Enhancement of radiation effects by combined treatment with Decitabine and Abacavir in a murine model of human pediatric medulloblastoma

The standard therapy for medulloblastoma (MB), the most common pediatric brain tumour, consists of surgical excision followed by irradiation (IR) and chemotherapy. Unfortunately, especially the SHH/p53-mutated and group 3 MBs have the worst prognosis with a 5-year overall survival of about 50% and 40% only. The majority of surviving patients suffers from long-term adverse effects like cognitive degeneration and endocrinal malfunction. Previous in vitro studies showed a significant combinatorial effect of IR, the demethylating agent 5-aza-2′deoxycytidine (5-aza-dC, decitabine), and the telomerase inhibitor abacavir (ABC) on the clonogenic MB cell death. In addition, the multimodal treatment tested in an ex vivo murine hippocampal tissue slice model revealed no neurotoxicity. Here, we translate those results in vivo in an orthotopic mouse model using a SHH/p53-mutated human MB PDX cell line (kindly provided by Dr. Kool, DKFZ Heidelberg).

MB-bearing mice were treated with 0.1 mg/kg/d of decitabine and/or 50 mg/kg/abacavir daily for 14 days and/or with a 2 Gy single dose IR at day 8. Mice were euthanized as soon as defined human endpoints occurred (= survival time). Brain tissue was cryo-preserved for further gene expression analyses by RT-PCR (angiogenesis: VEGF, CD31; tumour cell stemness: nestin, SOX-2, CD15, CD133; inflammation: IBA, CD68; proliferation: Ki67) and histological staining (classical histology: H/E; proliferation: Ki-67; angiogenesis: CD31).

Kaplan-Meier analyses (n = 10 per group) for overall survival showed mice receiving triple treatment had a significantly longer survival time (median 66 ± 9 d) compared to mice receiving only IR (49 ± 2 d, p = 0.02) or two-drug treatment without IR (decitabine + ABC) (48 ± 3 d, p = 0.02). First analyses of H/E staining showed invasive tumour growth composed of characteristically large, blue cells with an altered nucleo-cytoplasmic ratio. CD31 staining revealed both small and large tumour blood vessels with a significant lower amount in the triple-treated compared to the untreated group (16% reduction, p = 0.01). Ki-67 staining reliably identifies tumour tissue by enhanced proliferation index in comparison to surrounding unaffected brain tissue. Mice receiving triple therapy showed significantly less proliferation compared to control mice (19% reduction, p ≤ 0.001). Gene expression analyses are still ongoing.

The combined multimodal therapy with decitabine, abacavir, and IR significantly prolongs overall survival in an orthotopic SHH/p53-mutated MB mouse model. To verify this promising effect in other MB subgroups, we currently test the therapy concept in an orthotopic group 3 mouse model.