How do entry inhibitors work

Only two studies have described resistance developed in a non-subtype B virus clade G RU against maraviroc and vicriviroc [ , ]. There is a clear need for more studies characterizing small molecule resistance in non-subtype B isolates. Cross-resistance has been described for some but not all of the in vitro derived resistant variants.

Two maraviroc resistant variants remained susceptible to other small molecule CCR5 antagonists and the fusion inhibitor enfurvitide [ ]. Therefore the implications of the development of resistance to one small molecule CCR5 inhibitor to cross-resistance to other members of this class are not yet clear. In summary, mutations associated with resistance to CCR5 antagonists in in vitro derived resistant isolates have largely mapped to the V3 region of gp with the exception of two viruses with resistant mutations in other regions of gp However, resistance in clinical trial patients failing treatment with R5 tropic viruses have all been attributed to mutations in the V3 loop arising during therapy [ , ].

While cross-resistance has been reported for some resistant variants, others have remained susceptible to small molecule inhibitors. This is in significant contrast to inhibitors of reverse transcription and protease cleavage, which exhibit modest differences in intrinsic sensitivity across diverse HIV-1 isolates. Due to the high degree of diversity among HIV-1 env genes of the same or different HIV-1 subtypes, it is difficult to identify specific sequence variations that may be associated with variable sensitivity to entry inhibitors.

Evaluation of intrinsic sensitivity differences to T and TAK revealed that kinetic factors of fusion were largely responsible for variations in IC 50 [ ]. Sensitivity to T mapped to the V3 loop of env [ ], but mutations in the bridging sheet are also sufficient to modulate intrinsic susceptibility to these inhibitors [ , ].

Multiple factors are involved in the efficiency of host cell entry. Upon CD4 binding, structural rearrangements within the envelope occur which reveal the coreceptor binding site. The hypervariable V3 loop must evolve by balancing attempts to escape host humoral response with the need to engage the CCR5 coreceptor for host cell entry.

The affinity relationship between CCR5 and envelope, which can be modulated by the density of CCR5 on the cell surface, may be important in influencing the efficiency of entry. Some views hold that the major rate-limiting process in host cell entry is the formation of six-helix bundles [ ], but other data suggests that ternary complex formation is the major rate-limiting step of the entry process [ ].

The affinity relationship between CCR5 and V3 may be important in influencing the efficiency of entry through either of these pathways. Mechanisms involved in variable susceptibility to chemokines such as RANTES or their derivatives have not been evaluated. These inhibitors differ from small molecule CCR5 antagonists in their ability to occupy surface receptor as well as trigger internalization of CCR5 [ ].

Similar variations at position were observed when comparing intrinsic sensitivities of a different panel of primary isolate viruses to inhibition by TAK and SCH-C [ ].

Subsequent studies revealed that the most common polymorphisms at positions and e. In contrast, the more rare polymorphisms but still found as wild type sequence such as R and T were hypersusceptible to these entry inhibitors.

As a consequence of selected evolution, many primary HIV-1 isolates may be intrinsically resistant. However, this intrinsic resistance is highly context dependent. For example, substituting just the V3 loop coding region of a more sensitive primary HIV-1 isolate into a laboratory strain more resistant backbone resulted in demonstrable decrease in susceptibility to all entry inhibitors. Sensitivity to entry inhibitors increased when large segments of the env gene of primary HIV-1 isolates were substituted into the laboratory strain or when the primary isolates themselves were employed for drug susceptibility assays.

Variations in the efficiency of the entry process may have an important role in the context of inhibiting HIV-1 entry in vivo. Many studies have looked at the baseline susceptibility of primary isolate HIV-1 viruses to inhibition by a broad range of entry inhibitors.

The clinical relevance of such variations is unclear at this time. Furthermore, the mechanisms involved in these variations are not clear. The intrapatient population of viruses must infect cells in the presence of chemokine inhibitors, and some studies have suggested that viruses grow resistant to inhibition by chemokines over the course of HIV-1 infection in a single patient [ ].

