The purpose of the study is to evaluate the clinical effects of the investigational drug, SB-509, in subjects with diabetic neuropathy.

SB-509 contains the gene (DNA-a kind of biological "blueprint") for a protein. When a researcher injects SB-509 into your legs, the drug enters the muscle and nerve cells around the injection site and causes these cells to make a protein. This protein causes your cells to increase production of another protein called vascular endothelial growth factor (VEGF), which may improve the structure and function of nerves. In addition, there are changes in the levels of additional proteins in your cells. These proteins function to promote the growth of cells, are structures in cells, help synthesize products, and affect immune cells, and some have unknown functions. This increase in your own VEGF proteins may protect and repair the damaged nerves caused by diabetic neuropathy.

Key Inclusion Criteria:

• Clinical diagnosis of Diabetes Mellitus Type I or II for at least 12 months
• Clinical signs and symptoms of moderate to severe diabetic sensory-motor polyneuropathy of the lower extremities for at least 6 months that are not otherwise attributed to an etiology other than diabetes
• Measurable sural and peroneal response bilaterally
• HgbA1C level ≤ 10%
• LDL cholesterol ≤ 160 mg/dL
• Blood pressure ≤ 140/90 mm Hg
• Body mass index (BMI) ≤ 38

Key Exclusion Criteria:

• Moderate to severe ischemic heart disease or any history of congestive heart failure, or have had a myocardial infarction within the previous 6 months
• Evidence of cardiac enlargement and/or congestive heart failure
• Current diabetic foot or leg ulcer, gangrene in the lower extremity, or any amputation of the lower extremity
• History of malignancy, except for the following: adequately treated basal cell or squamous cell skin cancer, superficial bladder cancer, adequately treated Stage 1 or 2 cancer currently in complete remission, or any other cancer that has been in complete remission for at least 5 years
• Immune or immunodeficiency disorders or expected to require immunosuppressants for 30 days prior to, during, and for 30 days following administration of the investigational drug product
• History of or current proliferative retinopathy, macular edema or retinal neovascularization
• Pre-cancerous conditions or benign tumors which have the potential for clinically significant growth due to VEGF stimulation
• Family history of inherited neuropathy (e.g. Charcot Marie Tooth, Hereditary Predisposition to Pressure Palsy)