Anti-Cancer Drugs — Part 1

Correct Answer

C — Dihydrofolate reductase inhibitor

Rationale

Methotrexate is classified as a dihydrofolate reductase inhibitor. It competitively inhibits dihydrofolate reductase, the enzyme that reduces dihydrofolate to tetrahydrofolate, thereby depleting the active reduced folate pool required for thymidylate synthesis and de novo purine synthesis. Thymidylate synthase inhibitors (such as 5-fluorouracil active metabolite fluorodeoxyuridine monophosphate) target a downstream enzyme in the same pathway. Ribonucleotide reductase inhibitors (such as hydroxyurea and gemcitabine diphosphate) target deoxyribonucleoside diphosphate synthesis. Topoisomerase II inhibitors (such as etoposide and anthracyclines) target deoxyribonucleic acid strand-break resolution.

Correct Answer

A — Fluoropyrimidine

Rationale

5-Fluorouracil is classified as a fluoropyrimidine antimetabolite. Fluoropyrimidines are structural analogs of the pyrimidine uracil in which a fluorine atom replaces the 5-hydrogen, allowing the drug to enter pyrimidine metabolic pathways and generate active metabolites that inhibit thymidylate synthase or incorporate into nucleic acids. Purine analogs (such as 6-mercaptopurine and fludarabine) mimic purine bases and interfere with purine synthesis or incorporation. Folate antagonists (such as methotrexate and pemetrexed) inhibit folate-dependent enzymes. Alkylating agents form covalent deoxyribonucleic acid adducts and are mechanistically unrelated to antimetabolites.

Correct Answer

B — Oral fluoropyrimidine prodrug

Rationale

Capecitabine is classified as an oral fluoropyrimidine prodrug. It undergoes three-step enzymatic conversion to 5-fluorouracil after oral administration, with the final step catalyzed by thymidine phosphorylase, which is preferentially expressed in many tumor types. This distinguishes capecitabine from oral folate antagonists (such as methotrexate taken orally for non-oncologic indications), oral purine analogs (such as 6-mercaptopurine), and oral alkylating agents (such as temozolomide or melphalan), which are members of entirely different antimetabolite or alkylating subclasses.

Correct Answer

D — Reduced folate cofactor

Rationale

Leucovorin (5-formyltetrahydrofolate) is classified as a reduced folate cofactor. It is the pharmacologically active form of folate that can directly enter the intracellular tetrahydrofolate pool without requiring reduction by dihydrofolate reductase. In this role it serves either as a rescue agent after high-dose methotrexate or as a modulating agent with 5-fluorouracil, where it increases the intracellular reduced folate pool that stabilizes the thymidylate synthase inhibitory complex. Leucovorin is not a dihydrofolate reductase inhibitor, not a thymidylate synthase inhibitor, and not a folate antagonist — it is the opposite of a folate antagonist, providing the reduced folate that antagonists block.

Correct Answer

A — Fluorinated purine nucleoside analog

Rationale

Fludarabine is classified as a fluorinated purine nucleoside analog. It is a structural analog of adenine arabinoside in which a fluorine atom at the 2-position confers resistance to deamination by adenosine deaminase. After intracellular phosphorylation, it incorporates into deoxyribonucleic acid and ribonucleic acid and inhibits multiple enzymes in nucleoside metabolism. Fluoropyrimidines (such as 5-fluorouracil and capecitabine) are pyrimidine analogs. Folate antagonists (such as methotrexate and pemetrexed) target folate-dependent enzymes. Hypomethylating agents (such as azacitidine and decitabine) inhibit deoxyribonucleic acid methyltransferase and have a distinct mechanism from nucleoside analogs.

Correct Answer

C — Hypomethylating agent

Rationale

Azacitidine is classified as a hypomethylating agent (also called a deoxyribonucleic acid methyltransferase inhibitor). After incorporation into deoxyribonucleic acid, it forms an irreversible covalent bond with deoxyribonucleic acid methyltransferase 1, trapping and degrading the enzyme and causing progressive loss of cytosine methylation at CpG dinucleotides across successive cell divisions. This mechanism of epigenetic gene reactivation is distinct from purine nucleoside analogs (such as fludarabine and cladribine), fluoropyrimidines (such as 5-fluorouracil), and folate antagonists (such as methotrexate), despite azacitidine being a cytidine analog structurally.

