Abstract 4550: High throughput screen to identify compounds that reverse glucocorticoid resistance in pediatric leukemia.
Authors: Toscan Cara , Failes Tim , Arndt Greg , Lock Richard 
Published: August 2014

Abstract
Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy. Although cure rates are approaching 85%, it remains one of the most common causes of death from disease in children. Glucocorticoids (e.g. dexamethasone) form a critical component of chemotherapy regimens for pediatric ALL and the initial response to glucocorticoid therapy is a major prognostic factor, where resistance is predictive of poor outcome. We have previously established a clinically relevant ALL xenograft model, consisting of primary pediatric ALL biopsies engrafted in immune-deficient mice, in which in vitro and in vivo dexamethasone sensitivity significantly correlate with patient outcome. In this study we used a high throughput screen (HTS) to identify novel small compounds that reverse glucocorticoid resistance. A xenograft (ALL-19) derived from an aggressive and chemoresistant pediatric ALL that induced early fatality in the patient, and is representative of the most common pediatric ALL subtype (B-cell precursor, BCP-ALL, or c-ALL) was used to identify compounds that potentially will have wide applicability. Cells were exposed in vitro to dexamethasone at a concentration ineffective in cell killing against this xenograft (1 μM), but which is around 100-fold greater than the IC50 for other (sensitive) xenografts, simultaneously with one of 40,000 HTS library compounds (10 μM). Effective compounds were then tested alone and in the presence of dexamethasone, to identify those that sensitized the xenograft to glucocorticoids, rather than those that were toxic alone. The compound 2-(4-chlorophenoxy)-2-methyl-N-(2-(piperidin-1-yl)phenyl)propanamide showed little activity alone (IC50 22 μM), but when combined with dexamethasone caused a marked decrease in cell viability. Pre-incubating ALL-19 xenograft cells with either the compound or dexamethasone did not increase the sensitizing effect. Fixed-ratio combination assays were performed against a broad panel of dexamethasone-resistant and -sensitive xenografts representative of BCP-ALL, T-ALL and MLL-rearranged ALL, in order to determine whether the compound was broadly active. CalcuSyn software was employed to determine combination indices, and synergy was observed in all 4 BCP-ALL xenografts tested, which included a dexamethasone-sensitive BCP-ALL and a dexamethasone-resistant Philadelphia chromosome-positive ALL. Synergy was also observed in dexamethasone-resistant MLL-rearranged ALL xenograft cells. Additionally, despite an antagonistic effect observed in a dexamethasone-resistant T-ALL xenograft, the compound potentiated the effects of dexamethasone in a glucocorticoid-sensitive T-ALL. In conclusion, using a clinically relevant xenograft model and a HTS, this study has identified a novel compound that reverses dexamethasone resistance in BCP-ALL xenografts, and may have applications in other leukemia subtypes.
Citation Format: Cara Toscan, Tim Failes, Greg Arndt, Richard Lock. High throughput screen to identify compounds that reverse glucocorticoid resistance in pediatric leukemia. [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 4550. doi:10.1158/1538-7445.AM2013-4550