Elucidating the pathways of degradation of denagliptin

Stress testing or forced degradation studies of denagliptin (1) tosylate in solution and solid-state, its blends with excipients, and capsules were conducted in order to elucidate degradation pathways, aid formulation development, and generate data to support regulatory filings. In solution, denagliptin was stressed in acid, water, and base using organic cosolvents. In the solid-state, denagliptin was stressed under heat, humidity, and light. Blends of denagliptin with various excipients were stressed under heat and humidity in order to evaluate whether tablet was a viable dosage form. Capsules were stressed under heat, humidity, and light. It was found that denagliptin was stable in the solid-state, but degraded in solution, in blends with all excipients, and in capsules predominantly by cyclization to (3S,7S,8aS) amidine (2), which epimerized to (3S,7S,8aR) amidine (3). (3S,7S,8aR) amidine (3) subsequently hydrolyzed to the corresponding diketopiperazine (4). The purpose of this manuscript is to discuss the results of stress testing studies conducted during the development of denagliptin and the elucidation of its key degradation pathway.