276 PharmGKB summary PharmGKB summary: abacavir pathway Julia M. Barbarinoa, Deanna L. Kroetzc, Russ B. Altmana,b and Teri E. Kleina Pharmacogenetics and Genomics 2014, 24:276–282 Correspondence to Teri E. Klein, PhD, Department of Genetics, 318 Campus Drive, Clark Center Room S221A, MC: 5448, Stanford, CA 94305, USA Keywords: abacavir, HIV, HLA-B, HLA-B*57:01, HSP70-HOM, hypersensitivity, Tel: + 1 650 736 0156; fax: + 1 650 725 3863; e-mail: [email protected] pharmacodynamics, pharmacogenetics, pharmacokinetics, PharmGKB Received 4 December 2013 Accepted 5 February 2014 Departments of aGenetics, bBioengineering, Stanford University, Stanford and cDepartment of Bioengineering and Therapeutic Sciences, University of California, San Francisco, California, USA Introduction P450 (CYP) enzymes are predicted [2]. In addition, in- Abacavir is a nucleoside reverse transcriptase inhibitor vitro studies have shown that abacavir is unlikely to (NRTI) used for treatment of HIV infection. HIV inhibit CYP enzymes at clinically relevant concentra- infection is usually treated with antiretroviral therapy tions [14]. As non-NRTIs and protease inhibitors are regimens, which consist of three or more different drugs primarily metabolized by CYP enzymes [15], this may used in combination. Typical antiretrovirals used in these eliminate the potential for drug interactions with regimens include NRTIs, non-NRTIs, protease inhibi- these types of antiretrovirals. No clinically significant tors, and integrase strand inhibitors [1]. Abacavir makes pharmacokinetic changes were seen when abacavir was an ideal addition to these types of combination therapies administered with other NRTIs such as lamivudine and because of its dosing flexibility. It can be administered zidovudine [2,16]. As alcohol is also metabolized by either once or twice a day to match the dosing pattern of ADH, pharmacokinetic interactions between the drug other drugs and can also be administered as tablets that and ethanol have been analyzed, but no clinically contain other antiretroviral drugs such as lamivudine and significant changes or new adverse events have been zidovudine, allowing for a reduction in pill count [2]. reported [17]. Several studies have found a link between Abacavir is generally well tolerated, and common side abacavir administration and virologic response in hepatitis effects include nausea, headache, and diarrhea [2]. C patients being treated with ribavirin and pegylated However, B5–8% of patients experience a hypersensi- interferon who are also coinfected with HIV; in these tivity reaction (HSR) within the first 6 weeks of treat- patients, abacavir usage has been found to be significantly ment. Symptoms of an HSR include at least two of the associated with early virologic failure [18] and lack of following: fever, rash, cough, gastrointestinal symptoms, sustained virologic response [19,20]. However, subpopu- dyspnea, and fatigue [3]. These symptoms worsen with lation analyses from two of the studies found that the continued treatment, but typically improve within 24 h impact of abacavir on sustained virologic response was after discontinuation. However, drug rechallenge after only significant in patients with baseline hepatitis C viral discontinuation of abacavir because of an HSR can result RNA above a certain level [20], ribavirin daily doses in symptom recurrence within a matter of hours, and can below a certain level [20], or ribavirin trough concentra- lead to immediate and potentially fatal allergic reac- tions below a certain level [19]. In addition, a number of tions [4,5]. This hypersensitivity reaction is strongly studies have found no association between abacavir usage linked to the presence of the HLA-B*57:01 allele, and and virologic response [21–24]; therefore, it is uncertain testing for the allele before abacavir treatment is whether these two drugs have a significant and harmful recommended by the US Food and Drug Administration interaction. (FDA) [6], the European Medicines Agency [7], the Clinical Pharmacogenetics Implementation Consortium Pharmacokinetics [3], and the Dutch Pharmacogenetics Working Group [8]. A schematic representation of abacavir disposition within Abacavir was also reported to be associated with a higher the body is provided in Fig. 1. Abacavir is rapidly absorbed risk for myocardial infarction as compared with other following oral administration, and has a mean absolute NRTIs [9–12]. However, a meta-analysis carried out bioavailability of B83% [2,25]. The drug is lipophilic by the FDA in 2012 failed to find any such associa- yet also shows high water solubility, allowing it to cross tion [13]. cell membranes by passive diffusion alone. These proper- ties may