High-resolution microPET imaging of carcinoembryonic antigen-positive xenografts by using a copper-64-labeled engineered antibody fragment

Recombinant antibodies: from the laboratory to the clinic

The development of recombinant antibodies has facilitated the exploitation of the Ab-Ag interaction specificity for targeted therapies. A fully human antibody, with custom integrated designs, can be obtained in one-third the time, compared to development of antibodies by hybridoma technology. Recombinant antibodies can be tailored for specific applications, "armed" with cytotoxic agents in a controllable fashion, and used for extracellular and intracellular targeting. Multitargeted and combination therapies are rapidly evolving for the treatment of cancer. Antibody therapeutics, costly to develop and produce, have proven beneficial in the clinic.

Phospho-proteomic analysis of cellular signaling

Reversible protein phosphorylation plays an important role in the regulation of many different processes, such as cell growth, differentiation, migration, metabolism, and apoptosis. Identification of differentially phosphorylated proteins by means of phospho-proteomic analysis provides insight into signal transduction pathways that are activated in response to, for example, growth factor stimulation or toxicant-induced apoptosis. This review summarizes recent advances made in the field of phospho-proteomics and provides examples of how phospho-proteomic techniques can be combined to quantitatively investigate the dynamic changes in protein phosphorylation in time. By linking experimental data to clinical data (e.g., disease progression or response to therapy) new disease markers could be identified, which could then be validated for applications in disease diagnosis and progression or prediction of a response to drugs.

Molecular Imaging: A Primer for Interventionalists and Imagers

The characterization of human diseases by their underlying molecular and genomic aberrations has been the hallmark of molecular medicine. From this, molecular imaging has emerged as a potentially revolutionary discipline that aims to visually characterize normal and pathologic processes at the cellular and molecular levels within the milieu of living organisms. Molecular imaging holds promise to provide earlier and more precise disease diagnosis, improved disease characterization, and timely assessment of therapeutic response. This primer is intended to provide a broad overview of molecular imaging with specific focus on future clinical applications relevant to interventional radiology.

Successful high-resolution animal positron emission tomography of human Ewing tumours and their metastases in a murine xenograft model

PURPOSE

As primary osseous metastasis is the main adverse prognostic factor in patients with Ewing tumours, a NOD/scid mouse model for human Ewing tumour metastases has been established to examine the mechanisms of metastasis. The aim of this study was to evaluate the feasibility of diagnostic molecular imaging by small animal PET in this mouse model.

METHODS

Human Ewing tumour cells were transplanted into immune-deficient NOD/scid mice via s.c injection (n=17) or i.v. injection (n=17). The animals (mean weight 23.2 g) were studied 2-7 weeks after transplantation using a submillimetre resolution animal PET scanner. To assess glucose utilisation and bone metabolism, mice were scanned after intravenous injection of 9.6 MBq (mean) 2-[(18)F]fluoro-2-deoxy-D: -glucose (FDG) or 9.4 MBq (mean) [(18)F]fluoride. Whole-body PET images were analysed visually and semi-quantitatively [%ID/g, tumour to non-tumour ratio (T/NT)]. Foci of pathological uptake were identified with respect to the physiological organ uptake in corresponding regions.

RESULTS

Subcutaneously transplanted Ewing tumours demonstrated a moderately increased glucose uptake (median %ID/g 2.5; median T/NT 2.2). After i.v. transplantation, the pattern of metastasis was similar to that in patients with metastases in lung, bone and soft tissue. These metastases showed an increased FDG uptake (median %ID/g 3.6; median T/NT 2.7). Osseous metastases were additionally visible on [(18)F]fluoride PET by virtue of decreased [(18)F]fluoride uptake (osteolysis; median %ID/g 8.4; median T/NT 0.59). Metastases were confirmed immunohistologically.

CONCLUSION

Diagnostic molecular imaging of Ewing tumours and their small metastases in an in vivo NOD/scid mouse model is feasible using a submillimetre resolution PET scanner.

Prokaryotic expression of anti-carcinoembryonic single-chain variable fragment and its value in detection of gastric carcinoma cells

AIM: To study the prokaryotic expression of the anti-carcinoembryonic antigen (CEA) single-chain fragment variable (scFv) antibody T84.66 and its specific affinity to gastric cancer cell lines and tissues.

METHODS: The cDNA of anti-CEA scFv antibody was inserted into pCANTAB5E to obtain phage vector T84.66-scFv-pCANTAB5E, and then the vector was transferred into E. coli HB2151. Isopropyl-β-D-thiogalactoside (IPTG) was used to induce the expression of anti-CEA scFv antibody. SDS-PAGE and Western blot were used identify the anti-CEA scFv antibody. Human gastric cancer cells were cultured, and CEA was determined with the obtained scFv by immunohistochemistry in the cells and paraffin-embedded gastric carcinoma tissues.

RESULTS: SDS-PAGE and Western blot showed that the anti-CEA scFv antibody T84.66 was successfully expressed. T84.66 could bind to gastric carcinoma cell lines KATOⅢ, MKN45 and HGC-27, but not to SGC7901, GC803 and BGC823, suggesting that KATOⅢ, MKN45 and HGC-27 cells expressed CEA. For the 42 cases gastric carcinoma tissues, the positive rate of CEA in the early and progressive stage was 55% (6/11) and 61% (19/31), respectively, but no CEA expression was found in the 10 normal cases. CEA expression was significantly different between gastric cancer and normal tissues (P < 0.05).

CONCLUSION: The prokaryotic expression of anti-CEA scFv antibody T84.66 is successfully achieved, and can be used to identify CEA. CEA is highly expressed in gastric cancer, but not in normal mucosa.

Irreversibly binding anti-metal chelate antibodies: Artificial receptors for pretargeting

Antibodies against metal chelates may potentially be used in biomedical applications such as targeted imaging and therapy of cancer. Highly specific monoclonal antibodies can be developed, but their binding strength needs to be maximized for them to be of practical use. In general, the half-life for dissociation of an antibody-ligand complex is more than an order of magnitude lower than the half-lifetimes for decay of medically useful radiometal ions. Practically speaking, the metal chelate-based ligand will not be bound to its receptor long enough for all of the bound radiometal to decay. A novel approach to this problem is a combination of synthetic chemistry and site-directed mutagenesis, to position a mildly reactive group on the metal chelate adjacent to a complementary reactive group on the antibody when the complex is formed. The partners are chosen to be sufficiently unreactive so that they coexist with other molecules in living systems without undergoing reaction. When the antibody-chelate complex is formed the effective local concentrations of the two groups can be non-physically large, so that a permanent link is formed in the complex even though no reaction occurs when the partners are free in solution.

Mouse extrahepatic hepatoma detected on MicroPET using copper (II)-64 chloride uptake mediated by endogenous mouse copper transporter 1

PURPOSE

This study was conducted to assess the positron-emitting copper (II)-64 chloride ((64)CuCl(2)) as a probe for imaging mouse extrahepatic hepatoma expressing mouse copper transporter 1 (mCtr1) with positron emission tomography (PET).

PROCEDURES

Following the intravenous administration of (64)CuCl(2), athymic mice bearing extrahepatic hepatoma grafts were subjected to whole-body static PET imaging with a Concorde microPET R4 tomograph. Upon completion of the imaging study, immunohistochemistry (IHC) study of mCtr1 was performed with postmortem tissues.

RESULTS

The mouse extrahepatic hepatoma grafts were well visualized on static microPET images. Quantitative analysis demonstrated that the tracer concentration in the hepatoma was significantly higher than those in the soft tissue of the right shoulder opposite to the tumor site and the brain (p < 0.001). mCtr1 immunoreactivity in the hepatoma graft was approximately 70% of that in liver, whereas (64)CuCl(2) concentration in the graft was approximately 11% of the liver concentration.

CONCLUSIONS

The extrahepatic mouse hepatoma grafts may be visualized by Cu-64 PET, taking advantage of the (64)CuCl(2) uptake mediated by the functional endogenous mCtr1.

