Initially, 10 pharmacophore models were generated for each target, CXCR4 and CXCL12. During model construction, LigandScout ensured that discarded molecules could not geometrically match the query. Initial alignments were performed using a sophisticated pattern-matching method that allowed flexible inclusion of multiple pharmacophoric features. A database molecule was considered a hit only when all mapped features fell within the tolerance spheres of the corresponding query features. This study used the merged-feature pharmacophore alignment approach, with scoring based on pharmacophore fit and atomic overlap. To assess the structural contributions of active compounds, decoys, and test molecules, all structures were geometrically optimized using MMFF94 until the minimum potential energy was reached, yielding the most stable theoretical conformations (
28).
The 10 generated pharmacophore models were validated to select the optimal models. Internal validation assessed model consistency, whereas external validation evaluated generalizability to compounds beyond the training set (
16). The requirements for each validation parameter included TPR > 0.5, TNR > 0.5, ACC > 0.5, PPV > 0.5, NPV > 0.5, FPR < 0.5, FNR > 0.5, MR > 0.5, GH 0.7 - 0.8, AUC
100% ≥ 0.7, and EF
1%, with higher values considered better (
19).
Model 8 was identified as the most robust pharmacophore model for CXCR4, whereas model 3 showed the highest performance for CXCL12. Pharmacophore models can effectively filter large compound libraries to identify hits with features predictive of target activity. Because pharmacophores represent chemical activities and spatial interactions rather than chemical groups, hits are often structurally distinct, which is advantageous for scaffold hopping (
28). Compounds D41-D51 and D53-D57 met the screening criteria, with higher pharmacophore-fit scores indicating better alignment.
The RMSD values obtained from docking protocol validation were below 2 Å, indicating excellent agreement with the crystallographic positions of the ligands and supporting the suitability of the method for ligand screening (
29). Docking against CXCR4 and CXCL12 showed that D54 and D57 had more negative predicted docking scores than the other compounds and the reference ligands, suggesting more favorable predicted binding (
20). The CXCR4-D54 and CXCR4-D57 complexes interacted with all key CXCR4 residues reported as targets for anti-metastatic drugs: Trp94, Asp97, Trp102, Val112, Tyr116, Arg183, Ile185, Cys186, Asp187, and Glu288 (
14). The test ligands formed more extensive predicted interactions with CXCR4 than mavorixafor, which did not interact with Arg183. Meanwhile, D54, D57, and LIT-927 interacted with all key CXCL12 residues, including Glu15, Val18, Ala19, Asn22, Leu42, Asn45, Arg47, and Val49 (
15). These protein-ligand interactions suggest that D54 and D57 have more favorable predicted docking scores and binding interactions than the reference compounds.
MD simulations were used to investigate the time-dependent behavior of biomolecular systems, in which proteins, ligands, and solvents have full conformational flexibility (
30). MM-PBSA analysis showed that D54 and D57 had more negative predicted binding free energies than the reference ligands for both CXCR4 and CXCL12.
The backbone RMSD values consistently converged below 2 Å during MD simulations of the CXCR4-mavorixafor, CXCR4-D54, CXCR4-D57, CXCL12-LIT-927, CXCL12-D54, and CXCL12-D57 complexes. These results indicate an absence of substantial backbone deviations during the MD simulations and demonstrate the overall structural stability of the protein-ligand complexes under the simulated conditions. Among the residues in the active sites of the CXCR4 complexes, Trp94, Asp97, Trp102, Val112, Tyr116, Arg183, Ile185, Cys186, Asp187, and Glu288 showed low RMSF values (< 2 Å), indicating low flexibility in the binding pocket (
31). The CXCL12 complexes showed a comparable pattern, with critical interacting residues such as Glu15, Val18, Ala19, Asn22, Leu42, Asn45, Arg47, and Val49 showing smaller fluctuations than surrounding residues. These observations suggest that ligand binding can stabilize the local microenvironment and restrict residue movement within the binding pocket. This behavior is consistent with the interactions observed in both the MD and docking studies. However, the absence of apo (ligand-free) protein simulations is a limitation of this study. To partially address this limitation, reference ligand-protein complexes were included as comparative benchmarks for evaluating the stability of the simulated complexes.
