Evaluation of Tonabersat for DME

Key Inclusion Criteria 1. Adults with type 1 or 2 diabetes mellitus 2. At least one eye with: * Best corrected E-ETDRS visual acuity letter score ≥ 74 (i.e., 20/32 or better) * Ophthalmoscopic evidence of center-involved DME in study eye confirmed by central subfield thickness on spectral domain OCT * Zeiss Cirrus: ≥ 290 µm in females, ≥ 305 µm in males * Heidelberg Spectralis: ≥ 305 µm in females, ≥ 320 µm in males * Recruitment will be monitored with a goal to have equal proportions in the following categories above the CI-DME thresholds: \<75 μm, 75 μm to \<175 μm, ≥175 μm 3. Media clarity, pupillary dilation, and study participant cooperation sufficient for adequate OCT Key Exclusion Criteria * Macular edema is considered to be due to a cause other than DME * Major ocular surgery within prior 4 months, or anticipated after randomization * History of focal/grid laser or other ocular surgical, intravitreal, or peribulbar treatment for DR or DME within prior 1 year, and no more than 4 anti-VEGF injections within prior 3 years * Anticipated need to treat DME or DR during the first 6 months, or anticipated need for cataract surgery during study period * Any history of vitrectomy * Systemic anti-VEGF or pro-VEGF treatment within 12 months prior to randomization * History of chronic renal failure requiring dialysis or kidney transplant * History of moderate to severe hepatic impairment, including known liver function test (LFT) values \> 3x's the upper limit of normal