Monospecific bivalent scFv-SH: effects of linker length and location of an engineered cysteine on production, antigen binding activity and free SH accessibility

Development of tumor targeting pharmaceuticals on a modular platform is an attractive paradigm. Design choices for bispecific (anti-tumor and anti-chelate) pretargeting molecules are increased by the use of scFvs. Because a scFv is monovalent and small in size, its functional affinity and in vivo residence time can be improved through multimerization. ScFv multimers can be covalent or non-covalent. In vivo studies indicate that covalent scFv multimers are preferable. Attachment of scFv modules to scaffolds offers a wide range of possibilities for size and valency. A free thiol introduced at the C terminal end of a scFv (scFv-SH) allows for site-specific covalent attachment to a PEG scaffold without interfering with its antigen (Ag) binding. Although in theory, multimerization of 3 or 4 scFvs can be achieved by direct conjugation, as scFv-SH, to a tri or tetrafunctionalized PEG, it is not a practical option since homogeneous tri and tetrafunctionalized PEG are not readily available. However, the generation of (scFv)(3-4)-PEG molecules through attachment of combinations of di-scFv-SH (tandemly expressed scFvs) and scFv-SH or 2 di-scFv-SH to a bifunctional PEG is a sound approach that also allows for better control of the scFv-PEG conjugate molecular composition. Optimization of the molecular format of the di-scFv-SH module for production as soluble proteins in E. coli, Ag binding and conjugation is reported in this study. ScFvs in the VH-VL format were used for the di-scFv constructs since Fv domain inversion to VL-VH, while not yielding more protein, also abolished Ag binding. The effects on production yield, Ag binding and conjugation potential of the scFv joining linker length and the presence and location of an engineered cysteine were assessed in vitro. Our data indicate that for di-scFv-SH, an increase of the scFv joining linker length results in higher production and better Ag binding; a 20 aa long linker (G(4)S)(4) was the longest linker tested. For the engineered cysteine, three locations were tested; within the scFv joining linker, at the C terminus upstream of the E Tag and as the carboxy terminal aa. The accessibility of the free SH assessed by conjugation of di-scFv-SH to HRP-Mal demonstrated that di-scFv-HRP conjugates are formed with comparable efficiencies when the cysteine is located at the scFv carboxy end. This empirical work provides a framework for the development of bispecific scFv multimers via site-specific attachment of scFv-SH and di-scFv-SH modules to a scaffold.

Implantation of different malignant human cell lines in an athymic mouse does not alter success and growth rates of either xenograft

PURPOSE

Human xenografts in athymic mice are frequently used as preclinical models of cancer to investigate the targeting of drugs. In order to distinguish specific from nonspecific targeting of the xenograft, the mice can be implanted with different malignant cell lines. We studied in xenograft success and growth rates after implantation of human lymphoma and breast cancer cells to begin an assessment of the validity of this approach for distinguishing specific from nonspecific targeting. Investigations were undertaken to determine the effect of two different cell-line xenografts, and prior radiation needed for one of the xenografts, on implantation success and growth rates.

EXPERIMENTAL DESIGN

Female athymic mice were given 4 Gy of external beam radiation 4 days prior to subcutaneous (s.c.) abdominal implantation of 6 x 10(6) Raji human lymphoma cells. One week later, 3 x 10(6) hamster blood transfusion (HBT) 3477 human breast cancer cells were implanted s.c. in a contralateral abdominal site. Xenografts were evaluated frequently thereafter. Xenograft success and growth rates were compared to those observed in "historical" control groups, wherein only a single xenograft of each type was implanted.

RESULTS

Raji xenografts developed from 73.7% of the implantations, and 100% of the HBT 3477 xenografts were successful in the experimental group. The "historical" Raji xenograft success rate was 74.1% (+/-9.3%), and the "historical" HBT 3477 xenografts success rate was 99.0% (+/-1.1%). HBT 3477 xenografts did not affect the growth rate of the Raji xenografts, and the mean doubling time for the experimental Raji xenografts was 6.3 days (+/-4.5 days), compared to the "historical" control group mean of 5.1 days (+/-3.9 days; p = 0.2). Similarly, the growth rates for the HBT 3477 xenografts were not affected by the Raji xenografts and the pre-radiation needed for this model. Mean doubling time for HBT 3477 xenografts in the presence of Raji xenografts was 9.2 days (+/-17.6 days), compared to a doubling time of 1.4 days (+/-15.2 days; p = 0.55 and 0.94 studies 1 and 2, respectively). Mean HBT 3477 xenograft doubling time for the "historical" control group was 4.4 days (+/-6.0 days).

CONCLUSIONS

Implantation of Raji xenografts and HBT 3477 breast cancer xenografts in the same mouse did not affect xenograft success or growth rates, even when whole-body radiation of 4 Gy was used to promote Raji xenografts. These observations are not intended to imply an absence of differences in other biological parameters in this sytem or to encourage extrapolation of the conclusions indiscriminately to other preclinical models. Contrarily, our aim was to encourage other investigators to further validate these frequently used approaches.

Tailoring antibodies for radionuclide delivery

Therapeutic antibodies are well established as an important class of drugs in modern medicine. The exquisite specificity and affinity for a specific target offered by antibodies has also encouraged their development as delivery vehicles for agents such as radionuclides to target tissues, for radioimmunoimaging and radioimmunotherapy. Specifically, in nuclear medicine, radionuclide-conjugated antibody molecules make it possible to image diseased loci with greater sensitivity than other imaging modalities such as magnetic resonance imaging. Furthermore, two radionuclide-conjugated antibodies have recently been approved for the therapy of non-Hodgkin's lymphoma. However, optimal implementation of antibodies has been limited by the extended circulation persistence that is characteristic of native antibodies, which is responsible for increased background activity in radioimmunoimaging applications and dose-related normal organ toxicities in radioimmunotherapy. In this article the current status of radiolabelled intact antibodies is reviewed, focusing on strategies to improve their pharmacokinetic properties to suit a desired application. Examples from the literature that represent different approaches to accomplishing this task in terms of their successes as well as limitations, and perspectives for the future are discussed.

In vivo evaluation of 177Lu- and 67/64Cu-labeled recombinant fragments of antibody chCE7 for radioimmunotherapy and PET imaging of L1-CAM-positive tumors

PURPOSE

The L1 cell adhesion protein is overexpressed in tumors, such as neuroblastomas, renal cell carcinomas, ovarian carcinomas, and endometrial carcinomas, and represents a target for tumor diagnosis and therapy with anti-L1-CAM antibody chCE7. Divalent fragments of this internalizing antibody labeled with 67/64Cu and 177Lu were evaluated to establish a chCE7 antibody fragment for radioimmunotherapy and positron emission tomography imaging, which combines high-yield production with improved clearance and biodistribution properties.

EXPERIMENTAL DESIGN

chCE7F(ab')2 fragments were produced in high amounts (0.2 g/L) in HEK-293 cells, substituted with the peptide-linked tetraazamacrocycle 3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycyl-L-p-isothiocyanato-phenylalanine, and labeled with 67Cu and 177Lu. In vivo bioevaluation involved measuring kinetics of tumor and tissue uptake in nude mice with SK-N-BE2c xenografts and NanoPET (Oxford Positron Systems, Oxford, United Kingdom) imaging with 64Cu-3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycine-chCE7F(ab')2.

RESULTS

The 177Lu- and 67Cu-labeled immunoconjugates reached maximal tumor accumulation at 24 hours after injection with similar levels of 12%ID/g to 14%ID/g. Blood levels dropped to 1.0%ID/g for the 177Lu fragment and 2.3%ID/g for the 67Cu fragment at 24 hours. The most striking difference concerned radioactivity present in the kidneys, being 34.5%ID/g for the 177Lu fragment and 16.0%ID/g for the 67Cu fragment at 24 hours. Positron emission tomography imaging allowed clear visualization of s.c. xenografts and peritoneal metastases and a detailed assessment of whole-body tracer distribution.

CONCLUSIONS

67/64Cu- and 177Lu-labeled recombinant chCE7F(ab')2 revealed suitable in vivo characteristics for tumor imaging and therapy but displayed higher kidney uptake than the intact monoclonal antibody. The 67Cu- and 177Lu-labeled immunoconjugates showed different in vivo behavior, with 67/64Cu-3-(p-nitrobenzyl)-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetate-triglycine-F(ab')2 appearing as the more favorable conjugate due to superior tumor/kidney ratios.

