Noninvasive localization of experimental atherosclerotic lesions with mouse/human chimeric Z2D3 F(ab')2 specific for the proliferating smooth muscle cells of human atheroma. Imaging with conventional and negative charge-modified antibody fragments

Physicochemical Rules for Identifying Monoclonal Antibodies with Drug-like Specificity

The ability of antibodies to recognize their target antigens with high specificity is fundamental to their natural function. Nevertheless, therapeutic antibodies display variable and difficult-to-predict levels of nonspecific and self-interactions that can lead to various drug development challenges, including antibody aggregation, abnormally high viscosity, and rapid antibody clearance. Here we report a method for predicting the overall specificity of antibodies in terms of their relative risk for displaying high levels of nonspecific or self-interactions at physiological conditions. We find that individual and combined sets of chemical rules that limit the maximum and minimum numbers of certain solvent-exposed amino acids in antibody variable regions are strong predictors of specificity for large panels of preclinical and clinical-stage antibodies. We also demonstrate how the chemical rules can be used to identify sites that mediate nonspecific interactions in suboptimal antibodies and guide the design of targeted sublibraries that yield variants with high antibody specificity. These findings can be readily used to improve the selection and engineering of antibodies with drug-like specificity.

In vitro demonstration of enhanced prostate cancer toxicity: pretargeting with Bombesin bispecific complexes and targeting with polymer-drug-conjugates

BACKGROUND

Bombesin has been used to target Bombesin receptor, a growth receptor, which is over-expressed in many cancers, including prostate cancer. Polymer-anti-neoplastic-drug-conjugates (PDC) were also developed to reduce non-specific toxicity and increase tumor toxicity utilizing the enhanced permeability and retention effect, benefitting treatment of large tumors with well-established vasculature.

PURPOSE

If PDCs were delivered by targeted delivery to cancer cells, tumor toxicity would be enhanced and non-specific toxicity decreased.

METHODS

Cardiocyte toxicity was assessed in H9c2 cardiocytes with doxorubicin (Dox) or N-terminal DTPA-modified-Doxorubicin-loaded-polyglutamic acid polymers (D-Dox-PGA). Therapeutic efficacy of targeted D-Dox-PGA after pretargeting with Bombesin-conjugated anti-DTPA-antibody Bispecific Complexes (Bom-BiSpCx) was compared to that of Dox in PC3 cells. Bom-BiSpCx was generated by thioether bond between Bombesin to Anti-DTPA antibody.

RESULTS

D-Dox-PGA was demonstrated to have less cardiocyte toxicity (IC50 = 20 µg/ml) than free Dox (1.55 µg/ml, p < 0.001). However, after pre-targeting of human prostate cancer PC3 cells with Bom-BiSpCx and targeting with D-Dox-PGA, IC50 (13.2 µg/ml) was about two times less than that of Dox (28.5 µg/ml, p < 0.0001).

DISCUSSION

Targeted delivery of PDCs having lower cardiocyte toxicity enabled higher efficiency cancer cell therapy.

CONCLUSION

This study may allow development of very efficient targeted prostate cancer pro-drug therapy.

Molecular imaging of apolipoprotein B-100 in human coronary plaques by color fluorescent angioscopy and microscopy

Apolipoprotein B-100 (ApoB-100) is an important risk factor for coronary artery disease. However, its localization in human coronary plaques is not well understood. The present study was performed to visualize ApoB-100 in human coronary artery wall. Deposition of native ApoB-100 in excised human coronary plaques and normal segments classified by conventional angioscopy was investigated by color fluorescent angioscopy (CFA) and microscopy (CFM) using Nile blue dye (NB) which elicits a golden fluorescence characteristic of ApoB-100 as a biomarker. By CFA, the % incidence of ApoB-100 was 20 in 40 normal segments, 38 in 42 white, and 11 in 35 yellow plaques (P < 0.05 versus white plaques). There was no significant difference in detection sensitivity between CFA and luminal surface scan by CFM. By CFM transected surface scan, ApoB-100 deposited in superficial, deep, and/or in both layers. Deposition in both layers was frequently observed in white plaques and yellow plaques without necrotic core (NC), less frequently in normal segments, and rarely in yellow plaques with NC. (1) Taking into consideration the well known process of plaque growth, the results suggest that ApoB-100 begins to deposit before plaque formation, increasingly deposits with plaque growth, and disappears after necrotic core formation. (2) CFA is feasible for imaging of ApoB-100 in human coronary artery wall.