Correct Answer

B — Folylpolyglutamate synthetase adds multiple glutamate residues to methotrexate, forming polyglutamate derivatives that are retained intracellularly for weeks because they cannot exit through the reduced folate carrier

Rationale

After entering cells via the reduced folate carrier, methotrexate is modified by folylpolyglutamate synthetase, which sequentially adds multiple glutamate residues to the gamma-carboxyl group. The resulting methotrexate polyglutamates are highly charged anionic molecules that cannot exit through the reduced folate carrier and are not substrates for other export mechanisms, effectively trapping them intracellularly for weeks. Methotrexate polyglutamates retain potent dihydrofolate reductase inhibitory activity and additionally directly inhibit thymidylate synthase and an enzyme in de novo purine synthesis, extending and broadening the antiproliferative effect. The binding to dihydrofolate reductase is competitive and tight but not covalent; there is no lysosomal sequestration mechanism; and methotrexate does not undergo cytochrome P450-mediated conversion to an active metabolite.

Correct Answer

D — Leucovorin is a pre-reduced folate that enters the intracellular tetrahydrofolate pool directly without requiring reduction by the methotrexate-inhibited dihydrofolate reductase

Rationale

Leucovorin (5-formyltetrahydrofolate) is already in the reduced, active form — it does not require dihydrofolate reductase activity to function as a one-carbon donor in thymidylate and purine synthesis. By entering the tetrahydrofolate pool directly via a different transporter pathway, leucovorin bypasses the dihydrofolate reductase block that methotrexate has created and replenishes the reduced folate cofactors that normal cells need. Leucovorin does not displace methotrexate from its binding site on dihydrofolate reductase — the two do not compete at the same site. Leucovorin has no effect on hepatic methotrexate clearance. Leucovorin does not need to be reduced by dihydrofolate reductase; it is already in the active reduced form and enters the tetrahydrofolate pool without enzyme-mediated conversion.

Correct Answer

A — The continuous infusion patient will develop predominantly mucositis and hand-foot syndrome because sustained low concentrations favor thymidylate synthase inhibition, while the bolus patient will develop predominantly myelosuppression because high peak concentrations favor ribonucleic acid incorporation

Rationale

5-Fluorouracil exerts cytotoxicity through two distinct mechanisms, and the administration schedule determines which mechanism predominates. With bolus administration, high peak plasma concentrations favor conversion to fluorouridine triphosphate, which incorporates into ribonucleic acid and disrupts ribonucleic acid processing — this ribonucleic acid-directed mechanism produces predominantly myelosuppression. With continuous infusion, sustained lower concentrations favor formation of fluorodeoxyuridine monophosphate, which forms an irreversible ternary complex with thymidylate synthase and the folate cofactor — this deoxyribonucleic acid-directed mechanism produces predominantly mucositis and hand-foot syndrome (palmar-plantar erythrodysesthesia). The difference is mechanistic and schedule-dependent, not a function of total dose.

Correct Answer

C — Dihydropyrimidine dehydrogenase is the primary enzyme responsible for catabolizing more than 85% of administered 5-fluorouracil; deficiency impairs this elimination, causing drug to accumulate to toxic concentrations that far exceed normal therapeutic levels

Rationale

Dihydropyrimidine dehydrogenase is the rate-limiting enzyme of 5-fluorouracil catabolism, responsible for converting more than 85% of administered drug to dihydrofluorouracil, the first step in its elimination. The plasma half-life of 5-fluorouracil under normal dihydropyrimidine dehydrogenase activity is only approximately 10 to 20 minutes. In patients with complete dihydropyrimidine dehydrogenase deficiency, this primary elimination pathway is absent, and the drug accumulates to concentrations far exceeding the therapeutic range within hours of administration. The excess drug generates proportionally greater amounts of all active metabolites, causing severe myelosuppression, mucositis, neurotoxicity, and multi-organ failure at what would be standard doses in patients with normal enzyme activity. The mechanism is impaired elimination, not altered activation or receptor sensitivity.