Preparation and biological evaluation of copper-64-labeled tyr3-octreotate using a cross-bridged macrocyclic chelator

PURPOSE

Somatostatin receptors (SSTr) are expressed on many neuroendocrine tumors, and several radiotracers have been developed for imaging these types of tumors. For this reason, peptide analogues of somatostatin have been well characterized. Copper-64 (t(1/2) = 12.7 hours), a positron emitter suitable for positron emission tomography (PET) imaging, was shown recently to have improved in vivo clearance properties when chelated by the cross-bridged tetraazamacrocycle 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo(6.6.2)hexadecane (CB-TE2A) compared with 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA).

EXPERIMENTAL DESIGN

CB-TE2A and TETA were conjugated to the somatostatin analogue tyrosine-3-octreotate (Y3-TATE) for evaluation of CB-TE2A as a bifunctional chelator of 64Cu. The in vitro affinity of each compound for SSTr was determined using a homologous competitive binding assay. In vivo characteristics of both radiolabeled compounds were examined in biodistribution and microPET studies of AR42J tumor-bearing rats.

RESULTS

Cu-CB-TE2A-Y3-TATE (Kd = 1.7 nmol/L) and Cu-TETA-Y3-TATE (Kd = 0.7 nmol/L) showed similar affinities for AR42J derived SSTr. In biodistribution studies, nonspecific uptake in blood and liver was lower for 64Cu-CB-TE2A-Y3-TATE. Differences increased with time such that, at 4 hours, blood uptake was 4.3-fold higher and liver uptake was 2.4-fold higher for 64Cu-TETA-Y3-TATE than for 64Cu-CB-TE2A-Y3-TATE. In addition, 4.4-times greater tumor uptake was detected with 64Cu-CB-TE2A-Y3-TATE than with 64Cu-TETA-Y3-TATE at 4 hours postinjection. MicroPET imaging yielded similar results.

CONCLUSIONS

CB-TE2A appears to be a superior in vivo bifunctional chelator of 64Cu for use in molecular imaging by PET or targeted radiotherapy due to both improved nontarget organ clearance and higher target organ uptake of 64Cu-CB-TE2A-Y3-TATE compared with 64Cu-TETA-Y3-TATE.

Optimizing radiolabeled engineered anti-p185HER2 antibody fragments for in vivo imaging

We have recently described the in vivo properties of an iodinated anti-p185HER2 engineered antibody fragment [minibody (scFv-C(H)3)2; 80 kDa], made from the internalizing 10H8 monoclonal antibody. Although the 10H8 minibody showed excellent binding to the target in vitro, only modest tumor uptake [5.6 +/- 1.7% injected dose per gram (ID/g) of tissue] was achieved in nude mice bearing MCF7/HER2 breast cancer tumors. Here, in an attempt to improve targeting, the 10H8 minibody was conjugated to 1,4,7,10-tetraazacyclododecane-N, N', N'', N'''-tetraacetic acid (DOTA), radiometal labeled, and evaluated in vivo. The tumor uptake of 111In-DOTA 10H8 minibody was 5.7 +/- 0.1% ID/g, similar to the radioiodinated 10H8 minibody. However, in addition to the expected liver clearance, the kidneys had unexpectedly high activity (34.0 +/- 4.0% ID/g). A minibody derived from a second anti-p185(HER2) antibody (trastuzumab; hu4D5v8) was also made. Tumor uptakes, evaluated by quantitative microPET using 64Cu-DOTA hu4D5v8 minibody, were 4.2 +/- 0.5% ID/g. Furthermore, in non-tumor-bearing mice, 111In-DOTA hu4D5v8 minibody exhibited similar elevated uptake in the kidneys (28.4 +/- 6.5% ID/g). Immunohistochemical staining of kidneys from non-tumor-bearing mice showed strong specific staining of the proximal tubules, and Western blot analysis of kidney lysate confirmed the presence of cross-reactive antigen. To further improve tumor uptake and normal tissue distribution, a larger hu4D5v8 fragment [(scFv-C(H)2-C(H)3)2; 105 kDa] was made, engineered to exhibit rapid clearance kinetics. This fragment, when evaluated by microPET, exhibited improved tumor targeting (12.2 +/- 2.4% ID/g) and reduced kidney uptake (13.1 +/- 1.5% ID/g). Thus, by manipulating the size and format of anti-p185(HER2) antibody fragments, the kidney activity was reduced and high or low expression of p185HER2 in xenografts could be distinguished by microPET imaging.

Small animal positron emission tomography in food sciences

Positron emission tomography (PET) is a 3-dimensional imaging technique that has undergone tremendous developments during the last decade. Non-invasive tracing of molecular pathways in vivo is the key capability of PET. It has become an important tool in the diagnosis of human diseases as well as in biomedical and pharmaceutical research. In contrast to other imaging modalities, radiotracer concentrations can be determined quantitatively. By application of appropriate tracer kinetic models, the rate constants of numerous different biological processes can be determined. Rapid progress in PET radiochemistry has significantly increased the number of biologically important molecules labelled with PET nuclides to target a broader range of physiologic, metabolic, and molecular pathways. Progress in PET physics and technology strongly contributed to better scanners and image processing. In this context, dedicated high resolution scanners for dynamic PET studies in small laboratory animals are now available. These developments represent the driving force for the expansion of PET methodology into new areas of life sciences including food sciences. Small animal PET has a high potential to depict physiologic processes like absorption, distribution, metabolism, elimination and interactions of biologically significant substances, including nutrients, 'nutriceuticals', functional food ingredients, and foodborne toxicants. Based on present data, potential applications of small animal PET in food sciences are discussed.

Instrumentation Aspects of Animal PET

Biological research has been accelerated by the development of noninvasive imaging techniques and by use of genetically engineered mice to model human diseases and normal development. Because these mice can be expensive, noninvasive imaging techniques, such as high-resolution positron emission tomography (PET), that permit longitudinal studies of the same animals are very attractive. Such studies reduce the number of animals used, reduce intersubject variability, and improve the accuracy of biological models. PET provides quantitative measurements of the spatiotemporal distribution of radiotracers and is an extremely powerful tool in using molecular imaging to study biology, to monitor disease intervention, and to establish pharmacokinetics for new drugs. The design of animal PET scanners has improved significantly in the past decade and can provide adequate image resolution and sensitivity to study transgenic mice. This article reviews the fundamental and technical challenges of small-animal PET imaging, with a particular focus on the latest developments and future directions of detector technologies and system design.

Current status of cancer therapy with radiolabeled monoclonal antibody

Molecular targeting therapy has become a relevant therapeutic strategy for cancer. There are several monoclonal antibodies used for the treatment of malignant tumors. Radioimmunoconjugate is composed of antibody and radionuclide showing a synergistic effect of radiation and immunemediated cellular toxicity and thereby enabling increased efficacy and minimizing toxicity. Radioimmunotherapy using 131I- and 90Y-labeled anti-CD20 monoclonal antibodies is now indicated for the treatment of patients with CD20 antigen-expressing relapsed or refractory, low-grade or transformed non-Hodgkin's lymphoma (NHL), including patients who are refractory to anti-CD20 monoclonal antibody (rituximab) therapy in the United States. It has been exhibiting favorable anti-tumor efficacy in patients with NHL as compared with rituximab. Myelosuppression is the main side effect associated with the radioimmunotherapy but is usually reversible, and nonhematologic adverse reactions are mild to moderate. Following the impressive results of therapy using radiolabeled monoclonal antibodies for NHL, radioimmunotherapy for solid tumors has been examined; however, the results were unfavorable and did warrant further clinical trials as a single agent. Future studies on radioimmunotherapy for solid tumors should focus on the new strategies of targeting such as locoregional administration for intraperitoneal dissemination, and combination therapy with chemotherapy or cytostatic therapy. Although radioimmunotherapy for NHL has shown excellent results comparable to aggressive chemotherapy without severe adverse effects, additional clinical trials should be performed to define the proper role of radioimmunoconjugates as a relevant strategy for cure of NHL.

