Chapter 33 — Anti-Cancer Drugs Part I Pharmacology — Module 3 — Antimetabolites: Folate Antagonists, Fluoropyrimidines, Cytidine and Purine Analogs, and Hypomethylating Agents
1. After methotrexate enters a cell through the reduced folate carrier, the enzyme folylpolyglutamate synthetase (FPGS) adds glutamate residues to it, forming methotrexate polyglutamates (MTXPG). What is the principal pharmacologic consequence of this polyglutamation?
A) It accelerates efflux of methotrexate out of the cell, shortening intracellular exposure
B) It converts methotrexate into an inactive metabolite that is renally excreted
C) It traps methotrexate intracellularly and increases its affinity for folate-dependent enzymes, prolonging and intensifying the drug effect
D) It targets methotrexate for proteasomal degradation
E) It enables methotrexate to be transported across the blood-brain barrier
ANSWER: C
Rationale:
Polyglutamation adds negatively charged glutamate residues that cannot cross the cell membrane, so methotrexate polyglutamates (MTXPG) are retained intracellularly. The polyglutamated forms also bind dihydrofolate reductase and other folate-dependent enzymes with high affinity, intensifying and prolonging inhibition. Tumor cells that accumulate more MTXPG and have lower efflux are preferentially killed, which underlies the selectivity exploited by high-dose methotrexate with leucovorin rescue.
Option C is correct because intracellular trapping plus increased enzyme affinity is the defining consequence of polyglutamation.
Option A: Option A is incorrect because polyglutamation retains the drug rather than accelerating efflux.
Option B: Option B is incorrect because polyglutamation does not inactivate methotrexate; the polyglutamates remain pharmacologically active.
Option D: Option D is incorrect because polyglutamation does not mark methotrexate for proteasomal degradation.
Option E: Option E is incorrect because adding charged glutamates does not facilitate blood-brain barrier penetration; it impairs membrane crossing.
2. A patient receiving high-dose methotrexate develops acute kidney injury with a markedly elevated plasma methotrexate level at 48 hours despite aggressive leucovorin rescue. Which agent rapidly lowers the plasma methotrexate concentration by enzymatically cleaving the circulating drug?
A) Glucarpidase (carboxypeptidase G2)
B) Leucovorin (folinic acid)
C) Thymidine
D) Rasburicase
E) Sodium bicarbonate
ANSWER: A
Rationale:
Glucarpidase (carboxypeptidase G2) cleaves the terminal glutamate from circulating methotrexate, converting it to inactive metabolites and reducing plasma methotrexate by more than 98% within minutes. It is reserved for severe methotrexate toxicity or failure of enhanced leucovorin rescue, exactly the scenario of rising levels with renal impairment.
Option A is correct because glucarpidase is the enzyme that rapidly degrades circulating methotrexate.
Option B: Option B is incorrect because leucovorin rescues cells downstream of the dihydrofolate reductase block but does not lower the plasma methotrexate concentration; here it has already failed.
Option C: Option C is incorrect because thymidine can support rescue in some protocols but does not enzymatically clear methotrexate from plasma.
Option D: Option D is incorrect because rasburicase degrades uric acid for tumor lysis syndrome and has no effect on methotrexate levels.
Option E: Option E is incorrect because sodium bicarbonate alkalinizes urine to improve methotrexate solubility but does not cleave or rapidly clear the drug.
3. Before a high-dose methotrexate infusion, the team reviews the patient's medications to avoid agents that delay methotrexate clearance and prolong toxic exposure. Which group of drugs is most important to hold for this reason?
A) Beta-blockers, because they reduce renal blood flow through cardiac effects
B) Statins, because they compete for hepatic uptake transporters
C) Inhaled corticosteroids, because they alter methotrexate protein binding
D) NSAIDs (nonsteroidal anti-inflammatory drugs), proton pump inhibitors, and penicillins, because they impair renal methotrexate elimination, in part by competing at renal organic anion transporters
E) Antihistamines, because they increase methotrexate polyglutamation
ANSWER: D
Rationale:
Methotrexate is eliminated largely by the kidney, and several drugs delay its clearance and prolong toxic plasma exposure. NSAIDs and proton pump inhibitors are held before high-dose methotrexate, and penicillins compete for the renal organic anion transporter OAT3; each can slow methotrexate excretion and precipitate severe toxicity. These are discontinued for an appropriate interval before infusion.
