Photoacoustic imaging of multiple targets using gold nanorods.

Photoacoustic (PA) imaging has been used mainly for anatomical and functional imaging. Although functionalized nanoparticles also have been developed for PA molecular imaging, only single targeting has been demonstrated. In this study, PA imaging of multiple targets using gold nanorods is demonstrated experimentally using HER2 and CXCR4 as target molecules. The two corresponding monoclonal antibodies were conjugated to two types of gold nanorod with different aspect ratios. Gold nanorods with mean aspect ratios of 5.9 and 3.7 exhibited peak optical absorptions at 1000 and 785 nm, respectively. Appropriate selection of laser irradiation wavelength enhances PA signals by 7-12 dB and allows signals from gold nanorods corresponding to specific bindings to be distinguished. This approach potentially allows the expression levels of different oncogenes of cancer cells to be revealed simultaneously.

Increased MIBG uptake after transfer of the human norepinephrine transporter gene in rat hepatoma.

UNLABELLED: The transport of MIBG by the human norepinephrine transporter (hNET) seems to be the critical step in the treatment of MIBG-concentrating tumors. Therefore, we investigated whether the accumulation of MIBG may be induced by retroviral transfection of the hNET gene in Morris hepatoma cells. METHODS: A bicistronic retroviral vector for the transfer of the hNET coding sequence and the hygromycin resistance gene was generated. Morris hepatoma cells (MH3924A) were infected with the respective retroviral particles, and hNET-expressing cell lines MHhNEThyg1 to MHhNEThyg9 were obtained through hygromycin selection. The uptake of (3)H-norepinephrine or (131)I-MIBG and the efflux of (131)I-MIBG were determined in transfected and wild-type cells. In addition, the (131)I-MIBG distribution was monitored in nude mice and rats bearing wild-type and hNET-expressing hepatomas. RESULTS: hNET-expressing hepatoma cell lines accumulated up to 36 times more norepinephrine than did wild-type cells and 8 times more than did hNET-expressing neuroblastoma cell line SK-N-SH. The addition of nisoxetine, a selective inhibitor of noradrenaline uptake, inhibited norepinephrine uptake. Maximal (131)I-MIBG accumulation was observed 2 h after incubation and was followed by 43% efflux within 4 h after the (131)I-MIBG-containing medium had been removed. In vivo experiments performed with nude mice bearing both hNET-expressing and wild-type tumors showed a 10-fold-higher accumulation of (131)I-MIBG in transfected tumors than in wild-type tumors. The ex vivo calculations revealed doses of 605 and 75 mGy in hNET-expressing and wild-type tumor tissues, respectively. CONCLUSION: Transduction of the hNET gene enables Morris hepatoma cells to accumulate norepinephrine and MIBG. However, the retention of MIBG is brief; therefore, the absorbed dose of radiation in vivo is not expected to be therapeutically effective.

The novel retinoid AHPN/CD437 induces a rapid but incomplete apoptotic response in human myeloma cells.

The synthetic retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphthalene carboxylic acid (AHPN/CD437) appears to possess an apoptotic activity superior to classical retinoids in vitro as in vivo. Numerous studies have shown that CD437-induced apoptosis is independent of its nuclear receptor activity, suggesting that CD437 might have a unique mechanism of action. The purpose of this study was to compare CD437- and all-trans retinoic acid (atRA)-induced cell death. CD437 provoked a rapid apoptotic phenotype immediately followed by secondary necrosis in RPMI 8226, U266 and L363 human myeloma cell lines. Nuclear apoptotic features were observed upon both CD437 and atRA treatments. In contrast, membrane blebbing and the subsequent formation of apoptotic bodies, a classical apoptotic event, was only observed upon atRA treatment. In addition, CD437, contrary to atRA, was unable to induce tissue transglutaminase (tTG), an intracellular enzyme involved in the formation of cross-linked protein polymers contributing to apoptotic morphological changes. Taken together, these data suggest that CD437 induces rapid but incomplete apoptotic phenotype in human myeloma cells.

[99mTc]Demobesin 1, a novel potent bombesin analogue for GRP receptor-targeted tumour imaging.

