Antiviral Pharmacology

Correct Answer

C — An acyclic nucleoside analogue active against herpes simplex virus types 1 and 2 and varicella-zoster virus, but intrinsically inactive against cytomegalovirus

Rationale

Acyclovir is classified as an acyclic nucleoside analogue whose antiviral spectrum encompasses herpes simplex virus type 1, herpes simplex virus type 2, and varicella-zoster virus. Cytomegalovirus lacks the viral thymidine kinase required to phosphorylate acyclovir to its active monophosphate form, making cytomegalovirus intrinsically resistant to acyclovir. Foscarnet is the pyrophosphate analogue active against all human herpesviruses. Cidofovir is the acyclic nucleoside phosphonate. Brincidofovir is the lipid-conjugated prodrug of cidofovir.

Correct Answer

A — Valacyclovir

Rationale

Valacyclovir is the L-valyl ester prodrug of acyclovir. After oral administration it is hydrolyzed rapidly to acyclovir, achieving plasma acyclovir concentrations three to five times higher than oral acyclovir alone. Valganciclovir is the L-valyl ester prodrug of ganciclovir, following the same prodrug chemistry but targeting cytomegalovirus rather than herpes simplex virus and varicella-zoster virus. Brincidofovir is a lipid conjugate prodrug of cidofovir. Famciclovir is a prodrug of penciclovir.

Correct Answer

A — Valganciclovir

Rationale

Valganciclovir is classified as the oral L-valyl ester prodrug of ganciclovir. After oral absorption, intestinal esterases cleave the valyl ester linkage, releasing ganciclovir with bioavailability of approximately 60 percent — far superior to oral ganciclovir. Valacyclovir is the L-valyl ester prodrug of acyclovir. Famciclovir is the diacetyl ester prodrug of penciclovir. Brincidofovir is a lipid conjugate prodrug of cidofovir, representing a structurally distinct prodrug class.

Correct Answer

B — Foscarnet

Rationale

Foscarnet is classified as a pyrophosphate analogue that inhibits viral deoxyribonucleic acid polymerase directly at the pyrophosphate binding site without requiring intracellular phosphorylation. This activation-independent mechanism is what makes foscarnet active against thymidine kinase-deficient acyclovir-resistant herpes simplex virus strains and against UL97-mutant ganciclovir-resistant cytomegalovirus strains. Ganciclovir requires UL97 phosphorylation; acyclovir and cidofovir both require cellular phosphorylation steps, though cidofovir does not require viral enzymes.

Correct Answer

C — Cidofovir

Rationale

Cidofovir is classified as an acyclic nucleoside phosphonate — it is a cytosine nucleotide analogue with a phosphonate group in place of the normal phosphate ester bond, giving it greater metabolic stability. Acyclovir and ganciclovir are acyclic nucleoside analogues (without the preformed phosphonate group) that require viral or cellular kinases for initial phosphorylation. Foscarnet is classified as a pyrophosphate analogue and does not belong to the nucleoside or nucleotide analogue categories at all. Tenofovir and adefovir are the other clinically important acyclic nucleoside phosphonates.

Correct Answer

A — Brincidofovir

Rationale

Brincidofovir is classified as a lipid conjugate prodrug of cidofovir. The ether lipid linkage allows brincidofovir to enter cells via lipid transport pathways, dramatically reducing proximal tubular drug exposure and the severe nephrotoxicity that limits cidofovir use. Valganciclovir is the L-valyl ester prodrug of ganciclovir. Valacyclovir is the L-valyl ester prodrug of acyclovir. Famciclovir is the prodrug of penciclovir. Each of these represents a distinct prodrug class label in the herpesvirus antiviral classification framework.

Correct Answer

D — Viral thymidine kinase in herpes simplex virus-infected cells phosphorylates acyclovir far more efficiently than cellular kinases do, concentrating the active drug in infected cells; cytomegalovirus lacks a viral thymidine kinase entirely and cannot perform this initial phosphorylation

Rationale

Acyclovir selectivity rests on preferential phosphorylation by viral thymidine kinase (TK) encoded by herpes simplex virus and varicella-zoster virus. Viral TK converts acyclovir to acyclovir monophosphate roughly 40 to 100 times more efficiently than cellular kinases, creating a high concentration of active drug specifically within infected cells. Cellular kinases then complete the conversion to acyclovir triphosphate, which inhibits viral deoxyribonucleic acid polymerase. Cytomegalovirus does not encode a viral TK and therefore cannot perform this initial phosphorylation step, explaining cytomegalovirus intrinsic resistance to acyclovir. The other options describe mechanisms that do not reflect acyclovir pharmacology.

