Review CCR5 Antagonists: Host-Targeted Antiviral Agents for the Treatment of HIV Infection, 4 Years On

Antiviral Chemistry & Chemotherapy 2010 20:179–192 (doi: 10.3851/IMP1507)

Review CCR5 antagonists: host-targeted antiviral agents for the treatment of HIV infection, 4 years on

Mike Westby1* and Elna van der Ryst1

1Pfizer Global Research and Development, Sandwich, Kent, UK

*Corresponding author: e-mail: [email protected]

The chemokine coreceptor 5 (CCR5) antagonists are anti- are in various stages of clinical development. In 2005, we retroviral agents with an extracellular, host-targeted reviewed the limited clinical data then available on CCR5 mechanism of action against HIV. Maraviroc, the first- antagonists. In this follow-up review, we revisit the field and in-class CCR5 antagonist, received regulatory approval in assess the clinical and virological data that have emerged 2007, becoming the first oral antiretroviral from a new in the 4 years since, with particular reference to maraviroc class in more than 10 years. Other compounds in this class for which the most comprehensive data currently exist.

Introduction

Chemokine coreceptor 5 (CCR5) antagonists are a new supporting the current and future role of small-molecule class of antiretroviral agents that inhibit HIV replica- CCR5 antagonists in the treatment of HIV infection. tion by binding to CCR5 on the surface of host target cells and preventing entry of viral strains reliant on this Status of CCR5 antagonists in clinical coreceptor for infection. The first and only currently development in 2005 approved agent within this class, maraviroc, received marketing authorisation in the USA, Europe and other Of the three small-molecule CCR5 antagonists in regions of the world in 2007, with an indication for clinical development in 2005, only maraviroc (Figure treatment-experienced adult patients with only CCR5- 1A) is currently approved for clinical use. Develop- tropic (R5) HIV-1 detectable. Other compounds in this ment of aplaviroc was discontinued in 2006, follow- class are currently at various stages of clinical devel- ing reports of severe idiosyncratic hepatotoxicity in opment. In 2005, we reviewed the role of the CCR5 Phase IIb treatment-naive and Phase III treatment- receptor in HIV-1 entry and the potential of CCR5 experienced studies [2–4]. These reports of hepatotox- receptor antagonists as therapeutic anti-HIV-1 agents icity included cases of simultaneous increases in serum [1]. We reviewed the various categories of agents and alanine aminotransferase and bilirubin breaching the compared the three small-molecule CCR5 antagonists ‘Hy’s Law’ threshold, considered by the US Food and that had progressed to clinical development: maravi- Drug Administration (FDA) to identify drug-induced roc, vicriviroc and aplaviroc. At that time, several key hepatotoxicity of particular clinical concern [5], and a questions needed to be addressed by large-scale clini- case of increased alanine aminotransferase that dem- cal trials, concerning not only the clinical efficacy and onstrated a clear dechallenge-rechallenge reaction [2]. long-term safety of these agents, but also the pathways Development of vicriviroc [6] (Figure 1B) has been to CCR5 antagonist resistance in vivo and the effect slowed by poor early virological results in dose-ranging of therapeutic administration of CCR5 antagonists on studies and by safety concerns over a cluster of unex- CXCR4-using virus populations. The purpose of the plained malignancies in a Phase II trial of treatment- current review was to provide an update on develop- experienced patients. Development of vicriviroc for ments in the field of CCR5 antagonist therapy since treatment-naive patients was suspended for several years 2005, including the outcomes of the various Phase II/III following interim results from a Phase II dose-ranging trials conducted over the past 4–5 years, the progress study in which treatment-naive patients with CCR5- that has been made in our understanding of resistance tropic (R5) virus received either 600 mg of efavirenz to CCR5 antagonists and the evolving body of evidence once daily or 25, 50 or 75 mg vicriviroc once daily, each

in combination with a then standard-of-care Combivir rollover study for ACTG 5211 (study P04100AM4), at (zidovudine/lamivudine) backbone [7]. Patients in both a revised dose of 30 mg once daily in combination with the 25 mg and 50 mg vicriviroc arms had significantly background therapies containing a ritonavir-boosted higher rates of virological failure than did patients in protease inhibitor (PI) at a dose schedule modelled to the efavirenz comparator arm and the trial was termi- provide a vicriviroc exposure equivalent to 200 mg nated in late 2005. An interim review of a Phase II study once daily in the absence of ritonavir boosting [9]. No in treatment-experienced patients (ACTG 5211), which systemic malignancies were reported during 2 years of compared the effects of 5, 10 or 15 mg vicriviroc once follow-up of patients in the ACTG 5211 rollover study daily plus optimized background therapy (OBT) with [10]. At the time this review was written, a new Phase the effects of OBT plus placebo, recommended the ter- II/III study in treatment-naive patients, in which vicri- mination of the 5 mg treatment arm because of subop- viroc (30 mg once daily) plus atazanavir/ritonavir was timal responses. The study was subsequently unblinded being compared with atazanavir/ritonavir plus teno- early following reports of six cases of malignancy in the fovir/emtricitabine (Truvada), had entered its second vicriviroc arms and two cases in the placebo arm (one of stage following an interim safety analysis at 24 weeks whom received placebo for 7 months followed by vic- (clinicaltrials.gov identifier: NCT00551018). riviroc for 3 months prior to developing a malignancy Maraviroc was approved for clinical use by the US whilst off treatment 1 month after virological failure on FDA in August 2007 and by the European Agency vicriviroc) [8]. Vicriviroc Phase II and III clinical devel- for the Evaluation of Medicinal Products the follow- opment in treatment-experienced patients, includes a ing month. Maraviroc is licensed for administration

