Identification and Structural Characterization of New Degradation Products in Moxidectin Stressed Samples by LC-HRMS and NMR

Moxidectin (MOX) is a macrocyclic lactone which belongs to milbemycin family of antiparasitic endectocides. MOX is widely used in veterinary medicine for the treatment and control of external and internal parasites, MOX is also used for the treatment of onchocerciasis (river blindness) in humans. In this paper, MOX drug substance was subjected to acidic, alkaline, oxidation, thermal (solid and solution state), and photolytic (solid and solution state) stress degradation. Stress-degraded samples of MOX were analyzed by a reversed phase ultra performance liquid chromatography (RP-UPLC) method using HALO C18 column (100 x 2.1 mm, 2 mu m). MOX and all major DPs were adequately separated by a gradient elution using 0.1% formic acid in water as mobile phase-A and acetonitrile as mobile phase-B. Total 12 major DPs, including seven new DPs (2-carboxyl-19-hydroxyl, 2-carboxyl-3,4-epoxy-19-hydroxyl, 14,15-epoxide, 26,27-epoxide, 14-OOH, 15-OOH, and 27-OOH) not previously reported in the literature, were observed. Structural characterization of these DPs was performed using UPLC-high resolution mass spectrometry (HRMS) and by comparison of their fragmentation profile with parent compound. Six major DPs namely 3,4-epoxide, 14,15-epoxide, 26,27-epoxide, 14-OOH, 15-OOH, and 27-OOH were isolated and purified from the stressed samples using semi-preparative HPLC. The chemical structures of these DPs were further confirmed through comprehensive nuclear magnetic resonance (NMR) spectroscopy studies. The study results reported in this paper should be helpful to further understand degradation pathways of MOX under different conditions. These results are also beneficial to facilitate quality monitoring of MOX drug substance as well as identification of unknown degradation products in drug formulations made with MOX drug substance.