning (vinca alkaloids) are not doxorubicin mechanisms.
Option C: Option C is incorrect because RNA polymerase blockade (actinomycin D), purine analog incorporation (antimetabolites), and Topo I trapping (camptothecins) belong to other agents.
Option D: Option D is incorrect because tubulin binding, guanine alkylation, and acetylcholinesterase inhibition are unrelated to doxorubicin's actual mechanisms.
11. Epirubicin is an anthracycline closely related to doxorubicin. Which statement correctly describes how epirubicin differs?
A) Epirubicin has no cumulative dose ceiling because it is not cardiotoxic at any dose
B) Epirubicin is a topoisomerase I inhibitor, unlike doxorubicin
C) Epirubicin is a stereoisomer (4-prime-epimer) of doxorubicin that undergoes faster glucuronidation and clearance, has lower cardiotoxicity per unit dose, and therefore permits a higher cumulative dose ceiling than doxorubicin
D) Epirubicin cannot be used in breast cancer because of excessive cardiotoxicity
E) Epirubicin must always be combined with dexrazoxane because it is more cardiotoxic than doxorubicin per dose
ANSWER: C
Rationale:
Epirubicin is the 4-prime-epimer of doxorubicin; the altered stereochemistry of a hydroxyl group on the sugar moiety leads to faster glucuronidation and biliary clearance, a shorter half-life, and roughly lower cardiotoxicity per unit dose, which is why its cumulative dose ceiling is higher than that of doxorubicin.
Option A: Option A is incorrect because epirubicin is still cardiotoxic and does have a cumulative dose ceiling; it is simply higher than doxorubicin's.
Option B: Option B is incorrect because epirubicin, like doxorubicin, acts on topoisomerase II, not topoisomerase I.
Option D: Option D is incorrect because epirubicin is in fact used in breast cancer regimens; its favorable per-dose cardiac profile supports such use.
Option E: Option E is incorrect because epirubicin is less, not more, cardiotoxic per dose than doxorubicin and does not mandate routine dexrazoxane.
12. Pegylated liposomal doxorubicin alters the drug's distribution and toxicity. Which statement is correct?
A) The polyethylene glycol coating prolongs circulation and the enhanced permeability and retention effect (leaky tumor vasculature allowing nanoparticle accumulation) favors tumor uptake; cardiotoxicity and myelosuppression are reduced, while hand-foot syndrome becomes dose-limiting
B) Liposomal encapsulation increases cardiotoxicity, lowering the safe cumulative dose below that of conventional doxorubicin
C) The liposome converts doxorubicin into a topoisomerase I inhibitor
D) The formulation eliminates all dose-limiting toxicities, allowing unlimited dosing
E) Liposomal doxorubicin is cleared within minutes, requiring continuous infusion to maintain levels
ANSWER: A
Rationale:
Pegylated liposomal doxorubicin uses a polyethylene glycol coating to evade phagocytic clearance and prolong circulation, and it exploits the enhanced permeability and retention effect, whereby the leaky vasculature of tumors permits preferential accumulation of nanoscale particles. This shifts the toxicity profile: cardiotoxicity, myelosuppression, and alopecia are reduced, while palmar-plantar erythrodysesthesia (hand-foot syndrome) and mucositis become dose-limiting.
Option B: Option B is incorrect because liposomal encapsulation reduces, not increases, cardiotoxicity relative to conventional doxorubicin.
Option C: Option C is incorrect because the drug remains a topoisomerase II-active anthracycline; the liposome changes delivery, not mechanism.
Option D: Option D is incorrect because dose-limiting toxicities still exist (notably hand-foot syndrome), so dosing is not unlimited.
Option E: Option E is incorrect because pegylated liposomal doxorubicin has a markedly prolonged, not minutes-long, circulation time.
13. What is the molecular mechanism by which bleomycin damages cancer cells?
A) It cross-links DNA strands through covalent guanine adducts
B) It stabilizes the topoisomerase II cleavable complex
C) It inhibits dihydrofolate reductase, depleting reduced folate
D) It forms a complex with iron that, in the presence of oxygen, generates reactive oxygen species that directly cleave DNA strands, acting most strongly in the G2 and M phases of the cell cycle
E) It blocks microtubule assembly during mitosis
ANSWER: D
Rationale:
Bleomycin binds ferrous iron to form a complex that, in the presence of molecular oxygen, generates reactive oxygen species (including hydroxyl radical) that directly cleave DNA strands, producing predominantly single-strand and some double-strand breaks; its cytotoxicity is greatest in the G2 and M phases.
Option A: Option A is incorrect because covalent guanine cross-linking is the mechanism of alkylating agents and platinum compounds, not bleomycin.
Option B: Option B is incorrect because stabilizing the Topo II cleavable complex describes etoposide and the anthracyclines, not bleomycin.