Figure 1. Chemical structures of CCR5 antagonists

A F H3C F H N N N N N O B

H3C CH3 H3CO CH3

N CH3 H3C N N F3C C N N N O N N O CH H 3 N N

H3C F D CH3

H3C H N H C O 3 H C-SO H O O N 3 3 S O N

(A) Maraviroc, (B) vicriviroc, (C) PF-232798 and (D) TBR-652.

twice daily with other antiretrovirals in treatment- Virological and immunological efficacy experienced patients with R5 HIV-1. Approval was based on favourable 48-week results from two large An overview of published virological efficacy data Phase III studies (Maraviroc Plus Optimized Therapy for vicriviroc, maraviroc and aplaviroc is shown in in Viremic Antiretroviral Treatment Experienced Table 1. Data from studies of aplaviroc are some- Patients [MOTIVATE] 1 and 2) of maraviroc admin- what limited because of its early withdrawal from istered once or twice daily plus OBT compared with development. Early (12 week) descriptive data from placebo plus OBT in treatment-experienced patients the ASCENT [4] and EPIC [3] studies in treatment- with R5 virus determined at screening with pheno- naive patients indicate lower virological response rates typic testing [11,12]. Maraviroc has also demonstrated in patients receiving aplaviroc once or twice daily at efficacy in treatment-naive patients in the Phase III doses between 200 and 800 mg in combination with MERIT (Maraviroc versus Efavirenz Regimens as nucleoside analogue reverse transcriptase inhibitors Initial Therapy) study, although non-inferiority to (NRTIs) or lopinavir/ritonavir than in patients receiv- efavirenz was not demonstrated in the primary analy- ing an efavirenz or lopinavir/ritonavir active control. sis at week 48 [13]. However, in a post-hoc analy- Efficacy data for vicriviroc in its final dosing modality sis, favourable results for the MERIT end points at of 30 mg once daily in combination with a boosted weeks 48 [13] and 96 [14] were seen in a subset of PI are also limited, with 48-week blinded therapy data MERIT patients eligible for study inclusion based on [21] and 96-week open-label extension data [22] from a more sensitive screening tropism assay. Maraviroc the 116 patient Phase II VICTOR-E1 trial the most was granted a treatment-naive indication by the US complete available at time of writing. This study com- FDA in November 2009. pared the safety and efficacy of vicriviroc (20 or 30 mg Four other agents targeting the CCR5 coreceptor once daily) with placebo, each with an OBT containing are known to be in Phase II development. PF-232798 a boosted PI in treatment-experienced patients with R5 (Figure 1C) is a second-generation small-molecule oral virus. Mean changes from baseline in HIV-1 RNA and CCR5 antagonist with potency similar to maraviroc the proportions of patients with <50 copies/ml at week and the potential for once daily dosing. PF-232798 48 were significantly greater in both vicriviroc arms has shown activity against a laboratory-generated than in the placebo arm, and virological and immu- maraviroc-resistant R5 virus [15]. Another small- nological responses were preserved through week 96 molecule oral CCR5 antagonist, INCB9471 (Incyte, in those who were rolled over to open-label vicrivi- Wilmington, DE, USA), has shown promising results in roc (30 mg) at week 48. Better vicriviroc pharmaco- a Phase IIa trial in treatment-naive and treatment-ex- kinetic exposure and higher response rates among perienced patients with once daily dosing over 14 days patients with high baseline viraemia and fewer active [16]. However, Incyte announced last year that they OBT component drugs were seen at week 48 in those would not be initiating Phase IIb trials and were instead receiving the 30 mg dose that was carried forward into looking for a licensing partner to take the drug’s devel- Phase III testing. opment forward. PRO 140 is a humanized monoclonal Currently, the largest clinical data set for evaluating antibody that binds to CCR5 and potently inhibits R5 CCR5 antagonist efficacy is from the maraviroc clinical HIV-1 replication in vitro. A proof-of-concept study development programme, which to date has involved demonstrated potent antiviral efficacy after a single more than 2,000 individuals who have received mara- intravenous dose [17], and promising results from a viroc. Randomized comparative data on maraviroc short-term evaluation of weekly and biweekly subcu- treatment over at least 48 weeks are available for more taneous administration were recently presented [18]. than 1,900 treatment-experienced and treatment-naive PRO 140 has also shown evidence of activity against patients in three Phase III and one Phase IIb stud- escape mutants that demonstrate cross-class resist- ies (MOTIVATE 1 and 2, A4001029 and MERIT). ance to the small-molecule CCR5 antagonists [19]; Individual and pooled data from the pivotal Phase however, as a complex biological molecule requiring III MOTIVATE studies showed a more than twofold parenteral administration PRO 140 faces a different set greater decrease in viral load at week 48 in treatment- of development challenges to the orally available small- experienced patients with R5 virus who received OBT molecule compounds. Finally, TBR-652 (Figure 1D; with maraviroc (once or twice daily) than in patients previously designated as TAK-652) is a small-molecule who received OBT with a placebo [12]; this benefit was CCR5 antagonist licensed by the Takeda Pharmaceuti- maintained in all studied subgroups, including those cal Company to Tobira Pharmaceuticals. TBR-652 has with low CD4+ T-cell counts, a high level of viraemia pharmacokinetic data supportive of once daily dosing at baseline and weak or inactive OBT regimens [11]. and a good tolerability profile in single-dose studies in Additional MOTIVATE subanalyses that assessed healthy volunteers [20]. the association between OBT activity and maraviroc