Option D is correct because NSAIDs, proton pump inhibitors, and penicillins are the agents that impair renal methotrexate elimination.
Option A: Option A is incorrect because beta-blockers are not a recognized cause of delayed methotrexate clearance.
Option B: Option B is incorrect because the clinically important methotrexate interaction is at renal elimination, not statin-related hepatic uptake.
Option C: Option C is incorrect because inhaled corticosteroids do not meaningfully alter methotrexate protein binding or clearance.
Option E: Option E is incorrect because antihistamines do not increase methotrexate polyglutamation or delay its clearance.
4. Pemetrexed is described as a multitargeted antifolate, which distinguishes it from methotrexate. Which set of enzymes does pemetrexed inhibit?
A) Dihydrofolate reductase only
B) Thymidylate synthase, dihydrofolate reductase, and GARFT (glycinamide ribonucleotide formyltransferase)
C) Ribonucleotide reductase and thymidylate synthase only
D) DNA (deoxyribonucleic acid) methyltransferase and thymidylate synthase
E) Xanthine oxidase and dihydrofolate reductase
ANSWER: B
Rationale:
Pemetrexed inhibits three folate-dependent enzymes: thymidylate synthase (its principal target), dihydrofolate reductase, and GARFT (glycinamide ribonucleotide formyltransferase, an enzyme of de novo purine synthesis). This multitargeted profile, together with its extensive polyglutamation, distinguishes it from methotrexate, whose principal target is dihydrofolate reductase.
Option B is correct because pemetrexed inhibits thymidylate synthase, dihydrofolate reductase, and GARFT.
Option A: Option A is incorrect because that describes the predominant single target of methotrexate, not the multitargeted pemetrexed.
Option C: Option C is incorrect because pemetrexed does not act primarily through ribonucleotide reductase.
Option D: Option D is incorrect because pemetrexed does not inhibit DNA methyltransferase; that is the hypomethylating agents.
Option E: Option E is incorrect because xanthine oxidase is unrelated to pemetrexed's mechanism.
5. 5-Fluorouracil exerts cytotoxicity through two distinct active metabolites. Which statement correctly pairs each metabolite with its mechanism?
A) FdUMP (fluorodeoxyuridine monophosphate) is incorporated into RNA (ribonucleic acid); FUTP (fluorouridine triphosphate) inhibits thymidylate synthase
B) Both FdUMP and FUTP act exclusively by inhibiting dihydrofolate reductase
C) FdUMP inhibits ribonucleotide reductase; FUTP inhibits thymidylate synthase
D) Both FdUMP and FUTP act exclusively by incorporation into DNA (deoxyribonucleic acid)
E) FdUMP inhibits thymidylate synthase via a ternary complex; FUTP is incorporated into RNA (ribonucleic acid), disrupting RNA processing
ANSWER: E
Rationale:
5-Fluorouracil is activated to FdUMP, which forms the inhibitory ternary complex with thymidylate synthase, and to FUTP, which is incorporated into RNA (ribonucleic acid) and disrupts RNA processing and protein synthesis. These two mechanisms explain why administration schedule shifts the predominant toxicity.
Option E is correct because it accurately assigns thymidylate synthase inhibition to FdUMP and RNA incorporation to FUTP.
Option A: Option A is incorrect because it reverses the two mechanisms.
Option B: Option B is incorrect because neither metabolite acts by inhibiting dihydrofolate reductase.
Option C: Option C is incorrect because FdUMP inhibits thymidylate synthase, not ribonucleotide reductase.
Option D: Option D is incorrect because the two metabolites act by distinct mechanisms (thymidylate synthase inhibition and RNA incorporation), not solely by DNA incorporation.
6. Among the loss-of-function variants in the DPYD (dihydropyrimidine dehydrogenase) gene, which one is the splice-site variant that causes exon 14 skipping and complete loss of DPD activity in homozygotes, producing the most severe fluoropyrimidine toxicity?
