Eye drops made blind mice choose the dark again

Here’s the simple, audacious question: if the eye’s light-sensing cells have died, can you still make the remaining retinal circuit see light well enough to guide behavior? And if so, could it be done with something as ordinary as eye drops?

A research consortium led by the Institute for Bioengineering of Catalonia asked exactly that, and reports that photoswitchable small molecules restored saccadic eye movements and visually guided behavior in ambient white light in animal models. The study appears in the Journal of the American Chemical Society.

Who did this and why it’s a big target

The work was led by the Institute for Bioengineering of Catalonia (IBEC) and included the team of Pedro de la Villa at the University of Alcalá. The authors place their results in a field with a lot at stake: photoreceptor degeneration drives major causes of blindness, notably age-related macular degeneration (AMD) and retinitis pigmentosa (RP). Together, these affect about 200 million people worldwide and impose an economic burden estimated above US$400 billion each year.

The findings, published in the Journal of the American Chemical Society, build on more than 10 years of research and arrive shortly after the first-ever clinical trial of a photopharmacological drug for vision restoration was published (that drug targets a different protein).

Method: switching a drug on with light

The approach is photopharmacology: alter a drug’s structure so light flips a built-in molecular switch, changing the drug’s activity. In this case the team designed a family of compounds dubbed prosthe6. These are aimed at ON-bipolar neurons—retinal relay cells that normally take input from photoreceptors—and they bind a protein in that pathway (mGlu6).

Delivery was deliberately simple. The same molecules were active when put directly into the eye by injection and when applied as topical eye drops. No genetic modification and no implanted hardware were required.

What they measured, in what animals

Two testbeds did the heavy lifting. In blinded zebrafish larvae, the team looked for the return of saccadic eye movements called the optokinetic reflex—a standard assay of visual function. In mouse models of AMD and RP, they tested whether animals regained a hardwired behavior: healthy mice avoid bright spaces and prefer dark ones, a choice they lose when they cannot tell light from dark.

Illumination levels mattered. The authors assessed behavior under light like you’d find indoors or outside on an overcast day—conditions that are far more practical than the intense, specialized lighting some other approaches need.

What changed after dosing

With prosthe6 on board, blinded zebrafish larvae recovered the optokinetic reflex. In mice with retinal degeneration, innate light-avoidance behavior reappeared: after treatment they once again favored dark areas without any training, a clear sign they could detect light and use that information to guide action.

Two members of the series stood out—prosthe6-12 and prosthe6-15—and the effect appeared whether the compounds were injected into the eye or given as drops. Crucially, restored behavior was seen under indoor-like or overcast daylight illumination.

Item	Details
Publication	Journal of the American Chemical Society
Lead institution	Institute for Bioengineering of Catalonia (IBEC)
Key partners	Team led by Pedro de la Villa, University of Alcalá (UAH)
Approach	Photopharmacology; photoswitchable small molecules (prosthe6)
Primary targets	ON-bipolar neurons via mGlu6
Animal models	Blinded zebrafish larvae; mouse models of AMD and RP
Readouts	Optokinetic reflex restored (zebrafish); innate light-avoidance restored (mice)
Effective compounds	prosthe6-12; prosthe6-15
Administration	Intraocular injection; topical eye drops
Lighting conditions	Indoor-like or overcast daylight levels
Research timeline	Builds on >10 years of research
Context	Follows first-ever photopharmacology vision-restoration clinical trial (different target)

Limits, caveats, and what’s next

What’s reported here is restoration of visual behaviors in zebrafish and mouse models; the study does not report human outcomes. The team notes that the compounds have shown promising safety profiles, but how long a single dose lasts and how best to formulate the drops for durability are active lines of work.

The broader field is early but moving. The authors point out that a first-in-human photopharmacology trial targeting an unrelated protein has now been published, indicating the concept can reach clinics. This project itself received early patient-foundation support in 2016, and the doctoral research underpinning it earned a 2023–24 prize at the University of Barcelona—both markers of a long road from idea to intervention.

Why this could matter if it holds up

Photoreceptor degeneration underlies AMD and RP, and the numbers are sobering: roughly 200 million people live with these disorders, with a global cost north of US$400 billion annually. Gene therapies target narrow mutation windows, and implants are invasive. A drug that can be dropped into the eye, act on surviving retinal circuits, and work under everyday lighting would offer a different path—noninvasive and not tied to a specific mutation or device.

The team describes these compounds as “molecular prostheses”—a striking phrase, but also a plain description: the molecules let a degenerated retina respond to light again without hardware or genetic alteration. If further safety and durability work pan out, that could be a practical bridge to functional sight for people who currently have none.

Who kept the lights on

The project received early funding from the patients’ foundation Fundaluce in 2016. It also formed part of Rosalba Sortino’s doctoral work, which the University of Barcelona recognized with its Extraordinary Doctoral Prize for 2023–24.