Chapter 33 — Anti-Cancer Drugs Part I — Module 2 — Alkylating Agents
1. Alkylating agents are among the oldest classes of cytotoxic chemotherapy. Which statement best describes the fundamental mechanism by which these drugs produce their antitumor effect?
A) They inhibit the enzyme topoisomerase II, preventing the religation of double-strand DNA breaks during replication
B) They block microtubule assembly, arresting dividing cells in metaphase before chromosome separation
C) They form covalent bonds with DNA bases, producing adducts and cross-links that block replication and transcription
D) They competitively inhibit dihydrofolate reductase, depleting the reduced folates required for nucleotide synthesis
E) They intercalate between adjacent DNA base pairs without forming covalent bonds, distorting the helix reversibly
ANSWER: C
Rationale:
Option C is correct. Alkylating agents are electrophilic compounds that transfer alkyl groups to nucleophilic sites on DNA (deoxyribonucleic acid), most importantly the N7 (nitrogen-7) position of guanine. The resulting covalent adducts, and especially the interstrand cross-links formed by bifunctional agents, physically block the progression of replication and transcription machinery, triggering cell death. This covalent DNA-damaging action is the defining feature shared across all alkylating subclasses.
Option A: Option A is incorrect because topoisomerase II inhibition describes the mechanism of the epipodophyllotoxins (etoposide) and anthracyclines, not alkylating agents.
Option B: Option B is incorrect because microtubule-targeted metaphase arrest describes the vinca alkaloids and taxanes, which do not covalently modify DNA.
Option D: Option D is incorrect because dihydrofolate reductase inhibition describes the antifolate methotrexate, an antimetabolite, not an alkylating agent.
Option E: Option E is incorrect because reversible, non-covalent intercalation does not describe alkylating agents; the defining alkylator action is the formation of covalent, not reversible, DNA bonds.
2. Alkylating agents are described as either bifunctional or monofunctional. Why are bifunctional alkylating agents generally more cytotoxic than monofunctional agents?
A) They carry two reactive groups and can form interstrand cross-links, the most lethal lesion because it completely blocks replication fork progression
B) They are more lipophilic and therefore achieve higher intracellular concentrations in rapidly dividing tumor cells
C) They are activated more rapidly by hepatic cytochrome P450 enzymes, generating a larger burst of reactive intermediate
D) They preferentially alkylate RNA (ribonucleic acid) rather than DNA, halting protein synthesis in addition to replication
E) They resist inactivation by glutathione conjugation because their two reactive groups saturate cellular thiol pools
ANSWER: A
Rationale:
Option A is correct. A bifunctional alkylating agent bears two reactive groups, allowing it to bond to two separate DNA (deoxyribonucleic acid) sites. When these sites are on opposite strands, the result is an interstrand cross-link (ICL), the most lethal class of DNA lesion because it completely prevents strand separation and blocks replication fork progression. Most clinically important alkylators (cyclophosphamide, the platinum compounds, nitrosoureas, busulfan, mechlorethamine) are bifunctional, whereas monofunctional agents produce primarily single-strand adducts of lower cytotoxic potency per lesion.
Option B: Option B is incorrect because greater lipophilicity affects tissue distribution (notably CNS [central nervous system] penetration of nitrosoureas) but is not the reason bifunctional agents are more cytotoxic.
Option C: Option C is incorrect because the rate of cytochrome P450 activation governs prodrug kinetics, not the bifunctional-versus-monofunctional distinction; cross-link formation is the determinant of lethality.
Option D: Option D is incorrect because the principal cytotoxic target of alkylating agents is DNA, not RNA, and bifunctionality is defined by DNA cross-linking capability.
Option E: Option E is incorrect because glutathione conjugation inactivates alkylating intermediates and contributes to resistance; bifunctional agents are not protected from this detoxification by saturating thiol pools.
3. Cyclophosphamide is one of the most widely used cytotoxic drugs in oncology and rheumatology. Which statement correctly describes its activation?
A) It is administered as an already-active alkylating species that requires no metabolic conversion before binding DNA
B) It is activated by spontaneous non-enzymatic hydrolysis in plasma at physiological pH, independent of any enzyme
C) It is activated within tumor cells by tumor-specific esterases that are absent from normal host tissues
D) It is an inactive prodrug requiring hepatic cytochrome P450 oxidation to generate its active alkylating metabolite
E) It is activated by renal tubular enzymes during glomerular filtration, which is why it is dosed by creatinine clearance
ANSWER: D
Rationale:
Option D is correct. Cyclophosphamide is an inactive prodrug that must undergo hepatic oxidation by cytochrome P450 (CYP) enzymes, principally CYP2B6 with contributions from CYP3A4 and CYP2C9, to form 4-hydroxycyclophosphamide. This intermediate equilibrates with aldophosphamide, which decomposes to phosphoramide mustard, the active bifunctional alkylating species, and acrolein. Because activation is CYP-dependent, enzyme inducers (rifampin) and inhibitors (azole antifungals) alter both efficacy and toxicity.