Quantitative immuno-positron emission tomography imaging of HER2-positive tumor xenografts with an iodine-124 labeled anti-HER2 diabody

Positron emission tomography (PET) provides an effective means of both diagnosing/staging several types of cancer and evaluating efficacy of treatment. To date, the only U.S. Food and Drug Administration-approved radiotracer for oncologic PET is (18)F-fluoro-deoxyglucose, which measures glucose accumulation as a surrogate for malignant activity. Engineered antibody fragments have been developed with the appropriate targeting specificity and systemic elimination properties predicted to allow for effective imaging of cancer based on expression of tumor associated antigens. We evaluated a small engineered antibody fragment specific for the HER2 receptor tyrosine kinase (C6.5 diabody) for its ability to function as a PET radiotracer when labeled with iodine-124. Our studies revealed HER2-dependent imaging of mouse tumor xenografts with a time-dependent increase in tumor-to-background signal over the course of the experiments. Radioiodination via an indirect method attenuated uptake of radioiodine in tissues that express the Na/I symporter without affecting the ability to image the tumor xenografts. In addition, we validated a method for using a clinical PET/computed tomography scanner to quantify tumor uptake in small-animal model systems; quantitation of the tumor targeting by PET correlated with traditional necropsy-based analysis at all time points analyzed. Thus, diabodies may represent an effective molecular structure for development of novel PET radiotracers.

Noninvasive imaging of reporter gene expression in living subjects

The development of noninvasive imaging technologies designed specifically for use with small animals has provided new paradigms for cancer research. Traditional molecular biology techniques are being melded with noninvasive imaging technologies to develop a new research domain, "molecular imaging." One of the most exciting advances in this research area is the adaptation and application of conventional reporter-gene imaging techniques, used extensively by cell and molecular biologists, to living animals. Using these new assays, investigators can image noninvasively, repeatedly, and quantitatively the location, magnitude, and duration of reporter-gene expression in living animals. This review will describe the instrumentation used for noninvasive imaging of reporter genes, the reporter genes developed for noninvasive imaging with radio-nuclide-based assays such as positron emission tomography, and the reporter genes used for optically based noninvasive assays using sensitive charged-coupled device cameras. Applications of noninvasive, whole-animal imaging to gene therapy for cancer, to cell-based therapy for cancer, to lymphocyte activation, to cancer progression and dissemination in engrafted models, to tumor initiation, promotion and metastasis in conditional murine models of cancer induction, and to the noninvasive monitoring of tumor responses to a variety of therapies are described. New developments in multimodality molecular imaging are discussed, and the potential utility of noninvasive reporter gene expression in the diagnosis and management of human cancer is presented.

Real time non-invasive imaging of receptor-ligand interactions in vivo

Non-invasive longitudinal detection and evaluation of gene expression in living animals can provide investigators with an understanding of the ontogeny of a gene's biological function(s). Currently, mouse model systems are used to optimize magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and optical imaging modalities to detect gene expression and protein function. These molecular imaging strategies are being developed to assess tumor growth and the tumor microenvironment. In addition, pre-labeling of progenitor cells can provide invaluable information about the developmental lineage of stem cells both in organogenesis and tumorigenesis. The feasibility of this approach has been extensively tested by targeting of endogenous tumor cell receptors with labeled ligand (or ligand analog) reporters and targeting enzymes with labeled substrate (or substrate analog). We will primarily discuss MRI, PET, and SPECT imaging of cell surface receptors and the feasibility of non-invasive imaging of gene expression using the tumor microenvironment (e.g., hypoxia) as a conditional regulator of gene expression.

Molecular imaging applications for immunology

The use of multimodality molecular imaging has recently facilitated the study of molecular and cellular events in living subjects in a noninvasive and repetitive manner to improve the diagnostic capability of traditional assays. The noninvasive imaging modalities utilized for both small animal and human imaging include positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance imaging (MRI), ultrasound, and computed tomography (CT). Techniques specific to small-animal imaging include bioluminescent imaging (BIm) and fluorescent imaging (FIm). Molecular imaging permits the study of events within cells, the examination of cell trafficking patterns that relate to inflammatory diseases and metastases, and the ability to rapidly screen new drug treatments for distribution and effectiveness. In this paper, we will review the current field of molecular imaging assays (especially those utilizing PET and BIm modalities) and examine how they might impact animal models and human disease in the field of clinical immunology.

Covalent disulfide-linked anti-CEA diabody allows site-specific conjugation and radiolabeling for tumor targeting applications

An engineered anti-carcinoembryonic antigen (CEA) diabody (scFv dimer, 55 kDa) was previously constructed from the murine anti-CEA T84.66 antibody. Tumor targeting, imaging and biodistribution studies in nude mice bearing LS174T xenografts with radiolabeled anti-CEA diabody demonstrated rapid tumor uptake and fast blood clearance, which are favorable properties for an imaging agent. Current radiolabeling approaches result in random modification of the protein surface, which may impair immunoreactivity especially for smaller antibody fragments. Site-specific conjugation approaches can direct modifications to reactive groups located away from the binding site. Here, cysteine residues were introduced into the anti-CEA diabody at three different locations, to provide specific thiol groups for chemical modification. One version (with a C-terminal Gly-Gly-Cys) existed exclusively as a disulfide-bonded dimer. This cysteine-modified diabody (Cys-diabody) retained high binding to CEA and demonstrated tumor targeting and biodistribution properties identical to the non-covalent diabody. Furthermore, following reduction of the disulfide bond, the Cys-diabody could be chemically modified using a thiol-specific bifunctional chelating agent, for radiometal labeling. Thus, the Cys-diabody provides a covalently linked alternative to conventional diabodies, which can be reduced and modified site-specifically. This format will provide a versatile platform for targeting a variety of agents to CEA-positive tumors.

Imaging and therapy of tumors induced to express somatostatin receptor by gene transfer using radiolabeled peptides and single chain antibody constructs

The fields of radioimmunodetection and radioimmunotherapy began with an initial paradigm that a targeting molecule (eg, antibody) carrying a radioisotope had the potential of selectively imaging and delivering a therapeutic dose of radiation to tumor sites. A second paradigm was developed in which injection of the targeting molecule was separated from injection of a short-lived radioisotope-labeled ligand (so-called "pretargeting strategy"). This strategy has improved radioisotope delivery to tumors in animal models, enhanced radioimmune imaging in man, and therapeutic trials are in an early phase. We proposed a third paradigm to achieve radioisotopic localization at tumor sites by inducing tumor cells to synthesize a membrane expressed receptor with a high affinity for infused radiolabeled ligands. The use of gene transfer technology to induce expression of high affinity membrane receptors can enhance the specificity of radioligand localization, while the use of radioisotopes with the ability to deliver radiation damage across several cell diameters will compensate for less than perfect transduction efficiency. This approach was termed "Genetic Radioisotope Targeting Strategy." Using this strategy, induction of high levels of gastrin releasing peptide receptor or human somatostatin receptor subtype 2 expression and selective tumor uptake of radiolabeled peptides was achieved. The advantages of the genetic transduction approach are (1) constitutive expression of a tumor-associated antigen/receptor is not required; (2) tumor cells are altered to express a new target receptor or increased quantities of an existing receptor at levels that may significantly improve tumor targeting of radiolabeled ligands compared with normal tissues; (3) gene transfer can be achieved by intratumoral or regional injection of gene vectors; (4) it is feasible to target adenovirus vectors to receptors overexpressed on tumor cells by modifying adenoviral tropism (binding) so that the virus will be targeted specifically to the desired tumor; and (5) it is possible to coexpress the receptor gene and a therapeutic gene, such as cytosine deaminase, for molecular prodrug therapy to produce an enhanced therapeutic effect.

124I-labeled engineered anti-CEA minibodies and diabodies allow high-contrast, antigen-specific small-animal PET imaging of xenografts in athymic mice

Prolonged clearance kinetics have hampered the development of intact antibodies as imaging agents, despite their ability to effectively deliver radionuclides to tumor targets in vivo. Genetically engineered antibody fragments display rapid, high-level tumor uptake coupled with rapid clearance from the circulation in the athymic mouse/LS174T xenograft model. The anticarcinoembryonic antigen (CEA) T84.66 minibody (single-chain Fv fragment [scFv]-C(H)3 dimer, 80 kDa) and T84.66 diabody (noncovalent dimer of scFv, 55 kDa) exhibit pharmacokinetics favorable for radioimmunoimaging. The present work evaluated the minibody or diabody labeled with (124)I, for imaging tumor-bearing mice using a high-resolution small-animal PET system.

