Hypersensitivity syndrome
Hypersensitivity to abacavir is strongly associated with a specific allele at the human leukocyte antigen B locus namely HLA-B*5701.[12][13][14] The mechanism for this hypersensitivity reaction is due to abacavir binding in the antigen-binding cleft of HLA-B*57:01, allowing alternative peptides to bind, which appear as "non-self" when presented to T cells.[15] There is an association between the prevalence of HLA-B*5701 and ancestry. The prevalence of the allele is estimated to be 3.4 to 5.8 percent on average in populations of European ancestry, 17.6 percent in Indian Americans, 3.0 percent in Hispanic Americans, and 1.2 percent in Chinese Americans.[16][17] There is significant variability in the prevalence of HLA-B*5701 among African populations. In African Americans, the prevalence is estimated to be 1.0 percent on average, 0 percent in the Yoruba from Nigeria, 3.3 percent in the Luhya from Kenya, and 13.6 percent in the Masai from Kenya, although the average values are derived from highly variable frequencies within sample groups.[18]
Common symptoms of abacavir hypersensitivity syndrome include fever, malaise, nausea, and diarrhea. Some patients may also develop a skin rash.[19] Symptoms of AHS typically manifest within six weeks of treatment using abacavir, although they may be confused with symptoms of HIV, immune reconstitution syndrome, hypersensitivity syndromes associated with other drugs, or infection.[20] The U.S. Food and Drug Administration (FDA) released an alert concerning abacavir and abacavir-containing medications on 24 July 2008,[21] and the FDA-approved drug label for abacavir recommends pre-therapy screening for the HLA-B*5701 allele and the use of alternative therapy in subjects with this allele.[22] Additionally, both the Clinical Pharmacogenetics Implementation Consortium and the Dutch Pharmacogenetics Working Group recommend use of an alternative therapy in individuals with the HLA-B*5701 allele.[23][24]
Skin-patch testing may also be used to determine whether an individual will experience a hypersensitivity reaction to abacavir, although some patients susceptible to developing AHS may not react to the patch test.[25]
The development of suspected hypersensitivity reactions to abacavir requires immediate and permanent discontinuation of abacavir therapy in all patients, including patients who do not possess the HLA-B*5701 allele. On 1 March 2011, the FDA informed the public about an ongoing safety review of abacavir and a possible increased risk of heart attack associated with the drug.[26] A meta-analysis of 26 studies conducted by the FDA, however, did not find any association between abacavir use and heart attack[27][28]
