Template-directed synthesis of a small molecule-antisense conjugate targeting an mRNA structure.

The targeting of structural features in mRNA with specificity remains a great chemical challenge. A hairpin structure near exon 10 in the pre-mRNA encoding the tau protein controls its splicing, and dementia-causing mutations that disrupt this structure increase exon 10 splicing. We previously reported the discovery of small molecules, mitoxantrone (MTX) and analogs, which bind to the tau RNA hairpin structure and the design of bipartite antisense oligonucleotides (ASOs) that simultaneously bind to the discontinuous sequences that flank this hairpin. Herein we report the synthesis of a bipartite ASO conjugated to MTX using the tau RNA hairpin and flanking sequences as a template. A set of six MTX analogs, each containing a linker-azide, and a set of ten bipartite ASOs, each containing a branched linker-alkyne, were synthesized and tested in combinatorial fashion for their ability to conjugate in the presence or absence of template RNA. A single template-dependent MTX-ASO conjugate was identified from among the 60 reaction mixtures, demonstrating that the MTX and ASO precursors could simultaneously bind the RNA template and allow proper positioning of azide and alkyne for 1,3-cycloaddition. While the MTX-ASO conjugate proved too cytotoxic for cell-based assays, the conjugate inhibited tau exon 10 splicing under cell-free conditions more effectively than MTX or bipartite ASO alone.