Safety and Exploratory Efficacy Study of SF0166 for the Treatment of Diabetic Macular Edema (DME)

Inclusion Criteria: 1. Male or female, 18 years of age or older. 2. Retinal thickening secondary to type 1 or type 2 diabetes mellitus with Diabetic Macular Edema (DME) defined as central subfield thickness ≥325 microns (µm) on spectral domain OCT in the study eye. 3. Best-corrected Visual Acuity (BCVA) between 78 and 25 letters, inclusive, in the study eye at the screening/randomization visit using Early Treatment Diabetic Retinopathy Study (ETDRS) testing, with BCVA decrement primarily attributable to Diabetic Macular Edema (DME). 4. Treatment naïve (i.e., no previous anti--vascular endothelial growth factor \[VEGF\] treatment in the study eye) or previously treated study eye with adequate washout defined below: 1. Lucentis (ranibizumab): 30-day washout 2. Avastin (bevacizumab): 30-day washout 3. Eylea (aflibercept): 60-day washout 4. Macugen (pegaptanib): 45-day washout 5. Willing and able to return for all study visits. 6. Able to adhere to the study dosing requirements. 7. Understands and signs the written informed consent form. Exclusion Criteria: 1. Active proliferative diabetic retinopathy (PDR) in the study eye, such as neovascularization of the optic disc (NVD), neovascularization elsewhere (NVE), vitreous hemorrhage, or neovascular glaucoma. 2. Uncontrolled glaucoma or ocular hypertension in the study eye defined as an intraocular pressure (IOP) \>25 millimeter of mercury (mmHg) regardless of concomitant treatment with IOP-lowering medications. 3. Uncontrolled hypertension defined as systolic \>180 mmHg or \>160 mmHg on 2 consecutive measurements (during the same visit) or diastolic \>100 mmHg on optimal medical regimen. 4. Screening glycated hemoglobin (HbA1c) blood test \>12.0%. 5. Previous panretinal photocoagulation (PRP) in the study eye within 4 months of study enrollment, or the need for PRP during the study based on the Investigator's opinion. 6. Previous focal laser photocoagulation in the study eye, within the foveal avascular zone. 7. Intravitreal/periocular/topical ocular steroids of any type in the study eye within 90 days (3 months) prior to study enrollment. 8. Placement of Iluvien or Retisert (fluocinolone acetonide intravitreal implant) in the study eye within 36 months (3 years) prior to study enrollment. 9. Use of Ozurdex (dexamethasone intravitreal implant) in the study eye within 180 days (6 months) prior to study enrollment. 10. Significant epiretinal membrane, posterior hyaloidal traction, and/or vitreomacular traction in the study eye as determined by the optical coherence tomography (OCT) results. 11. Previous pars plana vitrectomy in the study eye. 12. Any intraocular surgery in the study eye within 90 days (3 months) prior to study enrollment. 13. Yttrium aluminium garnet (YAG) laser treatment in the study eye within 30 days (1 month) prior to study enrollment. 14. Concomitant use of any topical ophthalmic medications in the study eye, including dry eye or glaucoma medications, unless on a stable dose for at least 90 days (3 months) prior to study enrollment and expected to stay on stable dose throughout study participation. Artificial tears are allowed. 15. High myopia in the study eye, with a spherical equivalent of \>8.00 Diopters (D) at screening. 16. Chronic or recurrent uveitis in the study eye. 17. Ongoing ocular infection or inflammation in either eye. 18. A history of cataract surgery complicated by vitreous loss in the study eye. 19. Congenital eye malformations in the study eye. 20. A history of penetrating ocular trauma in the study eye. 21. Mentally handicapped. 22. Females of childbearing potential (i.e., who are not postmenopausal for at least 1 year or surgically sterile for at least 6 weeks prior to Visit 1 - Screening/Randomization) who are lactating, or who are pregnant as determined by a positive urine pregnancy test (UPT) at Visit 1 - Screening/Randomization. Women of childbearing potential must agree to use acceptable methods of birth control throughout the study. Acceptable methods of birth control include tubal ligation, transdermal patch, intrauterine devices/systems, oral/implantable/injectable or contraceptives, sexual abstinence, double barrier method, or vasectomized partner. 23. Participation in any other investigational device or drug clinical research study within 30 days of Visit 1 - Screening/Randomization. 24. Contraindication to the study medications or fluorescein dye. 25. Other ocular pathologies that in the Investigator's opinion would interfere with the subject's vision in the study eye.