Chapter 33 — Anti-Cancer Drugs Part I Pharmacology — Module 3 — Antimetabolites: Folate Antagonists, Fluoropyrimidines, Cytidine and Purine Analogs, and Hypomethylating Agents
1. Antimetabolites are a class of anti-cancer drugs that resemble normal cellular building blocks (such as folates, purines, and pyrimidines) and interfere with DNA (deoxyribonucleic acid) synthesis. With respect to the cell cycle, how are most antimetabolites best classified?
A) They are cell-cycle-specific agents that act predominantly during the S phase, when DNA is synthesized
B) They are cell-cycle-nonspecific agents that kill cells equally in all phases, including resting (G0) cells
C) They act predominantly during the M phase by preventing mitotic spindle formation
D) They act predominantly during the G1 phase by blocking the synthesis of structural proteins
E) They have no relationship to the cell cycle because they bind directly to chromosomal DNA in all phases
ANSWER: A
Rationale:
Antimetabolites interfere with the synthesis of nucleotides or their incorporation into nucleic acids. Because DNA replication occurs during the S (synthesis) phase of the cell cycle, drugs that starve the cell of nucleotides or insert fraudulent nucleotides exert their cytotoxic effect predominantly while cells are actively synthesizing DNA. This S-phase specificity is the basis for scheduling strategies such as continuous-infusion regimens, which keep drug present long enough to catch cells as they cycle into S phase.
Option A is correct because S-phase dependence is the defining cell-cycle property of the antimetabolite class.
Option B: Option B is incorrect because cell-cycle-nonspecific agents (for example, alkylating agents) damage cells in all phases including G0; antimetabolites characteristically spare cells that are not synthesizing DNA.
Option C: Option C is incorrect because M-phase spindle interference describes the vinca alkaloids and taxanes, not antimetabolites.
Option D: Option D is incorrect because antimetabolites do not act by blocking structural protein synthesis in G1; their target is nucleotide and nucleic acid synthesis.
Option E: Option E is incorrect because antimetabolites do not bind chromosomal DNA indiscriminately across all phases; their cytotoxicity is tied to active DNA synthesis in S phase.
2. Methotrexate (MTX) is the prototype folate antagonist used in malignancies such as acute lymphoblastic leukemia and osteosarcoma. What is the primary molecular target of methotrexate?
A) Thymidylate synthase, the enzyme that converts deoxyuridine monophosphate to deoxythymidine monophosphate
B) DNA (deoxyribonucleic acid) polymerase alpha, the enzyme that elongates the growing DNA strand
C) Ribonucleotide reductase, the enzyme that converts ribonucleotides to deoxyribonucleotides
D) Dihydrofolate reductase (DHFR), the enzyme that regenerates reduced (active) tetrahydrofolate
E) Xanthine oxidase, the enzyme that catabolizes purines to uric acid
ANSWER: D
Rationale:
Methotrexate is a tight-binding competitive inhibitor of dihydrofolate reductase (DHFR). DHFR reduces dihydrofolate back to tetrahydrofolate (THF), the active carrier of one-carbon units required for synthesis of thymidylate and purines. By blocking DHFR, methotrexate depletes the reduced folate pool, halting DNA synthesis.
Option D is correct because DHFR is the defining target of methotrexate.
Option A: Option A is incorrect because thymidylate synthase is the target of the fluoropyrimidines and pemetrexed; methotrexate inhibits it only indirectly by depleting the folate cofactor.
Option B: Option B is incorrect because methotrexate does not directly inhibit DNA polymerase alpha; that block characterizes incorporated nucleoside analogs such as cytarabine.
Option C: Option C is incorrect because ribonucleotide reductase is inhibited by gemcitabine and hydroxyurea, not by methotrexate.
Option E: Option E is incorrect because xanthine oxidase is relevant to purine catabolism and the allopurinol interaction, not to the mechanism of methotrexate.
3. Several antimetabolites converge on a single enzyme that performs the only de novo (newly synthesized, not salvaged) reaction producing thymidylate for DNA (deoxyribonucleic acid) synthesis. Which enzyme is the shared target of 5-fluorouracil, capecitabine, and pemetrexed?
