A Study to Compare the Glycemic Effects, Safety, and Tolerability of Exenatide Once Weekly to Those of Sitagliptin and Pioglitazone,in Subjects With Type 2 Diabetes Treated With Metformin (DURATION - 2)

Inclusion Criteria: * Has been diagnosed with type 2 diabetes mellitus * Has a hemoglobin-specific A1c fraction (HbA1c) of 7.1% to 11.0%, inclusive, at study start * Has a body mass index (BMI)of 25 kg/m2 to 45 kg/m2, inclusive, at study start * Has been on a stable treatment regimen of metformin for a minimum of 2 months prior to study start * Either is not treated with or has been on a stable treatment regimen with any of the following medications for a minimum of 2 months prior to study start: 1. Hormone replacement therapy (female subjects) 2. Oral contraceptives (female subjects) 3. Antihypertensive agents 4. Lipid-lowering agents 5. Thyroid replacement therapy 6. Antidepressant agents 7. Drugs known to affect body weight, including prescription medications (e.g. orlistat \[XENICAL®\], sibutramine \[MERIDIA®\], topiramate \[TOPAMAX®\]) and over-the-counter antiobesity agents Exclusion Criteria: * Has been previously exposed to exenatide once weekly * Has donated blood within 60 days of study start or is planning to donate blood during the study * Currently being treated, or is expected to require or undergo treatment with any of the following treatment-excluded medications: 1. Exenatide (BYETTA®) or any Dipeptidyl peptidase-4 DPP-4)inhibitor, sulfonylurea (SU), thiazolidinedione (TZD), or glucagon-like peptide (GLP)-1 analog within 3 months prior to study start 2. Alpha-glucosidase inhibitor, meglitinide, nateglinide, or pramlintide (SYMLIN®) within 30 days of study start 3. Insulin within 2 weeks of study start or for more than 1 week within 3 months of study start 4. Systemic corticosteroids by oral, intravenous, or intramuscular route; or potent, inhaled, or intrapulmonary (including ADVAIR®) steroids known to have a high rate of systemic absorption 5. Drugs interacting with the CYP2C8 enzyme system, including gemfibrozil (LOPID®) and rifampin * Has received any investigational drug within 1 month (or five half-lives of investigational drug, whichever is greater) of study start * Has previously experienced a clinically significant adverse event (e.g., significant edema) related to TZD or DPP-4 inhibitor use