Tumor Treating Fields (TTFields) combined with the drug repurposing approach CUSP9v3 induce metabolic reprogramming and synergistic anti-glioblastoma activity in vitro

BACKGROUND

Glioblastoma represents a brain tumor with a notoriously poor prognosis. First-line therapy may include adjunctive Tumor Treating Fields (TTFields) which are electric fields that are continuously delivered to the brain through non-invasive arrays. On a different note, CUSP9v3 represents a drug repurposing strategy that includes 9 repurposed drugs plus metronomic temozolomide. Here, we examined whether TTFields enhance the antineoplastic activity of CUSP9v3 against this disease.

METHODS

We performed preclinical testing of a multimodal approach of TTFields and CUSP9v3 in different glioblastoma models.

RESULTS

TTFields had predominantly synergistic inhibitory effects on the cell viability of glioblastoma cells and non-directed movement was significantly impaired when combined with CUSP9v3. TTFields plus CUSP9v3 significantly enhanced apoptosis, which was associated with a decreased mitochondrial outer membrane potential (MOMP), enhanced cleavage of effector caspase 3 and reduced expression of Bcl-2 and Mcl-1. Moreover, oxidative phosphorylation and expression of respiratory chain complexes I, III and IV was markedly reduced.

CONCLUSION

TTFields strongly enhance the CUSP9v3-mediated anti-glioblastoma activity. TTFields are currently widely used for the treatment of glioblastoma patients and CUSP9v3 was shown to have a favorable safety profile in a phase Ib/IIa trial (NCT02770378) which facilitates transition of this multimodal approach to the clinical setting.

EXTH-86. BH3 PROFILING IDENTIFIES ONC201/TIC10 AS A PROMISING PARTNER OF ABT-263 IN MEDULLOBLASTOMA IN VITRO

OBJECTIVE

Medulloblastoma represents one of the most common brain tumors in children. In this study, we identified by BH3 profiling that ONC201/TIC10 sensitizes for Bcl-xL/Bcl-2 inhibition in medulloblastoma and performed a preclinical testing of a combined treatment with ONC201/TIC10 and the Bcl-xL/Bcl-2 inhibitor ABT-263.

METHODS

BH3 profiling was performed to examine ONC201/TIC10-mediated dependencies on anti-apoptotic Bcl-2 family proteins. The combination treatment with ONC201/TIC10 and ABT-263 was tested in different medulloblastoma cells using MTT assays. Isobolograms were calculated to characterize the drug-drug interaction. Tumor spheroid and chorioallantoic membrane assays were used to examine the effects of the combination therapy in a 3D setting. Annexin V/PI staining and flowcytometry were used to detect pro-apoptotic effects. Western blot analyses and knockdown experiments with siRNA were performed for molecular analysis. Extracellular flux analyses served at examining effects on the tumor cell metabolism.

RESULTS

BH3 profiling showed that ONC201/TIC10 sensitizes medulloblastoma cells to Bcl-xL/Bcl-2 inhibition. In line with this finding, combined treatment with ONC201/TIC10 and ABT-263 led to a predominantly synergistic inhibitory effect on the cell viability of established (D425, D458, DAOY, HD-MB03), primary cultured (PC322) and stem-like (SC322) medulloblastoma cells in 2D and 3D models. The response towards the combination therapy was independent of baseline c-myc expression. On the molecular level, treatment with ONC201/TIC10 led to a dose-dependent decrease of Mcl-1. Moreover, the combination caused enhanced cleavage of caspases 9 and 3. On the metabolic level, the combination therapy led to a reduction in both, oxidative phosphorylation and the glycolytic rate and a reduced expression of respiratory chain proteins.

CONCLUSION

Combined treatment with ONC201/TIC10 and ABT-263 had a predominantly synergistic inhibitory effect on the cell viability of medulloblastoma cells. This effect was associated with downregulation of Mcl-1. Moreover, the combination treatment resulted in a metabolic reprogramming which likely creates a state of energy deprivation.