A) Dihydrofolate reductase (DHFR)
B) Thymidylate synthase (TS)
C) Hypoxanthine-guanine phosphoribosyltransferase (HGPRT)
D) DNA methyltransferase 1 (DNMT1)
E) Deoxycytidine kinase (dCK)
ANSWER: B
Rationale:
Thymidylate synthase (TS) catalyzes the reductive methylation of deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP), the sole de novo route to thymidylate. Both 5-fluorouracil (through its metabolite FdUMP) and pemetrexed inhibit TS, and capecitabine acts through 5-fluorouracil that it generates; loss of thymidylate halts DNA synthesis (thymine-less death).
Option B is correct because TS is the common target shared by these agents.
Option A: Option A is incorrect because DHFR is the target of methotrexate; pemetrexed also inhibits DHFR but the enzyme shared across all three listed drugs is TS.
Option C: Option C is incorrect because HGPRT activates the thiopurine 6-mercaptopurine and is unrelated to thymidylate synthesis.
Option D: Option D is incorrect because DNMT1 is the target of the hypomethylating agents azacitidine and decitabine, not of fluoropyrimidines or pemetrexed.
Option E: Option E is incorrect because deoxycytidine kinase activates cytidine analogs such as cytarabine and gemcitabine and is not the target of these drugs.
4. High-dose methotrexate (HD-MTX) is administered with a "rescue" agent given 24 to 42 hours later to protect normal tissues from prolonged folate depletion. Which agent is used for this rescue?
A) Glucarpidase, an enzyme that cleaves circulating methotrexate
B) Allopurinol, a xanthine oxidase inhibitor
C) Vitamin B12 (cobalamin) given intramuscularly
D) Mesna, a sulfhydryl compound that binds toxic metabolites in the bladder
E) Leucovorin (folinic acid), a reduced folate that bypasses the dihydrofolate reductase block
ANSWER: E
Rationale:
Leucovorin (5-formyltetrahydrofolate, folinic acid) is an already-reduced folate that enters the tetrahydrofolate pool directly, without requiring dihydrofolate reductase (DHFR). Because methotrexate blocks DHFR, leucovorin restores reduced folate to normal cells and rescues them, while tumor cells that accumulate more methotrexate polyglutamates remain preferentially killed. Rescue is timed by serial plasma methotrexate levels.
Option E is correct because leucovorin is the standard rescue agent after high-dose methotrexate.
Option A: Option A is incorrect because glucarpidase is reserved for severe methotrexate toxicity or failed leucovorin rescue, not for routine scheduled rescue.
Option B: Option B is incorrect because allopurinol is unrelated to methotrexate rescue and is itself implicated in a dangerous interaction with thiopurines.
Option C: Option C is incorrect because vitamin B12 supplementation reduces pemetrexed toxicity; it is not the methotrexate rescue agent.
Option D: Option D is incorrect because mesna protects the bladder from acrolein produced by oxazaphosphorine alkylating agents and has no role in methotrexate rescue.
5. 5-Fluorouracil (5-FU) is one of the most widely used cytotoxic drugs. More than 85% of an administered dose is eliminated by enzymatic catabolism rather than by renal excretion. Which enzyme is primarily responsible for catabolizing 5-FU?
A) Thymidine phosphorylase (TP)
B) Cytidine deaminase (CDA)
C) Dihydropyrimidine dehydrogenase (DPD)
D) Thiopurine methyltransferase (TPMT)
E) Glucuronosyltransferase (UGT)
ANSWER: C
Rationale:
Dihydropyrimidine dehydrogenase (DPD), located mainly in the liver, converts 5-fluorouracil to inactive dihydrofluorouracil and accounts for the great majority of 5-FU elimination. DPD activity is the dominant determinant of 5-FU half-life, and genetic DPD deficiency (DPYD variants) markedly increases the risk of severe toxicity.
Option C is correct because DPD is the principal catabolic enzyme for 5-FU.
Option A: Option A is incorrect because thymidine phosphorylase participates in activating capecitabine to 5-FU within tumor tissue rather than catabolizing 5-FU.
Option B: Option B is incorrect because cytidine deaminase inactivates cytidine analogs such as cytarabine and gemcitabine, not 5-FU.
Option D: Option D is incorrect because thiopurine methyltransferase catabolizes thiopurines such as 6-mercaptopurine and is unrelated to 5-FU.
