AT-01 * THERAPEUTIC TARGETING OF INI1 DEFICIENCY IN PEDIATRIC ATRT: A PRE-CLINICAL STUDY UTILIZING PATIENT DERIVED ORTHOTOPIC XENOGRAFT (PDOX) MODELS

Abstract

INTRODUCTION: Atypical teratoid rhabdoid tumors (ATRTs) are molecularly-defined by deletions or mutations in tumor suppressor INI1. INI1 deficiency causes dysfunction of chromatin-remodeling complexes which normally oppose EZH2, a epigenetic gene silencing protein. In the setting of INI1 deficiency, EZH2 is over-expressed, leading to ATRT oncogenesis. Using novel patient-derived orthotopic xenograft (PDOX) models, we investigated the pre-clinical utility of therapeutic EZH2 inhibition in ATRT using the drug DZNep. METHODS: Two PDOX models of ATRT were established by directly implanting tumor cells into mouse brains in the matched location with patient tumor. For in vitro testing, ATRT cells were exposed to DZNep at doses from 78 to 5000 nM for 2 weeks; cell viability was measured every 1- 3 days. For in vivo testing, 40 mice were divided into 4 treatment groups: control, DZNep (5 mg/kg i.p. daily x 14 days), standard of care (cranial radiation 2 Gy daily x 5 days, cisplatin 5 mg/kg i.p. on days 8 and 11), and combination therapy. Survival times were compared using log-rank analysis. RESULTS: DZNep showed dose- and time-dependent in vitro inhibition of tumor cell viability in both ATRT models tested. One of these models (IC-L1115ATRT) was then subjected to in vivo studies. ATRT tumors were present at the time of xenograft death in all 4 treatment groups including DZNep- and combination therapy-treated animals. Survival time did not differ between the control- and DZNep-treated groups or the standard of care- and combination therapy-treated groups (P > 0.05). CONCLUSIONS: While the EZH2 inhibitor DZNep is effective in vitro in killing ATRT tumor cells, it does not appear to have effects in vivo when tested in PDOX models of pediatric ATRT acting alone or in combination with combined chemo- and radio-therapies. Extensive further pre-clinical evidence should be obtained before an EZH2 inhibitor is brought into pediatric clinical trials.