Abacavir

Medical uses

Abacavir tablets and oral solution, in combination with other antiretroviral agents, are indicated for the treatment of HIV-1 infection.

Abacavir should always be used in combination with other antiretroviral agents. Abacavir should not be added as a single agent when antiretroviral regimens are changed due to loss of virologic response.

Side effects

Common adverse reactions include nausea, headache, fatigue, vomiting, diarrhea, loss of appetite and trouble sleeping. Rare but serious side effects include hypersensitivity reaction or rash, elevated AST and ALT, depression, anxiety, fever/chills, URI, lactic acidosis, hypertriglyceridemia, and lipodystrophy.

People with liver disease should be cautious about using abacavir because it can aggravate the condition. Signs of liver problems include nausea and vomiting, abdominal pain, dark-colored urine and yellowing of the skin or whites of the eyes. The use of nucleoside drugs such as abacavir can very rarely cause lactic acidosis. Signs of lactic acidosis include fast or irregular heartbeat, unusual muscle pain, fatigue, difficulty breathing and stomach pain with nausea and vomiting. Abacavir can also lead to immune reconstitution inflammatory syndrome, a change in body fat as well as an increased risk of heart attack.

Resistance to abacavir has developed in laboratory versions of HIV which are also resistant to other HIV-specific antiretrovirals such as lamivudine, didanosine and zalcitabine. HIV strains that are resistant to protease inhibitors are not likely to be resistant to abacavir.

Abacavir is contraindicated for use in infants under 3 months of age.

Little is known about the effects of Abacavir overdose. Overdose victims should be taken to a hospital emergency room for treatment.

Hypersensitivity syndrome

Hypersensitivity to abacavir is strongly associated with a specific allele at the human leukocyte antigen B locus namely HLA-B*5701. 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. 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.

Common symptoms of abacavir hypersensitivity syndrome include fever, malaise, nausea, and diarrhea. Some patients may also develop a skin rash. Symptoms of AHS typically manifest within six weeks of treatment using abacavir, although they may be confused with symptoms of HIV, immune restoration disease, hypersensitivity syndromes associated with other drugs, or infection. The U.S. Food and Drug Administration (FDA) released an alert concerning abacavir and abacavir-containing medications on July 24, 2008, 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. 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.

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.

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 March 1, 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. A meta-analysis of 26 studies conducted by the FDA, however, did not find any association between abacavir use and heart attack

Immunopathogenesis

The mechanism underlying abacavir hypersensitivity syndrome is related to the change in the HLA-B*5701 protein product. Abacavir binds with high specificity to the HLA-B*5701 protein, changing the shape and chemistry of the antigen-binding cleft. This results in a change in immunological tolerance and the subsequent activation of abacavir-specific cytotoxic T cells, which produce a systemic reaction known as abacavir hypersensitivity syndrome.

Interaction

Abacavir, and in general NRTIs, do not undergo hepatic metabolism and therefore have very limited (to none) interaction with the CYP enzymes and drugs that effect these enzymes. That being said there are still few interactions that can affect the absorption or the availability of abacavir. Below are few of the common established drug and food interaction that can take place during abacavir co-administration:

Mechanism of action

Abacavir is a nucleoside reverse transcriptase inhibitor that inhibits viral replication. It is a guanosine analogue that is phosphorylated to carbovir triphosphate (CBV-TP). CBV-TP competes with the viral molecules and is incorporated into the viral DNA. Once CBV-TP is integrated into the viral DNA, transcription and HIV reverse transcriptase is inhibited.

Pharmacokinetics

Abacavir is given orally and is rapidly absorbed with a high bioavailability of 83%. Solution and tablet have comparable concentrations and bioavailability. Abacavir can be taken with or without food.

Abacavir can cross the blood-brain barrier. Abacavir is metabolized primarily through the enzymes alcohol dehydrogenase and glucuronyl transferase to an inactive carboxylate and glucuronide metabolites. It has a half life of approximately 1.5-2.0 hours. If a person has liver failure, abacavir’s half life is increased by 58%.

Abacavir is eliminated via excretion in the urine (83%) and feces (16%). It is unclear whether abacavir can be removed by hemodialysis or peritoneal dialysis.

History

Abacavir was approved by the Food and Drug Administration (FDA) on December 18, 1998 and is thus the fifteenth approved antiretroviral drug in the United States. Its patent expired in the United States on 2009-12-26.

Robert Vince and Susan Daluge in the '80s along with a visiting scientist from China, Mei Hua, developed the medication.