Characterization of a novel p110β-specific inhibitor BL140 that overcomes MDV3100-resistance in castration-resistant prostate cancer cells

Background: Our previous studies demonstrated that the class IA PI3K/p110 beta is critical in castration-resistant progression of prostate cancer (CRPC) and that targeting prostate cancer with nanomicelle-loaded p110 beta-specific inhibitor TGX221 blocked xenograft tumor growth in nude mice, confirming the feasibility of p110 beta-targeted therapy for CRPCs. To improve TGX221's aqueous solubility, in this study, we characterized four recently synthesized TGX221 analogs. Methods: TGX221 analog efficacy were examined in multiple prostate cancer cell lines with the SRB cell growth assay, Western blot assay for AKTphosphorylation and cell cycle protein levels. Target engagement with PI3K isoforms was evaluated with cellular thermal shift assay. PI3K activity was determined with the Kinase-Glo Plus luminescent kinase assay. Cell cycle distribution was evaluated with flow cytometry after propidium iodide staining. Results: As expected, replacing either one of two major functional groups in TGX221 by more hydrophilic groups dramatically improved the aqueous solubility (about 40-fold) compared to TGX221. In the CETSA assay, all the analogs dramatically shifted the melting curve of p110 beta protein while none of them largely affected the melting curves of p110 alpha, p110 gamma, or Akt proteins, indicating target-specific engagement of these analogs with p110 beta protein. However, functional evaluation showed that only one of the analogs BL140 ubiquitously inhibited AKT phosphorylation in all CRPC cell lines tested with diverse genetic abnormalities including AR, PTEN, and p53 status. BL140 was superior than GSK2636771 ( IC50 5.74 vs 20.49 nM), the only p110 beta-selective inhibitor currently in clinical trials, as revealed in an in vitro Kinase-Glo assay. Furthermore, BL140 exhibited a stronger inhibitory effect than GSK2636771 on multiple CRPC cell lines including a MDV3100-resistant C4-2B cell subline, indicating BL140 elimination of MDV3100 resistance. Mechanistic studies revealed that BL140 blocked G(1) phase cell cycle entry by reducing cyclin D1 but increasing p27(kip1) protein levels. Conclusion: These studies suggested that BL140 is a promising p110 beta-specific inhibitor with multiple superb properties than GSK2636771 worthy for further clinical development.