Sarepta, Biomarin Move Over- Here Come tcDNAs

In a head-to-head comparative study in NatureMedicine, Goyenvalle and colleagues claim that tricycloDNAs (tcDNAs), a relatively unexplored constrained nucleic acid analogue commercialized by SYNTHENA have superior efficacy over 2’-O-methyl oligos and PMOs for therapeutic splice modulation in mouse models of Duchenne Muscular Dystrophy (DMD). 

In addition to ~3 to 5-fold enhancements in dystrophin exon skipping in various muscles compared to the competing chemistries, the authors report that only with tcDNAs there was splice modulation also for the dystrophin isoform expressed in the brain.  This was accompanied by improvements in the behavioral and cognitive abnormalities in this model.   Apparently, these neurological defects are another important treatment goal in DMD.

Splice modulation was accompanied by ~2 and 20ug/g oligo concentrations in brain and muscle, respectively, following the administration of relatively large weekly doses of 200mg/kg that were necessary particularly for the correction of the neurological symptoms.

Interestingly, such delivery and target modulation was achieved without encapsulation or further modification technologies.  Whereas a 2012 publication by some of the same authors in collaboration with ISIS Pharmaceuticals explored the use of tcDNAs for RNaseH-mediated gene knockdown in conjunction with the phosphorothioate backbone known to greatly improve biodistribution and cell uptake, tcDNAs here were applied in their naked form.

The authors attribute this to an apparently spontaneous tendency of tcDNAs to self-assemble into ~100nm nanoparticles.   

The results seem to warrant the development of tcDNA for DMD exon skipping.  Critical to their success in the clinic will be the safety and tolerability of tcDNAs in Man with the kidney predicted to be the dose-limiting organ.  

The study is also a reminder that there are now a number of other chemistries, including the high-affinity CRN chemistries by ISIS Pharmaceuticals and Marina Biotech, the latter with a stated focus on DMD, which similarly promise improvements over the trail-blazing 2’-O-methyl and PMO chemistries.