The purpose of the study is to determine the beneficial effects of CBX129801 (PEGylated synthetic human C-peptide) following weekly subcutaneous administration for 12 months in type 1 diabetes mellitus patients (T1DM) with mild to moderate diabetic peripheral neuropathy (DPN).

Key Inclusion Criteria:

• Give informed consent;
• 18-65 years old;
• Have type 1 diabetes mellitus for a minimum of 5 years, with a stable diabetic regimen (for at least 3 months);
• Have clinical signs of diabetic peripheral neuropathy at screening;
• Have abnormal sural nerve conduction observed bilaterally during screening;
• Be C-peptide deficient;
• Be in good general health (besides having type 1 diabetes mellitus);
• Practice effective contraception during and for at least 12 weeks after study participation;
• Have a body mass index (BMI) ≥18.0 and <35.0 kg/m2.

Key Exclusion Criteria:

• Any significant cardiovascular, hematological, lymphatic, immunologic, urologic, dermatologic, psychiatric, renal, hepatic, pulmonary, endocrine (except for diabetes mellitus), central nervous, gastrointestinal, or other major disease;
• Unstable or inadequate glucose control;
• Any clinically significant laboratory value at screening;
• Occurrence of a severe, unexplainable hypoglycemic event (defined as requiring the assistance of another individual) within 6 months of Day 0, or recurrent episodes of non-severe hypoglycemia (≥3 per week on average) that are deemed clinically significant by the Investigator;
• Have had an islet cell, kidney, and/or pancreas transplant;
• If female, is pregnant or lactating;
• History of alcohol or substance abuse within 2 years;
• Positive screen for hepatitis B, hepatitis C antibody, or human immunodeficiency virus (HIV) antibody;
• Initiation of treatment or change of dose of medication that could affect peripheral nerve function within 60 days;
• Previous treatment with CBX129801 or unmodified C-peptide;
• Receipt of an investigational product or therapeutic device, or participation in a drug research study, within a period of 30 days;
• Chronic use of oral steroids or use of Ampyra (dalfampridine) within 60 days.