Evaluation of Effect of Doxycycline Verses Placebo on Retinal Function and Diabetic Retinopathy

Inclusion Criteria: * age ≥ 18 years old * diagnosis of type 1 or type 2 diabetes mellitus (defined as current regular use of oral anti-hyperglycemia agents and/or insulin for the treatment of diabetes) * have a hemoglobin A1c \< 11% at pre-qualification visit * able and willing to give informed consent * best-corrected ETDRS visual acuity (10) in study eye ≥ 69 letters (20/40) * mild to moderate non-proliferative diabetic retinopathy (ETDRS levels 20 to 43) (11), and in whom retinal photocoagulation is not anticipated (by the investigator) within the subsequent 2 years * able to perform reliable visual field and dark adaptation testing * central subfield thickness on OCT ≤ 275 microns * media clarity and pupil dilation sufficient for high-quality fundus photographs * abnormal retinal function defined as: * abnormal FDP function as defined by a foveal sensitivity ≤ 30.91 dB Exclusion Criteria: * prior panretinal photocoagulation in the study eye * prior focal/grid laser photocoagulation in the macula in the study eye * intraocular pressure in the study eye \> 22 mmHg by Goldmann tonometry * history of pars plana vitrectomy in the study eye * systemic or intravitreal anti-VEGF agent to the study eye or the fellow eye within the past 3 months * peribulbar steroid injection to the study eye or the fellow eye within the past 6 months * intravitreal triamcinolone acetonide to the study eye within the past 4 months * expectation by the investigator that retinal photocoagulation or other treatment for diabetic retinopathy (e.g., focal/grid laser to study eye, intravitreal triamcinolone acetonide to study eye, intravitreal anti-VEGF agent to study or fellow eye, ruboxistaurin or systemic anti-VEGF agent for diabetic macular edema) will be administered in the subsequent 24months * an ocular condition (other than diabetes) is present in the study eye that, in the opinion of the investigator, might alter visual acuity during the course of the study (e.g., retinal vein occlusion, uveitis or other ocular inflammatory disease, neovascular glaucoma, Irvine-Gass Syndrome, etc) * anticipated need for cataract surgery in the study eye in the subsequent 24 months in the opinion of the investigator * history of major ocular surgery (including cataract surgery, scleral buckle, any intraocular surgery, etc) in the study eye within prior 6 months or anticipated within the subsequent 24 months following randomization * aphakia in the study eye * history of YAG capsulotomy performed in the study eye within 2 months prior to randomization