Therapeutics, Targets, and Chemical Biology FTY720 (Fingolimod) Sensitizes Prostate Cancer Cells to Radiotherapy by Inhibition of Sphingosine Kinase-1 Dmitri Pchejetski 1,4,5 , Torsten Bohler 2 , Leyre Brizuela 4 , Lysann Sauer 1 , Nicolas Doumerc 3 , Muriel Golzio 4,5 , Vishal Salunkhe 1 , Justin Teissié 4,5 , Bernard Malavaud 3,4,5 , Jonathan Waxman 1 , and Olivier Cuvillier 3,4,5 Abstract Radiotherapy is widely used as a radical treatment for prostate cancer, but curative treatments are elusive for poorly differentiated tumors where survival is just 15% at 15 years. Dose escalation improves local response rates but is limited by tolerance in normal tissues. A sphingosine analogue, FTY720 (fingolimod), a drug cur- rently in phase III studies for treatment of multiple sclerosis, has been found to be a potent apoptosis inducer in prostate cancer cells. Using in vitro and in vivo approaches, we analyzed the impact of FTY720 on sphingo- lipid metabolism in hormone-refractory metastatic prostate cancer cells and evaluated its potential as a radio- sensitizer on cell lines and prostate tumor xenografts. In prostate cancer cell lines, FTY720 acted as a sphingosine kinase 1 (SphK1) inhibitor that induced prostate cancer cell apoptosis in a manner independent of sphingosine-1-phosphate receptors. In contrast, γ irradiation did not affect SphK1 activity in prostate cancer cells yet synergized with FTY720 to inhibit SphK1. In mice bearing orthotopic or s.c. prostate cancer tumors, we show that FTY720 dramatically increased radiotherapeutic sensitivity, reducing tumor growth and metastasis without toxic side effects. Our findings suggest that low, well-tolerated doses of FTY720 could offer significant improvement to the clinical treatment of prostate cancer. Cancer Res; 70(21); 8651–61. ©2010 AACR. Introduction In the western world prostate cancer is now the most commonly diagnosed noncutaneous cancer in men and is the second leading cause of cancer-related death (1). In the United States the lifetime probability of developing prostate cancer is 1 in 6, and it is estimated that 192,280 new cases of prostate cancer were diagnosed during 2009, and there were 27,360 deaths. The management of prostate cancer is complex, but for the majority of patients radiotherapy remains a definitive treatment for early-stage disease. However, up to 85% of pa- tients with poor histologic subtypes of localized prostate cancer relapse (2). In radiotherapy research the current emphasis is on computerized planning to deliver increased dosage schedules. There is an opportunity to approach the subject of dose intensification from a different angle, and this is through the use of radiosensitizer treatment. Prostate cancer cell radioresistance has been linked with sustained activation of sphingosine kinase-1 (SphK1; ref. 3). SphK1 is a lipid converting enzyme responsible for the con- version of sphingosine into sphingosine-1-phosphate (S1P). S1P, and sphingosine and its precursor ceramide are lipid second messengers. In response to various stimuli ceramide and sphingosine mediate cell death, whereas S1P abrogates apoptosis and mediates cell proliferation and migration (4). The SphK1/S1P pathway contributes to cancer progression and leads to increased cell proliferation (5), impairment of apoptosis (6), and oncogenic transformation (7). SphK1 is a tumor-associated enzyme: high levels of SphK1 expression have been shown in various human tumor tissues (8, 9). Fur- thermore high levels of SphK1 expression and activity are as- sociated with a poor prognosis in breast cancer (10), glioma (11), and gastric cancer (12). These findings have highlighted the potential of SphK1 as a therapy target. A sphingosine analogue, dimethylsphingo- sine, was the first reported SphK inhibitor to induce prostate cancer cell apoptosis (3). These findings were further con- firmed by ourselves (13), showing a new selective SphK inhib- itor, SKI-II (first reported by French et al.; ref. 9), to have proapoptotic properties in prostate cancer cells. Work from our group in prostate cancer indicates that inhibition of the SphK1/S1P pathway has a synergistic effect with chemother- apy and has the potential to act as a molecular target for can- cer therapy (14–16). FTY720 is a sphingosine analogue and a potent immuno- suppressive drug that induces lymphopenia via an inhibition Authors' Affiliations: 1 Department of Surgery and Cancer, Imperial College London, London, United Kingdom; 2 INSERM U858/I2MR EQ 10, CHU Rangueil, 3 Hôpital Rangueil, Service d'Urologie et de Transplantation Rénale, 4 CNRS, Institut de Pharmacologie et de Biologie Structurale; and 5 Université de Toulouse, UPS, IPBS, Toulouse, France Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). Corresponding Authors: Dmitry Pshezhetskiy, 807 Cyclotron Building, Hammersmith Hospital, Ducane Road W120NN, London, United Kingdom. Phone: 07518361929; Fax: 02083835830; E-mail: d.pshezhetskiy@ imperial.ac.uk or Olivier Cuvillier, 205 Route de Narbonne, 31077 Toulouse, France. E-mail: cuvillio@me.com. doi: 10.1158/0008-5472.CAN-10-1388 ©2010 American Association for Cancer Research. Cancer Research www.aacrjournals.org 8651 Research. on November 24, 2021. © 2010 American Association for Cancer cancerres.aacrjournals.org Downloaded from Published OnlineFirst October 19, 2010; DOI: 10.1158/0008-5472.CAN-10-1388