A Predictive HQSAR Model for a Series of Tricycle Core Containing MMP-12 Inhibitors with Dibenzofuran Ring

MMP-12 is a member of matrix metalloproteinases (MMPs) family involved in pathogenesis of some inflammatory based diseases. Design of selective matrix MMPs inhibitors is still challenging because of binding pocket similarities among MMPs family. We tried to generate a HQSAR (hologram quantitative structure activity relationship) model for a series of MMP-12 inhibitors. Compounds in the series of inhibitors with reported biological activity against MMP-12 were used to construct a predictive HQSAR model for their inhibitory activity against MMP-12. The HQSAR model had statistically excellent properties and possessed good predictive ability for test set compounds. The HQSAR model was obtained for the 26 training set compounds showing cross-validated q 2 value of 0.697 and conventional r 2 value of 0.986. The model was then externally validated using a test set of 9 compounds and the predicted values were in good agreement with the experimental results (r pred 2 = 0.8733). Then, the external validity of the model was confirmed by Golbraikh-Tropsha and r m 2 metrics. The color code analysis based on the obtained HQSAR model provided useful insights into the structural features of the training set for their bioactivity against MMP-12 and was useful for the design of some new not yet synthesized MMP-12 inhibitors.


Introduction
Matrix metalloproteinases (MMPs) family enzymes can degrade extracellular matrix components by their proteolytic activity which depends on catalytic zinc ion [1]. The main role of macrophage metalloelastase (MMP-12) is degradation of elastin. Furthermore, MMP-12 is an interesting therapeutic target overexpressed in inflammatory pathological conditions (such as respiratory system diseases including asthma and chronic obstructive pulmonary disorder (COPD)) [2]. Effectiveness of MMP-12 inhibitors in reducing inflammation in respiratory system has been shown [3,4].
The active site is highly conserved among MMPs with the exception of a loop region called S1 . S1 pocket in MMPs active sites varies slightly among MMPs in both sequence and structure [5]. Despite available structural information, still the lack of selectivity remains as a main challenge for successfulness of MMPs inhibitors in clinical trials. Furthermore, intrinsic flexibility of MMPs active sites makes MMPs active site analysis more complicated [6,7]. Therefore, in this study, a ligand based approach was used to modify the side chain in a series of MMP-12 inhibitors. HQSAR (hologram quantitative structure activity relationship) is a method for QSAR (quantitative structure activity relationship) studies whose reliability has been established [8]. In the present study, a HQSAR study on a series of tricycle cores containing MMP-12 inhibitors was carried out.

Obtaining Biological Data and Generation of Molecular
Structures. The structures of 35 MMP-12 inhibitors and their biological activities for inhibition of MMP-12 were taken from the literatures ( Figure 1 and Table 1) [9,10]. As the 2 International Journal of Medicinal Chemistry  After generation of descriptors, partial least square (PLS) methodology was used to find the possible correlation between dependent variable (−pIC 50 ) and independent variable (descriptors generated by HQSAR structural features). LOO (leave-one-out) cross-validation method was used to determine the predictive value of the model. Optimum number of components was found out using results from LOO calculations. At this step, 2 and standard error obtained from leave-one-out cross-validation roughly estimate the predictive ability of the model. This cross-validated analysis was followed by a non-cross-validated analysis with the calculated optimum number of principle components. Conventional correlation coefficient 2 and standard error of estimate (SEE) indicated the validity of the model. The internal validity of the model was also tested by -randomization method [11]. In this test, the dependent variables are randomly shuffled while the independent variables (descriptors) are kept unchanged. It is expected that 2 and 2 calculated for these random datasets will be low. Finally, a set of compounds (which were not present in model development process) with available observed activity were used for external validation of the generated model. Predictive 2 ( 2 pred ) value was calculated using PRESS: sum of the squared deviation between predicted and actual pIC 50 for the test set compounds; SD: sum of the squared deviation between the actual pIC 50 values of the compounds from the test set and the mean pIC 50 value of the training set compounds.
The external validity of the model was also evaluated by Golbraikh-Tropsha [12] method and 2 [13] metrics. For an acceptable QSAR model, the value of "average 2 " should be >0.5 and "delta 2 " should be <0.2. The applicability domain of the generated model was evaluated for both test and prediction sets by Euclidean based method. It calculates a normalized mean distance score for each compound in training set in range of 0 (least diverse) to 1 (most diverse). Then, it calculates the normalized mean distance score for compounds in an external set. If a score is outside the 0 to 1 range, it will be considered outside of the applicability domain. The external validity tests (Golbraikh-Tropsha and Rm 2 ) and applicability domain test were done using tools available at http://dtclab.webs.com/software-tools.

