Aerts Hugo J W L, Dubois Ludwig, Perk Lars, Vermaelen Peter, van Dongen Guus A M S, Wouters Bradly G, Lambin Philippe
Department of Radiation Oncology (MAASTRO), GROW-School for Oncology and Developmental Biology, Maastricht University, Maastricht, The Netherlands.
J Nucl Med. 2009 Jan;50(1):123-31. doi: 10.2967/jnumed.108.054312. Epub 2008 Dec 17.
The epidermal growth factor receptor (EGFR) is highly expressed in a significant number of human malignancies, and its expression is associated with tumor aggressiveness and overall treatment resistance. The monoclonal antibody cetuximab is increasingly used in clinical settings as a treatment modality in combination with more conventional therapies, such as radio- and chemotherapy. Currently, little is known about tumor-specific uptake and overall pharmacokinetics. Noninvasive quantification of cetuximab uptake could provide important diagnostic information for patient selection and therapy evaluation. To this end, we have developed and validated a novel probe using cetuximab labeled with the long-lived positron emitter 89Zr for PET imaging.
Tumor cell lines with varying EGFR expression levels were used for in vivo tumor imaging experiments. PET with 89Zr-labeled cetuximab (3.75+/-0.14 MBq) was performed on tumor-bearing NMRI-nu mice at multiple time points after injection (ranging from 1 to 120 h) and quantified by drawing regions of interest on selected tissues. Uptake was compared by biodistribution gamma-counting, and ex vivo EGFR expression levels were quantified using Western blot analysis.
Uptake of 89Zr-labeled cetuximab was demonstrated in the EGFR-positive tumors. However, the EGFR levels measured in vivo did not correlate with the relative signal obtained by PET. Tumor-to-blood ratios were significantly higher in the cell lines with intermediate (compared with the high) EGFR expression starting from 24 h after injection. Normal tissue uptake was unaffected by the different tumor types. Ex vivo gamma-counting experiments confirmed the observed in vivo PET results. A similar disparity was found between 89Zr-labeled cetuximab tumor uptake and in vivo EGFR expression levels as demonstrated by Western blotting.
The 89Zr-labeled cetuximab imaging probe is a promising tool for noninvasive evaluation of cetuximab uptake. Our results demonstrate a disparity between in vivo EGFR expression levels and cetuximab uptake. In a general sense, the results indicate a disparity between antibody uptake and expression levels of a biologic target in a tumor, suggesting that additional pharmacokinetic or pharmacodynamic mechanisms influence tumor delivery of this therapy. These additional mechanisms may explain why receptor expression levels alone are not sufficient to predict patient response.
表皮生长因子受体（EGFR）在大量人类恶性肿瘤中高度表达，其表达与肿瘤侵袭性和整体治疗耐药性相关。单克隆抗体西妥昔单抗越来越多地在临床环境中作为一种治疗方式与更传统的疗法（如放疗和化疗）联合使用。目前，关于肿瘤特异性摄取和整体药代动力学知之甚少。西妥昔单抗摄取的非侵入性定量可为患者选择和治疗评估提供重要的诊断信息。为此，我们开发并验证了一种新型探针，该探针使用用长寿命正电子发射体89Zr标记的西妥昔单抗进行PET成像。
使用具有不同EGFR表达水平的肿瘤细胞系进行体内肿瘤成像实验。在注射后多个时间点（范围为1至120小时）对荷瘤NMRI-nu小鼠进行89Zr标记的西妥昔单抗（3.75±0.14 MBq）PET检查，并通过在选定组织上绘制感兴趣区域进行定量。通过生物分布γ计数比较摄取情况，并使用蛋白质印迹分析对离体EGFR表达水平进行定量。
在EGFR阳性肿瘤中证实了89Zr标记的西妥昔单抗的摄取。然而，体内测量的EGFR水平与PET获得的相对信号不相关。从注射后24小时开始，EGFR表达中等（与高表达相比）的细胞系中肿瘤与血液的比率显著更高。正常组织摄取不受不同肿瘤类型的影响。离体γ计数实验证实了观察到的体内PET结果。蛋白质印迹显示，89Zr标记的西妥昔单抗肿瘤摄取与体内EGFR表达水平之间也存在类似差异。
89Zr标记的西妥昔单抗成像探针是一种用于非侵入性评估西妥昔单抗摄取的有前途的工具。我们的结果表明体内EGFR表达水平与西妥昔单抗摄取之间存在差异。一般来说，结果表明肿瘤中抗体摄取与生物靶点表达水平之间存在差异，这表明额外的药代动力学或药效学机制影响了该疗法的肿瘤递送。这些额外的机制可能解释了为什么仅受体表达水平不足以预测患者反应。