Option E: Option E is incorrect because glucuronosyltransferase conjugates many drugs (for example, the active metabolite of irinotecan) but is not the catabolic enzyme for 5-FU.
6. A patient with colorectal cancer is prescribed an orally administered fluoropyrimidine that is itself inactive but undergoes a multi-step enzymatic conversion, with the final activating step occurring preferentially in tumor tissue. Which drug is being described?
A) Capecitabine
B) Cytarabine
C) Fludarabine
D) Azacitidine
E) Pemetrexed
ANSWER: A
Rationale:
Capecitabine is an oral prodrug of 5-fluorouracil. It is converted in three steps: intestinal carboxylesterases, then cytidine deaminase in liver and tumor, and finally thymidine phosphorylase, an enzyme present at higher levels in many tumors than in normal tissue. This tumor-preferential final step is the rationale for capecitabine's relatively favorable tolerability.
Option A is correct because capecitabine is the orally administered 5-FU prodrug described.
Option B: Option B is incorrect because cytarabine is a parenteral cytidine analog used in leukemia, not an oral 5-FU prodrug.
Option C: Option C is incorrect because fludarabine is a parenteral purine analog used in chronic lymphocytic leukemia, not a fluoropyrimidine prodrug.
Option D: Option D is incorrect because azacitidine is a hypomethylating cytidine analog given by injection, not an oral 5-FU prodrug.
Option E: Option E is incorrect because pemetrexed is a parenteral multitargeted antifolate, not an oral prodrug converted to 5-FU.
7. Which antimetabolite has been the cornerstone of acute myeloid leukemia (AML) induction for more than five decades, classically given as a 7-day continuous infusion combined with 3 days of an anthracycline (the "7+3" regimen)?
A) Methotrexate
B) Capecitabine
C) 6-Mercaptopurine
D) Cytarabine (ara-C)
E) Pemetrexed
ANSWER: D
Rationale:
Cytarabine (cytosine arabinoside, ara-C) is the defining drug of AML therapy. It is phosphorylated intracellularly to ara-CTP, incorporated into DNA (deoxyribonucleic acid), and terminates chain elongation. Its S-phase specificity is the rationale for the 7-day continuous infusion that, with 3 days of an anthracycline, forms the classic "7+3" induction regimen.
Option D is correct because cytarabine is the backbone of AML induction.
Option A: Option A is incorrect because methotrexate is central to acute lymphoblastic leukemia and osteosarcoma, not the "7+3" AML regimen.
Option B: Option B is incorrect because capecitabine is an oral fluoropyrimidine used in solid tumors such as colorectal cancer.
Option C: Option C is incorrect because 6-mercaptopurine is used in acute lymphoblastic leukemia maintenance, not AML induction.
Option E: Option E is incorrect because pemetrexed is used in mesothelioma and non-small cell lung cancer, not in AML.
8. A child in remission from acute lymphoblastic leukemia (ALL) is maintained on a daily oral thiopurine purine analog that requires intracellular activation by HGPRT (hypoxanthine-guanine phosphoribosyltransferase). Which drug is this?
A) Gemcitabine
B) 6-Mercaptopurine (6-MP)
C) 5-Fluorouracil
D) Decitabine
E) Methotrexate
ANSWER: B
Rationale:
6-Mercaptopurine (6-MP) is a thiopurine prodrug used primarily in acute lymphoblastic leukemia maintenance. It is activated by HGPRT to thioinosine monophosphate and then to thioguanine nucleotides, which are incorporated into DNA (deoxyribonucleic acid) and trigger cell death. Its catabolism by thiopurine methyltransferase and xanthine oxidase governs its dosing safety.
Option B is correct because 6-mercaptopurine is the HGPRT-activated thiopurine used in ALL maintenance.
Option A: Option A is incorrect because gemcitabine is a cytidine analog activated by deoxycytidine kinase, used in solid tumors.
Option C: Option C is incorrect because 5-fluorouracil is a pyrimidine analog targeting thymidylate synthase, not a thiopurine.
Option D: Option D is incorrect because decitabine is a hypomethylating cytidine analog, not a purine analog used in ALL maintenance.
Option E: Option E is incorrect because methotrexate is an antifolate; although it is used in ALL maintenance alongside 6-MP, it is not a purine analog and is not activated by HGPRT.