The vulnerable coronary plaque: update on imaging technologies

Several studies have been carried out on vulnerable plaque as the main culprit for ischaemic cardiac events. Historically, the most important diagnostic technique for studying coronary atherosclerotic disease was to determine the residual luminal diameter by angiographic measurement of the stenosis. However, it has become clear that vulnerable plaque rupture as well as thrombosis, rather than stenosis, triggers most acute ischaemic events and that the quantification of risk based merely on severity of the arterial stenosis is not sufficient. In the last decades, substantial progresses have been made on optimisation of techniques detecting the arterial wall morphology, plaque composition and inflammation. To date, the use of a single technique is not recommended to precisely identify the progression of the atherosclerotic process in human beings. In contrast, the integration of data that can be derived from multiple methods might improve our knowledge about plaque destabilisation. The aim of this narrative review is to update evidence on the accuracy of the currently available non-invasive and invasive imaging techniques in identifying components and morphologic characteristics associated with coronary plaque vulnerability.

Molecular imaging to identify the vulnerable plaque--from basic research to clinical practice

Cardiovascular disease (CVD) is still the leading cause of death in the Western World. Adverse outcomes of CVD include stroke, myocardial infarction, and heart failure. Atherosclerosis is considered to be the major cause of CVD and is estimated to cause half of all deaths in developed countries. Atherosclerotic lesions of the vessel wall may obstruct blood flow mechanically through stenosis, but rupture of atherosclerotic plaques causing formation of occlusive thrombi is far more prevalent. Unfortunately, conventional diagnostic tools fail to assess whether a plaque is vulnerable to rupture. Research over the past decade identified the biological processes that are implicated in the course towards plaque rupture, like cell death and inflammation. Knowledge about plaque biology propelled the development of imaging techniques that target biologic processes in order to predict the vulnerable plaque. This paper discusses novel and existing molecular imaging targets and addresses advantages and disadvantages of these targets and respective imaging techniques in respect of clinical application and socio-economic impact.

Cardiovascular Molecular Imaging

The goal of this review is to highlight how molecular imaging will impact the management and improved understanding of the major cardiovascular diseases that have substantial clinical impact and research interest. These topics include atherosclerosis, myocardial ischemia, myocardial viability, heart failure, gene therapy, and stem cell transplantation. Traditional methods of evaluation for these diseases will be presented first, followed by methods that incorporate conventional and molecular imaging approaches.

Activated alphavbeta3 integrin targeting in injury-induced vascular remodeling

There is currently no imaging modality to track the remodeling process, a common feature of a broad spectrum of vasculopathies, in vivo. alphavbeta3 Integrin is up-regulated in proliferating vascular cells. RP748, a novel peptidomimetic tracer, binds specifically to the activated alphavbeta3 conformer and exhibits favorable binding characteristics for in vivo imaging. In a model of injury-induced vascular remodeling in apoE null mice, RP748 localization to the injured carotid arteries parallels vascular cell proliferation, providing an opportunity to image the remodeling process in vivo.

Detection of inflamed atherosclerotic lesions with diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) and positron-emission tomography

Diadenosine-5',5'''-P(1),P(4)-tetraphosphate (Ap(4)A) and its analog P(2),P(3)-monochloromethylene diadenosine-5',5'''-P(1),P(4)-tetraphosphate (AppCHClppA) are competitive inhibitors of adenosine diphosphate-induced platelet aggregation, which plays a central role in arterial thrombosis and plaque formation. In this study, we evaluate the imaging capabilities of positron-emission tomography (PET) with P(2),P(3)-[(18)F]monofluoromethylene diadenosine-5',5'''-P(1),P(4)-tetraphosphate ([(18)F]AppCHFppA) to detect atherosclerotic lesions in male New Zealand White rabbits. Three to six months after balloon injury to the aorta, the rabbits were injected with [(18)F]AppCHFppA, and microPET imaging showed rapid accumulation of this radiopharmaceutical in the atherosclerotic abdominal aorta, with lesions clearly visible 30 min after injection. Computed tomographic images were coregistered with PET images to improve delineation of aortoiliac tracer activity. Plaque macrophage density, quantified by immunostaining with RAM11 against rabbit macrophages, correlated with PET measurements of [(18)F]AppCHFppA uptake (r = 0.87, P < 0.0001), whereas smooth-muscle cell density, quantified by immunostaining with 1A4 against smooth muscle actin, did not. Biodistribution studies of [(18)F]AppCHFppA in normal rats indicated typical adenosine dinucleotide behavior with insignificant myocardial uptake and fast kidney clearance. The accumulation of [(18)F]AppCHFppA in macrophage-rich atherosclerotic plaques can be quantified noninvasively with PET. Hence, [(18)F]AppCHFppA holds promise for the noninvasive characterization of vascular inflammation.

