Pharmacology · Antiviral Pharmacology

Herpesvirus Pharmacology

Mechanisms, resistance, toxicity, and clinical use across herpesvirus agents

Acyclovir's exceptional selectivity rests on a two-step activation requirement — viral thymidine kinase (TK) performs the first phosphorylation 40 to 100 times more efficiently in infected cells than cellular kinases do, concentrating the active triphosphate where it is needed; because CMV lacks viral TK, it is intrinsically resistant to acyclovir, which is why ganciclovir — activated instead by CMV's UL97 phosphotransferase — is the cornerstone of CMV therapy.
Foscarnet and cidofovir bypass both viral TK and CMV UL97 entirely, acting through mechanisms that require no viral enzyme activation — foscarnet binds the pyrophosphate site of viral DNA polymerase directly, while cidofovir is activated by cellular kinases alone; this is why both agents retain activity against TK-deficient acyclovir-resistant HSV and UL97-mutant ganciclovir-resistant CMV.
Acyclovir resistance in HSV/VZV almost always means the TK gene is mutated — and because all TK-dependent agents (acyclovir, valacyclovir, penciclovir, famciclovir) share this activation requirement, TK gene mutation confers simultaneous class-wide cross-resistance; the treatment pivot to foscarnet is therefore not optional but obligate, with cidofovir as the backup if DNA polymerase mutations add partial foscarnet resistance.
CMV resistance testing targets two genes with distinct clinical implications: UL97 mutations confer ganciclovir resistance while leaving foscarnet and cidofovir fully active; UL54 (DNA polymerase) mutations can cross-resist ganciclovir, foscarnet, and cidofovir; when both genes are mutated simultaneously (combined UL97+UL54), maribavir — which inhibits UL97 at a distinct allosteric site — or combination foscarnet plus ganciclovir are the options, and letermovir retains activity regardless because it targets a completely different viral enzyme.
Cidofovir's dose-limiting nephrotoxicity is caused by OAT1-mediated accumulation in proximal tubular cells — probenecid blocks OAT1 and is mandatory before every infusion, combined with saline pre-loading; brincidofovir, the lipid conjugate of cidofovir, avoids OAT1 uptake entirely and is FDA-approved for smallpox with substantially less nephrotoxicity.

Abbreviations: HSV = herpes simplex virus · VZV = varicella-zoster virus · CMV = cytomegalovirus · TK = thymidine kinase · EBV = Epstein-Barr virus · HSCT = hematopoietic stem cell transplant · SOT = solid organ transplant · OAT1 = organic anion transporter 1 · PTLD = post-transplant lymphoproliferative disorder · KSHV = Kaposi sarcoma herpesvirus

Herpesvirus Subfamily Classification and Drug Coverage

Alphaherpesviruses

HSV-1, HSV-2, VZV

Rapid replication cycle; latency established in sensory ganglia
All encode viral TK → acyclovir and related agents are activated with high efficiency in infected cells
Coverage: acyclovir, valacyclovir, famciclovir; foscarnet and cidofovir also active
Resistance primarily from TK gene mutations

Betaherpesviruses

CMV, HHV-6, HHV-7

Slow replication cycle; latency in hematopoietic cells
CMV lacks viral TK → intrinsically resistant to acyclovir and all TK-dependent agents
CMV uses UL97 phosphotransferase to activate ganciclovir
Coverage: ganciclovir, valganciclovir, foscarnet, cidofovir, letermovir, maribavir

Gammaherpesviruses

EBV, KSHV

Lymphotropic; associated with malignant transformation — PTLD (EBV), Kaposi sarcoma (KSHV)
EBV: low TK activity — acyclovir has minimal clinical benefit for EBV disease
No licensed antiviral reliably treats EBV or KSHV disease — management is immunological (reduce immunosuppression, rituximab for PTLD), not antiviral

Mechanism of Action — Key Agents

HSV / VZV Coverage

Acyclovir / Valacyclovir

Step 1 — Viral TKPhosphorylates acyclovir to monophosphate — 40–100× more efficient in virus-infected cells than in uninfected cells
Step 2 — Cellular KinasesConvert monophosphate to active triphosphate
Step 3 — Viral DNA PolymeraseCompetitively inhibited AND chain terminated — acyclovir lacks a 3′-OH on its acyclic sugar, so incorporation is irreversible
Selectivity: 10–30× preference for viral vs. cellular DNA polymerase
Valacyclovir = L-valyl ester prodrug; achieves 3–5× higher plasma levels than oral acyclovir

CMV Coverage

Ganciclovir / Valganciclovir

Step 1 — CMV UL97Phosphotransferase phosphorylates ganciclovir to monophosphate (CMV has no TK)
Step 2 — Cellular KinasesConvert to active triphosphate
Step 3 — CMV UL54Competitively inhibits CMV DNA polymerase; incorporation slows and terminates replication
Lower selectivity than acyclovir (~10×) → clinically significant myelosuppression (neutropenia, thrombocytopenia)
Valganciclovir = oral prodrug equivalent to IV ganciclovir exposure