EXTH-48. INHIBITION OF SREBP-1 IS SYNTHETICALLY LETHAL WITH BCL-XL/BCL-2 INHIBITION IN GLIOBLASTOMA IN VITRO

OBJECTIVE

A cellular homeostasis that is shifted away from apoptosis and a reprogramming of the lipid metabolism are both, features that are frequently encountered in glioblastoma. This study aimed at investigating whether interference with the lipid metabolism is synthetically lethal with inhibition of anti-apoptotic Bcl-2 family proteins in glioblastoma in vitro.

METHODS

Established (U251) and primary-cultured glioblastoma cells (PC38, PC40 and PC128) as well as glioblastoma stem-like cells (SC38 and SC40) were treated with the Bcl-xL/Bcl-2 inhibitor ABT-263 (navitoclax) and/or the SREBP-1 inhibitor Fatostatin. MTT-assays were performed to assess effects of the combination therapy on the cell viability. Isobolograms were calculated to characterize the drug-drug interaction. Spheroids were used to determine anti-proliferative effects in a 3-dimensional setting. Staining with annexin V/propidium iodide and flowcytometric analysis were performed to assess pro-apoptotic effects. For molecular analyses, Western blots and specific knock-down experiments with siRNA were performed.

RESULTS

Combined targeting of SREBP-1 and Bcl-xL/Bcl-2 led to a synergistic inhibitory effect on the cellular viability of established, primary-cultured and glioblastoma stem-like cells as well as spheroids. This effect was shown to be at least in part mediated by enhanced apoptosis and to occur in a caspase-dependent manner. On the molecular level, treatment with increasing concentrations of Fatostatin led to a downregulation of Mcl-1.

CONCLUSION

Our study indicates that combined inhibition of Bcl-xL/Bcl-2 and interference with the lipid metabolism targeting SREBP-1 synergistically induces caspase-dependent apoptosis in glioblastoma cells. This effect can also be observed in more complex 3-dimensional glioblastoma cell formations. Further studies will focus on deciphering the drug-induced alterations of the metabolic pathways that are responsible for the synergistic effect of this therapeutic strategy.

EXTH-72. TTFIELDS ENHANCE THE ANTINEOPLASTIC ACTIVITY OF THE DRUG-REPURPOSING APPROACH CUSP9V3 IN GLIOBLASTOMA IN VITRO

OBJECTIVE

Drug repurposing represents a promising strategy to safely accelerate the clinical use of therapeutics with antineoplastic activity. In this study, we examined whether Tumor Treating Fields (TTFields) enhance the biological effects of CUSP9v3, a treatment strategy including nine repurposed drugs, in an in vitro setting of glioblastoma.

METHODS

We performed MTT-assays to examine effects of the combination treatment on the viability of different glioblastoma cells. Tumor spheroids were used as a model to examine effects of the combination treatment in a 3-dimensional setting. Staining with annexin V/propidium iodide or MitoTrackerTM followed by flow cytometry was done to assess pro-apoptotic effects. Specific protein expression of caspases and members of the Bcl-2 family of proteins was determined by Western blot analyses.

RESULTS

TTFields had at least additive inhibitory effects on the cell viability of established (U251), primary cultured (PC38, PC40, PC128) and stem-like (SC38, SC40) glioblastoma cells when combined with CUSP9v3. In addition, flow cytometric analyses revealed that a simultaneous treatment with TTFields and CUSP9v3 significantly increased the fraction of annexin V-positive (apoptotic) glioblastoma cells. Moreover, the fraction of cells with a reduced mitochondrial outer membrane potential was significantly higher following a simultaneous treatment with TTFields and CUSP9v3. On the molecular level, these observations were associated with enhanced cleavage of effector caspase 3 and a reduced expression of the anti-apoptotic Bcl-2 family proteins Bcl-2 and Mcl-1.

CONCLUSION

These data suggest that TTFields enhance the susceptibility of glioblastoma cells towards CUSP9v3, potentially allowing significant dose reduction and decreased toxicity. This observation seems to rely at least in part on a caspase-dependent cell death mechanism. TTFields are widely used for the treatment of glioblastoma patients and CUSP9v3 was recently shown to have a favorable safety profile in a phase Ib/IIa trial (NCT02770378) which facilitates transition of a combined approach to the clinical setting.