Option A: Option A is incorrect because cyclophosphamide is not active as administered; it strictly requires metabolic conversion.
Option B: Option B is incorrect because cyclophosphamide depends on enzymatic CYP activation, not spontaneous hydrolysis; spontaneous decomposition describes the nitrosoureas and temozolomide.
Option C: Option C is incorrect because activation occurs in the liver via CYP enzymes, not by tumor-specific esterases.
Option E: Option E is incorrect because activation is hepatic and CYP-mediated, not renal; although the drug is renally cleared, glomerular filtration does not perform the activation step.
4. A patient receiving cyclophosphamide develops dysuria and hematuria. Which metabolite is the direct cause of cyclophosphamide-induced hemorrhagic cystitis?
A) Phosphoramide mustard, the active bifunctional alkylating species that cross-links urothelial DNA
B) Acrolein, a reactive aldehyde excreted in urine that produces direct urothelial cytotoxicity
C) Chloroacetaldehyde, a neurotoxic side-chain oxidation product that accumulates in the bladder wall
D) 4-hydroxycyclophosphamide, the initial cytochrome P450 oxidation product, acting directly on the bladder
E) Carboxyphosphamide, the aldehyde dehydrogenase product that concentrates in the urinary tract
ANSWER: B
Rationale:
Option B is correct. Acrolein (propenal) is a highly reactive alpha,beta-unsaturated aldehyde generated when aldophosphamide decomposes. It is excreted unchanged in urine and reaches high concentrations in the bladder lumen, where it directly injures the urothelium, producing the dysuria, hematuria, and (with chronic exposure) fibrosis and malignancy that define hemorrhagic cystitis (HC). Forced hydration and mesna, which binds acrolein in the urinary tract, are the preventive strategies.
Option A: Option A is incorrect because phosphoramide mustard is the antitumor alkylating species; it is not the agent responsible for urothelial injury, which is specifically attributable to acrolein.
Option C: Option C is incorrect because chloroacetaldehyde is the neurotoxic metabolite responsible for ifosfamide encephalopathy, not hemorrhagic cystitis.
Option D: Option D is incorrect because 4-hydroxycyclophosphamide is an upstream activation intermediate, not the urotoxic species reaching the bladder lumen.
Option E: Option E is incorrect because carboxyphosphamide is the non-toxic detoxification product formed by aldehyde dehydrogenase and does not cause cystitis.
5. Mesna (sodium 2-mercaptoethane sulfonate) is co-administered with ifosfamide and high-dose cyclophosphamide. How does mesna prevent hemorrhagic cystitis?
A) It inhibits hepatic cytochrome P450 enzymes, reducing the total amount of acrolein generated from the prodrug
B) It alkalinizes the urine, chemically neutralizing acrolein before it can contact the bladder urothelium
C) It accelerates renal excretion of the parent drug, shortening the time acrolein remains in the bladder
D) It scavenges free radicals systemically, protecting all normal tissues from oxidative alkylating injury
E) It binds acrolein in the urinary tract via a thiol group, forming a non-toxic conjugate that is excreted
ANSWER: E
Rationale:
Option E is correct. Mesna contains a free thiol (-SH) group that reacts with acrolein in the urinary tract through a thiol-disulfide type conjugation, forming a stable, non-toxic adduct that is eliminated renally. Because mesna acts in the urine rather than systemically, it detoxifies acrolein at the site of injury without compromising the antitumor activity of the alkylating metabolite. It is mandatory with ifosfamide at all doses and used with high-dose cyclophosphamide.
Option A: Option A is incorrect because mesna does not inhibit CYP enzymes or reduce acrolein generation; it neutralizes acrolein after formation.
Option B: Option B is incorrect because mesna works by covalent thiol binding of acrolein, not by urinary alkalinization.
Option C: Option C is incorrect because mesna does not act by accelerating excretion of parent drug; forced hydration promotes dilution and clearance, but mesna's mechanism is chemical binding of acrolein.
Option D: Option D is incorrect because mesna acts locally in the urinary tract and does not protect tissues systemically; broad free-radical scavenging in normal tissue describes amifostine, not mesna.
6. A patient is scheduled to receive cisplatin-based chemotherapy. Which toxicity most characteristically requires aggressive saline hydration and magnesium supplementation with every cycle?
