Identification and characterization of a novel aldehyde metabolite of WIN18,446 and associated WIN18,446-ALDH1A2 protein adducts using mass spectrometry.

ALDH1A family enzymes (ALDH1A1, ALDH1A2, and ALDH1A3) catalyze retinoic acid synthesis, and their dysregulation is linked to disease. Selective inhibitors of these enzymes have been tested in drug discovery programs and one such compound, WIN18,446, was found to irreversibly inhibit ALDH1A2. WIN18,446 is a reversible male contraceptive in humans and animals. The inhibition of spermatogenesis by WIN18,446 is thought to be due to inhibition of ALDH1A. However, the mechanism of irreversible inhibition of ALDH1A2 by WIN18,446 is not known. A crystal structure obtained after incubating ALDH1A2 with WIN18,446 revealed a WIN18,446-derived metabolite covalently adducted to the catalytic cysteine, C320. Inspection of this structure suggested that the observed adduct is unstable and may be a metabolic intermediate stabilized under crystallographic conditions. In the current work, we tested this hypothesis. We identified and characterized an aldehyde metabolite of WIN18,446, which we designated M-54. M-54 is likely the metabolite of the intermediate observed in the crystal structure. Using a range of proteomics techniques, we identified a WIN18,446-derived ALDH1A2 protein adduct of mass 292.07 Da on C319 of ALDH1A2. This adduct may result from the reaction of the crystal structure metabolic intermediate. Using the identified mass, we probed human liver samples from multiple donors and found WIN18,446 specific adducts on cysteines within the ALDH1A1 and ALDH2 active site regions. The current study provides new insight into the metabolism of WIN18,446 and the mechanism of inhibition of ALDH1A2. We also demonstrate a proteomics workflow for identifying and validating drug-protein adducts of unknown mass.