ProQR Announces Positive Results from Clinical Trial of QR-421a in Usher Syndrome and Plans to Start Pivotal Trials

  • QR-421a demonstrated a concordant benefit in multiple measures of vision, including best corrected visual activity (BCVA), static perimetry, and retinal imaging (OCT)
  • QR-421a observed to be well tolerated with no serious adverse events reported
  • Two pivotal Phase 2/3 trials are expected to start by the end of 2021

USH2A mediated Usher syndrome and retinitis pigmentosa

The news announced today is a substantial achievement in the clinical development focusing on Usher syndrome and RP.

Usher syndrome is a genetic condition characterized by both hearing loss up to deafness and the progressive loss of vision up to blindness. The loss of vision is an eye disease called retinitis pigmentosa (RP), which affects the light sensitive area of tissue on the back of the eye (the retina). There are three main types of Usher syndrome – type 1, type 2 and type 3. Each of these types has several different genes responsible for causing the syndrome.

There are currently no available treatments for the more than 16,000 patients with vision loss due to Usher syndrome 2A and RP due to USH2A exon 13 mutations.

QR-421a and the Stellar study

Our investigational RNA therapy, QR-421a, aims to stop vision loss in people with a mutation in a specific part of the USH2A gene, called exon 13. QR-421a works by binding to the mutated USH2A RNA and exclude exon 13 from the RNA. This approach is known as exon skipping. The cells in the retina can then produce a slightly shorter but functional USH2A protein.

The Stellar study is a Phase 1/2 clinical trial of QR-421a to test whether it is safe and leads to better vision. The interim findings from the trial were encouraging, announced in March 2020. Today, a year later, the positive results of the trial have been announced. With only one injection of QR-421a, the Stellar study showed benefit in the treated eye in comparison to the untreated eye across multiple eye tests. We have seen this benefit across all participants in the Stellar study, which included clinical trial participants with both advanced vision loss as well as early to moderate vision loss.

For more information on the safety & efficacy of the trial, please read our Community statement QR-421a program results (and translations in Dutch and French).

ProQR plans to advance QR-421a into final stage clinical testing. This will be our second clinical program to move into the final stage, following our final stage clinical testing in Leber Congenital Amaurosis type 10 (LCA10). The news today further validates our RNA therapy platform and our capabilities to design and efficiently take these programs through clinical development.


Benjamin R. Yerxa, PhD, Chief Executive Officer at the Foundation Fighting Blindness

At ProQR we partner with over 20+ retinal patient organizations around the world. A key partner of ours is the Foundation Fighting Blindness, for more information on our partnerships and work we do with the patient community please email


Robert Koenekoop, MD, MSc, PhD, FRCS(C), FARVO, a clinical scientist from the Montreal Children’s Hospital and Professor of the McGill University Faculty of Medicine and Department of Pediatric Surgery

What does this mean?

Today is a robust validation of our RNA therapies to treat the high unmet need in the inherited retinal disease community. It now means we have seen that our RNA therapies could provide visual benefit to clinical trial participants with both LCA10 and now Usher syndrome and RP.

What next?

Based on the news today, ProQR plans to start two final stage registration trials called Sirius and Celeste. Each trial could potentially serve as the sole registration trial depending on the findings.

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