Abstract 340: Talazoparib enhances low-dose temozolomide efficacy in flank glioblastoma models, but intracranial efficacy is constrained by limited brain distribution

BACKGROUND: Talazoparib (TAL) is a potent poly (ADP-ribose) polymerase (PARP) inhibitor that robustly enhances in vitro sensitivity to temozolomide (TMZ) in several tumor models. We assessed the in vitro and in vivo efficacy of TAL combined with TMZ in glioblastoma (GBM) models. METHODS: Established glioma cell lines (T98G, U251) and the GBM12 patient derived xenograft (PDX) line were treated in vitro with TAL (1-10 nM) ± TMZ (2-300 μM). Antitumor efficacy was assessed with CyQUANT, clonogenic and primary neurosphere assays; cell cycle analysis with flow cytometry; DNA damage signaling with phospho-specific western blots for pKap1, pChk1, pChk2 and fluorescent immunocytochemistry for γH2AX and replication protein A foci. Brain and plasma concentrations of TAL in wild-type (WT), Mdr1a/b(-/-), Bcrp(-/-) and Mdr1a/b(-/-)Bcrp(-/-) knockout (KO) mice were assessed with LC-MS/MS. The tolerability of TAL (0.05 - 0.30 mg/kg/day orally, divided twice daily (div. bid)) and TMZ (5-50 mg/kg/day orally) was tested in athymic nude mice. Subsequently, in vivo combination TAL/TMZ efficacy was assessed with survival analyses in intracranial and flank GBM12 PDX models. RESULTS: TAL 1-3 nM sensitizes the TMZ resistant T98G glioma cell line to TMZ at high TMZ concentrations (30-300 μM) and is associated with G2 cell cycle arrest and enhanced DNA damage signaling. TAL 1-3 nM also enhances the cytotoxic efficacy of lower concentrations of TMZ (10-30 μM) in the TMZ sensitive U251 cell line. At least 3 nM TAL is required to enhance the in vitro efficacy of low TMZ concentrations (2-10 μM) in the TMZ sensitive GBM12 PDX line. PK studies in non-tumor bearing WT FVB mice treated with a single dose of TAL 0.15 mg/kg revealed mean brain and plasma concentrations of 0.49 ng/g (1.3 nM) and 25.5 ng/ml (67.1 nM) respectively at 2 hours. The mean brain/plasma ratio at 2 hours was unchanged in Bcrp(-/-) KO mice compared to WT mice (1.2% vs. 2.0%), but was significantly increased in Mdr1a/b(-/-) KO mice (22.8%) and Mdr1a/b(-/-)Bcrp(-/-) KO mice (23.9%). Standard doses of TMZ (50 mg/kg/day, days 1-5) combined with low doses of TAL (0.05 mg/kg/day, div. bid) were toxic in nude mice. Higher doses of TAL (0.30 mg/kg/day, div. bid) required substantial TMZ dose reductions (5 mg/kg/day, days 1-5) for tolerability. The addition of TAL (0.30 mg/kg/day, div. bid) to low-dose TMZ (5 mg/kg/day) prolonged tumor stasis in flank GBM12 xenografts (median time to endpoint 76 vs. 50 days, p = 0.005). However this regimen was ineffective in intracranial GBM12 xenografts (median survival 37 vs. 30 days, p = 0.93). CONCLUSIONS: TAL is a potent PARP inhibitor that enhances the efficacy of TMZ in both in vitro GBM models and flank GBM12 PDX models, but not in the corresponding GBM12 intracranial xenografts. This lack of intracranial efficacy is associated with limited brain distribution due to active efflux mediated by Mdr1 at the blood-brain barrier. Citation Format: Sani H. Kizilbash, Kenneth Chang, Shiv K. Gupta, Ryo Kawashima, Karen Parrish, Ann C. Mladek, Brett L. Carlson, Katrina K. Bakken, Mark A. Schroeder, Gaspar J. Kitange, Paul A. Decker, Yuqiao Shen, William F. Elmquist, Jann N. Sarkaria. Talazoparib enhances low-dose temozolomide efficacy in flank glioblastoma models, but intracranial efficacy is constrained by limited brain distribution. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 340.