Objective:

Using TCGA database, we had demonstrated that aberrantly activated Forkhead box M1 (FOXM1) correlates to worse overall survival in a subgroup of platinum resistant patients. Application of thiostrepton, a natural thiazole antibiotics that inhibits FOXM1 transcription activity, improves the cytotoxic effect of paclitaxel and cisplatinum in ascites cells from platinum-resistant patients. However, the use of thiazole antibiotics in the clinic is hampered by difficulties in synthesis, degradation potential, and solubility issues. In this study, we aim to identify novel FOXM1 small molecule inhibitors which can be a new class of therapeutic agents to address the challenges in treating chemotherapy resistant EOC.

Methods:

The FOXM1 protein structure was obtained from the Protein Databank (pdb ID: 3GXU) and prepared for docking studies by Autodock Vina 1.0.2. Structural representations of the NCI Diversity Set II in were obtained from the NCI website and converted to a pdb format for screening. Individual pdb files were prepared for docking using the prepare_ligand.py scripts form MGLTools 1.5.4 using the largest non-bonded fragment present. The binding affinity was scored as the free energies of binding in kcal/mol.

Three selected compounds (one as negative control) were applied to SKOV3, OVCAR3 and ES2 cells for 72 hrs and sulfarodamine B assay was used to measure the cell viability and FOXM1 expression level was confirmed by RT-PCR and western blot.

Results:

The FOXM1 structure obtained from 3GXU represented the DNA binding region of FOXM1 and possessed the winged helix fold representative of the Forkhead family of enzymes with two wings in direct contact with DNA. For ease of representation, we described both wings as a dimer and a single wing as a monomer. From this structure, we hypothesized two main models of how thiostrepton binding to FOXM1 could possibly curtail its transcriptional activity. In the first model thiostrepton could bind either of the wings or both wings and prevent association to DNA. In the second model thiostrepton bind the FOXM1: DNA complex and weaken association of FOXM1 to DNA. Subsequently, small molecular inhibitors could also use either of the models to inhibit transcription. To account for both models, the NCI Diversity set was screened against the FOXM1 dimer:DNA complex (39 hits), dimer (11 hits) and monomer (14 hits). Those hits were further classified by chemical structure, biological function and chemical similarities to known molecules that target FOXM1. In cellular cytotoxicity assays, ZINC00990239 showed cytotoxicity with IC50 around 10uM, and downregulation of FOXM1 and transcription of its downstream molecules such as CCNB1.

Conclusion:

By illustrating the characteristics of an effective small molecule inhibitor of FOXM1 should contain, we have taken the first step towards identification of potential compounds with efficacy similar to thiostrepton.

Note: This abstract was not presented at the meeting.

Citation Format: Yi Chen, Eliza A. Ruben, Marcia M. Bieber, Jayakumar Rajadas, Nelson N.h Teng. In silico investigation of FOXM1 binding and novel inhibitors in epithelial ovarian cancer (EOC). [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5373. doi:10.1158/1538-7445.AM2014-5373