Development of novel erythromycin-based antibiotics has been one of the most studied topics in the past three decades. Such tremendous efforts have generated a number of beneficiary drugs such as clarithromycin, azithromycin and telithromycin. However, widespread application of antibiotics in clinical practice has triggered an increasing emergence of bacterial resistance. Therefore, discovery of novel macrolide antibiotics to suppress the resistance is urgent for human healthcare. This review focuses on advances in the area since 2004.

C hronic infection with the hepatitis C virus (HCV) is a major global health burden, infecting approximately 3.5 million individuals in the United States (1) and up to 150 million worldwide (2). HCV is estimated to infect approximately 3 to 4 million people each year, with a prevalence in the United States estimated at 1.3% (1) and prevalences as high as 22% in Egypt, 4.8% in Pakistan, and 3.2% in China (3). Chronic HCV infection can lead to cirrhosis of the liver and hepatocellular carcinoma, contributing to the deaths of 700,000 individuals each year (3).HCV is an enveloped, single-stranded, positive-sense RNA virus in the family Flaviviridae. The ϳ9.6-kb genome is translated into a single polyprotein that is subsequently processed into at least 10 structural and nonstructural proteins that are necessary for replication of viral RNA and assembly of new virions (4). At least six distinct HCV genotypes (GTs) and more than 50 subtypes have been characterized, with significant variability in their geographic distributions (5).Within the past 5 years, treatment options for HCV infection have improved dramatically, with approvals from the U.S. Food and Drug Administration for sofosbuvir, ledipasvir, daclatasvir, ombitasvir, paritaprevir, dasabuvir, simeprevir, grazoprevir, and elbasvir. However, it is becoming apparent that differences in potency among the existing approved drugs against various HCV genotypes, as well as their associated resistant variants, demonstrate a clear unmet medical need for additional approved therapies. EDP-239, a small-molecule inhibitor targeting the nonstructural protein 5A (NS5A) of HCV, was designed to address this unmet need.NS5A is a nonstructural HCV protein, with no known enzymatic activities, and is an essential component of the HCV replication complex (6). NS5A possesses RNA binding activity and is comprised of an amphipathic ␣-helix (amino acids 5 to 25) that is important for membrane localization, followed by three distinct structural domains (7-9). Domain I (amino acids 37 to 213) is essential for viral replication, possesses a zinc binding motif, and has been crystallized as a homodimer (9, 10). Domains II (amino acids 250 to 342) and III (amino acids 356 to 447) are less well characterized and exist in a natively unfolded conformation (11,12). Current evidence indicates that the NS5A protein is essential not only for genome replication but also for the assembly of infectious virions (13). Given its involvement in multiple stages of the viral life cycle, NS5A is an attractive target for small-molecule inhibition. Clinically, rapid and profound reductions of HCV RNA have been observed following monotherapy with several other NS5A inhibitors, including daclatasvir, ledipasvir, .EDP-239 is a novel NS5A inhibitor with half-maximal effective concentrations (EC 50 s) in vitro of 31 and 7 pM against genotype 1a (GT1a) and GT1b replicons, respectively. In this report, we demonstrate that EDP-239 does not experience a reduction in potency against replicons resistant to other host-tar...