A) UGT1A1*28
B) TPMT*3A
C) DPYD*2A (the IVS14+1G>A splice-site variant)
D) CYP2C9*3
E) HLA-B*57:01
ANSWER: C
Rationale:
DPYD*2A (IVS14+1G>A) is a splice-site mutation that causes skipping of exon 14 and complete loss of dihydropyrimidine dehydrogenase activity in homozygotes. Carriers require a substantial starting-dose reduction, and homozygotes should generally avoid fluoropyrimidines, because they cannot catabolize 5-fluorouracil and experience life-threatening toxicity.
Option C is correct because DPYD*2A is the splice-site variant causing complete DPD loss.
Option A: Option A is incorrect because UGT1A1*28 predicts irinotecan toxicity, not fluoropyrimidine toxicity.
Option B: Option B is incorrect because TPMT*3A is a thiopurine-metabolism variant relevant to 6-mercaptopurine.
Option D: Option D is incorrect because CYP2C9*3 affects metabolism of drugs such as warfarin and phenytoin, not 5-fluorouracil catabolism.
Option E: Option E is incorrect because HLA-B*57:01 predicts abacavir hypersensitivity and is unrelated to DPD.
7. A patient stabilized on warfarin is started on capecitabine. Within two weeks the international normalized ratio (INR) rises sharply. What is the mechanism of this interaction?
A) Capecitabine inhibits CYP2C9 (cytochrome P450 2C9), reducing clearance of the more potent S-warfarin enantiomer and raising the INR
B) Capecitabine displaces warfarin from plasma albumin, transiently increasing free drug
C) Capecitabine induces CYP3A4, increasing conversion of warfarin to an active metabolite
D) Capecitabine inhibits vitamin K epoxide reductase directly
E) Capecitabine impairs intestinal absorption of vitamin K
ANSWER: A
Rationale:
Capecitabine inhibits CYP2C9 (cytochrome P450 2C9), the enzyme that metabolizes the more potent S-warfarin enantiomer. Reduced S-warfarin clearance increases anticoagulant effect and can raise the INR to dangerous levels within days to weeks. INR must be monitored closely, and switching to a direct oral anticoagulant (not metabolized by CYP2C9) avoids the interaction.
Option A is correct because CYP2C9 inhibition of S-warfarin clearance is the established mechanism.
Option B: Option B is incorrect because the interaction is metabolic (CYP2C9 inhibition), not a transient protein-binding displacement.
Option C: Option C is incorrect because capecitabine does not induce CYP3A4 to create an active warfarin metabolite; the effect is enzyme inhibition raising warfarin exposure.
Option D: Option D is incorrect because capecitabine does not directly inhibit vitamin K epoxide reductase (the target of warfarin itself).
Option E: Option E is incorrect because the interaction is not explained by impaired vitamin K absorption.
8. Capecitabine undergoes a three-step conversion to 5-fluorouracil. The final step is catalyzed by an enzyme expressed at higher levels in many tumors than in normal tissue, which is the theoretical basis for tumor-preferential activation. Which enzyme catalyzes this final step?
A) Carboxylesterase
B) Thymidine phosphorylase (TP)
C) Dihydropyrimidine dehydrogenase (DPD)
D) Cytidine deaminase (CDA)
E) Deoxycytidine kinase (dCK)
ANSWER: B
Rationale:
The final step of capecitabine activation, conversion of 5-deoxy-5-fluorouridine to 5-fluorouracil, is catalyzed by thymidine phosphorylase (TP). Because thymidine phosphorylase is expressed at higher levels in many tumor types (for example, colorectal, breast, and gastric) than in surrounding normal tissue, more 5-fluorouracil is generated at the tumor site, the basis for capecitabine's tumor-preferential activation.
Option B is correct because thymidine phosphorylase catalyzes the tumor-preferential final activation step.
Option A: Option A is incorrect because carboxylesterase catalyzes the first step (in the intestine), not the final activation step.
Option C: Option C is incorrect because dihydropyrimidine dehydrogenase catabolizes the resulting 5-fluorouracil rather than activating capecitabine.
