When and how to use maraviroc in HIV-infected patients

Soriano, Vincenta; Perno, Carlo-Federicob; Kaiser, Rolfc; Calvez, Vincentd; Gatell, Jose Me; di Perri, Giovannif; Pillay, Deenang; Rockstroh, Juergenh; Geretti, Anna Maríai

doi: 10.1097/QAD.0b013e328332d32d
Editorial Review

CCR5 antagonists have recently entered the HIV armamentarium. This novel class of drugs inhibit viral entry blocking host cellular receptors, and therefore display unique mechanisms of resistance, different from other antiretroviral drugs. Maraviroc only blocks replication of R5 viruses and accordingly patients with X4 or D/M viruses do not or only marginally benefit from maraviroc therapy. Viral tropism has to be tested before considering maraviroc prescription. Phenotypic and more recently genotypic tools have been demonstrated to reliably estimate HIV-1 tropism in most cases and predict viral response. Beyond the initial approval only for antiretroviral-experienced patients, the pharmacokinetic properties and safety profile of maraviroc may support an earlier use of the drug. Studies using maraviroc in drug-naive patients and as part of switch strategies are warranted.

Author Information

aDepartment of Infectious Diseases, Hospital Carlos III, Madrid, Spain

bDepartment of Experimental Medicine, University of Rome Tor Vergata, Rome, Italy

cInstitute of Virology, University of Cologne, Cologne, Germany

dService of Virology, Hospital Pitié-Salpetrière, Paris, France

eInfectious Diseases Unit, Hospital Clinic, Barcelona, Spain

fUniversity of Torino, Torino, Italy

gDepartment of Infection, University College London, and Centre for Infections, Health Protection Agency, London, UK

hDepartment of Medicine, University of Bonn, Bonn, Germany

iDepartment of Virology, Royal Free Hampstead NHS Trust & University College London Medical School, London, UK.

Received 25 May, 2009

Revised 1 September, 2009

Accepted 9 September, 2009

Correspondence to Dr Vincent Soriano, Department of Infectious Diseases, Hospital Carlos III, Calle Sinesio Delgado 10, Madrid 28029, Spain. Tel: +34 91 453 25 00; fax: +34 91 7336614; e-mail: vsoriano@dragonet.es

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New classes of antiretroviral drugs that target different steps of the HIV replication cycle have recently been approved and others are in advanced stages of clinical development. Viral entry continues to be one of the most attractive targets in the search for new drugs against HIV infection [1,2]. Whereas fusion blockers, such as enfuvirtide, bind to the viral envelope, co-receptor antagonists represent an interesting class of HIV entry inhibitors since they do not directly target virus proteins, and therefore may select for drug resistance less frequently. Maraviroc (Celsentri, Selzentry; Pfizer Ltd., Sandwich, UK) is the first marketed chemokine (C-C motif) receptor 5 (CCR5) antagonist and the only oral HIV entry inhibitor approved to date for clinical use [3]. Maraviroc selectively inhibits the replication of CCR5-tropic (R5) HIV variants via an allosteric mechanism after binding to a transmembrane CCR5 co-receptor pocket [4,5]. This review summarizes the most recent clinical data regarding maraviroc, providing information about situations in which the use of this drug may prove particularly advantageous.

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HIV-1 tropism determination

CCR5 antagonists do not display activity against chemokine (CXC motif) receptor 4 (CXCR4)-using HIV variants. As a consequence, the presence of detectable X4 or R5/X4 dual-tropic viruses, either as dominant strains or as mixtures of CCR5 and CXCR4-using strains, has been associated with therapeutic failure using maraviroc [6–8]. Assessment of HIV-1 tropism is therefore required before recommending treatment with CCR5 antagonists. Several assays have been developed to determine HIV tropism in clinical samples [9,10]. The Trofile phenotypic assay (Monogram Biosciences, South San Francisco, California, USA), which is based on recombinant virus technology [11], has been the most widely used to date (Table 1). The test identifies X4 strains with a sensitivity of 10% when using clonal mixtures, but does not differentiate between dually tropic viruses and mixtures of X4 and R5 strains, reporting results as ‘dually or mixed’ (D/M) virus [11]. Although phenotypic assays such as Trofile are considered reliable for assessing HIV tropism, they remain far from perfect as diagnostic tests for clinical purposes. They are labour-intensive, slow, expensive and require special laboratory facilities and expertise. In recent years, efforts have been made to explore alternative testing approaches, mainly using genotypic predictors of viral tropism, as a guide to the use of maraviroc in clinical practice.

