The application of radiotherapy to the pediatric preclinical testing program: Results of a pilot study.

Abstract

9544 Background: The Pediatric Preclinical Testing Program (PPTP) has been successfully utilized to determine the efficacy of novel agents by testing via its mouse-flank in vivo model. We report on the feasibility and biologic outcomes of a pilot study using rhabdomyosarcoma (RMS) xenograft lines treated with radiotherapy (RT) alone and concurrently with the mTOR tyrosine kinase inhibitor, AZD8055, using the PPTP model. Methods: We developed a mouse flank irradiation device for daily delivery of RT in clinically relevant doses (2 Gy per fraction up to 40 Gy).Two RMS xenograft lines of the PPTP, Rh30 (alveolar) and Rh18 (embryonal), were implanted into SCID mice, grown to appropriate volumes and were subjected to fractionated RT. In a second study, daily co-administration of AZD8055 (5-20 mg/Kg, gavage) with RT was performed. Cure rates (durable complete response >12 weeks post-treatment) and RT dose densities (given dose / initial xenograft volume, Gy/cc) were compared between groups. Results: With RT alone at mean dose-densities of 59-60 Gy/cc, cure was achieved in only 4/18 (22%) of the Rh30-bearing mice and 9/12 (75%) of the Rh18-bearing mice (p=0.006). Profiling data revealed higher levels of Fanconi anemia pathway gene expression in Rh30 compared to the more sensitive Rh18. Since recent data showed conditional knockout of mTOR resulted in the loss of FANCD2 gene expression, we postulated that blockade of TORC1/TORC2 with AZD8055 would reduce FANCD2 and increase the RT-sensitivity of Rh30. The addition of AZD8055 to RT resulted in a selective sensitization of the Rh30 line. With a mean RT dose-density of 27 Gy/cc, the cure rate in Rh30-bearing mice improved to 11/15 (73%). For the Rh18 group, the cure rate was 7/15 (46%) at a mean dose density of 44 Gy/cc. Western blot analysis showed the co-administration of AZD8055 abrogated the brisk increase in mTOR signaling and FANCD2 expression after the first several 2 Gy fractions of RT; most strikingly in Rh30. Conclusions: This study demonstrates the feasibility of applying RT to the PPTP model. It recapitulated the expected clinical radiobiology and demonstrated its utility in preclinical testing and the discovery of novel mechanisms of RT resistance in pediatric tumors.