Endocrine Abstracts

Currently, no effective medical therapy exists for inoperable phaeochromocytomas (PCCs). Thus, we investigated the antitumor potential of the dual PI3K/mTOR inhibitor NVP-BEZ235 against PCC in vivo. We employed a spontaneous model of endogenous aggressive PCCs (MENX rats), which closely recapitulate the human tumours. Administration of NVP-BEZ235 to MENX rats for 14 days inhibited proliferation (assessed by Ki67, NuSAP staining), vascularity (CD31, VEGFA), and induced apoptosis (active caspase 3) of PCCs in vivo. Diffusion weighted magnetic resonance imaging (DW-MRI) with calculated mean apparent diffusion coefficient (ADC) values pre- and post-therapy were determined and used as surrogate markers of tumour cellularity. ADC values were significantly increased after drug treatment (P=0.0208), further supporting a cytotoxic role of NVP-BEZ235 in PCC. Gene expression profiling (by RT-PCR arrays) of tumours from drug-treated and vehicle-treated rats showed that NVP-BEZ235 affects the expression of genes involved in cell death and survival, cell proliferation and motility. Among the genes down-regulated in NVP-BEZ235-treated vs vehicle-treated rat tumours was Scl6a2, encoding the norepinephrine transporter (NET), a common target of positron emitted tomography (PET) imaging of PCC. Specifically, we observed a dose-dependent reduction of both Slc6a2 (by TaqMan) and NET (by immunohistochemistry) expression in rat PCCs following NVP-BEZ235 administration, which associated with decreased uptake of the radiolabeled norepinephrine analogue 18F-LMI1195 in vivo. To assess the relationship between NET levels and response to NVP-BEZ235, we generated a drug-resistant derivative of the MPC (mouse PCC) cell line. While incubation with NVP-BEZ235 reduced NET expression in MPC cells, no reduction was observed in the resistant derivative cells. This suggests that decreased NET expression associates with the ability of PCC cells to respond to PI3K/mTOR inhibition. Altogether, our data demonstrate that targeting PI3K/mTOR signalling is effective against PCCs and suggest that NET levels may represent a surrogate marker of therapy response to PI3K/mTOR inhibitors, which can be monitored by functional PET imaging.

Disclosure: This work was supported by SFB824, subproject B08, from the Deutsche Forschungs-gemeinschaft (DFG).