Option D: Option D is incorrect because cytidine deaminase catalyzes the second (intermediate) step, not the final tumor-preferential step.
Option E: Option E is incorrect because deoxycytidine kinase activates cytidine analogs such as cytarabine and gemcitabine, not capecitabine.
9. Cytarabine and gemcitabine are both cytidine analogs incorporated into DNA (deoxyribonucleic acid), but they terminate chain elongation differently. Which statement correctly contrasts them?
A) Cytarabine permits one additional nucleotide to be added before termination, whereas gemcitabine blocks elongation immediately
B) Both produce immediate, overt chain termination at the point of incorporation
C) Both produce masked chain termination, allowing one further nucleotide before stopping
D) Cytarabine produces an immediate (overt) chain-termination block, whereas gemcitabine permits one additional nucleotide before terminating (masked chain termination), making it more resistant to proofreading exonucleases
E) Neither agent terminates the chain; both act solely by inhibiting thymidylate synthase
ANSWER: D
Rationale:
Once incorporated, cytarabine produces an immediate, overt steric block to further elongation. Gemcitabine instead allows one more nucleotide to be added before elongation stops, a phenomenon called masked chain termination, which renders the incorporated gemcitabine relatively resistant to proofreading exonucleases. This contrast is a defining pharmacologic difference between the two cytidine analogs.
Option D is correct because it accurately assigns overt termination to cytarabine and masked termination to gemcitabine.
Option A: Option A is incorrect because it reverses the two behaviors.
Option B: Option B is incorrect because gemcitabine produces masked, not immediate, termination.
Option C: Option C is incorrect because cytarabine produces overt, not masked, termination.
Option E: Option E is incorrect because both agents do terminate DNA chain elongation and do not act solely through thymidylate synthase inhibition.
10. A 64-year-old undergoing high-dose cytarabine (HiDAC) develops ataxia, dysarthria, and nystagmus. Regarding this characteristic toxicity, which statement is correct?
A) It is an idiosyncratic reaction unrelated to dose, age, or renal function
B) It reflects peripheral neuropathy and resolves by reducing the infusion rate without other action
C) It is caused by cerebral cortical injury and presents as seizures rather than cerebellar signs
D) It is prevented entirely by corticosteroid eye drops and requires no neurologic monitoring
E) It is a dose-dependent cerebellar toxicity from Purkinje cell injury; age above 50 years and renal impairment are key risk factors, and cerebellar signs mandate immediate discontinuation
ANSWER: E
Rationale:
High-dose cytarabine causes a characteristic, dose-dependent cerebellar toxicity (ataxia, dysarthria, nystagmus) from Purkinje cell injury. Older age (above 50 years) and renal impairment are the principal risk factors, neurologic examination is required before each dose, and cerebellar signs mandate immediate discontinuation to prevent irreversible injury.
Option E is correct because it accurately describes the dose-dependence, risk factors, and required response.
Option A: Option A is incorrect because the toxicity is dose-dependent and linked to age and renal function, not idiosyncratic.
Option B: Option B is incorrect because the syndrome is cerebellar, not a peripheral neuropathy managed by slowing the infusion.
Option C: Option C is incorrect because the lesion is cerebellar (Purkinje cells), presenting with cerebellar signs rather than cortical seizures.
Option D: Option D is incorrect because corticosteroid eye drops do not prevent it; pre-dose neurologic monitoring is essential.
11. Both cytarabine and gemcitabine depend on a nucleoside transporter for cellular uptake, and the level of this transporter predicts sensitivity to these drugs. Which transporter is responsible?
A) P-glycoprotein (P-gp, an efflux pump)
B) The reduced folate carrier (RFC)
C) hENT1 (human equilibrative nucleoside transporter 1)
D) OAT3 (organic anion transporter 3)
E) The sodium-potassium ATPase
ANSWER: C
Rationale:
Cytarabine and gemcitabine are taken up predominantly by hENT1 (human equilibrative nucleoside transporter 1). High hENT1 expression predicts better drug uptake and sensitivity, whereas low expression contributes to resistance; at very high cytarabine doses, passive diffusion becomes proportionally more important.