METHODS

Labeling was conducted with 0.2-0.3 mg of protein and 65-98 MBq (1.7-2.6 mCi) of (124)I using an iodination reagent. Radiolabeling efficiencies ranged from 33% to 88%, and immunoreactivity was 42% (diabody) or >90% (minibody). In vivo distribution was evaluated in athymic mice bearing paired LS174T human colon carcinoma (CEA-positive) and C6 rat glioma (CEA-negative) xenografts. Mice were injected via the tail vein with 1.9-3.1 MBq (53-85 microCi) of (124)I-minibody or with 3.1 MBq (85 microCi) of (124)I-diabody and imaged at 4 and 18 h by PET. Some mice were also imaged using (18)F-FDG 2 d before imaging with (124)I-minibody.

RESULTS

PET images using (124)I-labeled minibody or diabody showed specific localization to the CEA-positive xenografts and relatively low activity elsewhere in the mice, particularly by 18 h. Target-to-background ratios for the LS174T tumors versus soft tissues using (124)I-minibody were 3.05 at 4 h and 11.03 at 18 h. Similar values were obtained for the (124)I-diabody (3.95 at 4 h and 10.93 at 18 h). These results were confirmed by direct counting of tissues after the final imaging. Marked reduction of normal tissue activity, especially in the abdominal region, resulted in high-contrast images at 18 h for the (124)I-anti-CEA diabody. CEA-positive tumors as small as 11 mg (<3 mm in diameter) could be imaged, and (124)I-anti-CEA minibodies, compared with (18)F-FDG, demonstrated highly specific localization.

CONCLUSION

(124)I labeling of engineered antibody fragments provides a promising new class of tumor-specific probes for PET imaging of tumors and metastases.

7α- and 17α-Substituted estrogens containing tridentate tricarbonyl rhenium/Technetium complexes: synthesis of estrogen receptor imaging agents and evaluation using microPET with technetium-94m

To develop technetium and rhenium-labeled imaging agents for estrogen receptor (ER) positive breast tumors, we have prepared tridentate metal tricarbonyl chelates substituted at the 7alpha- and 17alpha-positions of estradiol. Some of the Re(CO)(3) conjugates have high binding for the ER in vitro. The in vivo biodistribution of the highest affinity of these novel metal tricarbonyl conjugates, prepared as the (94m)Tc labeled analogue, was evaluated by tissue dissection and microPET imaging. Although target tissue-selective uptake was not apparent, it is notable that microPET imaging identified the stomach as a major site of activity deposition, a site that might have been missed by standard tissue distribution studies.

Molecular imaging: the application of small animal positron emission tomography

The extraordinary advances in genomic technologies over the last decade have led to the establishment of new animal models of disease. The use of molecular imaging techniques to examine these models, preferably with non-destructive imaging procedures, such as those offered by positron emission tomography (PET), are especially valuable for the timely advancement of research. With the use of small animal PET imaging it is possible to follow individual subjects of a sample population over an extended time period by using highly specific molecular probes and radiopharmaceuticals. In this Prospect small animal PET imaging will be described, specifically focusing on the current technologies, its applications in molecular imaging and the logistics of performing small animal PET.

Long-Lived Positron Emitters Zirconium-89 and Iodine-124 for Scouting of Therapeutic Radioimmunoconjugates with PET

Antibody-PET imaging might be of value for the selection of radioimmunotherapy (RIT) candidates to confirm tumor targeting and to estimate radiation doses to tumor and normal tissues. One of the requirements to be set for such a scouting procedure is that the biodistributions of the diagnostic and therapeutic radioimmunoconjugates should be similar. In the present study we evaluated the potential of the positron emitters zirconium-89 ((89)Zr) and iodine-124 ((124)I) for this approach, as these radionuclides have a relatively long half-life that matches with the kinetics of MAbs in vivo (t(1/2) 3.27 and 4.18 days, respectively). After radiolabeling of the head and neck squamous cell carcinoma (HNSCC)-selective chimeric antibody (cMAb) U36, the biodistribution of two diagnostic (cMAb U36-N-sucDf-(89)Zr and cMAb U36-(124)I) and three therapeutic radioimmunoconjugates (cMAb U36-p-SCN-Bz-DOTA-(88)Y-with (88)Y being substitute for (90)Y, cMAb U36-(131)I, and cMAb U36-MAG3-(186)Re) was assessed in mice with HNSCC-xenografts, at 24, 48, and 72 hours after injection. Two patterns of biodistribution were observed, one pattern matching for (89)Zr- and (88)Y-labeled cMAb U36 and one pattern matching for (124)I-, (131)I-, and (186)Re-cMAb U36. The most remarkable differences between both patterns were observed for uptake in tumor and liver. Tumor uptake levels were 23.2 +/- 0.5 and 24.1 +/- 0.7%ID/g for the (89)Zr- and (88)Y-cMAb U36 and 16.0 +/- 0.8, 15.7 +/- 0.79 and 17.1 +/- 1.6%ID/g for (124)I-, (131)I-, and (186)Re-cMAb U36-conjugates, respectively, at 72 hours after injection. For liver these values were 6.9 +/- 0.8 ((89)Zr), 6.2 +/- 0.8 ((88)Y), 1.7 +/- 0.1 ((124)I), 1.6 +/- 0.1 ((131)I), and 2.3 +/- 0.1 ((186)Re), respectively. These preliminary data justify the further development of antibody-PET with (89)Zr-labeled MAbs for scouting of therapeutic doses of (90)Y-labeled MAbs. In such approach (124)I-labeled MAbs are most suitable for scouting of (131)I- and (186)Re-labeled MAbs.

Micro-PET imaging and small animal models of disease

Positron emission tomography (PET) has been used clinically to measure enzyme reactions, ligand-receptor interactions, cellular metabolism and cell proliferation. Until recently, however, PET has not been suitable for small animal models because of resolution limitations. Development of micro-PET instrumentation for small animal imaging and the availability of positron-emitting tracers has made this technology accessible for the non-invasive, quantitative and repetitive imaging of biological function in living animals. The development of new probes and positron-imaging based reporter genes has extended micro-PET applications to investigations of metabolism, enzyme activity, receptor-ligand interactions, protein-protein interactions, gene expression, adoptive cell therapy and somatic gene therapy. Because small animal PET is immediately extrapolatable to the clinic, laboratory advances should rapidly be translated to clinical practice.

Preparation of humanized ovarian carcinoma anti-idiotypic minibody

Murine anti-idiotypic monoclonal antibody (MAb) 6B11 mimicking the tumor-associated antigen OC166-9 is used as a vaccine for the induction of an anti-tumoral immunity in experiments of in vitro and in vivo animal model with ovarian carcinoma. In this article, we have humanized 6B11 anti-idiotypic minibody using overlap polymerase chain reaction (PCR) and DNA recombinant technique, prokaryotic expression vector was produced by genetic fusion of 6B11V(L)-V(H) to human IgG1 hinge and CH3 region. Transformed E. coli BL21(DE3) were propagated and induced by isopropyl-beta D-thiogalactopyranoside (IPTG). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that a protein band with molecular weight of 50kD appeared as the expected size after transformation. Molecular weight of 100 kDa may be examined by electrophoresis in nondenaturing systems. The fusion protein was analyzed with enzyme-linked immunosorbant assay (ELISA), inhibition ELISA tests and Western blot, respectively. The humanized anti-idiotype minibody showed capacity of bivalent binding to ovarian cancer MAb COC166-9 and goat anti-human immunoglobulin IgG1. It is useful reagents for clinical use.