Correct Answer

B — Allopurinol inhibits xanthine oxidase, one of the primary catabolic enzymes for 6-mercaptopurine, blocking its elimination and raising plasma 6-mercaptopurine concentrations approximately fourfold, causing potentially fatal myelosuppression

Rationale

6-Mercaptopurine is catabolized primarily by two pathways: methylation by thiopurine methyltransferase to an inactive methylated metabolite, and oxidation by xanthine oxidase to 6-thiouric acid, which is excreted renally. Allopurinol is a xanthine oxidase inhibitor — the same enzyme class it inhibits to reduce uric acid production. By blocking xanthine oxidase-mediated catabolism of 6-mercaptopurine, allopurinol eliminates a major elimination pathway, causing 6-mercaptopurine plasma concentrations to rise approximately fourfold. The resulting excess thioguanine nucleotide accumulation causes life-threatening myelosuppression. The standard management is to reduce the 6-mercaptopurine dose to 25% of the usual dose when allopurinol cannot be avoided. Allopurinol does not inhibit thiopurine methyltransferase, does not affect plasma protein binding in a clinically meaningful way, and does not induce cytochrome P450 3A4.

Correct Answer

D — Gemcitabine must be phosphorylated to its active triphosphate form and incorporated into the growing strand by polymerase during active replication; cells in S phase are vulnerable while non-dividing cells are largely spared

Rationale

Gemcitabine is a prodrug that requires intracellular phosphorylation to its active triphosphate form. The active form is incorporated into the growing strand by polymerase during replication, where it causes chain termination and triggers cell death. Because this mechanism depends on active replication, only cells in S phase — when the genome is being copied — are vulnerable. Non-dividing cells, which have no ongoing replication, are largely spared. This S-phase specificity is the defining pharmacological basis for classifying gemcitabine as a cell cycle-specific agent. By contrast, alkylating agents cause strand damage independently of replication and are cell cycle non-specific. Gemcitabine does not bind tubulin and does not intercalate between base pairs.

Correct Answer

A — Without folic acid and vitamin B12 supplementation, pemetrexed causes severe and potentially life-threatening myelosuppression and mucositis in a disproportionately high fraction of patients; supplementation substantially reduces this toxicity without impairing antitumor activity

Rationale

Pemetrexed is a multitargeted antifolate that inhibits thymidylate synthase, dihydrofolate reductase, and an enzyme in de novo purine synthesis. In clinical trials, patients who did not receive folic acid and vitamin B12 supplementation experienced severe grade 3 and 4 myelosuppression and mucositis at substantially higher rates than those who received supplementation, with several toxic deaths in the unsupplemented group. Supplementation with folic acid 400 micrograms daily starting at least 5 days before the first dose and vitamin B12 1,000 micrograms intramuscularly approximately one week before the first dose (repeated every 3 cycles) reduces toxicity without reducing tumor response rates. The mechanism by which supplementation selectively protects normal tissues while preserving antitumor activity relates to differential folate metabolism between normal and malignant cells. Pemetrexed is administered intravenously, so gastrointestinal absorption and oral bioavailability are not relevant.

Correct Answer

C — Fludarabine is selectively toxic to CD4-positive T lymphocytes, depleting them to very low levels that persist for months to years after therapy, creating sustained susceptibility to Pneumocystis jirovecii pneumonia and other opportunistic infections

Rationale

Fludarabine is selectively toxic to CD4-positive T lymphocytes, the immune effector cells responsible for coordinating cellular immunity against opportunistic pathogens including Pneumocystis jirovecii, cytomegalovirus, and fungi. This CD4 depletion is profound and prolonged, persisting for months to years after the completion of fludarabine-containing therapy — far longer than the myelosuppression that recovers within weeks. Pneumocystis jirovecii pneumonia prophylaxis with trimethoprim-sulfamethoxazole is therefore mandatory during therapy and should be continued for at least 6 to 12 months afterward, until CD4 counts recover to levels that provide adequate immune protection. Fludarabine does not impair neutrophil myeloperoxidase, does not accumulate in alveolar macrophages, and its primary immunosuppressive effect is on T cells, not on immunoglobulin class switching in B cells.

