Abacavir is a nucleoside reverse transcriptase inhibitor (NRTI) used for treatment of human immunodeficiency virus (HIV) infection. HIV infection is usually treated with antiretroviral therapy (ART) regimens, which consist of three or more different drugs used in combination. Typical antiretrovirals used in these regimens include NRTIs, non-nucleoside reverse transcriptase inhibitors (NNRTIs), protease inhibitors (PIs) and integrase strand inhibitors (InSTIs) [Article:22820792]. Abacavir makes an ideal addition to these types of combination therapies due to its dosing flexibility. It can be given either once or twice a day to match other drugs' dosing patterns, and can also be given in tablets that contain other antiretroviral drugs such as lamivudine or zidovudine, allowing for a reduction in pill count [Article:18479171]. Abacavir is generally well tolerated, and common side effects include nausea, headache and diarrhea [Article:18479171]. However, approximately 5-8% of patients experience a hypersensitivity reaction (HSR) within the first 6 weeks of treatment. Symptoms of a HSR include at least two of the following: fever, rash, cough, gastrointestinal symptoms (e.g., nausea, vomiting, abdominal pain), dyspnea and fatigue [Article:22378157]. These symptoms worsen with continued treatment, but typically improve within 24 hours after discontinuation. However, drug rechallenge after discontinuation of abacavir due to HSR can result in symptom recurrence within a matter of hours, and potentially life-threatening allergic reactions [Articles:11888582, 10449301]. This hypersensitivity reaction is strongly linked to the presence of the HLA-B*57:01 allele, and testing for the allele prior to abacavir treatment is recommended by the U.S. Food and Drug Administration (FDA), the European Medicines Agency (EMA), the Clinical Pharmacogenetics Implementation Consortium (CPIC), and the Dutch Pharmacogenetics Working Group (DPWG). Abacavir was also reported to be associated with a higher risk of myocardial infarction as compared to other NRTIs [Articles:18387667, 19682101, 20660842, 18753925]. However, a meta-analysis conducted by the FDA in 2012 failed to find any such association [Article:22932321].
Since abacavir is primarily metabolized through cytosolic alcohol dehydrogenase (ADH) and uridine diphosphate glucuronosyltransferase (UGT) enzymes, no interactions between abacavir and inducers or inhibitors of cytochrome P450 (CYP) enzymes are predicted [Article:18479171]. Additionally, in vitro studies show that abacavir is unlikely to inhibit CYP enzymes at clinically relevant concentrations [Article:10898676]. Since NNRTIs and PIs are primarily metabolized by the CYP enzymes [Article:12416447], this may eliminate the potential for drug interactions with these types of antiretrovirals. No clinically significant pharmacokinetic changes were seen when abacavir was administered with other NRTIs such as lamivudine or zidovudine [Articles:18479171, 10320951]. Since alcohol is also metabolized by ADH, pharmacokinetic interactions between the drug and ethanol have been analyzed, but no clinically significant changes or new adverse events were reported [Article:10817729]. Additionally, concurrent treatment with UGT inducers such as rifampicin and phenobarbital has the potential to decrease abacavir plasma concentrations, but the clinical significance of these interactions is unclear [Article:14709631] [Antiretroviral Drugs and the Treatment of Tuberculosis] [EMA European public assessment report: Trizivir]. Several studies have found a link between abacavir administration and virologic response in hepatitis C patients being treated with ribavirin and pegylated interferon who are also co-infected with HIV; in these patients, abacavir usage was found to be a significantly associated with early virologic failure [Article:17460476] and lack of sustained virologic response (SVR) [Articles:18572756, 18854330]. However, subpopulation analyses from two of the studies found that the impact of abacavir on SVR was only significant in patients with baseline hepatitis C viral RNA above a certain level [Article:18854330], ribavirin daily doses below a certain level [Article:18854330], or ribavirin trough concentrations below a certain level [Article:18572756]. Additionally, a number of studies have found no association between abacavir usage and virologic response [Articles:20167995, 17938129, 19043930, 22781224], so it is uncertain whether these two drugs have a significant and harmful interaction.
