Chopra Arvind
National Center for Biotechnology Information, NLM, Bethesda, MD 20894
Most individuals suffering from pancreatic adenocarcinoma (PAC) do not survive for more than 1 year after diagnosis, and <1% of these patients live beyond 5 years (1). Although surgical resection of the cancer is a possible intervention for this disease, only 10%–25% of the patients are considered suitable for this treatment because, by the time that the neoplasm is detected, the malignancy has metastasized to other organs and the tumor load in the patient is too high to warrant surgery (2). Patients with nonresectable PAC are treated either with gemcitabine or radiotherapy or a combination of the two; however, these treatments are not curative because they only prolong survival and improve the quality of life of the patient (2). Early detection of this invasive cancer would facilitate proper staging of the disease so that a suitable treatment regimen can be initiated to possibly improve patient prognosis (3). In this regard, the monoclonal antibody (mAb) PAM4, which specifically targets mucin 1 (MUC1), a glycoprotein that is overexpressed only in PAC tumors, was developed, radiolabeled with I or In, and shown with scintigraphy to detect neoplastic tumors in patients with pancreatic malignancies (4). However, intact radiolabeled antibodies (5) are known to have incomplete tumor penetration due to their large size (~150 kDa), and they are of limited utility to visualize cancerous lesions with different imaging modalities because they have a long blood circulating half-life and usually generate a low signal/noise (S/N) ratio (5). To amplify the signal obtained from an imaging agent so that it can be used to detect malignant tumors noninvasively, investigators have developed and evaluated a variety of strategies in preclinical studies with animals. One such strategy involves the pretargeting of cancer lesions with a suitable mAb (or its derivative), allowing some time for the pretargeting Ab to bind to the cancerous tissue and clear from blood circulation; the animals subsequently are injected with an appropriate radiolabeled small molecular weight ligand that binds to the pretargeting mAb or its derivative. This technique has been shown to generate higher S/N ratios during imaging compared to ratios obtained with a directly labeled mAb alone (6-8). Use of the pretargeting technique with multivalent (i.e., containing more than one antigen binding site), multispecific (i.e., can bind more than a single type of antigen) Abs for the imaging and therapy of cancer has been discussed in detail elsewhere (9, 10). Cardillo et al. developed bsPAM4 (or bsmAb), a divalent, bispecific F(ab’) mAb, by cross-linking a PAM4 Fab’ fragment to a murine anti–In-diethylenetriamine pentaacetic acid (In-DTPA) mAb Fab’ fragment and used the unlabeled bsPAM4 to pretarget human CaPan-1 cell xenograft PAC tumors in nude mice (4). The animals were then exposed to a radiolabeled peptide hapten that bound to the In-DTPA mAb Fab’; imaging showed that this technique generated higher S/N ratios compared to the directly labeled bsmAb alone. From this study, the investigators concluded that the peptide was suitable for use with scintigraphy to target and visualize human xenograft PAC tumors in nude mice (4). In an effort to further improve the S/N ratio that can be obtained with the pretargeting technique, Gold et al. generated a recombinant trivalent (i.e., three binding sites) bsmAb, designated TF10, and evaluated its use with scintigraphy for the visualization of xenograft PAC tumors in mice (1). The TF10 bsmAb is divalent for binding to MUC1 and monovalent for binding to a histamine-succinyl-glycine (HSG) motif containing hapten (DOTA-d-Tyr-d-Lys(HSG)-d-Glu-d-Lys(HSG)-NH; IMP-288; HSG hapten), which was generated by linking two PAM4 Fab fragments to an anti-HSG hapten mAb Fab fragment as described elsewhere (1). Recently, the use of Y-labeled IMP-288 in the pretargeted radioimmunotherapy of mice bearing PAC tumors was shown to be superior to the application of Y-labeled PAM4 IgG alone in the animals (11). The biodistribution of In-labeled IMP-288 ([In]-IMP-288) and the visualization of PAC tumors in mice with this labeled hapten is discussed in this chapter. The biodistribution of I-labeled TF10 ([I]-TF10) was studied in mice bearing PAC tumors and is discussed in a separate chapter of MICAD (www.micad.nih.gov) (12).