Characterization of a single LSO crystal layer high resolution research tomograph

The purpose of this study was to determine the performance of a single lutetium oxy-orthosilicate (LSO) crystal layer High Resolution Research Tomograph (HRRT) positron emission tomography (PET) scanner. The HRRT is a high resolution PET scanner designed for human brain and small animal imaging. The scanner consists of eight panel detectors, which have one layer of 2.1 x 2.1 x 7.5 mm thick LSO crystals. Several phantom studies were performed to determine scanner characteristics, such as resolution, scatter fraction, count rate and noise equivalent count rates (NECR). NECR curves were measured according to both NEMA NU2-1994 and NU2-2001 for three different energy windows, i.e. lower level discriminators (lld) of 350, 400 and 450 keV and an upper level discriminator (uld) of 650 keV. Accuracy of scatter and single photon attenuation corrections was evaluated according to NU2-1994. Data were acquired using a ring difference of 67 and a span of 9. Reconstructions were performed using FORE + 2D FBP or OSEM. Transaxial resolution varied from 2.7 to 2.9 mm FWHM between I and 10 cm off centre locations, and axial resolution varied from 3.2 to 4.4 mm FWHM. Scatter fractions (NU2-1994) equalled 0.31, 0.42 and 0.54 for lld of 450, 400 and 350 keV, respectively. NECR data were highest for an lid of 400 keV and showed a maximum of 46 kcps at 38 kBq cm(-3). Lower NECR values were observed according to NU2-2001, but were still optimal for an lld of 400 keV. After scatter and attenuation corrections, pixel values within water, air and teflon inserts of the NU2-1994 phantom were 14, 4 and 35% of the background activity, respectively. The single layer LSO HRRT scanner shows excellent spatial resolution, making it suitable for small animal studies. The low count rate performance, due to the small amount of LSO, prohibits studies of the human brain, but is sufficient for studies in small laboratory animals.

The crystal structure of an anti-CEA scFv diabody assembled from T84.66 scFvs in V(L)-to-V(H) orientation: implications for diabody flexibility

Diabodies (scFv dimers) are small, bivalent antibody mimetics of approximately 55kDa in size that possess rapid in vivo targeting pharmacokinetics compared to the intact parent antibody, and may prove highly suitable for imaging and therapeutic applications. Here, we describe T84.66Di, the first diabody crystal structure in which the scFvs comprise V domains linked in the V(L)-to-V(H) orientation. The structure was determined by X-ray diffraction analysis to 2.6 A resolution. The T84.66Di scFv was constructed from the anti-carcinoembryonic antigen (anti-CEA) antibody T84.66 variable domains connected by an eight residue peptide linker to provide flexibility between Fv modules and promote dimer formation with bivalent affinity to the cell-surface target, CEA. Therefore, it was surprising to observe a close association of some Fv module complementarity-determining regions in the T84.66 diabody crystal, especially compared to other diabody structures all of which are linked in the opposite V(H)-to-V(L) orientation. The differences between the arrangement of Fv modules in the T84.66Di V(L)-to-V(H) linked diabody structure compared to the crystal structure of L5MK16 and other proposed V(H)-to-V(L) linked diabodies has been investigated and their potential for flexibility discussed. The comparison between V(H)-to-V(L) and V(L)-to-V(H) linked diabodies revealed in this study represents a limited repertoire of possible diabody Fv orientations, but one that reveals the potential flexibility of these molecules. This analysis therefore provides some signposts that may impact on future molecular designs for these therapeutic molecules with respect to diabody flexibility and avidity.

In vivo quantitative noninvasive imaging of gene transfer by single-photon emission computerized tomography

Systems aimed at detecting gene expression noninvasively in vivo are desirable for evaluating the outcome of gene transfer in clinical trials. Several approaches have been exploited using magnetic resonance imaging and spectroscopy ((31)P MRS), positron emission tomography (PET), single-photon emission tomography (SPECT), and detection of bioluminescent signals. An ideal system is based on transfer of a marker gene, the activity of which can be detected against a background from the target tissue without interfering with normal physiology or eliciting an immune response. The majority of approaches described to date use genes encoding a nonmammalian protein that can elicit immune responses or a transmembrane receptor as a marker gene whose ectopic expression may cause aberrant signaling in the target cell through binding to endogenous ligands. The dopamine transporter (DAT) is normally expressed at high levels, mainly in the dopaminergic neurons of the central nervous system. We previously synthesized a radioactive ligand, [(99m)Tc]TRODAT-1, that binds with high affinity to the dopamine transporter, allowing for SPECT imaging of the striatum in normal control subjects and individuals affected with Parkinson's disease. Here we describe a strategy to monitor gene transfer based on adeno-associated viral vector (AAV)-mediated transduction of DAT in murine muscle followed by [(99m)Tc]TRODAT-1 imaging by SPECT of cells expressing the transgene. We show that quantitative, noninvasive imaging of gene transfer is successfully achieved in vivo, using a single-photon computed tomography camera. This system employs a reporter gene encoding a mammalian protein that is absent in most tissues, has no enzymatic activity, and does not activate intracellular pathways. This should be useful to monitor gene transfer in the settings of gene therapy.

Applications of Positron Emission Tomography in the Development of Molecular Targeted Cancer Therapeutics

For molecular targeted cancer therapies to fulfill their promise in cancer treatment, innovative approaches are required to overcome significant obstacles that exist in the clinical development of these agents. Positron emission tomography (PET) is a functional imaging technology that allows rapid, repeated, noninvasive, in vivo assessment and quantification of many biological processes and in some cases molecular pathways targeted by these therapies. It is highly sensitive, with the capacity to detect subnanomolar concentrations of radiotracer and provides superior image resolution to conventional nuclear medicine imaging with gamma cameras. Novel PET radiotracers have been developed that allow visualisation of a variety of processes including tumour metabolism, cell proliferation, apoptosis, hypoxia and blood flow. Furthermore, specific molecular targets including cellular receptors can be identified using radiolabelled receptor ligands or specific monoclonal antibodies. Improvements in imaging technology leading to the development of small-animal PET scanners, with resolution capable of imaging commonly used mouse models of cancer, will enable PET to play an important role in preclinical proof-of-principle drug studies. Such improvements will also facilitate the validation of imaging protocols that can be readily translated to studies in humans. The greatest utility of PET in the development of molecular targeted therapeutics, however, lies in clinical studies, where PET may play a valuable role in a number of situations. These include selection of patients for therapy through noninvasive identification of the presence of specific molecular targets, pharmacokinetic studies with labelled drugs and pharmacodynamic evaluations of biological parameters to select the optimal biological dose, and assessment of response to therapies.

Non-invasive gamma camera imaging of gene transfer using an adenoviral vector encoding an epitope-tagged receptor as a reporter

A model epitope-tagged receptor was constructed by fusing the hemagglutinin (HA) sequence on the extracellular N-terminus of the human somatostatin receptor subtype 2 (hSSTr2) gene. This construct was placed in an adenoviral (Ad-HAhSSTr2) vector. This study evaluated Ad-HAhSSTr2 in vitro and in vivo using FACS, fluorescent microscopy, radioactive binding assays, and gamma camera imaging techniques. Infection of A-427 non-small cell lung cancer cells with Ad-HAhSSTr2 or Ad-hSSTr2 resulted in similar expression of hSSTr2 by FACS analysis and binding assays using a (99m)Tc-labeled somatostatin analogue ((99m)Tc-P2045). HAhSSTr2 expression in A-427 cells was specific for infection with Ad-HAhSSTr2. FITC-labeled anti-HA antibody (FITC-HA) confirmed surface expression in live A-427 cells and the absence of internalization. Gamma camera imaging and gamma counter analysis of normal mice showed significantly greater (P<0.05) liver uptake of (99m)Tc-labeled anti-HA antibody ((99m)Tc-anti-HA) in mice injected i.v. 48 h earlier with Ad-HAhSSTr2 (53.6+/-6.9% ID/g) as compared to mice similarly injected with Ad-hSSTr2 (9.0+/-1.3% ID/g). In a mouse tumor model, imaging detected increased tumor localization of (99m)Tc-anti-HA due to direct intratumor injection Ad-HAhSSTr2. Gamma counter analysis confirmed significantly greater (P<0.05) uptake of (99m)Tc-anti-HA in tumors injected with Ad-HAhSSTr2 (12.5+/-4.1% ID/g) as compared to Ad-hSSTr2-infected tumors (5.1+/-1.5% ID/g). These studies demonstrate the feasibility of using an epitope-tagged reporter receptor for non-invasively imaging gene transfer.

