Medical Pharmacology Chapter 36:  Antiviral Drugs

• Antiretroviral Drugs Used in Treating HIV Infection

  • HIV Life Cycle and Sites of Antiretroviral Drug Activity⁶

    Steps in HIV Viral Replication and Sites for Antiretroviral Drug Action (Red ellipses)  http://aidsinfo.nih.gov/education-materials/fact-sheets/19/73/the-hiv-life-cycle

  • There are four classes of antiretroviral agents targeting HIV-specific enzymes.⁷

    • These classes include:⁷

      • Nucleoside and nucleotide reverse-transcriptase inhibitors

      • Nonnucleoside reverse-transcriptase inhibitors

      • Protease inhibitors

      • Integrase inhibitors

    • Another class of anti-HIV drugs is are HIV cell entry inhibitors or CCR5 antagonists.

      • Here a specific, nonenzymatic protein is the pharmacological target.⁷

  • History and Review:¹

    • By review, the HIV virus targets cells based on the presence of the CD4 receptor.¹ 

      • Diagram of the HIV Virion⁹

        US National Institute of Health (redrawn by Henderson C) Diagram of the HIV Virus http://commons.wikimedia.org/wiki/File:HIV_Virion-en.png

      • The targeting is regulated by the viral envelope protein gp160 (env).

        • The CD4 receptor is present on both lymphocytes and macrophages.

        • HIV entry into host cells is also dependent on a chemokine receptor, either CCR5 or CXCR4.

          • CCR5 is associated with macrophage-type target cells.

            • Of the two chemokines, the CCR5 coreceptor is dominant although as the disease advances a shift from CCR5 to CXCR4 dependency has been observed.

            • Also, increased affinity of HIV-1 for the CXCR4 coreceptor allows T lymphocyte infection.

              • The transition between CCR5 to CXCR4 dependency is also correlated with an increased rate of loss of CD4+ helper T cells along with increasing likelihood of immunosuppression.¹

    • The gp41 domain of env regulates virus-lipid bilayer fusion with the host cell lipid bilayer (membrane).¹ 

      • HIV Viral Entry:  Membrane Fusion¹²

        Jones M Mechanism of Viral Entry/Membrane Fusion "1.  Inital interaction between gp120 and CD4. 2.  Conformational change in gp120 allows for secondary interaction with CCR5. 3.  The distal tips of gp41 are inserted into the cellular membrane. 4.  gp41 undergoes significant conformational change; folding in half and forming coiled-coils. This process pulls the viral and cellular membranes together, fusing them. The diagram above suggests that it is a T helper cell; however, the co-receptor on a T helper cell for this mechanism is CXCR4 not CCR5. If it contains a CCR5 receptor is is more likely to be a macrophage." Early in HIV infection, viruses tend to be macrophage-trophic, using the CCR5 co-receptor.11   Syncytial-forming viral variants, using the CXCR4 co-receptor, exhibit T-lymphotrophic preference, likely associated with rapid advancement to AIDS.11

      • After fusion, viral RNA enters the cytoplasm and undergoes replication to a transient RNA-DNA duplex.

        • The initial RNA is degraded by RNase H, permitting synthesis of a full-length double-stranded DNA "copy" of the viral genome.

          • The error prone characteristic of the HIV reverse transcriptase enzyme results in HIV virus mutations.

            • Mutations occur in about three bases per full-length (9300-base-pair) replication.

        • Following cytoplasmic-localized reverse transcription, viral DNA is translocated to the nucleus and integrated into a host chromosome by an HIV-viral integrase enzyme.

          • This viral integrase enzyme is a pharmacological anti-HIV target site.¹

            • More information about HIV viral-host cell integration may be found here.

    • After host cell genomic integration, the viral genome may remain latent i.e. not directing RNA or protein synthesis yet the viral genome is replicated as the cell divides.¹ 

      • Cells containing the viral genome as part of its own genome may be activated and at that time viral RNA and proteins would be synthesized.

      • Genomic viral RNA is housed in a nucleocapsid with envelope and structural proteins concentrated at the cell surface, localized in cholesterol-rich "lipid rafts.".

      • Nucleocapsid cores concentrate at these sites and bud across the cell membrane, thus creating new envelope HIV virions with two complete single-stranded RNA genomes.

      • When these virions infect new cell, reverse transcriptase may initiate a new cycle of viral replication.¹

        • HIV Replication Cycle¹²

          National Institute of Allergy and Infectious Diseases (NIAID) "Produced by the National Institute of Allergy and Infectious Diseases (NIAID), this illustration depicts the human immunodeficiency virus (HIV) replication cycle involving the host cell within the human body, beginning with a HIV virion attaching to the host cell wall, dumping its contents into the matrix of the host cell, conversion of the viral RNA to viral DNA, which combines with the host cell DNA, leading to the creation of new viral RNA, which migrates to the host cell periphery, and forms of vacuole around the new viral RNA using the host cell wall, thereafter,budding off is a newly-created mature HIV virion." http://phil.cdc.gov/phil/details.asp ID#:  18162

   