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Ultrasensitive phenotypic tropism assays

Monogram Biosciences has developed an enhanced sensitivity tropism assay (ESTA), which is 10–100-fold more sensitive for X4 minor populations when using clonal mixtures [12]. ESTA has been available since June 2008 and has replaced the original Trofile assay used in the pivotal clinical trials. It should be emphasized that the standard Trofile, which is no longer commercially available, is the only phenotypic assay that has been used prospectively to recruit patients in clinical trials and shown correlation with virological outcomes in this context. The performance of ESTA compared with the original Trofile was investigated in a retrospective analysis of maraviroc and vicriviroc clinical trials. In the MERIT trial, which evaluated maraviroc versus efavirenz (both with zidovudine and lamivudine) as initial antiretroviral therapy [13], a retrospective analysis using ESTA showed that 14.7% of patients reported as R5 with the original Trofile assay were classified as D/M using ESTA [14]. Similar results were obtained in the vicriviroc re-analysis with ESTA [15].

Since patients infected with detectable X4 variants may still have sufficient high proportion of R5 strains to experience a benefit when receiving maraviroc, the threshold of X4 variants which may predict virological failure to the drug must be established. Data from the phase 2b clinical study A4001029 of maraviroc in patients with a mixed tropic infection [6] showed no benefit in viral response at 48 weeks. However, the clinical response in the placebo arm was very poor, consistent with the fact that these patients were highly treatment-experienced. For the majority of these patients, therefore, maraviroc was the only active drug used and a clinically relevant virologic response in this setting is highly unlikely. Recently, Valdez et al. [16] showed that a weighted optimized background treatment susceptibility score, rather than a low level of X4 virus at baseline (as defined as a change in Trofile test result from R5 at screening to D/M at baseline), was the strongest predictor of virological response at 48 weeks in the MOTIVATE trials. This finding highlights the contribution to virus suppression of other antiretroviral drugs, such as most nucleoside analogues or protease inhibitors, for which partial activity may be recognized when confronting viruses with only a few drug-resistance mutations. It is the activity of the accompanying drugs which may permit to obtain benefit from maraviroc in patients with a low proportion of X4 variants. On the basis of these findings, it may be proposed that attempts at categorically excluding the presence of X4 strains at very low frequency within the viral population may lead to the unnecessary exclusion of a therapeutic option that could still provide at least partial activity.

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Genotypic tropism estimation

Although the Trofile technology has been the most widely used, it displays logistical and technical limitations that represent a serious obstacle to the widespread use of CCR5 antagonists, especially outside the United States, where the Monogram Biosciences laboratory is based. There is, therefore, a need for easier and more rapid viral tropism determination tools in routine clinical practice. HIV-1 co-receptor usage can be predicted using the amino acid sequence of the V3 region of gp120, which is the main determinant of viral tropism [17–19]. Indeed, several genotypic algorithms have been developed to predict HIV co-receptor usage based on V3 genetic sequences, and many of them are freely available via publicly accessible websites [9].

The reliability of genotypic tools to determine HIV tropism in clinical samples compared with phenotypic assays has been examined in several studies. Some of these comparisons noticed relatively poor concordances, mainly due to low sensitivity (<45%), to detect X4 variants by genotypic algorithms [19]; however, more recent studies have shown better sensitivity when employing certain genotypic tools and using phenotypic assays other than Trofile as a reference or ‘gold standard’ [20–23]. Recent studies have proposed different strategies to improve the sensitivity of genotypic methods to detect X4 variants. A few have substantially increased the sensitivity to detect X4 variants (up to 93%) through simple modifications in the interpretation algorithms [24], including structural/biochemical properties of the V3 loop and clinical parameters such as CD4 and CD8 cell counts, and plasma viremia. Using the latter approach within the geno2pheno co-receptor interpretation software, Strang et al. [25] recently showed 91% concordance with ESTA in a mixed population of antiretroviral-experienced and drug-naive patients. Another approach has combined the results given by different genotypic algorithms to produce a ‘pooled’ X4-sensitive tropism prediction [26].

