Comprehensive Stress Degradation Studies of Afoxolaner Drug Substance and Identification of Major Degradation Products Including its Degradation Pathways by LC–HRMS–MS and NMR

Afoxolaner, a novel insecticidal and acaricidal drug substance from the isoxazoline family, is efficacious against various parasites among companion animals. In general, afoxolaner drug substance is quite stable in formulated and unformulated forms, when stored under International Council for Harmonization (ICH) guideline storage conditions. Exhaustive stress degradation studies of afoxolaner drug substance were conducted under various stress conditions to obtain an in-depth understanding of potential degradation pathways, including formation mechanisms of potential degradations products of this compound. Stressed degradation studies were carried out in the presence of various type/class of acid, base, and oxidation agents, including thermal and light irradiation. An investigational ultra-high-performance liquid chromatography (UHPLC) method was developed for the purpose of this study. The UHPLC method provided adequate separation of afoxolaner and its major degradation products that were generated in the exhaustively stressed samples. A total of five major degradation products (DPs) were formed under acidic, basic, photolytic, and oxidative stress conditions. Various analytical techniques such as high-resolution tandem mass spectrometry (HRMS/MS–MS) and nuclear magnetic resonance (NMR) were used to identify and propose the most probable chemical structures of the key unknown degradation products in the stressed degradation samples. Adequate amounts of two DPs, namely DP-1 and DP-3, were isolated and purified by semi-preparative high-performance liquid chromatography (HPLC) methods. Subsequently, these two DPs were examined in detail using both 1D and 2D NMR spectroscopy. From the proposed chemical structures of identified degradation products, most probable degradation pathways and formation mechanism is proposed.