This is a multicenter, randomized, double-masked, placebo-controlled study to evaluate the effects of emixustat in subjects with PDR. Subjects will be randomly assigned to either emixustat or placebo arms and treated once daily (QD) for 12 weeks. Doses of emixustat will be doubled on a weekly basis until week 4 after which all subjects will be held at a stable dose for the remainder of the 12-week dosing regimen. Subjects in the placebo group will be mock-titrated on the same schedule as those in the emixustat arm.