9. Azacitidine and decitabine are used in myelodysplastic syndrome (MDS) and acute myeloid leukemia. Although they are cytidine analogs, their principal mechanism at clinical doses is not nucleotide starvation. How are these two agents best classified?
Azacitidine and decitabine are hypomethylating agents. After incorporation into DNA (deoxyribonucleic acid), they covalently trap and deplete DNA methyltransferase 1 (DNMT1), leading to progressive loss of promoter methylation and reactivation of epigenetically silenced tumor suppressor genes. At the doses used in MDS and AML, this epigenetic effect predominates over direct cytotoxicity.
Option E is correct because these agents are classified as hypomethylating (DNMT-inhibiting) drugs.
Option A: Option A is incorrect because folate antagonists such as methotrexate act on dihydrofolate reductase, a different mechanism.
Option B: Option B is incorrect because ribonucleotide reductase inhibition characterizes gemcitabine and hydroxyurea, not the hypomethylating agents.
Option C: Option C is incorrect because thymidylate synthase inhibition describes the fluoropyrimidines and pemetrexed.
Option D: Option D is incorrect because topoisomerase inhibition is the mechanism of agents covered in a later module, not of azacitidine or decitabine.
10. The active 5-fluorouracil metabolite FdUMP (fluorodeoxyuridine monophosphate) inhibits thymidylate synthase in a particularly durable way. Which description best explains why this inhibition is nearly irreversible?
A) FdUMP permanently alkylates the DNA (deoxyribonucleic acid) template strand, blocking replication
B) FdUMP is incorporated into RNA (ribonucleic acid), disrupting ribosome assembly
C) FdUMP forms a stable covalent ternary complex with thymidylate synthase and the folate cofactor that cannot complete catalysis
D) FdUMP irreversibly inhibits dihydrofolate reductase, depleting all reduced folate
E) FdUMP is dephosphorylated to a metabolite that competitively blocks nucleoside transporters
ANSWER: C
Rationale:
FdUMP binds thymidylate synthase together with the folate cofactor 5,10-methylenetetrahydrofolate, forming a covalent ternary complex. Because the fluorine at the C5 position cannot be transferred to complete the reaction, the complex is essentially locked, producing near-irreversible inhibition. Leucovorin increases the folate cofactor and thereby stabilizes and deepens this inhibition.
Option C is correct because the covalent ternary complex with the folate cofactor is the basis for the durable inhibition.
Option A: Option A is incorrect because FdUMP does not alkylate the DNA template; that mechanism belongs to alkylating agents.
Option B: Option B is incorrect because RNA incorporation is a separate mode of 5-FU toxicity (via FUTP) and does not explain the durability of thymidylate synthase inhibition.
Option D: Option D is incorrect because FdUMP targets thymidylate synthase, not dihydrofolate reductase.
Option E: Option E is incorrect because the inhibitory action results from the ternary complex, not from blockade of nucleoside transporters.
11. Leucovorin is used together with 5-fluorouracil (5-FU) in colorectal cancer regimens. In this setting, what is the effect of leucovorin on 5-FU activity, and how does it differ from leucovorin's role with high-dose methotrexate?
A) With 5-FU, leucovorin enhances cytotoxicity by increasing the folate cofactor that stabilizes the thymidylate synthase complex; with methotrexate, it rescues normal cells from folate depletion
B) With 5-FU, leucovorin rescues normal cells from toxicity; with methotrexate, it enhances tumor cell kill
C) With both drugs, leucovorin enhances cytotoxicity by the same mechanism
D) With both drugs, leucovorin rescues normal cells from toxicity by the same mechanism
E) With 5-FU, leucovorin inhibits dihydropyrimidine dehydrogenase, slowing 5-FU catabolism
ANSWER: A
Rationale:
With 5-fluorouracil, leucovorin is converted to 5,10-methylenetetrahydrofolate, the folate cofactor of the FdUMP-thymidylate synthase ternary complex; more cofactor means more stable, deeper thymidylate synthase inhibition and greater tumor cell kill. With high-dose methotrexate, the same molecule serves the opposite clinical purpose, supplying reduced folate to rescue normal cells from the dihydrofolate reductase block. The two uses are mechanistically distinct.
Option A is correct because it accurately contrasts potentiation (with 5-FU) and rescue (with methotrexate).
