Preclinical in vivo evaluation of rapamycin in human malignant peripheral nerve sheath explant xenograft Priya Bhola 1,2 , Sutapa Banerjee 3 , Joydeep Mukherjee 1,2 , Anand Balasubramanium 1,2 , Vedant Arun 1,2 , Zia Karim 1,2 , Kelly Burrell 1,2 , Sidney Croul 4 , David H. Gutmann 3 and Abhijit Guha 1,2 1 The Arthur and Sonia Labatt Brain Tumor Research Centre, Hospital for Sick Children, University of Toronto, Ontario, Canada 2 Div. of Neurosurgery, University Health Network, University of Toronto, Ontario, Canada 3 Department of Neurology, Washington University School of Medicine, St. Louis 4 Department of Pathology, University Health Network, University of Toronto, Canada Neurofibromatosis type 1 (NF1) patients are prone to the development of malignant tumors, the most common being Malignant Peripheral Nerve Sheath Tumor (MPNST). NF1-MPNST patients have an overall poor survival due to systemic metastasis. Currently, the management of MPNSTs includes surgery and radiation; however, conventional chemotherapy is not very effective, underscoring the need for effective biologically-targeted therapies. Recently, the NF1 gene product, neurofibromin, was shown to negatively regulate the phosphoinositide-3-kinase (PI3K)/Protein Kinase-B (Akt)/mammalian Target Of Rapamycin (mTOR) pathway, with loss of neurofibromin expression in established human MPNST cell lines associated with high levels of mTOR activity. We developed and characterized a human NF1-MPNST explant grown subcutaneously in NOD-SCID mice, to evaluate the effect of the mTOR inhibitor rapamycin. We demonstrate that rapamycin significantly inhibited human NF1-MPNST mTOR pathway activation and explant growth in vivo at doses as low as 1.0 mg/kg/ day, without systemic toxicities. While rapamycin was effective at reducing NF1-MPNST proliferation and angiogenesis, with decreased CyclinD1 and VEGF respectively, there was no increase in tumor apoptosis. Rapamycin effectively decreased activation of S6 downstream of mTOR, but there was accompanied increased Akt activation. This study demonstrates the therapeutic potential and limitations of rapamycin in NF1-associated, and likely sporadic, MPNSTs. Malignant Peripheral Nerve Sheath Tumors (MPNST) have an incidence of 0.001% in the general population, however, as many as 5–10% of individuals with the inherited cancer syndrome, neurofibromatosis type 1 (NF1) will develop an MPNST in their lifetime. In adults with NF1, MPNSTs are the most common malignancy and the major source of mor- bidity. 1 The poor survival associated with NF1-associated MPNST (NF1-MPNST) reflects the earlier age of onset, mul- tiple MPNSTs, and their overall resistance to conventional chemo- and radiation treatment. Unfortunately, pre- or post- operative adjuvant chemotherapy is not effective 2 and the five year survival after MPNST diagnosis is 20% for indi- viduals with NF1. 3 In addition, these tumors have a high propensity to metastasize to bone and lung, even after surgi- cal excision and local radiation therapy. 4 As such, there is a recognized need for effective targeted biological therapies for NF1-MPNSTs. With the identification of the NF1 protein (neurofibro- min), it became possible to consider treatments that reverse the biochemical abnormalities resulting from loss of neurofi- bromin function in NF1-MPNST. Neurofibromin functions in part as a Ras GTPase Activating Protein (Ras-GAP) by accelerating the conversion of active, GTP-bound p21-Ras to inactive, GDP-bound Ras and reducing p21-Ras stimulation of cell growth. We and others have previously shown that p21-ras-GTP levels are elevated in human NF1-MPNST tumors. 5,6 However, biological therapies targeting p21-Ras activation, such as farnesyltransferase inhibitors (FTIs), have had limited efficacy in human clinical trials. 7,8 Recently, neurofibromin has been shown to negatively reg- ulate the mammalian Target Of Rapamycin (mTOR) pathway in a p21-Ras and PI3K/Akt-dependent manner. 9,10 In these studies, both human and mouse neurofibromin-deficient pri- mary cells and tumors exhibited high levels of mTOR activa- tion. These observations have prompted several preclinical studies aimed at evaluating mTOR inhibitors for their ability to inhibit the growth of NF1-associated tumors in mice. 11–13 Key words: Malignant Peripheral Nerve Sheath Tumor, Neurofibromatosis, Mammalian Target of Rapamycin, Neurofibromin Additional Supporting Information may be found in the online version of this article Grant sponsors: PB was supported by scholarships from OSOTF while SB was supported by a Young Investigator Award from the Children’s Tumor Foundation. This work was supported by grants from the Children’s Tumor Foundation Drug Discovery Initiative to AG and DHG and Concept Award from Department of Defense to AG DOI: 10.1002/ijc.24783 History: Received 18 Feb 2009; Accepted 17 Jul 2009; Online 24 Jul 2009 Correspondence to: Abhijit Guha, 4W-446, Western Hosp, 399 Bathurst St, Toronto, Ontario, Canada M5T-2S8, E-mail: Abhijit.Guha@uhn.on.ca Cancer Therapy Int. J. Cancer: 126, 563–571 (2010) V C 2009 UICC International Journal of Cancer IJC