Atherosclerosis imaging on the molecular level

On the basis of clinical observations that acute coronary events often result from rupture of atherosclerotic plaques at sites with no or minor luminal narrowing, the search for techniques by which to identify vulnerable, rupture-prone lesions has developed into a quest for the holy grail of cardiovascular medicine. Vulnerable plaques may show characteristic morphologic features, but they may still differ in their biology and their activity, which ultimately leads to rupture. As a consequence, considerable efforts have been undertaken to identify biologic mechanisms of atherosclerotic lesions by use of molecular-targeted radiolabeled probes. A variety of approaches aiming at plaque inflammation, apoptosis, smooth muscle cell proliferation, extracellular matrix activation, or platelet binding have been introduced. Nevertheless, molecular imaging of atherosclerosis is still a work in progress. Challenges related to the best targeting approach, to translation of animal model results to the clinical setting, to adequate imaging methodology for visualization of coronary artery biology, and to a suitable target patient population will need to be overcome. But the field is steadily moving ahead and getting closer to the ultimate goal of an improved clinical risk assessment through in vivo assessment of vascular biology.

Vulnerable Plaque Imaging

The concept of vulnerable plaque is well established with increasing evidence from clinical and basic research. The paradigm has shifted from focusing exclusively on the hemodynamic effects of plaque (ie, resulting lumenal stenosis alone as a predictor of stroke risk) to assessment of the structure and composition of plaque (eg, denuded endothelium with inflammatory elements as a nidus for platelet-fibrin clumping). It is increasingly evident that methods to detect and characterize vulnerable plaque must be developed and optimized. Although MR imaging, CT, and ultrasound provide data regarding single lesions, future investigations relying heavily on nuclear medicine techniques may offer functional assessment of the entire cardiovascular system.

Radionuclide-Based Insights into the Pathophysiology of Ischemic Heart Disease: Beyond Diagnosis

This review article discusses the historical origin of cardiac radionuclide-based methods, the physiologic background that justifies their existence, as well as the basic pathophysiologic concepts of coronary artery disease and their connection with the technologic design and application of these methods. Most importantly, this review discusses the important insights that these methods have provided to the understanding of the mechanisms of ischemia, risk stratification, and both treatment choice and treatment efficacy in ischemic heart disease. Nuclear cardiology originated as an attempt to provide complementary physiologic information to the anatomic information provided by coronary angiography. To comprehend the design and applications of nuclear cardiology methods, one must have a basic understanding of coronary artery disease as an inflammatory process that may manifest as acute or chronic states. Basic concepts on myocyte metabolic pathways, coronary blood flow, ischemic cascade, ventricular remodeling, and ejection fraction become critical for this purpose. Insights into risk stratification may permit patient-tailored therapy approaches. Insights into prognosis have made nuclear cardiology a robust tool for outcome predictions, with an exceptionally high negative predictive value. Evaluation of prognosis in special patient populations such as diabetics has originated important pathophysiologic concepts. Most insights into phenomena such as myocardial hibernation, myocardial stunning, and viability have been generated by nuclear cardiology techniques. Finally, new applications of radionuclide-based methods such as molecular identification of "vulnerable" atherosclerotic plaques, "ischemic memory" using fatty acid imaging, and myocardial innervation imaging provide new avenues for insightful research.

Second Annual Mario S. Verani, MD, Memorial Lecture: Nuclear cardiology, the next 10 years

The nuclear cardiology of the future will be based on new clinical and biologic targets. It will be driven by modern concepts of molecular and cell biology and molecular genetics. A major effort involves detection of atherosclerosis and vascular vulnerability. Approaches include targeting proliferating smooth muscle cells, angiogenesis, vascular injury, inflammation through a variety of mechanisms, defining cell death and protease activation, and imaging gene expression. Another new clinical target involves imaging stem cells and various progenitor cells. To meet these new objectives, advanced imaging technology is required. This involves the development of micro-single photon emission computed tomography and micro-positron emission tomography systems as well as fusion technology involving radiologic computed tomography imaging together with nuclear imaging. Vascular lesion detection imaging may require intravascular detectors. The future of nuclear cardiology, based on molecular imaging, is extraordinarily exciting. The newly defined biologic targets will allow the answering of many of the key clinical questions that will dominate cardiovascular care in cardiovascular investigation over the next decade.

Defining the success of cardiac gene therapy: how can nuclear imaging contribute?

