| Description |
Tuberculosis (TB) remains one of the world’s deadliest infectious diseases, second only to Human Immunodeficiency Virus (HIV). The alarming global statistics on TB has prompted a renewed interest in the discovery and development of antitubercular drugs in the scientific community. As part of the growing efforts toward the development of new TB drugs, we have identified amicetin as a viable candidate for antitubercular drug discovery. A recent natural product library screening has identified amicetin, a ribosomal antibiotic isolated from Streptomyces vinaceusdrappus in 1953, to be active against TB, relatively noncytotoxic, and compatible with current antiretroviral therapy (ART). Here we present synthetic and biological studies on amicetin and its analogues. A modular synthetic route towards the formation of cytimidine, a derivative of amicetin, as well as some analogues via a one-pot copper catalyzed N-aryl amidations is described. This route allows the efficient and rapid diversification of the cytimidine core by exploiting the regioselective coupling of a 4-iodobenzamide with a 2-halopyrimidine affording the union of three fragments in a single synthetic manipulation. Synthetic efforts toward the synthesis of the disaccharide moiety using the Noyori-Achmatowicz reaction sequence were also presented. In line with our efforts, amicetin was expressed, isolated, and purified from Streptomyces vinaceusdrappus and was co-crystallized with the 70S subunit of Thermus thermophilus ribosome at 3.5 Å. From our crystallographic data, amicetin forms a Watson-Crick base pair through its cytosine moeity with G2262 (T. thermophilus numbering). Its aminosugar moeity forms cation-π interactions with the A2450 while its p aminobenzoyl group π-stacks with A2613. Its α methylserine moeity forms a hydrogen bond with the R18 of the ribosomal protein L16. Amicetin displaces the penultimate cytosine of the conserved CCA 3’ end of the P-site tRNA, trapping the P-site tRNA in a non-productive conformation, thereby inhibiting protein synthesis. Additionally, a number of synthesized analogues were found to exhibit good anti-mycobacterial property, the most potent of which, analogue 3, has an IC50 of 0.98 µM against M. tuberculosis H37Ra and selective cytotoxicity of IC50 > 100 µM against CEM-TART cell line. These active analogues were found to inhibit bacterial protein synthesis using luciferase assay. All amicetin analogues were found to exhibit a narrow spectrum of antibacterial activity and selective cytotoxicity against mammalian cells regardless of their antimycobacterial potency. Crystal structures of analogues 1 and 5 bound to the 70S subunit of the T. thermophilus ribosome were obtained at 3.1 Å. Both analogues were observed occupying the same binding site as amicetin. Analogue 1, the more potent of the two, however, exhibited a closer binding motif to amicetin, exhibiting all key interactions aforementioned while analogue 5 lacks the cation-π interaction with A2450. These discoveries provide modular routes to amicetin analogues as well as key insights into their biological activity. Current and future efforts are being directed towards the development of more potent amicetin antimycobacterial agents. |