Option C is correct because hENT1 is the uptake transporter governing sensitivity to these cytidine analogs.
Option A: Option A is incorrect because P-glycoprotein is an efflux pump and is not the uptake route for these nucleoside analogs.
Option B: Option B is incorrect because the reduced folate carrier transports antifolates such as methotrexate, not cytidine analogs.
Option D: Option D is incorrect because OAT3 is a renal organic anion transporter relevant to methotrexate elimination, not cytarabine or gemcitabine uptake.
Option E: Option E is incorrect because the sodium-potassium ATPase is an ion pump, not a nucleoside uptake transporter.
12. TPMT (thiopurine methyltransferase) activity is trimodally distributed in the population. For a patient found to have intermediate TPMT activity (heterozygous for one low-activity allele) who is starting 6-mercaptopurine, which dosing approach is appropriate?
A) Reduce the 6-mercaptopurine dose (commonly to roughly 30 to 50% of standard) because reduced methylation increases thioguanine nucleotide accumulation
B) Increase the dose above standard to compensate for faster drug inactivation
C) Use the standard dose unchanged, since only homozygous low-activity patients are at any risk
D) Avoid 6-mercaptopurine entirely, as intermediate activity is an absolute contraindication
E) Give a standard dose but add allopurinol to enhance efficacy
ANSWER: A
Rationale:
Intermediate TPMT activity means reduced methylation (inactivation) of 6-mercaptopurine, shunting more drug toward thioguanine nucleotide accumulation and raising myelotoxicity risk. The appropriate strategy is a dose reduction, commonly to roughly 30 to 50% of standard, with monitoring; patients with low or absent activity require far larger reductions.
Option A is correct because dose reduction matches the increased thioguanine nucleotide accumulation seen with intermediate activity.
Option B: Option B is incorrect because reduced methylation means slower, not faster, inactivation; increasing the dose would worsen toxicity.
Option C: Option C is incorrect because intermediate-activity patients are also at increased risk and warrant dose reduction, not unchanged dosing.
Option D: Option D is incorrect because intermediate activity calls for dose reduction, not absolute avoidance.
Option E: Option E is incorrect because adding allopurinol would dangerously raise 6-mercaptopurine levels rather than safely enhance efficacy.
13. A patient with chronic lymphocytic leukemia (CLL) and a pre-existing positive direct antiglobulin test (DAT) is receiving fludarabine and develops worsening autoimmune hemolytic anemia (AIHA). What is the appropriate response regarding the fludarabine?
A) Continue fludarabine unchanged, since hemolysis is unrelated to the drug
B) Discontinue fludarabine immediately, because continuation worsens the hemolysis
C) Double the fludarabine dose to suppress the autoimmune clone faster
D) Switch to high-dose cytarabine, which has the same risk profile
E) Add allopurinol, which reverses fludarabine-associated hemolysis
ANSWER: B
Rationale:
Fludarabine can trigger or worsen autoimmune hemolytic anemia (AIHA), particularly in chronic lymphocytic leukemia (CLL) patients with a pre-existing positive direct antiglobulin test (DAT). Development of AIHA during therapy is an indication to discontinue fludarabine immediately, because continuation worsens the hemolysis.
Option B is correct because immediate discontinuation is the established response to fludarabine-associated AIHA.
Option A: Option A is incorrect because the hemolysis is drug-related and continuing fludarabine is harmful.
Option C: Option C is incorrect because increasing the dose would worsen, not control, the autoimmune hemolysis.
Option D: Option D is incorrect because high-dose cytarabine is not the appropriate substitute and carries its own serious toxicities; the issue here is stopping the offending agent.
Option E: Option E is incorrect because allopurinol does not reverse fludarabine-associated hemolysis.
14. Which purine analog is the treatment of choice for hairy cell leukemia, producing durable complete remissions in the great majority of patients after a single short course?
A) Fludarabine
B) 6-Mercaptopurine
C) Clofarabine
D) Cladribine (2-chlorodeoxyadenosine)
E) Pemetrexed
ANSWER: D
Rationale:
Cladribine (2-chlorodeoxyadenosine) is the treatment of choice for hairy cell leukemia, producing durable complete remissions in more than 90% of patients after a single short continuous-infusion course. Its selectivity for lymphocytes reflects high deoxycytidine kinase activity and low 5-nucleotidase activity in these cells.
