In vivo imaging of intracranial human glioma xenografts comparing specific with nonspecific radiolabeled monoclonal antibodies

Clinical advances in TNC delivery vectors and their conjugate agents

Tenascin C (TNC), a glycoprotein that is abundant in the tumor extracellular matrix (ECM), is strongly overexpressed in tumor tissues but virtually undetectable in most normal tissues. Many TNC antibodies, peptides, aptamers, and nanobodies have been investigated as delivery vectors, including 20A1, α-A2, α-A3, α-IIIB, α-D, BC-2, BC-4 BC-8, 81C6, ch81C6, F16, FHK, Ft, Ft-NP, G11, G11-iRGD, GBI-10, 19H12, J1/TN1, J1/TN2, J1/TN3, J1/TN4, J1/TN5, NJT3, NJT4, NJT6, P12, PL1, PL3, R6N, SMART, ST2146, ST2485, TN11, TN12, TNFnA1A2-Fc, TNfnA1D-Fc, TNfnBD-Fc, TNFnCD-Fc, TNfnD6-Fc, TNfn78-Fc, TTA1, TTA1.1, and TTA1.2. In particular, BC-2, BC-4, 81C6, ch81C6, F16, FHK, G11, PL1, PL3, R6N, ST2146, TN11, and TN12 have been tested in human tissues. G11-iRGD and simultaneous multiple aptamers and arginine-glycine-aspartic acid (RGD) targeting (SMART) may be assessed in clinical trials because G11, iRGD and AS1411 (SMART components) are already in clinical trials. Many TNC-conjugate agents, including antibody-drug conjugates (ADCs), antibody fragment-drug conjugates (FDCs), immune-stimulating antibody conjugates (ISACs), and radionuclide-drug conjugates (RDCs), have been investigated in preclinical and clinical trials. RDCs investigated in clinical trials include 111In-DTPA-BC-2, 131I-BC-2, 131I-BC-4, 90Y-BC4, 131I81C6, 131I-ch81C6, 211At-ch81C6, F16124I, 131I-tenatumomab, ST2146biot, FDC 131I-F16S1PF(ab')2, and ISAC F16IL2. ADCs (including FHK-SSL-Nav, FHK-NB-DOX, Ft-NP-PTX, and F16*-MMAE) and ISACs (IL12-R6N and 125I-G11-IL2) may enter clinical trials because they contain components of marketed treatments or agents that were investigated in previous clinical studies. This comprehensive review presents historical perspectives on clinical advances in TNC-conjugate agents to provide timely information to facilitate tumor-targeting drug development using TNC.

Antibody-based immunotherapy for malignant glioma

Conventional therapy for malignant glioma (MG) fails to specifically eliminate tumor cells, resulting in toxicity that limits therapeutic efficacy. In contrast, antibody-based immunotherapy uses the immune system to eliminate tumor cells with exquisite specificity. Increased understanding of the pathobiology of MG and the profound immunosuppression present among patients with MG has revealed several biologic targets amenable to antibody-based immunotherapy. Novel antibody engineering techniques allow for the production of fully human antibodies or antibody fragments with vastly reduced antigen-binding dissociation constants, increasing safety when used clinically as therapeutics. In this report, we summarize the use of antibody-based immunotherapy for MG. Approaches currently under investigation include the use of antibodies or antibody fragments to: (1) redirect immune effector cells to target tumor mutations, (2) inhibit immunosuppressive signals and thereby stimulate an immunological response against tumor cells, and (3) provide costimulatory signals to evoke immunologic targeting of tumor cells. These approaches demonstrate highly compelling safety and efficacy for the treatment of MG, providing a viable adjunct to current standard-of-care therapy for MG.

Affinity reagents against tumour-associated extracellular molecules and newforming vessels

Here we report some recent results of tumour targeting using extracellular matrix components of tumour stroma as targets. The possibility of using human recombinant antibodies in tumour targeting is also described. Preliminary results indicate that neovasculature markers can be targeted by recombinant antibodies and that they allow long residence time in tumours, thus exploiting the avidity properties of multivalent recombinant fragments.

