Basic Principles of Classification
Modern pharmacology groups antiviral agents based on their chemical structure and mechanism of action. Globally, all drugs are divided into four major categories:
- Chemotherapeutic agents — synthetic molecules for causal treatment that selectively target the pathogen.
- Interferons — endogenous body proteins with natural antiviral activity.
- Interferon inducers — specific substances that stimulate the patient's body to actively produce its own interferons.
- Immunomodulators — agents designed to correct and restore the immune response.
The main objective of synthetic chemotherapy is to selectively interrupt virion replication without significantly damaging the healthy tissues of the host organism.
Targets and Sites of Action
The viral life cycle consists of several sequential stages. Chemical classes of drugs correspond to specific stages they are able to block:
- Deproteinization (uncoating of the genome): disrupted by adamantane derivatives.
- Synthesis of 'early' viral proteins: blocked by guanidine.
- Nucleic acid synthesis: genome replication is effectively halted by nucleoside analogues.
- Synthesis of 'late' viral proteins: inhibited by peptide derivatives.
- Assembly of new virions: the target for thiosemicarbazone derivatives.
Abnormal Nucleosides: The Foundation of Therapy
This is the most extensive group of agents disrupting nucleic acid synthesis. By incorporating into the growing chain, they render further replication impossible.
- Zidovudine (AZT): a fundamental drug for treating HIV infection (AIDS).
- Acyclovir, vidarabine, idoxuridine: effective against herpes simplex viruses (types 1 and 2) and Varicella Zoster.
- Ganciclovir and cytarabine: used for cytomegalovirus infection (ganciclovir is also active against herpes simplex type 1).
- Ribavirin: exhibits a broad spectrum, active against hepatitis C, Lassa fever, and respiratory syncytial virus.
- Trifluridine: used in ophthalmology for keratitis of adenoviral and herpetic etiology.
Specific Chemical Classes
Depending on indications, other structural groups are used in clinical practice:
- Adamantane derivatives. Target respiratory viruses. Amantadine and rimantadine are effective against influenza A; adapromine covers influenza types A and B; deitiforin has a complex action (influenza A, parainfluenza, RSV infection). An exception is tromantadine, which is used for herpes.
- Synthetic amino acids. Amben and aminocaproic acid are prescribed for influenza, ARVI, parainfluenza, and RSV infection.
- Pyrophosphate analogues. Foscarnet is used for severe systemic infections: cytomegalovirus, hepatitis B, HIV, and herpes (types 1 and 6).
- Thiosemicarbazone derivatives. Methisazone and marboran are historically important drugs once used to combat smallpox.
- Virucidal agents. Local preparations (ointments, drops) acting against extracellular virions. Oxoline, tebrofen, and flurenacyl are used for local forms of herpes, rhinitis, and viral keratitis.
Challenges of Modern Chemotherapy
Despite massive production of antimicrobial agents, clinicians face two primary problems. The first is the emergence of resistant viral strains that quickly adapt to therapy and reduce treatment efficacy.
The second problem is the shortage of etiotropic drugs. While a robust foundation exists for treating HIV, hepatitis, and herpes, humanity still lacks reliable specific molecules for treating a range of enteroviral infections and dangerous viral encephalitides.