One possible consequence of this hypothesis is that virus variants found at acute infection may be more susceptible to entry inhibitors than variants found late in infection. Variations in coreceptor affinity may be involved in determining sensitivity to coreceptor inhibitors. HIV-1 envelope-mediated entry is a highly cooperative process.

The efficiency of this process can be modulated by factors specific to the host cell, including receptor density on the surface, as well as expression of receptors in antigenically permissive conformations. Entry efficiencies are further modulated by the HIV-1 envelope both through quantitative factors such as receptor affinity as well as through qualitative factors of receptor interaction e.

Several approaches have been taken to assess cooperativity in HIV-1 envelope mediated membrane fusion. The earliest study evaluated the effect of varying CCR5 expression levels on virus infection, and estimated that six CCR5 molecules were involved in a stable entry complex [ ].

By titrating out functional envelope trimers using a dominant negative gp variant, it has been suggested that HIV-1 entry can be achieved by a single trimer [ ], and further that two subunits of the trimer are sufficient for entry [ ]. However, previous studies have implicated a role for at least four CD4 interactions in entry [ ], and similar dominant negative gp approaches have suggested the involvement of between four and five trimers [ ].

There are several implications to the multiple receptor stoichiometry of HIV-1 entry. One assumption is that receptor density will play a significant role in entry efficiency, and furthermore, affinity relationships between envelope and CD4 as well as envelope and coreceptor will have implications on the rate and efficiency of entry. A stoichiometry requiring the aggregation of multiple CCR5 molecules to form a stable entry complex will be significantly inhibited in environments with limiting amounts of CCR5, such as in the presence of inhibitor.

This prolongation of the entry step may allow for greater efficacy of various virus deactivation processes, such as endocytosis or complement mediated lysis. The efficiency of the HIV-1 entry process is thought to have implications for overall viral replicative fitness [ , ]. HIV-1 exists as a genetically variable population, or quasispecies [ , ] as a consequence of a high mutation rate 3. Fitness is a complex factor within an infected host.

In a simple model, intrinsic rates of viral replication determine the relative proportion of an HIV-1 clone in the overall population. Although low fitness variants can contribute to the genetic pool in the quasispecies theory, the most fit variants will dominate the population due to their competitive replicative advantage.

Host selection is a major determinant of fitness in vivo. However, replicative fitness, also termed replication capacity a measure of the growth kinetics of a virus has important implications on viral fitness and pathogenesis.

For example, viruses derived from long term nonprogressor cohorts have been shown to have lower replicative fitness than viruses from individuals with normal disease progression [ ]. Viruses lacking the nef gene also have a replication defect in vitro , and these viruses are frequently associated with less severe disease symptoms. Efficiency of virus entry may be an important determinant of viral replicative fitness, and consequently of viral pathogenesis.

In the case of primary HIV-1 isolates, there is a direct relationship between replicative fitness and sensitivity to many entry inhibitors. These same selective pressures could result in viruses with increased binding affinity to co-receptor and higher rate of entry kinetics and as a consequence outcompete inhibitor or simply utilize inhibitor-bound CCR5 receptor. To support this hypothesis, HIV-1 envelopes derived from patients with elite suppression displayed reduced entry efficiencies, slow entry kinetics, and poor CCR5 utilization [ ].

Lobritz et al. The latter three drugs likely inhibit through a competitive mechanism whereas TAK may be more of an allosteric, noncompetitive inhibitor. As a consequence, primary HIV-1 isolates are unlikely to evolve to utilize a coreceptor bound to a synthetic drug without a natural analog.

Thus, resistance to drugs such as VCV or MCV might follow more classical selection of drug resistance and result in a fitness decrease.

However, two in vitro studies suggest that a decrease in fitness resulting from CCR5 inhibitor resistance may not be as common as with the fitness loss observed with resistance to almost all other antiretroviral drugs. Anastassopoulou et al. Clearly many questions remain in our understanding of HIV-1 entry. It has become essential to answer many of these questions as inhibitors of the HIV-1 entry process begin widespread use in the treatment of HIV infection.