Tumor antigens in astrocytic gliomas

Gliomas affect 15,000 to 17,000 Americans every year and carry a dismal prognosis. The potential of immunologically mediated diagnosis and therapy, although greatly enhanced since the advent of monoclonal antibodies, has not been fully realized due to significant problems, most especially the challenge of identifying antigenic molecules specific to glial tumors. Other problematic issues include antigen-associated factors such as heterogeneity, modulation, shedding, and cross-reactivity with normal cells, and factors associated with therapeutic agent delivery, typically variable tumor perfusion and unfavorable diffusional forces in tumor microenvironment. An understanding of these problems called for the delineation of operationally specific antigens (tumor-associated antigens not expressed by the normal central nervous system) combined with the use of compartmental therapeutic approaches to increase the specificity of therapy. Numerous antigens have been identified and are classified as extracellular/matrix-associated, membrane-associated, and intracellular antigens. Nevertheless, only a few have been demonstrated to be of significant therapeutic and diagnostic utility. These few include the extracellular matrix-associated antigens tenascin and GP 240, defined by the monoclonal antibodies 81C6 and Mel-14, both of which are now in Phase I clinical trials, and membrane-associated ganglioside molecules, primarily 3', 6'-isoLD1, defined by the antibody DMAb-22. Recent identification of the overexpression of a deletion variant of the epidermal growth factor receptor (EGFRvIII) in up to 50% of the more malignant glial tumors and the subsequent creation of monoclonal antibodies that are specific to this molecule and do not recognize the wild-type EGFR provide the most exciting development yet in the design of specific antiglioma immunoconjugates. In addition, the tumor-specific nature of EGFRvIII combined with improved knowledge of immune mechanisms, especially in the context of the central nervous system, will facilitate the design of highly selective cell-mediated therapeutic approaches with a view toward obtaining tumor-specific immunity.

Tenascin distribution in human brain tumours

Using a monoclonal antibody specific for human tenascin (TN), 180 intracranial growths were immunohistochemically studied. In 69 cases of meningioma, neoplastic cells were negative, with some positivity being observed only in the perivascular and the supporting stroma, especially in anaplastic meningiomas. In 57 cases of glioma different degrees of reactivity occurred in both the cellular conglomerates and the stromal components of the tumours. A higher variability in reactivity was observed in anaplastic astrocytomas and glioblastomas. The most constant finding of the study was the staining of the stroma, which was observed in all types of growths, including metastasis, abscess and tuberculoma. The results are consistent with the hypothesis that tenascin is a stromal marker rather than a true marker of malignant tumours. The heterogeneous distribution of TN in anaplastic gliomas may be a factor in the variable response to treatment with radiolabelled anti-TN monoclonal antibodies.

Phase I studies of treatment of malignant gliomas and neoplastic meningitis with 131I-radiolabeled monoclonal antibodies anti-tenascin 81C6 and anti-chondroitin proteoglycan sulfate Me1-14 F (ab')2--a preliminary report

