In the same research group
In 2017, the same research group released a free internet accessible database (HemeOxDB, http://www.researchdsf.unict.it/hemeoxdb), to make it easier the analysis of the literature and, consequently, the design of potent and selective HO-1 inhibitors. The database reported the entire set of HO-1 and HO-2 inhibitors known until then [, , ]. The HemeOxDB collects two-dimension (2D) structural information, computed physicochemical and experimental pharmacological properties along with the procedure protocols, extracted from the literature. The database can be searched by chemical structure, smiles, computed physicochemical properties, pharmacological properties, reference, and HemeOxDB-ID. Matching queries obtained on a results page are shown in tabulated format and can be personalized. 2D or 3D graphs for the defined axes can be built, and individual compounds can be visualized in a summary page. Analysis of the entire collection enables users to understand the chemical features involved in the recognition and inhibition of these enzymes. In fact, a careful analysis of HemeOxDB showed that only 32 compounds were very potent HO-1 inhibitors (HO-1 IC50 < 1 μM), and, among them, only 4 were highly selective for HO-1 vs. HO-2 (HO-2/HO-1 ratio >100). Moreover, it emerged that compounds with an alcoholic moiety in the central alkyl chain were generally more selective for HO-1 than the corresponding ketone analogs . Based on this information, a new series of imidazole compounds with a phenylethanolic connecting chain (Table 5, compounds 13a–g) was designed and synthesized . The majority of these novel compounds (13a–e) showed very good HO-1 inhibition with an IC50 < 1 μM, confirming that the phenylethanolic chain is a critical chemical feature for optimal HO-1 interaction. The hydrophobic portion of the molecules further influenced the potency of new compounds. In particular, Calcium Colorimetric Assay Kit 13a, possessing a 3-Br linked to the phenyl ring, was the most potent compound of the series (HO-1 IC50 = 0.40 μM). Replacement of 3-Br with a 3-phenyl as in compound 13b, slightly decreased activity (HO-1 IC50 = 0.95 μM), but remarkably increased selectivity towards HO-2 isoform (IC50 > 100 μM). Further modifications of the hydrophobic moieties afforded more hindered compounds such as diphenyl ether 13c, or benzyloxy derivatives, 13d–f. The high HO-1 inhibitory potency of these compounds (IC50s < 1 μM for compounds 13c–e) showed that larger hydrophobic groups were generally well allocated by the distal hydrophobic pocket of the enzyme. An exception is represented by compound 13d in which the presence of the bulkier 4-I residue is detrimental for HO-1 activity (HO-1 IC50 = 24.50 μM). Among the benzyloxy series (13d–f), compound 13e stand out in addition to the potency for its selectivity vs. HO-2 (IC50 > 100 μM) showing a similar profile with compound 13b. Docking poses were examined to explore the binding mode with the enzyme and showed interactions similar to that previously described. In a following paper, three novel compounds (Table 5, compound 13g, Fig. 4 compounds 14 and 15) were described and all of them showed a reduced capacity of inhibiting HO-1 . It was investigated the effect of the benzylation of compound 13a that afforded 13g (Table 5, HO-1 IC50 = 80.0 μM). The transformation of the keto function of compound 5 (Fig. 2) into an ethanolic spacer afforded compound 14 (Fig. 4, HO-1 IC50 = 67.6 μM) and the transformation of the keto function of compound 7 (Fig. 2) into an amide residue gave compound 15 with an HO-1 IC50 of 28.8 μM. Overall, these substitutions did not permit optimal interactions to the enzyme, as explained by docking studies reported in section 3. Recently, Subashini et al. reported the identification of a small series of quinoline-based imidazole derivatives, exemplified by compound 16 (Fig. 5), that emerged as rule breakers . This compound, using Lineweaver-Burk plot studies, has been shown to behave as competitive inhibitor being able to displace the substrate from the catalytic site of the enzyme, dissimilarly to the classical azole-based inhibitors. Although imidazole nucleus is present in their structure, these molecules represent the only example of HO-1 inhibitors structurally and functionally different from classical Azalanstat-like compounds, in which the imidazole ring is substituted at the 2-, 4-, and 5-positions.