The purpose of this study is to assess safety and tolerability and determine the pharmacokinetics of subcutaneous single and multiple (up to 12) dose administration of CBX129801 (long-acting synthetic C-peptide) in type 1 diabetes patients.

This study will be conducted in two parts (Part 1 and Part 2). Part 1 will be conducted in three sequential dose cohorts with dose level escalated for each successive cohort. In Part 2 of the study, a dose regimen determined from the pharmacokinetic data from Part 1 will be administered in up to 40 additional subjects to achieve a target plasma concentration range that represents the most commonly observed physiological C-peptide levels in healthy, non-diabetic individuals.

Key Inclusion Criteria:

• Give informed consent
• Aged 18 to 55 years (Part 1), Ages 18 to 65 years (Part 2)
• Stable type 1 diabetes mellitus (T1DM) for a minimum of 5 years
• Body mass index of 18-35 kg/m2
• Be C-peptide deficient (assessed by fasting concentration level)
• Normal renal function (assessed by serum creatinine)
• Be in good general health (besides T1DM)
• Have abnormal sural nerve conduction velocity observed bilaterally (Part 2 only)

Key Exclusion Criteria:

• Any significant cardiovascular, hematological, renal, hepatic, pulmonary, endocrine (except for diabetes mellitus), central nervous, or gastrointestinal condition or disease
• Unstable glucose control
• Have had a islet cell, kidney, and/or pancreas transplant
• Blood loss or blood donation within 56 days
• Drug or alcohol abuse (within 2 years) or recreational drug use (within 3 months)
• History or positive test result for Hepatitis B, C, and/or HIV
• Treatment with medication for peripheral neuropathy within 30 days