Due to lack of a gold diagnostic standard, studies to date that have compared predictions by one tropism method to the other have significant limitations. The validation of genotypic tropism prediction methods requires not so much evidence of perfect concordance with the Trofile (or ESTA) assay, but rather evidence of a similar ability to correctly identify patients that will benefit from the use of maraviroc. It is therefore most significant that the use of genotypic tropism prediction based upon V3 sequencing data alone has recently shown similar ability to Trofile to predict virological responses to maraviroc within clinical trials [27]. In the context of cohort studies, encouraging data from the Berlin cohort have also been released showing that genotypic tropism testing can reliably guide clinical practice [28].

The use of pyrosequencing technology has allowed investigation of whether improvements in prediction of X4 variants could be obtained searching a larger number of genomes than using conventional (‘bulk’) sequencing. This technology may provide a unique opportunity to enhance the sensitivity of identifying minority variants, including those from X4 viruses [29] and a unique tool to explore composition of the viral quasispecies [30,31]. Ultradeep sequencing, however, is a sophisticated and expensive method, only available in a few research facilities. Interpretation of the large amount of sequencing data generated for each sample remains challenging. Furthermore, reliability of results at very low frequency has been recently called into question. Whereas further studies are in progress, it seems unlikely that ultradeep sequencing will become helpful for providing information on viral tropism in routine diagnostic settings in the short term.

A further consideration regarding the interpretation of results of ultradeep sequencing is again related to the optimal sensitivity threshold for X4 variants that is clinically relevant. In this regard, a re-analysis of the maraviroc A4001029 study, in which all enrolled patients had baseline evidence of X4 or D/M viruses by Trofile, demonstrated using ultradeep sequencing that there is an inverse relationship between the proportion of plasma variants and the extent of virological responses to maraviroc [32]. Overall, patients with a low prevalence of X4 variants (<10%) showed a substantial viral load decline (–2.6 log HIV-RNA copies/ml at week 8), regardless of the result provided by Trofile. If these results are confirmed, patients with less than 10% X4 variants might benefit from maraviroc therapy.

The potential for using maraviroc without knowing the result of a preceding tropism assay is also under debate. This approach could be especially interesting for antiretroviral-naive patients in which R5 variants are by far predominant [33], with only 18–26% of patients showing D/M or X4 viruses by either Trofile or other testing methodologies [33,34]. Another interesting subset of patients are those infected with HIV-1 subtypes showing low propensity for CXCR4 use, as it has been shown for clade C [35].

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Resistance to CCR5 antagonists

The antiviral activity of CCR5 antagonists may be evaded by two main mechanisms: outgrowth of X4 or dual tropic viruses usually pre-existing any drug pressure as a minority viral population; and selection of mutations in the viral envelope gp120 molecule that may allow HIV-1 to regain its binding affinity for the CCR5 co-receptor in the presence of maraviroc [1,36,37]. The evaluation of the time to treatment failure suggest that selection of pre-existing X4 variants explain earlier failures, whereas selection of gp120 changes may account more often for later failures [7].

Data from the MOTIVATE trials showed that the majority of patients failing maraviroc experienced a shift from R5 to D/M or X4 viruses, which was seen rarely in the placebo arm (57 versus 6%, respectively, at 48 weeks) [7]. On the contrary, at 24 weeks, 43% of patients who failed maraviroc harbouring R5 viruses showed selection of gp120 changes, mainly within the variable regions (V1–V5) [38]. More recently, a post-hoc analysis from 331 patients from the MOTIVATE trials identified maraviroc resistance in 22 (35%) out of 62 virological failures with R5 virus. Lack of other active drugs in the combination regimen or use of a single active nucleos(t)ide analogue was seen in 16 of 22 (73%) of these patients [39]. Overall, these data indicate that the genetic barrier to resistance of maraviroc is probably intermediate rather than low.

Mutations in the V3 loop appear to play a key role in conferring maraviroc resistance in R5 viruses. The impact of changes in gp120 outside the V3 loop is currently unclear [40]. These changes enable the mutant virus to interact with the maraviroc-bound disrupted form of the second extracellular loop (ECL2) of the CCR5 receptor. Although mutations tend to occur in the stem of the V3 loop (Fig. 1), the patterns of amino acid changes seem to be quite heterogeneous, currently creating an obstacle to genotypic testing for maraviroc resistance and making phenotypic resistance testing necessary. The problem is further compounded by the observation that mutations reported to be selected in patients failing maraviroc with R5 viruses have been found in 7% of patients never exposed to maraviroc [40]. Moreover, the same V3 amino acid changes may produce maraviroc resistance in some but not all patients. At this time, baseline screening of maraviroc-associated resistance mutations in patients who are naive to the drug is not warranted. Furthermore, the potential clinical utility of maraviroc resistance testing at the time of virological failure is uncertain. The potential for cross-resistance between maraviroc and vicriviroc – another CCR5 antagonist in advanced stages of clinical development – is expected, given that both drugs interact with the same binding site at the CCR5 co-receptor. Indeed, viruses isolated from patients who have failed vicriviroc have shown high-level resistance to maraviroc [41].