A) Nephrotoxicity from direct proximal tubular injury, accompanied by magnesium and potassium wasting
B) Thrombocytopenia that is predictably related to the area under the concentration-time curve
C) Cold-triggered peripheral sensory neuropathy beginning within hours of the infusion
D) Delayed myelosuppression with a nadir occurring 4 to 6 weeks after administration
E) Hemorrhagic cystitis from a reactive aldehyde concentrated in the bladder lumen
ANSWER: A
Rationale:
Option A is correct. Cisplatin causes dose-dependent, cumulative nephrotoxicity through direct injury to proximal tubular cells and the loop of Henle, producing a fall in glomerular filtration rate along with renal magnesium, potassium, and calcium wasting; hypomagnesemia is frequently dose-limiting before overt renal failure. Prevention requires pre- and post-hydration with normal saline and routine magnesium supplementation in every cycle.
Option B: Option B is incorrect because AUC-related thrombocytopenia is the dose-limiting toxicity of carboplatin, not the characteristic toxicity prompting saline-and-magnesium protocols for cisplatin.
Option C: Option C is incorrect because cold-triggered sensory neuropathy is the signature toxicity of oxaliplatin, not cisplatin.
Option D: Option D is incorrect because a delayed 4-to-6-week myelosuppressive nadir characterizes the nitrosoureas, not cisplatin.
Option E: Option E is incorrect because hemorrhagic cystitis is caused by acrolein from cyclophosphamide and ifosfamide, not by cisplatin.
7. Carboplatin is often substituted for cisplatin when its toxicity profile is preferable. What is the dose-limiting toxicity of carboplatin?
A) Nephrotoxicity requiring mandatory saline hydration with every administered cycle
B) Sensorineural high-frequency hearing loss that is cumulative and largely irreversible
C) Myelosuppression, predominantly thrombocytopenia, related to drug exposure as area under the curve
D) Cumulative peripheral neuropathy in a stocking-glove distribution that limits total dose
E) Hepatic sinusoidal obstruction syndrome presenting with tender hepatomegaly and ascites
ANSWER: C
Rationale:
Option C is correct. Because carboplatin's bidentate cyclobutanedicarboxylate ligand is hydrolyzed slowly, it produces a less reactive platinum species with much less renal, neural, and otologic toxicity than cisplatin. Its dose-limiting toxicity is myelosuppression, predominantly thrombocytopenia, and the degree of suppression tracks predictably with the area under the concentration-time curve (AUC), which is the basis for AUC-targeted (Calvert formula) dosing.
Option A: Option A is incorrect because mandatory saline-hydration nephrotoxicity is the cisplatin profile; carboplatin is substantially less nephrotoxic.
Option B: Option B is incorrect because cumulative ototoxicity is characteristic of cisplatin, not carboplatin.
Option D: Option D is incorrect because cumulative stocking-glove neuropathy is the dose-limiting toxicity of oxaliplatin, not carboplatin.
Option E: Option E is incorrect because hepatic sinusoidal obstruction syndrome is associated with high-dose busulfan conditioning, not carboplatin.
8. The nitrosoureas (carmustine, lomustine) occupy a distinct niche in the treatment of primary brain tumors. Which property accounts for their usefulness in central nervous system malignancies?
A) They are actively transported across the blood-brain barrier by a carrier that is upregulated in glial tumors
B) Their high lipophilicity allows passive diffusion across the blood-brain barrier into the central nervous system
C) They are administered intrathecally because systemic formulations cannot reach brain tissue at therapeutic levels
D) They disrupt blood-brain barrier tight junctions, transiently increasing permeability to themselves and other drugs
E) They are converted to active metabolites only within neural tissue by brain-specific cytochrome P450 enzymes
ANSWER: B
Rationale:
Option B is correct. Nitrosoureas are highly lipophilic, with a log P substantially above that of most other alkylating agents. This high lipid solubility permits passive diffusion across the blood-brain barrier (BBB), so that carmustine achieves cerebrospinal fluid concentrations roughly 15 to 30 percent of plasma, adequate for treating glioblastoma, anaplastic glioma, and primary CNS (central nervous system) lymphoma.
Option A: Option A is incorrect because CNS entry is by passive lipophilic diffusion, not an active tumor-upregulated transporter.
Option C: Option C is incorrect because nitrosoureas are given systemically (oral lomustine, intravenous carmustine) and do reach the CNS; intrathecal administration is not how they are used.
Option D: Option D is incorrect because nitrosoureas do not work by disrupting tight junctions; their CNS access reflects their own lipophilicity.
Option E: Option E is incorrect because nitrosoureas decompose spontaneously in aqueous solution to their reactive species rather than requiring brain-specific CYP activation.
9. Temozolomide is the standard alkylating agent in the Stupp protocol for newly diagnosed glioblastoma. Which combination of properties characterizes it?
