Design, synthesis and biological evaluation of novel l-ascorbic acid-conjugated pentacyclic triterpene derivatives as potential influenza virus entry inhibitors

Since the influenza viruses can rapidly evolve, it is urgently required to develop novel anti-influenza agents possessing a novel mechanism of action. In our previous study, two pentacyclic triterpene derivatives ( Q8 and Y3 ) have been found to have anti-influenza virus entry activities. Keeping the potential synergy of biological activity of pentacyclic triterpenes and l -ascorbic acid in mind, we synthesized a series of novel l -ascorbic acid-conjugated pentacyclic triterpene derivatives ( 18 – 26 , 29 – 31 , 35 – 40 and 42 – 43 ). Moreover, we evaluated these novel compounds for their anti-influenza activities against A/WSN/33 virus in MDCK cells. Among all evaluated compounds, the 2,3- O , O -dibenzyl-6-deoxy- l -ascorbic acid-betulinic acid conjugate ( 30 ) showed the most significant anti-influenza activity with an EC 50 of 8.7 μM, and no cytotoxic effects on MDCK cells were observed. Time-of-addition assay indicated that compound 30 acted at an early stage of the influenza life cycle. Further analyses revealed that influenza virus-induced hemagglutination of chicken red blood cells was inhibited by treatment of compound 30 , and the interaction between the influenza hemagglutinin (HA) and compound 30 was determined by surface plasmon resonance (SPR) with a dissociation constant of K D  = 3.76 μM. Finally, silico docking studies indicated that compound 30 and its derivative 31 were able to occupy the binding pocket of HA for sialic acid receptor. Collectively, these results suggested that l -ascorbic acid-conjugated pentacyclic triterpenes were promising anti-influenza entry inhibitors , and HA protein associated with viral entry was a promising drug target.