East to West not North-West: Structure-Based Mechanistic Resolution of 8-Hydroxyl Replacement and Resulting Effects on the Activities of Imidazole-Based Heme Oxygenase-1 Inhibitors

Upregulation of Heme Oxygenase-1 (HO-1) has been widely implicated in cancer growth and chemoresistance. This explains the numerous drug discovery efforts aimed at mitigating its pro-carcinogenic roles till date. In a recent study, two selective azole-based HO-1 inhibitors (Cpd1 and Cpd2) were synthesized, which exhibited differential inhibitory potencies of ~200μm. Interestingly, variations in the affinities of these compounds were determined by their positioning across specific regions of the HO-1 binding domain, pin-pointing a pharmacological interrelationship that remains unresolved. Therefore, in this study, using molecular dynamics simulations and binding free energy calculations, we investigate how dynamical orientations of these compounds influence their binding affinities at the active HO-1 domain. Findings revealed favorable binding for the bromobenzene and imidazole substituents of Cpd1 at the western and eastern regions of the HO-1 active domain. The constituent hydroxyl group was coordinated by residues Asp140 and Arg136 over the simulation period. On the contrary, stable binding of the bromobenzene and imidazole substituents were negated by the optimal orientations of the benzyl substituent, which extended into the northeastern region. These were supported by the displacement of Asp140 and Arg136, crucial for hydrogen bond formation in Cpd1. Also, we observed that Cpd2 exhibited high deviations indicative of an unstable binding relative to Cpd1. This further supports the presumption that Cpd2 was systematically oriented away from the active HO-1 region, a phenomenon that was due to the optimal motions of the benzyl group at the northeastern regions. The highlight of our findings is that the benzyl substituent in Cpd2 elicited negative effects on HO-1, vis a vis, instability, displacement of crucial residues, and low binding energy when compared to Cpd1. Findings pave the way for future drug discovery efforts related to HO-1 inhibition in cancer therapy.