explain its high bioavailability, as well as Drug interactions its ability to easily penetrate into tissues such as the As abacavir is primarily metabolized by cytosolic alcohol blood–brain barrier [2,26]. After absorption, abacavir is dehydrogenase (ADH) and uridine diphosphate glucur- extensively metabolized within the liver, with less than onosyltransferase (UGT) enzymes, no interactions be- 2% of the drug excreted unchanged in the urine [27]. tween abacavir and inducers or inhibitors of cytochrome ADH and UGT are the primary enzymes responsible for 1744-6872 c 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins DOI: 10.1097/FPC.0000000000000040 Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. PharmGKB summary: abacavir pathway Barbarino et al.277 Fig. 1 Abacavir Hepatocyte UGT 361W94 Minor Abacavir metabolites Elimination through urine and feces ADH 2269W93 Host cell P ??? P (−−)-Carbovir GUK1 Abacavir PP (−−)-Carbovir ADK PCK1 Abacavir CK PGK1 NME Abacavir Abacavir PK HLA-B∗57:01 HLA-B∗57:01 57:01 ∗ “Self” P P P peptide −− HLA-B ( )-Carbovir Abacavir Alteration of peptide binding repertoire Abacavir ∗ HLA-B 57:01 ER “Self” peptide perceived as foreign Double- Viral stranded mRNA HIV RT Abacavir hypersensitivity DNA reaction Translation Genomic Integration RNA Proviral Viral RNA DNA proteins Transcription Uncoating Nucleus Genomic RNA HIV Assembly HIV Schematic representation of abacavir pharmacokinetics and pharmacodynamics. The potential mechanism of an abacavir hypersensitivity reaction is also shown and is drawn using dashed lines as it is not currently well established. A fully interactive version is available online at http://www.pharmgkb.org/pathway/PA166104634. abacavir metabolism within hepatocytes. Metabolism by balance study found that 83% of the original dose was ADH results in the inactive carboxylate metabolite eliminated in the urine and 16% in the feces. Of the 83% 2269W93; metabolism by the UGT enzymes results in eliminated through urine, 36% of the dose recovered was the inactive glucuronide metabolite 361W94 [27]. A mass the glucuronide metabolite, and 30% was the carboxylate Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited. 278 Pharmacogenetics and Genomics 2014, Vol 24 No 5 metabolite. The remaining dose was either the parent incorporation of CBV-TP into the nucleotide chain, its drug or trace metabolites [2,27]. lack of a 30-OH on the deoxyribose sugar on which subsequent nucleotides are added blocks continuing The parent drug that is not metabolized by hepatocytes synthesis of viral DNA [28,31]. A comparison between undergoes metabolism within viral-infected cells by a the structures of carbovir triphosphate and deoxyguano- different set of intracellular enzymes, converting the drug sine triphosphate, as well as the structure of abacavir, is into its pharmacologically active metabolite. Initially, shown in Fig. 2. abacavir is metabolized to abacavir 50-monophosphate by the enzyme adenosine phosphotransferase (encoded by the CBV-TP is particularly well suited for this role as it is ADK gene). It then undergoes deamination by an unknown highly selective for reverse transcriptase as compared 0 cytosolic enzyme to form (–)-carbovir 5 -monophosphate; with DNA polymerases a, b, g, and E. Indeed, Ki values no diphosphates or triphosphates of abacavir have been for DNA polymerases a, b, g, and E were 90-, 2900-, detected within cells [26,28]. (–)-Carbovir 50-monophos- 1200-, and 1900-fold greater, respectively, than the 0 phate is then converted to (–)-carbovir 5 -diphosphate by Ki value for HIV-RT [26]. This selectivity for reverse the enzyme guanylate kinase (GUK1), which is stereo- transcriptase prevents the potentially toxic side effects selective for the (–) enantiomer of carbovir [28–30]. One that occur when DNA polymerases are inhibited. Many study in particular found that (–)-carbovir 50-monophos- antiretroviral NRTIs are associated with a range of phate was 7000 times more efficient as a substrate adverse events attributed to mitochondrial dysfunction, for guanylate kinase than the (+) enantiomer [30]. such as lactic acidosis and hepatic steatosis. These are (–)-Carbovir 50-diphosphate is then converted to the active believed to result from the inhibition of mitochondrial 50-triphosphate form by various cellular kinases. These DNA polymerase g by these drugs, leading to altered include creatine kinases (represented by the CK gene mitochondrial DNA replication and resulting in mito- group in Fig. 1), pyruvate kinases (PK gene group), chondrial myopathy and toxicity. Abacavir has the lowest nucleoside diphosphate kinases (NME gene group), inhibition rate for DNA polymerase g, whereas zalcita- phosphoglycerate kinase (PGK1),