Synthesis, properties, and biological activity of boranophosphate analogs of the mRNA cap: versatile tools for manipulation of therapeutically relevant cap-dependent processes

Modified mRNA cap analogs aid in the study of mRNA-related processes and may enable creation of novel therapeutic interventions. We report the synthesis and properties of 11 dinucleotide cap analogs bearing a single boranophosphate modification at either the α-, β- or γ-position of the 5′,5′-triphosphate chain. The compounds can potentially serve either as inhibitors of translation in cancer cells or reagents for increasing expression of therapeutic proteins in vivo from exogenous mRNAs. The BH(3)-analogs were tested as substrates and binding partners for two major cytoplasmic cap-binding proteins, DcpS, a decapping pyrophosphatase, and eIF4E, a translation initiation factor. The susceptibility to DcpS was different between BH(3)-analogs and the corresponding analogs containing S instead of BH(3) (S-analogs). Depending on its placement, the boranophosphate group weakened the interaction with DcpS but stabilized the interaction with eIF4E. The first of the properties makes the BH(3)-analogs more stable and the second, more potent as inhibitors of protein biosynthesis. Protein expression in dendritic cells was 2.2- and 1.7-fold higher for mRNAs capped with m(2)(7,2′-O)Gpp(BH3)pG D1 and m(2)(7,2′-O)Gpp(BH3)pG D2, respectively, than for in vitro transcribed mRNA capped with m(2)(7,3′-O)GpppG. Higher expression of cancer antigens would make mRNAs containing m(2)(7,2′-O)Gpp(BH3)pG D1 and m(2)(7,2′-O)Gpp(BH3)pG D2 favorable for anticancer immunization.