Antitumor Activity And Molecular Mechanism Of Action Of The PI3K And HDAC Dual Inhibitor CUDC-907 In Prostate Cancer

Prostate cancer is one of the most commonly diagnosed cancer in men.The androgen receptor(AR)signaling pathway is critical for the growth and survival of prostate cancer cells,including those of lethal castrate-resistant prostate cancer(CRPC),demonstrating that AR is a valid therapeutic target for treating prostate cancer.Accordingly,androgen-deprivation therapy has been the standard treatment for patients with metastatic disease,and the next-generation AR-targeting inhibitors,such as abiraterone and enzalutamide,have been the standard of care for patients with CRPC.Although the AR-targeted therapies prolong the median survival of patients with CRPC,resistance to these AR-targeting therapies emerges and represents a great challenge for the treatment of these patients,highlighting the need for new therapies.The PI3 K pathway plays an important role in cancer cell proliferation,growth,and survival and is frequently activated in different types of cancer,including prostate cancer.In addition,aberrant activation of the PI3 K pathway is associated with poor prognosis,metastasis,and resistance to therapy in prostate cancer patients.Although small molecule inhibitors targeting the PI3 K pathway have demonstrated promising preclinical activity,clinical efficacy has been limited.These disappointing results are likely due to cross-talk between the PI3 K pathway and other survival pathways,suggesting that PI3 K inhibitors must be used in combination with inhibitors targeting other survival pathways to ensure successful eradication of cancer cells.Previous studies have shown that histone deacetylases(HDACs)are upregulated in prostate cancer and their expression levels correlate positively with Gleason scores and cell proliferation.Preclinical studies have revealed that combination of PI3 K inhibitors with HDAC inhibitors(HDACIs)show promising antitumor activityagainst solid tumors,including prostate cancer.As a result,CUDC-907,an orally available small molecule that dually inhibits HDAC and PI3 K enzymes,was rationally designed and synthesized.CUDC-907 has demonstrated promising antitumor activity in multiple preclinical models of both solid tumors and hematological malignancies and clinical efficacy in relapsed/refractory lymphoma and multiple myeloma.Furthermore,CUDC-907 is being tested in Phase I and II clinical trials for the treatment of multiple myeloma,lymphoma,and advanced/relapsed solid tumors.In this study,we investigated the antitumor activity and underlying molecular mechanisms of CUDC-907 in prostate cancer cell lines in vitro and castration-resistant LuCaP 35 CR patient-derived xenograft(PDX)mouse model in vivo.First,we tested the antitumor activity of CUDC-907 in prostate cancer cell lines in vitro and in the castration resistant LuCaP 35 CR PDX mouse model in vivo.We found that CUDC-907 treatment significantly decreased viable prostate cancer cells,determined by SRB assay.CUDC-907 treatment also induced apoptosis of prostate cancer cells determined by Annexin V/PI staining and flow cytometry analysis.Furthermore,CUDC-907 treatment also induced DNA damage in prostate cancer cells determined by alkaline comet assays.When tested in vivo,CUDC-907 treatment significantly inhibited tumor growth in the castration resistant LuCaP 35 CR PDX mouse model.These results demonstrate that CUDC-907 has excellent anti-prostate cancer activity both in vitro and in vivo.Next,we investigated the molecular mechanisms underlying the antitumor activity of CUDC-907 in prostate cancer cell lines.As a PI3 K and HDAC dual inhibitor,CUDC-908 indeed caused inactivation of both PI3 K and HADCs.However,it increased the protein levels for p-ERK1/2,indicating that CUDC-907 treatment caused ERK activation,which may represent a potential resistance mechanism to CUDC-907 treatment in prostate cancer.Indeed,ERK-selective inhibitor SCH772984 not only abolished CUDC-907-induced upregulation of p-ERK1/2,but also enhanced the antitumor activity of CUDC-907.Results from additional studies revealed thatCUDC-907 downregulates Mcl-1,Bcl-xL and c-Myc,and increases Bim.Ectopic overexpression of Mcl-1,Bcl-xL,c-Myc or shRNA knockdown of Bim partially rescued cells from CUDC-907-induced apoptosis,demonstrating that these proteins play important roles in CUDC-907-induced cell death.Further studies on the mechanism of DNA damage induced by CUDC-907 revealed that the drug downregulates DNA damage response proteins Wee1,CHK1,and ribonucleotide reductase(RR)subunits RRM1 and RRM2.Overexpression of Wee1,CHK1,RRM1 or RRM2 not only significantly attenuated DNA damage induced by CUDC-907,but also rescued prostate cancer cells from CUDC-907-induced apoptosis.These results demonstrate that CUDC-907 down-regulates Wee1,CHK1,RRM1,and RRM2,leading to DNA damage and apoptosis.In summary,our results demonstrate that CUDC-907 induces apoptosis along with DNA damage in prostate cancer cells and shows potent antitumor activity in the LuCaP 35 CR PDX model.CUDC-907-induced apoptosis is partially dependent on Mcl-1,Bcl-xL,Bim,Wee1,CHK1,RRM1,RRM2 and c-Myc.Furthermore,downregulation of Wee1,CHK1,RRM1 and RRM2 contributes to CUDC-907-induced DNA damage and apoptosis,as well.These findings support the clinical development of CUDC-907 for the treatment of prostate cancer.