Demobesin 1 is a potent new GRP-R-selective bombesin (BN) analogue containing an open chain tetraamine chelator for stable technetium-99m binding. Following a convenient labelling protocol, the radiopeptide, [(99m)Tc]Demobesin 1, formed in nearly quantitative yields and with high specific activities. Both unlabelled and labelled peptide demonstrated high-affinity binding in membrane preparations of the human androgen-independent prostate adenocarcinoma PC-3 cell line. The IC(50) values determined for Demobesin 1 and [Tyr(4)]BN were 0.70+/-0.08 n M and 1.5+/-0.20 n M, respectively, while the K(d) defined for [(99m)Tc/(99g)Tc]Demobesin 1 was 0.67+/-0.10 n M. [(99m)Tc]Demobesin 1 was rather stable in murine plasma, whereas it degraded rapidly in kidney and liver homogenates. After injection in healthy Swiss albino mice, [(99m)Tc]Demobesin 1 accumulated very efficiently in the target organs (pancreas, intestinal tract) via a GRP-R-mediated process, as shown by in vivo receptor blocking experiments. An equally high and GRP-R-mediated uptake was exhibited by [(99m)Tc]Demobesin 1 after injection in PC-3 tumour-bearing athymic mice. The initial high radioligand uptake of 16.2+/-3.1%ID/g in the PC-3 xenografts at 1 h p.i. remained at a similar level (15.61+/-1.19%ID/g) at 4 h p.i. Even after 24 h p.i., when the radioactivity had cleared from all other tissues, a value of 5.24+/-0.67%ID/g was still observed in the tumour. The high and prolonged localization of [(99m)Tc]Demobesin 1 at the tumour site and its rapid background clearance are very promising qualities for GRP-R-targeted tumour imaging in man.

Uptake of [18F]fluorodeoxyglucose in human monocyte-macrophages in vitro.

The fact that fluorine-18 fluorodeoxyglucose ([(18)F]FDG) accumulates in inflammatory lesions as well as in tumours reduces the diagnostic specificity of positron emission tomography (PET) in oncology. The aim of this study was to characterise the uptake of [(18)F]FDG in isolated human monocyte-macrophages (HMMs) in vitro in comparison with that in human glioblastoma (GLI) and pancreatic carcinoma cells (PAN). The purity of HMM preparations was determined by immunohistochemical staining and their functional integrity was assessed by long-term incubation with iodine-131 acetylated bovine serum albumin. [(18)F]FDG uptake in HMMs was quantified as percent of whole [(18)F]FDG activity per well (% ID) or as % ID in relation to total protein mass. [(18)F]FDG uptake in HMMs significantly increased with culture duration, yielding 7.5%+/-0.9% (% ID/100 micro g) at day 14. Stimulation by lipopolysaccharide further enhanced [(18)F]FDG uptake in HMMs by a factor of 2. [(18)F]FDG uptake significantly decreased with increasing glucose concentration in the medium. Radio-thin layer chromatography of intracellular metabolites revealed that [(18)F]FDG was trapped by HMMs mainly as [(18)F]FDG-6-phosphate and [(18)F]FDG-1,6-diphosphate. [(18)F]FDG uptake was in the range of uptake values measured in GLI and PAN. By accumulating [(18)F]FDG in a manner analogous to uptake by tumour cells, activated HMMs may contribute to the [(18)F]FDG uptake values measured by PET in neoplasms.

In vitro and in vivo evaluation of a 99mTc(I)-labeled bombesin analogue for imaging of gastrin releasing peptide receptor-positive tumors.

A new radiolabeled bombesin analogue, [99mTc(I)-PADA-AVA]bombesin (7-14), was synthesized and in vitro and in vivo characterized. High affinity and rapid internalization were obtained in binding assays. A specific binding towards gastrin releasing peptide receptors-positive tissues, pancreas and tumor, was observed in CD-1 nu/nu mice bearing PC-3 prostate adenocarcinoma xenografts. We therefore conclude that [99mTc(I)-PADA-AVA]bombesin (7-14) might have promising characteristics for applications in nuclear medicine, namely for diagnosis of GRP receptor overexpressing tumors.

The type 2 human somatostatin receptor as a platform for reporter gene imaging.

The human somatostatin receptor subtype 2A (hSSTr2) is under evaluation as a reporter gene for molecular imaging applications. Two approved somatostatin analogues are already available for imaging expression of the reporter gene following delivery with adenoviral (Ad) vectors or other genetic targeting strategies. In animal models, Ad-mediated expression of hSSTr2 in subcutaneous and intraperitoneal tumors was detected by non-invasive gamma camera imaging. This review discusses the rationale and strategy for using the hSSTr2 reporter gene as a platform for imaging applications.

