Medical Pharmacology Chapter 36:  Antiviral Drugs

Entry Inhibitors

• Antiretroviral Drugs Used in Treating HIV Infection

  •  →Entry Inhibitors:

    • HIV viral entry may be inhibited by two drugs classified as "entry inhibitors" and these agents act by different mechanisms.⁶  

      • One agent, enfuvirtide (Fuzeon) prevents viral and cell membrane fusion, a process mediated by molecular interactions between viral gp41 and the target cell surface binding site CD4.

      • The second agent, maraviroc (Selzentry or Celsentri), is classified as a chemokine receptor inhibitor, binding to host cell CCR5 receptors with the consequent effect of  preventing HIV viral gp120 protein association.⁶

    • The drug enfuvirtide, derived from gp41, binds to gp41 in a way that prevents the structural change needed for fusion.

  •  

    Site of Enfuvirtide (Fuzeon) Anti-Viral Activity⁹

    a: The lifecycle of HIV-1, showing the step at which enfuvirtide acts, and also the steps targeted by other drugs.9  b:  "Simplified schematic of the mechanism by which enfuvirtide inhibits viral entry into host cells.9    The envelope glycoprotein of HIV-1 consists of two non-covalently associated subunits, gp120 and gp41. After attachment of HIV-1 to its target cells carrying the CD4 receptor, gp120 interacts with the CD4 receptor, which initiates a series of conformational changes in gp41 and gp120 that lead to the insertion of a region of gp41 into the membrane of the host cell, and formation of a ' pre-hairpin intermediate' (top). Further changes in the confirmation of gp41 bring the viral and cellular membranes into close enough proximity for membrane fusion (bottom left). Enfuvirtide binds to a region of gp41 that mediates this conformational change from pre-hairpin intermediate to the fusion-active structure, thereby preventing fusion and viral entry (bottom right). Abbreviations: NNRTI, non-nucleoside reverse transcriptase inhibitor; NRTI, nucleoside reverse transcriptase inhibitor; PI, protease inhibitor" Attribution:  Slight modification of Figure 1 from:  LaBonte J Lebbos J Kirkpatrick nature Reviews Drug Discovery 2, 345-346 (May 2003).9  Original figure attribution before modification:  Kilby JM Hopkins S Venetta TM DiMassima B Cloud GA Lee JY Alldredge L Hunter E Lambert D Bolognesi D Matthews T Johnson MR Nowak MA Shaw GM Saag MS Potent suppression of HIV-1 replication in humans by T-20, a peptide inhibitor of gp41-mediated virus entry. Nat. Med 4(11): 1302-1307, 1998.

     

  • Enfuviritide (Fuzeon) is a synthetic peptide consisting of 36 amino acids.⁶ 

    • Enfuviritide (Fuzeon)

    • Part of its amino acid sequence was modelled  based on the sequence of the transmembrane gp41 HIV-1 protein important in viral membrane fusion with the target cell membrane.

      • Enfuviritide exhibits selectivity against HIV-1 laboratory and clinical variants; moreover, this drug is not active against HIV-2.

      • The mechanism of anti-HIV activity was determined associated with inhibition of HIV-mediated membrane fusion.

      • Enfuviritide exhibits a unique pharmacodynamic anti-retroviral mechanism, blocking the interaction between the N36 and C34 sequences of the gp41 glycoprotein.⁶  

      • The peptide inhibition is due to association in the hydrophobic groove associated with the N36 coil.

    • Because of its distinctive mechanism of antiretroviral action, enfuviritide may continue to exhibit antiretroviral activity in patients in which their viral isolates exhibit resistance to antiretroviral drugs associated with other classes.

      • However, HIV-enfuvirtide resistance may develop as a result of mutations in the enfuvirtide- gp41 binding domain.

      • Patients exhibiting virologic failure while on enfuvirtide treatment usually (94%) present with mutations in this gp41 region (in vitro).

      • Single amino acid substitutions at one of two common mutation sites may increase resistance by 450 fold in laboratory tests.

      • However,  clinical resistance typically presents with two or more amino acid alterations.

    • Enfuviritide (Fuzeon) was as of 2011 the only approved antiretroviral agent requiring parenteral administration, with the bioavailability of subcutaneously administered enfuvirtide estimated at about 80% compared to the IV dose.⁶  The approximate elimination half-life (t_½) of the parenteral drug is about four hours, thus requiring twice-daily dosing.

      • Adverse effects:⁶  

        • Injection site reactions represent the most prominent adverse effect with nearly all patients exhibiting injection site pain, erythema and induration.

        • About 5% of patients discontinue therapy due to these local reactions.

      • Clinical indications:⁶  

        • Enfuvirtide (Fuzeon) is FDA-approved only for use in those adults exhibiting HIV replication despite ongoing antiretroviral drug treatment.