Option B: Option B is incorrect because it reverses the two roles.
Option C: Option C is incorrect because the mechanisms are not the same; only the 5-FU use is potentiation.
Option D: Option D is incorrect because only the methotrexate use is rescue; the 5-FU use is potentiation.
Option E: Option E is incorrect because leucovorin does not inhibit dihydropyrimidine dehydrogenase; it acts by supplying the folate cofactor.
12. Before starting 6-mercaptopurine, current practice is to assess a patient's activity of a specific catabolic enzyme, because low activity causes thioguanine nucleotides to accumulate and produces life-threatening myelosuppression at standard doses. Which enzyme is assessed?
A) Dihydropyrimidine dehydrogenase (DPD)
B) Dihydrofolate reductase (DHFR)
C) Cytidine deaminase (CDA)
D) Thiopurine methyltransferase (TPMT)
E) Thymidylate synthase (TS)
ANSWER: D
Rationale:
Thiopurine methyltransferase (TPMT) methylates 6-mercaptopurine to an inactive metabolite. Patients with low TPMT activity cannot inactivate the drug efficiently, shunting it toward thioguanine nucleotide accumulation and severe myelosuppression. TPMT genotyping or phenotyping before therapy allows dose reduction in intermediate and low-activity patients and is a paradigm of pharmacogenomic dosing.
Option D is correct because TPMT activity governs 6-mercaptopurine dosing safety.
Option A: Option A is incorrect because dihydropyrimidine dehydrogenase deficiency governs fluoropyrimidine (5-FU, capecitabine) toxicity, not thiopurine toxicity.
Option B: Option B is incorrect because dihydrofolate reductase is the target of methotrexate, unrelated to thiopurine catabolism.
Option C: Option C is incorrect because cytidine deaminase inactivates cytidine analogs, not thiopurines.
Option E: Option E is incorrect because thymidylate synthase is a drug target, not the enzyme whose activity determines 6-mercaptopurine safety.
13. Cytarabine must be converted intracellularly to its active triphosphate (ara-CTP) to be cytotoxic. Which step is rate-limiting for this activation, such that its loss is the primary mechanism of cytarabine resistance?
A) Uptake by the equilibrative nucleoside transporter hENT1
B) Phosphorylation by deoxycytidine kinase (dCK)
C) Deamination by cytidine deaminase (CDA)
D) Incorporation into DNA (deoxyribonucleic acid) by DNA polymerase
E) Methylation by thiopurine methyltransferase (TPMT)
ANSWER: B
Rationale:
The rate-limiting activating step for cytarabine is the first phosphorylation, catalyzed by deoxycytidine kinase (dCK). Loss of dCK activity is the principal cause of cytarabine resistance because the drug can no longer be converted to ara-CTP for incorporation into DNA (deoxyribonucleic acid).
Option B is correct because dCK phosphorylation is the rate-limiting activation step.
Option A: Option A is incorrect because although hENT1 uptake influences sensitivity, the rate-limiting activation step is dCK phosphorylation, and dCK loss is the primary resistance mechanism.
Option C: Option C is incorrect because cytidine deaminase catabolizes (inactivates) cytarabine rather than activating it; its increased activity is a resistance mechanism but it is not the activating step.
Option D: Option D is incorrect because incorporation into DNA is the downstream cytotoxic event, not the rate-limiting activation step.
Option E: Option E is incorrect because thiopurine methyltransferase acts on thiopurines and has no role in cytarabine activation.
14. Gemcitabine shares deoxycytidine kinase activation with cytarabine but has an additional mechanism that makes its own incorporation into DNA (deoxyribonucleic acid) more efficient. Which mechanism accounts for this "self-potentiation"?
A) Gemcitabine inhibits dihydrofolate reductase, increasing the folate pool
B) Gemcitabine inhibits thymidylate synthase, depleting thymidylate
C) Gemcitabine diphosphate inhibits ribonucleotide reductase, lowering the competing dCTP pool
D) Gemcitabine inhibits cytidine deaminase, preventing its own catabolism
E) Gemcitabine inhibits DNA methyltransferase, reactivating tumor suppressor genes
ANSWER: C
Rationale:
Gemcitabine diphosphate inhibits ribonucleotide reductase, the enzyme that produces deoxyribonucleotides. This lowers the intracellular pool of deoxycytidine triphosphate (dCTP), the natural substrate that competes with gemcitabine triphosphate for incorporation by DNA polymerase. With less competing dCTP, gemcitabine triphosphate is incorporated more efficiently, a self-potentiating effect unique among the cytidine analogs.