Comparative uptake of polyamines by prostate and non-prostate cancer cell lines.

The Km and Vmax of [14C]-radiolabeled polyamines were determined for PC-3 and AT3B-1 cell lines. With PC-3 Km values are in the following order: ornithine> spermidine> spermine> putrescine, while with AT3B-1 it was spermidine> ornithine> spermine> putrescine. To determine which of these polyamines exhibit higher accumulation, the relative uptake of all the four amines was studied with prostate (PC-3, AT3B-1, LNCaP) and non-prostate (MCF-7, KLN-205, OVCAR) cell lines at 10 and 20 microM after 1 hour. Spermine and spermidine accumulated at higher levels in prostate (AT3B-1 and LNCaP) over non-prostate cell lines (p < 0.01). Putrescine accumulated more in PC-3 and LNCaP than the non-prostate cancer cells.

Effects of irradiation on 99m Tc sestamibi and 201Tl uptake in a human papillary thyroid carcinoma cell line.

Both 99mTc sestamibi and 201Tl have been used in conjunction with 131I scintigraphy for follow-up of patients with thyroid cancer. The aim of the study was to determine if irradiation affects tracer uptake in papillary thyroid cancer cells. The human papillary carcinoma cell line (PAP/ES-1) used in this study was generated from a papillary thyroid tumour obtained after surgery. For the in vitro uptake studies cells were seeded at 2 x 105 cells/well into 12-well microtitre plates. Irradiation was performed with a 60Co source (total dose, 2 Gy and 10 Gy). After incubation at 37 degrees C the supernatants were saved for determination of the unincorporated activity. The reaction was stopped by washing the cells four times in ice cold phosphate buffered saline. Total cellular uptake was determined by measuring cell lysate radioactivity in a Compugammasystem and was expressed as per cent uptake per mg of total cellular protein. At continuous incubation 201Tl uptake was significantly (P<0.01) higher after radiation whereas no effect of irradiation was found on 99mTc sestamibi uptake. Pulsed experiments revealed that irradiated cells displayed a faster 201Tl efflux. The net tracer retention at 90 min was similar to 201Tl to that of 99mTc sestamibi. We conclude that 99mTc sestamibi kinetics in thyroid cancer are not affected by irradiation and may therefore be superior to 201Tl in the follow-up of thyroid cancer shortly after radiotherapy.

Use of the incretin hormone glucagon-like peptide-1 (GLP-1) for the detection of insulinomas: initial experimental results.

The non-invasive detection of insulinomas remains a diagnostic problem that is not solved by means of somatostatin receptor scintigraphy. We investigated the biokinetics and specificity of uptake and degradation of the incretin hormone glucagon-like peptide-1 (GLP-1) in a rat insulinoma cell line (RINm5F) in order to ascertain whether radiolabelled GLP-1 may be suitable for specific visualisation of insulinomas in vivo. GLP-1 (7-36)amide was radioiodinated according to the iodogen method. The specificity of the uptake of [(125)I]GLP-1(7-36)amide by RINm5F cells was investigated. Degradation products of GLP-1 (7-36)amide in the cell medium were purified by HPLC. Their masses and amino acid sequences were determined by (252)Cf-plasma desorption mass spectrometry. Lysosomal degradation was inhibited and after differential centrifugation the amount of radiotracer incorporated into lysosomes was determined. Biodistribution studies were performed in a rat insulinoma model (NEDH rats and RINm5F cells) with [(123)I]GLP-1(7-36)amide and its more stable agonist [(123)I]exendin 3. The uptake of radiotracer into insulinoma cells reached a maximum within 5 min. It was inhibited by an excess of unlabelled peptide. [(125)I]GLP-1(7-36)amide accumulated in the cells if lysosomal degradation was inhibited. Degradation products of the peptide were found in the cell medium. We determined their mass and derived their amino acid sequence. Radiolabelling of exendin 3 was more difficult than that of GLP-1 because of the lack of tyrosine in its primary structure. Biodistribution studies showed rapid blood clearance and uptake of the radiotracer into the tumour and the pancreas. It was also possible to detect insulinomas in an animal model by external scintigraphy using radioiodinated GLP-1 (7-36)amide and exendin 3. GLP-1 (7-36)amide is specifically internalised into insulinoma cells by a receptor-mediated mechanism. Our results demonstrate that GLP-1 receptor-directed scintigraphy may be a new method for the detection of insulinomas in vivo. Due to the short half-life of GLP-1, its more stable analogue exendin 3 may better suit this purpose in vivo.