A) Intravenous-only administration with negligible central nervous system penetration, used mainly for melanoma
B) Spontaneous decomposition in plasma to a vesicant species requiring central venous access for delivery
C) Activation exclusively by hepatic cytochrome P450, precluding use when liver function is impaired
D) Oral bioavailability with effective central nervous system penetration, used as standard therapy in glioblastoma
E) Requirement for intrathecal administration to achieve adequate cerebrospinal fluid drug concentrations
ANSWER: D
Rationale:
Option D is correct. Temozolomide (TMZ) is an orally bioavailable imidazotetrazine prodrug that undergoes spontaneous non-enzymatic hydrolysis at physiological pH to the active methylating species MTIC. It is essentially completely absorbed orally, crosses the blood-brain barrier effectively because of its small size and lipophilicity, and achieves cerebrospinal fluid concentrations roughly 30 to 40 percent of plasma. It is standard therapy for newly diagnosed glioblastoma in the Stupp protocol.
Option A: Option A is incorrect because TMZ is oral and penetrates the CNS (central nervous system) well; the intravenous-only, poorly CNS-penetrant triazene used in melanoma is dacarbazine.
Option B: Option B is incorrect because TMZ is not a vesicant requiring central access; the reactive, vesicant nitrogen mustard requiring immediate handling is mechlorethamine.
Option C: Option C is incorrect because TMZ activates by spontaneous hydrolysis, not hepatic CYP metabolism, which is minimal.
Option E: Option E is incorrect because TMZ is given orally and reaches therapeutic CNS levels without intrathecal dosing.
10. Twenty-four hours into an ifosfamide infusion, a patient becomes confused, somnolent, and ataxic. Which statement correctly links the cause of this syndrome to its treatment?
A) The cause is acrolein-mediated central nervous system injury, treated by increasing the mesna dose
B) The cause is direct phosphoramide mustard neurotoxicity, treated with high-dose corticosteroids
C) The cause is chloroacetaldehyde, a side-chain oxidation metabolite, and the treatment is intravenous methylene blue
D) The cause is hyponatremia from the ifosfamide infusion fluid, treated with hypertonic saline correction
E) The cause is cerebral edema from blood-brain barrier disruption, treated with mannitol and hyperventilation
ANSWER: C
Rationale:
Option C is correct. Ifosfamide undergoes a higher rate of side-chain oxidation (N-dechloroethylation) than cyclophosphamide, generating chloroacetaldehyde, a neurotoxic metabolite that causes ifosfamide encephalopathy (confusion, somnolence, cerebellar ataxia, hallucinations, seizures, or coma) typically 12 to 48 hours after infusion onset. The treatment is intravenous methylene blue (50 mg every 4 to 8 hours), which acts as an electron acceptor reversing the metabolic disturbance; ifosfamide should be discontinued.
Option A: Option A is incorrect because the encephalopathy is caused by chloroacetaldehyde, not acrolein; mesna prevents urothelial injury and does not treat the encephalopathy.
Option B: Option B is incorrect because the neurotoxic species is chloroacetaldehyde, not phosphoramide mustard, and corticosteroids are not the treatment.
Option D: Option D is incorrect because the syndrome is metabolite-mediated encephalopathy, not infusion-fluid hyponatremia.
Option E: Option E is incorrect because the mechanism is a neurotoxic metabolite rather than cerebral edema from barrier disruption, and mannitol/hyperventilation are not the management.
11. MGMT (O6-methylguanine-DNA methyltransferase) promoter methylation status is tested in newly diagnosed glioblastoma. How does MGMT status predict temozolomide benefit?
A) A methylated MGMT promoter silences the repair enzyme, so O6-guanine lesions persist and the tumor responds better
B) A methylated MGMT promoter increases enzyme expression, accelerating repair and conferring temozolomide resistance
C) MGMT methylation predicts response by increasing cellular uptake of temozolomide across the tumor membrane
D) MGMT methylation predicts response by blocking efflux of temozolomide through drug-resistance transporters
E) MGMT methylation has no effect on temozolomide response but predicts sensitivity to platinum agents instead
ANSWER: A
Rationale:
Option A is correct. MGMT is a suicide repair enzyme that reverses O6-guanine alkylation by transferring the methyl group to its own active-site cysteine, after which it is inactivated. When the MGMT promoter is methylated, the enzyme is silenced, so temozolomide-induced O6-methylguanine lesions persist, generate lethal mismatches, and kill the cell. Methylated tumors are therefore more than twice as likely to respond, with markedly longer median survival in the pivotal trial.
Option B: Option B is incorrect because promoter methylation silences rather than increases MGMT expression; high (unmethylated) expression is what confers resistance.