Correct Answer

B — Ibuprofen inhibits renal prostaglandin synthesis, reducing glomerular filtration rate and impairing tubular secretion of methotrexate, causing plasma methotrexate concentrations to rise to toxic levels

Rationale

Methotrexate is eliminated approximately 80 to 90% unchanged in urine through glomerular filtration and active tubular secretion via organic anion transporters. Nonsteroidal anti-inflammatory drugs such as ibuprofen inhibit cyclooxygenase enzymes in the renal vasculature, reducing prostaglandin-mediated afferent arteriolar dilation and thereby decreasing glomerular filtration rate. Nonsteroidal anti-inflammatory drugs may also compete for organic anion transporter-mediated tubular secretion. Together these effects impair methotrexate renal clearance, prolonging plasma half-life and raising concentrations to levels that produce the classic triad of myelosuppression, mucositis, and mucous membrane toxicity. This interaction can be life-threatening even at the low doses of methotrexate used in rheumatoid arthritis. Methotrexate is not a significant cytochrome P450 substrate, ibuprofen does not compete at dihydrofolate reductase, and ibuprofen does not affect folylpolyglutamate synthetase.

Correct Answer

A — Thiopurine methyltransferase deficiency impairs methylation-based inactivation of 6-mercaptopurine, shunting virtually all drug through the activation pathway and causing thioguanine nucleotides to accumulate to concentrations that cause severe myelosuppression

Rationale

6-Mercaptopurine is catabolized by two competing pathways: methylation by thiopurine methyltransferase to inactive S-methylated metabolites, and oxidation by xanthine oxidase to 6-thiouric acid. When thiopurine methyltransferase is absent due to homozygous low-activity genotype, the methylation inactivation pathway is eliminated. The drug is instead shunted almost entirely through the hypoxanthine-guanine phosphoribosyltransferase-mediated activation pathway, converting 6-mercaptopurine to thioguanine nucleotides at a far higher rate than occurs in patients with normal thiopurine methyltransferase activity. The resulting thioguanine nucleotide accumulation to very high intracellular concentrations causes severe myelosuppression at doses that are safe in patients with functional thiopurine methyltransferase. Xanthine oxidase converts 6-mercaptopurine to 6-thiouric acid, not thiopurine methyltransferase; hypoxanthine-guanine phosphoribosyltransferase is the activation enzyme, not impaired by thiopurine methyltransferase deficiency; and there is no lysosomal depot mechanism.

Correct Answer

D — High-dose cytarabine causes cerebellar toxicity through Purkinje cell loss; the drug must be discontinued immediately because continued administration causes irreversible cerebellar damage

Rationale

The triad of ataxia, dysarthria, and nystagmus in a patient receiving high-dose cytarabine is the hallmark presentation of cytarabine cerebellar toxicity. High-dose cytarabine achieves cytotoxic concentrations in the cerebrospinal fluid and causes selective injury to cerebellar Purkinje cells, which are particularly vulnerable to high ara-cytosine triphosphate concentrations. Cerebellar toxicity occurs in approximately 10 to 25% of patients receiving high-dose cytarabine regimens and requires neurological examination before each dose cycle specifically to detect early signs. Upon recognition, high-dose cytarabine must be discontinued immediately — continued administration causes progressive and largely irreversible Purkinje cell loss and permanent cerebellar dysfunction. This is not a peripheral neuropathy, not a metabolic alkalosis presentation, and not a chemical meningitis; the correct recognition and immediate discontinuation are the only appropriate response.

Correct Answer

C — Capecitabine inhibits cytochrome P450 2C9, the primary enzyme responsible for metabolizing the pharmacologically active S-warfarin enantiomer, causing S-warfarin plasma concentrations to rise and the international normalized ratio to increase

Rationale

Warfarin is administered as a racemic mixture of R- and S-enantiomers, with S-warfarin being approximately three to five times more potent as a vitamin K epoxide reductase inhibitor. S-warfarin is metabolized primarily by cytochrome P450 2C9. Capecitabine is a potent inhibitor of cytochrome P450 2C9, and when added to a stable warfarin regimen it impairs the clearance of S-warfarin, causing plasma S-warfarin concentrations to rise substantially within days to weeks. The result is an unpredictable and potentially dangerous increase in the international normalized ratio that requires weekly or more frequent monitoring. Capecitabine does not inhibit vitamin K epoxide reductase directly, does not affect warfarin-albumin binding through thymidine phosphorylase, and does not meaningfully deplete intestinal vitamin K synthesis.