Classification and Mechanism of Action
The nucleoside reverse transcriptase inhibitor (NRTI) class includes synthetic analogues of natural nitrogenous bases required for DNA synthesis. Chemically, these drugs are divided into several subgroups:
- Thymidine analogues: stavudine.
- Guanine analogues: abacavir.
- Adenine analogues: didanosine.
- Cytidine analogues: zalcitabine, lamivudine, and emtricitabine.
The general antiviral mechanism of the entire group is identical to that of the prototype drug, zidovudine. Upon entering the affected cell, these false nucleosides are recognized by the viral enzyme and used for viral DNA synthesis. The incorporation of an abnormal molecule prevents the attachment of the next nucleotide. Chain termination occurs, effectively suppressing HIV replication.
Pharmacological Profile of Lamivudine
Lamivudine is a cytidine analogue distinguished by a favorable safety profile. Its main advantage is that it has a significantly weaker inhibitory effect on host cell mitochondrial DNA polymerase. Consequently, specific toxicity is reduced, and complications such as neutropenia occur only when therapeutic doses are exceeded.
The drug's pharmacokinetics show excellent absorption: oral bioavailability reaches 86%. Its relative safety allows for widespread use in pediatrics, and it is officially approved for treating children starting from 3 months of age.
The antiviral spectrum includes both human immunodeficiency virus and hepatitis B virus. However, clinicians face a major challenge: viral drug resistance to lamivudine develops extremely rapidly. For this reason, monotherapy with this agent is strictly contraindicated. It is prescribed exclusively as part of combination therapy, most commonly paired with zidovudine or non-nucleoside reverse transcriptase inhibitors.
Tenofovir: The Sole Nucleotide Inhibitor
Tenofovir differs fundamentally from other members of the group. It is the only nucleotide (rather than nucleoside) reverse transcriptase inhibitor used in HIV therapy. Chemically, it is an acyclic nucleotide—an adenosine monophosphate analogue featuring an incomplete ribose ring.
Its mechanism of action unfolds in three sequential steps:
- Upon entering the cell, the drug undergoes phosphorylation and converts into its active form, tenofovir diphosphate.
- The resulting active metabolite directly competes with the natural substrate, endogenous deoxyadenosine monophosphate.
- The false nucleotide incorporates into the growing viral DNA chain, causing immediate chain termination and blocking further viral assembly.
The spectrum of activity of tenofovir is broad: it exhibits high activity against HIV-1, HIV-2, and hepatitis B virus. Unlike lamivudine, this drug is used primarily for treating adult patients.
General Drawbacks and Class Toxicity
Despite the vital necessity of using abnormal nucleosides and nucleotides in HIV treatment, these drugs possess numerous adverse effects. The primary cause of toxicity lies in their negative impact on normal host cellular processes.
Most drugs in this class (like the classic zidovudine) lack absolute selectivity. They can inhibit mitochondrial DNA synthesis in healthy human cells. This interference with cellular energy metabolism leads to class-wide adverse effects, the most severe of which is marked bone marrow suppression (impaired hematopoiesis).