Inclusion Criteria: * Written informed consent must be obtained prior to all assessments. * Age ≥18 years at screening. * Type 1 or type 2 diabetes mellitus at screening. * Study eye criteria: * Decreased visual acuity attributable primarily to DME. * DME involves the macular center. Exclusion Criteria: * Active proliferative diabetic retinopathy in the study eye. * Any current or previous ocular disease in the study eye other than DME that could interfere with macular evaluation or affect central vision at screening or baseline. * Any active intraocular or periocular infection or active intraocular inflammation of the study eye at screening or baseline. * History of idiopathic or autoimmune uveitis in the study eye at screening or baseline. * Prior gene therapy in either eye. * History of vitreoretinal surgery in the study eye. * Uncontrolled blood pressure at screening or baseline defined as systolic blood pressure (SBP) ≥160 mmHg or diastolic blood pressure (DBP) ≥100 mmHg. * History of treated or untreated malignancy of any organ system within the past 5 years. * Pregnant or lactating women. * Women of reproductive age, defined as all women who are biologically capable of being pregnant, unless they use highly effective contraceptive methods between the time of study drug administration and 3 months after EOS (end of study).