To investigate , in a proof-of-concept clinical trial , whether low-dose oral doxycycline monohydrate can ( 1 ) slow the deterioration of , or improve , retinal function or ( 2 ) induce regression or slow the progression of DR in patients with severe nonproliferative DR ( NPDR ) or non-high-risk proliferative ( PDR ) , and to determine the potential usefulness of visual function end points to expedite the feasibility of conducting proof-of-concept clinical trials in patients with DR.