The discovery and successful development of CCR5 entry inhibitors proves viral entry is a viable target for therapeutic intervention in HIV-1 patients.

The propensity of HIV to develop drug resistance however, will ultimately result in treatment failures to this new class of inhibitors. Therefore it is of upmost importance to understand how resistance develops and by what mechanism s the virus is able to subvert inhibition.

The research highlighted here provides a starting point towards understanding the complex mechanisms of resistance to HIV-1 entry inhibitors. National Center for Biotechnology Information , U. Journal List Viruses v. Published online Apr Michael A. Lobritz , Annette N. Ratcliff , and Eric J. Author information Article notes Copyright and License information Disclaimer. This article has been cited by other articles in PMC.

Abstract Entry inhibitors represent a new class of antiretroviral agents for the treatment of infection with HIV Table 1. Open in a separate window. Figure 1. HIV-1 Receptors and Implications of Tropism The concept of inhibiting HIV-1 replication by preventing the virus from entering the host cell has been contemplated since the first identification of the major receptor, CD4 [ 4 , 14 ].

Figure 2. Table 2. Investigational Entry Inhibitors. Fusion Inhibitors and Mechanisms of Resistance The crystal structure of the gp41 ectodomain [ 29 ] and of the ectodomain partnered with an inhibitory peptide C34 [ 89 ] revealed that the fusion active conformation of gp41 was a six-helix bundle in which three N-helices form an interior, trimeric coiled-coil onto which three antiparallel C-helices pack.

Chemokine Analogs Discovery of the chemokine coreceptors was facilitated by the observation that native chemokines can inhibit HIV-1 replication [ 44 ]. Resistance to Coreceptor Inhibitors As with any antiretroviral, resistance to entry inhibitors will inevitably arise.

Figure 3. Tropism Switching The major concern in the therapeutic administration of coreceptor inhibitors is the possibility that resistance will manifest by a change in coreceptor tropism from CCR5 to CXCR4, or that an outgrowth of an X4-tropic virus subset will come to dominate the intrapatient virus population.

Competitive Resistance Model: Increased Coreceptor Affinity If coreceptor switching is not the main mechanism of resistance to small molecule coreceptor inhibitors, then it is important to understand by what mechanism resistance does arise. Figure 4. Non-competitive Resistance Model: Utilization of Inhibitor-bound Coreceptor The second model of resistance to coreceptor inhibitors is noncompetitive resistance Figure 4.

Table 3. V3 loop resistance mutations of CCR5 small molecule resistant viruses. Conclusions Clearly many questions remain in our understanding of HIV-1 entry. References and Notes 1. Lower in vivo mutation rate of human immunodeficiency virus type 1 than that predicted from the fidelity of purified reverse transcriptase. Chemokine receptors as HIV-1 coreceptors: roles in viral entry, tropism, and disease.

HIV enters cells via endocytosis and dynamin-dependent fusion with endosomes. Involvement of clathrin-mediated endocytosis in human immunodeficiency virus type 1 entry. Wyatt R, Sodroski J. The HIV-1 envelope glycoproteins: fusogens, antigens, and immunogens. Folding and assembly of viral membrane proteins. Folding, interaction with GRPBiP, assembly, and transport of the human immunodeficiency virus type 1 envelope protein. Moulard M, Decroly E. Maturation of HIV envelope glycoprotein precursors by cellular endoproteases.

The structural biology of type I viral membrane fusion. Cell Biol. Structural definition of a conserved neutralization epitope on HIV-1 gp HIVinduced cell fusion is mediated by multiple regions within both the viral envelope and the CCR-5 co-receptor.

EMBO J. Structure of an HIV gp envelope glycoprotein in complex with the CD4 receptor and a neutralizing human antibody. Rizzuto C, Sodroski J.