Salvage — No Viral Enzyme Required

Foscarnet — Pyrophosphate Analogue

Does NOT require viral TK or UL97 for activation — directly active against resistant strains
Binds pyrophosphate-binding site of viral DNA polymerase → blocks pyrophosphate release → halts chain elongation
Broad spectrum: HSV-1, HSV-2, VZV, CMV, HHV-6; active against acyclovir-resistant HSV and ganciclovir-resistant CMV
Nephrotoxicity in up to 30%; electrolyte disturbances (hypocalcemia, hypomagnesemia, hypokalemia)
No myelosuppression — preferred when ganciclovir neutropenia is severe

Broad-Spectrum — Cellular Activation

Cidofovir — Nucleoside Phosphonate

Does NOT require viral TK or UL97 — activated by cellular enzymes alone to cidofovir diphosphate
Broad spectrum: CMV, HSV, VZV, adenovirus, BKPyV, orthopoxviruses (smallpox, mpox)
Dose-limiting nephrotoxicity via OAT1 transporter accumulation in proximal tubular cells
Mandatory protocol before every infusion: probenecid (OAT1 inhibitor) + saline pre-load — never omit
Brincidofovir = lipid conjugate; avoids OAT1 uptake → reduced nephrotoxicity; FDA-approved for smallpox

Resistance Mechanisms and Treatment Pivots

HSV / VZV Resistance

Acyclovir Resistance

Occurs almost exclusively in immunocompromised patients on prolonged acyclovir — suspect with progressive lesions after 5–7 days of adequate IV acyclovir
Primary mechanism: TK gene mutations (TK-null or TK-partial) → acyclovir not phosphorylated
Cross-resistance: ALL TK-dependent agents (acyclovir, valacyclovir, penciclovir, famciclovir) lose activity simultaneously — class-wide failure
Treatment pivot: foscarnet (does not need TK) — first choice
Less common: DNA polymerase mutations → may add partial foscarnet resistance → cidofovir only option

CMV Resistance

Ganciclovir Resistance

Suspect when CMV viral load fails to decline ≥1 log10 after 2 weeks of adequate ganciclovir — obtain UL97 + UL54 genotyping
UL97 Mutations (codons 460, 594, 595)Ganciclovir resistance only; foscarnet and cidofovir remain fully active
UL54 MutationsMay cross-resist ganciclovir, foscarnet, and cidofovir — more limited options
Combined UL97+UL54High-level multidrug resistance → maribavir (inhibits UL97 at distinct allosteric site) or combination foscarnet + ganciclovir
Letermovir: no cross-resistance with any resistance profile above — targets terminase complex

Clinical Priorities — Special Populations

Emergency Protocol

Neonatal HSV

IV acyclovir 20 mg/kg every 8 hours immediately — do not delay for PCR confirmation
Duration by syndrome: SEM disease 14 days; CNS encephalitis or disseminated disease 21 days
Follow with oral acyclovir suppression 300 mg/m² three times daily for 6 months after CNS or disseminated disease — reduces recurrence and improves neurodevelopmental outcomes

Immunocompromised Patients

Threshold for IV Acyclovir

Lower threshold for IV acyclovir in immunocompromised patients with severe or visceral disease
Disseminated HSV/VZV, HSV encephalitis, visceral VZV: IV acyclovir 10–12 mg/kg every 8 hours
Acyclovir-resistant HSV/VZV (progressive lesions despite adequate IV acyclovir 5–7 days): switch to foscarnet IV immediately

Transplant Recipients

CMV Prophylaxis Strategy

SOT donor-positive/recipient-negative (D+/R−, highest risk): universal prophylaxis with valganciclovir for 3–6 months
HSCT CMV-seropositive recipient: letermovir prophylaxis day 0–100 — no myelosuppression, does not compromise engraftment monitoring
Pre-emptive strategy: monitor CMV PCR and treat when viral load exceeds threshold before symptoms develop
Late-onset CMV: occurs after prophylaxis discontinuation — more common with D+/R− SOT; surveillance required

Cidofovir Mandatory Protocol — Never Skip

Every cidofovir infusion requires: probenecid 2 g orally 3 hours before infusion, then 1 g at 2 hours and 8 hours after; 1 liter normal saline IV over 1–2 hours before infusion; serum creatinine and urine protein checked within 48 hours of each dose. Hold cidofovir if creatinine rises 0.3 mg/dL or more above baseline, or if proteinuria reaches 2+ or greater. Cidofovir is absolutely contraindicated when CrCl is below 55 mL/min — use an alternative agent.

Suggested References

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Brunton L, Knollmann B, Hilal-Dandan R, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. — Chapter 56: Antiviral Agents (Non-Retroviral)	McGraw-Hill; 2023
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