Gene therapy for choroideremia using an adeno-associated viral vector encoding Rab escort protein 1: the REGENERATE open-label trial
Authors: Cehajic-Kapetanovic Jasmina , Bellini Marco P , Taylor Laura J , Yusuf Imran H , Soomro Taha , da Cruz Lyndon , MacLaren Robert E 
Published: December 2023

Background
Choroideremia is an X-linked inherited retinal degeneration that begins in childhood with nyctalopia and loss of peripheral vision, and gradually progresses to blindness in adulthood. Choroideremia is caused by null mutations in the CHM gene, which encodes Rab escort protein 1.

Objective
Assessment of the efficacy and safety of a single subretinal injection of an adeno-associated virus serotype 2 vector encoding Rab escort protein 1 in patients with choroideremia.

Design
Multicentre open-label clinical trial of a gene therapy for choroideremia using an adeno-associated virus serotype 2-Rab escort protein 1 vector.

Setting
This study (NCT02407678) was conducted at two NHS eye hospitals in the UK.

Participants
Males aged 18 years or above, having a clinical diagnosis of choroideremia with genetic confirmation of CHM gene mutation or molecular confirmation of Rab escort protein 1 protein deficiency and having best corrected visual acuity better than or equal to 6/60 (20/200; LogMAR 1.0).

Intervention
Adeno-associated virus serotype 2-Rab escort protein 1 vector suspension (1 × 1012 vector particles per ml) was supplied by Nightstar Therapeutics (London, UK), now part of Biogen Inc. (Cambridge, MA, USA). Up to 0.1 ml of adeno-associated virus serotype 2-Rab escort protein 1 vector suspension, corresponding to a dose of up to 1 × 1011 vector particles, was administered to the treated eye by subretinal injection. Selection of treated eyes was randomised in participants having relatively symmetrical retinal degeneration.

Main outcome measures
The primary safety-related outcome was change from baseline in best corrected visual acuity in treated eyes at 24 months post treatment, with prospective efficacy evaluated by comparative change from baseline in best corrected visual acuity in treated and untreated contralateral (control) eyes. Secondary outcomes included comparative change from baseline in mean retinal sensitivity (microperimetry) and retinal anatomy (area of autofluorescence) in treated and control eyes. Visual assessments were conducted by masked assessors.

Results
The primary efficacy-related outcome (comparative change from baseline in best corrected visual acuity in treated and control eyes at 24 months post treatment) was not statistically different between treated eyes (−2.63 letters, standard error of the mean 2.76) and control eyes (+2.67 letters, standard error of the mean 0.768) in all 30 participants (p = 0.08). Greater loss of visual fields, possibly surgery-induced, was observed in treated eyes. Six serious adverse events were reported in the treated eyes of four participants: one surgery-related and two inflammation-related serious adverse events involving clinically significant decreases in best corrected visual acuity, and three serious adverse events in one participant involving reduction in central retinal sensitivity, but with best corrected visual acuity remaining stable.

Limitations
No evidence of possible efficacy of the intervention was observed, as a meaningful difference in comparative change from baseline in best corrected visual acuity in treated and control eyes was not discernible at 24 months post treatment. As choroideremia is a very slow degeneration, best corrected visual acuity in control eyes did not decline significantly during the assessment period.

Conclusion
Although this study has not presented evidence that reduction in visual fields caused by the intervention would be justified by the possible rescue of best corrected visual acuity, a more definitive assessment may be provided by long-term monitoring of trial participants in an observational study (NCT03584165).

Trial registration
This study is registered as ISRCTN15602229 (www.isrctn.com/) and NCT02407678 (https://clinicaltrials.gov/).

Funding
This award was funded by the National Institute for Health and Care Research (NIHR) Efficacy and Mechanism Evaluation (EME) programme (NIHR award ref: 12/66/35) and is published in full in Efficacy and Mechanism Evaluation; Vol. 11, No. 9. See the NIHR Funding and Awards website for further award information.