Fine definition of a conserved CCR5-binding region on the human immunodeficiency virus type 1 glycoprotein AIDS Res. Conformational changes induced in the human immunodeficiency virus envelope glycoprotein by soluble CD4 binding. Gallaher WR. Detection of a fusion peptide sequence in the transmembrane protein of human immunodeficiency virus. Evidence that the transition of HIV-1 gp41 into a six-helix bundle, not the bundle configuration, induces membrane fusion. The ectodomain of HIV-1 env subunit gp41 forms a soluble, alpha-helical, rod-like oligomer in the absence of gp and the N-terminal fusion peptide.

Heptad repeat sequences are located adjacent to hydrophobic regions in several types of virus fusion glycoproteins. A general model for the transmembrane proteins of HIV and other retroviruses.

Core structure of gp41 from the HIV envelope glycoprotein. Atomic structure of the ectodomain from HIV-1 gp Soluble CD4 molecules neutralize human immunodeficiency virus type 1. HIV infection is blocked in vitro by recombinant soluble CD4.

Effect of recombinant soluble CD4 in rhesus monkeys infected with simian immunodeficiency virus of macaques. A phase I—II escalating dosage trial. High concentrations of recombinant soluble CD4 are required to neutralize primary human immunodeficiency virus type 1 isolates. Envelope glycoproteins from biologically diverse isolates of immunodeficiency viruses have widely different affinities for CD4.

Agents Chemother. Evidence for a soluble inhibitor. International union of pharmacology. Nomenclature for chemokine receptors. Identification of a major co-receptor for primary isolates of HIV Envelope glycoproteins from human immunodeficiency virus types 1 and 2 and simian immunodeficiency virus can use human CCR5 as a coreceptor for viral entry and make direct CD4-dependent interactions with this chemokine receptor.

An orphan seven-transmembrane domain receptor expressed widely in the brain functions as a coreceptor for human immunodeficiency virus type 1 and simian immunodeficiency virus. Two orphan seven-transmembrane segment receptors which are expressed in CD4-positive cells support simian immunodeficiency virus infection. A new classification for HIV Change in coreceptor use correlates with disease progression in HIVinfected individuals.

Differential syncytium-inducing capacity of human immunodeficiency virus isolates: frequent detection of syncytium-inducing isolates in patients with acquired immunodeficiency syndrome AIDS and AIDS-related complex. Biological phenotype of human immunodeficiency virus type 1 clones at different stages of infection: progression of disease is associated with a shift from monocytotropic to T-cell-tropic virus population. Homozygous defect in HIV-1 coreceptor accounts for resistance of some multiply-exposed individuals to HIV-1 infection.

Entry inhibitors are a promising therapeutic option for HIV-infected patients carrying drug-resistant viruses. Enfuvirtide is the first fusion inhibitor approved for clinical use, but many other compounds are currently in the advanced stages of clinical development. The knowledge of the mechanisms of action of each of these molecules is crucial to understand and predict the corresponding resistance pathways.

While resistance to enfuvirtide is largely dependent on the selection of changes in residues 36—45 within the gp41 HR1 region, the resistance profile for other entry inhibitors is expected to be more complex. The variability of the env gene among the different HIV strains, in conjunction with the different structures and mechanisms of action of the whole family of entry inhibitors, is the major factor responsible for this complexity.

Multiple changes in different gp domains have been associated with resistance to entry inhibitors. The authors express that they don't have any conflicts of interest, financial or otherwise, regarding this manuscript.

Enfuvirtide, the first fusion inhibitor to treat HIV infection. AIDS Rev ; 7 : — Nature ; : —7. T-lymphocyte T4 molecule behaves as the receptor for human retrovirus LAV. Nature ; : —8. Sattentau Q, Weiss R.

Cell ; 52 , —3. Hoxie J. Assignment of intrachain disulfide bonds and characterization of potential glycosylation sites of the type 1 recombinant HIV envelope glycoprotein gp expressed in Chinese hamster ovary cells. J Biol Chem ; : — J Virol ; 78 : — Structure of an HIV gp envelope glycoprotein in complex with the CD4 receptor and a neutralizing human antibody.

