This is an open label dose-escalation study to evaluate the safety and treatment benefits of MS-553 in treatment-naive diabetic retinopathy patients with central involved macular edema. Fifteen subjects with diabetic macular edema will be enrolled into each of three dose cohorts and will receive oral administration of MS-553 for 8 weeks.

Inclusion Criteria:

• Diagnosis of diabetes mellitus (type 1 or type 2) according to ADA or WHO diabetic diagnosis criteria
• Subject has the ability to follow the study instructions and is likely to complete all required study procedures and visits;
• All males and females must consent to pregnancy prevention during the study and qualified measures of birth control. All females of childbearing potential must consent to a pregnancy test before entering the study.
• Presence of central involved macular edema associated with diabetic retinopathy, i.e. diabetic macular edema (DME), as assessed by spectral domain optical coherence tomography (sd-OCT) of the central retina subfield thickness (CRT) at the Screening Visit (Centration must be confirmed by Investigators with signatures);
• Best Corrected Visual Acuity (BCVA) score ≥ 34 letters (approximately 20/200 to 20/20 Snellen equivalent or better) using the Early Treatment Diabetic Retinopathy Study (ETDRS) visual acuity protocol, and assessed at the Screening Visit Visual acuity (VA);
• Confirmation by the investigator that laser photocoagulation and anti-VEGF treatments are either declined by the patients or not needed and can be withheld for at least 3 months after the Screening Visit;
• Ocular media and pupil dilation adequate to permit good quality retinal imaging as assessed at Screening Visit.

Exclusion Criteria:

• Subjects with unstable metabolic or blood pressure control
• Subject with emaciation or obesity at Screening Visit: body mass index (BMI) <18.5kg/m2 or >28kg/m2
• Current use or likely need for medications know to be toxic to the lens, retina or optic nerve, including Deferoxamine, Chloroquine / hydroxychloroquine (Plaquenil), Tamoxifen, Phenothiazines and Ethambutol -
• History of myocardial infarction or other cardiac event requiring hospitalization (unstable angina pectoris, etc.), cerebrovascular accident, transient ischemic attack, treatment for acute congestive heart failure or any arrhythmia within 4-months prior to Screening Visit;
• Any situation that may in the opinion of the investigator preclude the safe administration of the study medication, adherence to the scheduled study visits, safe participation in the study or affect the results of the study as assessed at Screening Visit;