Anti-Cancer Drugs — Part 1

Correct Answer

A — Nitrogen mustard

Rationale

Cyclophosphamide is classified as a nitrogen mustard alkylating agent. Nitrogen mustards generate aziridinium ion intermediates that alkylate deoxyribonucleic acid and include cyclophosphamide, ifosfamide, mechlorethamine, melphalan, chlorambucil, and bendamustine. Platinum compounds (cisplatin, carboplatin, oxaliplatin) form platinum-deoxyribonucleic acid adducts after aquation. Nitrosoureas (carmustine, lomustine) are highly lipophilic agents with central nervous system penetration. Alkyl sulfonates (busulfan) have selectivity for hematopoietic stem cells.

Correct Answer

C — Platinum compound

Rationale

Cisplatin is classified as a platinum compound. Platinum compounds form covalent platinum-deoxyribonucleic acid adducts after aquation and include cisplatin, carboplatin, and oxaliplatin. Nitrogen mustards (cyclophosphamide, ifosfamide) generate aziridinium ion intermediates. Nitrosoureas (carmustine, lomustine) are lipophilic agents used for central nervous system tumors. Triazenes (temozolomide, dacarbazine) generate methylating species after metabolic or spontaneous activation.

Correct Answer

B — Triazene

Rationale

Temozolomide is classified as a triazene (specifically an imidazotetrazine) that generates a methylating species through spontaneous hydrolysis at physiological pH. The triazene and hydrazine subgroup, which also includes dacarbazine and procarbazine, requires metabolic or spontaneous activation to produce methylating intermediates. Nitrogen mustards generate aziridinium ions. Nitrosoureas (carmustine, lomustine) are lipophilic central nervous system-penetrating alkylating agents. Platinum compounds form platinum-deoxyribonucleic acid adducts after aquation.

Correct Answer

D — Nitrosourea

Rationale

Carmustine is classified as a nitrosourea. Nitrosoureas, which also include lomustine and streptozocin, are distinguished by their high lipophilicity enabling passive diffusion across the blood-brain barrier, and by their characteristically delayed and prolonged myelosuppression nadir at 4 to 6 weeks. Alkyl sulfonates (busulfan) have selectivity for hematopoietic stem cells. Platinum compounds (cisplatin, carboplatin, oxaliplatin) form platinum-deoxyribonucleic acid adducts. Nitrogen mustards (cyclophosphamide, ifosfamide) generate aziridinium intermediates.

Correct Answer

A — Alkyl sulfonate

Rationale

Busulfan is classified as an alkyl sulfonate. The alkyl sulfonates have particular selectivity for hematopoietic stem cells, which is the basis for busulfan's primary clinical use as a myeloablative conditioning agent before hematopoietic stem cell transplantation. Nitrosoureas (carmustine, lomustine) penetrate the central nervous system and produce delayed myelosuppression. Nitrogen mustards (cyclophosphamide, ifosfamide, melphalan) generate aziridinium intermediates. Triazenes (temozolomide, dacarbazine) generate methylating species.

Correct Answer

C — Uroprotective thiol agent

Rationale

Mesna (sodium 2-mercaptoethane sulfonate) is classified as a uroprotective thiol agent. It is administered alongside ifosfamide and high-dose cyclophosphamide to prevent hemorrhagic cystitis. Mesna itself has no antitumor activity and is not an alkylating agent, a colony-stimulating factor, or an antiemetic. Granulocyte colony-stimulating factors (filgrastim, pegfilgrastim) prevent febrile neutropenia. Neurokinin-1 receptor antagonists (aprepitant) prevent delayed-phase chemotherapy-induced nausea.

