Chemistry and Biology of Dihydroisoxazole Derivatives: Selective Inhibitors of Human Transglutaminase 2 Original Research Article

3-Halo-4,5-dihydroisoxazoles are attractive warheads for the selective inhibition of nucleophilic active sites in biological systems. A series of 3-bromo-4,5-dihydroisoxazole compounds were prepared and tested for their ability to irreversibly inhibit human transglutaminase 2 (TG2), an enzyme that plays an important role in the pathogenesis of diverse disorders including Celiac Sprue and certain types of cancers. Several compounds showed high specificity for human TG2 (kinh/KI > 2000 min−1M−1) but essentially no reactivity (k < 1 min−1M−1) toward physiological thiols such as glutathione. The pharmacokinetic and pharmacodynamic properties of a prototype dihydroisoxazole inhibitor, 1b, were evaluated; in mice the compound showed good oral bioavailability, short serum half-life and efficient TG2 inhibition in small intestinal tissue, and low toxicity. It also showed excellent synergism with N,N′-bis(2-chloroethyl)-N-nitrosourea (BCNU, carmustine) against refractory glioblastoma tumors in mice. A fluorescent dihydroisoxazole inhibitor 5 facilitated microscopic visualization of TG2 endocytosis from the extracellular surface of HCT-116 cells. Together, these findings demonstrate the promise of dihydroisoxazole compounds as probes for the biology of TG2 and its role in human disease.