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Immunological benefit of CCR5 antagonists

Patients who received maraviroc in the MOTIVATE trials, both in the once daily and twice daily dosing schedule, experienced significantly greater CD4 gains from baseline than did patients who received placebo (+116, +124 and +61 cells/μl, respectively; P < 0.0001 for each maraviroc group versus placebo) [42]. This was also true for patients on maraviroc who experienced virological failure, as they showed a greater mean CD4 gain compared to the placebo arm {mean [95% confidence interval (CI)]: +64 (47–82), +74 (56–92) and +24 (10–40) cells/μl, respectively}. Interestingly, patients on maraviroc who experienced failure with selection of X4 or D/M viruses had smaller CD4 gains as compared to patients who experienced failure with R5 viruses [mean (95% CI) +47 (27–66) versus +77 (40–115) cells/μl, and +57 (32–82) versus +133 (88–178) cells/μl for once and twice daily doses, respectively]. However, the mean CD4 gain in patients who experienced failure with X4 or D/M viruses was still higher than in the overall placebo group [+47 (27–66), +57 (32–82) and +24 (10–40) cells/μl, respectively] [7]. These results are reassuring, since they rule out concerns about a putative detrimental effect on CD4 cells associated with expansion of CXCR4-using viruses in patients failing maraviroc. A further reassuring observation was that treatment discontinuation after failure was followed by the disappearance of X4 or D/M viruses from the dominant quasispecies with re-emergence of R5 strains, indicating lack of fitness advantage for the CXCR4-using strains.

Altogether, these data suggest that treatment with maraviroc may provide an immunological benefit beyond its direct antiviral activity. This phenomenon has already been reported in HIV patients failing the fusion inhibitor enfuvirtide [43–46]. This suggests that immunological benefit unrelated to suppression of viral load might be a class effect of entry inhibitors. The underlying mechanisms are still unknown. CD4+ T cells might increase as a result of either proliferation or increased survival. The inhibition of gp120 binding to CCR5 by maraviroc may disrupt the chain of events that leads to cell death. In this regard, CCR5 inhibitors may increase CD4 cells as result of blocking gp120-driven apoptosis, thereby increasing cell survival [47,48].

The particular immune benefit provided by maraviroc therapy may suggest benefit in considering its use as part of intensification strategies, especially in patients considered as nonimmunological responders, such as those failing to reach CD4 cell counts above 200 cells/μl despite achieving complete viral load suppression on HAART. This benefit, however, has not been confirmed in recent studies [49,50]. Given that this subset of patients remains at increased risk for developing opportunistic complications, pursuing a greater CD4 recovery is warranted.

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The big picture: broader use of maraviroc

The particular features of maraviroc may allow the recognition of specific scenarios and/or patient populations in which the drug may be of choice.

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Drug-naive patients

Although maraviroc was initially approved for patients with triple-class resistance, based upon results from the MOTIVATE trials [7,36], evidence indicates that only approximately half of these patients have dominant R5 viruses and are susceptible to maraviroc. In contrast, R5 viruses predominate in the acute phase and most of the chronic phase of HIV-1 infection [33,34]. As a result, the population that may overall gain the greatest benefits from the use of maraviroc is that in the early rather than very advanced stages of infection [8].