A schematic representation of abacavir disposition within the body is provided in the pathway figure above. Abacavir is rapidly absorbed following oral administration, and has a mean absolute bioavailability of approximately 83% [Articles:18479171, 10453964]. The drug is lipophilic yet also shows high water solubility, allowing it to cross cell membranes by passive diffusion alone. These properties may explain its high bioavailability, as well as allowing the drug to easily penetrate into tissues such as the blood-brain barrier [Articles:9145874, 18479171]. After absorption, abacavir is extensively metabolized within the liver, with less than 2% of the drug excreted unchanged in the urine [Article:10582871]. ADH and UGT are the primary enzymes responsible for abacavir metabolism within hepatocytes. Metabolism by ADH results in the inactive carboxylate metabolite 2269W93; metabolism by the UGT enzymes results in the inactive glucuronide metabolite 361W94 [Article:10582871]. A mass balance study found that 83% of the original dose was eliminated in the urine, and 16% in the feces. Out of the 83% eliminated via urine, 36% of the dose recovered was the glucuronide metabolite, and 30% was the carboxylate metabolite. The remaining dose was either the parent drug or trace metabolites [Article:10582871].
Parent drug that is not metabolized by hepatocytes undergoes anabolism within viral-infected cells by a different set of intracellular enzymes, converting the drug into its pharmacologically active metabolite. Initially, abacavir is anabolized to abacavir 5'-monophosphate by the enzyme adenosine phosphotransferase (encoded by the ADK gene). It then undergoes deamination by an unknown cytosolic enzyme into (-)-carbovir 5'-monophosphate; no di- or triphosphates of abacavir have been detected within cells [Articles:9145874, 9145876]. Carbovir 5'-monophosphate is then converted to carbovir 5'-diphosphate by the enzyme guanylate kinase (GUK1), which is stereoselective for the (-) enantiomer of carbovir 5'-monophosphate [Articles:1383219, 9145876, 10066527]. One study in particular found that (-)-carbovir 5'-monophosphate was 7,000 times more efficient as a substrate for guanylate kinase than the (+) enantiomer [Article:1383219]. Carbovir 5'-diphosphate is then converted to the active 5'-triphosphate form by various cellular kinases. These include creatine kinases (represented by the CK gene group on pathway), pyruvate kinases (PK gene group), nucleoside diphosphate kinases (NME gene group), phosphoglycerate kinase (PGK1), and phosphoenolpyruvate carboxykinase (PCK1). Given the stereoselectivity of guanylate kinase, only (-)-carbovir 5'-triphosphate is formed in any significant quantities. However, the (+) enantiomer has equivalent anti-viral activity [Articles:1383219, 9145876].
Abacavir can also be converted to carbovir 5'-monophosphate by other, minor pathways, such as transformation into carbovir, followed by phosphorylation of carbovir into carbovir 5'-monophosphate by inosine phosphotransferase. Since these pathways make up less than 2% of abacavir anabolism, they are not shown on the figure [Article:9145876].
A stylized depiction of the mechanism of action of abacavir within host cells is provided in pathway figure. Carbovir 5'-triphosphate (CBV-TP) represents the active form of abacavir, and works by blocking the action of HIV reverse transcriptase (HIV-RT). HIV replicates by taking advantage of the host cell's existing genetic machinery as well as the viral enzyme reverse transcriptase, which is responsible for taking viral RNA and converting it into double-stranded DNA. The viral DNA can then be incorporated into the host DNA, at which point the host machinery may convert the DNA into viral RNA. The viral RNA is then translated into viral proteins, which assemble to form the HIV virus [Article:11309630]. CBV-TP acts a guanosine analog, and competes for incorporation into the nucleotide chain being produced by HIV-RT from the viral RNA [Article:9145874]. Other NRTIs can act as derivatives of different nucleosides, leading to the same type of inhibition. Examples of this are didanosine and adenosine, zalcitabine and cytidine and zidovudine and thymidine [Article:10929917]. After incorporation of CBV-TP into the nucleotide chain, its lack of a 3'-OH on the ribose sugar on which to add the subsequent nucleoside blocks continuing synthesis of viral DNA [Articles:9145876, 11309630]. A comparison between the structures of carbovir triphosphate and guanosine triphosphate, as well as the structure of abacavir, can be seen below in Figure 1.