Correct Answer

B — Hepatic cytochrome P450 2B6 oxidizes cyclophosphamide to 4-hydroxycyclophosphamide, which decomposes to yield phosphoramide mustard as the active alkylating species and acrolein as the toxic urinary byproduct

Rationale

Cyclophosphamide is an inactive prodrug requiring hepatic oxidation, principally by cytochrome P450 2B6, to generate 4-hydroxycyclophosphamide. This intermediate spontaneously decomposes in tissues to yield two products: phosphoramide mustard, the bifunctional alkylating species responsible for antitumor activity, and acrolein, a reactive aldehyde excreted unchanged in urine that accumulates in the bladder lumen and causes direct urothelial cytotoxicity. The site of activation is hepatic, not renal or intestinal, and acrolein is generated as an obligate byproduct of the same decomposition step that produces the active drug.

Correct Answer

D — Mesna contains a free thiol group that reacts with acrolein in the urinary tract to form a stable, non-toxic conjugate that is excreted renally

Rationale

Mesna (sodium 2-mercaptoethane sulfonate) exerts its uroprotective effect through a direct chemical reaction in the urinary tract. Its free thiol group undergoes nucleophilic addition to acrolein, forming a stable and non-toxic thioether conjugate that is excreted in the urine without causing urothelial injury. This reaction occurs in the bladder lumen after mesna is filtered and excreted, not systemically. Mesna does not alkalinize urine, does not inhibit cytochrome P450 2B6, and has no prostaglandin-related mechanism; it is purely a chemical scavenger of the toxic acrolein byproduct.

Correct Answer

A — Cisplatin causes dose-dependent tubular injury; saline diuresis dilutes urinary cisplatin concentrations and reduces tubular exposure, while magnesium replacement corrects the renal magnesium wasting that results from tubular damage

Rationale

Cisplatin nephrotoxicity is dose-dependent and cumulative, caused by direct tubular injury that reduces glomerular filtration rate and impairs tubular transport functions. Pre-hydration with at least 1 to 2 liters of normal saline before cisplatin administration dilutes urinary cisplatin concentrations, promotes diuresis, and reduces the duration of tubular exposure to the drug. Renal tubular injury specifically impairs magnesium reabsorption in the thick ascending limb, producing obligatory urinary magnesium wasting that persists between cycles; supplemental magnesium must therefore be given with every cycle regardless of serum levels. Cisplatin does not precipitate in acidic urine, does not displace protein binding through saline competition, and does not activate the renin-angiotensin-aldosterone system as its primary toxicity mechanism.

Correct Answer

C — Carboplatin is eliminated almost exclusively by renal filtration, so glomerular filtration rate is the primary determinant of drug exposure; the Calvert formula uses target area under the curve multiplied by the sum of glomerular filtration rate and 25 to achieve predictable dosing

Rationale

Carboplatin is eliminated almost entirely by renal glomerular filtration. Because individual variation in renal function determines how quickly the drug is cleared, a fixed body surface area-based dose produces widely varying drug exposures — underdosing patients with high glomerular filtration rates and overdosing those with impaired renal function. The Calvert formula directly addresses this by using target area under the curve as the dosing objective: dose (mg) = target area under the curve × (glomerular filtration rate + 25). The constant 25 accounts for non-renal clearance. This approach ties the dose to individual renal function and achieves predictable thrombocytopenic toxicity and antitumor exposure regardless of body habitus or renal status variation. Carboplatin does not undergo significant hepatic metabolism and its protein binding does not determine dose.

Correct Answer

B — Nitrosoureas produce myelosuppression with a nadir at 4 to 6 weeks rather than the 10 to 14 days seen with most cytotoxic drugs; administering a second cycle before the nadir resolves risks overlapping suppressions that can cause aplasia

Rationale

The defining pharmacotoxicological feature of nitrosoureas is delayed and prolonged myelosuppression. Unlike most cytotoxic agents whose myelosuppressive nadir occurs 10 to 14 days after administration, nitrosoureas produce a nadir at 4 to 6 weeks, with full marrow recovery requiring 6 to 8 weeks. The mechanism is not fully characterized but relates to the alkylation of slowly dividing hematopoietic progenitors. Administering a subsequent cycle on a standard 3-week schedule would deliver the second dose before the first cycle's nadir has occurred, producing cumulative overlapping suppressions that can cause life-threatening aplasia. The 6-week minimum interval is determined by this toxicity kinetic, not by drug elimination half-life, central nervous system retention, or deoxyribonucleic acid repair timelines.

