A Study About the Safety of a Single ASP2020 Eye Injection and if it Helps People With Vision Loss From Stargardt-type Eye Conditions

Macular dystrophies are a group of inherited eye conditions that affect the macula. The macula is in the center of the retina, the light sensitive part at the back of the eye. In people with macular dystrophies, some of the cells in the macula gradually stop working and may die over time. This leads to loss of central vision, which can make it harder to read, recognize faces or see fine details. What's seen out of the corner of the eye (peripheral vision) is mostly unaffected. Stargardt disease (STGD) is a type of macular dystrophy which is caused by 1 faulty gene (ABCA4). Vision loss most typically begins in childhood or teenage years but may also develop in adulthood. As well as STGD, there are other macular dystrophies that look very similar to STGD and are called STGD-like macular dystrophies. These are caused by many other different genes. Together, STGD and STGD-like conditions can be called STGD-type macular dystrophies. This is an early development study of ASP2020 in adults, teenagers, and children with STGD-type macular dystrophies. ASP2020 are human stem cells which have been changed into cells found in the macula. In this study ASP2020 will be given to people for the first time. The main aim of the study is to check the safety of ASP2020 and how well people tolerate it. Other aims are to learn if people have an immune reaction to ASP2020, and if there are signs that the stem cells replace damaged cells in the retina, and vision improves for people with STGD-type macular dystrophies. ASP2020 will be given as a single injection into the eye, under the retina. This requires a surgical procedure where the person is put to sleep by a general anesthetic. At the end of surgery, a steroid will be injected into the eye to reduce any swelling. The study has 2 parts. In Part 1, different small groups will receive a lower to higher dose of ASP2020. This is done to find a suitable dose to use in Part 2. The adults will receive the lower dose and higher dose before the teenagers. There will be a 6-month gap between the last adult receiving the lower dose of ASP2020 and the first teenager receiving the same lower dose. This will also happen for the last adult receiving the higher dose of ASP2020 and the first teenager receiving the higher dose of ASP2020. Any medical problems will be recorded for each dose in each group. Children will not receive ASP2020 in Part 1. In Part 2, different groups of adults, teenagers and children will receive the most suitable dose of ASP2020 worked out from Part 1. People will be in the study for about 1 year and they will visit the clinic several times. In both parts of the study, safety checks will be done at each visit, and the study doctors will continue to check for any medical problems throughout the study. Various eye tests and eye imaging will be done throughout the study. Blood tests will also be done at some of the visits during the study.

Inclusion Criteria: * Documented clinical diagnosis of macular dystrophy with a STGD-type clinical presentation and molecular confirmation, defined as either: * STGD: presence of biallelic (pathogenic or likely pathogenic) ABCA4 variants, or one definite disease-causing ABCA4 variant together with a typical phenotype consistent with STGD. * STGD-like macular dystrophy: presence of one or more pathogenic variants in a gene known to cause macular dystrophy, as appropriate for its expected inheritance mode. * Sufficiently clear ocular media and adequate pupillary dilation to allow for all imaging procedures. * Intraocular pressure (IOP) of ≤ 21 mmHg * Participant has a spherical equivalent refractive error between +8.00 D and -10.00 D. * BCVA ranging from 20/500 to 20/40 (equivalent to 15 to 70 ETDRS letters) * For participants in the \> 20/80 to ≤ 20/40 BCVA range (moderate visual impairment \[MVI\]): presence of a visible definite or probable residual ellipsoid zone (EZ) on SD-OCT, and a total retinal SD-OCT central subfield thickness ≥ 150 micrometers (µm) * For participants in the ≥ 20/500 to ≤ 20/80 BCVA range (severe visual impairment): Presence of a residual ONL within the macular optical coherence tomography (OCT) scan area and Evidence of RPE disease/damage by means of SD-OCT (hypertransmission defect) and/or FAF imaging (questionably decreased autofluorescence/definitely decreased autofluorescence). Exclusion Criteria: * Participant has a known history of significant systemic disease that could impact ocular health or confound study assessments, based on medical history or prior clinical documentation. * Participant has an autoimmune condition that requires treatment with immunomodulatory therapy and/or biologics that cause immunosuppression. * Participant has known diagnosis of diabetes mellitus with a documented glycated hemoglobin (HbA1c) value ≥ 7% 3 months prior to screening and based on available medical records. * Participant has a history or evidence of severe cardiac disease, cardiovascular or cerebrovascular disease, including a history of stroke within 12 months prior to screening. * Participant has any complicating systemic disease or active malignancy. * Participant has a known history of any systemic or metabolic condition, or physical examination finding that may significantly affect ocular health or interfere with the interpretation of study assessments. * Participant has presence of another known or suspected molecular diagnosis of macular or retinal disease that could confound interpretation of study outcomes, indicate a second concomitant retinal condition, or suggest a different etiology for the macular disease. * Participant has macular atrophy due to any cause other than a genetically or clinically confirmed diagnosis of STGD or STGD-like macular dystrophy. * Participant has evidence or history of choroidal neovascularization. * Participant has diagnosis of any form of uncontrolled glaucoma (for high-tension glaucoma IOP \> 25 mmHg). * Participant has a history of steroid-induced IOP elevation or known steroid responder status. * Participant has and/or is receiving treatment for thyroid eye disease. * Participant has diabetic retinopathy in excess of mild nonproliferative diabetic retinopathy * Participant has any other disease(s) affecting the optic nerve. * Participant has a history of anterior or posterior uveitis and/or presence of intraocular inflammation (trace anterior chamber cell or flare), or history of idiopathic or autoimmune-associated uveitis in either eye. * Participant has media opacities impeding the visualization of the fundus and/or the reliable performance of the visual function tests required by the protocol. * Participant has aphakia. * Participant has a clinically significant epiretinal membrane or evidence of clinically significant vitreomacular traction syndrome. * Participant has any other disorders which could interfere with or confound visual acuity and other ocular assessments, including OCT or FAF. * Participant has history of any of the following procedures: posterior vitrectomy, retinal detachment surgery, glaucoma filtering surgery, glaucoma drainage device implantation, selective laser trabeculoplasty, full-thickness or partial- thickness corneal transplant. * Participant has had any intraocular surgery within 3 months of screening. * Participant has a history of intraocular metallic foreign bodies. * Participant has received any treatment including gene therapy, stem cell therapy, surgical implantation of prosthetic retinal chips, or any prior intravitreal treatment for any indication in either eye that may be considered to potentially interfere with the study participation or its conduct. * Participant has received within 1 month prior to screening or is receiving concomitant treatment with any ocular or systemic medication known to be toxic to the lens, retina, or optic nerve. * Participant has received any investigational therapy within 3 months prior to screening. * Participant has any condition, which makes the participant unsuitable for study participation.