|Figure 1. Structures for abacavir, carbovir triphosphate (CBV-TP) and guanosine triphosphate (GTP). Note the similarities in structure between CBV-TP and GTP, excluding the absence of the critical free 3'-OH group on the ribose sugar ring in CBV-TP. This absence prevents the addition of any additional nucleotides and blocks further viral DNA synthesis. Structures adapted from [Article:18479171] and ChemSpider.|
CBV-TP is particularly well suited for this role since it is highly selective for reverse transcriptase as compared to DNA polymerases α, β, γ, and ε. Indeed, Ki values for DNA polymerases α, β, γ, and ε were 90, 2900, 1200 and 1900-fold greater, respectively, than the Ki value for HIV-RT [Article:9145874]. This selectivity for reverse transcriptase avoids the potentially toxic side effects that occur when DNA polymerases are inhibited. Many antiretroviral NRTIs are associated with a range of adverse events attributed to mitochondrial dysfunction, such as lactic acidosis and hepatic steatosis. This is believed to result from the inhibition of the mitochondrial DNA polymerase γ by these drugs, leading to altered mitochondrial DNA replication and resulting mitochondrial myopathy and toxicity. Abacavir has the lowest inhibition of polymerase γ, while zalcitabine, didanosine and stavudine have the highest [Article:10929917].
The pharmacogenetics of abacavir are well established, and almost exclusively relate to the human leukocyte antigen B (HLA-B) gene and its variant allele *57:01. This particular allele has been strongly associated with abacavir HSR over a large number of studies, as discussed below, However, the positive predictive value for this allele is approximately 50% [Article:18256392], indicating that other genetic factors may be involved in the development of an HSR. Limited research has been conducted in this area, though some potential exists for a variant in the gene HSP70-HOM (also known as HSP1AL), whose protein is hypothesized to have direct involvement in the stimulation of an immune reaction to abacavir [Article:17545699].
The HLA-B gene is a member of the major histocompatibility complex (MHC) region located on chromosome 6. This genomic region encodes three groups of genes involved in the immune system. HLA-B is part of the Class I group, along with HLA-A and HLA-C, all of which code for their eponymous proteins. The HLA-B protein and the other Class I group members are cell-surface molecules responsible for the presentation of endogenous peptides to immune system cells, and exist on almost all nucleated cells. This is in contrast to Class II molecules, which display exogenous peptides and are present only on antigen presenting cells (APCs) such as macrophages or dendritic cells. Briefly, Class I molecules such as HLA-B are heterodimers consisting of an α chain, encoded by the HLA-B gene, and a protein known as β2-microglobulin, which is encoded on chromosome 15. The alpha chain of HLA-B has four domains: one cytoplasmic, one transmembrane, one which binds to CD8+ cytotoxic T cells, and the last which makes up a peptide-binding groove, where the peptide to be presented is nestled. This particular region of the gene is highly polymorphic, allowing for the presentation of a wide variety of peptides. Most of the peptides that HLA-B presents come from the normal breakdown of cellular proteins, and are recognized by the immune system as such (i.e. "self" peptides). However, when a cell becomes infected by a pathogen, the proteins presented will be from the pathogen and recognized as foreign or "non-self". T cell antigen receptors (TCRs) on CD8+ cytotoxic T cells are responsible for this recognition, and will stimulate an immune reaction and destroy the cell [Immunobiology, Janeway, Chapters 3 and 5].
