Current therapeutics against HCV

Abstract

Hepatitis C is a positive stranded enveloped RNA virus belonging to the Flaviviridae family. HCV infection leads to severe liver diseases, cirrhosis and hepatocellular carcinoma worldwide. Although treatments have been available for a while, due to its complexity and genetic diversity, only few are reported to be effective against all HCV genotypes. Here, we review the HCV life cycle and its immunogenic potential and various mechanisms via which the virus interferes in the signalling process. A comprehensive overview of current anti-HCV therapeutics, such as, Direct Acting Antiviral (DAA) as well as Host Targeting Agents (HTA), along with their scope, known mechanism of action and limitations are presented.

Supplementary information: The online version contains supplementary material available at 10.1007/s13337-021-00697-0.

Keywords: DAA; HCV; HTAs; Therapeutics.

Fig. 1

HCV entry and life cycle. Circulating lipoviroparticles enter hepatocytes in a multi-step process involving multiple interactions as depicted. DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin) and L-SIGN (liver/ lymph node-specific intercellular adhesion molecule-3-grabbing integrin) have been shown to bind with HCV envelope glycoprotein E2 and deliver the virus to the liver. Circulating LVPs enter the liver via sinusoidal blood pass through the space of Disse, and interact with the receptors on the hepatocyte surface. LVPs near to the surface of hepatocytes are initially captured by HSPG which enables its interaction with LDLR. Other entry receptors include SRB1 (scavenger receptor class B type 1),CD81, the TJ (tight junction) proteins such as occludin (OCLN) and claudin-1 (CLDN1), EGFR (epidermal growth factor receptor), iron receptor protein TfR1 (transferrin receptor 1), RTKs (receptor tyrosine kinases), EphA2 (ephrin receptor A2) and NPC1L1 (Niemann-Pick C1-like 1 cholesterol uptake receptor). The initial attachment of LVPs is with SRB1 which rearranges lipoprotein on HCV particles and exposes the hidden E1E2 epitopes, which enable E1E2 binding to other receptors. Following attachment with CD81, CLDN1 and OCLN join to form a complex and the whole is internalized via clathrin and dynamin mediated endocytosis. Viral and host membrane fusion leads to the viral genome release into the cytosol which results in the initiation of viral replication and translation. Viral envelope proteins E1 and E2 play the key role in the membrane fusion where E1 acts as a chaperone. A fusion pore is formed due to the conformational changes in the glycoproteins and the viral genome is released in the cytoplasm. The acidic pH along with the optimum temperature within the endosomal compartment triggers this process of penetration of host cell membrane and uncoating. Host membrane protein NPC1L1 modulates and rearranges the lipid composition in the membrane which leads to membrane fusion. After entry, the virus undergoes replication within a membranous web adjacent to the ER. Viral replication machinery consists of NS3/4A, NS4B, NS5A, and NS5B. A negative strand intermediate is synthesized by NS5B using the RNA genome as template. A microenvironment is created in the cytoplasm by NS4B which involves massive rearrangements of intracellular membranes to form a ‘membranous web’, where viral replication takes place. Upon translation, the HCV proteins are associated with membranes derived from the endoplasmic reticulum (ER). Nascent RNA genomes are translated to produce new viral proteins and also serve as new/additional RNA templates for further RNA replication which are progressively assembled to form infectious virions. Thereafter the virus assembles at the ER surface and egresses via the secretory pathway. Red stop dash arrows indicate points of intervention by DAAs

Fig. 2

Innate immune response to HCV infection and sites of intervention. Innate immune response to trigger interferon signalling upon HCV infection is depicted. Presence of viral RNA activates PRR (pattern recognition receptors) on the cell surface, cytoplasm, and endosomes and help to initiate an immunogenic response via PAMP (pathogen-associated molecular pattern). IRES (internal ribosome entry site) of HCV is recognized by RNA-dependent PKR (protein kinase R) whereas RIG-I (retinoic acid inducible gene I), another PRR recognizes unique features of sequences present in 3’ and 5’ region of HCV RNA. After viral recognition by RIG-I and PKR, longer HCV dsRNA intermediates activate endosomal TLR 3 (toll-like-receptor 3). Upon activation, PKR and RIG-I bind to MAVS (Mitochondrial antiviral signalling protein, also called VISA, IPS-1 and CARDIF) which via ubiquitinylation of Traf6 ultimately results in upregulation of interferon synthesis via the JNK pathway and activation of NFκB. Simultaneously, TLR3 signals are transmitted via the adapter molecule IFN-β /TRIF (Toll/IL-1 receptor domain-containing adapter inducing interferon-β). ds RNA activated protein kinase R (PKR) plays a major role in host antiviral defence, by phosphorylating and thereby inhibiting eIF2α (eukaryotic elongation initiation factor 2α subunit). Inhibition of eIF2α decreases cellular mRNA translation which may ultimately culminate in induction of apoptosis. HCV can however escape via induced phosphorylation of protein kinase R thereby inactivating it and is one of the probable reason for HCV persistence. Either way, viral protein synthesis is not affected by PKR induced stall in mRNA translation due to an IRES (internal ribose entry site) dependent manner. HCV can also escape immune response by NS3-4A serine protease complex mediated cleavage of MAVS and TRIF. This blocks the production of IFN-β in infected liver cells. Circulating dendritic cells and macrophages are also activated to produce an interferon response upon activation by PAMPS in the hepatocytes. Ultimately in response to activation by NFκB and JNK pathway, interferons are secreted which aim to create an antiviral state. Red stop dash arrows indicate points of intervention by viral proteins to stop the onset of immune response