CUDC907, a dual phosphoinositide-3 kinase/histone deacetylase inhibitor, promotes apoptosis of NF2 Schwannoma cells.

Neurofibromatosis Type 2 (NF2) is a rare tumor disorder caused by pathogenic variants of the merlin tumor suppressor encoded by NF2. Patients develop vestibular schwannomas (VS), peripheral schwannomas, meningiomas, and ependymomas. There are no approved drug therapies for NF2. Previous work identified phosphoinositide-3 kinase (PI3K) as a druggable target. Here we screened PI3K pathway inhibitors for efficacy in reducing viability of human schwannoma cells. The lead compound, CUDC907, a dual histone deacetylase (HDAC)/PI3K inhibitor, was further evaluated for its effects on isolated and nerve-grafted schwannoma model cells, and primary VS cells. CUDC907 (3 nM IG50) reduced human merlin deficient Schwann cell (MD-SC) viability and was 5-100 fold selective for MD over WT-SCs. CUDC907 (10 nM) promoted cell cycle arrest and caspase-3/7 activation within 24 h in human MD-SCs. Western blots confirmed a dose-dependent increase in acetylated lysine and decreases in pAKT and YAP. CUDC907 decreased tumor growth rate by 44% in a 14-day treatment regimen, modulated phospho-target levels, and decreased YAP levels. In five primary VS, CUDC907 decreased viability, induced caspase-3/7 cleavage, and reduced YAP levels. Its efficacy correlated with basal phospho-HDAC2 levels. CUDC907 has cytotoxic activity in NF2 schwannoma models and primary VS cells and is a candidate for clinical trials.

Effect of AR42 in Primary Vestibular Schwannoma Cells and a Xenograft Model of Vestibular Schwannoma.

HYPOTHESIS: AR42, a histone deacetylase (HDAC) inhibitor, reduces viability of primary vestibular schwannoma (VS) cells and delays tumor progression and hearing loss (HL) in a xenograft model of VS. BACKGROUND: The impact of HDAC expression on AR42 response in primary VS cells is unknown, as well as the effects of AR42 on VS-associated HL and imbalance. METHODS: Primary human VS cells (n = 7) were treated with AR42 (0-3.0 µM), and viability assays were conducted. Immunohistochemistry and western blotting for phosphorylated-HDAC2 (pHDAC2) were performed on tumor chunks. Pharmacokinetic studies were conducted in Fischer rats using mass spectrometry. Merlin-deficient Schwann cells were grafted onto cochleovestibular nerves of immunodeficient rats and treated with vehicle (n=7) or AR42 (25 mg/kg/day for 4weeks; n=12). Tumor bioluminescence imaging, auditory brainstem response (ABR), and rotarod tests were conducted to 6weeks. Final tumor weight and toxicities were measured. RESULTS: AR42 caused dose-dependent reductions in viability of VS cells. Tumors with higher pHDAC2:HDAC2 ratios had greater reductions in viability with AR42. On pharmacokinetic studies, AR42 reached peak levels in nerve ~24 hours after oral administration. Although AR42-treated rats demonstrated mean ABR threshold shifts ~10 to 20 dB lower than controls, this did not persist nor reach significance. When compared to controls, AR42 did not affect tumor bioluminescence, tumor weight, and rotarod measurements. CONCLUSIONS: Response of primary VS cells to AR42 may be influenced by pHDAC2 expression in tumor. Although AR42 may delay HL in our xenograft model, it did not halt tumor growth or vestibular dysfunction. Further investigations are warranted to evaluate the AR42 effectiveness in NF2-associated VS.

Primary Vestibular Schwannoma Cells Activate p21 and RAD51-Associated DNA Repair Following Radiation-Induced DNA Damage.

HYPOTHESIS: Vestibular Schwannoma (VS) can avoid cell death following radiation injury by entering cell cycle arrest and activating RAD51-related DNA repair. BACKGROUND: Although the radiobiology of various cancers is well-studied, the radiobiological effects in VS are poorly understood. In this study, we describe how VS cells enter cell cycle arrest (through p21 expression), activate DNA repair (through RAD51 upregulation), and avoid cell death after radiation-induced double-stranded breaks (DSB) in DNA (as measured by γ-H2AX). METHODS: Primary human VS cells were cultured on 96-well plates and 16-well culture slides at 10,000 cells/well and exposed to either 0 or 18 Gray of radiation. Viability assays were performed at 96 h in vitro. Immunofluorescence for γ-H2AX, RAD51, and p21 was performed at 6 h. RESULTS: Radiation (18 Gy) induced the expression of γ-H2AX, p21, and RAD51 in six cultured VS, suggesting that irradiated VS acquire DSBs, enter cell cycle arrest, and initiate RAD51 DNA repair to evade cell death. However, viability studies demonstrate variable responses in individual VS cells with 3 of 6 VS showing radiation resistance to 18 Gy. On further analyses, radiation-resistant VS cells expressed significantly more p21 than radiation-responsive tumors. CONCLUSIONS: In response to radiation-induced DNA damage, primary VS cells can enter cell cycle arrest and express RAD51 DNA repair mechanisms to avoid cell death. Radioresistant VS cells may mount a more robust p21 response to ensure sufficient time for DNA repair. Further investigation into DNA repair proteins and cell cycle checkpoints may provide important insight on the radiobiology of VS and mechanisms for resistance.