The advent of monoclonal antibody (MAb) technology has made Ehrlich's postulate of the 'magic bullet' an attainable goal. Although specific localization of polyvalent antibodies to human gliomas was demonstrated in the 1960s, the lack of specific, high affinity antibody populations and of defined target antigens of sufficient density precluded therapeutic applications. Not until the identification of operationally specific tumor-associated antigens (present in tumor tissue but not normal central nervous system tissue); production of homogeneous, high affinity MAbs to such antigens; and the use of compartmental administration (intrathecal or intracystic), has the promise of passive immunotherapy of primary and metastatic central nervous system neoplasms been recognized. We report here preliminary data from Phase I studies of the compartmental administration of the anti-tenascin MAb 81C6 and F(ab2)2 fragments of MAb Me1-14, which recognizes the proteoglycan chondroitin sulfate-associated protein of gliomas and melanomas, to patients with primary central nervous system tumors or tumors metastatic to the central nervous system. Phase I dose escalation studies of intracystically administered 131I-labeled anti-tenascin MAb 81C6 to either spontaneous cysts of recurrent gliomas or surgically created cystic resection cavities have resulted in striking responses. Of five patients with recurrent cystic gliomas treated, four had partial responses, clinically or radiographically. Similarly, in patients with surgically created resection cavities, a partial response at the treatment site and extended stable disease status has been obtained following intracystic administration of 131I-labeled 81C6. No evidence of hematologic or neurologic toxicity has been observed in either patient population, with the exception of transient exacerbation of a pre-existing seizure disorder in a single patient. Dosimetry calculations indicated high intracystic retention for four to six weeks with little or no systemic dissemination; estimated total doses intracystically ranged from 12,700-70,290 rad. Intrathecal administration of labeled MAbs to patients with neoplastic meningitis is more difficult to assess in terms of clinical responsiveness. Of patients so treated with either 131I-labeled 81C6 or 131I-labeled Me1-14 (F(ab)2, cerebrospinal fluid and radiographic responses have been achieved, and survival prolongation through maintenance of stable disease has been observed in several cases. Initial results from pHase I dose escalation trials are encouraging in terms of the proportion of cases of disease stabilization and partial and complete responses obtained. Importantly, neurotoxicity has been virtually nonexistent, and hematologic toxicity rare and rapidly responsive to treatment.(ABSTRACT TRUNCATED AT 400 WORDS)

Generation and characterization of a mouse/human chimeric antibody directed against extracellular matrix protein tenascin

The murine anti-tenascin monoclonal antibody 81C6, following iodination, has been shown to be an efficient localizing and therapeutic agent in both subcutaneous and intracranial human glioma xenograft models in athymic mice and rats. Similarly, effective monoclonal antibody 81C6 localization has been demonstrated in glioma patients, and Phase I trials with the intact murine IgG2b kappa molecule are currently in progress. In order to maximize the potential for repeated administration by minimizing murine Fc-mediated immunogenicity and reducing Fc-mediated immune effects, we created murine 81C6 variable region/human IgG2 chimeric monoclonal antibodies by the molecular cloning of the variable region genes of mouse 81C6 and their genetic linkage to human constant region exons. The resulting chimeric constructs were introduced into SP2/0 cells, and stable transfectomas were selected by G418 and mycophenolic acid resistance. The resistant clones were screened for anti-tenascin activity on tenascin-coated plates by enzyme-linked immunosorbent assay. The N-terminal amino acid sequence of both heavy and light chains of the purified chimeric 81C6 antibody matched exactly with that of the native mouse 81C6 as well as with that deduced from the nucleotide sequence. The production level of chimeric 81C6 (13.9 mg/ml) from ascites in the highest expressing transfectoma was much higher than that of native mouse 81C6 (2.5 mg/ml). The chimeric antibody showed the same specificity and equivalent affinity for human intact tenascin or tenascin-expressing cells as the native mouse 81C6 antibody. Direct comparison of radioiodinated chimeric and radioiodinated mouse 81C6 biodistribution in subcutaneous and intracranial xenograft-bearing mice showed higher tumor-to-normal tissue ratios for chimeric 81C6 as compared with native mouse 81C6. The improved localizing and clearance characteristics of chimeric 81C6 in xenograft model systems suggests that chimeric 81C6 would be an improved reagent for intracompartmental therapy of tenascin-expressing tumors in the human central nervous system.

Migration of human malignant astrocytoma cells in the mammalian brain: Scherer revisited

Fresh suspensions of human glioblastoma multiforme were preincubated in the plant lectin Phaseolus vulgaris leucoagglutinin (PHAL) and implanted into cortical pockets in adult rat brain. Brains were investigated periodically over 30 postoperative days and the migration of the human glioblastoma cells was traced with anti-PHAL immunofluorescence or the overexpression of human specific p185c-neu a specific marker of a class of human malignant astrocytoma cells. The principal pathway of migration of the implanted human cells in the rat brain was ventrally through cortical gray matter and into the corpus callosum, with rapid lateral distribution in this and other parallel and intersecting white matter fascicles. Human glioblastoma cells also migrated on basement membrane lined blood vessels, pia-glia membrane and spaces of Virchow-Robin, as well as the subependymal space of the ventricles. These paths of migration of human glioblastoma cells in the rat brain are consistent with the pathways of spread of glioblastoma in the human brain as described by Scherer over 50 years ago, indicating that multifocal malignant astrocytomas have common migratory pathways in mature mammalian brain.

