I-HuCC49deltaCH2 for TAG-72 antigen-directed positron emission tomography (PET) imaging of LS174T colon adenocarcinoma tumor implants in xenograft mice: preliminary results

Background: F-fluorodeoxyglucose positron emission tomography (F-FDG-PET) is widely used in diagnostic cancer imaging. However, the use of F-FDG in PET-based imaging is limited by its specificity and sensitivity. In contrast, anti-TAG (tumor associated glycoprotein)-72 monoclonal antibodies are highly specific for binding to a variety of adenocarcinomas, including colorectal cancer. The aim of this preliminary study was to evaluate a complimentary determining region (CDR)-grafted humanized CH2-domain-deleted anti-TAG-72 monoclonal antibody (HuCC49deltaCH2), radiolabeled with iodine-124 ( I), as an antigen-directed and cancer-specific targeting agent for PET-based imaging. Methods: HuCC49deltaCH2 was radiolabeled with I. Subcutaneous tumor implants of LS174T colon adenocarcinoma cells, which express TAG-72 antigen, were grown on athymic Nu/Nu nude mice as the xenograft model. Intravascular (i.v.) and intraperitoneal (i.p.) administration of I-HuCC49deltaCH2 was then evaluated in this xenograft mouse model at various time points from approximately 1 hour to 24 hours after injection using microPET imaging. This was compared to i.v. injection of F-FDG in the same xenograft mouse model using microPET imaging at 50 minutes after injection. Results: At approximately 1 hour after i.v. injection, I-HuCC49deltaCH2 was distributed within the systemic circulation, while at approximately 1 hour after i.p. injection, I-HuCC49deltaCH2 was distributed within the peritoneal cavity. At time points from 18 hours to 24 hours after i.v. and i.p. injection, I-HuCC49deltaCH2 demonstrated a significantly increased level of specific localization to LS174T tumor implants (p = 0.001) when compared to the 1 hour images. In contrast, approximately 50 minutes after i.v. injection, F-FDG failed to demonstrate any increased level of specific localization to a LS174T tumor implant, but showed the propensity toward more nonspecific uptake within the heart, Harderian glands of the bony orbits of the eyes, brown fat of the posterior neck, kidneys, and bladder. Conclusions: On microPET imaging, I-HuCC49deltaCH2 demonstrates an increased level of specific localization to tumor implants of LS174T colon adenocarcinoma cells in the xenograft mouse model on delayed imaging, while F-FDG failed to demonstrate this. The antigen-directed and cancer-specific I-radiolabled anti-TAG-72 monoclonal antibody conjugate, I-HuCC49deltaCH2, holds future potential for use in human clinical trials for * Correspondence: [email protected]; [email protected]; [email protected] Division of Pharmaceutics, College of Pharmacy, The Ohio State University, Columbus, Ohio, 43210, USA Division of Surgical Oncology, Department of Surgery, Arthur G. James Cancer Hospital and Richard J. Solove Research Institute and Comprehensive Cancer Center, The Ohio State University, Columbus, Ohio, 43210, USA Full list of author information is available at the end of the article Zou et al. World Journal of Surgical Oncology 2010, 8:65 http://www.wjso.com/content/8/1/65 WORLD JOURNAL OF SURGICAL ONCOLOGY © 2010 Zou et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. preoperative, intraoperative, and postoperative PET-based imaging strategies, including fused-modality PET-based imaging platforms. Background The origin of positron imaging dates back to the early 1950’s [1], culminating in the development of positron emission tomography (PET) and its subsequent evolution over the last 40 years [1-4]. The clinical application of PET-based imaging strategies to the field of oncology has had a significant impact upon the care of cancer patients [5-11]. Therefore, the development and selection of the most appropriate and specific radiotracer for PET-based imaging is critical to its success in oncology [12-15]. F-fluorodeoxyglucose (F-FDG) is currently the most widely used radiotracer for PET-based imaging strategies [16]. In this regard, F-FDG-PET-based imaging is considered state-of-the-art for the diagnostic imaging, staging, and follow-up of a wide variety of malignancies, including colorectal cancer [10,11]. However, there are several intrinsic limitations related to the