Antiviral Chemistry & Chemotherapy 20.5 181 M Westby & E van der Ryst VCV Study

NS, not significant; OBT, optimized background therapy; qd, once daily; RTV, ritonavir. tropism determined by using an enhanced Trofile assay (Monogram Biosciences, South San Francisco, CA, USA). bid, twice daily; DM, dual- or mixed-tropic virus; EFV, efavirenz; MVC, maraviroc; NR/ND, not reported/not determined; A4001029 MERIT 1 and 2 MOTIVATE MVC a Table 1. Summary of HIV-1 virological responses in published clinical studies of CCR5 antagonists ASCENT APL ACTG 5211 PO3802 Highest dose arm only included for vicriviroc (VCV) and aplaviroc (APL) dose-ranging studies. a a

treatment CCR5 antagonist Combivir ( n OBT ( n 300 mg MVC bid + OBT ( n 300 mg MVC qd + 300 mg MVC bid + OBT ( n or 300 mg MVC bid + OBT ( n 300 mg MVC qd + Combivir bid ( n 800 mg APL bid + 100–800 mg RTV ( n 15 mg VCV qd + Combivir ( n 75 mg VCV qd + =52) =57) =426) =414) =360; n= 311 =23) =37) =30) c ) Comparator treatment Placebo + OBT ( n 600 mg EFV qd + Placebo + OBT ( n bid ( n Combivir ( n EFV qd + Combivir RTV ( n Placebo + 100–800 mg EFV + Combivir ( n =20) =28) =361; n= 303 =58) =209) =24) c ) Population/tropism experienced/DM Treatment- naive/R5 Treatment-naive/R5 experienced/R5 Treatment- Treatment- Treatment-naive/R5 experienced/R5 Treatment- b Study prematurely terminated because of treatment-emergent idiosyncratic hepatotoxicity. weeks Time, 48 24 48 24 48 48 12 24 24 b

antagonist arm CCR5 -1.11 -1.20 -0.62 -0.91 NR/ND -1.84 -1.68 -2.80 (0.94) -2.65 (0.80) -1.68 Change from baseline in HIV-1 RNA, mean ( ± d s Comparator arm -0.84 -0.97 -0.84 -0.97 NR/ND -0.79 -3.12 (0.48) -3.20 (0.57) -0.29 ) log 10 copies/ml P NR/ND NS NR/ND NS – < NS 0.02 < -value

0.001 0.01 antagonist arm CCR5 27 27 18 21 65.3; 68.5 46 43 NR/ND NR/ND 27 Patients with < c

c Data for patients with post-hoc R5 screening 50 copies HIV-1 RNA/ml, % Comparator arm 69.3; 68.3 22 16 22 16 17 NR/ND NR/ND 7 c

P NR/ND NS NS NS NS < – – < -value 0.001 0.05 Reference [13] [26] [12] [4] [7] [8]

outcomes have identified a 70% virological response Consistently greater increases in CD4+ T-cells have rate (<50 copies HIV-1 RNA/ml) in those receiving mar- been noted in patients who received maraviroc than in aviroc with at least two fully active agents other than those who received either a placebo or efavirenz compa- NRTIs, increasing to 80% in those initiating maraviroc rator across all patient groups [12,26,28–30] (Table 2). with two active agents and a baseline CD4+ T-cell count This effect of CCR5 antagonism, which was also appar- >50 cells/mm3 [23]. These results are similar to those ent in a meta-analysis of recent Phase II and III trials reported for other new agents, such as darunavir [24] evaluating newer agents in treatment-experienced indi- and raltegravir [25] in treatment-experienced patients, viduals [31], appears to be independent of virological and extend the range of options now available to this suppression [28,31] and is not linked to the presence patient population to enable the goal of maximal virus of PIs or other potent background agents [29]. Factors suppression to be achieved. that potentially contribute to this immune benefit of In contrast with the results in treatment-experienced CCR5 antagonists include alterations in cell traffick- patients with R5 virus, no virological benefit of treating [32], reductions in HIV-1-mediated syncytia [33], ment with maraviroc over that with placebo was apoptosis associated with CCR5 surface density [34] observed when both were given with OBT to patients and suppression of chronic cytokine-associated T-cell with non-R5 virus (dual- or mixed-tropic [DM] or activation [35,36], although no definite mechanism has CXCR4-tropic [X4] virus) in the Phase IIb study been established. A4001029 [26]. However, a significantly greater Although large-scale data are currently limited to increase in CD4+ T-cell count was seen with maraviroc maraviroc, the results indicate that the CCR5 antago- than with placebo in the primary analysis at week 24. nist class confers significant virological and immuno- This difference was also noted at week 48, but it was logical efficacy in treatment-experienced patients with not statistically significant [26]. R5 virus, similar to that observed recently with other Efficacy in treatment-naive patients with R5 virus was agents. Although all members of the class appear to demonstrated in the MERIT Phase III study [13], in which share this efficacy, meaningful comparisons between patients were randomly assigned to receive maraviroc them are not yet possible. Comprehensive Phase III once or twice daily or efavirenz once daily each with a clinical trial data for vicriviroc will be needed for com- fixed background of Combivir. Once daily maraviroc was parison with maraviroc and, similarly, Phase IIb/III data discontinued following an interim analysis in the first 205 on PRO 140 in combination regimens will be needed patients at week 16, which concluded that non-inferiority before comparisons with either of these small-molecule to efavirenz had not been established in this arm. Rand- CCR5 antagonists can be made. omization to the efavirenz and twice daily maraviroc arms continued unchanged. In the primary analysis at week 48, CCR5 antagonist safety and tolerability the maraviroc arm was found to be non-inferior to efavirenz at the coprimary study end point of <400 HIV-1 CCR5 antagonists represent the first class of antiretro- RNA copies/ml, but not non-inferior at the <50 copies/ viral agents to target a host cellular receptor rather ml coprimary end point. Subsequent rescreening of the than the virus and concerns have been raised about MERIT patient set with a more sensitive tropism assay the safety of long-term CCR5 blockade. CCR5 is than that used at initial enrolment identified a 15% inci- implicated in the immunological response to a variety dence of non-R5 virus in screening samples that had been of pathogens [37] and evidence indicates that, despite present below the limit of detection of the original assay. having an apparently normal phenotype, individuals Exclusion of these patients from a post-hoc reanalysis of homozygous for the CCR5-∆32 deletion who lack the main study end points resulted in greater response functional CCR5 might have an increased susceptibil- rates in the maraviroc arm, which fell within the criteria ity to disease following West Nile virus infection [38] defining non-inferiority [13]. Overall, the number of dis- or tick-borne encephalitis [39]. In addition, conflict- continuations in each treatment arm at both time points ing reports of associations between CCR5-∆32 and were similar, with more virological discontinuations on various malignancies [40–45] combined with a clus- maraviroc balanced by a higher number of adverse event ter of lymphomas observed in the Phase IIb ACTG discontinuations on efavirenz. With study centres that 5211 study of vicriviroc have also caused concern included sites in both South Africa and South America, regarding the potential for an increase in malignan- this study enrolled a significant proportion of patients cies. Data demonstrating that CCR5-∆32 mice have with non-clade B infections (including more than 400 an increased susceptibility to concanavalin A-induced patients [~30%] infected with clade C HIV-1). Thus, the hepatitis [46,47], when considered together with the study provided clinical confirmation of the in vitro data severe idiosyncratic hepatotoxicity observed during showing maraviroc to be active against viruses with enve- aplaviroc development, have also raised concerns that lopes of both B and non-B origin [27]. hepatotoxicity could be a class effect.

