Degradation kinetics and mechanism of an oxadiazole derivative, design of a stable drug product for BMS-708163, a γ-secretase inhibitor drug candidate.

The stability of a 1,2,4-oxadiazole derivative, BMS-708163, A, was studied in the cosolvent mixture of acetonitrile-water at various pH values, in the solid state and in the presence of various excipients. The objective of this study was to develop a deep understanding of the stability of compound A based on its degradation kinetics and mechanism to enable design of a robust drug product. A series of isotopically (13) C- and (15) N-labeled compounds were synthesized and their degradation was studied by liquid chromatography-mass spectrometry and nuclear magnetic resonance to prove the degradation mechanism. Compound A exhibited maximum stability at a pH range of 3-5. In forced degradation studies, higher or lower pH resulted in an increase in degradation rate. At low pH, the N-4 atom on the 1,2,4-oxadiazole ring is protonated, followed by nucleophilic attack on the activated methine carbon to cause ring opening to form aryl nitrile degradation product, compound B. At high pH, the nucleophilic attack occurs on the methine carbon to generate an anion on N-4. Subsequent proton capture from a proton donor, such as ambient water, facilitates ring opening to generate compound B. In the absence of a proton donor, such as in dry acetonitrile, anion on N-4 will go back to compound A. Therefore, compound A is stable in absence of proton donor. This study defines the package condition and microenvironmental pH under which compound A can be formulated into a stable product.