use of F-FDG-PET imaging that remain a challenge and a concern to those involved in the care of cancer patients [6-9,16-24]. First, false positive results can occur with F-FDG-PET imaging in the presence of any pathologic conditions in which there is a high rate of glucose metabolism, such as inflammatory or infectious processes. Second, false negative results can occur with F-FDG-PET imaging secondary to poor avidity of FFDG to certain tumor types and secondary to impaired uptake of F-FDG in patients with elevated blood glucose levels. Third, due to system resolution limitations, F-FDG-PET imaging is generally limited in its ability to detect small-volume, early-stage primary disease or to detect microscopic disease within the lymph nodes. Fourth, F-FDG-PET imaging can produce either false positive or false negative results secondary to the normal physiologic accumulation of F-FDG within certain tissues with an elevated level of glucose metabolism (most striking in the brain and heart, and to a lesser degree in the mucosa and smooth muscle of the stomach, small intestine and colon, as well as in liver, spleen, skeletal muscle, thyroid, and brown fat) and secondary to the excretion and accumulation of F-FDG within the urinary tract (kidneys, ureters, and bladder). Overall, these factors have a negative impact on optimizing the specificity and sensitivity of F-FDG-PET for accurate diagnostic cancer imaging [6-9,16-24]. A PET-based imaging approach that specifically targets the cancer cell environment would clearly have a significant potential advantage for improving the accuracy of diagnostic cancer imaging over that of the more nonspecific nature of F-FDG. In that regard, tumorassociated glycoprotein-72 (TAG-72) is a mucin-like glycoprotein complex that is overexpressed by many adenocarcinomas, including colorectal, pancreatic, gastric, esophageal, ovarian, endometrial, breast, prostate, and lung [22,24-27]. Such overexpression of TAG-72 is noted in up to approximately 90% of these various adenocarcinomas [24]. In xenograft mice bearing subcutaneous tumor implants of the TAG-72-expressing human colon adenocarcinoma cell line, LS174T [27-29], antiTAG-72 monoclonal antibodies have been shown to accumulate up to 18-fold higher in LS174T tumor implants than in normal tissues [25,30,31]. Over the last 25 years, our group at The Ohio State University, as well as others, have evaluated a variety of radioiodine labeled anti-TAG-72 monoclonal antibodies for tumorspecific antigen targeting at the time of surgery for known primary, recurrent, and metastatic disease, as well as for targeting occult disease and affected lymph nodes in colorectal cancer patients [22,24,32-59]. Most recently, we have evaluated the complimentary determining region (CDR)-grafted humanized CH2domain-deleted anti-TAG-72 monoclonal antibody, HuCC49deltaCH2 [60-63], radiolabeled with iodine-125 (I), for intraoperative tumor detection of colorectal cancer in both a preclinical xenograft mouse model and in a human clinical trial [22,24,57-59]. Collectively, our experience with radiolabeled anti-TAG-72 monoclonal antibodies in combination with a handheld gamma detection probe has clearly shown that this technology provides the surgeon with real-time intraoperative information for more precise tumor localization and resection and has demonstrated improved long-term patient survival after surgery [22,24]. Because of the drawbacks of using I as the radioiodine label for anti-TAG-72 monoclonal antibodies, including the extremely long physical half-life of I of approximately 60 days (which generates handling, storage, and disposal issues within the operating room environment and in the surgical pathology department) and the inability of I to allow for diagnostic imaging capabilities, other radionuclides have been sought for use with anti-TAG-72 monoclonal antibodies. One such alternative is iodine-124 (I) [64]. In this regard, I is a positron emitting radionuclide that has a physical halflife of approximately 4.2 days, for which its positron emitting properties makes it well-suited for PET-based imaging and for which its shorter physical half-life simplifies the handling, storage, and disposal issues. Zou et al. World Journal of Surgical Oncology 2010, 8:65 http://www.wjso.com/content/8/1/65 Page 2 of 13