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

Abstract

<p>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 <i>in vivo</i> in the zebrafish model by targeting the <i>no tail</i> 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.</p>