Gene therapy is a promising modality for the treatment of various cardiovascular diseases such as ischaemia, heart failure, restenosis after revascularisation, hypertension and hyperlipidaemia. An increasing number of approaches are moving from experimental and preclinical validation to clinical application, and several multi-centre trials are currently underway. Despite the rapid progress in cardiac gene therapy, many basic tools and principles remain under development. Questions with regard to the optimal method for gene delivery in a given situation remain open, as do questions concerning therapeutic efficacy and the time course and magnitude of gene expression in target and remote areas. Nuclear imaging provides valuable tools to address these open issues non-invasively. Functional effects of molecular therapy at the tissue level can be identified using tracers of blood flow, metabolism, innervation or cell death. The use of reporter genes and radiolabelled reporter probes allows for non-invasive assessment of location, magnitude and persistence of transgene expression in the heart and the whole body. Co-expression of a reporter gene will allow for indirect imaging of the expression of a therapeutic gene of choice, and linkage of measures of transgene expression to downstream functional effects will enhance the understanding of basic mechanisms of cardiac gene therapy. Hence, nuclear imaging offers great potential to facilitate and refine the determination of therapeutic effects in preclinical and clinical cardiovascular gene therapy.

Targeting of peptides to restenotic vascular smooth muscle cells using phage display in vitro and in vivo

Restenosis after angioplasty occurs in 30-40% of the treated patients. To develop a strategy to deliver drugs to restenotic lesions, we selected phages that bind to proliferating vascular smooth muscle cells (VSMC), from a random constraint 15-mer peptide phage display library. Phages were selected for binding to cultured primary aortic VSMC (in vitro biopanning) and selected for binding to denudated carotid arteries in mice (in vivo biopanning). In vitro biopanning did not result in a consensus sequence, but recurring FLGW and LASR amino acid motifs were identified. In vivo biopanning resulted in two consensus peptides 5G6 (CNIWGVVLSWIGVFPEC) and 5E5 (CESLWGGLMWTIGLSDC). Surprisingly, these two sequences were recovered after both in vitro and in vivo biopanning, but predominantly in vivo. Moreover, a strong recurring motif, IGR, was identified in the in vivo clones. The consensus phages 5G6 and 5E5 bind selectively to VSMC compared to other cell types. Furthermore, they bind preferentially to proliferating VSMC compared to VSMC that were growth arrested, and are effectively internalized by their target cells. The specific binding capacities of 5G6 and 5E5 phages suggest that these peptide sequences can be used for targeting of restenotic lesions, in which proliferating VSMC are the dominant cell type.

Pharmacokinetic characteristics and biodistribution of radioiodinated chimeric TNT-1, -2, and -3 monoclonal antibodies after chemical modification with biotin

To improve the clinical potential of monoclonal antibodies (MAbs), new methods are required to augment antibody uptake in the tumor while minimizing binding in normal tissues. Our laboratory has pioneered the use of chemical modification to accomplish this goal. Using three chimeric MAbs, chTNT-1, chTNT-2, and chTNT-3, which target solid tumors by binding to common antigens found in the central necrotic core, we now demonstrate the potential of chemical modification to improve the pharmacokinetic characteristics of these unique MAbs. To identify optimal modification conditions, TNT MAbs were reacted with biotin at various ratios and tested by clearance and biodistribution analyses. The biodistribution results revealed that the numbers of biotin molecules per MAb yielding optimal tumor uptake were 3:1 for chTNT-1, 5:1 for chTNT-2, and 8:1 for chTNT-3. Biotinylated MAbs were found to have faster whole body clearance times and better biodistribution profiles compared to unmodified antibodies. Although chTNT-2 showed only a modest improvement after biotinylation, biodistribution results indicated that this MAb had the highest uptake in tumor. By reducing the charge of the antibody molecule, chemical modification appears to be a useful method for improving the pharmacokinetics and biodistribution of TNT antibodies directed to the necrotic region of solid tumors.

18F FDG uptake in the large arteries: a correlation study with the atherogenic risk factors