  • Viral Pathogenesis:¹

    • HIV Disease Pathogenesis¹⁴

      "Pathogenic events in HIV disease. 1. "On entry into the body, HIV probably encounters a dendritic cell, which is either infected or carries the virus to a CD4+ T cell in a lymphoid organ. 2. "Infection is then established in the lymphoid organ and a burst of viraemia seeds virus throughout the body. 3. "Viral replication is partially controlled, but the remaining replication almost invariably leads to escape from immune control, and establishment of chronic persistent infection in lymphoid tissue. "The net level of viral replication is regulated by a balance between factors that induce virus replication and factors that contain or suppress it. "In most patients, replication accelerates over a period of time, leading to destruction of the immune system. "The white dots in the lymphoid tissue represent individual cells expressing HIV viral RNA, as determined by in situ hybridization. Trapped virus (large clumps of white staining in the peripheral regions of the lymph node) is extracellular." Attribution:  This figure corresponds to Fig. 1 in reference 14 [Fauci AS Host factors and the pathogenesis of HIV-induced disease.  Nature 384 (6609):  529-534 Dec 12, 1996.]

    • HIV infection due to sexual activity may be initiated by one or only a few infectious virions.¹ 

      • After this event, a short, rapid replication phase occurs in the 2-4 week time frame.¹ 

        • During this brief period ≥ 10⁹ cells become infected.

          • This peak of infectious activity is accompanied by a temporary reduction in peripheral CD4⁺ (helper) T lymphocytes.

          • New host immune response combined with HIV-target cell depletion leads to an approximate steady-state level of infectious HIV virions.

          • This level is mirrored by plasma HIV RNA concentration, also reflecting "viral load".

          • This apparently relatively constant level of HIV virus activity results from a number of factors such as the host immune response, the pathogenicity of the particular HIV viral strain and defines a "set point".

            • This set point can be thought of as a balance between virion replication which increases the viral burden and clearance which reduces it.¹

    • The early burst of activity is associated with acute symptoms, significant viremia and large numbers of infected target cells (CD4⁺ T cells) in blood.¹³ 

      • The early antiviral immune response involves antibodies and cytotoxic T cells and reduces circulating virus by at least a factor of 100, initiating the clinically latent phase characterized by low, yet constant presence of HIV virions and infected cells in circulation.

        • During this latent phase the number of CD4+ T cells gradually decline.

          • Clinical latency may extend for about 10 years.

          • Eventually, the number of CD4 + T cells is reduced to extremely low values coinciding with the development of acquired immunodeficiency syndrome (AIDS).

          • The development of AIDS is also associated with rapidly increasing viral activity and increasing circulating infected cells.¹³

          •  

            Viral Load and Cell Number Kinetics in HIV-Infected Patients¹³

            In the landmark publication by Coffin (13) noted that prior to antiretroviral treatment, the concentration of HIV virions in circulation appeared constant over a time frame of week two months. However, after antiretroviral treatment was initiated, a rapid reduction in circulating virion concentration was observed. This reduction was associated with a t½ (half-time) of about two days. Patient to patient variation of this response was found to be very small. Furthermore, virion concentration with antiviral treatment diminished to as little as 1% of starting values. Following the initial two week period of drug administration, the HIV virion concentration began to increase, as estimated by viral RNA levels. This increase was accounted for by HIV mutant strains resistant to the antiretroviral drug. With respect to CD4+ T cell levels, the initial increase in total CD4 T-cell levels declined back to antiretroviral drug pretreatment values as free mutant virion levels climbed. (See figure above)13 The figure above corresponds to Fig. 1 in reference 13 and is a composite of results from several studies that followed the concentration in blood of both wild-type and mutant (drug-resistant) HIV genomes.  The genomic levels were evaluated by B-DNA or RT-PCR (Reverse Transcriptase-Polymerase Chain Reaction) The results in the figure included trials involveing the nucleoside RT inhibitors 3TC and AZT (zidovudine), nonnucleoside inhibitors nevaripine an delaviridine and two protease inhibitors.13  (http://www.ncbi.nlm.nih.gov/pubmed/7824947)

      • As noted earlier, most individuals progress to clinical AIDS in about a decade patients subsequent to sexual acquisition of CCR5-tropic HIV-1 disease (without antiretroviral treatment) .¹ 

        • However, a subset of patients, described as "long-term nonprogressors" have been identified as having HIV infection for over 20 years without exhibiting immunosuppression or notable reduction in CD4 T cell counts.^1,14   

          • Long-term nonprogression may occur as a consequence of an interplay of individual immunogenetics and immune responses.^1,14 

            •  

              Factors Influencing HIV Replication and HIV-Related Disease Progression¹⁴

              "Control of HIV replication and the progress of HIV-induced disease by the balance of host factors. "Cellular activation and proinflammatory cytokines drive viral replication. "These are counterbalanced by inhibitory factors, including both the immune system and nonspecific cytokines such as IL-10 and TGF-β, as well as co-receptor ligands such as the β-chemokines and SDF-1."14 Attribution:  This figure corresponds to Fig. 4 in reference 14. [Fauci AS Host factors and the pathogenesis of HIV-induced disease.  Nature 384 (6609):  529-534 Dec 12, 1996.]

          • Some long-term nonprogressors (probably a small subset) exhibit mutations of the CCR5 coreceptors; these mutations reduce HIV infectivity.^14,15 

          • Other factors may include different mitochondrial DNA haplotypes (group of genes inherited from one parent) which influences rates of HIV-1 disease progression.¹⁶