Synthesis of Phosphorodiamidate Morpholino Oligonucleotides Using Trityl and Fmoc Chemistry-A New Method Amenable to Automated Synthesizer

Phosphorodiamidatemorpholino oligonucleotides (PMO) are routinely used for gene silencing and the recently developed PMO-based drug “Exondys51” has highlighted the importance of PMO as excellent antisense reagents. However, the synthesis of PMO has remained challenging. Here a method for the synthesis of PMO using either trityl or Fmoc-protected active morpholino monomers using chlorophosphoramidate chemistry in the presence of a suitable coupling agent on a solid support has been reported. After screening several coupling agents (tetrazole, 1,2,4-triazole, ETT, iodine, LiBr and dicyanoimidazole), ETT and iodine were found to be suitable for efficient coupling. Fmoc chemistry was not known for PMO synthesis because the preparation of Fmoc-protected chlorophosphoramidate monomers was not trivial. Synthesis of Fmoc-protected activated monomers and their use in PMO synthesis is reported for the first time. 25-mer PMO has been synthesized using both the methods and validated in vivo in the zebrafish model by targeting the no tail gene. Methods have been transferred in DNA synthesizer which has become user friendly for PMO synthesis and opened a new avenue to explore PMO for various applications. Fmoc chemistry could be suitable for scalable approach of PMO synthesis using peptide synthesizer as it is a neutral oligomer like peptide.