Option C: Option C is incorrect because MGMT governs DNA repair, not temozolomide uptake.
Option D: Option D is incorrect because MGMT is a repair enzyme, not a drug-efflux determinant.
Option E: Option E is incorrect because MGMT methylation is the strongest predictor of temozolomide benefit, not a platinum-sensitivity marker.
12. Carboplatin is dosed differently from most cytotoxic drugs. Which statement correctly describes the basis of carboplatin dosing?
A) It is dosed in milligrams per kilogram of actual body weight, like most weight-based cytotoxic agents
B) It is dosed empirically by tumor type, with fixed milligram doses assigned to each cancer indication
C) It is dosed by body surface area in milligrams per square meter, identical to the cisplatin approach
D) It is dosed by serum creatinine alone, without reference to a target drug-exposure value
E) It is dosed to a target area under the curve using the Calvert formula, which incorporates renal function
ANSWER: E
Rationale:
Option E is correct. Carboplatin is eliminated almost entirely by renal filtration as the intact complex, so glomerular filtration rate (GFR) is the primary determinant of drug exposure. The Calvert formula (dose = target AUC x [GFR + 25]) sets the dose to achieve a target area under the concentration-time curve, individualizing for renal function. This AUC-based approach is standard because carboplatin's dose-limiting thrombocytopenia tracks with exposure.
Option A: Option A is incorrect because carboplatin is dosed to target AUC, not by milligrams per kilogram of body weight.
Option B: Option B is incorrect because dosing is exposure-based via the Calvert formula, not fixed empiric milligrams per indication.
Option C: Option C is incorrect because body-surface-area dosing is the cisplatin approach; carboplatin uses AUC-targeted dosing.
Option D: Option D is incorrect because the Calvert formula uses GFR within a target-AUC calculation rather than serum creatinine alone with no exposure target.
13. A patient receiving FOLFOX (folinic acid, fluorouracil, oxaliplatin) for colorectal cancer reports that touching cold objects triggers uncomfortable tingling in the hands within hours of each infusion. Which toxicity does this describe, and is it the dose-limiting toxicity of oxaliplatin?
A) Nephrotoxicity; yes, it is the dose-limiting toxicity and requires saline hydration each cycle
B) Peripheral sensory neuropathy; yes, it is the dose-limiting toxicity of oxaliplatin
C) Ototoxicity; yes, high-frequency hearing loss is the dose-limiting toxicity of oxaliplatin
D) Hemorrhagic cystitis; no, the dose-limiting toxicity of oxaliplatin is myelosuppression
E) Hepatotoxicity; no, the dose-limiting toxicity of oxaliplatin is pulmonary fibrosis
ANSWER: B
Rationale:
Option B is correct. Oxaliplatin's dose-limiting toxicity is peripheral sensory neuropathy with two phases: an acute, cold-triggered dysesthesia occurring within hours of infusion and reversible over days, and a cumulative, dose-dependent stocking-glove sensory neuropathy that is only partially reversible. The cold-triggered tingling described is the acute presentation of this signature toxicity.
Option A: Option A is incorrect because oxaliplatin is not significantly nephrotoxic at standard doses; nephrotoxicity characterizes cisplatin.
Option C: Option C is incorrect because ototoxicity is a cisplatin toxicity and is not observed with oxaliplatin at standard doses.
Option D: Option D is incorrect because the described syndrome is neuropathy, not hemorrhagic cystitis, and the oxaliplatin dose-limiting toxicity is neuropathy rather than myelosuppression.
Option E: Option E is incorrect because the dose-limiting toxicity is neuropathy, not hepatotoxicity; pulmonary fibrosis is a cumulative carmustine toxicity.
14. Cisplatin circulates largely as the neutral parent compound in plasma but becomes reactive after entering the cell. What explains this activation?
A) Plasma esterases cleave a protecting group from cisplatin, and the unmasked compound becomes reactive in tissue
B) Cisplatin is a prodrug requiring hepatic cytochrome P450 oxidation before it can bind DNA in target cells
C) The low intracellular chloride concentration promotes aquation, replacing chloride ligands with water to form the reactive species
D) Intracellular alkaline pH hydrolyzes cisplatin to its active form, which is suppressed by acidic extracellular conditions
E) Cisplatin must first be reduced by intracellular glutathione to a platinum species capable of binding guanine
ANSWER: C
Rationale:
Option C is correct. In the chloride-rich extracellular environment (about 100 mEq/L), aquation is suppressed and cisplatin circulates as the neutral parent. Once inside the cell, where chloride falls to roughly 4 mEq/L, the chloride ligands are sequentially replaced by water (aquation), producing the electrophilic aquated platinum species that attacks N7 of guanine to form the cytotoxic intrastrand and interstrand adducts.