Correct Answer

D — An unmethylated promoter allows robust O6-methylguanine-deoxyribonucleic acid methyltransferase expression; the enzyme directly reverses O6-guanine alkylation in a stoichiometric repair reaction, removing the lesions that would otherwise cause cell death

Rationale

Temozolomide kills tumor cells by methylating deoxyribonucleic acid at the O6 position of guanine, producing lesions that trigger futile mismatch repair cycles and ultimately cause double-strand breaks and apoptosis. O6-methylguanine-deoxyribonucleic acid methyltransferase is a repair enzyme that directly and stoichiometrically reverses O6-guanine alkylation, consuming one enzyme molecule per repair event. When the O6-methylguanine-deoxyribonucleic acid methyltransferase gene promoter is unmethylated, the gene is expressed at normal or high levels, the protein is abundant, and O6-guanine lesions are efficiently repaired before they can cause lethal deoxyribonucleic acid damage. Promoter methylation silences the gene, eliminates this repair capacity, and sensitizes the tumor. The mechanism is direct enzymatic reversal of the drug's primary cytotoxic lesion — not efflux, not mismatch repair, and not a change in drug activation rate.

Correct Answer

A — Methylene blue intravenously — it corrects the mitochondrial dysfunction caused by the neurotoxic ifosfamide metabolite chloroacetaldehyde

Rationale

Ifosfamide encephalopathy is caused by chloroacetaldehyde, a neurotoxic metabolite generated during ifosfamide side-chain oxidation. Chloroacetaldehyde disrupts mitochondrial function in neurons, impairing cellular energy production. Methylene blue is given intravenously to correct this mitochondrial dysfunction — it acts as an electron carrier that bypasses the block and restores energy metabolism in affected neurons. It is administered at 50 mg every 4 to 8 hours until symptoms resolve, and is also used prophylactically in subsequent cycles when encephalopathy has previously occurred. N-acetylcysteine is used for ifosfamide-induced hemorrhagic cystitis prevention as a mucosal thiol donor, not for encephalopathy. Flumazenil reverses benzodiazepine sedation and does not address chloroacetaldehyde toxicity. Sodium thiosulfate and bicarbonate are used for cisplatin nephrotoxicity rescue and cyanide poisoning, not for ifosfamide encephalopathy.

Correct Answer

C — Oxaliplatin produces structurally distinct platinum-strand adducts that are not recognized by mismatch repair proteins, allowing cytotoxicity to proceed independently of mismatch repair status

Rationale

Cisplatin and carboplatin form platinum-strand adducts that are recognized by mismatch repair proteins. In cells with intact mismatch repair, the proteins bind these adducts and ultimately signal apoptosis; in mismatch repair-deficient cells, the adducts are not recognized and the apoptotic signal is not generated, producing resistance. Oxaliplatin forms structurally distinct platinum-strand adducts that the mismatch repair machinery does not recognize, so its cytotoxicity does not depend on mismatch repair recognition — oxaliplatin kills both mismatch repair-proficient and mismatch repair-deficient cells. This difference explains the clinical utility of oxaliplatin in colorectal cancer, a tumor type with frequent mismatch repair deficiency. Differential transporter expression, glutathione inactivation selectivity, and active transport upregulation do not account for this distinction.

