Design and Synthesis of Antiviral Iminoribitol C-Nucleosides

Abstract

Infections caused by RNA viruses, such as Ebola and Zika, continue to exist worldwide as significant public health problems. In response to the urgent need for safer and more efficacious treatment options to treat infections caused by RNA viruses, the pharmaceutical and biotechnology industries have devoted significant efforts over the last two decades to discovering and developing new antiviral agents. One such antiviral, Sofosbuvir®, was approved by the US Federal Drug Administration (FDA) in 2014 and has revolutionized the treatment of Hepatitis-C. Sofosbuvir® was the second largest selling drug in the world in 2016 and in just twenty-one months Gilead reported sales worth $26.6 billion USD.The strategy of using nucleoside analogues to inhibit viral RNA dependent RNA polymerase(RdRp)has been pursued since the 1970s, and exemplified bythe discovery and development of ribavirin. The natural substrates of RNA polymerases are nucleoside triphosphates and often the efficacy of nucleoside analogues as antivirals are dependent on their ability to be converted by the host or virus to mono-, di-, and ultimately tri-phosphate analogues which block the active site of RNA polymerase as an analogue of the substrate causing chain termination. Recently Biocryst Pharmaceuticals (Biocryst) described the anti-viral properties of Immucillin-A (Galidesivir), an iminoribitol based nucleoside analogue, which was found to have broad spectrum antiviral activity especially against RNA viruses including Ebola. Researchers at the Ferrier Research Institute (Ferrier) have synthesizedan analogue of Immucillin-A, 8-aza-Immucillin-A (AIA) which shows comparable activityto Immucillin-A, in anti-viral screens against Ebola, and this antiviral activity forms part of a US patent application. The Ferrier is keen to further exemplify this compound class through the synthesis of analogues of both Immucillin-A and AIA as well as improve the overall synthesis of the lead compound AIA.Included as part of this study is the synthesis of pro-drugs of these iminoribitol based nucleoside analogues. Prodrugs are metabolized inside the body and are often converted to the corresponding pharmacologically active form. In general, prodrug strategies have improved the bioavailability and efficacy of many drugs. In particular, prodrugs strategies involving nucleoside analogue antivirals, which target RNA polymerase, have been particularly effective as they ensure conversion to the monophosphate in vivo. Conversion to the 5’-monophosphate form of a nucleoside analogue is the rate limiting step to the inhibition of the RNA polymerase –prior to its conversion to the triphosphateanalogue. The prodrug is effectively a protected monophosphate, and is then readily converted to monophosphate by the host and then onto the di-and tri-phosphate by kinases in both the host and virus. ProTide prodrugs, such as Sofosbuvir® provide a verified strategy for improving anti-viral activity and hence our desire to synthesize pro-drugs of all our iminoribitol based nucleoside analogues. This research thesis also involved repeating the known synthesis of the Immucillins, in particular, Immucillin-H (Forodesine), which requires in excess of 20 linear synthetic steps to make. The linear synthetic route to Immucillin-H was used instead of the more convenient convergent method developed by the Ferrier as several key synthetic intermediates in this progress were utilized in the attempted synthesis of some of the planned nucleoside analogues of AIA. As part of this work the candidate learned aspects of scaling up chemical reactions andthe critical analysis of both reaction hazards and reagent compatibilities at scale. Where possible and given the number of synthetic steps involved the candidate was also interested in improving the yields of the building blocks involved in the synthesis of the Immucillins with limited success.