Shen, Yuqiao

Abstract Poly ADP-ribose polymerase (PARP) inhibitors, including talazoparib, potentiate temozolomide efficacy in multiple tumor types; however, talazoparib-mediated sensitization has not been evaluated in orthotopic glioblastoma (GBM) models. This study evaluates talazoparib ± temozolomide in clinically relevant GBM models. Talazoparib at 1–3 nmol/L sensitized T98G, U251, and GBM12 cells to temozolomide, and enhanced DNA damage signaling and G2–M arrest in vitro. In vivo cyclical therapy with talazoparib (0.15 mg/kg twice daily) combined with low-dose temozolomide (5 mg/kg daily) was well tolerated. This talazoparib/temozolomide regimen prolonged tumor stasis more than temozolomide alone in heterotopic GBM12 xenografts [median time to endpoint: 76 days versus 50 days temozolomide (P = 0.005), 11 days placebo (P < 0.001)]. However, talazoparib/temozolomide did not accentuate survival beyond that of temozolomide alone in corresponding orthotopic xenografts [median survival 37 vs. 30 days with temozolomide (P = 0.93), 14 days with placebo, P < 0.001]. Average brain and plasma talazoparib concentrations at 2 hours after a single dose (0.15 mg/kg) were 0.49 ± 0.07 ng/g and 25.5±4.1 ng/mL, respectively. The brain/plasma distribution of talazoparib in Bcrp−/− versus wild-type (WT) mice did not differ, whereas the brain/plasma ratio in Mdr1a/b−/− mice was higher than WT mice (0.23 vs. 0.02, P < 0.001). Consistent with the in vivo brain distribution, overexpression of MDR1 decreased talazoparib accumulation in MDCKII cells. These results indicate that talazoparib has significant MDR1 efflux liability that may restrict delivery across the blood–brain barrier, and this may explain the loss of talazoparib-mediated temozolomide sensitization in orthotopic versus heterotopic GBM xenografts. Mol Cancer Ther; 16(12); 2735–46. ©2017 AACR.

Abstract 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.

Abstract Poly (ADP-ribose) polymerase (PARP) plays an important role in several DNA repair processes, and previous studies with various PARP inhibitors have demonstrated robust enhancement of sensitivity to temozolomide (TMZ) chemotherapy in pre-clinical models of glioblastoma multiforme (GBM). The focus of this study was to evaluate the antitumor effects of a novel PARP inhibitor, BMN 673, as a single agent and in combination with TMZ in the established U251 glioma cell line, neurosphere cultures from the primary Mayo GBM xenograft line GBM12, and TMZ-resistant cells derived from either line (U251TMZ, GBM12TMZ(MGMT high) and GBM12TMZ(MGMT low)). In a biochemical assay of PARP inhibition, BMN 673 potently suppressed poly-ADP-ribosylation of cellular proteins at concentrations of 3 nM following hydrogen peroxide treatment of U251 cells. Single agent treatment had modest anti-tumor activity with 30 nM BMN 673 reducing absorbance in a CyQUANT cell proliferation assay relative to control treatment in both U251 and U251TMZ cells (Table 1). BMN 673 co-treatment also enhanced the efficacy of TMZ in both U251-derivative lines at a clinically relevant concentration of TMZ. Similarly, treatment with BMN673 alone or in combination with TMZ inhibited primary neurosphere formation in all three GBM12 models. Notably, the addition BMN 673 clearly re-sensitized both resistant GBM12 sub-lines to relatively low concentrations of TMZ. Consistent with a mechanism involving disruption of DNA repair, combined treatment with BMN 673 and TMZ resulted in robust activation of ATM-mediated KAP1 phosphorylation (Ser824) and ATR-mediated Chk1 phosphorylation (Ser345) as compared to TMZ or BMN 673 alone 24 hours after treatment. In conclusion, BMN673 is a promising PARP inhibitor with high potency and cytotoxicity in vitro against both TMZ sensitive and TMZ resistant GBM cells. Table 1 - In vitro antitumor effects of TMZ combined with BMN673 TMZ, microM 0 nM BMN673 30 nM BMN673 CyQUANT U251 Relative absorbance Relative absorbance 0 1.00 0.58 30 0.80 0.30 U251TMZ 0 1.00 0.69 30 1.02 0.21 Primary Neurosphere Assay GBM12 Relative Count Relative Count 0 1.00 0.66 5 0.32 0 10 0.01 0 GBM12TMZ (MGMT high) 0 1.00 0.32 5 1.11 0.02 10 1.04 0.01 GBM12TMZ (MGMT low) 0 1.00 0.25 5 0.86 0.03 10 0.75 0 Citation Format: Sani H. Kizilbash, Ryo Kawashima, Shiv Gupta, Ann C. Mladek, Yuqiao Shen, Jann N. Sarkaria. BMN 673 potentiates efficacy of temozolomide in sensitive and resistant glioblastoma models. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2067. doi:10.1158/1538-7445.AM2013-2067