Malignant gliomas display altered plasma membrane potential and pH regulation--interaction with Tc-99m-MIBI and Tc-99m-Tetrofosmin uptakes.

Two radiopharmaceuticals, Tc-99m-MIBI (MIBI) and Tc-99m-Tetrofosmin (Tfos), are currently used for in vivo non-invasive monitoring of the MultiDrug Resistant (MDR) status of tumours. As gliomas are highly multidrug resistant, it is expected that the tracers would be poorly retained in those cells, but the in vivo and in vitro studies to date have shown that Tfos was highly retained in malignant gliomas. The high degree of malignancy of tumour cells is linked to alterations of physiological parameters as plasma membrane potential and intracellular pH. In order to elucidate the contribution of those parameters to Tfos and MIBI uptakes in malignant gliomas, we used several glioma cell lines--G111, G5, G152, and 42 MG-BA. These cells showed to be chemoresistant with a high level of expression and activity of the Multidrug Resistant associated Protein 1 (MRP1). They also had an alkaline intracellular pH (pHi) related to the Na+/H+ antiporter (NHE-1) expression and depolarised plasma membranes (-45 to -55 mV). In spite of their chemoresistance, we have found a high accumulation of both radiotracers in gliomas, more important for Tfos than MIBI, related to the presence and activity of NHE-1. In conjunction, the uptakes of the tracers were only partially dependent upon the plasma membrane potential of the glioma cell lines, again Tfos uptake being less dependent on this parameter than MIBI uptake. In conclusion, the evidence accumulated in this study suggests that Tfos could be a suitable glioma marker in vivo.

Ultrasound-mediated disruption of cell membranes. I. Quantification of molecular uptake and cell viability.

Ultrasound-mediated drug delivery is a nonchemical, nonviral, and noninvasive method for targeted transport of drugs and genes into cells. Molecules can be delivered into cells when ultrasound disrupts the cell membrane by a mechanism believed to involve cavitation. This study examined molecular uptake and cell viability in cell suspensions (DU145 prostate cancer and aortic smooth muscle cells) exposed to varying peak negative acoustic pressures (0.6-3.0 MPa), exposure times (120-2000 ms), and pulse lengths (0.02-60 ms) in the presence of Optison (1.7% v/v) contrast agent. With increasing pressure and exposure time, molecular uptake of a marker compound, a calcein, increased and approached equilibrium with the extra cellular solution, while cell viability decreased. Varying pulse length produced no significant effect. All viability and molecular uptake measurements collected over the broad range of ultrasound conditions studied correlated with acoustic energy exposure. This suggests that acoustic energy exposure may be predictive of ultrasound's nonthermal bioeffects.

Ultrasound-mediated disruption of cell membranes. II. Heterogeneous effects on cells.

Ultrasound has been shown to reversibly and irreversibly disrupt membranes of viable cells through a mechanism believed to involve cavitation. Because cavitation is both temporally and spatially heterogeneous, flow cytometry was used to identify and quantify heterogeneity in the effects of ultrasound on molecular uptake and cell viability on a cell-by-cell basis for suspensions of DU145 prostate cancer and aortic smooth muscle cells exposed to varying peak negative acoustic pressures (0.6-3.0 MPa). exposure times (120-2,000 ms), and pulse lengths (0.02-60 ms) in the presence of Optison (1.7% v/v) contrast agent. Cell-to-cell heterogeneity was observed at all conditions studied and was classified into three subpopulations: nominal uptake (NUP), low uptake (LUP), and high uptake (HUP) populations. The average number of molecules within each subpopulation was generally constant: 10(4)-10(5) molecules/cell in NUP, approximately 10(6) molecules/cell in LUP, and approximately 10(7) molecules/cell in HUP. However, the fraction of cells within each subpopulation showed a strong dependence on both acoustic pressure and exposure time. Varying pulse length produced no significant effect. The distribution of cells among the three subpopulations correlated with acoustic energy exposure, which suggests that energy exposure may govern the ability of ultrasound to induce bioeffects by a nonthermal mechanism.

Factors affecting chemistry of reduction-mediated 99mTc-labelling of monoclonal antibodies and polyclonal human immunoglobulins.

