This study will compare the effect of a new oral agent for type 2 diabetes, sitagliptin, in comparison to thiazolidinediones as the third-line oral agent, in patients with type 2 diabetes mellitus.

The aim of this protocol is to determine the non-inferiority of the effectiveness of sitagliptin compared to a control group of patients treated with thiazolidinediones as add-on therapy, in low-income ethnic minority type 2 diabetic patients who are failing to maintain adequate control with maximal doses of metformin and a sulfonylurea agent.

Inclusion Criteria:

• Male or female, age 18-75
• Type 2 diabetes
• At least 3 months of treatment with maximum tolerated doses of metformin and a sulfonylurea agent, and inadequate glycemic control (Hemoglobin A1c (HbA1c) >7.0%)
• Able to comply with all scheduled visits and requirements of the protocol

Exclusion Criteria:

• Any contraindications to the use of metformin or a sulfonylurea agent
• Extreme hyperglycemia or symptoms of polyuria or polydipsia
• Current or previous chronic use of insulin (other than for treatment of gestational diabetes)
• History of confirmed (or clinical suspicion of) type 1 diabetes mellitus
• Episodes of symptomatic hypoglycemia averaging greater than once per day
• Estimated glomerular filtration rate (eGFR) < 60 mL/min
• Subjects with active hemolytic anemias or hemoglobin variants that render the measurement of HbA1c unreliable
• History of any clinically significant hepatic, cardiovascular (including the use of digoxin), or other major systemic disease that may make the use of sitagliptin unsafe, or otherwise make the interpretation of the data difficult.
• Female subjects of childbearing potential who are sexually active and not using a reliable form of contraception
• Current pregnancy or lactation.
• Subjects who will likely require or initiate therapy with drugs that may interfere with glucose metabolism during the course of the study.
• Subjects who are in another investigational study or have received another investigational medication within 30 days of study entry
• Subjects who are unable or unwilling to give informed consent, comply with all components of the study protocol, attend all scheduled follow-up visits, or present other barriers that would make the implementation of the protocol unusually difficult.