Antiviral Chemistry & Chemotherapy 20.5 183 M Westby & E van der Ryst

Table 2. Summary of CD4+ T-cell count increases in published clinical studies of CCR5 antagonists

Change from baseline in CD4+ T-cell count, mean (±s d ) cells/mm3 CCR5 Comparator Population/ Time, CCR5 Comparator Difference Study antagonist treatment treatment tropism weeks antagonist arm arm (95% CI) P-value Reference

VCVa PO3802 75 mg VCV qd + EFV qd + Treatment- 24 158 (171) 102 (152) NR/ND NS [7] Combivir (n=23) Combivir (n=24) naive/R5 ACTG 5211 15 mg VCV qd + Placebo + 100–800 mg Treatment- 24 142 –9 NR/ND 0.002 [8] 100–800 mg RTV (n=30) RTV (n=28) experienced/R5 APLa ASCENT 800 mg APL bid + EFV qd + Treatment- 12b 117 87 NR/ND NR/ND [4] Combivir (n=37) Combivir (n=20) naive/R5 (-39– -465)c (-12– -257)c MVC MOTIVATE 300 mg MVC qd + Placebo + Treatment- 48 116 61 55 (36–74) <0.001 [12] 1 and 2 OBT (n=414) OBT (n=209) experienced/R5 300 mg MVC qd + 124 63 (44–82) OBT (n=426) MERIT 300 mg MVC bid + 600 mg EFV qd + Treatment- 48 170 144 26 (7–46) 0.0075 [13] Combivir (n=360) Combivir (n=361) naive/R5 A4001029 300 mg MVC qd + Placebo + OBT (n=58) Treatment- 24 60 36 24 (-1–49) 0.06 [26] OBT (n=57) experienced/DM 48 65 51 NR/ND NS 300 mg MVC bid + 24 62 36 27 (1–52) 0.04 OBT (n=52) 48 78 51 NR/ND NS aHighest dose arm only included for vicriviroc (VCV) and aplaviroc (APL) dose-ranging studies. bStudy prematurely terminated because of treatment-emergent idiosyncratic hepatotoxicity. cMedian values (range). bid, twice daily; CI, confidence interval; DM, dual- or mixed-tropic virus; EFV, efavirenz; MVC, maraviroc; NR/ND, not reported/not determined; NS, not significant; OBT, optimized background therapy; qd, once daily; RTV, ritonavir.

Despite these concerns, data from Phase IIb/III studies bilirubin increases in patients receiving maraviroc than of maraviroc following more than 2,000 patients over in those receiving placebo [50]. Similarly, no systematic periods in excess of 96 weeks have shown it to be well signs of hepatotoxicity with maraviroc were observed tolerated with an adverse event profile similar to placebo in treatment-naive patients in the MERIT study or in [12,26,48] and superior to efavirenz [13]. Although studies of vicriviroc to date. postural hypotension (at unit doses of 600 mg or more) An association between vicriviroc and malignancy was identified as a signature dose-limiting adverse event remains unproven and additional clusters have not for maraviroc in Phase I clinical trials, there was no evi- been reported during its further development involving dence of an excess of adverse events related to postural up to 4 years of patient follow-up to date [51]. There hypotension in the Phase IIb/III clinical development is no evidence that treatment with vicriviroc increases programme at FDA-approved dosing [48]. replication of Epstein–Barr virus, which has been linked The hepatotoxicity demonstrated by aplaviroc, to lymphomas in immunocompromised patients [52]. although still of unknown cause, appears to be a Ninety-six week exposure-adjusted data from the MOTI- compound-related rather than a mechanism-related VATE studies showed a lower rate of malignancies and event [2]. Two confirmed cases of severe hepatotoxic- a significantly lower risk of overall malignancies, AIDS- ity – one in a patient receiving 600 mg maraviroc once defining malignancies and coinfection-related malignan- daily in a Phase I study and one in a patient from the cies in patients who received maraviroc than in those who discontinued once daily maraviroc arm of MERIT – received placebo, despite a 10-fold greater exposure to were confounded by concomitant infection and the use maraviroc than to placebo. Multivariate analysis dem- of other potentially hepatotoxic agents, respectively onstrated that the greater increase in CD4+ T-cells with [49]. Unconfirmed hepatic failure in one patient with maraviroc treatment was protective against the develop- a history of chronic hepatitis C virus (HCV) infection ment of new malignancies [53]. Similarly, a low incidence and hepatic angioma, who discontinued treatment on of malignancies was reported in treatment-naive MERIT the twice daily maraviroc arm of MOTIVATE 2 on day patients who received maraviroc with a Combivir back- 337, was considered to be possibly drug-related. Expo- bone [14]. Finally, there was no evidence of treatment- sure-adjusted data from the MOTIVATE studies showed related malignancies in the terminated aplaviroc clinical similar or slightly lower incidences of transaminase and development programme [3,4].

