How does AZT work?

October 7, 2025 •

3 min read

Approved by the FDA in 1987, azidothymidine (AZT) was the very first antiretroviral medication for treating HIV. To understand how does AZT work, one must first grasp its role as a nucleoside analog that disrupts the virus's replication cycle by targeting a key viral enzyme.

Quick Summary

AZT, a nucleoside analog reverse-transcriptase inhibitor, interferes with the HIV replication process. It causes DNA chain termination, halting the virus's ability to reproduce. This action effectively slows disease progression by reducing the viral load. AZT is a vital component of combination antiretroviral therapy.

Key Points

NRTI Class: AZT is a nucleoside analog reverse-transcriptase inhibitor (NRTI) that interferes with HIV's replication process.
Intracellular Activation: Cellular enzymes phosphorylate AZT into its active form, AZT triphosphate (AZT-TP), which is the molecule that fights the virus.
Reverse Transcriptase Inhibition: The active AZT-TP molecule is a molecular mimic that competitively inhibits the HIV viral enzyme, reverse transcriptase.
DNA Chain Termination: By replacing a natural nucleotide, AZT-TP lacks a crucial chemical group ($3'$-OH) needed for DNA elongation, thus terminating the viral DNA chain.
Combination Therapy: To combat resistance and increase efficacy, AZT is now used in combination with other antiretroviral drugs, not as a standalone treatment.
Mitochondrial Toxicity: A significant side effect is mitochondrial damage, caused by AZT inhibiting mitochondrial DNA polymerase at high concentrations.
Role in Prevention: AZT is most famously known for its role in preventing mother-to-child transmission of HIV during pregnancy and childbirth.

The HIV Replication Cycle: A Target for AZT

To grasp the mechanism of action for azidothymidine (AZT), it is essential to first understand the replication cycle of the human immunodeficiency virus (HIV). As a retrovirus, HIV has a unique lifecycle that relies on a specific set of enzymes and processes to reproduce within a host's CD4+ T-cells. This cycle involves several key steps:

Entry and Fusion: The virus binds to and enters a host cell.
Reverse Transcription: HIV's reverse transcriptase enzyme converts viral RNA into DNA, a primary target for AZT.
Integration: Viral DNA is incorporated into the host cell's DNA.
Transcription and Translation: New viral RNA and proteins are produced.
Assembly and Budding: New virus particles are assembled and released from the host cell.

AZT's Mechanism of Action: The Mimic and Terminator

AZT (zidovudine, also known as Retrovir) disrupts the reverse transcription stage, preventing viral replication.

Intracellular Activation

AZT is converted inside the cell by cellular enzymes into its active form, AZT triphosphate (AZT-TP), by adding three phosphate groups.

Competitive Inhibition of Reverse Transcriptase

AZT-TP is a nucleoside analog similar to thymidine, a natural DNA building block. HIV's reverse transcriptase mistakenly uses AZT-TP instead of thymidine triphosphate (TTP) when building viral DNA.

The Chain-Terminating Effect

The critical difference is that AZT lacks the $3'$-hydroxyl ($3'$-OH) group necessary for adding subsequent nucleotides to the DNA chain. When AZT-TP is incorporated, the viral DNA chain terminates, halting replication.

Comparing AZT and Natural Nucleotides

The table below highlights the differences between natural thymidine triphosphate (TTP) and AZT triphosphate (AZT-TP).


Feature	Natural Thymidine Triphosphate (TTP)	AZT Triphosphate (AZT-TP)
Molecular Role	A natural building block for DNA synthesis	A nucleoside analog and DNA chain terminator
Presence of $3'$-OH Group	Yes, has a $3'$-OH group	No, lacks a $3'$-OH group, replaced with an azido ($N_3$) group
Effect on DNA Synthesis	Allows for continuous DNA chain elongation	Causes irreversible termination of the DNA chain
Primary Target Enzyme	Human and viral DNA polymerases	Primarily HIV reverse transcriptase due to its selective affinity
Cellular vs. Viral	Used by both human and viral enzymes	Acts primarily on the viral enzyme, but can cause host toxicity at high levels

The Evolution of AZT Therapy: From Monotherapy to ART

AZT's effectiveness as a single treatment (monotherapy) was limited due to the rapid development of viral resistance and the need for high doses that caused side effects. This led to the adoption of Antiretroviral Therapy (ART), which uses a combination of several drugs to target HIV at different stages. ART significantly reduces resistance and improves treatment outcomes. Today, AZT is mainly used in combination therapies, notably for preventing mother-to-child transmission.

Side Effects and Mitochondrial Toxicity

A significant side effect of AZT, particularly at high doses, is mitochondrial toxicity. This occurs because AZT-TP can inhibit human mitochondrial DNA polymerase gamma. This can lead to various issues, including muscle weakness (myopathy), low blood cell counts (anemia and neutropenia), and liver problems (lactic acidosis and hepatic steatosis). Modern ART regimens use lower doses or alternative drugs, reducing these risks.

Conclusion: The Legacy of AZT

AZT was the first major breakthrough in HIV treatment. Its mechanism of action, terminating the viral DNA chain, was foundational for antiretroviral drug development. Although no longer a first-line treatment and primarily used in combination therapy, particularly for preventing mother-to-child transmission, AZT remains a significant part of medical history and a testament to the progress in combating HIV. For more information on HIV treatment guidelines, consult the Department of Health and Human Services.

Frequently Asked Questions

AZT is an abbreviation for azidothymidine. Zidovudine is the official generic name for the drug, and Retrovir was its original brand name. They all refer to the same medication.

The key enzyme targeted by AZT is HIV reverse transcriptase (RT). This enzyme is essential for converting the virus's RNA into DNA, a crucial step in its replication cycle.

AZT lacks the $3'$-hydroxyl ($3'$-OH) group that is required to add the next nucleotide to a growing DNA chain. When HIV reverse transcriptase incorporates AZT into the viral DNA chain, the chain-building process comes to a halt.

AZT is no longer used alone due to the rapid development of drug resistance by HIV. As the virus mutates, its reverse transcriptase enzyme can become less susceptible to AZT's effects. Modern treatment uses combination therapy to overcome this.

At high doses, the active form of AZT (AZT-TP) can inhibit human mitochondrial DNA polymerase gamma, which is necessary for replicating mitochondrial DNA. This leads to mitochondrial dysfunction and toxicity.

AZT was the first drug proven to significantly reduce the risk of HIV transmission from a pregnant parent to their baby. This landmark discovery fundamentally changed prenatal and newborn HIV care.

Yes, AZT is still used, but almost exclusively as part of combination antiretroviral therapy (ART). It is no longer a first-line treatment but remains an important component, particularly for preventing mother-to-child transmission.