The MERIT trial compared the safety and efficacy of maraviroc versus efavirenz, each in combination with zidovudine–lamivudine, in drug-naive patients, and concluded that the maraviroc arm did not meet the criteria for noninferiority compared with the efavirenz arm over 48 weeks when viral load suppression less than 50 HIV-RNA copies/ml was considered [13]. However, the noninferiority was demonstrated for viral load suppression less than 400 HIV-RNA copies/ml. Moreover, a careful analysis of the MERIT data showed that 4% of patients experienced a change in the Trofile result from R5 to D/M viruses between the screening sample and the baseline sample collected at the time of therapy initiation. When the virological response at 48 weeks was analysed according to the baseline tropism result and only R5 patients were included in the analysis, maraviroc was noninferior to efavirenz (69.3 versus 68%, respectively, showed a viral load <50 HIV-RNA copies/ml) [51]. These data suggest the presence of CXCR4-using viruses within the quaspecies population at a rate that was just below (or just above) the sensitivity threshold of the Trofile assay. Consistent with this hypothesis, re-analysis of the MERIT trial using ESTA reclassified as D/M nearly 15% of samples originally scored as R5 by Trofile. Following this new assignment, the proportion of patients achieving less than 50 copies/ml at 48 weeks was the same (68%) in both the maraviroc and efavirenz arms (Fig. 2). Since the proportion of patients harbouring R5 variants was 80% in this drug-naive population [14], the results of the MERIT trial may make the use of maraviroc as first-line therapy more attractive, especially in patients presenting with nonadvanced immune deficiency [8]. It should be noted, however, that not all specimens from MERIT could be re-tested using ESTA. More recently, the 96-week results of the MERIT trial were reported only for the subset of patients with baseline R5 using ESTA, and maraviroc kept as noninferior to efavirenz in terms of plasma HIV-RNA less than 50 copies/ml (59 versus 62%, respectively); however, more patients discontinued efavirenz due to side effects, whereas more patients failed virologically on the maraviroc arm [52].

In drug-naive patients, the sensitivity for detecting X4 viruses remains an important consideration, as one would wish to avoid development of resistance to the nucleoside analogue backbone in the few patients who may not achieve undetectable viremia. Whereas clinical validation data for genotypic tropism are awaited in this setting, treatment strategies that may be considered include use of a four-drug combination that combines a ritonavir-boosted protease inhibitor in the induction phase, followed by maintenance on triple therapy after virological suppression is achieved, or use of nucleoside analogue backbone with a high genetic barrier to resistance (e.g. tenofovir and zidovudine) for the induction phase, followed by maintenance on a lamivudine (or emtricitabine)-based backbone. The main advantages of this approach would be to preserve drug susceptibility while reducing toxicity associated with the use of efavirenz or protease inhibitors. Alternatively, maraviroc may be considered in less conventional drug combinations to replace, for example, tenofovir in patients at risk for renal disease, or abacavir in those at risk for cardiovascular disease. Whereas all these strategies look interesting, they await support from clinical trials [8].

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Simplification strategies

The long-term use of the most commonly prescribed antiretroviral drugs has been associated with a broad range of adverse events, including metabolic abnormalities [53], increased cardiovascular risk [54,55], lipodistrophy [56], hepatotoxicity [57], gastrointestinal disturbances [58] and neuropsychiatric conditions [59]. In contrast, maraviroc has demonstrated an excellent safety profile in clinical trials. Data from the MERIT trial showed that fewer patients discontinued maraviroc than efavirenz due to adverse events (4.2 versus 13.6%, respectively) [13]. Moreover, lipid abnormalities occurred less frequently in patients taking maraviroc than efavirenz [13]. Since maraviroc can be given once a day (600 mg) as two pills [60], causes minimal side effects and displays easily manageable drug interactions, its convenience makes the drug attractive for simplification purposes. Although, more information about potential long-term toxicities, including predisposition to more severe infections [49,61], must be ruled out, altogether the current data support considering maraviroc as part of switch strategies in patients having suppressed HIV replication with regimens that are less well tolerated.

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Immunological nonresponders

A subset of HIV patients with low CD4 cell counts may show an impaired CD4 gain despite prolonged suppression of HIV replication with HAART. These patients may continue to be at increased risk for developing opportunistic complications. Strategies such as treatment intensification with antiretroviral drugs [62,63] or use of interleukin (IL)-2 or IL-7 have been explored in these individuals, generally with poor results in terms of overall clinical benefit even when CD4 gains have occurred [64,65]. In this regard, reports of CD4 gains using enfuvirtide, another HIV entry inhibitor, in the face of virological failure [43,66] have encouraged the exploration of whether maraviroc could be useful in the subset of HIV patients experiencing a lack of appropriate CD4 recovery while receiving HAART. Additionally, since maraviroc is given orally, it would be preferred over enfuvirtide for intensification strategies. Although preliminary results have shown a null or minimal benefit in this specific situation [49,50], further studies including larger number of patients and longer follow-up are warranted.

