Safety and Efficacy Study of Dutogliptin/PHX1149T in Subjects With Type 2 Diabetes Mellitus on a Background Medication of Metformin

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