Boddy, Alan V.

Abstract PARP inhibition can enhance the efficacy of temozolomide and prolong survival in orthotopic glioblastoma (GBM) xenografts. The aim of this study was to evaluate the combination of the PARP inhibitor rucaparib with temozolomide and to correlate pharmacokinetic and pharmacodynamic studies with efficacy in patient-derived GBM xenograft models. The combination of rucaparib with temozolomide was highly effective in vitro in short-term explant cultures derived from GBM12, and, similarly, the combination of rucaparib and temozolomide (dosed for 5 days every 28 days for 3 cycles) significantly prolonged the time to tumor regrowth by 40% in heterotopic xenografts. In contrast, the addition of rucaparib had no impact on the efficacy of temozolomide in GBM12 or GBM39 orthotopic models. Using Madin-Darby canine kidney (MDCK) II cells stably expressing murine BCRP1 or human MDR1, cell accumulation studies demonstrated that rucaparib is transported by both transporters. Consistent with the influence of these efflux pumps on central nervous system drug distribution, Mdr1a/b−/−Bcrp1−/− knockout mice had a significantly higher brain to plasma ratio for rucaparib (1.61 ± 0.25) than wild-type mice (0.11 ± 0.08). A pharmacokinetic and pharmacodynamic evaluation after a single dose confirmed limited accumulation of rucaparib in the brain is associated with substantial residual PARP enzymatic activity. Similarly, matrix-assisted laser desorption/ionization mass spectrometric imaging demonstrated significantly enhanced accumulation of drug in flank tumor compared with normal brain or orthotopic tumors. Collectively, these results suggest that limited drug delivery into brain tumors may significantly limit the efficacy of rucaparib combined with temozolomide in GBM. Mol Cancer Ther; 14(12); 2735–43. ©2015 AACR.

Abstract Poly (ADP-ribose) polymerase (PARP) inhibition can enhance the efficacy of temozolomide (TMZ) and has been shown to significantly prolong survival in orthotopic GBM xenografts. Here the effects of PARP inhibitor AG014699 (AG699) on the efficacy of TMZ was evaluated and corresponding pharmacokinetic and pharmacodynamic studies were performed in the MGMT methylated GBM12 primary GBM xenograft line. AG699 effectively sensitized GBM12 cells to TMZ in vitro in a neurosphere formation assay: IC50 of 1.56 M for TMZ when combined with 1 μM AG699 compared to 8.16 M TMZ when used alone). Using these same cells, orthotopic xenografts were established and mice were randomized to 3–28 day cycles of placebo or TMZ (50 mg/kg days 1–5) with or without AG699 (1 mg/kg IP days 1–5). Although this model is responsive to other TMZ/PARP inhibitor combinations, the addition of AG699 was ineffective (median survival: placebo 16 days, TMZ 68 days and TMZ/AG699 81 days, p=0.883). In contrast, AG699 significantly potentiated TMZ-induced tumor regrowth delay in a flank tumor model with GBM12 (median time for tumors to reach endpoint: placebo 32 days, TMZ 86 days and TMZ/AG699 121 days, p<0.01). To more clearly understand the discrepancy between intracranial and flank studies, drug accumulation and target inhibition were assessed. Plasma and tissue distribution studies in mice bearing GBM12 orthotopic xenografts showed that 0.5 h after AG699 treatment its level in brain and tumor xenograft was much lower compared with that in liver (see table). In an assessment of catalytic activity, PARP activity in liver was inhibited more than 95% of control at least 2 h after AG699 treatment, while it was inhibited less than 50% and 30% in orthotopic xenograft tissue and normal brain, respectively (Table). Collectively, these data support the hypothesis that the efficacy of AG699 combined with TMZ is limited in the orthotopic model due to limited drug accumulation within the brain, and the mechanisms of which are under investigation. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr A77.