PP2A Activating Drugs (PAD): Anti-Leukemic and Non-Toxic Activity of Two Novel and Non-Immunosuppressive FTY720 Derivatives

Abstract 2901

FTY720 is a sphingosine analog proposed by the FDA for treating Multiple Sclerosis patients because of its immunosuppressive activity, which depends on its ability to prevent lymphocyte egress into the peripheral blood. To act as an immunosuppressive drug, FTY720 undergoes sphingosine kinase 2 (SPHK2) phosphorylation and internalization upon interaction with the sphingosine-1-phosphate receptor 1 (S1PR1). FTY720 also acts as a potent activator of protein phosphatase 2A (PP2A), a tumor suppressor found inactivated in chronic and blast crisis CML with wild type or imatinib/dasatinib-resistant BCR-ABL1, Ph⁺ B-ALL, Kit^D816V AML, Jak2^V617F+ MPDs and other leukemias/lymphomas. FTY720 treatment of cell lines and primary progenitors isolated from bone marrow of patients with these malignancies, markedly suppressed leukemic cell proliferation/survival and induced apoptosis in a PP2A-dependent manner. Notably, long-term treatment with FTY720 of mice carrying these hematopoietic malignancies significantly prolonged survival and restored normal myelopoiesis without exerting any toxic effect in hematopoietic and non-hematopoietic organs. However, in vivo administration of FTY720 strongly, albeit reversibly, decreases the number of circulating B and T lymphocytes.

Here we report that a synthetically phosphorylated FTY720 (FTY720-P) is unable to induce neither PP2A activation nor apoptosis of BCR-ABL1-, Jak2^V617F-, or Kit^D816V-expressing myeloid precursors, indicating that FTY720 phosphorylation is dispensable for its anti-leukemic activity. Thus, we functionally characterize two FTY720 derivatives, QC-FTYSM and OSU-2S, which were synthesized as molecules unable to undergo SPHK2 phosphorylation. Treatment (2.5 uM; 24h) of FTY720-sensitive 32D-BCR/ABL cells with QC-FTYSM and OSU2S results in ∼80% and 40%, respectively, more efficient suppression of BCR-ABL1 expression and kinase activity than that observed with FTY720. Moreover, QC-FTYSM, OSU-2S and FTY720 (2.5uM; 0–60h) induce a progressive block of proliferation and marked induction of apoptosis of 32D-BCR/ABL cells. In fact, a 96%, 98%, and 79% decrease in viability is observed after treatment with QC-FTYSM, OSU-2S and FTY720, respectively. Notably, viability of non-transformed myeloid 32Dcl3 cells is not significantly affected by treatment with FTY720 or its derivatives. Consistent with the ability of FTY720 to induce apoptosis through rescue of PP2A activity, phosphatase assays show identical ability of FTY720, QC-FTYSM and OSU-2S to restore PP2A functionality. In fact, comparable and marked decrease in the amount of inactive Y307-phosphorylated PP2Ac was detected in 32D-BCR/ABL cells treated with FTY720 or its derivatives. To formally demonstrate that QC-FTYSM and OSU-2S lack immunosuppressive activity, we first assessed their ability to be internalized upon interaction/association with the S1PR1 receptor. Thus, cells were transduced with a GFP-tagged S1PR1 and treated with either QC-FTYSM, OSU-2S, or, as positive control, FTY720-P. Confocal microscopy revealed that treatment FTY720-P resulted in a strong S1PR1 internalization. Conversely, exposure of the cells to QC-FTYSM and OSU-2S did not alter the S1PR1 membrane localization, indicating that these molecules did not undergo SHPK2 phosphorylation. Further demonstration of the inability of these compounds to act as immunosuppressive molecules was gained upon in vivo administration of a single dose of FTY720, QC-FTYSM or OSU-2C (10 mg/kg) to wild type FVB/N mice. As expected, percentage of B220+/CD19+ circulating B-cells decreased of ∼90% in FTY720-treated animals. Conversely, the percentage of B-cells after exposure to QC-FTYSM and OSU-2S remained unchanged (≤ 1% decrease). Likewise, the number or CD4+ and CD8+ cells also was not affect by treatment with the QC-FTYSM compound. Note that effect of OSU-2S on T-cells and the toxicity profile and anti-leukemic activity of these drugs in healthy animals and mouse models of deadly leukemias (e.g. T315I+ and blast crisis CML and Ph+ ALL) as well as Ph⁻ MPDs are currently being assessed.

Altogether our data indicate that QC-FTYSM and OSU-2S represent two potentially powerful and safe drugs which could be introduced in the current therapeutic protocols for different types of hematopoietic and non-hematopoietic malignancies characterized by functional inactivation of the PP2A tumor suppressor.