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Gastroenteropancreatic neuroendocrine tumors (GEP-NET) are slow-growing neoplasms that arise from the neuroendocrine system of the gastrointestinal tract and pancreas. These may be classified based on location into the following: pheochromocytomas and parangangliomas; carcinoids; and pancreatic endocrine tumors. The majority of these tumors are nonfunctional, and thus, molecular imaging methods are critical in detection and staging of disease. Meta-iodobenzyl guanidine (MIBG) is a norepinephrine analog taken up by norepinephrine transporters that are overexpressed in the majority of GEP-NET. Radioactive MIBG can be used to image GEP-NET. The isotopes suitable for imaging include iodine-123 and iodine-131, using single-photon cameras, and iodine-124, using positron emission tomography (PET). Imaging is usually carried out a day or more after administration of the radiotracer, and serial and tomographic imaging may be necessary for optimal delineation. MIBG imaging is more useful for detecting pheochromocytoma, with reported accuracies greater than 80%, than for detecting carcinoid tumors, where the accuracy has been ∼70% and is reportedly higher in mid-gut tumors. MIBG imaging has been invaluable in the accurate staging of children with neuroblastoma, a lethal childhood tumor of the sympathetic nervous system. An important application of MIBG imaging is to demonstrate targeting of therapeutic I-131 MIBG. Imaging is thus useful in the detection of disease as well as in the demonstration of adequate targeting for therapy — either qualitatively or quantitatively with dosimetry. The latter will probably be feasible with PET using isotopes like iodine-124, and perhaps with single photon emission computed tomography/computed tomography. Imaging with MIBG will continue to be the mainstay for detection and staging of GEP-NET. More importantly, perhaps, imaging with MIBG will form part of an imaging continuum, including assessment of glycolytic rate and somatostatin receptor status, that will enable assessment of tumor phenotype and guide management.

Recent developments in tumor imaging, made possible by advances in instrumentation and radiopharmaceuticals, has led to an increasing need for accurate anatomic correlation of single photon emission computed tomography (SPECT) and positron emission tomography (PET) images. Fusion imaging permits the functional strengths of SPECT and PET to be combined with the anatomic resolution of computed tomography (CT) and magnetic resonance imaging (MRI). Clinical applications of fusion imaging include the evaluation of brain tumors, lymphoma, hepatic lesions and monoclonal antibody studies. The continued development of these techniques will eventually allow fusion imaging to become a routine part of nuclear medicine practice.

Gallium-67-citrate is one of the most useful radiopharmaceuticals to detect tumors, stage extent of the disease, monitor response to treatment and distinguish recurrent disease from post-treatment changes. Gallium is likewise very sensitive to detect and locate infections and inflammatory foci. This application is extremely important in the management of immunocompromised cancer patients. Image interpretation should be tempered with full knowledge of the patients clinical condition, anatomic alterations due to prior surgery and correct timing of image acquisition. Early imaging at 4-6 h is useful to detect gastrointestinal infections, whereas 24 h imaging is used to evaluate chest infections. Although gallium-67 is a non-specific agent, the identification of the etiology of the inflammation may be improved by adequate clinical and laboratory information as well as correlation with other imaging modalities such as sonography and computerized x-ray tomography (CT). High dose (10 mCi) gallium-67 single photon emission computed tomography (SPECT) imaging with image co-registration is important for accurate uptake localization.