It has been reported that there is a high correlation between fluorodeoxyglucose (FDG) uptake in the aorta and macrophage content of atherosclerotic lesions in an experimental rabbit model. We evaluated the frequency of FDG uptake in the large arteries in relation to the atherogenic risk factors. We also investigated whether FDG uptake of the large arteries is related to clinically known coronary artery disease. The presence of FDG uptake was assessed in the abdominal aorta (AA), iliac (IA), and proximal femoral arteries (FAs) in 156 patients. Medical history of the atherogenic risk factors (age, cigarette smoking, hypertension, diabetes, high cholesterol, and obesity) and coronary artery disease (CAD) was identified for each patient. The frequency of vascular FDG uptake was compared between the patients without risk factors (Group I, 23 patients) and those with at least 1 risk factor (Group II, 133 patients). The correlation of each risk factor and known CAD with arterial FDG uptake was also assessed in the 3 different arteries. There was a significant difference in the frequency of FDG uptake between the 2 groups for the FA (22% vs 70%) and IA (30% vs 54%), but not for the AA (35% vs 53%). Among all risk factors, age was the most significant and consistent factor correlating with FDG uptake in all 3 arteries. Hypercholesterolemia also correlated consistently with FDG uptake in all 3 arteries. The correlation between the remaining risk factors and arterial FDG uptake was rather artery specific than consistent throughout all 3 arteries. A higher frequency of FDG uptake in the FA was seen in patients with CAD compared with those without CAD. Not all risk factors correlated with FDG uptake in different arteries. Among the risk factors, age and hypercholesterolemia most consistently correlated with FDG uptake in the AA, and the IA and proximal FAs. The positive correlation of arterial FDG uptake with the atherogenic risk factors suggested a promising role for FDG-PET imaging in the diagnosis of atherosclerosis and follow-up after treatment intervention.

Assessment of peripheral arterial vascular disease with radionuclide techniques

Various radioisotopic imaging techniques for noninvasive detection of vessel stenosis and for functional investigation of reduced blood flow and follow-up have been developed during the last decade in peripheral vascular disease (PVD), with the aim of replacing invasive techniques and complementing standardized methods. Radionuclide assessment of PVD is divided into 2 major groups: imaging of perfusion and metabolic investigations. The measurement of arterial blood flow and muscle perfusion is intended to show the morphology, to evaluate the functional consequences of PVD, and to quantify the latter. The application of radiolabeled tracers was developed as a noninvasive alternative to angiography in morphologic imaging. Treadmill testing has been used to assess the functional effects of reduced blood flow in PVD where the onset of pain indicates the stage of disease, but the results can be confused by other symptoms. Scintigraphic measurement of muscle perfusion should detect insufficient nutritional blood flow in peripheral muscle and thus have a higher specificity for PVD than treadmill testing alone. Although there are very promising theoretical and experimental data in animals, the clinical use of radionuclide investigations is limited by different technical problems, such as methodologic differentiation between skin and muscle perfusion, the lack of controlled and prospective studies, and incomplete correlation with other standardized routine techniques. Among the great number of radioisotopic metabolic imaging techniques, only radiolabeled platelets and lipoproteins, to some extent, have shown a limited potential clinical use. Some other approaches seem to have a high potential from a theoretical point of view. They are limited, however, by a great number of problems. Correlation with sonographic or magnetic resonance imaging (MRI) findings may identify a potential metabolic value. Correlation with angiography reflecting the extent of the disease makes no sense. So far with PVD, neither radioisotopic perfusion studies nor metabolic imaging techniques are able to achieve a level of routine application or wider meaningful interpretation of the clinical condition of a specific patient. Competing techniques are easier to perform, less expensive, faster, more widely available, and do not carry the radiation burden. Positron emission tomography is still in its early stages of application, with great theoretical potential but at a high price. A great deal of work is still required to transform in vitro and experimental experience into more meaningful routine radioisotopic investigations in patients with PVD.

Drug targeting

The main problems currently associated with systemic drug administration are: even biodistribution of pharmaceuticals throughout the body; the lack of drug specific affinity toward a pathological site; the necessity of a large total dose of a drug to achieve high local concentration; non-specific toxicity and other adverse side-effects due to high drug doses. Drug targeting, i.e. predominant drug accumulation in the target zone independently on the method and route of drug administration, may resolve many of these problems. Currently, the principal schemes of drug targeting include direct application of a drug into the affected zone, passive drug targeting (spontaneous drug accumulation in the areas with leaky vasculature, or Enhanced Permeability and Retention-EPR-effect), 'physical' targeting (based on abnormal pH value and/or temperature in the pathological zone), magnetic targeting (or targeting of a drug immobilized on paramagnetic materials under the action of an external magnetic field), and targeting using a specific 'vector' molecules (ligands having an increased affinity toward the area of interest). The last approach provides the widest opportunities. Such pharmaceutical carriers as soluble polymers, microcapsules, microparticles, cells, cell ghosts, liposomes, and micelles have been successfully used for targeted drug delivery in vivo. Though the direct conjugation of a drug molecule with a targeted moiety is also possible (immunotoxin), the use of microreservoir-type systems provides clear advantages, such as high loading capacity, possibility to control size and permeability of drug carrier systems and use relatively small number of vector molecules to deliver substantial quantities of a drug to the target. The practical use of the listed systems and approaches for the delivery of therapeutic and diagnostic agents will be considered.