        • Adding enfuvirtide to and optimize antiretroviral treatment plan increases the fraction of patients with undetectable plasma HIV-1 RNA concentrations evaluated after six months of treatment.

          • The magnitude of this effect is described as an increase from about 6% of patients with undetectable plasma HIV-1 RNA to about 16%.

          • In this consideration undetectable plasma HIV-1 RNA is defined as <50 copies/cc.

    • Background:  Mechanism of Drug Action⁷

      • Gp41 and gp120 constitute the envelope (Env) glycoprotein in HIV-1.

        • Gp120 is the surface subunit binding to host cell receptors; whereas gp41 is the transmembrane unit that inserts into host cell membrane promoting membrane fusion.

        • Gp41 exhibits two helical regions with one residing near the amino terminus (N helix) and one near the carboxyterminus (C helix).

        • These two helices interact in an anti-parallel fashion, like a hairpin bringing the amino and carboxy termini together.

        • The membrane fusion structure gp41 has been described as three gp41 hairpins (trimer-of-hairpins) with the N helices from three gp41 domains forming a central three-stranded coiled coil while being surrounded by three antiparallel C helices.⁷

        • Therefore, it may not be surprising that synthetic peptides derived from these regions of gp41 inhibit HIV-1 viral infection.⁷ 

    • Mutations and Resistance to Enfuvirtide:

      • Resistance to enfuvirtide is mediated by mutations in the enfuvirtide binding region of the Gp41 subunits of the HIV envelope protein complex.

        • A functional Gp41 transmembrane protein is required for host cell infection.

        • Patients experiencing virologic failure during enfuvirtide treatment typically (>90) exhibited gp41 mutations with the most frequently noted mutations being a V38A or N43D substitution.⁶ 

          • V38A:  wild-type valine changed to an alanine amino acid.¹³

          • N43B:  wild-type asparagine changed to an aspartate amino acid.¹³

        • "High-level" clinical resistance often involves 2 or more amino acid mutations, although single-amino acid substitutions in the in vitro setting may induce up to a 450-fold resistance.⁶

      •  

        Locations of Gene Mutations and Resistance to the Antiretroviral Drug Enfuvirtide¹³

        Amino acid abbreviations: A:  Alanine C:  Cysteine D:  Aspartate E:  Glutamate F:  Phenylalanine G:  Glycine H:  Histidine I:  isoleucine K:  Lysine L:  Leucine	M:  Methionine N:  Asparagine P:  Proline Q:  Glutamine R:  Arginine S:  Serine T:  Threonine V:  Valine W:  Tryptophan Y:  Tyrosine

    • Pharmacokinetics:

      • Enfuvirtide is administered parenterally e.g. subcutaneously or by intravenous administration.¹²

        • Administration may require twice-daily injections; however, the location of the subcutaneous injection site does not appear to influence drug pharmacokinetics.⁶

        • In vitro metabolic studies using hepatocytes and human microsomes suggest that enfuvirtide is hydrolyzed to form a deaminated metabolite at the C-terminal phenylalanine residue.¹¹  

          • This metabolite is designated M3,  which is detectable in human plasma following enfuvirtide administration.

          • Enfuvirtide half-life is about 4 hours after a single 90-mg subcutaneous dose within associated apparent clearance of about 30 ml/h/kg.¹¹

    • Adverse Effects:

      • Local injection site reactions represent the most common adverse effect associated with enfuvirtide treatment.¹²  

        • This reaction is characterized by "painful erythematous nodules".

          •  

            Injection Site Reactions Associated with Enfuvirtide Administration¹⁶

            Image Attribution:  Correll T, PharmD, BCPS, Clinical Pharmacy Specialist, Infectious Disease/HIV University of North Carolina Hospitals November 2006 From Slide 51   http://www.slideshare.net/flaviog/11-1822498

          • This adverse reaction only rarely necessitates drug discontinuation.

      • Headache, dizziness, insomnia nausea represent other side effects that may occur.

      • Furthermore, enfuvirtide hypersensitivity reactions may also rarely present.

      • The principal laboratory abnormality following enfuvirtide administration is an eosinophilia. Drug-drug interactions that would require dosage adjustment have not been noted.¹²

    • Therapeutics:

      • Enfuvirtide (Fuzeon) has received FDA approval for use in treatment-experienced adults presenting with significant, continuing HIV replication in the face of continuing standard antiretroviral treatment.⁶  

        • Beneficial response with enfuvirtide is more likely to occur in those patients receiving at least two other effective antiretroviral agents in the protocol.

        • For several reasons including administration convenience, cutaneous toxicity described above, and cost, enfuvirtide is usually given only to those patients who have "failed all other feasible antiretroviral regimens".⁶