Antiherpesvirus Agents — Questions

a. Cytomegalovirus (CMV) disease, here CMV hepatitis; the owl’s-eye inclusions are its cytopathic hallmark.

b. First-line is ganciclovir (intravenous) or oral valganciclovir, with the full blood count monitored for neutropenia. When disease is refractory or resistant (the viral load fails to fall, with a confirmed UL97 mutation), switch to foscarnet or cidofovir, or to maribavir for resistant disease; reducing immunosuppression where possible aids control.

a. Oral valganciclovir for 6 months (intravenous ganciclovir if oral therapy is not tolerated). Treating central-nervous-system-involved symptomatic disease improves hearing and neurodevelopmental outcomes.

b. Neutropenia; the full blood count is monitored through the course.

c. Asymptomatic infants, and those with isolated hearing loss without other disease, are generally not treated, because the benefit does not outweigh the toxicity; the decision is specialist-led.

• Mechanism. Cidofovir is an acyclic nucleotide (phosphonate) analogue that already carries the equivalent of a first phosphate, so cellular kinases alone activate it to the diphosphate, which inhibits the viral DNA polymerase as a delayed chain terminator. Its long intracellular half-life allows dosing only once every week or two.
• Activity against resistant CMV. Because it needs no viral kinase, it stays active against UL97-mutant ganciclovir-resistant CMV, although UL54 polymerase mutations can confer cross-resistance.
• Limiting toxicity. Its dose-limiting nephrotoxicity is reduced by giving probenecid with intravenous saline hydration to cut proximal-tubular uptake of the drug.

• Mechanism. Letermovir inhibits the CMV terminase complex (UL56, UL89 and UL51), which cleaves and packages replicated viral DNA into capsids; the target is unique to cytomegalovirus.
• Place in therapy. Oral and well tolerated, used for CMV prophylaxis (notably in seropositive allogeneic stem-cell transplant recipients), with no cross-resistance to the polymerase-targeting agents.
• Caveat. A low genetic barrier (UL56 resistance) makes it a prophylactic rather than a treatment agent, and it inhibits CYP3A (interaction with cyclosporine).

• Activation. Aciclovir is an inert prodrug until the viral thymidine kinase of HSV (herpes simplex virus) or VZV (varicella-zoster virus) monophosphorylates it; cellular kinases then add the second and third phosphates to make aciclovir triphosphate. Host thymidine kinase barely activates it, so active drug concentrates only where the virus is replicating.
• Inhibition. Aciclovir triphosphate inhibits the viral DNA polymerase far more strongly than host polymerases and, lacking a 3’-hydroxyl, acts as an obligate chain terminator that freezes the enzyme in a dead-end complex.
• Selectivity. The two layers, selective activation by a viral kinase and selective inhibition of the viral polymerase, together make aciclovir toxic to the virus but not to the host.
• Side effects. Aciclovir is very well tolerated. The main concern is crystalluria with renal injury at high intravenous doses (prevented by good hydration and slow infusion), with rare neurotoxicity in renal impairment; it is not marrow-toxic at ordinary doses.

• Mechanism. Foscarnet is a pyrophosphate analogue, not a nucleoside analogue. It inhibits the viral DNA polymerase directly, binding the pyrophosphate-binding site and blocking release of pyrophosphate during chain elongation, and it is not incorporated into the DNA.
• No activation step. It requires no intracellular phosphorylation by a viral kinase, so it stays active against thymidine-kinase-deficient (aciclovir-resistant) HSV (herpes simplex virus) and VZV (varicella-zoster virus) and UL97-mutant (ganciclovir-resistant) CMV. This is why it is the principal second-line agent for resistant herpesvirus disease.
• Toxicities. Nephrotoxicity is the most frequent. It also causes electrolyte disturbance, notably hypocalcaemia from chelation of ionised calcium (paraesthesia, tetany, seizures), with hypomagnesaemia and hypokalaemia, plus painful genital ulceration. It is given intravenously only.

