A Novel, Double-Clamp'' Binding Mode for Human Heme Oxygenase-1 Inhibition

The development of heme oxygenase (HO) inhibitors is critical in dissecting and understanding the HO system and for potential therapeutic applications. We have established a program to design and optimize HO inhibitors using structure-activity relationships in conjunction with X-ray crystallographic analyses. One of our previous complex crystal structures revealed a putative secondary hydrophobic binding pocket which could be exploited for a new design strategy by introducing a functional group that would fit into this potential site. To test this hypothesis and gain further insights into the structural basis of inhibitor binding, we have synthesized and characterized 1-(1H-imidazol-1-yl)-4,4-diphenyl-2-butanone (QC-308). Using a carbon monoxide (CO) formation assay on rat spleen microsomes, the compound was found to be similar to 15 times more potent (IC50 = 0.27 +/- 0.07 mu M) than its monophenyl analogue, which is already a potent compound in its own right (QC-65; IC50 = 4.0 +/- 1.8 mu M). The crystal structure of hHO-1 with QC-308 revealed that the second phenyl group in the western region of the compound is indeed accommodated by a definitive secondary proximal hydrophobic pocket. Thus, the two phenyl moieties are each stabilized by distinct hydrophobic pockets. This double-clamp'' binding offers additional inhibitor stabilization and provides a new route for improvement of human heme oxygenase inhibitors.