Correct Answer

B — Foscarnet does not require any intracellular phosphorylation, and cidofovir is activated by cellular enzymes independently of viral thymidine kinase; neither agent relies on the enzyme that is mutated in acyclovir-resistant strains

Rationale

Acyclovir resistance in clinical practice results predominantly from mutations in viral thymidine kinase (TK) that impair acyclovir phosphorylation. Foscarnet bypasses this entirely — it is a pyrophosphate analogue that inhibits viral deoxyribonucleic acid polymerase directly without requiring any phosphorylation step. Cidofovir is an acyclic nucleoside phosphonate activated by cellular kinases without involvement of viral TK. Because neither agent depends on viral TK for activity, thymidine kinase-null or thymidine kinase-partial mutations have no effect on their susceptibility. This provides a clear therapeutic escape route: when acyclovir fails due to TK mutation, foscarnet (preferred) or cidofovir (alternative) retains full activity against the resistant strain.

Correct Answer

A — Ganciclovir has considerably lower selectivity for virus-infected cells than acyclovir; cellular kinases in rapidly dividing bone marrow cells phosphorylate ganciclovir to inhibitory concentrations, suppressing hematopoietic progenitor proliferation

Rationale

Acyclovir achieves high selectivity because viral thymidine kinase phosphorylates it far more efficiently than cellular kinases, concentrating active drug in infected cells. Ganciclovir relies on cytomegalovirus UL97 phosphotransferase for initial phosphorylation, but cellular kinases also phosphorylate ganciclovir at a rate sufficient to produce inhibitory concentrations in rapidly dividing uninfected cells — particularly hematopoietic progenitor cells in bone marrow. This lower selectivity is the mechanistic basis for ganciclovir myelosuppression. Neutropenia is the most common and dose-limiting manifestation, often requiring granulocyte colony-stimulating factor support or dose reduction during prolonged therapy.

Correct Answer

C — Foscarnet accumulates in renal tubular cells and causes direct tubular toxicity; saline prehydration increases urine flow and reduces tubular foscarnet concentration, limiting proximal tubular exposure

Rationale

Foscarnet nephrotoxicity results from accumulation of the drug in renal proximal tubular cells, producing direct tubular toxicity. Vigorous saline prehydration before each infusion increases urine flow rate, dilutes the tubular foscarnet concentration, and reduces the duration of proximal tubular exposure during drug elimination. This substantially reduces the risk of nephrotoxicity and is a mandatory component of foscarnet administration. Foscarnet also chelates ionized calcium (option D has some basis) but the primary nephrotoxicity mechanism is direct tubular toxicity, not hypocalcemia-mediated reduction in glomerular filtration. Hypocalcemia is an electrolyte complication of foscarnet requiring separate monitoring.

Correct Answer

D — Probenecid inhibits the organic anion transporters in proximal tubular cells that actively take up cidofovir from the tubular lumen and peritubular capillaries, reducing cidofovir accumulation at the site of toxicity

Rationale

Cidofovir nephrotoxicity is caused by concentration of the drug in renal proximal tubular cells, where it impairs mitochondrial function and causes tubular apoptosis and progressive renal failure. Proximal tubular cidofovir concentrations are driven by active uptake via organic anion transporters present in both the luminal and basolateral membranes. Probenecid, a classic inhibitor of organic anion transporters, blocks this active uptake and substantially reduces intracellular cidofovir accumulation in proximal tubular cells, lowering the nephrotoxicity risk. Despite mandatory probenecid and saline preloading, serum creatinine must be checked before every dose and cidofovir must be withheld if creatinine rises above the defined threshold.

Correct Answer

B — UL97 mutations impair ganciclovir phosphorylation to its monophosphate form, preventing conversion to the active triphosphate; foscarnet does not require phosphorylation and therefore remains fully active

Rationale

Ganciclovir requires UL97 phosphotransferase to catalyze the first phosphorylation step — conversion of ganciclovir to ganciclovir monophosphate. UL97 resistance mutations reduce the efficiency of this step, impairing the generation of active ganciclovir triphosphate in cytomegalovirus-infected cells. Because foscarnet is a pyrophosphate analogue that inhibits viral deoxyribonucleic acid polymerase directly without any phosphorylation requirement, UL97 mutations have no effect on foscarnet activity. Cidofovir, which is activated by cellular enzymes rather than UL97, also retains full activity against UL97-mutant strains. UL54 mutations — in the cytomegalovirus deoxyribonucleic acid polymerase gene — are required to produce resistance to foscarnet or cidofovir and may cause cross-resistance among agents.

Correct Answer

A — Intravenous acyclovir at 20 mg per kilogram every 8 hours for 14 days, followed by oral acyclovir suppression for 6 months to reduce neurological sequelae

Rationale

All three forms of neonatal herpes simplex virus disease — skin, eye, and mouth (SEM) disease; encephalitis; and disseminated disease — require intravenous acyclovir at 20 mg per kilogram every 8 hours. This high-dose intravenous regimen is mandatory even for SEM disease, which may appear limited but carries risk of progression and neurological sequelae without treatment. Duration is syndrome-dependent: 14 days for SEM disease and 21 days for encephalitis or disseminated disease. Following intravenous therapy, all forms are treated with 6 months of oral acyclovir suppression at 300 mg per square meter three times daily, which reduces neurological deterioration and HSV recurrences. Oral therapy alone or topical treatment is not appropriate for any form of neonatal HSV.

