Structure–activity relationships of hydrophobic small molecule irreversible inhibitors of tissue transglutaminase

Tissue transglutaminase (TG2) is both an enzyme and a G-protein that is implicated in many diseases, such that small molecule inhibitors of TG2 have broad potential as drugs or research tools. Previous work has demonstrated how the structure of EB-2-16, a highly potent irreversible inhibitor of TG2, has been optimised with respect to its warhead, tether and bridge moieties. In this work, we studied the structure-activity relationships of the pendant hydrophobic group of the scaffold. This confirmed the superior affinity conferred by the parent adamantyl moiety, over other cycloalkyl, aryl, biaryl and bridged biaryl groups. Additionally, some substituted adamantyl derivatives were shown to exhibit superior inhibitory efficiency over the parent inhibitor, with <i>k</i> _inact/<i>K</i> _I values over 10⁶ M^-1 min^-1. The best inhibitors were shown to exhibit excellent lipid membrane permeability, but evaluation of their human hepatocyte stability revealed a sharp distinction between them. Despite the bromo- and iodoadamantyl derivatives being more efficient inhibitors, chloroadamantyl inhibitor 25b exhibits the best overall properties (<i>k</i> _inact = 1.69 min^-1, <i>K</i> _I = 1.79 μM, <i>k</i> _inact/<i>K</i> _I = 941 × 10³ M^-1 min^-1, <i>P</i> _e = 1.41 × 10^-6 cm s^-1, CL_int = 6.91 μL min^-1/10⁶ cells) and suitability for potential applications <i>in vivo</i>.