Selective targeting of tumoral vasculature: comparison of different formats of an antibody (L19) to the ED-B domain of fibronectin

We recently demonstrated that a human recombinant scFv, L19, reacting with the ED-B domain of fibronectin, a marker of angiogenesis, selectively targets tumoral vasculature in vivo. Using the variable regions of L19, we constructed and expressed a human "small immunoprotein" (SIP) and a complete human IgG1 and performed biodistribution studies in tumor-bearing mice to compare the blood clearance rate, in vivo stability and performance in tumor targeting of the 3 L19 formats [dimeric scFv (scFv)(2), SIP and IgG1]. The accumulation of the different antibody formats in the tumors studied was a consequence of the clearance rate and in vivo stability of the molecules. Using the SIP, the %ID/g in tumors was 2-5 times higher than that of the (scFv)(2), reaching a maximum 4-6 hr after injection. By contrast, the accumulation of IgG1 in tumors constantly rose during the experiments. However, due to its slow clearance, the tumor-blood ratio of the %ID/g after 144 hr was only about 3 compared to a ratio of 10 for the (scFv)(2) and 70 for the SIP after the same period of time. The different in vivo behavior of these 3 completely human L19 formats could be exploited for different diagnostic and/or therapeutic purposes, depending on clinical needs and disease. Furthermore, the fact that ED-B is 100% homologous in human and mouse, which ensures that L19 reacts equally well with the human and the murine antigen, should expedite the transfer of these reagents to clinical trials.

Small animal imaging. current technology and perspectives for oncological imaging

Advances in the biomedical sciences have been accelerated by the introduction of many new imaging technologies in recent years. With animal models widely used in the basic and pre-clinical sciences, finding ways to conduct animal experiments more accurately and efficiently becomes a key factor in the success and timeliness of research. Non-invasive imaging technologies prove to be extremely valuable tools in performing such studies and have created the recent surge in small animal imaging. This review is focused on three modalities, PET, MR and optical imaging which are available to the scientist for oncological investigations in animals.

Molecular imaging of gene expression and protein function in vivo with PET and SPECT

Molecular imaging is broadly defined as the characterization and measurement of biological processes in living animals, model systems, and humans at the cellular and molecular level using remote imaging detectors. One underlying premise of molecular imaging is that this emerging field is not defined by the imaging technologies that underpin acquisition of the final image per se, but rather is driven by the underlying biological questions. In practice, the choice of imaging modality and probe is usually reduced to choosing between high spatial resolution and high sensitivity to address a given biological system. Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) inherently use image-enhancing agents (radiopharmaceuticals) that are synthesized at sufficiently high specific activity to enable use of tracer concentrations of the compound (picomolar to nanomolar) for detecting molecular signals while providing the desired levels of image contrast. The tracer technologies strategically provide high sensitivity for imaging small-capacity molecular systems in vivo (receptors, enzymes, transporters) at a cost of lower spatial resolution than other technologies. We review several significant PET and SPECT advances in imaging receptors (somatostatin receptor subtypes, neurotensin receptor subtypes, alpha(v)beta(3) integrin), enzymes (hexokinase, thymidine kinase), transporters (MDR1 P-glycoprotein, sodium-iodide symporter), and permeation peptides (human immunodeficiency virus type 1 (HIV-1) Tat conjugates), as well as innovative reporter gene constructs (herpes simplex virus 1 thymidine kinase, somatostatin receptor subtype 2, cytosine deaminase) for imaging gene promoter activation and repression, signal transduction pathways, and protein-protein interactions in vivo.

Molecular imaging of cancer with positron emission tomography

The imaging of specific molecular targets that are associated with cancer should allow earlier diagnosis and better management of oncology patients. Positron emission tomography (PET) is a highly sensitive non-invasive technology that is ideally suited for pre-clinical and clinical imaging of cancer biology, in contrast to anatomical approaches. By using radiolabelled tracers, which are injected in non-pharmacological doses, three-dimensional images can be reconstructed by a computer to show the concentration and location(s) of the tracer of interest. PET should become increasingly important in cancer imaging in the next decade.

A carcinoembryonic antigen-specific diabody produced in tobacco

The feasibility of using tobacco for production of a recombinant antibody (T84.66/GS8 diabody) directed against the carcinoembryonic antigen (CEA) and used for tumor imaging was investigated. Two constructs were generated for targeting the protein either to the apoplast or to the endoplasmic reticulum. Expression of the diabody in tobacco leaves after vacuum-assisted infiltration of engineered Agrobacteria (agro-infiltration) and in regenerated transgenic tobacco plants was analyzed and compared. Results in terms of protein expression and accumulation between both systems showed a good correlation. His6-tagged T84.66 diabody was readily purified from agro-infiltrated tobacco leaves and from transgenic plants by immobilized metal ion affinity chromatography. The purified protein was analyzed by polyacrylamide gel electrophoresis, Western blot, gel filtration, electrospray mass spectrometry, direct and competition ELISA, electrophoretic mobility shift assay, and staining of CEA-positive colon adenocarcinoma cell line LS174T. Our results demonstrate that tobacco is a competent production system for this clinically relevant diabody.

Novel chelating agents for potential clinical applications of copper

Copper offers a unique selection of radioisotopes ((60)Cu, (61)Cu, (62)Cu, (64)Cu, and (67)Cu) with half-lives ranging from 9.8 min to 61.9 h suitable for imaging and/or radiotherapy. In peptide/antibody targeted radiotherapy one of the most studied chelating agents for copper, 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid (TETA), has been employed in clinical trials, but transchelation to ceruloplasmin and/or superoxide dismutase in vivo has been noted. In this study, a series of novel hexadentate chelating agents based on N,N',N"-tris(2-pyridylmethyl)-1,3,5-cis,cis,-triaminocyclohexane (tachpyr) have been synthesized and the serum stability of their copper complexes was evaluated as compared to TETA. Copper complexes of tachpyr modified at the 3, 4, or 5 position or with replacement of pyridine by imidazole have serum stability comparable to Cu[TETA]. When the complexes were cross-challenged, Cu[TETA] versus tachpyr or 1,3,5-cis,cis,-triaminocyclohexane- N,N',N"-tris-(2-methyl-(N-methylimidazole)) (IM), tachpyr and IM appear to have superior copper chelation ability to TETA. When challenged by a large excess of non-radioactive copper, copper exchange with the tachpyr radio-copper complex was observed. However, tachpyr clearly exhibited a significant preference for Cu(II) over Zn(II) or Fe(III). Therefore, tachpyr, 1,3,5-cis,cis,-triaminocyclohexane-N,N',N"-tri-(3-methyl-2-methylpyridineimine) (tachpyr(3-Me)), 1,3,5-cis,cis,-triaminocyclohexane-N,N',N"-tri-(4-methyl-2-methylpyridineimine) (tachpyr(4-Me)), 1,3,5-cis,cis,-triaminocyclohexane-N,N',N"-tri-(5-methyl-2-methylpyridineimine) (tachpyr(5-Me)) and IM easily form copper complexes with high stability. These novel chelating agents provide an attractive lead for creation of new copper radiopharmaceuticals for diagnosis and therapy applications.

Magnetic labeling of activated microglia in experimental gliomas

Microglia, as intrinsic immunoeffector cells of the central nervous system (CNS), play a very sensitive, crucial role in the response to almost any brain pathology where they are activated to a phagocytic state. Based on the characteristic features of activated microglia, we investigated whether these cells can be visualized with magnetic resonance imaging (MRI) using ultrasmall superparamagnetic iron oxides (USPIOs). The hypothesis of this study was that MR microglia visualization could not only reveal the extent of the tumor, but also allow for assessing the status of immunologic defense. Using USPIOs in cell culture experiments and in a rat glioma model, we showed that microglia can be labeled magnetically. Labeled microglia are detected by confocal microscopy within and around tumors in a typical border-like pattern. Quantitative in vitro studies revealed that microglia internalize amounts of USPIOs that are significantly higher than those incorporated by tumor cells and astrocytes. Labeled microglia can be detected and quantified with MRI in cell phantoms, and the extent of the tumor can be seen in glioma-bearing rats in vivo. We conclude that magnetic labeling of microglia provides a potential tool for MRI of gliomas, which reflects tumor morphology precisely. Furthermore, the results suggest that MRI may yield functional data on the immunologic reaction of the CNS.

