Identification of a Crystalline Drug Metabolite in Tissue Sections using Microscopy and Maldi-MS

Abstract

Development of a potential drug candidate was discontinued when rats treated with the compound developed severe renal toxicity after one week of daily dosing. Tubular degeneration in the kidney was accompanied by deposition of large amounts of crystalline material, which was also present in spleen and pancreas (Fig. 1). It was presumed that the crystals were precipitated parent drug or one of its metabolites. Using microscopy and mass spectrometry, it was confirmed that the crystals were indeed derived from the drug and not endogenously formed; in addition, the precise metabolite that had precipitated in the tissues was identified, providing clues to the metabolic pathways involved.Frozen sections of phosphate-buffered formaldehyde-fixed kidney, pancreas, and spleen were evaluated by polarized light microscopy (PLM), scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDXS), and matrix assisted laser desorption ionization-mass spectrometry (MALDI-MS). For reference, crystals of the trifluoroacetate salt of the parent drug were also analyzed using these methods.