The antitumor effect of the atypical retinoid ST1926 in human Glioblastoma

Abstract

Title: The antitumor effect of the atypical retinoid ST1926 in human glioblastoma Abstract

Background: Glioblastoma Multiforme (GBM) is the most aggressive form of malignant brain tumors. It accounts for 70% of newly diagnosed malignant primary brain tumors in adults. The current standard therapy comprises surgical resection with adjuvant radiotherapy, followed by the administration of Temozolomide, an alkylating chemotherapeutic agent. Despite the available therapies, the median survival rate does not exceed two years, since patients tend to relapse and eventually develop resistance. Tumor heterogeneity and the restrictive nature of the blood-brain barrier (BBB) are major obstacles faced in GBM management. Therefore, there is an eminent need to develop more adapted and efficient treatments. Natural and synthetic retinoids have been investigated in several cancer types and showed promising antitumor effects through the regulation of proliferation, differentiation, and apoptosis. Synthetic retinoids offered an enhanced specificity and reduced the toxicity of their natural counterparts. The atypical adamantyl synthetic retinoid ST1926 induces apoptosis and growth inhibition in different cancer types at sub-micromolar concentrations independently of retinoid receptor signaling pathway. We have recently found that ST1926 is an inhibitor of the catalytic subunit of DNA polymerase alpha (POLA1) which is involved in DNA synthesis initiation. Our in silico analysis revealed elevated levels of POLA1 expression in glioblastoma tumor tissues versus normal counterparts. This suggests POLA1 as a relevant and attractive target for ST1926 in GBM.

Aims: We investigated the antitumor activities and the mechanism of action of ST1926 on a panel of human GBM cell lines. In addition, we tested the therapeutic properties of ST1926 using a mouse GBM xenograft model.

Methods: Experiments were conducted both in vitro and in vivo to explore the efficacy of ST1926 as a potential therapeutic agent against human GBM. Several human GBM cell lines of different p53 status were used in this study namely U87MG (WT p53), U251 (mutated p53), U118 (mutated p53), and A172 (WT p53) cells. We measured cell viability assays by MTT and SRB, flow cytometry by propidium iodide staining of DNA, and apoptosis by TUNEL assay. Western blot were performed to study the mechanism of action of ST1926 on GBM cell extract. We finally investigated the effect of ST1926 in a xenografted mouse model of human glioblastoma cells.

Results: ST1926 reduced cell viability in all tested human GBM cell lines with IC50 values at sub-micromolar levels. ST1926 induced early DNA damage and reduction of POLA1 protein levels. Flow cytometry results of ST1926-treated GBM cells showed that ST1926 exhibited G1 phase cell cycle arrest and sub-G1 cell accumulation. ST1926 induced apoptosis was further confirmed by PARP cleavage and TUNEL. ST1926 reduced tumor volume in a GBM xenograft model. Experiments are still in progress to optimize the therapeutic properties of ST1926 in a xenograft model and to elucidate its mechanism of action by proteomic analysis of treated GBM cell extracts.

Conclusion: ST1926 showed favorable preclinical efficacy and thus is worth to be tested in GBM orthotopic animal models.