For individuals living with advanced retinitis pigmentosa, the world often shrinks into a narrow tunnel of vision before fading into near-total darkness. This genetic degeneration destroys the rods and cones—the photoreceptors responsible for capturing light—leaving the remaining architecture of the retina unable to communicate with the brain. However, a first-in-human clinical trial is testing a provocative new approach: bypassing the dead cells entirely to turn the remaining retinal neurons into light sensors.
The ABACUS-1 trial, conducted in Adelaide, Australia, evaluated the safety and feasibility of intravitreal photoswitch therapy for advanced retinitis pigmentosa. The treatment utilizes a specialized molecule called KIO-301, designed to act as a chemical “switch” that confers light sensitivity to retinal ganglion cells (RGCs). Because these ganglion cells often survive even after the primary photoreceptors have perished, enabling them to respond to light could potentially restore a rudimentary level of vision to those with severe impairment.
As a physician and medical writer, I find the mechanism of KIO-301 particularly compelling. Unlike traditional gene therapies that attempt to repair or replace damaged photoreceptors, this photoswitch approach uses an azobenzene compound. In laboratory settings, this molecule changes shape when exposed to specific wavelengths of light, triggering an electrical signal in the RGCs that the brain can interpret as a visual stimulus. This effectively transforms the inner retina into a primary light-detecting layer.
The Architecture of the ABACUS-1 Trial
The study was designed as an open-label, phase 1 dose-escalation trial, meaning the primary objective was not to prove the treatment’s efficacy, but to ensure it was safe for human leverage. The trial took place between November 2022 and September 2023 across two clinical sites in Adelaide.
Participants included adults aged 18 to 80 who were diagnosed with advanced retinitis pigmentosa and suffered from severe visual impairment. In clinical terms, this included individuals with vision categorized as “hand motion” (HM), “count fingers” (CF), “light perception” (LP), or “no light perception” (NLP). To ensure safety, the researchers excluded anyone with active ocular infections, prior retinal detachments, or other significant comorbidities that could compromise the eye’s stability.
The intervention involved a single 50-µl intravitreal injection of KIO-301. To find the optimal balance between safety and potential activity, the researchers used a sequential dosing strategy:
- Part 1: Participants received an initial dose in the right eye, either at 7.5 µg or 25 µg.
- Part 2: After reviewing the safety data from the first phase, the contralateral (left) eye was treated with a higher dose—either 25 µg or 50 µg.
Advancement to higher doses was strictly contingent on the absence of dose-limiting adverse events, prioritizing patient safety over any observed functional gains.
Measuring Vision in the Absence of Sight
One of the greatest challenges in treating profound blindness is measuring success. Standard eye charts are useless for patients who cannot see letters. To solve this, the ABACUS-1 team employed a battery of specialized assessments designed for those with rudimentary vision.
Visual acuity was measured using the Berkeley Rudimentary Vision Test (BRVT), a tool specifically validated for people with profound visual impairment. The team used manual Goldmann perimetry with blue light stimuli to map “kinetic visual fields,” attempting to determine if the therapy expanded the area of the environment the patient could perceive.
Beyond clinical charts, the researchers focused on “functional vision”—the ability to use sight in real-world scenarios. Participants were asked to perform orientation and mobility tasks, such as identifying the location of a window, finding a door, or walking in a specific direction under varying levels of illumination (45, 125, and 350 lux).
To provide objective evidence of the therapy’s impact on the brain, the study utilized functional MRI (fMRI) scans. By using BOLD contrast imaging, researchers could monitor whether visual stimuli delivered to the treated eye actually triggered responses in the visual cortex of the brain, providing a biological confirmation of light perception that does not rely on a patient’s verbal report.
From Lab to Eye: The Preclinical Foundation
The transition to human trials was informed by extensive preclinical work, including studies using retinal explants from rd1 mice. These experiments confirmed that azobenzene photoswitches could confer rapid and reversible light responsiveness to RGCs at low concentrations. Crucially, this biological activity was observed at doses significantly lower than those associated with ocular toxicity in animal models.
These findings led the researchers to adopt a conservative starting dose for the human trial. Preclinical data also indicated that the effects of the photoswitch are transient, meaning the light-responsiveness declines over days or weeks. This pharmacodynamic profile explains why the clinical trial prioritized early post-dosing assessments to capture these fleeting windows of potential functional gain.
Summary of Clinical Assessment Tools
| Assessment Type | Tool/Method | Purpose |
|---|---|---|
| Safety | OCT &. Slit-lamp Biomicroscopy | Monitor retinal structure and intraocular pressure |
| Acuity | BRVT (Berkeley Rudimentary Vision Test) | Quantify vision in profoundly impaired eyes |
| Neural Response | fMRI (BOLD contrast) | Verify cortical activation in the brain |
| Daily Living | Orientation & Mobility Tasks | Test real-world navigation and spatial awareness |
What So for the Future
While the ABACUS-1 trial was a safety-led investigation, it provides a vital proof-of-concept for photoswitch therapy. By demonstrating that KIO-301 can be administered safely via intravitreal injection, the study opens the door for future trials focused on maximizing the duration and quality of the restored vision.
The use of National Eye Institute standardized questionnaires to track quality of life further emphasizes the goal of these therapies: not necessarily to restore 20/20 vision, but to provide enough visual information to increase independence and safety for those living in darkness.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.
The next critical step for this research will be the analysis of the full pharmacokinetics and cortical response data to inform the dosing and frequency of subsequent phase 2 trials. Updates on the progression of KIO-301 will likely be shared through official clinical trial registries and peer-reviewed medical literature.
Do you or a loved one live with retinitis pigmentosa? We invite you to share your experiences or questions in the comments below.