In HIV patients with undetectable viremia, examination of viral tropism in plasma is currently not commercially available. Two alternative strategies may be proposed: first, retrospective examination of viral tropism in plasma specimens stored before attainment of viral suppression with HAART; second, determination of viral tropism using current proviral DNA. In both cases, however, it is crucial to determine the dynamics of viral tropism during prolonged HIV suppression under HAART and the extent of correlation in viral tropism between plasma and cellular compartments. Fortunately, emerging data provide evidence that both approaches may be helpful for predicting tropism by phenotypic or genotypic tools [67,68]. Shifts in HIV tropism under prolonged suppressive HAART seem to be uncommon and correlation between plasma RNA and proviral DNA appears good, probably with an increased detection of X4 variants in proviral DNA. As a result, maraviroc might be confidently used as part of simplification or intensification strategies as long as viral tropism has excluded X4 variants in retrospective plasma specimens or testing of current proviral DNA.

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Other medical conditions with CCR5 pathogenic involvement

Individuals who are homozygous (∼1% of Caucasian) for a deletion of 32 base pairs (32-bp) within the CCR5 gene (CCR5 Δ32) seem to be protected from HIV infection [69]. Moreover, the presence of this mutation in heterozygosis (4–15% of Caucasian) has been associated with slower HIV disease progression [70]. The critical role of the CCR5 co-receptor for sustaining HIV-1 infection has been recently demonstrated in one HIV patient with leukaemia who underwent bone marrow transplantation with stem cells from a compatible homozygous CCR5 Δ32 donor. Nearly 2 years after transplantation, the patient remained without evidence of HIV relapse even after removing antiretroviral therapy [71]. This pivotal observation constitutes a further argument for the benefit of CCR5 antagonists as HIV therapy.

CCR5 is the receptor for the chemokines MIP1α, MIP1β and RANTES, which are involved in inflammatory responses and participate in the recruitment of T cells that confront infectious agents. The density of CCR5 molecules on the T-cell surface determines the intensity of T-cell recruitment and therefore, influences the extent of inflammatory responses. Since individuals heterozygous for CCR5 Δ32 express a low number of CCR5 molecules on the cell surface, it has been postulated that these individuals may show a reduced susceptibility to conditions in which inflammation plays an important role. This is the case for chronic viral hepatitis, atherosclerosis, rheumatoid arthritis, and some neurological illnesses [72–76].

Since hepatitis viruses B and C share transmission mechanisms with HIV, coinfection is quite common. The presence of the CCR5 Δ32 polymorphism has been associated with a reduced risk of viral persistence following exposure to hepatitis B virus (HBV) and amelioration of liver disease progression in chronic HBV carriers [72,77]. Moreover, HBV fails to establish chronic infection in patients homozygous for the deletion [72]. Thus, CCR5 antagonists might be useful in patients with chronic hepatitis B alone or in HIV/HBV-coinfected patients. For hepatitis C virus (HCV) infection, available data are more conflicting, with some studies showing protection from establishment of chronic infection and liver inflammation [73,78], and others not supporting the beneficial effect [79].

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Maraviroc is the first CCR5 antagonist approved for the treatment of HIV-1 infection. The drug inhibits viral entry blocking host cellular receptors, and therefore the mechanisms of resistance are different that for the rest of antiretroviral drugs, with lack of cross-resistance. Maraviroc only blocks replication of R5 viruses and accordingly patients with X4 or D/M viruses do not or only marginally benefit from maraviroc therapy. Viral tropism has to be tested before considering maraviroc prescription. Phenotypic and more recently genotypic tools have shown to accurately estimate HIV-1 tropism and predict viral response. The pharmacokinetic properties and safety profile of maraviroc may support an earlier use of the drug in HIV-1 infection beyond the initial approval for antiretroviral-experienced patients with advanced HIV disease. Studies using maraviroc in drug-naive patients and as part of switch strategies are currently under investigation.

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The work was supported in part by an unrestricted grant from Pfizer and Fundacion Investigacion y Educacion en SIDA (IES). We would like to thank Dr Eva Poveda for helpful comments.

Author's role: all authors participated in a 1-day meeting held in Paris on 11 December 2008. Each author prepared and addressed orally his main conclusions to the rest, which discussed its accuracy. Thereafter, V.S. wrote the draft summarizing the main conclusions. All authors reviewed the text and provided their suggestions. The final document was then circulated electronically and when needed further changes were made.

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chemokine (C–C motif) receptor 5 antagonists; HIV; maraviroc; resistance; tropism

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