Restricted delivery of talazoparib across the blood-brain barrier limits the sensitizing effects of poly (ADP-ribose) polymerase inhibition on temozolomide therapy in glioblastoma

Poly (ADP-ribose) polymerase PARP inhibitors, including talazoparib (TAL), potentiate temozolomide (TMZ) efficacy in multiple tumor types, however TAL-mediated sensitization has not been evaluated in orthotopic glioblastoma (GBM) models. This study evaluates TAL ± TMZ in clinically relevant GBM models. TAL at 1-3 nmol/L sensitized T98G, U251 and GBM12 cells to TMZ, and enhanced DNA damage signaling and G2/M arrest in vitro. In vivo cyclical therapy with TAL (0.15 mg/kg twice daily) combined with low dose TMZ (5 mg/kg daily) was well tolerated. This TAL/TMZ regimen prolonged tumor stasis more than TMZ alone in heterotopic GBM12 xenografts [median time to endpoint: 76 days vs. 50 days TMZ (p=0.005), 11 days placebo (p <0.001)]. However, TAL/TMZ did not accentuate survival beyond that of TMZ alone in corresponding orthotopic xenografts (median survival 37 vs. 30 days with TMZ (p=0.93), 14 days with placebo, p<0.001). Average brain and plasma TAL 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 TAL in Bcrp-/- versus WT mice did not differ, while 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 TAL accumulation in MDCKII cells. These results indicate that TAL has significant MDR1 efflux liability that may restrict delivery across the blood-brain barrier, and this may explain the loss of TAL-mediated TMZ sensitization in orthotopic versus heterotopic GBM xenografts.

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