| How Do Targeted Anti-HCV Drugs Work? |
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Thursday, 04 February 2010 23:26
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By Liz Highleyman THE HCV LIFE CYCLE HCV is a small virus consisting of a genome encased in a capsid shell and surrounded by an outer envelope. HCV life cycle: a) Virus binding and internalization, b) cytoplasmic release and uncoating, c) IRES-mediated translation and polyprotein processing, d) RNA replication, e) packaging and assembly, f) virion maturation and release. The topology of HCV structural and nonstructural proteins at the endoplasmic reticulum membrane is shown schematically. HCV RNA replication occurs in a specific membrane alteration, the membranous web. Note that IRES-mediated translation and polyprotein processing as well as membranous web formation and RNA replication, illustrated here as separate steps for simplicity, may occur in a tightly coupled fashion (from Moradpour D. et al. Nat. Rev. Microbiol. 2007;5:453-463.) http://www.chuv.ch/imul/imu_recherche _moradpour.htm Click Here to view an animation of the HCV Life Cycle http://www.hcvadvocate.org/news/newsLet ... 209.html#3 The HCV genome, or genetic material, takes the form of positive single-strand RNA, which serves as a “blueprint” for the production of proteins and enzymes that make up the virus. In order to replicate, or reproduce, HCV must enter a host cell and take over its machinery. The virus first attaches itself to receptors on the host cell’s surface, penetrates the cell membrane, and uncoats itself by shedding its outer layers. Using the cell’s ribosomes tiny protein-production factories the RNA is translated, or used to create new viral proteins. The viral genome copies itself using genetic building blocks present in the host cell. Finally, these newly produced proteins and RNA are assembled to form complete virus particles that bud out of the host cell and go on to infect additional cells. HCV PROTEASE Specific viral enzymes are required to carry out certain steps in the replication process. Compounds that interfere with the action of these enzymes can therefore slow or halt viral reproduction. The STAT-C drugs furthest along in development target the HCV protease and polymerase enzymes. When viral RNA is translated, it initially produces a single large polyprotein containing about 3,000 amino acids. This polyprotein must then be cleaved, or cut up into smaller pieces that can be used to assemble new viral particles. This is the job of protease enzymes, which act as “molecular scissors.” The hepatitis C drug candidates expected to emerge first from the development pipeline, Vertex’s telaprevir and Schering-Plough’s boceprevir, are both covalent NS3/4A serine protease inhibitors. These compounds interfere with the HCV non-structural NS3 serine protease and the NS4A cofactor that facilitates protease function. The “next-generation” protease inhibitor narlaprevir belongs to the same class. These agents bind covalently to an active site on the protease enzyme, preventing it from carrying out its normal activity. Other promising agents, including RG7227 (also known as ITMN-191) and MK-7009, work similarly, but with non-covalent binding to the HCV protease. Many other protease inhibitor candidates are in earlier stages of development. HCV POLYMERASE Different enzymes are needed to copy viral RNA, the other crucial component of new virus particles. Genetic material is composed of a chain of building blocks known as nucleotides. HCV’s positive RNA strand is used as a template to produce a complementary negative (antisense) strand, which in turn is used to make more positive strands. HCV accomplishes this using an enzyme called RNA-dependent RNA polymerase, meaning it uses RNA to produce RNA (human cells, in contrast, use DNA to produce DNA, while retroviruses like HIV use RNA to produce DNA). The HCV NS5B RNA-dependent RNA polymerase produces new RNA by adding successive nucleotides in a chain. Nucleoside or nucleotide analog drugs act as defective building blocks, or chain terminators. When added to a growing RNA chain, they prevent the addition of further nucleotides, thereby bringing production to a halt. Some promising experimental agents, including RG7128, are nucleoside analog HCV polymerase inhibitors, as was the now-discontinued valopicitabine. (Ribavirin and successors like taribavirin are also nucleoside analogs but, as noted, seem to work by other mechanisms in anti-HCV therapy). The experimental agents PSI-7851 and IDX184 are examples of nucleotide analogs, which require less processing in the body than nucleoside analogs before they can be used. Drugs may