The humanized monoclonal antibody A33 (huA33) is a potential targeting agent against colorectal carcinoma since the A33 antigen is highly and homogenously expressed in95% of all colorectal cancers, both primary tumors and metastases. The aim of this study was to determine the biodistribution and tumor-targeting ability of (177)Lu-labeled huA33.huA33 was labeled with the beta-emitting therapeutic nuclide (177)Lu using the chelator CHX-A"-DTPA, and the properties of the (177)Lu-CHX-A"-huA33 ((177)Lu-huA33) conjugate was determined both in vitro and in vivo in a biodistribution study in nude mice xenografted with colorectal SW1222 tumor cells.The (177)Lu-huA33 conjugate bound specifically to colorectal cancer cells in vitro (with a K(D) value of 2.3+/-0.3 nM, determined by a saturation assay) and in vivo. The tumor uptake of (177)Lu-huA33 was very high, peaking at 134+/-21%ID/g 72 h postinjection (pi). Normal tissue uptake was low; radioactivity concentration in blood (which had the second highest radioactivity concentration) was lower than in tumor at all time points studied (8 h to 10 days). The tumor-to-blood ratio increased with time, reaching 70+/-30, 10 days pi. Throughout the study, the uptake of (177)Lu in bone (known to accumulate free (177)Lu) was low, and the fraction of protein-bound (177)Lu in plasma samples was high (95% to 99%). This indicates high stability of the (177)Lu-huA33 conjugate in vivo.The (177)Lu-huA33 conjugate shows a very favorable biodistribution, with an impressively high tumor uptake and high tumor-to-organ ratios, indicating that the conjugate may be suitable for radioimmunotherapy of colorectal cancer.

Multidrug resistance (MDR) in tumors is associated with P-glycoprotein (Pgp) expression. In vivo quantitation of Pgp may allow MDR to be evaluated noninvasively prior to treatment planning. The purpose of this study was to radiolabel MRK-16, a monoclonal antibody that targets an external epitope of P-glycoprotein, and perform in vivo quantitation of P-glycoprotein in a MDR xenograft nude mouse model. MRK-16 was labeled with 125 I by the iodogen method, with subsequent purification by size exclusion chromatography. Groups of 10 Balb/c mice were each xenografted with colchicine-resistant or -sensitive neuroblastoma cell lines, respectively. Whole body clearance and tumor uptake over time was quantitated by gamma camera imaging, and biodistribution studies were performed with [ 125 I]MRK-16 and an isotype matched control antibody, A33. Quantitative autoradiography and immunohistochemistry analysis of tumors was also evaluated to confirm specific targetting of [ 125 I]MRK-16. Peak tumor uptake was at 2–3 days post-injection, and was significantly greater in resistance compared to sensitive tumors (mean % injected dose/g ± SD) (18.76 ± 2.94 vs 10.93 ± 0.96; p p 125 I]MRK-16 was confirmed by comparison to [ 131 I]A33 in biodistribution studies, and localized to cellular components of tissue stroma by comparison of histologic and autoradiographic sections of sensitive and resistant tumors. Immunoblot analysis demonstrated a 4.5-fold difference in P-glycoprotein expression between sensitive and resistant cell lines without colchicine selective pressure. We conclude that in vivo quantitation of P-glycoprotein in MDR tumors can be performed with [ 125 I]MRK-16. These findings suggest a potential clinical application for radiolabeled MRK-16 in the in vivo evaluation of multidrug resistance in tumors.

Radioimmunotherapy with radiolabeled monoclonal antibodies is increasingly effective for hematopoietic tumors, with a number of investigators reporting persistent major responses. Radioimmunotherapy for solid tumors has been more difficult and only an occasional major response has been reported and these have so far not been persistent. Toxicity is predominantly hematopoietic, with platelets being most sensitive to the effects of radiation. Even at ultra-high doses (up to 28 mCi/kg of 131I), second organ toxicity has not been reached. Rational approaches to dose planning are becoming possible with improvements in dosimetry, based on quantitative SPECT and PET imaging. Current therapeutic indices for tumor/marrow, the most radiosensitive organ, are in the range of 5-10 to 1. This is probably still too low for curative treatment of solid tumors, and further refinements, perhaps based on novel antibody formulations, are needed.

Ten patients with colorectal cancer metastases received 125I-B72.3 and 131I-CC49 prior to laparotomy (five patients received 1 mg, and five 20 mg of each mAb). Tumor:serum ratios of 131I-CC49 were better than those of 125I-B72.3 (P < 0.01 at 1 mg; P = 0.05 at 20 mg; P < 0.01 at both doses). All known lesions ⩾ 1 cm in diameter were visualized at the 20 mg dose. There was no difference in absolute tumor uptake of 125I-B72.3 or 131I-CC49. We conclude that mAb CC49 has better relative uptake in colorectal cancers than mAb B72.3.