Immunodetection of thyroid tumors: role of immuno aggregates

To understand the inconsistent immunodetection of tumors in vivo, a labelled monoclonal antibody (MAb) against a human follicular cancer cell line (UCLA RO 82 W-1) was used as a model for in vitro and in vivo studies. The 131I labelled MAb x RO 82 W-1 bound to its target cells (10% to 70%) mainly because of the generation of immunoglobulin aggregates. Aggregates generated by the freezing process were shown by polyacrylamide gel electrophoresis (PAGE) and their removal by filtration. When the aggregated 131I MAb x RO 82 W-1 was injected into BALB/c mice bearing UCLA RO 82 W-1 tumors, a high tumor/blood ratio was found in the large tumors. The tracer concentrated in the macroscopically visible necrotic part of the tumor was largely responsible for the scintigraphic detection. Irrelevant 131I-IgG also concentrated in necrotic regions of tumors. Scintigraphic detection of thyroid tumors in this model was related to the degree to which labeled aggregates of IgG, regardless of their specificity, localized in necrotic regions of the tumors.

Monoclonal antibodies in neuro-oncology

Many studies have suggested the possible existence of tumor-associated antigens in brain gliomas. Strong evidence for the existence of such cell determinants was provided by recent investigations using hybridoma technology. The possibility of obtaining monoclonal antibodies (MAbs) against glioma-associated antigens should help to allow their identification, purification, and characterization. Utilizing MAbs as reagents of predefined specificity, a number of central and peripheral nervous system antigens could be detected. The molecules recognized by MAbs in glioma cells can be subdivided into four categories: [1] biochemical defined proteins, [2] specificities shared by nervous system-lymphoid cells, [3] oncoembryonic-oncofetal determinants, and [4] tumor-restricted antigens. Of greater significance is the heterogeneity of antigen expression among various individual glioma cells observed in frozen sections of tumor biopsies. Using a panel of MAbs, the phenotypic heterogeneity, i.e., the variation in antigen expression can be documented within and among malignant gliomas and cell lines derived from them. In spite of this the characteristic pattern of antibody binding to brain tumors makes MAbs the potentially best reagents for immuno-histochemical application in surgical neuropathology. Moreover, immuno-cytological screening of tumor cells in the cerebrospinal fluid has also proved to be valuable. The localization of radio-labelled MAbs in experimental and human gliomas growing subcutaneously and intracranially in athymic nude mice were explored by radioscintigraphy and autoradiography. Imaging experiments with 131I-labelled MAbs recognizing epitopes on the glioma cell surface showed high levels of specific activity in xenografts. Preliminary data indicate that administration of 131I-MAbs as well as drug conjugates (daunomycin-MAbs) causes a depression of glioma cell proliferation in vitro as well as delayed tumor growth and thus prolonged survival time of tumor-bearing mice. The mechanisms of antibody delivery and transport of "immunotoxins" from the vascular compartment to intracerebral tumor tissue are presently a subject of discussion. The complexity of this area necessitates comprehensive experimental work in order to define the factors involved in the delivery of MAbs to brain to tumor tissue and thus optimize the rate of blood-to-tumor transport. Current investigations have shown that it is possible to image malignant human gliomas using radio-labelled antibodies. The next step will be to attain target immunotherapy. The use of MAbs as carrier molecules for clinical applications might soon be possible.