Option A: Option A is incorrect because activation is by aquation driven by the intracellular chloride gradient, not by plasma esterase cleavage of a protecting group.
Option B: Option B is incorrect because cisplatin is not activated by hepatic CYP metabolism; the relevant step is chemical aquation.
Option D: Option D is incorrect because the activating step is chloride-dependent aquation, not pH-dependent hydrolysis.
Option E: Option E is incorrect because glutathione conjugates and inactivates platinum (a resistance mechanism); it does not generate the active DNA-binding species.
15. A patient starting procarbazine as part of a lymphoma regimen is counseled about dietary and drug restrictions. What pharmacologic property underlies these warnings?
A) Procarbazine inhibits aldehyde dehydrogenase, so the dietary restriction is limited to avoiding tyramine-containing foods
B) Procarbazine strongly induces cytochrome P450, so the warning concerns loss of efficacy of co-administered drugs only
C) Procarbazine is a potent serotonin reuptake inhibitor, so the only relevant restriction is avoidance of other antidepressants
D) Procarbazine is a weak monoamine oxidase inhibitor, creating risk of hypertensive crisis and serotonin syndrome with certain foods and drugs
E) Procarbazine irreversibly blocks dopamine beta-hydroxylase, so the warnings concern orthostatic hypotension with standing
ANSWER: D
Rationale:
Option D is correct. Procarbazine is metabolized in part by monoamine oxidase (MAO) and is itself a weak MAO inhibitor. This creates clinically important interactions: concomitant sympathomimetics, tricyclic antidepressants, or selective serotonin reuptake inhibitors, and tyramine-rich foods (aged cheeses, red wine, fermented meats) risk hypertensive crisis or serotonin syndrome. Procarbazine also produces a disulfiram-like reaction with alcohol through aldehyde dehydrogenase inhibition.
Option A: Option A is incorrect because, although procarbazine does cause a disulfiram-like alcohol reaction, the dietary and drug warnings stem chiefly from its weak MAO inhibition, not solely tyramine avoidance via aldehyde dehydrogenase.
Option B: Option B is incorrect because the key interaction risk is MAO inhibition, not potent CYP induction.
Option C: Option C is incorrect because procarbazine is a weak MAO inhibitor, not a potent serotonin reuptake inhibitor.
Option E: Option E is incorrect because procarbazine does not act by blocking dopamine beta-hydroxylase; the interaction basis is MAO inhibition.
16. Lomustine is prescribed for a patient with recurrent glioma. Why is the standard cycle interval for nitrosoureas every 6 weeks rather than every 3 to 4 weeks?
A) Their myelosuppressive nadir is delayed to 4 to 6 weeks, so shorter intervals risk severe cumulative suppression
B) Their renal clearance is slow, so a 6-week interval is required to avoid drug accumulation and nephrotoxicity
C) Their antitumor effect requires a 6-week interval to allow DNA cross-links to mature before the next dose
D) Their oral absorption is so erratic that a 6-week interval is needed to confirm adequate exposure each cycle
E) Their hepatic metabolism saturates rapidly, so a 6-week interval prevents accumulation of a toxic metabolite
ANSWER: A
Rationale:
Option A is correct. Unlike most cytotoxic drugs, whose nadir occurs at 10 to 14 days, nitrosoureas produce a delayed myelosuppressive nadir at 4 to 6 weeks with recovery requiring 6 to 8 weeks, reflecting the slow rearrangement of chloroethyl adducts to interstrand cross-links. Dosing more often than every 6 weeks risks giving a second cycle before the first nadir, producing dangerous overlapping cumulative myelosuppression.
Option B: Option B is incorrect because the 6-week interval is dictated by the delayed marrow nadir, not by slow renal clearance.
Option C: Option C is incorrect because the interval protects the marrow from cumulative toxicity, not to allow cross-link maturation between doses.
Option D: Option D is incorrect because the interval is set by delayed myelosuppression kinetics, not by erratic oral absorption.
Option E: Option E is incorrect because the basis is the delayed hematologic nadir, not saturable hepatic metabolism of a toxic metabolite.
17. A patient undergoing high-dose busulfan conditioning before allogeneic hematopoietic stem cell transplantation develops tender hepatomegaly, jaundice, weight gain with ascites, and thrombocytopenia. Which complication does this describe?
