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Antiherpetic Drugs
For medical students2 min readUpdated 2026-10-10
Antiherpetic drugs are synthetic pharmacological agents derived from natural nucleosides. They act as "false" substrates that block herpes virus replication through structural similarity and competition during DNA synthesis.
Natural BaseSynthesis is based on deoxyguanosine and deoxycytidine.
NucleosidesAcyclovir, valacyclovir, and ganciclovir featuring an open carbohydrate chain.
NucleotidesCidofovir already contains a preformed phosphoryl group.
TargetInhibition of replication and viral DNA chain termination.
Chemical Structure and Origin
The development of antiviral agents is based on natural compounds — deoxyguanosine and deoxycytidine. These natural substrates consist of a nitrogenous base and a closed deoxyribose ring, forming the foundation for DNA synthesis. Scientists used them to create various pharmacological agents capable of interfering with viral life cycles.
Classification of Antiherpetic Drugs
Modern antiherpetic agents are divided into several categories depending on their structural features:
Acyclic nucleosides: Acyclovir, Valacyclovir, and Ganciclovir. They are synthetic analogs of purine nucleosides (primarily guanine), but with a key difference: instead of a complete sugar ring, they have an open (acyclic) carbohydrate chain. Valacyclovir is a special ester modification of acyclovir with the addition of a valine group.
Acyclic nucleotides: Cidofovir is a prominent representative. Its main difference is that its molecular structure initially contains a phosphoryl group (phosphonate), meaning it does not require initial phosphorylation.
Mechanism of Action at the Cellular Level
The mechanism of antiherpetic drugs is based on chemical similarity to natural cellular components:
Mimicry: the drugs act as abnormal nucleosides or "false" substrates.
Competition: due to structural similarity to natural nucleosides, the drug compounds actively compete with them for incorporation into the viral DNA chain.
Blockade: successful incorporation of the acyclic analog instead of natural deoxyguanosine leads to irreversible disruption of viral replication, causing chain termination or complete blockade of the key enzyme.
Mnemonic
Acyclovir is acyclic (open chain), cidofovir is a nucleotide (phosphonate is already in place).
Frequently asked questions
What are the pharmacokinetic absorption features of valacyclovir in the gastrointestinal tract?
Valacyclovir absorption upon oral administration occurs via active transport, as it is a substrate for the oligopeptide transporter.
Pharmacokinetic features:
Presystemic metabolism — under the action of esterases, the prodrug is rapidly converted into active acyclovir.
Bioavailability — thanks to transporter involvement, it increases up to 70% (rising by more than 30% compared to the parent drug).
Activation — finally converted into the active compound in the liver.
Which herpes virus enzymes perform the initial phosphorylation of acyclovir?
Initial phosphorylation of acyclovir is performed by a specific enzyme — viral thymidine kinase.
Process features:
Localization — primary activation occurs exclusively in infected cells, ensuring safety for healthy tissues.
Reaction product — the enzyme catalyzes the conversion of acyclovir into acyclovir monophosphate.
Dependency — drug efficacy directly depends on the activity of this enzyme; activation is impaired in its deficiency.
Subsequent phosphorylation to the active triphosphate is carried out by host cell kinases.
How do acyclic nucleosides differ structurally from natural analogs?
They are based on purine nucleosides (guanine), but instead of a complete closed sugar ring, they have an open acyclic carbohydrate chain.
What is the main structural difference between cidofovir and acyclovir?
Cidofovir is an acyclic nucleotide and already possesses a phosphoryl group (phosphonate) in its structure, whereas nucleosides require preliminary phosphorylation.
What is valacyclovir from a chemical standpoint?
Valacyclovir is an ester modification of acyclovir distinguished by the presence of a specific valine group.
What is the ultimate result of incorporating a "false" substrate into viral DNA?
It leads to the inhibition of herpes virus replication, causing DNA chain termination or enzyme blockade.
Go deeper
The role of phosphorylation in the activation of acyclic nucleosides
Pharmacokinetic differences between valacyclovir and acyclovir
Mechanisms of cidofovir action as a nucleotide
Mechanisms of viral resistance development against replication inhibitors
Comparative analysis of drug affinity for viral versus cellular DNA polymerase