In developing a new method for preparing a radiopharmaceutical for clinical investigation, a thorough understanding of reaction stoichiometry is crucial in optimizing the labelling chemistry. Factors determining labelling efficiency of the 2-mercaptoethanol (2-ME)-mediated 99mTc-labelling of antibody molecules were elucidated using anti-tumor monoclonal antibodies of different IgG subclasses (i.e. IOR-CEA(IgG1), M170(IgG1), 3F8(IgG3) and EMD (IgG2a)) and polyclonal human immunoglobulins (Sandoglobulin). Antibodies which were sensitive to 2-ME reduction (i.e. required 500-1000 molar excess of 2-ME) could tag 99mTc with high efficiency since they possessed abundant reactive sites (i.e. sulfydryl groups) for 99mTc binding. Reduction sensitivity of antibodies was unlikely to be affected by IgG subclass and could be rated as follows: Sandoglobulin > IOR-CEA > 3F8 > M170 > EMD. Concentrations of the reduced antibodies for effective labelling appeared to be related to the reduction sensitivity, i.e. 0.2, 0.4 and 0.6 mg/ml were required for labelling of IOR-CEA, 3F8 and M170 respectively. In addition, susceptibility to 2-ME reduction seemed to reflect the rate of antibody labelling. For 2-ME resistant molecules, i.e. M170 and EMD, successful labelling could be achieved by using a slow 99mTc reducing agent such as SnCl2 instead of SnF2 which reacted more rapidly. Since 2-ME generates reactive sulfhydryl groups that are distal to antigen binding sites, the immunoreactivity of the modified antibody was not affected by the effect of reduction.

Detection and measurement of in vitro gene transfer by gamma camera imaging.

The purpose of this work was to develop a high capacity method to image gene transfer to cancer cells growing as monolayers in cell culture plates. A sensitive and high capacity nuclear-imaging method for detection of gene transfer in vitro will allow rapid validation of vectors in different cell lines under various conditions. Human cancer cell lines (A-427 non-small cell lung, SKOV3.ip1 ovarian, MDA-MB-468 breast, and BxPC-3 pancreatic) were infected with a replication-incompetent adenoviral vector encoding the human type 2 somatostatin receptor (Ad-hSSTr2). Expression of the hSSTr2 reporter protein in cells was detected by imaging an internalized 99mTc-labeled, hSSTr2 binding peptide (P2045, Diatide, Inc.). Imaging provided an accurate measure of internally bound 99mTc as evidenced by equivalence of results for imaging region of interest (ROI) analyses and gamma counter measurements. Internally bound 99mTc-P2045 was linearly correlated (R2 = 0.98) with the percentage of hSSTr2-positive cells following gene transfer. Excess P2045 blocked binding and internalization of the 99mTc-P2045, indicating the specificity of the technique. Up to four 96-well plates could be imaged simultaneously, thereby demonstrating the high capacity of the system. This novel in vitro approach provides a new method to test enhanced gene transfer as new vectors are developed.

High-efficiency astatination of antibodies using N-iodosuccinimide as the oxidising agent in labelling of N-succinimidyl 3-(trimethylstannyl)benzoate.

Monoclonal antibodies C215, reactive with colorectal carcinomas, and MOv18, reactive with most of the ovarian carcinomas, were radiohalogenated with [211At]astatine. The radiohalogen was conjugate coupled to antibodies via the intermediate labelling reagent N-succinimidyl-3-(trimethylstannyl)benzoate (m-MeATE) in a two-step, single-pot reaction. Optimisation of the labelling of the reagent was achieved using N-iodosuccinimide, NIS, as the oxidising agent. The yields ranged from 69-95% in the labelling of 0.1-1.0 nmole of the m-MeATE precursor. Subsequent conjugation to antibodies resulted in yields of 58+/-7%. In vitro binding to tumour cells showed that the immunoreactivity of both antibodies was retained after astatine labelling.

In vivo model mimicking natural history of dog prostate cancer using DPC-1, a new canine prostate carcinoma cell line.

