Li Fiona, Hicks Justin W, Yu Lihai, Desjardin Lise, Morrison Laura, Hadway Jennifer, Lee Ting-Yim
Department of Medical Biophysics, The University of Western University, 1151 Richmond Street North, London, ON, N6A 3K7, Canada.
Lawson Health Research Institute, Grosvenor Campus, 268 Grosvenor Street, London, ON, N6A 4V2, Canada.
EJNMMI Res. 2020 Nov 23;10(1):141. doi: 10.1186/s13550-020-00705-2.
In molecular imaging with dynamic PET, the binding and dissociation of a targeted tracer is characterized by kinetics modeling which requires the arterial concentration of the tracer to be measured accurately. Once in the body the radiolabeled parent tracer may be subjected to hydrolysis, demethylation/dealkylation and other biochemical processes, resulting in the production and accumulation of different metabolites in blood which can be labeled with the same PET radionuclide as the parent. Since these radio-metabolites cannot be distinguished by PET scanning from the parent tracer, their contribution to the arterial concentration curve has to be removed for the accurate estimation of kinetic parameters from kinetic analysis of dynamic PET. High-performance liquid chromatography has been used to separate and measure radio-metabolites in blood plasma; however, the method is labor intensive and remains a challenge to implement for each individual patient. The purpose of this study is to develop an alternate technique based on thin layer chromatography (TLC) and a sensitive commercial autoradiography system (Beaver, Ai4R, Nantes, France) to measure radio-metabolites in blood plasma of two targeted tracers-[F]FAZA and [F]FEPPA, for imaging hypoxia and inflammation, respectively.
Radioactivity as low as 17 Bq in 2 µL of pig's plasma can be detected on the TLC plate using autoradiography. Peaks corresponding to the parent tracer and radio-metabolites could be distinguished in the line profile through each sample (n = 8) in the autoradiographic image. Significant intersubject and intra-subject variability in radio-metabolites production could be observed with both tracers. For [F]FEPPA, 50% of plasma activity was from radio-metabolites as early as 5-min post injection, while for [F]FAZA, significant metabolites did not appear until 50-min post. Simulation study investigating the effect of radio-metabolite in the estimation of kinetic parameters indicated that 32-400% parameter error can result without radio-metabolites correction.
TLC coupled with autoradiography is a good alternative to high-performance liquid chromatography for radio-metabolite correction. The advantages of requiring only small blood samples (~ 100 μL) and of analyzing multiple samples simultaneously, make the method suitable for individual dynamic PET studies.
在动态正电子发射断层扫描（PET）的分子成像中，靶向示踪剂的结合和解离通过动力学建模来表征，这需要准确测量示踪剂的动脉血浓度。放射性标记的母体示踪剂一旦进入体内，可能会发生水解、去甲基化/去烷基化及其他生化过程，导致血液中产生并积累不同的代谢物，这些代谢物可与母体示踪剂标记相同的PET放射性核素。由于PET扫描无法区分这些放射性代谢物和母体示踪剂，因此在通过动态PET的动力学分析准确估算动力学参数时，必须消除它们对动脉血浓度曲线的影响。高效液相色谱法已被用于分离和测量血浆中的放射性代谢物；然而，该方法劳动强度大，且对每个患者实施起来仍然是一项挑战。本研究的目的是开发一种基于薄层色谱法（TLC）和灵敏的商业放射自显影系统（Beaver，Ai4R，法国南特）的替代技术，以测量两种靶向示踪剂——[F]FAZA和[F]FEPPA血浆中的放射性代谢物，分别用于成像缺氧和炎症。
使用放射自显影技术，在TLC板上可检测到猪血浆2 μL中低至17 Bq的放射性。在放射自显影图像中，通过每个样本（n = 8）的线轮廓可区分出对应于母体示踪剂和放射性代谢物的峰。两种示踪剂均观察到放射性代谢物产生存在显著的个体间和个体内变异性。对于[F]FEPPA，早在注射后5分钟，50%的血浆活性来自放射性代谢物，而对于[F]FAZA，直到注射后50分钟才出现显著的代谢物。研究放射性代谢物对动力学参数估计影响的模拟研究表明，不进行放射性代谢物校正可能导致32% - 400%的参数误差。
TLC结合放射自显影是用于放射性代谢物校正的高效液相色谱法的良好替代方法。该方法仅需少量血样（约100 μL）且可同时分析多个样本，这些优点使其适用于个体动态PET研究。