Correct Answer

C — The transplanted organ carries latent cytomegalovirus in hematopoietic and endothelial cells; the seronegative recipient has no prior cytomegalovirus-specific immunity, so reactivation driven by transplant-related inflammation goes unchecked by cellular immune memory

Rationale

Cytomegalovirus establishes lifelong latency in hematopoietic progenitor cells, monocytes, and endothelial cells. A seropositive donor organ carries this latent virus. In a seronegative recipient, transplant-related inflammatory signals and the required immunosuppression trigger cytomegalovirus reactivation from the donor organ. Because the recipient has never been exposed to cytomegalovirus, there is no pre-existing cytomegalovirus-specific T cell memory or antibody response to contain reactivation. The result is primary cytomegalovirus infection in an immunosuppressed host — the highest-risk scenario. Universal prophylaxis with valganciclovir for 3 to 6 months after transplantation is standard of care for D+/R− recipients, reducing CMV disease incidence from 50 to 80 percent to below 10 percent.

Correct Answer

B — Foscarnet, because it inhibits viral deoxyribonucleic acid polymerase directly without requiring thymidine kinase for activation and therefore remains fully active against thymidine kinase-null strains

Rationale

Foscarnet is the first-line treatment for acyclovir-resistant herpes simplex virus caused by thymidine kinase mutations. Because foscarnet is a pyrophosphate analogue that inhibits viral deoxyribonucleic acid polymerase directly at the pyrophosphate binding site — without requiring any intracellular phosphorylation — it retains full activity against thymidine kinase-null and thymidine kinase-partial mutants. Increasing the acyclovir or valacyclovir dose cannot overcome thymidine kinase-null resistance because the enzyme required for the first phosphorylation step is absent. Ganciclovir requires cytomegalovirus UL97, not present in herpes simplex virus. Cidofovir is an alternative to foscarnet in this setting; brincidofovir delivers cidofovir's active metabolite but does not restore thymidine kinase function.

Correct Answer

D — Switch to foscarnet, because UL97 mutations impair ganciclovir phosphorylation but do not affect foscarnet, which inhibits viral deoxyribonucleic acid polymerase without requiring phosphorylation

Rationale

UL97 mutations confer ganciclovir resistance by reducing UL97 phosphotransferase efficiency, impairing conversion of ganciclovir to its active monophosphate and triphosphate forms. Increasing the ganciclovir dose or switching to valganciclovir cannot overcome this because both depend on UL97 for initial phosphorylation. Foscarnet inhibits cytomegalovirus deoxyribonucleic acid polymerase directly at the pyrophosphate binding site without any phosphorylation requirement, and UL97 mutations therefore have no effect on foscarnet activity. Cidofovir also retains activity against UL97-mutant strains and is an alternative, though it carries its own nephrotoxicity burden. The appropriate management of confirmed UL97-only resistance is to switch from ganciclovir-based therapy to foscarnet or cidofovir while continuing to monitor for UL54 mutations that could compromise these agents.

Correct Answer

A — Acyclovir and valacyclovir are classified as safe in pregnancy based on animal data and extensive human registry experience; treatment is indicated for primary genital herpes simplex virus in pregnancy

Rationale

Acyclovir and valacyclovir are classified as safe for use throughout pregnancy. The Acyclovir in Pregnancy Registry found no increase in birth defects among over 1,800 first-trimester exposures, and current guidelines support their use for primary genital herpes simplex virus, varicella-zoster virus pneumonia, and disseminated herpesvirus infections in pregnancy. Suppressive valacyclovir from 36 weeks of gestation reduces herpes simplex virus shedding and recurrent lesions at delivery, lowering the rate of cesarean delivery performed for active herpes simplex virus disease. Ganciclovir is not safe in pregnancy and is not used for herpes simplex virus infections. Withholding all antiviral therapy would be inappropriate given established drug safety and the clinical risks of untreated primary herpes simplex virus in pregnancy.

Correct Answer

C — Serum creatinine must be checked before every cidofovir dose; a rise to 1.5 mg/dL in a patient with baseline 0.9 mg/dL exceeds the threshold for withholding the dose due to cidofovir proximal tubular toxicity

Rationale

Cidofovir causes dose-dependent nephrotoxicity by accumulating in renal proximal tubular cells and impairing mitochondrial function. Serum creatinine must be measured before every dose; cidofovir must be withheld if creatinine rises by 0.3 to 0.4 mg/dL above baseline or exceeds a defined threshold. A rise from 0.9 to 1.5 mg/dL represents an increase of 0.6 mg/dL — well above the withholding threshold. Despite mandatory probenecid and saline preloading, progressive renal toxicity can still occur. When cidofovir is withheld due to creatinine rise, the dose is permanently reduced for subsequent administrations if and when therapy resumes. A creatinine rise is not a marker of resistance and does not indicate failure of antiviral efficacy.