Abacavir

Abacavir (brand name Ziagen) is an antiretroviral medication used as part of combination therapy to manage HIV-1 infection. It belongs to the class of nucleoside reverse transcriptase inhibitors (NRTIs), which are essential components of many HIV treatment regimens.

Abacavir is approved for use in adults and children with HIV-1 infection. By mimicking natural nucleosides, it is incorporated by the viral reverse transcriptase enzyme into the viral DNA chain, causing premature termination and preventing the virus from replicating.

The primary gene relevant to abacavir response is HLA-B, which encodes a Major Histocompatibility Complex (MHC) class I protein involved in presenting peptides to the immune system. A specific variant of this gene, HLA-B*57:01, has been linked to an increased risk of abacavir hypersensitivity.

Hypersensitivity reactions are mediated by the immune system recognizing drug–peptide complexes on the surface of cells. Carriers of HLA-B*57:01 present abacavir-derived peptides differently, triggering a severe immune response. Testing for this variant helps identify patients at risk before starting therapy.

Individuals who carry the HLA-B*57:01 allele do not metabolize abacavir differently in the liver, but they have a markedly increased risk of developing a potentially life-threatening hypersensitivity reaction. Those without the allele generally tolerate standard dosing without this immune-mediated adverse effect.

HLA-B*57:01 Positive

• Effect on drug levels: No significant change in abacavir plasma concentrations
• Clinical consequence: High risk of severe hypersensitivity reaction requiring immediate drug discontinuation
• Side effects: Hypersensitivity reactions – severe, potentially life-threatening; incidence approximately 5–8% in carriers

Indeterminate/Not Available

• Effect: Unknown
• Clinical consequence: No specific guidance; follow standard dosing with clinical monitoring

HLA-B Genotype and Abacavir Guidance

For patients who test positive for HLA-B*57:01, alternative NRTIs or backbone agents—such as tenofovir, lamivudine, or emtricitabine—may be used in place of abacavir. These examples are drawn from guideline recommendations and should be considered in consultation with a healthcare provider.

• FDA – Table of Pharmacogenetic Associations