Abstract 477: Inhibition of MEK confers hypersensitivity to X-radiation in the context of BRAF mutation in a model of childhood astrocytoma

Abstract

Abstract Patients with recurrent low-grade brain tumors or anaplastic astrocytoma may receive radiation treatment, however, the long-term sequellae from radiation treatment can be severe. For pediatric astrocytomas, low-grade tumors are associated with activation of BRAF through a tandem duplication that results in the KIAA1549-BRAF fusion (Grade 1) or through an activating point mutation of BRAF (predominantly V600E; Grade 2-4) and is the most common lesion in low- and intermediate-grade astrocytoma. Previous studies have indicated synergy between MEK inhibition and ionizing radiation therapy (XRT), although the proposed mechanisms for synergy differ. We have explored the combination of ionizing radiation with MEK inhibition in a model of BRAF-mutant anaplastic childhood astrocytoma. The regulation of TORC1 signaling by BRAF was examined in BT-40 (BRAF mutant) and BT-35 (BRAF wild type) xenografts, in a cell line derived from the BT-40 xenograft and two adult BRAF mutant (V600E) glioblastoma cell lines (DBTRG-05MG, AM380c1). The effect of MEK inhibition (selumetinib, AZD6244, ARRY-142886), XRT (2 Gy daily fractionated doses), or the combination of selumetinib and XRT was evaluated in subcutaneous BT-40 xenografts. Inhibition of MEK signaling by selumetinib, suppressed TORC1 signaling only in the context of the BRAF-mutant both in vitro and in vivo. Inhibition of MEK signaling in BT-40 cells or in xenografts lead to a complete suppression of TORC1 signaling and decreased levels of FANCD2 protein, whereas inhibition of TORC1 signaling was less in DBTRG-05MG cells and even less in AM380c1 cells. The level of TORC1 inhibition by selumetinib was associated with loss of clonogenic survival, and suppression of FANCD2 in vitro. In vivo we examined the sensitivity of BT-40 xenografts to selumetinib (75 mg/kg BID x 42 days, PO), XRT (10 Gy delivered as 2 Gy daily fractions), or selumetinib combined with XRT. For control tumors, median time to event was 17 days, whereas for XRT alone it was 80 days (P<0.0001). Selumetinib treatment induced complete regression of most tumors with progression starting around week 5 of the 6-week treatment period, consistent with previous data, with median time to event being 51 days (P<0.0001 vs control). In contrast to XRT or selumetinib as single agents, the combination caused complete regression of all tumors, with only 4 of 10 (40%) recurrent tumors within the 24 weeks observation period, and significantly better than radiation or selumetinib alone (P<0.0001 for both comparisons). These data add to studies that indicate synergy between XRT and MEK inhibitors, and suggest the possibility of potentiating the effect of XRT selectively in tumor cells by MEK inhibition in the context of mutant BRAF. Combining MEK inhibition with XRT may allow maintenance of tumor control at lower doses of XRT that would decrease long-term sequellae for children with BRAF-mutant brain tumors. Citation Format: Adam W. Studebaker, Kathryn Bondra, Star Seum, Justin Leasure, Christopher Chronowski, Changxian Shen, Doris Phelps, Paul D. Smith, Raushan T. Kurmasheva, Xiaokui Mo, Peter J. Houghton. Inhibition of MEK confers hypersensitivity to X-radiation in the context of BRAF mutation in a model of childhood astrocytoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 477. doi:10.1158/1538-7445.AM2015-477