Poethko Thorsten, Schottelius Margret, Thumshirn Georgette, Hersel Ulrich, Herz Michael, Henriksen Gjermund, Kessler Horst, Schwaiger Markus, Wester Hans-Jürgen
Nuklearmedizinische Klinik und Poliklinik, Klinikum rechts der Isar, Technische Universität München, München, Germany.
J Nucl Med. 2004 May;45(5):892-902.
Routine application of (18)F-labeled peptides for quantitative in vivo receptor imaging of receptor-expressing tissues and quantification of receptor status using PET is limited by the lack of appropriate radiofluorination methods for routine large-scale synthesis of (18)F-labeled peptides. To satisfy this demand, a new (18)F-labeling methodology based on the chemoselective oxime formation between an unprotected aminooxy-functionalized peptide and an (18)F-labeled aldehyde or ketone was investigated and optimized with respect to peptide conjugation.
4-[(18)F]Fluorobenzaldehyde ([(18)F]FB-CHO) was prepared from the 4-formyl-N,N,N-trimethylanilinium precursor via direct no-carrier-added (18)F-fluorination (dimethyl sulfoxide, 60 degrees C, 15 min) and purified using a cation-exchange/reversed-phase cartridge system. Radiochemical yields (RCYs) of N-(4-[(18)F]fluorobenzylidene)oxime ([(18)F]FBOA) formation with various aminooxy-modified peptides such as minigastrin, RGD, and octreotate analogs were investigated as a function of reaction time and temperature, peptide concentration, and pH. Biodistribution studies were performed with an [(18)F]FBOA-RGD dimer ((c(RGDfE)HEG)(2)-K-Dpr-[(18)F]FBOA, 60 and 120 min after injection) and a gylcosylated [(18)F]FB-Tyr(3)-octreotate (Gluc-S-Dpr([(18)F]FBOA)TOCA), 10 and 60 min after injection) using M21 and M21L human melanoma and AR42J rat pancreatic tumor-bearing nude mice, respectively.
[(18)F]FB-CHO was obtained in a nonoptimized RCY of 50% within 30 min. At low peptide concentrations (0.5 mmol/L), optimal [(18)F]FBOA-labeling efficiencies (60%-80%) were obtained within 15 min at 60 degrees C and pH 2-3, independently of the peptide used, affording the [(18)F]FBOA-peptides in overall RCYs of up to 40% (from end of bombardment) after purification. Both (c(RGDfE)HEG)(2)-K-Dpr-[(18)F]FBOA and Gluc-S-Dpr([(18)F]FBOA)TOCA showed pharmacokinetics suitable for early (<or=60 min) high-contrast PET imaging, high tumor uptake (2.48 +/- 0.15 %ID/g [RGD] and 21.8 +/- 1.4 %ID/g [TOCA] at 60 min after injection, where %ID/g = percentage injected dose per gram), and tumor-to-organ ratios that compared well with the corresponding [(18)F]fluoropropionyl analogs [(18)F] Galacto-RGD and Gluc-Lys([(18)F]FP)TOCA, which are prepared via multistep procedures.
Oxime formation between aminooxy-functionalized peptides and an (18)F-labeled aldehyde or ketone-in this case, [(18)F]FB-CHO-combines fast 1-step, high-yield synthesis of an (18)F-labeled prosthetic group stable against in vivo defluorination with rapid, 1-step chemoselective conjugation to unprotected peptides under mild conditions. Thus, it allows fast and straightforward large-scale production of (18)F-labeled peptides for clinical routine PET application. Furthermore, it opens new perspectives to peptide radiohalogenation in general, permitting labeling of the same precursor both with diagnostic ((18)F, (124)I, (120 g)I, (123)I) and therapeutic ((211)At, (131)I) radiohalogens.
由于缺乏适用于常规大规模合成¹⁸F标记肽的放射性氟化方法，¹⁸F标记肽在表达受体组织的体内定量受体成像及使用PET定量受体状态方面的常规应用受到限制。为满足这一需求，研究并优化了一种基于未保护的氨氧基官能化肽与¹⁸F标记的醛或酮之间化学选择性肟形成的新型¹⁸F标记方法，以用于肽偶联。
4-[(¹⁸F]氟苯甲醛([(¹⁸F]FB-CHO)由4-甲酰基-N,N,N-三甲基苯胺前体通过直接无载体添加¹⁸F氟化反应（二甲基亚砜，60℃，15分钟）制备，并使用阳离子交换/反相柱系统纯化。研究了用各种氨氧基修饰的肽（如小胃泌素、RGD和奥曲肽类似物）形成N-(4-[(¹⁸F]氟亚苄基)肟([(¹⁸F]FBOA))的放射化学产率（RCYs）与反应时间、温度、肽浓度和pH的关系。分别使用M21和M21L人黑色素瘤及AR42J大鼠荷胰腺肿瘤裸鼠，对[(¹⁸F]FBOA-RGD二聚体（(c(RGDfE)HEG)₂-K-Dpr-[(¹⁸F]FBOA，注射后60和120分钟）和糖基化[(¹⁸F]FB-Tyr³-奥曲肽（Gluc-S-Dpr([(¹⁸F]FBOA)TOCA)，注射后10和60分钟）进行生物分布研究。
在30分钟内以50%的非优化RCY获得[(¹⁸F]FB-CHO。在低肽浓度（0.5 mmol/L）下，在60℃和pH 2 - 3条件下，15分钟内可获得最佳[(¹⁸F]FBOA标记效率（60% - 80%），与所用肽无关，纯化后得到的[(¹⁸F]FBOA - 肽的总RCY高达40%（从轰击结束时起）。(c(RGDfE)HEG)₂-K-Dpr-[(¹⁸F]FBOA和Gluc-S-Dpr([(¹⁸F]FBOA)TOCA)均显示出适合早期（≤60分钟）高对比度PET成像的药代动力学，肿瘤摄取高（注射后60分钟时分别为2.48±0.15 %ID/g [RGD]和21.8±1.4 %ID/g [TOCA]，其中%ID/g =每克注射剂量的百分比），肿瘤与器官的比值与通过多步程序制备的相应[(¹⁸F]氟丙酰类似物[(¹⁸F]半乳糖-RGD和Gluc-Lys([(¹⁸F]FP)TOCA)相当。
氨氧基官能化肽与¹⁸F标记的醛或酮（在这种情况下为[(¹⁸F]FB-CHO）之间的肟形成，将快速的一步高产率合成对体内脱氟稳定的¹⁸F标记辅基与在温和条件下与未保护肽的快速一步化学选择性偶联相结合。因此，它允许快速直接地大规模生产用于临床常规PET应用的¹⁸F标记肽。此外，它总体上为肽放射性卤化开辟了新的前景，允许用诊断性（¹⁸F、¹²⁴I、¹²⁰gI、¹²³I）和治疗性（²¹¹At、¹³¹I）放射性卤素标记相同的前体。
