An early-stage update on Adverum Biotechnologies intravitreal gene therapy has triggered investor concern, after patients with wet age-related macular degeneration (AMD) saw their vision deteriorate, despite signs that the treatment is improving retinal anatomy.

Adverum, on Wednesday, unveiled 24-week data from the OPTIC trial of its experimental therapy, ADVM-022, in six patients who have been administered with one dose of the therapy. On average, patients in the trial had severe disease with an average of 6.2 anti-VEGF injections in the eight months prior to screening and an average annualized injection frequency of 9.3 injections.

Over the six month period, patients did not require any anti-VEGF rescue injections and five of six patients saw a complete response with a total resolution of fluid following the Adverum injection. There were no serious adverse events, and the majority of side-effects were mild.

However, patients lost visual acuity by two letters on average, with a 90% confidence interval of -9.1 letters to +5.1 letters.

The range of individual patient data were not presented, though the wide confidence interval suggests that some patients may have experienced a loss of more than 10 letters during the course of the trial lack of rescue injections is difficult to square with declining vision. SVB Leerinks Mani Foroohar wrote in a note.

However, the study investigator insisted no loss in vision was due to wet AMD pathology and observed loss of visual acuity is due to normal variabilityin a small set of patients an assertion that, if proved out with additional follow-up, would very substantially improve the implied quality of this dataset.

Shares of the company which spectacularly failed years ago when it was christened Avalanche Biotechnologies $ADVM were down about 6.8% to $5.56 in Friday premarket trading. The stock sank on Thursday, evaporating millions from its market value.

This data suggest ADVM-022 is potentially active in delivering an expressible gene cassette in wet AMD, but mixed signals in this small dataset should lift some of the competitive overhang on RGNX shares, Foroohar added. RegenexBio experimental gene therapy for wet AMD, RGX-314, is currently in a Phase I/II trial.

Wet AMD, which is characterized by blurred vision or a blind spot in an individuals visual field, is typically caused by abnormal growth of blood vessels that leak fluid or blood into the macula. Macular degeneration is the leading cause of severe, irreversible vision loss in the elderly. Anti-VEGF injections such as Regenerons $REGN flagship Eylea, as well as Roches $RHBBY Lucentis and Avastin, are commonly used to treat wet AMD.

In April, the FDA imposed a clinical hold on an application to test ADVM-022 in humans, asking for additional data on Adverums chemistry, manufacturing and control process. In May, the hold was lifted. Late last year, the biotech abandoned its then lead experimental drug, ADVM-043, for the treatment of A1AT deficiency.

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Early snapshot of Adverum's eye gene therapy sparks concern about vision loss - Endpoints News