Approval of retinal gene therapies in the US and Europe based on visual acuity and microperimetry.

Introduction: Gene therapy is an emerging technology for the treatment of inherited retinal diseases. Whilst the development of delivery vectors and genotyping is progressing at speed, outcome measures used for regulatory approval are slow to change and hinder progress clinical trial stage. Traditional measures of visual function such as best corrected visual acuity (BCVA) and low luminance visual acuity (LLVA) may only be useful across a very short window and in late stages of disease. They are unsuitable measures for early- to mid-stage disease where foveal function, and so letter reading, is primarily unaffected. In such cases, microperimetry is an accurate and repeatable measure of retinal function of the whole of the macular region. Areas covered: This article provides evidence-based guidance on criteria for microperimetry outcome measures, drawing on experience from long-term clinical trials in RPGR-related retinitis pigmentosa. Expert opinion: Microperimetry provides a sensitive and repeatable measure of retinal function, with mean sensitivity across the macula or central 16-point region offering a more reliable metric than single-point requirements recommended by the FDA. A 2.5dB gain in mean sensitivity is equivalent to a 13-letter increase in low-luminance ETDRS representing a clinically significant change and aligning closely with regulatory standards.