Type 1 myotonic dystrophy (DM1) is an autosomal dominantly inherited multi-systemic disease that is primarily manifested as myotonia, muscular dystrophy, cataracts, and cardiac conduction abnormalities. DM1 is mechanistically caused by an expanded trinucleotide repeats of CUG in the 3’-untranslated region of DMPK transcripts. The pathogenic transcripts of DMPK sequester RNA binding proteins and thereby cause mRNA spliceopathies. Currently, there is no approved therapy treating the genetic cause of DM1 although there are drugs in the early stage of clinical development. Efficacious and safe therapeutics remain unmet medical needs.
To develop a therapeutic for DM1, we designed siRNA conjugates to silence DMPK mRNA in skeletal muscles. The pharmacodynamic (PD) properties of the conjugates were evaluated in cynomolgus monkeys. We identified the 2 best siRNA conjugates, ARO-DM1 and S-ARO-DM1 which are comparable in PD properties but target different regions of DMPK transcripts. ARO-DM1 was intensely characterized for clinical development, and S-ARO-DM1 was selected for preclinical pharmacological studies due to its sequence complementarity to the transgene in the TREDT960i/HSA-rtTA mouse disease model of DM1, which expresses a partial human DMPK transgene with 960 CUG repeats (CUG960) in the 3’ UTR in skeletal muscle under the control of doxycycline-inducible system. In a pharmacological study, DMPK transgene expression was induced by doxycycline administration and the pathogenic transcripts were detectable as was substantial mis-splicing after 2 weeks of doxycycline induction. Transgenic mice received vehicle or S-ARO-DM1 respectively by weekly intravenous injections. Skeletal muscles were collected at week-5 after the S-ARO-DM1 treatment. The transgene expression was analyzed by RT-qPCR and mRNA splicing was analyzed by ddPCR and RNAseq. The transgene expression was moderately reduced. However, the mis-splicing abnormalities of mRNA caused by transgene expression were substantially corrected.