In 2002, two separate research groups published evidence that an allele known as HLA-B*57:01 was present in a significantly higher percentage of patients showing an abacavir HSR compared to patients with no reaction. One was conducted in a North American population [Article:11943262], and the other in a population known as the Western Australian HIV Cohort [Article:11888582]. Both included 200 patients. This was confirmed by another study within a UK population of 64 participants [Article:15247625]. However, these three studies were conducted using predominantly Caucasian males, limiting their scope. Despite this limitation, several clinics began implementing prospective screening of these alleles to great success [Articles:18025891, 17356469, 16758424]. A later study recognized the significance of the allele in White female and Hispanic populations, but did not find any significant associations in Black populations [Article:15016610]. This was likely due to the lower number of Black patients within this study (as compared to Hispanics or Whites) and the fact that Black populations have a lower carriage rate of the allele [Articles:11543903, 15016610]. European populations have a *57:01 allele frequency of about 6-7%, but African populations often have allele frequencies of less than 2.5%. Additionally, some Asian populations, such as the Japanese or South Koreans, have extremely low allele frequencies of 0.5% or less; in contrast, some Indian populations can have *57:01 frequencies of greater than 16% [Article:22378157]. In 2007, a study known as SHAPE (which included a similar number of White and Black participants) found that Black patients did have fewer cases of abacavir HSRs. However, in patients with immunologically confirmed HSRs, 100% of both White and Black patients were positive for the HLA-B*57:01 allele. This suggested that though immunologically confirmed HSRs are rare among Black populations due to the reduced carriage of the allele, HLA-B*57:01 allele has the same clinical implications in both populations [Article:18444831]. A definitive association between this allele and abacavir HSRs came in 2008 with the results of the PREDICT-1 study, a double-blind, prospective, randomized study with 1956 patients from 19 countries. Patients were observed for six weeks and separated into two categories: those that underwent screening for the HLA-B*57:01 allele and were eliminated if they tested positive, and those that underwent standard care without any screening. Abacavir HSRs were immunologically confirmed with skin patch testing. The results of the study showed that screening completely eliminated HSRs - 0% of the patients screened had a HSR, while 2.7% of the control population did. This gave the screening a negative predictive value of 100%. However, the study found a positive predictive value of 47.9%, indicating that about half of all patients who are HLA-B*57:01 positive will not develop an abacavir HSR [Article:18256392]. This indicates that other genes are likely involved in the development of the HSR. This paper, along with the large amount of other existing evidence, led the FDA to implement a boxed warning in 2008, detailing the risk of a HSR for abacavir-treated patients with the HLA-B*57:01 allele. The FDA, along with the EMA, CPIC, and DPWG also recommended that all patients be screened before being treated, and to not use abacavir in HLA-B*57:01-positive individuals.
The HLA-B protein has no direct effect on abacavir pharmacokinetics or pharmacodynamics, and it is still unclear how the HLA-B*57:01 allele affects susceptibility to drug hypersensitivity. Several hypotheses exist. One theory is the hapten concept, which suggests that small compounds such as drugs (called "haptens"), bind to the peptides bound to immune receptors such as HLA-B, causing T cells to react and stimulate an immune reaction [Article:17075282]. Another theory is the p-i concept (pharmacological interactions with immune receptors), which suggests that drugs bind directly and reversibly to immune receptors, stimulating an immune reaction [Article:17075282]. Recent evidence seems to support an alternative hypothesis. Two studies, both published in 2012, found that abacavir can bind non-covalently and with specificity in the F pocket of the peptide-binding groove of HLA-B*57:01 [Articles:22645359, 22722860]. Due to the amino acid residues unique to *57:01, abacavir can bind only to this particular form of HLA-B. The binding of abacavir to HLA-B*57:01 is believed to change the shape and chemistry of the antigen-binding cleft, and consequently the repertoire of peptides which can bind the molecule. Indeed, both of these papers, as well as an additional paper by Norcross et al., all identified particular changes in the peptides presented by HLA-B*57:01 in the presence of abacavir, as compared to when the drug is absent [Articles:22645359, 22722860, 22617051]. Conventional HLA-B*57:01 epitopes frequently possess large hydrophobic amino acids such as tryptophan or phenylalanine at their C-terminus; Illing et al., Ostrov et al., and Norcross et al. all found that peptides eluted under the presence of abacavir showed a preference for isoleucine or leucine at this position [Articles:22722860, 22645359, 22617051]. This binding and subsequent peptide alteration is shown on the pathway figure - a dashed line is used, as this mechanism is not currently well established. The typical cycle of peptide loading and transport to the cell surface plasma membrane ([Article:14511229]) is also shown. Since T cells are trained to be tolerant to a particular repertoire of peptides during their development in the thymus, the alteration in the peptides that can be presented may mean that these new peptides are perceived as foreign. This change would stimulate CD8+ T cell production and response, and would manifest as an abacavir HSR [Articles:22722860, 22645359]. Indeed, CD8+ T cells are abundant in skin biopsies of patients who present with a rash during an abacavir HSR [Article:18549801].