Correct Answer

B — Mesna must reach the bladder lumen at sufficient concentration to conjugate acrolein; inadequate hydration reduced urinary mesna concentration below the level needed to protect the bladder epithelium

Rationale

Mesna protects the bladder by conjugating acrolein in the urine before it can contact and damage the urothelium. For this mechanism to work, mesna must be present in the bladder lumen at a high enough concentration to neutralize the acrolein being excreted. Adequate hydration serves two purposes: it dilutes acrolein in the urine, reducing its contact concentration with the bladder wall, and it maintains urinary flow that delivers mesna to the bladder in sufficient volume. When hydration is severely reduced, urine production falls and the mesna reaching the bladder is insufficient to conjugate all the acrolein present, allowing bladder injury to occur. This is why vigorous hydration is an essential component of the protective regimen alongside mesna — mesna alone at reduced urinary volume is not reliably protective. Plasma acrolein concentration is not the critical variable, dehydration does not cause mesna accumulation and inactivation in blood, and acrolein does not concentrate preferentially in the bladder wall.

Correct Answer

D — Nitrosoureas produce delayed myelosuppression with a nadir at 4 to 6 weeks after dosing, substantially later than the 10 to 14 day nadir seen with most other cytotoxic agents, because they damage early hematopoietic progenitors that take weeks to deplete the mature cell pool

Rationale

The timing of myelosuppression after chemotherapy depends on which level of the hematopoietic hierarchy is affected. Most cytotoxic agents damage actively dividing precursors in the mid-to-late progenitor compartment; because these cells are days to a week from maturity, the blood count nadir appears at 10 to 14 days. Nitrosoureas — including lomustine and carmustine — are unusual in that they damage early hematopoietic progenitors and possibly hematopoietic stem cells. Because early progenitors must progress through multiple cell divisions before generating mature blood cells, the depletion of circulating cells takes considerably longer to manifest, producing a nadir at 4 to 6 weeks. This delayed nadir is the pharmacological rationale for the minimum 6-week interval between nitrosourea cycles — a shorter interval risks dosing during active marrow suppression from the previous cycle. Delayed hepatic activation, fat tissue accumulation and slow release, and selective late progenitor targeting do not explain the characteristic delayed nadir of this drug class.

Correct Answer

A — Oxaliplatin forms platinum-strand adducts that are not recognized by mismatch repair proteins, so its cytotoxicity is preserved in microsatellite instability-high tumors where cisplatin loses activity

Rationale

Mismatch repair proteins recognize the platinum-strand adducts formed by cisplatin and carboplatin as part of the signaling process that ultimately triggers apoptosis. When mismatch repair is absent — as in microsatellite instability-high tumors — cisplatin adducts are not recognized, the apoptotic signal is not generated, and the tumor is resistant. Oxaliplatin forms structurally distinct adducts that the mismatch repair machinery does not recognize, which means cytotoxicity is generated through a mismatch repair-independent pathway and is preserved regardless of mismatch repair status. This mechanistic distinction is the pharmacological basis for preferring oxaliplatin in mismatch repair-deficient colorectal cancer. Nephrotoxicity differences, mucositis interactions, and differential transporter efflux are not the explanation for this clinical preference in microsatellite instability-high disease.

Correct Answer

C — Cisplatin and carboplatin form identical platinum-deoxyribonucleic acid adducts, so resistance mechanisms confer complete cross-resistance; additionally, the curative outcomes in germ cell tumors are established specifically for cisplatin-containing regimens and have not been replicated with carboplatin substitution

Rationale

Cisplatin and carboplatin both form N7-guanine platinum-deoxyribonucleic acid adducts — the adducts are chemically identical, differing only in their carrier ligand. Because the cytotoxic lesions are the same, resistance mechanisms that arise against one agent confer complete cross-resistance to the other. Beyond cross-resistance, the landmark cure rates for metastatic testicular germ cell tumors exceeding 80% even in disseminated disease were established with BEP using cisplatin; prospective data demonstrating equivalent curative outcomes with carboplatin substitution do not exist. In curative-intent settings, substitution based on toxicity preference rather than equivalent efficacy data is not acceptable. Carboplatin is not limited by tissue distribution, does not have a shorter half-life than cisplatin in the relevant pharmacokinetic sense, and is not a prodrug requiring hepatic activation.