Techniques characterizing the coronary atherosclerotic plaque: influence on clinical decision making?

The composition of the atherosclerotic lesion rather than the degree of stenosis is currently considered to be the most important determinant for acute clinical events. Modalities capable of characterizing the atherosclerotic lesion may be helpful in understanding its natural history and detecting lesions with high risk for acute events. Speaking grossly, three histologic features of the vulnerable plaque have been reported: size of the atheroma, thickness of the fibrous cap, and inflammation. Imaging techniques are currently being deployed and are under development to aid visualization of the vulnerable coronary plaque. Most of these diagnostic modalities have the potential to detect locally one or more of the three histologically defined features of vulnerable plaque. This review will focus on imaging techniques that have been developed to characterize the atherosclerotic lesion. Most catheter-based visualization techniques will provide insight into components of the local atherosclerotic plaque which may limit their predictive value for the occurrence of a clinical event. Therefore, the clinical relevance of these imaging tools will be discussed.

Imaging techniques to predict cardiovascular risk

Conventional cardiovascular imaging, with a focus on identifying flow-limiting stenoses, does not directly image the atherosclerotic lesion. Recent clinical and pathobiologic data indicate that stenosis severity does not dictate cardiovascular risk and that there are functional, structural, and biologic features of atherosclerosis that are associated with cardiovascular events. Imaging technologies, such as ultrasound, light, x-ray, magnetic resonance, and targeted contrast agents, have been developed to characterize directly the atherosclerotic vessel wall. They provide promising approaches to predict cardiovascular risk and facilitate further study of the mechanisms of atherosclerosis progression and its response to therapy.

Current status of radionuclide tracer imaging of thrombi and atheroma

The imaging of thrombi and atherosclerotic plaques has great potential for decision making in the management of patients with all types of disease within the circulatory system. This importance is owing to the developments showing that areas of moderate stenosis with underlying atheroma are physiologically reactive and capable of causing reversible clinical symptoms that can progress to irreversible end-organ damage if not effectively treated. Identifying and quantifying areas of smaller vulnerable plaque and areas of acute thrombosis will assist in identification of patients at risk and help determine when and how to treat these patients. Initial efforts in this area used nonspecific constituents of thrombi and atheroma that were radiolabeled using long-lived isotopes, which had high background activity that required imaging over 48 to 72 hours. Newer approaches have focused on the use of small antibody fragments or small peptides, so-called molecular recognition units, that specifically target antigens present only in areas of thrombosis or active atherogenesis. These compounds are labeled Technetium-99 m (99mTc) and provide excellent images. Efforts to image thrombi have been directed at the IIB/IIIA receptor, which is present in low concentration on the cell membrane of circulating quiescent platelets, but on stimulation and active thrombosis, more than 80,000 potential binding sites per platelet appear. One such peptide has been clinically approved for imaging of deep vein thrombophlebitis. Parallel efforts are being made for imaging areas of active atherogenesis by targeting smooth muscle cells and other constituents unique for vulnerable plaques. Efforts in developing these modalities are important to expand the applications to new areas in nuclear cardiology.

Antibodies as delivery systems for diagnostic functions

Antibodies are highly specific targeting agents. Therefore, they are invaluable for in vitro and in vivo diagnostic applications. With the advent of monoclonal antibody technology, the utilization of antibodies has increased dramatically in almost every field of biological sciences. The present review describes the utility of monoclonal antibodies primarily in the cardiovascular diseases. Monoclonal antimyosin antibodies have been developed for noninvasive scintigraphic imaging of equivocal acute myocardial infarction. They have been negative charge modified to provide quicker in vivo visualization of the targeted antibody, as well as being applied for diagnosis of other cardiomyopathies that have disruption of myocardial cell membrane as an obligatory component of the disease. The radiolabeling techniques developed initially for myocardial necrosis imaging have also been applied for imaging of intravascular blood clots and atherosclerotic lesions. The applications of antimyosin, antifibrin and anti-atherosclerotic lesion specific monoclonal antibodies have all achieved initial clinical verification of their efficacy to target the respective lesions. However, to date, only antimyosin has been approved by the FDA for commercialization. Others must await additional studies to unequivocally verify the clinical utilities.

