City Therapeutics doses first participant in phase 1 trial of RNAi therapy CITY-RBP4 for Stargardt disease

City Therapeutics has dosed the first participant in its phase 1 trial of CITY-RBP4, an investigational RNA interference (RNAi) therapeutic for Stargardt disease type 1 (STGD1).¹ Initial phase 1 data is expected in mid-2027, according to the company.¹

The milestone arrives at a pivotal moment for the field. There are no approved treatments for Stargardt disease, but the FDA has accepted Belite Bio’s new drug application for oral tinlarebant—another therapy that targets RBP4—under priority review, with a target action date of February 12, 2027.^2,3

“Stargardt disease is a devastating condition that can lead to significant vision loss early in life,” said Andy Orth, president and CEO of City Therapeutics, in the release.¹ Orth noted that CITY-RBP4 is the company’s second program to enter the clinic, with a third expected to begin clinical testing through an investigator-initiated trial by year-end.¹

Silencing RBP4 at the source

Stargardt disease is caused by mutations in the ABCA4 gene, which lead to accumulation of toxic vitamin A byproducts in the retina and progressive, irreversible central vision loss.¹ RBP4 is the primary carrier of retinol from the liver to the eye. In plasma, retinol circulates as a complex with RBP4 and transthyretin (TTR), and the retinal pigment epithelium (RPE) takes it up through the RBP4 receptor STRA6.^1,4

CITY-RBP4 is a GalNAc-conjugated small interfering RNA (siRNA) designed to silence RBP4 expression in the liver. By reducing how much RBP4 hepatocytes produce, the therapy aims to limit vitamin A delivery to the eye and, in turn, slow retinal damage and vision loss.¹ According to the company, preclinical studies showed potent, durable RBP4 suppression and reduced circulating retinol, and pharmacodynamic modeling suggests the drug could support infrequent subcutaneous dosing.¹

That sets CITY-RBP4 apart from tinlarebant, a once-daily oral small molecule that disrupts the RBP4–TTR interaction and promotes renal elimination of RBP4.⁴ Both strategies converge on the same goal—lowering circulating retinol—but differ in route, dosing frequency, and how the protein is depleted. City cites third-party clinical proof-of-mechanism data in support of RBP4 reduction as a therapeutic approach.¹

Phase 1 trial design and end points

The phase 1 study is a first-in-human, multicenter, randomized, double-masked, placebo-controlled trial enrolling both healthy volunteers and individuals with Stargardt disease.¹ Its primary objectives are to evaluate the safety, tolerability, pharmacokinetics, and pharmacodynamics of single and multiple subcutaneous doses of CITY-RBP4.

Pharmacodynamic end points include RBP4 suppression and the resulting reduction in circulating retinol. Participants with Stargardt disease may enroll in a planned open-label extension that will assess long-term safety and preliminary efficacy, including exploratory retinal imaging and visual function measures.¹

A crowded and accelerating pipeline

Stargardt disease is the most common inherited macular dystrophy and the leading inherited cause of central vision loss in children and young adults.¹ An estimated 50,000 to 60,000 people in the United States are living with the condition.⁵

The most advanced RBP4-targeted candidate is tinlarebant. The phase 3 DRAGON trial (NCT05244304) randomly assigned 104 patients aged 12 to 20 years to oral tinlarebant 5 mg daily or placebo for 24 months. Tinlarebant reduced the growth rate of atrophic lesions on fundus autofluorescence by 35.7% vs placebo (P = .0033).³ Xanthopsia, a yellowish tint to vision, was among the most commonly reported drug-related adverse events, consistent with the drug’s mechanism.⁴

Other programs act at different points in the disease process. In June 2026, Alkeus Pharmaceuticals dosed the first participant in its pivotal phase 3 NORTHSTAR trial of oral gildeuretinol, which is designed to reduce vitamin A dimerization without modulating the visual cycle.⁵ Ocugen’s OCU410ST, a modifier gene therapy delivered by a single subretinal injection, is being evaluated in the phase 2/3 GARDian3 trial.⁶ Gene-based approaches have been complicated by the size of ABCA4, which is too large to fit into the standard viral vectors used for gene replacement.³

Interpretive considerations

Phase 1 will primarily establish safety and the depth and durability of RBP4 knockdown; any efficacy signal will come from exploratory measures in the open-label extension. Visual acuity is an insensitive end point in Stargardt disease. In the ProgStar natural history study, best-corrected visual acuity declined by only about 0.55 ETDRS letters per year, so structural measures such as lesion growth on autofluorescence are likely to remain central.⁷ As City notes, preclinical and early clinical results may not predict later outcomes.¹

The program’s ambitions extend beyond Stargardt disease. City is also exploring CITY-RBP4 for geographic atrophy (GA) secondary to age-related macular degeneration—the same indication in which tinlarebant is being studied in the phase 3 PHOENIX trial.^1,7 Separately, City has partnered with Bausch + Lomb to develop an intravitreal RNAi candidate for retinal diseases, including GA, an agreement that includes milestone payments of up to $485 million.⁸

References

1. City Therapeutics announces first participant dosed in phase 1 clinical trial of CITY-RBP4 for the treatment of Stargardt disease. News release. City Therapeutics, Inc. November 10, 2026. Accessed November 11, 2026. https://investors.citytx.com/news-releases/news-release-details/city-therapeutics-announces-first-participant-dosed-phase-1

2. FDA grants priority review to tinlarebant NDA for Stargardt disease type 1. Ophthalmology Times. August 11, 2026. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/fda-grants-priority-review-to-tinlarebant-nda-for-stargardt-disease-type-1

3. AAO 2026: Tinlarebant in Stargardt disease: Jacque Duncan, MD, on DRAGON results. Ophthalmology Times. October 12, 2026. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/aao-2026-tinlarebant-in-stargardt-disease-jacque-duncan-md-on-dragon-results

4. Tinlarebant earns Swiss orphan drug status for Stargardt disease. Ophthalmology Times. May 20, 2026. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/tinlarebant-earns-swiss-orphan-drug-status-for-stargardt-disease

5. Phase 3 NORTHSTAR trial launches for gildeuretinol in Stargardt disease. Ophthalmology Times. June 10, 2026. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/northstar-trial-gildeuretinol-stargardt-disease

6. Ocugen completes dosing early in phase 2/3 GARDian3 trial for Stargardt disease. Ophthalmology Times. April 1, 2026. Accessed November 11, 2026. https://ophthalmologytimes.com/view/ocugen-completes-dosing-early-in-phase-2-3-gardian3-trial-for-stargardt-disease

7. Slowing Stargardt: what a 36% reduction in lesion growth could mean for adolescent patients. Ophthalmology Times. October 9, 2026. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/slowing-stargardt-what-a-36-reduction-in-lesion-growth-could-mean-for-adolescent-patients

8. City Therapeutics and Bausch + Lomb collaborate on GA treatment candidate. Ophthalmology Times. January 13, 2025. Accessed November 11, 2026. https://www.ophthalmologytimes.com/view/city-therapeutics-and-bausch-lomb-collaborate-on-ga-treatment-candidate