A) Ifosfamide-type encephalopathy from a neurotoxic metabolite accumulating during conditioning
B) Cisplatin-pattern acute tubular necrosis from platinum accumulation in renal tubular cells
C) Carmustine-type pulmonary fibrosis from cumulative alkylator injury to lung parenchyma
D) Cyclophosphamide-induced hemorrhagic cystitis from acrolein concentrated in the bladder
Option E is correct. The most serious non-hematologic toxicity of high-dose busulfan conditioning is hepatic sinusoidal obstruction syndrome (SOS), formerly called veno-occlusive disease (VOD). It results from narrowing and occlusion of hepatic sinusoids and presents with tender hepatomegaly, jaundice, fluid retention with ascites, and thrombocytopenia. Therapeutic drug monitoring of busulfan AUC reduces the risk, and defibrotide is approved for established severe SOS.
Option A: Option A is incorrect because the picture is hepatic, not the neurologic syndrome of ifosfamide encephalopathy.
Option B: Option B is incorrect because the findings are hepatic rather than the renal failure of cisplatin tubular injury.
Option C: Option C is incorrect because the presentation is hepatic congestion, not the cumulative pulmonary fibrosis seen with carmustine.
Option D: Option D is incorrect because the syndrome is hepatic SOS, not acrolein-mediated hemorrhagic cystitis.
18. In non-small cell lung cancer, high tumor expression of ERCC1 (excision repair cross-complementation group 1) is associated with reduced benefit from cisplatin. What is the mechanistic explanation?
A) ERCC1 increases cellular efflux of platinum through copper-transporting ATPases, lowering intracellular drug
B) ERCC1 conjugates platinum to glutathione, forming water-soluble adducts that are exported from the cell
C) ERCC1 is an endonuclease of nucleotide excision repair that removes platinum-DNA adducts, conferring resistance
D) ERCC1 blocks the aquation of cisplatin, preventing formation of the reactive platinum species inside the cell
E) ERCC1 methylates the MGMT promoter, silencing a repair enzyme and paradoxically protecting the tumor from platinum
ANSWER: C
Rationale:
Option C is correct. ERCC1, partnered with XPF, is the principal incision endonuclease of nucleotide excision repair (NER), the pathway that removes bulky platinum-DNA adducts. Tumors with high ERCC1 expression repair platinum lesions more efficiently and are therefore more resistant to cisplatin, which is why ERCC1 level correlates inversely with platinum sensitivity in lung, ovarian, and gastric cancers.
Option A: Option A is incorrect because platinum efflux is mediated by copper-transporting ATPases (ATP7A/ATP7B), not by ERCC1.
Option B: Option B is incorrect because glutathione/GST conjugation is a separate detoxification mechanism; ERCC1 is a repair endonuclease, not a conjugating enzyme.
Option D: Option D is incorrect because ERCC1 acts on already-formed DNA adducts; it does not block aquation.
Option E: Option E is incorrect because ERCC1 is an NER nuclease and does not methylate the MGMT promoter; MGMT silencing is a distinct, methylating-agent-specific mechanism.
19. A patient with ovarian cancer has progressed on carboplatin-based therapy. Which statement about platinum cross-resistance is correct?
A) Cisplatin and carboplatin show no cross-resistance, so cisplatin substitution reliably restores response
B) Cisplatin and carboplatin share essentially complete cross-resistance, while oxaliplatin cross-resistance is only partial
C) Oxaliplatin shows complete cross-resistance with carboplatin, so it should never be tried after carboplatin failure
D) Cross-resistance among all three platinums is complete, making any platinum substitution futile in every tumor type
E) Carboplatin resistance predicts enhanced sensitivity to cisplatin because the two form chemically distinct adducts
ANSWER: B
Rationale:
Option B is correct. Cisplatin and carboplatin form identical platinum-DNA adducts and share essentially complete cross-resistance, so substituting one for the other after progression generally does not restore response. Oxaliplatin's bulky DACH (diaminocyclohexane)-platinum adducts evade mismatch repair recognition and partially evade ERCC1-XPF repair, so its cross-resistance with cisplatin/carboplatin is partial rather than complete, and it may retain activity in selected settings such as colorectal cancer.
Option A: Option A is incorrect because cisplatin and carboplatin do show cross-resistance; substitution does not reliably restore response.
Option C: Option C is incorrect because oxaliplatin cross-resistance is partial, not complete.
Option D: Option D is incorrect because cross-resistance is complete only between cisplatin and carboplatin; oxaliplatin is partially non-cross-resistant.
Option E: Option E is incorrect because cisplatin and carboplatin form the same adducts and are cross-resistant, not reciprocally sensitizing.
20. A broad mechanism of resistance affects nitrogen mustards, platinum compounds, and other electrophilic alkylating agents alike. Which mechanism is this?
