**3. NA-based antiviral strategies**

404 Myocarditis

immunohistochemical and viral PCR analyses of cardiac tissues (L. T. Cooper et al., 2007). In case of confirmed autoimmune-related disease and lack of detectable viral infection, an immunosuppressive treatment combining corticoids and azathioprine may be beneficial to the patients (Frustaci et al., 2003). However, if the disease is primarily caused by viral infections, more specific antiviral agents would be the ideal drugs of choice. In recent years, the search for such antiviral drugs has become a new trend in drug development for treatment of viral myocarditis. The strategies for developing such antivirals include i) screening chemical compounds, such as Pleconaril, capable of interacting with picornavirus (particularly human rhinovirus) antireceptor to block viral entry of the host cells (Groarke& Pevear, 1999; Kaiser et al., 2000; Reisdorph et al., 2003), ii) application of herb medicine to reduce viral load or boost immune responses to limit viral replication (Si et al., 2007; Y. F. Wang et al., 2009), iii) development of small peptide inhibitors of viral proteases to block CVB3 replication cycle (Maghsoudi et al., 2010) and iv) production of recombinant soluble protein of coxsackievirus-adenovirus receptor (CAR) fused to a human immunoglobulin (sCAR-Fc) to block coxsackievirus B3 entry (Pinkert et al., 2009; Werk et al., 2009; Yanagawa et al., 2003; Yanagawa et al., 2004). Another very attractive and promising trend in drug development is the nucleic acid (NA)-based approach to target viral genome or cellular genes to block viral translation and transcription. These strategies include design and synthesis of antisense oligonucleotide (ASON), ribozyme, short interfering RNA (siRNA) and artificial microRNA (miRNA). In this chapter we will focus our discussion on the recent state of this group of antiviral agents for the treatment of myocarditis caused by CVB3 and

other viruses that have been recently reported as causal agents of myocarditis.

CVB3, a member of enterovirus in the *Picornaviridae* family, is a positive single-stranded, non-enveloped RNA virus. Its genome is ~7.4 kb long containing a single long open reading frame (encoding 11 proteins) flanking by the 5' and 3'untranslated regions (UTRs) (Klump et al., 1990). The 5'UTR is unusually long (741 nucleotides (nt)) and harbors a number of *cis*acting translational elements, such as internal ribosomal entry site (IRES) and cloverleaf sequence (Cheung et al., 2007; Z. Liu et al., 1999; Verma et al., 2010; Yang et al., 1997), which are crucial structures for viral translation and transcription. The 3'UTR is a 99-nt long segment attached with a poly-A tail. The 3'UTR folds to form kissing-loop tertiary structures, which are believed to play a role in facilitating viral transcription of the negative strand of CVB3 replication intermediate (Melchers et al., 1997; J. Wang et al., 1999). The viral genomic RNA can directly serve as a mRNA template for translation of a single long polyprotein, which is processed by viral proteases to produce eleven individual proteins, among which four are structural proteins, VP1-VP4, and seven are non-structural proteins including proteases, 2A and 3C, as well as a RNA-dependent RNA polymerase 3D. These three enzymatic proteins play important roles in viral life cycle and pathogenesis

CVB3 is a cardiotropic virus. It infects cardiomyocytes by endocytosis through viral receptor CAR (coxsackie and adenovirus receptor) colocalized with tight junction protein (e.g., occludin) (Raschperger et al., 2006). Structural analysis of CAR D1 domain supports the proposed function of CAR as a mediator of cell adhesion (Honda et al., 2000) in the junction complexes of epithelial cells in many tissues (Cohen et al., 2001). It is also known that CAR binding site (anti-receptor) on CVB3 particle lies in the canyon on the capsid surface. Upon

**2. CVB3 genome organization and its receptor** 

(Knowlton, 2008).