大多数胰腺腺癌（PAC）患者在确诊后存活时间不超过1年，这些患者中不到1%能存活5年以上（1）。尽管手术切除癌症是针对这种疾病的一种可能干预措施，但只有10% - 25%的患者被认为适合这种治疗，因为在检测到肿瘤时，恶性肿瘤已经转移到其他器官，患者体内的肿瘤负荷过高，无法进行手术（2）。无法进行手术切除的PAC患者接受吉西他滨或放疗或两者联合治疗；然而，这些治疗并非治愈性的，因为它们只能延长患者的生存期并改善生活质量（2）。早期检测这种侵袭性癌症将有助于对疾病进行适当分期，从而启动合适的治疗方案，有可能改善患者的预后（3）。在这方面，开发了单克隆抗体（mAb）PAM4，它特异性靶向粘蛋白1（MUC1），一种仅在PAC肿瘤中过度表达的糖蛋白，用碘或铟进行放射性标记，并通过闪烁显像显示可检测胰腺恶性肿瘤患者的肿瘤（4）。然而，完整的放射性标记抗体（5）由于其尺寸较大（约150 kDa），已知肿瘤穿透不完全，并且它们在使用不同成像方式可视化癌性病变方面的效用有限，因为它们在血液中的循环半衰期长，通常产生低信号/噪声（S/N）比（5）。为了放大从成像剂获得的信号，以便能够用于无创检测恶性肿瘤，研究人员在动物的临床前研究中开发并评估了多种策略。一种这样的策略涉及用合适的单克隆抗体（或其衍生物）对癌性病变进行预靶向，让预靶向抗体有一些时间与癌组织结合并从血液循环中清除；随后给动物注射与预靶向单克隆抗体或其衍生物结合的合适的放射性标记小分子配体。与单独使用直接标记的单克隆抗体相比，该技术在成像过程中已显示产生更高的S/N比（6 - 8）。在其他地方已详细讨论了使用具有多价（即含有多个抗原结合位点）、多特异性（即能结合不止一种类型的抗原）抗体的预靶向技术进行癌症的成像和治疗（9, 10）。Cardillo等人通过将PAM4 Fab’片段与鼠抗铟 - 二乙烯三胺五乙酸（In - DTPA）单克隆抗体Fab’片段交联，开发了二价、双特异性F(ab’)单克隆抗体bsPAM4（或bsmAb），并使用未标记的bsPAM4对裸鼠体内的人CaPan - 1细胞异种移植PAC肿瘤进行预靶向（4）。然后给动物注射与In - DTPA单克隆抗体Fab’结合的放射性标记肽半抗原；成像显示与单独使用直接标记的bsmAb相比，该技术产生更高的S/N比。从这项研究中，研究人员得出结论，该肽适用于闪烁显像，以靶向并可视化裸鼠体内的人异种移植PAC肿瘤（4）。为了进一步提高通过预靶向技术可获得的S/N比，Gold等人制备了一种重组三价（即三个结合位点）bsmAb，命名为TF10，并评估其在闪烁显像中用于可视化小鼠体内异种移植PAC肿瘤的用途（1）。TF10 bsmAb对MUC1的结合是二价的，对含有组胺 - 琥珀酰 - 甘氨酸（HSG）基序的半抗原（DOTA - d - Tyr - d - Lys(HSG) - d - Glu - d - Lys(HSG) - NH；IMP - 288；HSG半抗原）的结合是单价的，该半抗原是通过将两个PAM4 Fab片段与抗HSG半抗原单克隆抗体Fab片段连接而产生的，如其他地方所述（1）。最近，在携带PAC肿瘤的小鼠的预靶向放射免疫治疗中使用Y标记的IMP - 288已显示优于在动物中单独应用Y标记的PAM4 IgG（11）。本章讨论了铟标记的IMP - 288（[In] - IMP - 288）的生物分布以及用这种标记的半抗原对小鼠体内PAC肿瘤的可视化。在携带PAC肿瘤的小鼠中研究了碘标记的TF10（[I] - TF10）的生物分布，并在MICAD（www.micad.nih.gov）的单独一章中进行了讨论（12）。