Option C is correct because ribonucleotide reductase inhibition reduces the competing dCTP pool and potentiates gemcitabine incorporation.
Option A: Option A is incorrect because gemcitabine does not inhibit dihydrofolate reductase.
Option B: Option B is incorrect because thymidylate synthase inhibition belongs to the fluoropyrimidines and pemetrexed, not gemcitabine.
Option D: Option D is incorrect because gemcitabine is a substrate for cytidine deaminase, not an inhibitor of it.
Option E: Option E is incorrect because DNA methyltransferase inhibition describes the hypomethylating agents, not gemcitabine's self-potentiation.
15. A patient with non-squamous non-small cell lung cancer is to begin pemetrexed. Which supplementation is required to substantially reduce its hematologic and mucosal toxicity?
A) Folic acid and vitamin B12 (cobalamin)
B) Leucovorin rescue 24 hours after each dose
C) Mesna with each dose
D) Allopurinol started before the first dose
E) Glucarpidase with each dose
ANSWER: A
Rationale:
Pemetrexed toxicity (myelosuppression and mucositis) is markedly reduced by routine supplementation with folic acid (started before the first dose) and vitamin B12. Dexamethasone is also given to reduce rash, but the supplements that reduce hematologic and mucosal toxicity are folic acid and B12.
Option A is correct because folic acid and vitamin B12 supplementation is the required toxicity-reducing measure for pemetrexed.
Option B: Option B is incorrect because scheduled leucovorin rescue is used with high-dose methotrexate, not as routine pemetrexed supplementation.
Option C: Option C is incorrect because mesna protects against hemorrhagic cystitis from oxazaphosphorine alkylators, not pemetrexed toxicity.
Option D: Option D is incorrect because allopurinol is not part of pemetrexed supplementation and carries a dangerous interaction with thiopurines.
Option E: Option E is incorrect because glucarpidase is a rescue enzyme for methotrexate toxicity, not a pemetrexed supplement.
16. A patient with higher-risk myelodysplastic syndrome (MDS) is started on azacitidine. The clinician counsels that response should not be expected quickly. How many treatment cycles are generally recommended before declaring the hypomethylating agent a failure, and why?
A) One cycle, because demethylation occurs immediately after the first dose
B) One to two cycles, because the drug is directly cytotoxic like intensive chemotherapy
C) Two cycles, because the drug acts mainly by alkylating DNA (deoxyribonucleic acid)
D) Eight to ten cycles, because the drug must first be cleared from bone marrow stores
E) Four to six cycles, because passive demethylation accumulates only across repeated cell divisions
ANSWER: E
Rationale:
Hypomethylating agents work through passive demethylation, in which methylation marks are progressively lost from daughter strands across successive cell divisions. This requires repeated cycles of drug exposure, so the median time to response is several cycles. A minimum of four to six cycles is recommended before declaring failure, provided hematologic tolerance allows continuation.
Option E is correct because the epigenetic mechanism requires four to six cycles to manifest a response.
Option A: Option A is incorrect because demethylation does not occur immediately; it accumulates over divisions.
Option B: Option B is incorrect because at clinical doses the agents act epigenetically rather than as rapid direct cytotoxics.
Option C: Option C is incorrect because these drugs do not act primarily by alkylating DNA.
Option D: Option D is incorrect because the delay reflects the demethylation mechanism, not clearance from marrow stores.
17. 5-Fluorouracil has two cytotoxic modes: thymidylate synthase inhibition (favored by sustained low concentrations) and RNA (ribonucleic acid) incorporation (favored by high peak concentrations). Applying this principle, how does the predominant toxicity differ between bolus and continuous-infusion 5-FU?