A) Glutathione and glutathione S-transferase conjugation, which inactivates electrophilic intermediates and exports them
B) MGMT (O6-methylguanine-DNA methyltransferase) overexpression, which reverses O6-guanine alkylation specifically
C) Mismatch repair deficiency, which prevents recognition of platinum adducts and triggers tumor apoptosis
D) Topoisomerase II downregulation, which reduces the number of enzyme-mediated double-strand breaks
E) Loss of the copper transporter CTR1, which selectively reduces uptake of platinum compounds only
ANSWER: A
Rationale:
Option A is correct. Glutathione (GSH), a thiol-bearing tripeptide present at millimolar concentrations, reacts both non-enzymatically and via glutathione S-transferase (GST) isoenzymes with electrophilic alkylating intermediates to form water-soluble conjugates that are exported (for example, by MRP1). Because all alkylating agents share an electrophilic reactive species, elevated GSH or GST-pi overexpression confers broad-spectrum resistance across nitrogen mustards, platinums, and others.
Option B: Option B is incorrect because MGMT overexpression is specific to methylating and chloroethylating agents (temozolomide, nitrosoureas), not a broad mechanism across all alkylators including platinums.
Option C: Option C is incorrect because mismatch repair deficiency tends to reduce platinum-triggered apoptosis (a resistance route for some agents) but is not the broad glutathione-based detoxification mechanism described, and it is selective rather than universal.
Option D: Option D is incorrect because topoisomerase II governs resistance to topoisomerase poisons, not alkylating agents.
Option E: Option E is incorrect because loss of CTR1 reduces platinum uptake selectively and does not explain resistance to non-platinum alkylators.
21. Streptozocin is a nitrosourea used for metastatic pancreatic neuroendocrine tumors. Which pair of features distinguishes it from the other nitrosoureas?
A) It penetrates the central nervous system best of all nitrosoureas, and its dose-limiting toxicity is delayed myelosuppression
B) It is given intrathecally, and its dose-limiting toxicity is cumulative pulmonary fibrosis
C) It selectively spares pancreatic islet cells, and its dose-limiting toxicity is hemorrhagic cystitis
D) It shows selective tropism for pancreatic beta cells via the GLUT2 transporter, and its dose-limiting toxicity is nephrotoxicity
E) It requires hepatic cytochrome P450 activation, and its dose-limiting toxicity is ototoxicity
ANSWER: D
Rationale:
Option D is correct. Streptozocin carries a glucose moiety that promotes uptake through the GLUT2 (glucose transporter 2) transporter, giving it selective tropism for pancreatic beta (islet) cells and its role in pancreatic neuroendocrine tumors. Unlike the other nitrosoureas, its dose-limiting toxicity is nephrotoxicity (tubular injury) rather than myelosuppression.
Option A: Option A is incorrect because streptozocin is defined by beta-cell tropism and renal dose-limiting toxicity, not by superior CNS (central nervous system) penetration with myelosuppression.
Option B: Option B is incorrect because it is given systemically, and its dose-limiting toxicity is nephrotoxicity, not pulmonary fibrosis (a carmustine toxicity).
Option C: Option C is incorrect because streptozocin targets, rather than spares, beta cells, and its dose-limiting toxicity is nephrotoxicity, not hemorrhagic cystitis.
Option E: Option E is incorrect because nitrosoureas decompose spontaneously rather than requiring CYP activation, and streptozocin's dose-limiting toxicity is nephrotoxicity, not ototoxicity.
22. A patient who has received multiple cycles of carmustine for glioma is monitored for a specific cumulative, dose-related organ toxicity that constitutes an absolute contraindication to further treatment once it develops. Which toxicity is this?
A) Cumulative cardiomyopathy that becomes irreversible above a defined lifetime anthracycline-equivalent dose
B) Cumulative peripheral neuropathy in a stocking-glove distribution that worsens with each successive cycle
C) Cumulative nephrotoxicity from tubular platinum accumulation requiring saline hydration each cycle
D) Cumulative ototoxicity producing irreversible high-frequency sensorineural hearing loss
E) Cumulative pulmonary fibrosis, typically after high total carmustine doses, contraindicating further treatment
ANSWER: E
Rationale:
Option E is correct. Carmustine causes cumulative pulmonary fibrosis, characteristically appearing after high total doses (above roughly 1,200 mg/m^2). It is a dose-related toxicity, and once established it represents an absolute contraindication to additional carmustine, making cumulative-dose tracking and pulmonary monitoring essential.
Option A: Option A is incorrect because cumulative cardiomyopathy with a lifetime dose ceiling describes the anthracyclines, not carmustine.
Option B: Option B is incorrect because cumulative stocking-glove neuropathy is the oxaliplatin profile, not a carmustine toxicity.
Option C: Option C is incorrect because cumulative tubular nephrotoxicity requiring hydration is the cisplatin profile, not carmustine.
Option D: Option D is incorrect because cumulative sensorineural ototoxicity is characteristic of cisplatin, not carmustine.
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