The positive predictive value of ~50% for the HLA-B*57:01 allele and abacavir HSRs indicates the need for further studies to elucidate other genes that may affect the development of a HSR. Research in this area has been scarce, but several studies have suggested a member of the 70 kilodalton heat shock protein (HSP70) family as a potential factor. The HSP70 proteins are responsible for protecting cells from stress, as well as other cellular activities, such as assisting in protein folding [Article:9222585]. Three genes within the human MHC region encode members of the HSP70 family: HSP70-1, HSP70-2 and HSP70-HOM [Article:1356099]. HSP70-1 and -2 encode identical heat-inducible protein products, while HSP70-HOM encodes a similar but non-heat-inducible protein [Article:1356099]. A study using the Western Australian HIV Cohort found that the reference C allele at rs2227956 in the HSP70-HOM gene (which results in a threonine at residue 493 as opposed to a methionine) was found in combination with the HLA-B*57:01 allele in 94.4% of immunologically-confirmed hypersensitive cases and 0.4% of controls, while the HLA-B*57:01 allele appeared on its own in 94.4% of hypersensitive cases and 1.7% of controls. The authors suggested that consideration of the HSP70-HOM allele in addition to HLA-B*57:01 may therefore increase the ability to discriminate between patients who would develop a HSR and tolerant controls. The population consisted of 230 controls and 18 patients with a HSR, and the alleles were found to be in strong linkage disequilibrium [Article:15024131]. Further studies in larger populations are needed to verify this association. However, a later study did find that the HSP70-HOM protein co-localized with both the HLA-B*57:01 protein and abacavir within the endoplasmic reticulum. This implies that the HSP70-HOM 493Thr variant might lead to a protein that somehow facilitates the presentation of abacavir antigens to CD8+ T cells, perhaps by chaperoning the drug in antigen processing [Article:17545699].
The implementation of HLA-B*57:01 testing prior to abacavir treatment is one of the best examples of pharmacogenetic research being used in the clinic, and genotyping for this allele is widely available in the Western world. Despite this, further research should be conducted into other genes that lead to a propensity for an abacavir HSR. This could increase the positive predictive value, allowing more patients to be given abacavir who could benefit from treatment. Currently there is very little evidence for the involvement of other genes, and only a variation in the HSP70-HOM gene has emerged as a potential factor. Further advancement of our understanding in this area could prevent inappropriate denial of abacavir to patients who would tolerate it, and hopefully help further elucidate the mechanism by which abacavir elicits its hypersensitivity reaction.
Barbarino Julia M, Kroetz Deanna L, Altman Russ B, Klein Teri E. "PharmGKB summary: abacavir pathway" Pharmacogenetics and genomics (2014).
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Entities in the Pathway
Drugs/Drug Classes (1)
Relationships in the Pathway
|Arrow From||Arrow To||Controllers||PMID|
|(-)-carbovir 5'-diphosphate||(-)-carbovir 5'-triphosphate||CKB, CKM, NME1, NME2, PCK1, PGK1, PKLR, PKM||1383219, 9145876|
|(-)-carbovir 5'-monophosphate||(-)-carbovir 5'-diphosphate||GUK1||10066527, 1383219, 9145876|
|Abacavir-HLA-B*57:01:01 ()||Black Box: Alteration of peptide binding repertoire|
|abacavir 5'-monophosphate||(-)-carbovir 5'-monophosphate||Cytosolic deaminase||9145874, 9145876|
|abacavir||2269W93||ADH1A, ADH1B, ADH1C||10582871|
|abacavir||abacavir 5'-monophosphate||ADK||9145874, 9145876|
|"Self" peptide||Abacavir-HLA-B*57:01:01 ()||14511229|
|Abacavir-HLA-B*57:01:01 ()||Abacavir-HLA-B*57:01:01-self peptide ()||14511229|
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