HQSAR Model Predictivity.
The statistics for developed HQSAR model were shown in Table 2. The statistical parameters, 2 , 2 , SEE, and 2 pred , showed the validity of our model. The best hologram model was generated using histogram length of 199 having six optimum components. Descriptors used for model generation were atoms, connections, and hydrogen atoms. The best generated model had crossvalidated 2 of 0.697 and non-cross-validated 2 value of 0.986 with a standard error of 0.93. The total collection of the generated models for various histogram lengths comprises ensemble, and the ensemble value for 2 was found to be 0.528. The -randomization results indicated that the calculated 2 Table 1 and the experimental pIC 50 against the values predicted by the HQSAR models are plotted ( Figure 2).

HQSAR Atomic Contribution
Plot. The generated model can be accessed through atomic contribution plot. The various colors of each atom correspond to various degrees of contribution towards the overall biological activity. Red, red orange, and orange depicted that the color belonging atoms were contributing negatively to the generated HQSAR model while colors reflecting yellow, green, and green blue were contributing positively to the model. Intermediate contributions were reflected by gray atom. The maximum common substructure was shown in cyan. Figure 3 depicts the contribution of the most potent compound 20 as well as compound 19.

Prediction Set (Design of New Virtual Compounds).
This work allowed prediction of the activity of a set shown in Table 3 (not yet synthesized molecules   compounds in inhibition of MMP-12. In Table 3, the docking scores of the 3 new designed compounds and 3 molecules from train set were reported. The binding positions of all new compounds were inspected for their binding conformation and interactions in MMP-12 active site. For n3, 2D diagram of ligand-receptor interaction was presented (Figure 4(c)). The various heterocyclic rings substituted on the dibenzofuran scaffold do not seem to have strong interactions with the binding pocket of MMP-12 as it was suggested previously by X-ray crystallography [9]. However, if they have undesired properties they cannot fit in the narrow deep S1 pocket of MMP-12. On the other hand, they can induce steric hindrance that prevents other parts of the molecule to have strong interactions with residues in the binding pocket. The conformation of the heterocyclic ring upon ligand binding is demonstrated in Figure 5.

Discussion
We successfully developed a HQSAR model for prediction of some MMP-12 inhibitors with good internal and external validity. Subsequently, the model was used to predict the activity of new MMP-12 inhibitors. The binding energy of new not yet synthesized molecules was evaluated by molecular docking.
Crystal structures have provided useful information for developing selective inhibitors toward particular MMPs including MMP-12. The segment 241-245 of MMPs (MMP-1 numbering) has the highest sequence variability among the various MMP enzymes and could be a target for designing selective inhibitors. However, this segment is very flexible which makes the molecular modeling predictions using 3D structures of MMPs inaccurate [14,15]. Only some small differences in the sizes of hydrophobic side chains were seen. For example, Val 235 in MMP-12 is replaced by Leu214 in MMP-13 which makes the MMP-13 binding pocket smaller and more hydrophobic. The series of MMP-12 inhibitors employed in this study had carboxylic acid zinc binding group. Changing the R group that was placed in hydrophobic pocket of MMP-12 active site altered the potency and selectivity of these inhibitors. We modified this R group for fine tuning and designed new compound with promising MMP-12 inhibitory activity.
In the present study, we used HQSAR approach for a set of MMP-12 inhibitors. Contribution plot (Figure 3) showed that the green aromatic carbon was contributing positively to the model. Oxygen atom in furan ring was depicted by green or yellow and was contributing to the biological activity. Hydrogen molecules were rendered green, yellow, or white indicating that they showed intermediate contribution.
The bulky group (methyl) was green indicating that it was contributing positively to the generated model, and it can be explained from the example that compound 20 was showing high potency. The developed model was used for the design of 5 new molecules. Overall docked conformation of the training set of inhibitors and new compounds in MMP-12 active site was similar to one determined by crystallography. Among new designed compounds, compounds n1, n2, and n3 had low SEP and their predicted activities were more reliable. Furthermore, compound n3 has the lowest docked energy.

Conclusion
In summary, we have developed a reliable HQSAR model for a series of tricycle cores containing MMP-12 inhibitors with dibenzofuran ring using activity data reported earlier [9,10]. We used HQSAR analysis to design new not yet synthesized potent MMP-12 inhibitors. Their binding energies were evaluated by docking studies but for further validation it needs synthesize of the proposed new compounds and subsequent enzyme inhibition study.

Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.