Sustained Inhibition of Receptor Tyrosine Kinases and Macrophage Depletion by PLX3397 and Rapamycin as a Potential New Approach for the Treatment of MPNSTs

Abstract

Purpose Malignant peripheral nerve sheath tumor (MPNST) is a highly aggressive tumor type that is resistant to chemotherapy and there are no effective therapies. MPNSTs have been shown to have gene amplification for receptor tyrosine kinases (RTKs), PDGFR and c-Kit. We tested the c-Kit inhibitor, imatinib, and PLX3397, a selective c-Fms and c-Kit inhibitor, to evaluate their efficacy against MPNST cells in vitro and in vivo. Experimental Design We tested the efficacy of imatinib or PLX3397 either alone or in combination with TORC1 inhibitor rapamycin in a cell proliferation assay in vitro and by immunoblotting to determine target inhibition. Immunoblotting and immunohistochemical analysis was further carried out using xenograft samples in vivo. Results Our in vitro studies show that imatinib and PLX3397 similarly inhibit cell growth and this can be enhanced with rapamycin with comparable target specificity. However, in vivo studies clearly demonstrate that compared to imatinib, PLX3397 results in sustained blockade of c-Kit, c-Fms and PDGFRβ, resulting in significant suppression of tumor growth. Moreover, staining for Iba-1, a marker for macrophages, indicates that PLX3397 results in significant depletion of macrophages in the growing tumors. The combination of PLX3397 and rapamycin results in even greater macrophage depletion with continued growth suppression, even when the drug treatment is discontinued. Conclusions Taken together, our data strongly suggests that PLX3397 is superior to imatinib in the treatment of MPNST, and the combination of PLX3397 with a TORC1 inhibitor could provide a new therapeutic approach for the treatment of this disease.

Therapeutic Peptide Vaccine-Induced CD8 T Cells Strongly Modulate Intratumoral Macrophages Required for Tumor Regression

“…The typical steep shrinkage of tumors that starts 10 days after SLP vaccination was abrogated in the presence of PLX3397, an inhibitor of the myeloid growth receptors CSF-1R and c-Kit (21,22), and in the presence of PLX5622, a weaker but selective CSF-1R inhibitor (24). Upon inhibition of intratumoral macrophages an initial stabilization of tumor sizes was followed by rapid regrowth, instead of shrinkage.…”

MLN0128, an ATP-Competitive mTOR Kinase Inhibitor with Potent In Vitro and In Vivo Antitumor Activity, as Potential Therapy for Bone and Soft-Tissue Sarcoma

“…Malignant peripheral nerve sheath tumor (MPNST, ST8814) and rhabdomyosarcoma cell line RMS-559 were supplied by Dr. Jonathan Fletcher (Dana Farber Cancer Institute, Boston, MA). RMS-559 (22), MPNST and ST8814 (34) cell lines were authenticated as previously described. Ewing Sarcoma (CHP100, A673) cell lines were obtained from Dr. Melinda S. Merchant (Center for Cancer Research, NCI/NIH, Bethesda, MD).…”

CSF-1/CSF-1R targeting agents in clinical development for cancer therapy

“…A similar mechanistic explanation of increased CSF-1 expression derived from the irradiated tumor cells followed by enhanced recruitment of TAM and myeloid-derived suppressor cells (MDSC) was provided by the authors. The combination of PLX3397 together with the mTOR inhibitor rapamycin showed higher efficacy than each monotherapy in malignant peripheral nerve sheath tumor cell lines and xenograft model [14]. TAM, particularly the perivascular Tie2 expressing subpopulation, have been implicated in angiogenesis and evasion of anti-angiogenic therapies [41,42].…”

“…Possible combination partners have been evaluated in pre-clinical models with clinically well-established treatment modalities such as chemotherapeutic agents [31,38,39] and irradiation [40]. Further combinations included targeted therapies [14], anti-angiogenic therapies [42][43][44], adoptive T cell transfer [50,51] as well as immune checkpoint inhibitors [52 ].…”