Abstract 3243: Synergistic Effects of Connexin 43 Inhibitor and PI3K Isoform-Selective Inhibitors in Temozolomide-Resistant Glioblastoma

Abstract Glioblastoma is the most common central nervous system cancer and is responsible for approximately one-half of all malignant brain tumors. With a 5-year survival rate of 6.8% and a median observed survival time of only 8 months, recurrent glioblastoma is among the world’s most lethal malignancies. Temozolomide (TMZ) has become a standard chemotherapeutic for glioblastoma due to its alkylating effects and ability to cross the blood-brain barrier. Unfortunately, many patients with glioblastoma develop resistance to TMZ, severely limiting therapeutic options. Recent studies have found a link between ectopic connexin 43 (Cx43) and TMZ-resistant glioblastoma. Further studies have shown success in overcoming this resistance by combining TMZ with alpha carboxyl terminus 1 (αCT1) peptide, a clinically tested Cx43 carboxyl terminus mimetic peptide inhibitor; however, because of the short half-life of αCT1, therapeutic potential is limited, and new approaches are needed. In this study, we hypothesized that simultaneously targeting Cx43 with αCT1 and phosphoinositide 3-kinase (PI3K) using PI3K isoform-selective inhibitors would synergistically sensitize glioblastoma to TMZ treatment because p110β, one of the four PI3K catalytic subunits, is a prime survival factor for TMZ-resistant glioblastoma. We cultured newly dissected primary glioblastoma cells, glioblastoma cell lines, and glioblastoma stem cells and dosed them with various combinations of αCT1, TMZ, and one of two clinically tested p110β inhibitors (TGX-221 or GSK2636771). Cell viability was measured using an MTS viability assay. Glioblastoma cells that expressed high levels of Cx43 and p110β experienced a significant decrease in viability when dosed with a triple combination. The additive properties of these combinations were measured using an excess over Bliss (EOB) score derived from Bliss independence modeling, which showed synergistic effects as high as 81% for triple therapy combinations in TMZ-resistant cell lines. The EOB was significantly higher in triple therapies than the double or single therapies. To support our findings, we used the same combinations in primary glioblastoma cells and measured the activity of cleaved caspase 3/7 using a luminescence assay. There was a synergistic increase in caspase 3/7 activity in the triple therapy that was not seen in any of the other combinations. Identical treatment in non-pathological human astrocytes suggests that our drug combinations do not exacerbate non-selective TMZ toxicity in the brain. Collectively, our data demonstrate that simultaneous inhibition of Cx43 and PI3K is an effective approach for overcoming TMZ resistance in glioblastoma. Citation Format: Kasen R. Hutchings, Kevin J. Pridham, Min Liu, Joseph Owens, Zhi Sheng. Synergistic effects of connexin 43 inhibitor and PI3K isoform-selective inhibitors in temozolomide-resistant glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3243.