A Unified Phosphoramidite Platform for the Synthesis of Morpholino Oligonucleotides and Diverse Chimeric Backbones

Phosphorodiamidate Morpholino Oligonucleotides (PMOs) have been well established in the milieu of FDA-approved oligonucleotide-based drugs in the past decade. Given their relevance in antisense therapeutics, a DNA/RNA synthesizer-compatible modular synthesis protocol of PMOs is long awaited to explore next-generation PMO chimeras with other therapeutically proven oligonucleotide backbones. Herein, we demonstrate a streamlined 5' → 3'phosphoramidite approach for the synthesis of PMOs using tert-butyl-protected 5'-^tBu-morpholino phosphoramidites, which were synthesized from 5'-OH morpholino monomers derived from commercially available ribonucleosides. 2-Cyanoethyl (CE)-protected 5'-CE-morpholino phosphoramidites were also synthesized to generate thiophosphoramidate (TMO) and phosphoramidate (MO) morpholino oligonucleotides. Full-length PMOs and TMOs in exceptional overall yields were obtained with operational simplicity and compatibility with automated DNA/RNA synthesizers utilizing controlled pore glass (CPG) as the solid support and CH₃CN as the solvent. Importantly, this method has opened the possibility of designing various biologically relevant ASO design modalities, such as PMO-TMO and PMO-MO, which were inaccessible otherwise. Moreover, DNA nucleotides were also incorporated to generate PMO-psDNA and PMO-DNA using commercially available 5'-DNA phosphoramidites. The biophysical properties of all synthesized oligonucleotides were analyzed using UV melting and circular dichroism studies. The serum interaction profile and innate immune response of the PS-modified polythymidine oligonucleotides were analyzed and it was found that the mixed backbone oligonucleotides had a balanced profile compared to fully charged or neutral extremities. Overall, the synthesis is amenable to fast generation of various types of PMO chimeras for biological screening, which will expedite their therapeutic exploration and transition to clinic.