Abstract A09: Preclinical trial using the combination of panobinostat with bortezomib in human orthotopic xenograft mouse models of rhabdomyosarcoma

Background: Rhabdomyosarcoma (RMS) is an aggressive malignancy of childhood which has a poor prognosis when it has metastasized. Although there has been improvement in overall survival for low and intermediate risk patients, there has been no significant improvement in outcome in the past 25 years for patients with high risk disease. Orthotopic patient derived mouse xenografts (O-PDXs) have been created from patients with high risk disease to facilitate preclinical trials and provide useful information regarding efficacy and toxicity of novel therapeutic agents. Histone deacetylases (HDAC) have emerged as a relevant clinical target in RMS and other pediatric solid tumors, and the synergistic combination of HDAC inhibitors with proteasome inhibitors has led to increased cellular stress and apoptosis in other malignancies. The HDAC inhibitor panobinostat combined with bortezomib, a proteasome inhibitor, is a combination currently in consideration for clinical trial development for this patient population. Methods: Orthotopic xenografts were created by injecting patient derived RMS cells into the hind-leg muscle of CD-1 nude mice. Detailed molecular and histopathologic characterization of the tumors was performed to ensure they recapitulate human disease. Primary cultures of dissociated O-PDXs were used for high-throughput drug screening in dose response. Pharmacokinetic studies on RMS tumor-bearing mice were performed to determine matched human AUC-guided dosing and schedule of chemotherapy agents. Preclinical phase I tolerability studies were performed for all drug combinations. The following preclinical phase II study was performed using 2 different O-PDXs (SJRBH000026_X1, SJRHB012_Y) both derived from patients with metastatic recurrent embryonal RMS: (SJRBH000026_X1, SJRHB012_Y) Group 1 (n = 5, n = 4) Panobinostat/Bortezomib Group 2 (n = 3, n = 3) Vincristine/Irinotecan (relapse standard of care) Group 3 (n = 3, n = 3) Control (no chemotherapy) Enrollment into the 3 treatment groups was randomized for each of the O-PDXs tested. Two courses of therapy were given on a clinically relevant schedule with each course consisting of 21 days. The study endpoint was reached and mice were classified as progressive disease when tumor approached 20% of body weight. Results: Both O-PDXs tested in vitro were found to be highly sensitive to panobinostat and bortezomib with EC50 values in the nanomolar range for both drugs. Drug combinations were found to be well tolerated in preclinical phase I studies at the doses tested with only temporary mild dehydration observed in the panobinostat/bortezomib group. In the preclinical phase II study, mice in group 1 were found to have an average survival of 16.5 days (avg. of 11 days, 22 days respectively). Mice receiving the standard of care therapy (group 2) survived an average of 40.7 days (avg. of 49 days, 32.3 days respectively). Mice in group 3 were determined to have progressive disease on average at 17 days (avg. of 12.3 days, 21.7 days respectively). Conclusions: On the basis of preclinical in vitro data, combining HDAC and proteasome inhibitors is an attractive strategy for RMS and was found to be tolerable in preclinical phase I studies. However, the use of this combination in O-PDXs failed to show meaningful efficacy. The lack of response in relevant RMS models raises concerns about the sensitivity observed in in vitro studies and additional biologic and pharmacokinetic studies are indicated to identify factors leading to this discrepancy prior to further clinical development of these agents for this patient population. Citation Format: Elizabeth Stewart, Cori Bradley, Anang Shelat, Alberto Pappo, Michael A. Dyer. Preclinical trial using the combination of panobinostat with bortezomib in human orthotopic xenograft mouse models of rhabdomyosarcoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr A09.