Gray Institute for Radiation Oncology and Biology, University of Oxford, Old Road Campus Research Building, Off Roosevelt Drive, OX3 7DQ Oxford, UK.
Nucl Med Biol. 2012 Oct;39(7):986-92. doi: 10.1016/j.nucmedbio.2012.04.006. Epub 2012 Jun 8.
[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.
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.
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.
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.
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.
[18F]氟脱氧葡萄糖正电子发射断层扫描（[18F]FDG-PET）广泛用于监测临床治疗反应，最近已被提议作为长期反应的早期标志物。这依赖于这样一个假设，即葡萄糖消耗的变化与活力和长期生长潜力的降低平行。然而，细胞可能利用除葡萄糖以外的其他底物，并且由于许多治疗药物直接干扰葡萄糖代谢，因此，阳性[18F]FDG-PET 反应可能与长期生长无关是完全合理的。此外，代谢和增殖的变化可能发生在不同的时间尺度上，因此限制了[18F]FDG-PET 具有预测性的时间窗口。PI3K 致癌信号通路是多种细胞过程的主调节因子，包括葡萄糖代谢、增殖和细胞存活。已经表明抑制 PI3K 可降低几种肿瘤类型中[18F]FDG 的摄取，但该途径对葡萄糖代谢和增殖的相对影响尚未完全确定。
我们提出了（i）评估[18F]FDG 作为测量 PI3K 抑制反应的示踪剂的适用性，以及（ii）确定最佳成像方案，在体外。我们使用多细胞肿瘤球体，这是一种出色的无血管肿瘤的 3D 体外模型，来研究 PI3K 抑制剂 NVP-BKM120 和 NVP-BEZ235 对[18F]FDG 摄取及其与 3D 生长的关系的影响。
从两条具有组成性激活的 PI3K/Akt 通路的细胞系 EMT6（高度增殖的小鼠乳腺）和 FaDu（中度增殖的人鼻咽）中制备球体。治疗包括 24 小时暴露于抑制剂中的一种，并且在接下来的 7 天中监测生长。为了模拟[18F]FDG-PET 的潜在成像方案，测量了每个活细胞的平均[18F]FDG 摄取（a）直接在 24 小时暴露后，（b）在额外的 24 小时恢复期后，或（c）在 48 小时暴露后。
在两种细胞系的球体中，生长均以剂量依赖性方式受到显著限制（p<0.0001）。在高度增殖的细胞系 EMT6 中，[18F]FDG 摄取在所有浓度的抑制剂中均显著降低。对于中度增殖的细胞系 FaDu，[18F]FDG 呈剂量依赖性方式受到影响，但程度较小。为了评估[18F]FDG 摄取对长期生长限制的预测性，为每个成像方案计算了 Pearson 相关系数。这些表明不同细胞系之间的最佳成像方案不同。
这项研究表明，[18F]FDG 可能是我们研究的细胞系中 PI3K 抑制反应的合适标志物。我们的数据支持这样一种假设，即成像方案应根据肿瘤类型进行优化。
