Mechanism of Action: From Prodrug to DNA Chain Termination
The drug's mechanism relies on sequential bioactivation. Initially, the molecule is an inactive compound.
- Penetration and Selectivity: The drug crosses the cell membrane. Its primary activation occurs only in infected cells. This is driven by a key enzyme — viral thymidine kinase (vTK) — which converts the drug into acyclovir monophosphate (AMP). This process barely occurs in healthy cells, ensuring a high safety profile.
- Phosphorylation Cascade: Host cell enzymes (cellular kinases) then complete the molecule's transformation: acyclovir monophosphate $\rightarrow$ acyclovir diphosphate $\rightarrow$ acyclovir triphosphate. The triphosphate is the active form of the drug.
- Nuclear Target: The active triphosphate enters the nucleus, acting as a structural analog of the natural nucleotide deoxyguanosine (dG). It competes for incorporation into the elongating viral DNA chain.
- Result: When viral enzymes mistakenly incorporate acyclovir into the chain, chain termination occurs (halting replication). This happens because the drug molecule lacks the necessary chemical groups to attach the next incoming nucleotide. Synthesis of viral DNA is completely halted.
Spectrum of Activity and Biochemical Resistance
The pharmacodynamics of the drug exhibit a clear efficiency gradient: its activity varies across different herpesvirus subfamilies, which directly correlates with the presence of pathogen-specific thymidine kinase.
- Alpha-herpesviruses (High sensitivity): This group includes herpes simplex virus types 1 and 2 (HSV-1, HSV-2) and the varicella-zoster virus (Varicella Zoster). They are thymidine kinase-positive. Minimal inhibitory concentrations (IC50) required to suppress HSV-1 are as low as 0.15 µM.
- Beta- and gamma-herpesviruses (Low sensitivity): This includes Cytomegalovirus (CMV) and Epstein-Barr virus. Their resistance mechanism stems from being thymidine kinase-deficient. Consequently, intracellular phosphorylation of the drug is severely impaired. Suppressing CMV requires maximum concentrations exceeding 100 µM.
Conclusion: The difference in required dosage spans orders of magnitude, making the drug the agent of choice for HSV, but largely ineffective against CMV.
Pharmacokinetics: Disposition in the Body
- Absorption: Oral (per os) absorption is incomplete, with a low bioavailability of only 15–30%. Topical application to the skin results in moderate systemic absorption.
- Distribution: The drug penetrates tissues and organs exceptionally well. It successfully crosses tissue barriers, enters the central nervous system, crosses the placenta, and is excreted in breast milk.
- Metabolism and Excretion: 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 value rises up to 20 hours, necessitating mandatory dosage adjustments.
Clinical Application and Safety Profile
The choice of formulation depends on the localization and severity of the pathological process.
- Topical Application: For skin and mucous membrane lesions, 5% ointments and creams are used. In ophthalmology, a specialized 3% eye ointment is applied to the conjunctival sac for herpetic keratitis. Adverse effects are minimal; transient burning or local inflammation may occasionally occur.
- Enteral (Oral) Administration: Tablet formulations are prescribed for shingles (Herpes Zoster) and chickenpox (Varicella). Potential adverse reactions include gastrointestinal disturbances (nausea, vomiting, diarrhea) and CNS symptoms (headache, dizziness).
- Parenteral Administration: Intravenous infusion is used for severe and disseminated infections. Indications include meningoencephalitis, severe localized forms (e.g., severe genital herpes), and varicella or zoster in immunocompromised patients. Infusions may cause injection-site reactions (phlebitis), reversible nephrotoxicity, neurological disorders (seizures, hallucinations), and infrequent hematologic abnormalities (leukopenia, thrombocytopenia).