3D tumour spheroids as a model to assess the suitability of [18F]FDG-PET as an early indicator of response to PI3K inhibition.

BACKGROUND: [18F] Fluorodeoxyglucose Positron Emission Tomography ([18F]FDG-PET) is widely used to monitor response to therapy in the clinic and has, more recently, been proposed as an early marker of long term response. This relies on the assumption that a change in glucose consumption parallels a reduction in viability and long term growth potential. However, cells may utilise substrates other than glucose and as many therapeutics interfere with glucose metabolism directly, it is entirely plausible that a positive [18F]FDG-PET response may be unrelated to long term growth. Furthermore, changes in metabolism and proliferation may take place on different temporal scales, thus restricting the time window in which [18F]FDG-PET is predictive. The PI3K oncogenic signalling pathway is a master regulator of multiple cellular processes including glucose metabolism, proliferation and cell survival. Inhibition of PI3K has been shown to reduce [18F]FDG uptake in several tumour types but the relative influence of this pathway on glucose metabolism and proliferation is not fully established. AIM: We proposed to (i) assess the suitability of [18F]FDG as a tracer for measuring response to PI3K inhibition and (ii) determine the optimum imaging schedule, in vitro. We used multicellular tumour spheroids, an excellent 3D in vitro model of avascular tumours, to investigate the effects of the PI3K inhibitors, NVP-BKM120 and NVP-BEZ235, on [18F]FDG uptake and its relation to 3D growth.
METHODS: Spheroids were prepared from two cell lines with a constitutively active PI3K/Akt pathway, EMT6 (highly proliferative mouse mammary) and FaDu (moderately proliferate human nasopharyngeal). Treatment consisted of a 24h exposure to either inhibitor, and growth was monitored over the following 7 days. To mimic potential imaging regimens with [18F]FDG-PET, average [18F]FDG uptake per viable cell was measured (a) directly following the 24h exposure, (b) following an additional 24h recovery period, or (c) following a 48 h exposure.
RESULTS: Growth was restricted significantly (p<0.0001) in a dose-dependent fashion in spheroids from both cell lines treated with either inhibitor. In the highly proliferative cell line EMT6, [18F]FDG uptake was significantly reduced at all concentrations of inhibitor. For the moderately proliferative cell line FaDu, [18F]FDG was affected in a dose-dependent fashion, but to lesser degree. To assess the predictivity of [18F]FDG uptake for long term growth restriction, Pearson correlation coefficients were calculated for each imaging regimen. These indicated that the optimal imaging schedules differed between cell lines.
CONCLUSION: This study suggests that [18F]FDG may be a suitable marker of response to PI3K inhibition in the cell lines that we have studied. Our data support the hypothesis that imaging schedules should be optimised on a tumour type-specific basis.