Figure 2. Maraviroc resistance is characterized by dose–response curves with plateaus in maximal percentage inhibition

100 Susceptible to maraviroc Virus replication not fully inhibited

Resistant to maraviroc eplication, % -1 r Inhibition of HIV

Maraviroc concentration

Taken together, these data are reassuring and suggest able to enter cells even when the density of antagonist- that the antagonism of CCR5 does not give rise to class- free receptors is low [61]. These mutants would be wide signature toxicities over the durations of treatment expected to have a lower susceptibility to CCR5 antag- studied thus far. onists than would variants with lower affinity for free receptors, regardless of the CCR5 antagonist used, and Development of resistance to CCR5 indeed this has been observed in laboratory analyses of antagonists panels of viruses [56]. Resistance to CCR5 antagonists more typically Given the innate variability of the HIV envelope gene manifests itself phenotypically as a reduction in the [54] and the complexity of interactions between the maximum level of viral inhibition achievable at any virus and host cell during HIV entry, resistance to concentration of drug (Figure 2) [62]. Such reduc- CCR5 antagonists is complex [55]. As would be antici- tions in the maximum percentage inhibition (MPI) are pated, mechanisms of viral escape from CCR5 antag- characteristic of viruses that have acquired the ability onists, which target an immutable host protein, are to use antagonist-bound receptors for cellular entry fundamentally different from the resistance seen with and, therefore, cannot be fully inhibited even when no antiretroviral drugs that bind viral targets. Two path- unbound receptors are present [63]. The level of MPI ways of virological escape from CCR5 antagonists have in drug susceptibility assays reflects the difference in been identified: selection of R5 virus that can use drug- the affinity of the virus for antagonist-free as opposed bound CCR5 and ‘un-masking’ of pre-existing minor- to antagonist-bound forms of CCR5, with the MPI ity populations of dual-tropic or X4 virus (collectively becoming progressively lower as the efficiency of the called ‘CXCR4-using’ viruses). These pathways are virus at using antagonist-bound receptor increases summarized below and are reviewed in greater detail [55,56,63]. elsewhere [56,57]. Reductions in MPI, which have been observed with maraviroc [62], vicriviroc [7,64] and aplaviroc [58], CCR5 antagonist treatment failure with R5 virus are consistent with these drugs acting as allosteric, non- For CCR5 antagonists, shifts in the 50% HIV inhibitory competitive entry inhibitors [65]. Small-molecule CCR5 concentration (IC50), characteristic of resistance to other antagonists are thought to interact with coreceptors in antiretrovirals, have only rarely been observed at viro- a way that stabilizes or induces a CCR5 receptor con- logical failure [58–60]. Such shifts probably represent formation not recognised efficiently by the HIV enve- variants with an increased affinity for CCR5 that are lope [62,66–68]. Although very different in structure

Antiviral Chemistry & Chemotherapy 20.5 185 M Westby & E van der Ryst

and shape, the CCR5 antagonists investigated to date Figure 3. HIV envelope sequence changes associated with appear to share a common binding site that is located maraviroc-resistant R5 virus in the second extracellular loop of CCR5 [65,67,69]. The specific interactions between each compound and amino acid lining the binding pocket differ between the agents, as might be anticipated from their different G/A electrostatic shapes and polarities [69]. GRA Although demonstrated both in vitro and in vivo, P G F I Tip the results of clinical studies suggest that R5 virological Tip Y

failure with reduced susceptibility to CCR5 antagonists A I is not the most common mechanism of failure. In the T

S I

MOTIVATE studies in treatment-experienced patients, K G a reduced MPI was observed in only ~40% of R5 viro- R E

I Stem Stem T I logical failures with maraviroc over the first 24 weeks a G

D of treatment; poor or intermittent adherence was the N

most probable reason for failure in the remaining 60% a I N a R

of R5 failures without maraviroc resistance [59]. Simi- Q N

larly, low numbers of R5 failure isolates with a reduced a

P A MPI were observed in the MERIT study of treatment- Base a Base naive patients [70] and in studies of aplaviroc [58,71] R H

