Mechanism of action: from prodrug to DNA termination
The drug's mechanism of action is based on its sequential bioactivation. Initially, the molecule is an inactive compound.
- Penetration and selectivity: The drug crosses the cell membrane. Its initial activation occurs only in infected cells. This is mediated by a key enzyme, viral thymidine kinase (vTK), which converts the drug into acyclovir monophosphate (AMP). In healthy cells, this process barely occurs, ensuring a high safety profile.
- Phosphorylation cascade: Next, host cell enzymes (cellular kinases) come into play. They further build the molecule: aciclovir monophosphate $\rightarrow$ aciclovir diphosphate $\rightarrow$ aciclovir triphosphate. The triphosphate form is the active drug.
- Nuclear target: Active triphosphate enters the cell nucleus, where it acts as a structural analogue of the natural nucleotide deoxyguanosine (dG). This creates competition for incorporation into the elongating viral DNA.
- Result: When viral enzymes mistakenly incorporate aciclovir into the chain, chain termination (replication arrest) occurs. This happens because the drug molecule lacks the necessary chemical groups to attach the next nucleotide. Synthesis of viral DNA is completely halted.
Spectrum of activity and biochemical resistance
The drug's pharmacodynamics exhibit a clear efficacy gradient: its activity varies across different herpesvirus subfamilies. This is directly related to the presence of a specific thymidine kinase in the pathogen.
- Alpha-herpesviruses (high sensitivity): This group includes herpes simplex virus types 1 and 2 (HSV-1, HSV-2), as well as varicella-zoster virus (Varicella Zoster). They are thymidine kinase-positive. Inhibiting HSV-1 requires minimal inhibitory concentrations (IC50) of only 0.15 µM.
- Beta- and gamma-herpesviruses (low sensitivity): This group includes Cytomegalovirus (CMV) and Epstein-Barr virus. Their mechanism of resistance is due to a deficiency in thymidine kinase. Consequently, intracellular phosphorylation of the drug is severely hindered. Inhibiting CMV requires maximum concentrations exceeding 100 µM.
Conclusion: The difference in required dosage spans several orders of magnitude. This makes the drug the agent of choice for HSV, but largely ineffective against CMV.
Pharmacokinetics: pathway in the body
- Absorption: Following oral administration (per os), absorption is incomplete, with low bioavailability of only 15–30%. When applied topically to the skin, systemic absorption is moderate.
- Distribution: The drug exhibits high penetration into tissues and organs. It successfully crosses blood-tissue barriers, penetrating brain tissue, crossing the placenta, and being excreted into breast milk.
- Metabolism and elimination: Partially metabolized in the liver, but the primary route of elimination is renal. Normally, the elimination half-life ($T_{1/2}$) is 2.5–3.3 hours. A clinically significant fact: in chronic kidney disease (CKD), this figure increases to 20 hours, necessitating mandatory dosage adjustment.
Clinical application and safety profile
The choice of a specific drug formulation depends on the localization and severity of the pathological process.
- Topical (external) use: Ointments and creams at a concentration of 5% are used to treat skin and mucous membrane lesions. In ophthalmology, a specialized 3% eye ointment is applied into the conjunctival sac for herpetic keratitis. Side effects are minimal: occasional burning or local inflammatory reactions.
- Systemic (enteral) use: Tablet formulations are prescribed for shingles (Herpes Zoster) and chickenpox (Varicella). Oral administration may cause adverse gastrointestinal reactions (nausea, vomiting, diarrhea) and central nervous system effects (headache, dizziness).
- Parenteral use: Intravenous infusion is used for severe and generalized forms of infection. Indications include meningoencephalitis, severe localized forms (e.g., severe genital herpes), as well as chickenpox and shingles in immunocompromised patients. Infusions may cause local reactions (phlebitis), reversible nephrotoxicity, neurological disorders (seizures, hallucinations), and infrequent blood count changes (leukopenia, thrombocytopenia).
Briefly: answers to topical questions
What is the mechanism of acquired resistance of herpes simplex viruses to aciclovir?
The mechanism of acquired resistance of herpes simplex viruses to aciclovir is due to specific genetic mutations and enzyme deficiency.
Main causes of resistance development:
- Enzyme deficiency — a shortage of viral thymidine kinase, which is critically required to activate the drug inside the cell.
- Genetic mutations — mutations in the thymidine kinase or DNA polymerase genes of herpes simplex virus (Herpes Simplex).
The rule of three T's for aciclovir: Thymidine kinase (viral) initiates activation, Triphosphate (of aciclovir) is the active form, Termination of the DNA chain kills the virus.
Class and exam questions
Why is acyclovir safe for healthy body cells?
Initial phosphorylation of the drug is carried out exclusively by viral thymidine kinase. Uninfected cells lack this enzyme, so the drug remains in the form of an inactive prodrug.
Is the drug effective for cytomegalovirus infection?
No, cytomegalovirus is deficient in thymidine kinase. Its inhibition requires aciclovir concentrations (over 100 µM) that are hundreds of times higher than standard ones.
How to adjust therapy in renal disease?
In chronic kidney disease, the half-life of aciclovir increases from 2.5–3.3 hours to 20 hours. This requires a mandatory dose reduction to avoid nephrotoxicity.
What else to study on this topic
- Comparative characteristics of alpha-, beta-, and gamma-herpesviruses.
- Mechanisms of resistance to nucleoside analogues.
- Features of treating herpetic infections in immunosuppression.
- Prevention of nephrotoxicity during intravenous administration of antiviral drugs.
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