The purpose of this study is to demonstrate the efficacy of dutogliptin over 26 weeks (as evidenced by placebo-corrected changes in HbA1c relative to baseline), to demonstrate safety and tolerability of dutogliptin, and to demonstrate changes in fasting plasma glucose over 26 weeks.

Inclusion Criteria:

• Type 2 diabetes mellitus, diagnosed at least 4 months prior to Screening (Visit 1)
• Age 18 to 85 years, inclusive.
• Male and non-pregnant, non-lactating (and not planning to become pregnant during the study) female subjects with a BMI of 20 to 48 kg/m2, inclusive
• Current treatment of Type 2 diabetes mellitus with a stable dose of metformin of ≥ 2000 mg (or the highest tolerated dose) used in accordance with product labeling for at least 6 weeks prior to screening (Visit 1)
• HbA1c 7.0% - 10.0%, inclusive; and fasting plasma C peptide greater than 0.26 nmol/L (> 0.8 ng/mL; > 281 pmol/L) at screening (Visit 1)

Exclusion Criteria:

• Type 1 diabetes mellitus