T and vicriviroc [7]. C

The observation of a low clinical incidence of R5 C virus with reduced susceptibility to CCR5 antagonists is consistent with the complexity of the genetic changes involved in its development. Unlike resistance to virally Deletion/change Change Insertion/change targeted antiretrovirals, CCR5 antagonist resistance in R5 virus is conferred by complex and generally unpre- dictable changes across a number of regions in the HIV The HIV envelope sequence changes associated with maraviroc resistance were envelope [57,62,63,72] and no predictive signature concentrated in the stem and tip of the V3 loop [73]. aResidues associated with a mutations or combinations have been identified to date. conserved site for binding sulfotyrosine on the CCR5 N terminus. The consensus B sequence is based on polymorphism predominance. Interpatient sequence diversity of resistant R5 virus has been reported for clinical isolates from both maraviroc [60,73] (Figure 3) and aplaviroc [58] virological failures, which suggests the existence of multiple genetic disease [78–83]. Such emergence is coincident with more pathways to resistance and implies that the mutations rapid CD4+ T-cell depletion and acceleration of disease observed might be virus-specific [55]. Sequencing has progression, but it remains unclear whether this indi- also revealed different patterns of adaptive amino acid cates a cause-and-effect relationship [84]. Longitudinal changes in the HIV envelope under selection pressure analyses of envelope sequences in clinical isolates that from different CCR5 antagonists, including changes evolved to use CXCR4 in the absence of antiretroviral outside the V3 loop [57,58,72,74,75]. These sequence drug pressure indicate that R5 to X4 tropism ‘switch- changes might, however, come at a cost to viral fitness ing’ requires an ordered accumulation of multiple muta- [76]. Viral affinity for CCR5 bound by one antagonist tions, which differ both in composition and in the order will not necessarily confer affinity for CCR5 bound of appearance between patients and that may or may by another, given the different amino acid interactions not include changes in the V3 loop [85]. Interestingly, involved. Although there is evidence of cross-resistance ‘ultra-deep’ clonal sequencing has identified CXCR4- between different CCR5 antagonists [19,77], this does using minority populations in most or all clinical R5 not appear to be a universal phenomenon [1]. samples studied [86]. This finding, therefore, raises the question of whether a simple threshold level is Failure with CXCR4-using virus required for X4 outgrowth under CCR5-antagonist Although theoretical concerns about whether prolonged selection pressure or whether more complex interac- therapy with a CCR5 antagonist will accelerate viral tions are involved. It also raises the possibility that the evolution toward CXCR4 use still exist, this has not emergence of CXCR4-using virus later in the course of been observed in clinical studies to date. Emergence of infection could be a consequence of disease-associated CXCR4-using virus in the absence of CCR5 antagonist changes in the host environment that favour a shift pressure tends to dominate later in the course of HIV toward CXCR4-mediated replication.

Evaluation of tropism data over 48 weeks in the with a CCR5 antagonist, the clinical relevance of these MOTIVATE studies indicated that more than 50% of observations remains unclear given the small sample treatment-experienced patients experiencing virological sizes, the short duration of follow-up involved and the failure on maraviroc did so with CXCR4-using virus, lack of a clinically validated algorithm to distinguish which was derived from the unmasking and outgrowth ‘susceptible’ from ‘non-susceptible’ DM samples. of pre-existing minority populations present at baseline [11,87]. This outgrowth was not associated with any Future directions short-term adverse immunological outcomes; greater gains in CD4+ T-cell counts were seen in patients failing The introduction of several new, potent antiretrovirals maraviroc with CXCR4-using virus relative to patients in the past few years – darunavir, tipranavir, etravirine, failing in the placebo arm with any viral tropism [11]. raltegravir and maraviroc – has significantly improved Similarly, in MERIT, greater CD4+ T-cell increases were HIV treatment and made full virological suppression an seen in treatment-naive patients with CXCR4-using achievable objective for almost all patients, including virological failure on maraviroc plus zidovudine/lami- those with extensive prior treatment experience. In all vudine, relative to those failing on efavirenz with the cases, clinical studies have consistently demonstrated same nucleosides and any viral tropism [70]. Further- the importance of using these new drugs in combina- more, in the MOTIVATE studies, no association was tion with at least two other fully active agents to achieve observed between CXCR4-using virus at failure and durable suppression [94–98]. The increasing availability the development of Category C (AIDS-defining) events, of more potent later options, coupled with the generally and most patients with more than 1 month of off-drug favourable safety profiles of these newer agents, presents follow-up reverted back to R5 virus infection following the opportunity to combine them directly and avoid the withdrawal of maraviroc [88]. practice of recycling nucleoside analogues between reg- Clinical data from the A4001029 study (maraviroc imens. Many planned and ongoing studies, including plus OBT compared with OBT with placebo in patients study ACTG A5241 (OPTIONS; clinicaltrials.gov iden- with non-R5 infection) clearly showed that, at a popu- tifier: NCT00537394) and the POEM observational lation level, patients with DM virus at screening did cohort study (study A4001067 [NCT00665561]), will not derive a virological benefit from maraviroc [26]. explore combinations of these newer agents and pro- However, interestingly, evidence suggests that DM vide valuable information on optimal drugs to partner virus populations are not homogeneous in their natural with maraviroc under various conditions. resistance to CCR5 antagonists. It has been shown that Other strategies for maraviroc use are being explored, dual-tropic viruses from clinical samples exhibit a broad including NRTI-sparing once daily dosing in treatment- range of affinities for CCR5- and CXCR4-mediated cell naive patients (study A4001078 [NCT00827112]) and entry: the majority of dual-tropic strains show a nota- regimen intensification in acute primary infection (MAR- bly greater preference for CCR5 (dual-R5 variants) and AVIBOOST study [NCT00808002]) or in established the minority show an increased affinity for CXCR4 infection in patients with virological suppression on their (dual-X4) [58,89–92]. Furthermore, an analysis of the current regimen (study MMA-0612 [NCT00703586]). proportion of CCR5-using and CXCR4-using viruses These studies will determine whether the addition of in DM populations from antiretroviral-naive patients maraviroc to conventional triple therapy or its inclusion showed that the majority of clones used CCR5 only in a four-class initial regimen can accelerate HIV-1 reser- [92]. Together, these observations indicate that some voir decay in latently infected cells, reduce residual rep- DM populations might be responsive to CCR5 antago- lication in gut-associated lymphoid tissue (GALT) and/ nists in vivo to a greater or lesser extent [90]. Clinical or improve immune reconstitution in peripheral blood data also appear to support this inference. For exam- and GALT. Another study plans to examine the effects ple, a retrospective analysis of A4001029 identified a of therapeutic intensification with both maraviroc and greater short-term virological response (approximately raltegravir together with immune modulators on the twofold at week 12) among maraviroc patients with proviral and GALT-associated viral reservoirs (study <10% CXCR4-using virus at screening, by ultra-deep EraMune02 [NCT00976404]). The potential immune sequencing, than those with >10% [93]. Furthermore, benefits of CCR5 blockade are being further explored in in the EPIC study of aplaviroc [3], week 12 viral load studies involving patients with low CD4+ T-cell counts reductions among 11 evaluable patients with DM virus despite long-term virological suppression (studies at screening visit ranged from -1.89 to -3.49 log10 cop- ANRS 145 MARIMUNO [NCT00944541] and CCTG ies/ml, suggesting that some patients experienced rapid 590 [NCT00925756]), as well as in an intensive GALT virological declines despite detectable CXCR4-using sampling study to examine the effect of maraviroc (with virus [3]. Although these reports suggest that some or without raltegravir) on GALT immune cell recovery patients with DM virus might respond to treatment (study CH-2009.01 [NCT00935480]).

