Due to the lack of metabolic labeling studies, the stability or rate of turn-over of cccDNA in infected hepatocytes is unknown. triggers a caveolin-1-mediated endocytosis to internalize the virus into hepatocytes 21. Recently, using a high-throughput infectious cell culture model enabling RNA interference-mediated loss-of-function screening of hepatitis delta virus (HDV) entry and infection, Verrier and colleagues identified glypican 5 as an additional host cell entry factor for HBV and HDV 22. Apparently, specific disruption of the interactions between HBL and host cellular receptor and entry factors will selectively inhibit HBV infection. For instance, myrcludex B, a synthetic HBV preS1 domain-derived lipopeptide, binds to NTCP and efficiently inhibits HBV and HDV infection of hepatocytes in culture and in humanized chimeric uPA mouse model 23, 24. In a phase I clinical trial, myrcludex B showed excellent tolerability and dose-dependent pharmacokinetics 25. In a phase Ib/IIa clinical trial in chronically HDV infected patients, myrcludex B monotherapy for 24 weeks significantly reduced HDV RNA serum levels and induced ALT normalization. Furthermore, combination therapy with pegylated IFN- demonstrated synergistic effects in reducing HDV RNA and HBV DNA serum levels 26. Hence, as a first-in-class HBV and HDV entry inhibitor, myrcludex B is very promising for further clinical development. Open in a separate window Figure 1 HBV replication cycle in hepatocyte and targets of currently available antiviral therapeuticsBriefly, HBV infects Glyburide hepatocyte by binding to its cellular receptor NTCP and entering into the cells endocytosis. Upon arriving at the cytoplasm, nucleocapsid delivers viral relaxed circular (rc) DNA into the nucleus to be converted into cccDNA by cellular DNA repair machinery. The cccDNA serves as a transcriptional template for production of all viral RNAs, which are subsequently exported to the cytoplasm to translate viral proteins and serve as a pgRNA to be packaged into nucleocapsid. Within the nucleocapsids, pgRNA is reverse transcribed into single-stranded (ss) DNA and then rcDNA. In addition, capsid protien can also assemble into viral RNA/DNA-free empty capsids. Interestingly, while rcDNA-containing mature nucleocapsids can be enveloped and secreted as virions multi-vesicle bodies (MVB), pgRNA containing nucleocapsids as well as empty capsids can also been enveloped and secreted as virion-like particles. In the blood of HBV infected individuals, the number of empty capsid-containing virion-like particles is approximately 100-fold more than virions. However, the number of pgRNA-containing virions is approximately 10 to 100-fold less than virions. In addition to virion-like particles, HBV-infected hepatocytes also secrete empty envelope particles or filaments in the amounts of 1000 to 10,000-fold more than virions. In addition to myrcludex B, several classes of structurally distinct small molecules, including bile salt analogues and cyclosporin A derivatives, have been shown to bind NTCP and inhibit HBV and HDV infection of hepatocytes 27, 28. However, their antiviral efficacy has not been demonstrated in animal models modulation of capsid protein interaction, which results in altered nucleocapsid assembly pathway and formation of either empty capsids or non-capsid polymers. As illustrated in Figure 2, five chemotypes of nucleocapsid assembly inhibitors have been reported thus far. While heteroaryldihydropyrimidines (HAPs), such as Bay 41-4109 and GSL-4, misdirect capsid assembly to form non-capsid polymers of core proteins 41C44, all other nucleocapsid inhibitors induce the formation of morphologically normal capsids devoid of viral pgRNA and DNA polymerase 45C48. Interestingly, crystal structure analyses of HAPs or phenylpropenamides (PPAs) in complex with core protein and capsids revealed that both compounds bind a hydrophobic pocket, designated as HAP pocket, at the dimer-dimer interface near the C-terminal of core protein subunits, Glyburide with contribution from two neighboring core protein dimmers. Binding of these molecules in the HAP pocket induces large scale allosteric conformational changes in core protein subunits and results in quaternary and/or tertiary structure changes of capsids 49, 50. Moreover, a V124W mutation fills the HAP pocket and renders resistance to the inhibition of nucleocapsid assembly by HAPs and PPAs 51. Intriguingly, it was demonstrated recently that Rabbit Polyclonal to CCDC45 NZ-4, a derivative of bis-heterocycle tandem pairs, induces the formation of morphologically normal, but genome-free capsids in Glyburide a core protein C-terminal arginine-rich domain-dependent manner 47. Pharmacologically, as anticipated, by specifically targeting core protein interaction, all the reported nucleocapsid inhibitors showed excellent antiviral activity against the NUC-resistant strains of HBV 45, 48. Thus far, several nucleocapsid assembly inhibitors, such as,.