Nature ; : — Structures of HIV-1 gp envelope glycoproteins from laboratory-adapted and primary isolates. Structure ; 8 : — The antigenic structure of the HIV gp envelope glycoprotein.

J Infect Dis ; : —9. J Infect Dis ; : —5. J Virol ; 66 : — J Infect Dis ; : — Reeves J, Piefer A. Emerging drug targets for antiretroviral therapy. Drugs ; 65 : — The anti-HIV potency of cyclotriazadisulfonamide analogs is directly correlated with their ability to down-modulate the CD4 receptor.

Mol Pharmacol ; 63 : — Virology ; : — Biochemical and genetic characterizations of a novel HIV type 1 inhibitor that blocks gpCD4 interactions. J Virol ; 77 : — Identification and characterization of a novel inhibitor of HIV-1 entry-I: virology and resistance. Abstract 9. Abstract Curr Pharm Des ; 10 : — Protection of macaques from vaginal SHIV challenges by vaginally delivered inhibitors of virus-cell fusion. Nature ; : 99 — AIDS ; 18 : — Comier E, Dragic T.

J Virol ; 76 : —7. A new classification for HIV Nature ; : J Virol ; 72 : — Interaction of chemokine receptor CCR5 with its ligands: multiple domains for HIV-1 gp binding and a single domain for chemokine binding. J Exp Med ; : — J Virol ; 75 : — Abstract 1. Discovery and characterization of vicriviroc SCH , a novel CCR5 antagonist with potent activity against human immunodeficiency virus type 1.

Antimicrob Agents and Chemother ; 49 : —9. J Virol ; 77 : —8. Hitchcock C. De Clercq E. New approaches toward anti-HIV chemotherapy. J Med Chem ; 10 ; — Evaluation of dosing frequency and food effect on viral load reduction during short-term monotherapy with UK,, a novel CCR5 antagonist.

Abstract B Binding of 2-aryl-4 piperidin-1yl butanmines and 1, 3, 4-trisubstituted pyrrolidines to human CCR5: a molecular modeling-guide mutagenesis study of the binding pocket. Biochemistry ; 42 : — Abstract a.

Mol Pharmacol ; 67 : — Nat Med ; 4 : 72 —7. Abstract 2. Abstract A Abstract LbA J Virol ; 72 : —8. J Virol ; 71 : — Antivir Ther ; 10 : S The molecular target of bicyclams, potent inhibitors of HIV replication. J Virol ; 70 : — J Virol ; 72 : —7. Clonal analysis detects pre-existing R5X4-tropic virus in patient demonstrating populations-level tropism shift on monotherapy. Shift of clinical human immunodeficiency virus type 1 isolates from X4 to R5 and prevention of emergence of the syncytium-inducing phenotype by blockade of CXCR4.

J Virol ; 73 : — Weiss C. HIV-1 gp Mediator of fusion and target for inhibition. AIDS Rev ; 5 : — A synthetic peptide inhibitor of HIV replication: correlation between solution structure and viral inhibition. Peptides corresponding to a predictive alpha-helical domain of HIV type 1 are potent inhibitors of virus infection.

HIV-1 inhibition by a peptide. A synthetic peptide from HIV-1 gp41 is a potent inhibitor of virus mediated cell-cell fusion. N Engl J Med ; : — Robertson D. Nat Biotechnol ; 21 : —1. Kilby J, Eron J. Novel therapies based on mechanisms of HIV-1 cell entry. Abstract 14Ib. Pausing or changing ARVs can do more harm than good. The medications may become less effective or the virus may become resistant to the drugs altogether. Entry inhibitors are medications that have made HIV management possible.

It is important that the appropriate antiretroviral drug ARV regimen for HIV treatment is carefully selected, depending on your medical history, other illnesses, prior HIV treatment, stage of infection, and individual preferences. If you have side effects from antiretroviral therapy ART , there are some tips you can try to manage them. More importantly, talk to your healthcare provider for suggestions and recommendations. Your healthcare provider may also change your treatment plan to help relieve side effects.

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