A) Bolus and continuous infusion produce identical toxicity because the total dose is the same
B) Continuous infusion produces predominantly myelosuppression, while bolus produces predominantly mucositis
C) Bolus produces predominantly hand-foot syndrome, while continuous infusion produces predominantly myelosuppression
E) Neither schedule causes mucositis because 5-FU spares the gastrointestinal mucosa
ANSWER: D
Rationale:
Bolus 5-fluorouracil generates high peak concentrations that favor incorporation into RNA (ribonucleic acid), striking rapidly dividing marrow cells and producing predominantly myelosuppression. Continuous infusion maintains sustained lower concentrations that favor FdUMP-mediated thymidylate synthase inhibition, producing predominantly mucositis and hand-foot syndrome. This is a direct application of the two-mode concept.
Option D is correct because it correctly maps each schedule to its predominant toxicity.
Option A: Option A is incorrect because schedule changes which mode predominates, so toxicity is not identical.
Option B: Option B is incorrect because it reverses the schedule-toxicity relationship.
Option C: Option C is incorrect because hand-foot syndrome is associated with continuous infusion, not bolus dosing.
Option E: Option E is incorrect because 5-FU does cause mucositis; it does not spare the gastrointestinal mucosa.
18. A patient on standard-dose 6-mercaptopurine is about to be started on allopurinol for hyperuricemia. Knowing that xanthine oxidase is a catabolic enzyme for 6-mercaptopurine, what is the correct management of this interaction?
A) No change is needed because allopurinol does not affect 6-mercaptopurine
B) Reduce the 6-mercaptopurine dose to approximately 25% of the usual dose, or use rasburicase instead of allopurinol
C) Double the 6-mercaptopurine dose to overcome reduced absorption
D) Stop 6-mercaptopurine permanently because the combination is always fatal
E) Add leucovorin rescue to counteract the interaction
ANSWER: B
Rationale:
Allopurinol inhibits xanthine oxidase, a primary catabolic enzyme for 6-mercaptopurine. Blocking this pathway raises 6-mercaptopurine levels several-fold, dramatically increasing thioguanine nucleotide accumulation and the risk of fatal myelosuppression. The standard management is to reduce the 6-mercaptopurine dose to about 25% when allopurinol cannot be avoided, or to use rasburicase for urate lowering instead.
Option B is correct because dose reduction to roughly 25% (or substituting rasburicase) is the established management.
Option A: Option A is incorrect because the interaction is real and dangerous, not negligible.
Option C: Option C is incorrect because the problem is reduced catabolism causing accumulation, so increasing the dose would worsen toxicity.
Option D: Option D is incorrect because the combination can be managed with dose reduction; permanent discontinuation is not mandatory.
Option E: Option E is incorrect because leucovorin does not counteract this thiopurine-xanthine oxidase interaction.
19. Using the principle that dihydropyrimidine dehydrogenase (DPD) catabolizes 5-fluorouracil, which pre-treatment strategy best identifies a patient at high risk of severe fluoropyrimidine toxicity before the first dose?
A) DPYD genotyping (or a phenotypic measure such as plasma uracil) to detect DPD deficiency before starting 5-FU or capecitabine
B) Measuring thiopurine methyltransferase activity
C) Genotyping for UGT1A1 status
D) Checking dihydrofolate reductase expression in the tumor
E) Measuring deoxycytidine kinase activity in peripheral blood
ANSWER: A
Rationale:
Because DPD is the principal enzyme catabolizing 5-fluorouracil, deficient patients clear the drug poorly and suffer severe toxicity at standard doses. Detecting DPD deficiency in advance, by DPYD genotyping or a phenotypic surrogate such as plasma uracil, identifies the at-risk patient before exposure and allows dose individualization. This applies the catabolism concept established earlier in the set.
Option A is correct because DPYD/DPD assessment targets the relevant catabolic enzyme for fluoropyrimidines.
Option B: Option B is incorrect because thiopurine methyltransferase governs thiopurine, not fluoropyrimidine, safety.
Option C: Option C is incorrect because UGT1A1 status is relevant to irinotecan toxicity, not fluoropyrimidines.
Option D: Option D is incorrect because dihydrofolate reductase expression relates to methotrexate, not 5-FU catabolism.
Option E: Option E is incorrect because deoxycytidine kinase activity relates to cytidine analog activation, not fluoropyrimidine catabolism.
20. Both cytarabine and gemcitabine are cytidine analogs, but each carries a characteristic non-hematologic toxicity. Which pairing of drug to its characteristic toxicity is correct?
