Despite recent developments in platinum-based combination chemotherapy, patients with CCC of the ovary, especially in advanced stage or recurrent disease, have a worse progression free survival and overall survival when compared with patients with a serous histology (5
). Therefore, to improve survival, new strategies are necessary to more effectively treat CCC.
In the present study, we observed activation of mTOR in 86.6% of CCC of the ovary (). Importantly, the frequency of strong phospho-mTOR immunoreactivity in CCCs was significantly higher than that found in SACs, indicating that CCCs are more strongly dependent on mTOR signaling for tumor progression than are SACs. In addition, mTOR was frequently activated in both stage III-IV CCCs (76%) and stage I-II CCCs (96%). Therefore, mTOR appears to be a promising target for the treatment of patients with both early and advanced stage CCC. In contrast, phospho-mTOR expression was uncommon in early stage SACs but was significantly increased in advanced stage SACs. The very high frequency of mTOR activation observed in early stage CCCs suggests that hyperactivation of mTOR kinase is an early event in the development of CCCs. This is noteworthy in light of the fact that activated AKT/mTOR signaling has been reported in ovarian endometriosis, from which CCC is thought to arise (24
). We have recently demonstrated that the mTOR inhibitor RAD001 markedly inhibited tumor onset and progression in a transgenic mouse model of ovarian cancer that develops ovarian SACs with activated AKT/mTOR signaling. Thus, mTOR might be a reasonable target for the chemoprevention of CCC in patients with ovarian endometriosis.
Our data demonstrate that treatment with RAD001 effectively attenuates the phosphorylation of p70S6K in vitro
and markedly inhibits the proliferation of ovarian CCC cells. There exists a concern in inhibiting mTOR, in that mTOR inhibition may trigger a feedback mechanism that activates AKT to potentially promote tumor growth and may consequently reduce the anti-tumor effect of mTOR inhibitors (32
). Although such a feedback has been observed in several cancer cell types including breast cancer (32
), rhabdomyosarcoma (33
), non-small cell lung cancer (34
), and multiple myeloma (35
), in the current study treatment with RAD001 did not induce activation of AKT in ovarian CCC cells ().
We also evaluated the efficacy of RAD001 in vivo, employing s.c. xenograft models (). In mice inoculated s.c. with RMG1 or KOC7C cells, treatment with RAD001 significantly inhibited tumor growth. Moreover, orally administrated RAD001 in our treatment schedule was well tolerated. Taken together, these findings indicate that RAD001 could have significant anti-tumor effects as a single agent for CCC in a setting of front-line therapy.
An additional important finding in our study is the anti-tumor activity of RAD001 in cisplatin-resistant CCC. In general, patients with platinum-resistant recurrent epithelial ovarian cancer have been treated with anti-neoplastic agents that do not exhibit cross-resistance with platinum-agents. However, these patients have dismal prognosis, with overall response rate ranging from 9% to 33% (41
). Unfortunately, the prognosis of patients with cisplatin-resistant CCCs is even worse. For example, in one study, the response rate for salvage chemotherapy for cisplatin-resistant CCC was only 1% (13
), indicative of the urgent need of new treatment strategies for recurrent CCC of the ovary.
In this study, we found that cisplatin-resistant CCC cell lines exhibit enhanced phospho-mTOR expression compared to the corresponding cisplatin-sensitive parental cell lines (). The increased phospho-mTOR expression was associated with increased activation of AKT. The involvement of AKT in the resistance to cisplatin has been reported previously (39
). Although we and others have previously reported that inhibition of AKT activity sensitizes human ovarian cancer cells to conventional anticancer agents such as cisplatin (39
) and paclitaxel (43
), there are concerns associated with inhibiting AKT, because AKT also mediates certain biologically important cell processes such as glucose metabolism (44
). Thus, a safer approach may be to target downstream therapeutic effectors such as mTOR. Interestingly, our cisplatin-resistant CCC cells showed significantly higher sensitivity to RAD001 in vitro
, compared with the respective cisplatin-sensitive parental cell lines. Furthermore, the in vivo
anti-tumor effect of RAD001 was also greater in cisplatin resistant cell-derived tumors than in cisplatin sensitive cell-derived tumors (). It has been previously reported that AKT activation may be a biomarker to predict the sensitivity to mTOR inhibitors (20
). Although AKT activation is not the sole determinant of sensitivity to mTOR inhibition (26
), our results indicate that enhanced sensitivity to mTOR inhibitors in cisplatin-resistant CCC cells is associated with, at least in part, the activation of AKT/mTOR signaling. Since the RMG1-CR and KOC7C-CR cells used in this study mimic the clinical situation of resistance development in cisplatin-treated patients, our results may suggest that a mTOR inhibitor might have efficacy for the clinical management of cisplatin-resistant CCCs.
We should note, however, that a potential limitation of our experimental design is the use of a subcutaneous xenograft model. Peritoneal dissemination is the main process involved in the progression in human ovarian cancer. Thus, intra-peritoneal injection of cancer cells would more accurately model advanced disease. Therefore, further investigation using an intra-peritoneal model or a genetically engineered mouse model of ovarian cancer would be helpful.
Our results indicate that RAD001 is a promising agent for the treatment of CCC of the ovary both as a front-line treatment and as a salvage treatment for recurrence after platinum-based chemotherapy. A recent phase III study demonstrated that RAD001 had significant activity in some patients with advanced renal cell carcinoma (45
). For patients with recurrent ovarian cancer, the Southwest Oncology Group (SWOG) will soon initiate a randomized phase II trial of carboplatin and paclitaxel with or without everolimus in patients with ovarian cancer in first relapse. We believe that our data support the scientific justification or this and future clinical trials with RAD001 in patients with CCC of the ovary, a chemoresistant histological subtype characterized by frequent hyperactivation of mTOR pathway.