Dimeric and trimeric antibodies: high avidity scFvs for cancer targeting

Recombinant antibody fragments can be engineered to assemble into stable multimeric oligomers of high binding avidity and specificity to a wide range of target antigens and haptens. This review describes the design and expression of diabodies (dimers), triabodies (trimers) and tetrabodies (tetramers). In particular we discuss the role of linker length between V-domains and the orientation of the V-domains to direct the formation of either diabodies (60 kDa), triabodies (90 kDa) or tetrabodies (120 kDa), and how the size, flexibility and valency of each molecules is suited to different applications for in vivo imaging and therapy. Single chain Fv antibody fragments joined by polypeptide linkers of at least 12 residues irrespective of V-domains orientation predominantly form monomers with varying amounts of dimer and higher molecular mass oligomers in equilibrium. A scFv molecule with a linker of 3-12 residues cannot fold into a functional Fv domain and instead associates with a second scFv molecule to form a bivalent dimer (diabody, approximately 60 kDa). Reducing the linker length below three residues can force scFv association into trimers (triabodies, approximately 90 kDa) or tetramers ( approximately 120 kDa) depending on linker length, composition and V-domain orientation. A particular advantage for tumour targeting is that molecules of 60-100 kDa have increased tumour penetration and fast clearance rates compared with the parent Ig (150 kDa). We highlight a number of cancer-targeting scFv diabodies that have undergone successful pre-clinical trials for in vivo stability and efficacy. We also briefly review the design of multi-specific Fv modules suited to cross-link two or more different target antigens. Bi-specific diabodies formed by association of different scFv molecules have been designed as cross-linking reagents for T-cell recruitment into tumours (immunotherapy), viral retargeting (gene therapy) and as red blood cell agglutination reagents (immunodiagnostics). The more challenging trispecific multimers (triabodies) remain to be described.

Protein-protein interactions in hematology and phage display

Phage display, which exploits fundamental tools and principles of immune repertoire diversity, antigen-antibody interactions, and clonal and immunologic selection, is used increasingly to advance experimental and clinical hematology. Phage display is based on the ability of bacteriophage to present engineered proteins on their surface coat. Diverse libraries of proteins such as peptides, antibody fragments, and protein domains corresponding to gene fragments or cDNAs may be displayed. Interactions between phage-displayed proteins and target antigens can be identified rapidly and characterized using high throughput methodologies. Peptide and gene fragment libraries are particularly useful to characterize binding interactions between proteins, such as ligand-receptor interactions. This approach allows rapid generation of human antibodies, often against nonimmunogenic, conserved proteins. Phage antibodies against surface and intracellular antigens are used as reagents for flow cytometry, in vivo imaging, and therapeutic targeting. Phage-derived antibodies also facilitate analyses of the humoral antibody response. Finally, cellular delivery of phage-displayed peptides and gene fragments can be used to modulate functional pathways and molecules in vitro and in vivo. The combinatorial power of phage display enables identification of candidate epitopes without knowledge of the protein interaction, a priori. Overall, these capabilities provide a versatile, high-throughput approach to develop tools and reagents useful for a plethora of experimental hematology applications. This paper focuses on current and future applications of antibody and epitope phage display technology in hematology.

Use of positron emission tomography in animal research

Among the several imaging technologies applied to in vivo studies of research animals, positron emission tomography (PET) is a nuclear imaging technique that permits the spatial and temporal distribution of compounds labeled with a positron-emitting radionuclide to be determined noninvasively. It can be viewed as an in vivo analog of classic autoradiographic methods. Many different positron-labeled compounds have been synthesized as tracers that target a range of specific markers or pathways. These tracers permit the measurement of quantities of biological interest ranging from glucose metabolism to gene expression. PET has been extensively used in imaging studies of larger research animals such as dogs and nonhuman primates. Now, using newly developed high-resolution dedicated animal PET scanners, these types of studies can be performed in small laboratory animals such as mice and rats. The entire whole-body biodistribution kinetics can be determined in a single imaging study in a single animal. This technique should enable statistically significant biodistribution data to be obtained from a handful of animals, compared with the tens or hundreds of animals that might be required for a similar study by autoradiography. PET also enables repeat studies in a single subject, facilitating longitudinal study designs and permitting each animal to serve as its own control in experiments designed to evaluate the effects of a particular interventional strategy. This paper provides a basic overview of the methodology of PET imaging, a discussion of the advantages and drawbacks of PET as a tool in animal research, a description of the latest generation of dedicated animal PET scanners, and a review of a few of the many applications of PET in animal research to date.

Performance evaluation of the microPET P4: a PET system dedicated to animal imaging

The microPET Primate 4-ring system (P4) is an animal PET tomograph with a 7.8 cm axial extent, a 19 cm diameter transaxial field of view (FOV) and a 22 cm animal port. The system is composed of 168 detector modules, each with an 8 x 8 array of 2.2 x 2.2 x 10 mm3 lutetium oxyorthosilicate crystals, arranged as 32 crystal rings 26 cm in diameter. The detector crystals are coupled to a Hamamatsu R5900-C8 PS-PMT via a 10 cm long optical fibre bundle. The detectors have a timing resolution of 3.2 ns, an average energy resolution of 26%, and an average intrinsic spatial resolution of 1.75 mm. The system operates in 3D mode without inter-plane septa, acquiring data in list mode. The reconstructed image spatial resolution ranges from 1.8 mm at the centre to 3 mm at 4 cm radial offset. The tomograph has a peak system sensitivity of 2.25% at the centre of the FOV with a 250-750 keV energy window. The noise equivalent count rate peaks at 100-290 kcps for representative object sizes. Images from two phantoms and three different types of laboratory animal demonstrate the advantage of the P4 system over the original prototype microPET. including its threefold improvement in sensitivity and a large axial FOV sufficient to image an entire mouse in a single bed position.

Tumor targeting of radiometal labeled anti-CEA recombinant T84.66 diabody and t84.66 minibody: comparison to radioiodinated fragments

Recombinant antibody fragments offer potential advantages over intact monoclonal antibodies in the radioimmunoscintigraphy (RIS) of solid tumors. Due to their smaller molecular size, antibody fragments have shown rapid tumor targeting and blood clearance, a more uniform tumor distribution and a lower potential to elicit a human immune response. Previously, we have expressed two genetically engineered antibody fragments, the T84.66 diabody (scFv dimer) and the T84.66 minibody (scFv-CH3 dimer), specific to carcinoembryonic antigen (CEA). When radioiodinated, both antibody fragments exhibited rapid tumor targeting and rapid blood clearance in xenografted mice. To extend and optimize their future clinical RIS utility with radiometals, these antibody fragments were conjugated with the macrocycle 1,4,7,10-tetraazacyclododecane N,N',N' ',N' "-tetraacetic acid (DOTA) and labeled with 111In. Tumor targeting and biodistribution studies were carried out in athymic mice xenografted with a human colorectal tumor cell line, LS174T. The [111In]T84.66 diabody (55 kDa) exhibited very rapid tumor targeting with 12.5 +/- 0.4% injected dose per gram (% ID g(-1) +/- standard error) at 2 h and reached a maximum of 13.3 +/- 0.9% ID g(-1) at 6 h. However, kidney uptake was observed to reached a peak of 183.5 +/- 21.0% ID g(-1) at 6 h, a result similar to that reported by others for other low molecular weight fragments labeled with radiometals. Preadministration of an oral dose of D-lysine resulted in a 59% lowering of the renal accumulation at 6 h, but was accompanied by a 31% reduction of tumor uptake to 9.2 +/- 1.2% ID g(-1). The second recombinant antibody fragment, the [111In]T84.66 minibody (80 kDa), displayed rapid tumor targeting of 14.2 +/- 6.1% ID g(-1) at 2 h, and reached a maximum activity of 24.5 +/- 6.1% ID g(-1) by 12 h. Renal uptake achieved a plateau of 12-13% ID g(-1) which cleared to 7.2% ID g(-1) at 72 h. However, hepatic uptake was elevated and reached a maximum of 26.0 +/- 1.0% ID g(-1) at 12 h in these xenograft-bearing mice. Experiments in nontumor bearing mice showed a reduction of hepatic activity at 12 h to 16.6 +/- 1.5% ID g(-1), indicative of an intrinsic hepatic accumulation of the [111In]DOTA-T84.66 minibody or metabolites. While the anti-CEA [111In]DOTA-T84.66 diabody and T84.66 minibody retain the rapid tumor targeting properties of the radioiodinated form, the normal organ accumulation (kidneys and liver, respectively) of the [111In]DOTA forms appeared problematic for RIS and RIT applications. Development of alternative blocking strategies or new metabolizable chelates are under investigation to enhance the utility of the radiometal form of these and other promising recombinant antibody fragments.