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Abacavir is a nucleoside reverse-transcriptase inhibitor with activity against the human immunodeficiency virus (HIV) that is manufactured by GlaxoSmithKline and sold under the brand name of Ziagen®. Abacavir is a component of two combination antiretroviral drugs, Trizivir and Epzicom.

The most important adverse effect of abacavir that limits its use in therapy is an immunologically mediated hypersensitivity reaction affecting 5 to 8% of patients during the first 6 weeks of treatment. Symptoms include combinations of fever, rash, constitutionalsymptoms, gastrointestinal tract symptoms, and respiratory symptoms that become more severe with continued dosing. Immediate and permanent discontinuation of Abacavir results in a rapid reversal of symptoms. Subsequent rechallenge with Abacavir is contraindicated, since it can result in a more severe, rapid, and potentially life-threatening reaction.

A recent study confirmed that a particular human leukocyte antigen allele, HLA-B*5701, is a genetic marker for this immunologically mediated hypersensitivity reaction (NEJM 2008;358:568-79). FDA recommends screening for this allele prior to starting therapy with Abacavir. Patients testing positive for HLA-B*5701 should be treated with an alternative medication.

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