A Phase IIa, Dose-finding, Double-blind, Placebo-controlled, Double-dummy, Randomized, Eightfold Cross-over Study to Investigate the Glucose Lowering Effects of Dextromethorphan Alone or in Combination With Sitagliptin in Subjects With Type 2 Diabetes Mellitus (T2DM) After an Oral Glucose Tolerance Test - ICH GCP - Clinical Trials Registry