The study has a randomized, double-blind, placebo-controlled, double dummy and eight-way cross-over design. Males with T2DM on a stable metformin monotherapy will be screened for participation in the study. Eligible subjects will be randomized to receive DXM 30 mg, DXM 60 mg, DXM 90 mg alone or in combination with sitagliptin 100 mg, sitagliptin 100 mg alone, or placebo (for DXM and sitagliptin) on in total eight treatment days. An OGTT will be started 1 hour after study drug administration and blood glucose will be measured over the next 4 hours. There will be a 3 to 14-day washout period between doses.

This study is performed to determine the optimal dose of DXM that, compared to placebo, decreases glucose excursions following an OGTT. Doses of 30 mg, 60 mg and 90 mg DXM were chosen. Since DXM 60 mg was effective in the previous study in increasing insulin secretion, this dose will be kept. A lower dose (30 mg) will be given to see whether effects on insulin secretion are present also at lower doses and then translate into a BG lowering action. A higher dose (90 mg) will be added to see whether this way an effect on glucose excursion reduction occurs following an OGTT, (like previously demonstrated with 270 mg DXM) that can be related to an increase of insulin secretion.

Inclusion Criteria:

1. Signed and dated written informed consent obtained before any study-related activities
2. Male subjects with a diagnosis of type 2 diabetes mellitus according to ADA criteria at least 4 months prior to screening
3. Medical history without major pathology (with the exception of type 2 diabetes)
4. On a stable regimen of metformin monotherapy for at least 3 months
5. Aged between 45 and 70 years of age, both inclusive
6. Body mass index (BMI) between 25 and 35kg/m2, both inclusive

Exclusion Criteria:

1. Subjects with type 1 diabetes, MODY or secondary forms of diabetes such as due to pancreatitis
2. History of pancreatitis
3. Current or previous treatment with insulin therapy
4. Treatment with any hypoglycemic medication other than metformin within the three months prior to screening
5. Mean QTc> 450 msec