MK-0893, developed by Merck, is a reversible and competitive antagonist with high binding affinity (IC(50) of 6.6 nM) and functional cAMP activity (IC(50) of 15.7 nM). MK-0893 reached Phase 2; 12 weeks of once daily oral dosing led to significant and dose-dependent reductions in fasting and post-prandial plasma glucose and in HbA1c, with similar low incidences of hypoglycaemia as a metformin cohort. Combinations with metformin and sitagliptin were also trialled, and risk for hypoglycaemia was assessed in healthy males which showed the drug caused lengthening of recovery times. However, plasma levels of LDL cholesterol and liver transaminases were increased in some studies, as was body weight and blood pressure, all of which were not evident pre-clinically, and MK-0893 was discontinued.

Sitagliptin (MK-0431), chemically (2R)-4-Oxo-4-[3- (trifluoromethyl)-5,6-dihydro[1,2,4]triazolo[4,3-a]pyrazin- 7(8H)-yl]-1-(2,4,5-trifl uorophenyl)butan-2-amine has a very high selectivity towards DPP-4, with an IC(50) of 18 nM. There is no affinity towards other DDP enzymes (DPP- 8 and DPP-9). It has been approved for the treatment of type 2 diabetes in the USA and Europe and is registered by the name Januvia (Merck Pharmaceuticals, Whitehouse Station, NJ, USA). In healthy volunteers and in patients with type 2 diabetes of different ethnic background, the tolerability of different doses given once or twice daily is good. The drug works to competitively inhibit a protein/enzyme, dipeptidyl peptidase 4 (DPP-4), that results in an increased amount of active incretins (GLP-1 and GIP), reduced amount of release of glucagon (diminishes its release) and increased release of insulin. Sitagliptin is an incretin enhancer and the first marketed medication belonging to the gliptin class. In fact, no published literature exists regarding incidence or severity of hypoglycemia when sitagliptin is used off-label in combined with insulin therapy. However, is recommended to use methods to avoid hypoglycemia when using this off-label combination. Approximately 79% of sitagliptin is excreted unchanged in the urine with metabolism being a minor pathway of elimination. Elimination of sitagliptin occurs primarily via renal excretion and involves active tubular secretion. Sitagliptin is a substrate for human organic anion transporter-3 (hOAT-3), which may be involved in the renal elimination of sitagliptin