gms | German Medical Science

Objective: Targeted therapies against activated signaling pathways are important in the treatment of human malignancies. In glioblastomas, the inactivated tumor suppressor PTEN causes activation of mTOR, explaining the rationale of mTOR inhibitors to reduce proliferation, migration, transcription and protein synthesis.

Method: In immunodeficient nude- and NSG-mice subcutaneous and intracerebral patient-derived human xenografts were established. Mice were treated with everolimus (5mg/kg p.o. daily) in controlled series (each with 8 animals). Temozolamide and PBS served as controls. After 14 days, the experiments were terminated. The influence of everolimus on tumor size, morphology, tumor cell proliferation (Ki-67), vascularization (CD31), vascular pericytes (NG-2), apoptosis (caspase-3) and on the expression of PTEN, mTOR, p-mTOR, S6-kinase and p-S6-kinase were evaluated.

Results: With increasing concentrations of everolimus, cell viability of tumor cells decreased progressively and significantly in vitro. Everolimus caused apoptosis in all of these cell types. The migration rate of glioma cells decreased significantly (p<0.01). Mice with subcutaneously and intracerebrally growing tumors were treated with everolimus. This substance led to markedly (p<0.001) reduced growth rates in all glioblastoma xenografts. Everolimus was the most effective substance, behind temozolamide, and compared to bevacizumab, sorafenib, pazopanib, 5-azacytidine and valproic acid. No changes in mRNA expression of mTOR and S6-kinase resulted from everolimus, but immunohistochemistry revealed a significant decrease in phosphorylation of mTOR protein and a reduced expression of the downstream S6-kinase (p<0.001), thereby proving the effect of mTOR inhibition on mTOR functionality. Tumor proliferation rate (Ki-67 index) was significantly decreased by everolimus and correlated with the growth curve (p<0.001). There was an increase of caspase-3 apoptosis score in tumor cells compared to the PBS controls (p<0.01). Everolimus led to lower numbers of CD31-positive endothelial cells and NG2-positive pericytes (p<0.001) in glioblastomas. Increased endothelial apoptosis was detected in everolimus treated tumors (8.4%, p<0.001 vs. 1.3% in PBS group).

Conclusions: Experimental results show a significant tumor inhibition. The administration of everolimus – at least to a circumscribed patient group – in the context of personalized tumor therapy seems to be a promising approach.