PS930 SENSITIZATION OF PH+ ALL RESISTANT TO IMATINIB BY TARGETING SPHINGOLIPID METABOLISM

Field: Pharmacology, Toxicology and Pharmaceutics (Life Sciences)

Background: Acute lymphoblastic leukemia (ALL) is formed by accumulation of lymphoblasts in bone marrow and peripheral blood. The most common subtype of ALL is philadelphia positive ALL (Ph+ ALL) which is characterized by having BCR/ABL protein. For the treatment of Ph+ALL tyrosine kinase inhibitors (TKI) such as imatinib are used. However, 60–75% of the patients can still develop resistance against the TKIs and die within 5 years. Bioactive sphingolipids are a group of lipid family having ceramide as the central molecule, that play roles in many cellular mechanisms including proliferation, apoptosis, inflammation, metastasis. Ceramide is known to induce apoptosis which makes this sphingolipid a useful tool in cancer treatment. Sphingolipids can be generated through different pathways including de novo synthesis, recycling and salvage pathways. Previous studies showed that bioactive sphingolipids and genes in the pathway have been involved in response to TKI treatment in Ph+ALL. Aims: Here, we studied the roles of bioactive sphingolipids on the growth inhibitory effects of imatinib and exploit sphingolipid metabolism by majorly inhibiting glucosylceramide synthase (GCS) to further sensitize cells to imatinib and/or overcome resistance to imatinib in Ph+ ALL. Methods: Imatinib resistant cell line SD‐1R is generated by clonal selection from parental SD‐1 cell line by exposing the cells to gradually increased concentration of imatinib. Intracellular lipid levels were measured by HPLC/MS/MS. Cell viability is detected by MTT and trypan blue exclusion assay. Gene and protein expression levels were detected by real time PCR and western blotting, respectively. Results: We detected that intracellular levels of sphingolipids: ceramide, sphingosine and sphingomyelin is increased in response to imatinib in parental SD‐1 cells but not in newly generated SD‐1R cells. The data showed that these changes are not detectable in SD‐1 cells when imatinib is applied in presence of myriocin which is a palmitoyltransferase inhibitor that blocks the first step of de novo synthesis pathway in sphingolipids suggesting that imatinib induced changes in sphingolipids are due to induction of de novo synthesis pathway and a defect in this pathway is contributing the regulation of imatinib resistance in SD‐1R cells. Interestingly, another sphingolipid hexosylceramide which is a product of further metabolization of ceramide by the enzyme GCS, levels were elevated in both SD‐1 and SD‐1R cells. Knowing that, an FDA approved drug eliglustat, which is a GCS inhibitor used in Gaucher's disease is used to block the conversion of ceramide to hexosylceramide to accumulate apoptotic ceramide levels to sensitize cells to imatinib. Indeed, at the lipid levels, this combination resulted in a greater increase in ceramide and sphingosine levels. The reflection of these changes on cell growth was also detected and in parental SD‐1 cells, eliglustat application with imatinib is sensitized the cells to imatinib induced growth inhibition by 50% whereas eliglustat and imatinib cotreatment was able to overcome imatinib resistance in SD‐1R cells and inhibiting cell growth by 60–70% at 48 h. Summary/Conclusion: Taken together for the first time in the literature, we showed that pharmacological inhibition of GCS by eliglustat and imatinib cotreatment has synergistic cytostatic effects on imatinib resistant Ph+ ALL cells. Additionally, inducing the accumulation of ceramide (and/or sphingosine) by this cotreatment might be used as an approach to sensitize intrinsically / acquired resistant Ph+ ALL.