Option C: Option C is incorrect because dihydrofolate reductase inhibition describes methotrexate, an antifolate, not bleomycin.
Option E: Option E is incorrect because blocking microtubule assembly describes the vinca alkaloids, not the oxidative DNA-cleaving mechanism of bleomycin.
14. Which set of factors correctly identifies recognized risk factors for bleomycin pulmonary toxicity and the test used to monitor for it?
A) Risk rises with low cumulative dose and young age; monitored by serum troponin
B) Risk rises with high cumulative dose, age over 40, prior or concurrent thoracic radiation, renal impairment, and high inspired oxygen; monitored by the diffusing capacity for carbon monoxide (DLCO)
C) Risk is unrelated to cumulative dose and depends only on the infusion rate; monitored by echocardiography
D) Risk is determined solely by UGT1A1 genotype; monitored by liver function tests
E) Risk rises only in patients under age 18; monitored by electrocardiography
ANSWER: B
Rationale:
Bleomycin pulmonary toxicity risk increases with higher cumulative dose (classically above about 400 units), age over 40, prior or concurrent thoracic radiation, renal impairment (bleomycin is renally eliminated), and high inspired oxygen concentration; the diffusing capacity for carbon monoxide (DLCO) falls before overt symptoms and is the standard monitoring test.
Option A: Option A is incorrect because risk rises with high (not low) cumulative dose and older (not younger) age, and troponin monitors cardiac, not pulmonary, injury.
Option C: Option C is incorrect because risk is strongly dose-related and the monitoring test is DLCO, not echocardiography.
Option D: Option D is incorrect because UGT1A1 genotype governs irinotecan handling, not bleomycin lung risk, and liver function tests are not the monitoring tool.
Option E: Option E is incorrect because risk increases with older age over 40 rather than being confined to minors, and electrocardiography does not monitor pulmonary toxicity.
15. Which statement most accurately characterizes actinomycin D (dactinomycin)?
A) It is a topoisomerase I inhibitor used mainly in colorectal cancer
B) It alkylates DNA and is used principally for its myelosuppressive effect
C) It is an antifolate that depletes reduced folate pools
D) It stabilizes microtubules to arrest cells in mitosis
E) It intercalates DNA at GC base pairs, blocking RNA polymerase progression to inhibit transcription, and acts as a potent radiation sensitizer
ANSWER: E
Rationale:
Actinomycin D intercalates into DNA preferentially at GC-rich sequences and physically blocks RNA polymerase progression, inhibiting transcription of all RNA species; it is also a potent radiation sensitizer because it impairs transcription of DNA repair enzymes needed after radiation damage.
Option A: Option A is incorrect because actinomycin D is not a topoisomerase I inhibitor and is used in pediatric tumors such as Wilms tumor and rhabdomyosarcoma, not primarily colorectal cancer.
Option B: Option B is incorrect because actinomycin D acts by intercalation and transcription blockade, not DNA alkylation, and it is not used for a myelosuppressive effect (myelosuppression is a toxicity, not a therapeutic goal).
Option C: Option C is incorrect because antifolate folate depletion describes methotrexate, not actinomycin D.
Option D: Option D is incorrect because microtubule stabilization describes the taxanes, not actinomycin D.
16. Which statement correctly describes the mechanism of dexrazoxane as an anthracycline cardioprotectant?
A) It scavenges anthracycline in plasma by forming an inert covalent adduct before the drug reaches the heart
B) It accelerates renal clearance of doxorubicin to reduce cardiac exposure
C) It enters cells and is hydrolyzed to an open-ring form (ADR-925) that chelates intracellular free iron, preventing the iron-dependent generation of reactive oxygen species that injure cardiomyocytes
D) It stimulates regeneration of cardiomyocytes lost to prior anthracycline therapy
E) It blocks anthracycline uptake into all cells, including tumor cells, which is how it protects the heart
ANSWER: C
Rationale:
Dexrazoxane is taken up by cells and hydrolyzed to ADR-925, an open-ring EDTA-like chelator that binds intracellular free iron, removing the iron required to catalyze reactive oxygen species formation and thereby reducing oxidative injury to cardiomyocytes; this intracellular iron-chelation mechanism is the basis of its cardioprotection.
Option A: Option A is incorrect because dexrazoxane does not act by forming a plasma adduct with circulating anthracycline; it acts intracellularly on iron chemistry.
Option B: Option B is incorrect because it does not protect by accelerating renal clearance of doxorubicin.
Option D: Option D is incorrect because cardiomyocytes are postmitotic and dexrazoxane prevents injury rather than regenerating lost heart muscle.
Option E: Option E is incorrect because dexrazoxane is not intended to block anthracycline uptake into tumor cells (which would compromise efficacy); its protection comes from intracellular iron chelation.
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