BACKGROUND: Dog prostate cancer is usually considered to be highly relevant to human prostate cancer. We report the isolation of a new canine prostate cancer epithelial cell line designated DPC-1. METHODS: Primary cultures were established from a canine poorly differentiated prostatic adenocarcinoma. Population doubling time was determined by counting nuclei after cell lysis. Tumorigenicity was assessed in nude mice and in one adult immunodeficient dog. Immunoscintigraphy was performed in both models using a monoclonal antibody (mAb) raised against the [44-62] sequence of human PSMA. RESULTS: DPC-1 cells have a rapid growth in vitro (doubling time, 27 hr) which is not stimulated by androgens. In addition, DPC-1 displays immunoreactivity to human PSA and PSMA. DPC-1 was found to be highly tumorigenic not only in nude mice but also for the first time after orthotopic seeding in an immunodeficient dog. This allograft mimicked, in a compressed form, the aggressive biological behavior of spontaneous dog prostate adenocarcinoma. Immunoscintigraphy using a (131)Iodine-labeled PSMA mAb clearly visualized induced tumors in nude mice and in the dog allograft. CONCLUSIONS: This study suggests that DPC-1 may constitute a powerful model for assessing new diagnostic and/or therapeutic tools in the management of prostate cancer.

Sonoporation of monolayer cells by diagnostic ultrasound activation of contrast-agent gas bodies.

Human (A431 epidermoid carcinoma) cells were grown as monolayers on 5 microm thick Mylar sheets, which formed the upper window for a 1-mm thick, 23-mm diameter disc-shaped exposure chamber. A 3.5-MHz curved linear-array transducer was aimed upward at the chamber, 7 cm away, in a 37 degrees C water bath. The chamber contained phosphate-buffered saline (PBS) with 10 mg/mL fluorescent dextran and 1% Optison ultrasound (US) contrast agent. Significant fluorescent cell counts, indicative of membrane damage (i.e., sonoporation), up to about 10% of cells within a 1-mm diameter field of view, were noted for spectral Doppler and two-dimensional (2-D) scan mode with or without a tissue-mimicking phantom. The effect was only weakly dependent on pulse-repetition frequency or exposure duration, but was strongly dependent on contrast agent concentration below 2%. Thus, diagnostic US activation of contrast-agent gas bodies can produce cell membrane damage.

Ultrasound imaging of apoptosis: high-resolution non-invasive monitoring of programmed cell death in vitro, in situ and in vivo.

A new non-invasive method for monitoring apoptosis has been developed using high frequency (40 MHz) ultrasound imaging. Conventional ultrasound backscatter imaging techniques were used to observe apoptosis occurring in response to anticancer agents in cells in vitro, in tissues ex vivo and in live animals. The mechanism behind this ultrasonic detection was identified experimentally to be the subcellular nuclear changes, condensation followed by fragmentation, that cells undergo during apoptosis. These changes dramatically increase the high frequency ultrasound scattering efficiency of apoptotic cells over normal cells (25- to 50-fold change in intensity). The result is that areas of tissue undergoing apoptosis become much brighter in comparison to surrounding viable tissues. The results provide a framework for the possibility of using high frequency ultrasound imaging in the future to non-invasively monitor the effects of chemotherapeutic agents and other anticancer treatments in experimental animal systems and in patients.

Reduction-mediated 99mTc-labeling of antitumor monoclonal antibodies: effect of increasing specific activity on antibody binding kinetics.

Reduction-mediated 99mTc-labeling of antibodies has gained widespread acceptance in preparation of tumor imaging agents. Increased specific activity to enhance detection signals has raised the question of whether such an attempt would cause change in antibody binding kinetics. To answer this question, two antitumor monoclonal antibodies, i.e. IOR-CEA (IgG1) and EMD (IgG2a) were labeled with 99mTc to yield specific activities ranging from 549-4414 MBq/mg. Regression analysis of the binding data revealed that the binding kinetics of IOR-CEA were shifted from monovalent to bivalent binding upon increasing the specific activities. This phenomenon of affinity enhancement was confirmed by the dissociation study where we found soluble CEA had greater difficulty in extracting the cell-bound IOR-CEA labeled at higher specific activity. The bivalent bindings was further supported by the finding that IOR-CEA with higher specific activities delivered less than expected radioactivity to tumor targets despite their immunoreactivities being well preserved. For EMD, the kinetics seemed to be shifted from bivalent to monovalent interaction. At higher specific activities, adverse changes in immunoreactivity were recognized. Breakage of EMD into 99mTc-Fab fragments was likely to occur and was supported by the observation that EMD delivered more than expected radioactivities to target cells upon increasing specific activity. Precaution should be taken when one deals with high specific activity labeling since this might alter the antibody binding kinetics either favorably or unfavorably.