Option D is correct because cladribine is the agent of choice for hairy cell leukemia.
Option A: Option A is incorrect because fludarabine is used mainly in chronic lymphocytic leukemia and low-grade lymphoma, not as the hairy cell leukemia agent of choice.
Option B: Option B is incorrect because 6-mercaptopurine is used in acute lymphoblastic leukemia maintenance.
Option C: Option C is incorrect because clofarabine is approved for relapsed or refractory pediatric acute lymphoblastic leukemia, not hairy cell leukemia.
Option E: Option E is incorrect because pemetrexed is an antifolate used in solid tumors, not a purine analog for hairy cell leukemia.
15. Azacitidine and decitabine are both hypomethylating agents, but a structural difference determines where each is incorporated. Which statement correctly distinguishes them?
A) Decitabine is a ribonucleoside incorporated into both RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); azacitidine is incorporated only into DNA
B) Both are ribonucleosides incorporated only into RNA (ribonucleic acid)
C) Azacitidine is a ribonucleoside incorporated into both RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); decitabine is a deoxyribonucleoside incorporated only into DNA
D) Both are deoxyribonucleosides incorporated only into DNA (deoxyribonucleic acid)
E) Neither is incorporated into nucleic acids; both act purely by extracellular enzyme inhibition
ANSWER: C
Rationale:
Azacitidine is a ribonucleoside and is incorporated into both RNA (ribonucleic acid) and, after reduction by ribonucleotide reductase, into DNA (deoxyribonucleic acid); its RNA incorporation adds mechanisms such as disruption of RNA processing. Decitabine is a deoxyribonucleoside incorporated exclusively into DNA, which focuses its DNMT (DNA methyltransferase) trapping and demethylating effect.
Option C is correct because azacitidine is the ribonucleoside (RNA and DNA) and decitabine the deoxyribonucleoside (DNA only).
Option A: Option A is incorrect because it reverses the two agents.
Option B: Option B is incorrect because decitabine is incorporated into DNA, and azacitidine enters DNA as well as RNA.
Option D: Option D is incorrect because azacitidine is a ribonucleoside with RNA incorporation, not a DNA-only deoxyribonucleoside.
Option E: Option E is incorrect because both agents are incorporated into nucleic acids and trap DNMT on DNA; they do not act purely extracellularly.
16. A patient on gemcitabine develops microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. Which complication does this represent, and what is the appropriate action regarding the drug?
A) Tumor lysis syndrome; continue gemcitabine and add rasburicase
B) Cerebellar toxicity; reduce the gemcitabine dose by half
C) Autoimmune hemolytic anemia; continue gemcitabine with corticosteroids
D) Pulmonary toxicity; continue gemcitabine with bronchodilators
E) Hemolytic uremic syndrome (a thrombotic microangiopathy); discontinue gemcitabine immediately
ANSWER: E
Rationale:
The combination of microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury in a patient on gemcitabine indicates gemcitabine-induced hemolytic uremic syndrome, a thrombotic microangiopathy. Although rare, it is potentially fatal and requires immediate discontinuation of the drug.
Option E is correct because the triad defines hemolytic uremic syndrome (thrombotic microangiopathy) and the required action is immediate discontinuation.
Option A: Option A is incorrect because tumor lysis syndrome presents with metabolic derangements (hyperkalemia, hyperuricemia, hyperphosphatemia), not this microangiopathic triad.
Option B: Option B is incorrect because cerebellar toxicity (ataxia, dysarthria, nystagmus) is associated with high-dose cytarabine, not this presentation.
Option C: Option C is incorrect because the picture is a thrombotic microangiopathy requiring drug discontinuation, not an autoimmune hemolytic anemia managed by continuing gemcitabine.
Option D: Option D is incorrect because gemcitabine pulmonary toxicity presents with dyspnea and interstitial infiltrates, not the hemolytic-thrombocytopenic-renal triad.
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