High-dose cytarabine produces a characteristic cerebellar toxicity (ataxia, dysarthria, nystagmus) from Purkinje cell injury, with risk rising in older patients and renal impairment; neurologic examination is required before each dose. Gemcitabine instead causes a distinctive pulmonary toxicity and, rarely, a hemolytic uremic syndrome (thrombotic microangiopathy). Distinguishing these is the discrimination being tested.
Option E is correct because it correctly assigns cerebellar toxicity to high-dose cytarabine and pulmonary/hemolytic uremic toxicity to gemcitabine.
Option A: Option A is incorrect because it reverses the two toxicities.
Option B: Option B is incorrect because high-dose cytarabine causes cerebellar toxicity, not pulmonary fibrosis, and gemcitabine does not characteristically cause cerebellar ataxia.
Option C: Option C is incorrect because cerebellar ataxia is characteristic of cytarabine, not both drugs.
Option D: Option D is incorrect because hemolytic uremic syndrome is associated with gemcitabine, not characteristically with cytarabine.
21. Fludarabine and cladribine are purine analogs that produce profound, sustained depletion of CD4 (cluster of differentiation 4) T-helper lymphocytes lasting months to years. Based on this immunosuppression, which prophylaxis is standard during and after therapy?
A) No prophylaxis is needed because purine analogs do not impair immunity
B) Iron supplementation only
C) Prophylactic anticoagulation for venous thromboembolism
D) Prophylaxis against Pneumocystis jirovecii pneumonia (PCP), typically with trimethoprim-sulfamethoxazole, plus antiviral prophylaxis against herpesvirus reactivation
E) Routine granulocyte transfusions with each cycle
ANSWER: D
Rationale:
Because fludarabine and cladribine deplete CD4 T-helper lymphocytes for a prolonged period, patients are at risk for opportunistic infections. Standard practice is Pneumocystis jirovecii pneumonia (PCP) prophylaxis, usually with trimethoprim-sulfamethoxazole, together with antiviral prophylaxis against herpesvirus reactivation. This applies the immunosuppression concept directly to a preventive strategy.
Option D is correct because PCP and antiviral prophylaxis address the predictable opportunistic-infection risk from CD4 depletion.
Option A: Option A is incorrect because these purine analogs cause clinically important immunosuppression requiring prophylaxis.
Option B: Option B is incorrect because iron supplementation does not address opportunistic infection risk.
Option C: Option C is incorrect because anticoagulation does not address the immunosuppression caused by these drugs.
Option E: Option E is incorrect because routine granulocyte transfusions are not standard prophylaxis; the risk being prevented relates to T-cell-mediated opportunistic infection.
22. Antimetabolite resistance arises through several recurring themes. Applying the mechanisms established in this module, which pairing correctly matches a resistance mechanism to its consequence?
A) Increased reduced folate carrier (RFC) expression causes methotrexate resistance
B) Increased deoxycytidine kinase (dCK) expression causes cytarabine resistance
C) Loss of an activating enzyme (such as dCK, HGPRT, or RFC) and amplification of a drug target (such as DHFR or thymidylate synthase) are two distinct mechanisms that both reduce antimetabolite efficacy
E) Loss of dihydrofolate reductase (DHFR) expression causes methotrexate resistance
ANSWER: C
Rationale:
Two major resistance themes operate across the antimetabolite class: loss of an activating enzyme or transporter (for example, deoxycytidine kinase for cytidine analogs, HGPRT [hypoxanthine-guanine phosphoribosyltransferase] for thiopurines, or the reduced folate carrier for methotrexate), and amplification or alteration of the drug target (for example, dihydrofolate reductase amplification, or thymidylate synthase amplification). Both reduce efficacy though by different routes.
Option C is correct because it accurately distinguishes loss of activation from target amplification as two separate efficacy-reducing mechanisms.
Option A: Option A is incorrect because resistance results from reduced, not increased, reduced folate carrier expression (which lowers methotrexate uptake).
Option B: Option B is incorrect because loss, not increase, of deoxycytidine kinase causes cytarabine resistance, since dCK is required for activation.
Option D: Option D is incorrect because increased, not decreased, cytidine deaminase causes gemcitabine resistance by accelerating inactivation.
Option E: Option E is incorrect because methotrexate resistance is associated with dihydrofolate reductase amplification, not loss; loss of the target would not confer resistance to a drug that inhibits it.
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