A new chemically modified chimeric TNT-3 monoclonal antibody directed against DNA for the radioimmunotherapy of solid tumors

In the last several years, our laboratory has developed a new approach to the radioimmunotherapy of solid tumors, designated Tumor Necrosis Treatment (TNT), that exploits the presence of degenerating and necrotic cells within tumors by utilizing MAbs directed against universal, intracellular antigens. The first TNT MAb developed by our laboratory, designated TNT-1, was directed against nucleosomal determinants consisting of histone H1 and DNA. Since absolute tumor accretion of MAb is a critical determinant of antitumor efficacy in radioimmunotherapy, we sought to identify new antinuclear antibodies that displayed high tumor localization properties. In the present study, we describe a murine antinuclear antibody, TNT-3, which demonstrates 3-fold higher tumor uptake than TNT-1. Because of this characteristic, a chimeric derivative designated chTNT-3 was developed and evaluated for antigen binding and tumor targeting. ELISA studies using a series of nuclear antigens confirmed that TNT-3 is directed against single-stranded DNA and does not cross react with TNT-1. Immunohistology reveals predominantly nuclear staining reactivity in human tissues and tumors. Since it was shown by our laboratory that charge modification can significantly improve the pharmacokinetic performance of monoclonal antibodies, chTNT-3 was chemically modified with biotin to generate an improved therapeutic reagent designated chTNT-3/B. Comparative studies with unmodified MAb demonstrated that biotinylation significantly shortened its clearance time in mice and produced lower normal tissue levels, while maintaining an equal amount of uptake in tumor xenografts for up to 10 days. These in vivo characteristics suggest that chTNT-3/B is an improved TNT reagent for the radioimmunotherapy of solid tumors.

Rapid noninvasive detection of experimental atherosclerotic lesions with novel 99mTc-labeled diadenosine tetraphosphates

The development of a noninvasive imaging procedure for identifying atherosclerotic lesions is extremely important for the clinical management of patients with coronary artery and peripheral vascular disease. Although numerous radiopharmaceuticals have been proposed for this purpose, none has demonstrated the diagnostic accuracy required to replace invasive angiography. In this report, we used the radiolabeled purine analog, 99mTc diadenosine tetraphosphate (Ap4A; AppppA, P1,P4-di(adenosine-5')-tetraphosphate) and its analogue 99mTc AppCHClppA for imaging experimental atherosclerotic lesions in New Zealand White rabbits. Serial gamma camera images were obtained after intravenous injection of the radiolabeled dinucleotides. After acquiring the final images, the animals were sacrificed, ex vivo images of the aortas were recorded, and biodistribution was measured. 99mTc-Ap4A and 99mTc AppCHClppA accumulated rapidly in atherosclerotic abdominal aorta, and lesions were clearly visible within 30 min after injection in all animals that were studied. Both radiopharmaceuticals were retained in the lesions for 3 hr, and the peak lesion to normal vessel ratio was 7.4 to 1. Neither of the purine analogs showed significant accumulation in the abdominal aorta of normal (control) rabbits. The excised aortas showed lesion patterns that were highly correlated with the in vivo and ex vivo imaging results. The present study demonstrates that purine receptors are up-regulated in experimental atherosclerotic lesions and 99mTc-labeled purine analogs have potential for rapid noninvasive detection of plaque formation.

Noninvasive detection of atherosclerotic lesions by 99mTc-based immunoscintigraphic targeting of proliferating smooth muscle cells

BACKGROUND

Mouse/human chimeric antibody Z2D3 identifies an antigen produced exclusively by proliferating smooth muscle cells of human atheroma, and also cross reacts with experimentally induced atherosclerotic lesions in rabbits. Fab' fragments of Z2D3 antibody were labeled with (99m)Tc using glucaric acid as a weak transchelator. The potential role of (99m)Tc-labeled Z2D3 scintigraphy was explored for noninvasive imaging of experimental atherosclerotic lesions.

METHODS AND RESULTS

(99m)Tc-Z2D3 Fab' was utilized for noninvasive imaging in four rabbits with experimentally induced atherosclerotic lesions and in one control rabbit. In addition, (99m)Tc-labeled nonspecific 103D2 Fab' was used for comparison in four other rabbits with atherosclerotic lesions. The atherosclerotic lesions were induced by balloon de-endothelialization of the infradiaphragmatic abdominal aorta and 12 weeks of hyperlipidemic diet. An aliquot of 15 mCi (550 mBq) of (99m)Tc pertechnetate was incubated with 6.25 mg of glucaric acid for 30 min followed by incubation of (99m)Tc glucarate with 375 microg of Z2D3 Fab' or 103D2 Fab' for an additional 30 min. Instant thin-layer chromatography demonstrated almost complete radiolabeling. (99m)Tc-Z2D3 was administered IV and gamma imaging was performed at the time of injection, 3, 6, 9, and 12 h, followed by ex vivo imaging of the excised aorta, and biodistribution was performed. Unequivocal visualization of atherosclerotic lesions was possible in all four animals at 9 to 12 h with Z2D3 Fab'. Quantitative uptake, as represented by mean lesion-to-liver count density ratio, was 0.6+/-0.05. Imaging with nonspecific 103D2 Fab' did not show any localization in the abdominal aorta (lesion-to-liver ratio, 0.45+/-0.02, p=0.02). Ex vivo lesion-to-normal aortic segment ratio was 4.3+/-0.9 for Z2D3 and 1.04+/-0.08 for nonspecific 103D2 Fab' (p=0.01). Biodistribution studies demonstrated 0.03+/-0.003% injected Z2D3 dose per gram in the atherosclerotic lesions as compared with 0.01+/-0.003% in the nondenuded thoracic aorta of atherosclerotic rabbits (p=0.008). However, only 0.008+/-0.002% of the mean injected dose per gram was obtained in the atherosclerotic lesions (p=0.001) as compared with 0.005+/-0.003% in the normal aortic segments with 103D2. No Z2D3 uptake in normal rabbits was observed on either the in vivo or ex vivo images.

