QR-421a, an antisense oligonucleotide, for the treatment of retinitis pigmentosa due to USH2A exon 13 mutations

Abstract

Purpose : Pathogenic mutations in the USH2A gene cause syndromic and non-syndromic forms of RP. USH2A is expressed in retinal photoreceptors where it is required for their maintenance. Exon 13 mutations are some of the most common USH2A mutations and are estimated to be present in around 16,000 patients in the Western World (US, Canada, Europe and Australia). Skipping of exon 13 from the USH2A mRNA results in an in-frame transcript that is expected to be translated into a functional (albeit shorter) usherin protein. It is hypothesized that restoration of the usherin protein in photoreceptors will stop vision loss or even restore vision in patients with RP caused by mutations in exon 13 of the USH2A gene. Here we report data on a lead candidate drug for RP due to USH2A exon 13 mutations.

Methods : A panel of antisense oligonucleotides was designed to skip exon 13 from USH2A mRNA and screened for safety and efficacy. The ability of the lead candidate to exclude exon 13 from USH2A mRNA was tested in patient iPSC derived retinal organoids. A zebrafish model, homozygous for exon 13 premature stop codon mutation, was used to assess the activity of the shortened usherin protein resulting from the AON-mediated skipping of exon 13 and restoration of visual function. As the USH2A gene is well conserved in mammalian species, in vivo studies were also performed in wild-type mice and monkeys. A pharmacokinetic study in monkeys investigated the time course of the AON in the retina.

Results : The screening approach has identified QR-421a as the best performing AON. QR-421a treatment in vitro resulted in concentration-dependent USH2A exon 13 skip in patient derived retinal organoids. Exon 13 skipping at the mRNA level was shown to result in restoration of usherin protein expression and restoration of ERG signal in an ush2a mutant zebrafish model. In vivo mouse and monkey studies with QR-421a showed a dose-dependent USH2A exon skip with long duration of action lasting at least 3 months post single intravitreal injection. The duration of the effect of QR-421a correlated with its long half-life in the retina.

Conclusions : Demonstration of preclinical efficacy, functionality of the shortened protein, effective retinal delivery following intravitreal delivery, and pharmacokinetic properties make QR-421a a promising candidate for future clinical development.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.