• Thymidine kinase mutations (commonest). Loss of the enzyme (TK-negative), reduced production (TK-partial), or altered substrate specificity (TK-altered) prevent the first phosphorylation; TK-negative is the usual mechanism.
• DNA polymerase mutations (less common). Changes in the UL30 polymerase reduce binding of the drug.
• Context and consequence. Resistance is mainly a problem in the immunocompromised (advanced HIV, allogeneic transplant). TK-deficient isolates remain susceptible to foscarnet and cidofovir, which need no viral kinase.

A complete answer covers the first-line nucleoside analogues, the agents for resistant disease, the newer class, and the resistance mechanism.

First-line. Aciclovir, its better-absorbed prodrug valaciclovir, and famciclovir (the penciclovir prodrug) are activated by the viral thymidine kinase and terminate viral DNA synthesis.

Resistant disease. Thymidine-kinase-deficient, aciclovir-resistant HSV, seen mainly in the immunocompromised, is treated with foscarnet or cidofovir, which need no viral kinase.

Newer option. The helicase-primase inhibitors (pritelivir, amenamevir) act on a different target and retain activity against aciclovir- and foscarnet-resistant HSV.

Aciclovir resistance. Most resistance is loss or alteration of the viral thymidine kinase, removing the activation step shared by aciclovir and famciclovir; DNA polymerase mutations are a less common route.

Herpes simplex encephalitis is a medical emergency, so intravenous aciclovir is started empirically the moment it is suspected, because delay worsens outcome and the drug is safe; the standard course is 10 mg/kg every 8 hours for 14 to 21 days.

Waiting for PCR loses critical time, oral therapy gives inadequate central nervous system levels, and ganciclovir and foscarnet are neither first-line for HSV nor free of greater toxicity.

Cytomegalovirus lies beyond aciclovir’s reach because it makes no thymidine kinase to activate the drug, relying instead on the UL97 kinase that ganciclovir exploits.

Aciclovir is active, to a decreasing degree, against HSV-1, then HSV-2, then VZV.

Thymidine-kinase-deficient HSV is cross-resistant to the kinase-dependent drugs, but foscarnet needs no viral kinase and stays active (as do cidofovir and pritelivir).

Valaciclovir, famciclovir, penciclovir and ganciclovir all depend on a viral kinase and are escaped.

A brivudine metabolite irreversibly inhibits dihydropyrimidine dehydrogenase, the enzyme that clears 5-fluorouracil, so co-administration causes fatal fluoropyrimidine toxicity, and the two must be separated by several weeks.

The other classes have no comparable interaction with brivudine.

Foscarnet chelates ionised calcium, and co-administered pentamidine potentiates the resulting hypocalcaemia, which can cause paraesthesia, tetany or seizures.

The other agents listed do not share this interaction.

Brincidofovir is a lipid-ester prodrug of cidofovir, well absorbed orally and, by not concentrating in the renal tubule, largely free of the nephrotoxicity that limits cidofovir (its dose-limiting toxicity is diarrhoea instead).

It shares cidofovir’s mechanism and broad DNA-virus activity, and is now licensed for smallpox rather than a herpesvirus.

Letermovir is a moderate CYP3A inhibitor, so it raises levels of the calcineurin inhibitors, particularly cyclosporine (and tacrolimus and several statins), requiring monitoring and dose adjustment.

The other interactions listed are not features of letermovir.

Trifluridine is phosphorylated by cellular rather than viral kinases, so it does not depend on the viral thymidine kinase and stays active against thymidine-kinase-deficient, aciclovir-resistant HSV.

It does not act on the terminase or on viral entry, and it is not selectively activated by the virus.

Intravenous ganciclovir is preferred when gut absorption cannot be relied on (vomiting, gut graft-versus-host disease, critical illness) or when disease is severe or sight-threatening; oral valganciclovir suits stable patients and outpatient prophylaxis.

Ganciclovir is not obsolete; only the old oral ganciclovir capsule was superseded. A UL97 mutation calls for a different drug rather than a different route, and prophylaxis is usually oral.

Aciclovir lacks the 3’-hydroxyl needed to add the next nucleotide, so once it is incorporated DNA synthesis halts absolutely and the polymerase is frozen in a dead-end complex.

Ganciclovir, penciclovir and cidofovir retain a hydroxyl equivalent and are delayed (non-obligate) terminators that allow a few further bases before stalling; foscarnet is a pyrophosphate analogue that is not incorporated into the DNA at all.

