Klivényi G, Schuhmacher J, Patzelt E, Hauser H, Matys R, Moock M, Regiert T, Maier-Borst W
Department of Diagnostic and Therapeutic Radiology, German Cancer Research Center, Heidelberg.
J Nucl Med. 1998 Oct;39(10):1769-76.
Recently, we demonstrated the feasibility of combining improved tumor-to-tissue contrasts and PET imaging for immunoscintigraphic tumor localization using a multistep targeting technique that consists of the administration of an antitumor/antihapten bispecific monoclonal antibody (BS-MAb), a blocker to saturate the antihapten binding sites of the BS-MAb that are still present in the circulation, and a low molecular weight Ga chelate, labeled with positron emitter 68Ga, serving as the hapten. Due to this technique, the biodistribution of the radiolabeled hapten is governed mainly by the binding characteristics of both the antitumor and the antihapten part of the BS-MAb. For a future clinical implementation of the method, we investigated MAb VFF18, which is reactive with the adhesion molecule CD44V6, a tumor-associated antigen, and up-regulated in colon, squamous cell and pancreas carcinoma, and two anti-Ga chelate MAbs, which are highly selective for only one of the two enantiomers (optical isomers) of the inherently racemic Ga chelate.
From the VFF18 MAb and the anti-Ga chelate MAbs, two BS-MAbs containing the same antitumor parts, but different antihapten parts, were prepared and tested for multistep targeting in human colon carcinoma-bearing nude mice.
Despite identical biodistributions of both BS-MAbs and their very similar affinities for the corresponding Ga chelate enantiomers, tumor uptake of the two enantiomers 1 hr postinjection was significantly different [8.7 +/- 1.9% versus 5.8% +/- 1.6% of the injected dose/g (%i.d./g)], with tumor-to-blood ratios being higher for the BS-MAb showing the lower tumor uptake (7.6 +/- 1.6 versus 4.7 +/- 0.6). From data obtained with each BS-MAb, a similar initial tumor binding of approximately 15.5%i.d./g, but different in vivo half-lives of the corresponding BS-MAb-enantiomer immune complexes, could be estimated. Pretargeting with a mixture of both BS-MAbs followed by the administration of the racemic Ga chelate resulted in the lowest tumor uptake (3.9% +/- 1.5%i.d./g). PET imaging of nude mice with the enantiomeric, as well as with the racemic, 68Ga chelate demonstrated a clear delineation of tumors against blood pool background.
Multistep immunoscintigraphy with BS-MAbs markedly increases tumor-to-tissue ratios in nude mice and enables PET imaging. Using a BS-MAb containing MAb VFF18, a more sensitive localization of CD44V6-positive tumors in patients should also be obtained.
最近，我们证明了使用多步靶向技术将改进的肿瘤与组织对比度和PET成像相结合用于免疫闪烁肿瘤定位的可行性，该技术包括给予抗肿瘤/抗半抗原双特异性单克隆抗体（BS-MAb）、一种阻断剂以饱和循环中仍存在的BS-MAb的抗半抗原结合位点，以及一种用正电子发射体68Ga标记的低分子量Ga螯合物作为半抗原。由于该技术，放射性标记半抗原的生物分布主要由BS-MAb的抗肿瘤和抗半抗原部分的结合特性决定。为了该方法未来的临床应用，我们研究了与粘附分子CD44V6（一种肿瘤相关抗原）反应且在结肠癌、鳞状细胞癌和胰腺癌中上调的单克隆抗体VFF18，以及两种抗Ga螯合物单克隆抗体，它们对固有外消旋Ga螯合物的两种对映体（光学异构体）中的仅一种具有高度选择性。
从VFF18单克隆抗体和抗Ga螯合物单克隆抗体中制备了两种包含相同抗肿瘤部分但不同抗半抗原部分的BS-MAb，并在荷人结肠癌裸鼠中进行多步靶向测试。
尽管两种BS-MAb的生物分布相同且它们对相应Ga螯合物对映体的亲和力非常相似，但注射后1小时两种对映体的肿瘤摄取量显著不同[分别为注射剂量/克（%i.d./g）的8.7±1.9%和5.8%±1.6%]，肿瘤摄取量较低的BS-MAb的肿瘤与血液比值更高（分别为7.6±1.6和4.7±0.6）。根据每种BS-MAb获得的数据，可以估计最初肿瘤结合量相似，约为15.5%i.d./g，但相应的BS-MAb-对映体免疫复合物的体内半衰期不同。先用两种BS-MAb的混合物进行预靶向，然后给予外消旋Ga螯合物，导致肿瘤摄取量最低（3.9%±1.5%i.d./g）。用对映体以及外消旋68Ga螯合物对裸鼠进行PET成像，显示肿瘤在血池背景下清晰可辨。
用BS-MAb进行多步免疫闪烁显著提高了裸鼠的肿瘤与组织比值，并实现了PET成像。使用包含单克隆抗体VFF18的BS-MAb，也应能在患者中更灵敏地定位CD44V6阳性肿瘤。