also interfere with polymerase activity in a different way, by binding to the enzyme and preventing it from working properly. Several non-nucleoside HCV polymerase inhibitors are in earlier stages of development, including Abbott’s ABT-072, Anadys Pharmaceuticals’ ANA598, Japan Tobacco’s JTK-003, and Vertex’s VCH-916. OTHER DRUG TARGETS While protease and polymerase inhibitors are furthest along in development, agents targeting other steps in the HCV life cycle are also being studied. Compared with other viruses, researchers know relatively little about how HCV enters host cells. Nevertheless, drug developers are working on agents that interfere with viral attachment to cells by blocking either cell receptors or viral envelope proteins. Once inside a host cell, HCV sheds its outer layers to release its genetic material. Agents that interfere with this uncoating process are also potential drug candidates. Unlike some other viruses, HCV does not integrate its genetic material into the host cell’s genome, so integrase inhibition is not a potential drug target. In order to produce new proteins, HCV uses the host cell’s ribosomes and a replication complex where gene translation takes place. The viral genome contains an internal ribosomal entry site (IRES) at one end to enable this process. Companies are working on early development of various agents that interfere with IRES and HCV messenger RNA, including antisense oligonucleotides, ribozymes, and short interfering RNA sequences (siRNA). After the HCV polyprotein is cleaved by a protease, some of the component pieces must be further processed before they can be used to assemble new virus particles. Various compounds such as castanospermine and its derivative celgosivir disrupt these processes. Helicase enzymes are “motor” proteins that separate strands of genetic material. The function of the HCV helicase is not fully understood, but it represents another potential drug target. Finally, anti-HCV agents could interfere with the final step of replication, inhibiting viral assembly or budding from host cells. STAT-C TOXICITY Drugs that specifically target HCV are less likely to cause systemic or whole body side effects such as those seen with interferon, which affects immune response rather than the virus itself. However, experience with antiretroviral therapies for HIV which work by some of the same mechanisms as anti-HCV agents indicates a need for caution. HCV protease inhibitors, for example, have the potential to mimic the activity of human protease inhibitors. Nucleoside or nucleotide analogs could potentially interfere with copying of human as well as viral genetic material. Though the underlying mechanisms are not fully understood, drugs targeting HIV protease and reverse transcriptase (a form of polymerase) have been linked to a variety of metabolic and mitochondrial toxicities, so researchers should be on the lookout for similar side effects with anti-HCV agents. PREVENTING DRUG RESISTANCE Compared with DNA viruses, RNA viruses like HCV mutate frequently as they replicate, and lack a “proofreading” mechanism to eliminate such errors. Many viral mutations are either irrelevant or detrimental, but others can enable the virus to overcome the action of drugs. For example, a small change in the structure of the HCV protease binding pocket can mean protease inhibitors no longer fit into the pocket to disrupt protease function. For all protease and polymerase inhibitors in development, single amino acid changes have been identified that reduce viral sensitivity to the drug. But combining agents that work by different mechanisms can slow or prevent such resistance. To overcome the effects of multiple drugs, HCV would have to produce multiple mutations, which tends to reduce viral fitness. Pharmaceutical companies are testing several STAT-C combinations to determine whether the drugs have synergistic activity. Furthest along is a combination of the HCV protease inhibitor RG7227 plus the polymerase inhibitor RG7128. In the Phase 1 INFORM-1 trial, the two drugs demonstrated potent antiviral activity over 14 days in both treatment-naïve and interferon-experienced genotype 1 chronic hepatitis C patients. In the future, it is likely that hepatitis C treatment will come to increasingly resemble therapy for HIV or hepatitis B virus, using combinations of small oral agents that attack the virus from multiple angles simultaneously, thereby improving the chances of disrupting viral replication over the long term. http://www.hcvadvocate.org/news/newsLet ... 209.html#3 Add this page to your favourite Social Bookmarking websites;         
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