Ganciclovir and zidovudine are both myelosuppressive, so together they cause severe additive neutropenia, often precluding combined use without growth-factor support.

Foscarnet adds nephrotoxicity rather than marrow toxicity, and the other pairings do not share this effect.

• UL97 (commoner). Mutations impair the phosphotransferase that activates ganciclovir. UL97-mutant CMV stays susceptible to foscarnet and cidofovir (which need no kinase) and to maribavir.
• UL54 (DNA polymerase). Mutations can confer cross-resistance across ganciclovir, cidofovir and sometimes foscarnet; combined with a UL97 mutation they give high-level resistance.
• Switching. Confirm by genotyping the UL97 and UL54 genes. Letermovir (terminase) and maribavir (UL97 kinase) act at targets the polymerase mutations do not affect.

• Mechanism. Maribavir is a direct competitive inhibitor of the CMV (cytomegalovirus) UL97 protein kinase; it is not phosphorylated and does not act on the DNA polymerase. Blocking UL97 impairs viral DNA synthesis and the nuclear egress of new capsids.
• Indication. Oral treatment of post-transplant CMV refractory to or resistant against ganciclovir, valganciclovir, foscarnet and cidofovir.
• Interaction with ganciclovir. Ganciclovir needs UL97 to be activated, and maribavir inhibits UL97, so maribavir antagonises ganciclovir and the two should not be combined.

• Indication. Treatment and prevention of cytomegalovirus (CMV) disease in the immunocompromised (retinitis, gastrointestinal disease, transplant CMV), and human herpesvirus 6 disease.
• Mechanism. Monophosphorylated in CMV-infected cells by the viral UL97 protein kinase (not a thymidine kinase); cellular kinases complete the triphosphate, which inhibits the viral DNA polymerase as a delayed chain terminator.
• Principal toxicity. Bone-marrow suppression, above all neutropenia (and thrombocytopenia), requiring full-blood-count monitoring; additive with zidovudine.
• Dosing. Intravenous 5 mg/kg twice daily for induction; oral valganciclovir gives equivalent exposure for maintenance and less severe disease.

• Indication. HSV (herpes simplex virus) disease (genital herpes, herpes labialis, suppression) and VZV (varicella-zoster virus) disease (herpes zoster), and a step-down option in some severe presentations.
• Mechanism. The L-valyl ester prodrug of aciclovir, hydrolysed after absorption to aciclovir; the mechanism is then identical, with thymidine-kinase activation and obligate chain termination.
• Reason for use. Much better oral bioavailability, three to five times that of aciclovir, allowing less frequent dosing.

• Mechanism. Famciclovir is the oral prodrug of penciclovir, which the viral thymidine kinase activates; penciclovir triphosphate inhibits the viral DNA polymerase as a non-obligate (delayed) chain terminator and has a long intracellular half-life.
• Spectrum. HSV (herpes simplex virus) and VZV (varicella-zoster virus); it shares aciclovir’s thymidine-kinase dependence and therefore its resistance.
• Side effects. Very well tolerated, with headache and nausea the main complaints.

A complete answer covers the polymerase-targeting agents, the two newer agents with novel targets, and the resistance logic that selects between them.

First-line. Ganciclovir and its oral prodrug valganciclovir are first-line. They are activated by the viral UL97 kinase, and their dose-limiting toxicity is bone-marrow suppression.

Agents for resistant or intolerant disease. Foscarnet (a pyrophosphate analogue) and cidofovir (a nucleotide analogue) need no viral kinase, so both remain active against UL97-mutant ganciclovir-resistant CMV; both are nephrotoxic.

Newer agents. Letermovir inhibits the viral terminase and is used for prophylaxis, with no cross-resistance to the polymerase drugs but a low resistance barrier. Maribavir inhibits the UL97 kinase and is used for refractory or resistant disease, and must not be combined with ganciclovir.

Resistance-directed choice. UL97 mutations (commonest) retain foscarnet, cidofovir and maribavir; UL54 polymerase mutations can cross-resist the polymerase agents but not letermovir or maribavir. Genotyping the UL97 and UL54 genes guides the switch.