Rg was used to assess molecular compactness and flexibility during the MD simulations. Stable Rg values without a significant increase over time were observed, indicating no structural expansion or unfolding and supporting cohesive, conformationally stable complexes (
32). The Rg values of D54 and D57 were comparable to those of the reference ligands, mavorixafor and LIT-927, suggesting that neither compound caused substantial conformational changes in CXCR4 or CXCL12. Although D57 showed a slightly higher Rg in one CXCL12 simulation (13.58 ± 0.61 Å), the variation overlapped with that of the reference ligand and did not indicate systematic or sustained expansion of the protein structure.
SASA was calculated to evaluate solvent accessibility and complex stability. In the CXCR4 and CXCL12 systems, D54 and D57 had SASA values similar to those of the reference ligands, suggesting that both compounds maintained complex stability comparable to that of the reference ligands under the simulated conditions. Higher SASA indicates greater solvent exposure, whereas lower values indicate deeper sequestration within the protein (
32). The lower SASA values in the CXCL12 complexes reflect the smaller molecular size of CXCL12 compared with CXCR4.
H-bond occupancy analysis further supported the stability of the protein-ligand complexes. Persistent H-bonds contribute to maintaining spatial configuration throughout a simulation (
33). D54, D57, and the reference drugs occupied several active-site residues in CXCR4 and CXCL12, although variability was observed between replicates. These residues included Trp94, Asp97, Arg183, Asp187, and Glu288 in CXCR4 and Glu15, Val18, Ala19, Asn22, Asn45, Arg47, and Val49 in CXCL12. These findings indicate stable ligand retention within the active sites and support the prioritization of D54 and D57 as lead scaffolds for further investigation.
The toxicity and pharmacokinetic profiles of D54 and D57 demonstrated both potential and limitations as oral drug candidates. Both compounds were predicted to be free of hepatotoxic, mutagenic, and cytotoxic risks (
26). However, D54 was predicted to exhibit immunotoxicity, whereas D57 showed a positive carcinogenicity prediction, indicating potential safety concerns that require further experimental validation. Despite low solubility and Caco2 permeability, both compounds were predicted to have high intestinal absorption, suggesting that absorption may occur through a transport-mediated mechanism. However, D57, as a P-glycoprotein substrate, may have limited bioavailability relative to D54. The low VDss (< -0.15) indicates that D54 and D57 are predicted to remain more in plasma than in tissues. Moderate BBB and CNS permeability suggest that these compounds may enter the brain without excessive accumulation, supporting their potential to reach metastatic targets. However, violations of Lipinski's rule, particularly the high molecular weight and lipophilicity of D57, together with the predicted toxicity liabilities, may limit their suitability as drug candidates and warrant further optimization and experimental validation (
27). Furthermore, the findings of this study are based solely on computational approaches and have not been experimentally validated. Further studies should therefore include cytotoxicity testing and evaluation of anti-metastatic activity using Western blot analysis of the CXCR4/CXCL12 signaling pathway and wound-healing assays to validate the predicted biological activity of D54 and D57.
5.1. Conclusions
In conclusion, D54 and D57 emerged as promising lead compounds from computational screening targeting the CXCR4/CXCL12 axis in lung cancer. Their structural frameworks and favorable computational binding profiles support their potential as lead compounds. However, the predicted immunotoxicity of D54, the predicted carcinogenicity of D57, and overall pharmacokinetic and drug-likeness limitations indicate that further structural optimization and experimental validation, including cytotoxicity testing, Western blot analysis, and wound-healing assays, are required before their therapeutic potential can be fully established.