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Complications with current antiretroviral treatment was provided by healthy volunteer data showing that regimens – including drug resistance, poor adherence maraviroc not only reaches similar concentrations in and formulation issues – have left many children and plasma and cervicovaginal fluid to many other anti- adolescents with limited therapeutic options. Paediatric retroviral agents, but it also reaches higher levels in the studies are therefore planned (vicriviroc study IMPAACT female genital tract [111]. An analogous study (study P1071 [NCT00766597]) or underway (maraviroc study IRB 08-0418 [NCT00775294]) is underway in men to A4001031 [NCT00791700]) to evaluate CCR5 antag- determine the extent of maraviroc exposure in blood, onists in this important patient group. Another group saliva, seminal fluid and rectal mucosal tissue. of patients of special interest is those with hepatitis B Maraviroc’s extracellular mode of action also makes virus (HBV) or HCV coinfection. These patients have it a promising candidate for inclusion in a vaginal an increased risk of drug-induced hepatotoxicity and, microbicide to prevent HIV transmission. In early 2008, although no increases in hepatotoxic effects among coin- Pfizer granted a non-exclusive, royalty-free license to fected patients have been observed in studies of maravi- develop maraviroc as a microbicide to the International roc [49] or vicriviroc [99] to date, these analyses were Partnership for Microbicides (IPM), a non-profit prod- based on relatively small numbers of trial participants. uct development partnership. Although no clinical tri- More detailed information on maraviroc safety in hepa- als have yet been performed, pre-clinical studies by the titis coinfection will be gathered from an ongoing Phase IPM in human genital and colorectal tissue explants IV study (A4001080 [NCT00782301]) comparing the have shown that maraviroc effectively blocks ex vivo effects of maraviroc with those of etravirine, each in infection in these tissues and that potency is increased combination with darunavir/ritonavir and raltegravir, in by sustained delivery [112]. approximately 120 treatment-experienced patients with Finally, the need for a pre-therapeutic tropism HBV or HCV coinfection. assessment has been a barrier to CCR5 antagonist use Larger and more specific trials of CCR5 antagonists in some patients. Currently, most HIV-1 tropism screen- in coinfection are also warranted on the basis of recent ing involves centrally provided recombinant phenotype evidence implicating CCR5 function and its modulation assays and this remains the only form of tropism testing in the etiopathogenesis of viral hepatitis. For example, to have been validated in large-scale prospective clini- spontaneous clearance of HBV occurs more frequently cal studies to date. However, there is also considerable among individuals without functional CCR5 and with interest in developing improved genotypic methods genetically elevated levels of the CCR5 ligand RANTES (reviewed by Low et al. [113]), which have been shown [100,101]. In addition, CCR5 expression is detectable to predict outcomes after CCR5 antagonist administra- in hepatic stellate cells and in areas of liver inflam- tion [114]. Recent data [115,116] have raised hopes mation and fibrogenesis in HIV–HCV coinfection. that ongoing improvements in genotypic methodologies Moreover, stellate cell chemotaxis and secretion of the and algorithms might soon render them a viable alter- pro-inflammatory cytokine MCP-1 increase on expo- native to phenotypic assays. Such genotypic platforms sure to CCR5-tropic gp120 [102]. These effects are pre- would be analogous to genotypic resistance testing vented by pre-incubation with a CCR5 antagonist [102]. methods, in that they are typically less expensive, more Functional CCR5 expression has also been observed to portable, locally implementable, less reliant on either promote hepatic fibrosis in a mouse model [103]. Thus, individual operator skill or highly specialized equip- there is evidence to indicate that the faster progression ment and typically have faster turnaround times than to liver fibrosis and cirrhosis in HCV-infected patients their phenotypic counterparts. Quality-controlled local who are coinfected with HIV [104,105] might at least in sequencing might also assist the development and vali- part be attributable to CCR5–HIV interactions that are dation of clinical algorithms to differentiate those DM preventable with the use of CCR5 antagonists. virus subpopulations that retain significant sensitivity The potential use of CCR5 antagonists for pre- and to CCR5 antagonists. post-exposure prophylaxis is an active area of research In conclusion, the attractiveness of viral entry as a given that R5 viruses are by far the most common types target for HIV drug discovery has now yielded two present in sexual transmission [79–81] and early infec- new classes of extracellular antiretroviral agents with tion [78,106,107]. Proof-of-concept for CCR5 antago- the introduction of fusion inhibitors in 2003 and nist prophylaxis has been demonstrated in a non-human CCR5 antagonists in 2007. The CCR5 antagonists primate model using a novel oral CCR5 antagonist have established the clinical viability of host-targeted (CMPD167) [108,109]. There is also one reported case interventions and have provided some interesting hints study of successful post-exposure prophylaxis with a that such interventions could provide additional ben- maraviroc-containing regimen following occupational efits in modifying the course of HIV disease beyond needle stick injury [110]. Pharmacokinetic support for the suppression of viral replication alone. More delib- examining prophylactic use of maraviroc in women erate disease-modifying host interventions are likely

