In the article by Roberts and colleagues (1), the authors demonstrated that inhibition of protein phosphatase 2A (PP2A) was required for survival of myeloid cells expressing mutant KIT. Restoration of PP2A activity by FTY720 (fingolimod; Novartis) was cytotoxic to these cells. The U.S. Food and Drug Administration (FDA) has just approved fingolimod as treatment for relapsing forms of multiple sclerosis (MS) at 0.5 mg daily dose based on the results of 2 phase 3 clinical trials (2, 3). In view of the FDA approval and the work by Roberts and colleagues, there is clear potential for FTY720 as a therapeutic agent for KIT-expressing tumors.
Our group focuses on preclinical and clinical development of new therapeutic approaches for patients with gastrointestinal stromal tumor (GIST), a sarcoma driven by KIT gain-of-function mutations. The standard-of-care for patients with advanced GIST is imatinib mesylate but most patients develop resistance after about 2 years (4). As such, we believe that dephosphorylation and inactivation of KIT induced by PP2A activation offers an attractive therapeutic strategy for patients with GIST. However, we are cautiously optimistic about the clinical applicability of FTY720 as proposed by Roberts and colleagues. Specifically, the IC50 of FTY720 for murine FDC-P1 cells expressing the imatinib-resistant D816V mutant KIT was found to be 2.4 μmol/L (737.9 ng/mL). Similarly, the IC50 ranged between 3.4 and 4.9 μmol/L in human leukemia cell lines expressing mutant KIT. Although effective in vitro, these concentrations exceed that obtained in clinical trials and would require higher oral dosing than has been reported. In a pooled pharmacokinetic analysis of the phase III studies for MS the average steady-state plasma concentrations of FTY720 at doses of 0.5 and 1.25 mg were 2 to 3 and 5 to 6 ng/mL, respectively (2, 3).
This prompted us to evaluate FTY720 in a panel of previously-described GIST cells (Table 1) harboring imatinib-sensitive and -resistant KIT mutations (5). Although FTY720 was significantly cytotoxic, achieving 80% to 95% inhibition in all GIST cell lines, the concentration required for antitumor efficacy was 10 μmol/L. The IC50 of FTY720 at 72 hours ranged between 2.9 and 3.9 μmol/L in GIST cells.
Cytotoxicity of FTY720 in human GIST cells
| GIST cell line . | KIT mutation(s) . | FTY720 ID50a, μmol/L . |
|---|---|---|
| GIST-T1 | V560-Y579del | 3.8 ± 0.8 |
| GIST882 | K642E | 2.9 ± 0.9 |
| GIST48IM | V560D, D820A | 3.4 ± 0.8 |
| GIST430IM | V560-L576del, V654A | 3.9 ± 0.8 |
| GIST cell line . | KIT mutation(s) . | FTY720 ID50a, μmol/L . |
|---|---|---|
| GIST-T1 | V560-Y579del | 3.8 ± 0.8 |
| GIST882 | K642E | 2.9 ± 0.9 |
| GIST48IM | V560D, D820A | 3.4 ± 0.8 |
| GIST430IM | V560-L576del, V654A | 3.9 ± 0.8 |
aConcentration of FTY720 required to reduce GIST cell viability by 50% at 72 hours, calculated using nonlinear regression equation for log(inhibitor) versus normalized response, Y = 100/(1+10⁁((X-LogIC50).
In summary, we feel that PP2A-reactivation is a promising approach in GIST but will require higher doses of FTY720 than that observed in patients with MS. Although we hope that this agent will prove active in the treatment of patients with GIST, we also hope that these issues are considered in the design of clinical trials for patients with GIST.
See the Response, p. 2404
Acknowledgments
GIST-T1 cells were kindly provided by Drs. Andrew Godwin and Takahiro Taguchi, GIST882 by Dr. Jonathan Fletcher, and GIST48IM and GIST430IM by Dr. Anette Duensing.