CONCLUSIONS

The present study demonstrates that (99m)Tc-based immunoimaging of the vascular lesions may be feasible by the use of smaller antibody fragments. Earlier visualization is possible at the expense of a lower absolute antibody uptake in the lesions as compared to the use of intact antibody or larger fragments with longer circulating time.

In vitro targeting of antibody-conjugated echogenic liposomes for site-specific ultrasonic image enhancement

Tissue-specific ultrasonic enhancement can be used for the detection and characterization of atherosclerosis. We have previously demonstrated the generation of inherently echogenic (acoustically reflective) liposomes solely by varying lipid composition and controlling the method of production. In this study, echogenic liposomes composed of phosphatidylcholine (PC), 4-(p-maleimidophenyl) butyryl phosphatidylethanolamine (MPB-PE), phosphatidylglycerol (PG), and cholesterol were conjugated to human gamma globulin to determine the effect of antibody conjugation on liposomal acoustic reflectivity. The liposomes remained highly echogenic following antibody conjugation. Echogenic liposomes were also conjugated to rabbit antihuman fibrinogen to study their ability to target fibrin. Antibody-conjugated liposomes were targeted to fibrin-coated filter paper and slides, thrombi made in vitro, and segments of atheroma in an animal model of atherosclerosis. Liposomes were detected by scanning electron microscopy, radiolabeling, and imaging with intravascular ultrasound. Electron microscopy revealed attachment of antibody-conjugated liposomes to fibrin on slides and to the fibrous plaques of the arterial segments, whereas unconjugated liposomes did not attach. Similarly, conjugated liposomes did not attach to normal arteries, indicating their binding to the arterial segment is directed towards a component of the fibrous plaque. Ultrasound imaging of the thrombi demonstrated surface attachment of the acoustic conjugated liposomes. 125I-Labeled liposomes conjugated to rabbit anti-human were targeted to fibrin-coated paper. Counting specifically bound radioactivity showed that > 84% of applied liposomes remained attached to the fibrin after washing with saline. These results demonstrate the potential of acoustically reflective liposomes for site-specific targeting and acoustic enhancement.

Plug and seal: prevention of hypoxic cardiocyte death by sealing membrane lesions with antimyosin-liposomes

The hallmark of cell death is the development of cell membrane lesions. Such lesions in the myocardium are usually associated with acute myocardial infarction. Minimizing myocardial necrosis by thrombolytic reperfusion therapy constitutes the only major treatment to date. We envisioned a method to seal these membrane lesions using immunoliposomes as a novel adjunctive approach. An antigen to intracellular cytoskeletal myosin in hypoxic embryonic cardiocytes is used as an anchoring site, and a specific antibody on immunoliposomes as the anchor to plug and to seal the membrane lesions. H9C2 cells were used because they are cardiocytes and are propagated in tissue culture and their viability may be assessed by various methods. Viability assessed by [3H]thymidine uptake in hypoxic cardiocyte cultures (n = 6 each) treated with antimyosin-immunoliposomes (3.26 +/- 0.483 x 10(6) c.p.m.) was similar to that of normoxic cells (3.68 +/- 0.328 x 10(6) c.p.m.), but was greater than those of untreated hypoxic cells (0.115 +/- 0.155 x 10(6) c.p.m.) or hypoxic cells treated with plain liposomes (1.140 +/- 0.577 x 10(6) c.p.m.). These results were reconfirmed by trypan blue exclusion and by fluorescent, confocal and transmission electron microscopy. They indicated that cell death in hypoxic cardiocytes can be prevented by targeted cell membrane sealing. This concept of cell salvage should be applicable in the prevention of cell death in different biological systems.