8. Gulick RM, Su Z, Flexner C, et al. Phase 2 study of the to undergo investigation in clinical trials over time, safety and efficacy of vicriviroc, a CCR5 inhibitor, in HIV- opening up an entirely new direction in HIV medicine. 1-Infected, treatment-experienced patients: AIDS Clinical Whereas small-molecule CCR5 antagonists have always Trials Group 5211. J Infect Dis 2007; 196:304–312. had certain theoretical advantages over intracellular 9. Keung A, Prasad P, Martinho M, et al. A pharmacokinetic- pharmacodynamic (PK-PD) disease model to describe and virus-targeted agents, the clinical data accumulated short- and long-term antiviral activity of vicriviroc (VCV). in the past 4 years has established this new drug class as 46th Interscience Conference on Antimicrobial Agents and Chemotherapy. 27–30 September 2006, San Francisco, CA, an important therapeutic option in the long-term treat- USA. Abstract A-0387. ment of HIV disease. 10. Gulick R, Haas D, Collier AC, Lennox J, Parker C, Greaves W. Two-year follow-up of treatment-experienced patients on vicriviroc (VCV). 47th Interscience Conference Note added in proof on Antimicrobial Agents and Chemotherapy. 17–20 September 2007, Chicago, USA. Abstract H-1030. Please note that since the submission of this review, 11. Fatkenheuer G, Nelson M, Lazzarin A, et al. Subgroup analyses of maraviroc in previously treated R5 HIV-1 Merck has announced (17 February 2010) that it infection. N Engl J Med 2008; 359:1442–1455. will not submit a New Drug Application for vicri- 12. Gulick R, Lalezari J, Goodrich J, et al. Maraviroc for viroc in treatment-experienced patients at this time. previously treated patients with R5 HIV-1 infection. N Engl Their decision follows poor initial efficacy results in J Med 2008; 359:1429–1441. the VICTOR-E3 and E4 phase III studies reported as 13. Cooper DA, Heera J, Goodrich J, et al. Maraviroc versus efavirenz, both in combination with zidovudine/lamivudine, a late-breaking presentation at the 17th Conference for the treatment of antiretroviral-naive subjects with on Retroviruses and Opportunistic Infections in San CCR5-tropic HIV-1. J Infect Dis 2010; 201:803–813. Francisco 2010 [117]. 14. Heera J, Ive P, Botes M, et al. The MERIT study of maraviroc in antiretroviral-naive patients with R5 HIV-1: 96-week results. 5th IAS Conference on HIV Pathogenesis, Treatment and Prevention. 19–22 July 2009, Cape Town, Acknowledgements South Africa. Abstract TUAB103. 15. Dorr P, Westby M, McFadyen L, et al. PF-232798, a second The study was sponsored by Pfizer Global Research generation oral CCR5 antagonist. 15th Conference on and Development and was funded by Pfizer, Inc. Retroviruses and Opportunistic Infections. 3–6 February Editorial assistance was provided by Nick Fitch and 2008, Boston, USA. Abstract 737. 16. Erickson-Viitanen S, Ambremski K, Solomon K, et al. Caroline Minshull of Health Interactions, with fund- Co-receptor tropism, ENV genotype, and in vitro ing by Pfizer, Inc. susceptibility to CCR5 antagonists during a 14-day monotherapy study with INCB9471. 15th Conference on Retroviruses and Opportunistic Infections. 3–6 February Disclosure statement 2008, Boston, USA. Abstract 862. 17. Jacobson JM, Saag MS, Thompson MA, et al. Antiviral MW and EvdR are full-time employees of Pfizer. activity of single-dose PRO 140, a CCR5 monoclonal antibody, in HIV-infected adults. J Infect Dis 2008; 198:1345–1352. References 18. Thompson M, Lalezari J, Saag M, et al. Weekly and biweekly subcutaneous PRO 140 demonstrates potent, 1. Westby M, van der Ryst E. CCR5 antagonists: host-targeted sustained antiviral activity. 16th Conference on Retroviruses antivirals for the treatment of HIV infection. Antivir Chem and Opportunistic Infections. 8-11 February 2009, Chemother 2005; 16:339–354. Montreal, Canada. Abstract 571a. 2. 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Boucher C, Schapiro JM, Kuritzkes D, et al. Genotypic 7. Landovitz RJ, Angel JB, Hoffmann C, et al. Phase II study and phenotypic weighted optimized background therapy of vicriviroc versus efavirenz (both with zidovudine/ susceptibility scores are similarly strong predictors lamivudine) in treatment-naive subjects with HIV-1 of virologic response <50 copies/mL at Week 48 in infection. J Infect Dis 2008; 198:1113–1122. MOTIVATE 1 and 2. Antivir Ther 2009; 14 Suppl 1:A52.

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Received 25 November 2009, accepted 6 January 2010