Immunobiology of brain tumors

In summary, many actual interactions between tumors in the CNS and the immune system have been demonstrated. The normal brain does not possess a lymphatic system and is partially hidden from the systemic immune system by the BBB, furthermore brain cells do not express MHC antigens which are necessary for the initiation of an immune response. In pathological conditions however, immunocompetent cells may find their way through transformed endothelial cells. Microglia and astrocytes may function as antigen presenting cells. Glioma cells when stimulated by cytokines such as IFN gamma can be induced to express MHC class I and class II antigens, thus making them more susceptible to an immune attack. In addition glioma cells are capable of secreting several cytokines including IL 1, IL 3 and IL 6 also involved in the generation of an immune response. Indeed, a functional analysis of lymphocytes infiltrating gliomas has revealed the accumulation at the tumor site of cytotoxic T lymphocytes as well as NK cells. However host-immune responses against gliomas seem to be weak in comparison to other cancers. Glioma cells are known to secrete TGF beta 2 and PGE 2 which may in part be responsible for this lack of immune response, thus shielding themselves from immune attack. In order to be recognized by the immune system the tumor cells must express TAA in addition to MHC antigens, and such TAA have been identified by MAbs. These MAbs can be used for "targeted" therapy when coupled to toxic agents or radionuclides. Preclinical studies have shown that, after intravenous or intracarotid injection, there is specific accumulation of the MAb in the tumor but in insufficient amounts for therapeutic use. The relatively small amount of MAb binding to the tumor in vivo can be due to several factors: not all the cells in a single tumor express a given tumor-associated antigens, the MAb may have a low affinity for the antigen, the BBB may hinder the passage of the MAb. Attempts have been made to overcome these drawbacks by opening the BBB for example. In addition MAbs can readily be used for the treatment of carcinomatous meningitis. There has been little success in the development of immunotherapy with IFN beta 1 and even less with adoptive immunotherapy using LAK cells plus IL 2. TIL as well as LAK cells can be expanded in vitro with IL2 and it is feasible to reinject these cells into the tumor site.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunology of central nervous system tumors

With progress in cellular immunology and the development of hybridoma technology, the idea of manipulating host-tumor immune interactions to improve the prognosis of brain tumors has aroused renewed interest. Although no brain tumor-specific antigens have been found, and in spite of the wide antigenic heterogeneity of brain tumor cells, some monoclonal antibodies possessing restricted specificity have been isolated and their potential clinical applications investigated. One of the most pronounced changes in the cellular immune responses of brain tumor patients is a profound depression of the T4-helper lymphocytes. Clinical and laboratory trials are under way to assess the ability of lymphokines, such as gamma-interferon or interleukin-2, to restore immunologic competence in these patients and potentiate a specific anti-tumor immunologic response. Recent work suggests that the endothelium-astrocyte complex may have a pivotal role in assisting the escape of brain tumors from the host's immunologic responses, since it is responsible for the intracerebral sequestration of antigens and local anti-tumor responses. In this review, the data on the antigenic properties of central nervous system tumors and the host's humoral and cellular immune responses to them are analyzed and potential immunologic therapies are discussed.

Experimental transplantation gliomas in the adult cat brain. 1. Experimental model and neuropathology

Tumours were produced in the adult cat brain by injection of the rapidly growing anaplastic rat glioma clone F98 in order to study their neuropathology, pathophysiology, regional biochemistry and magnetic reasonance imaging. We report here the neuropathological behaviour of cell suspensions in the basal ganglia and the left cerebral hemisphere one, two, three, four and six weeks after stereotactic implantation with respect to tumour growth, immunological tumour regression and alterations of the blood-brain barrier with associated vasogenic brain oedema. Injected cell suspensions produce consistently growing tumours during the first, second and third weeks. Tumour sizes varied according to the survival time and were only slightly dependent on the inoculated cell number, i.e., 3 and 6 x 10(6) tumour cells, respectively. Immunohistochemistry with respect to proteins of the cytoskeleton and other cell markers showed positive tumour cell immunoreactions for vimentin and S 100, but not for GFAP, Leu-7, Leu-M1 and MBP. While leucocyte infiltration is apparent after only one week, major tumour regression phenomena develop after three weeks in conjunction with severe lymphocytic reactions of the host, resulting in complete tumour rejection with scar gliosis after four and six weeks, respectively. This transplantation glioma model is accompanied by vasogenic brain oedema both within the tumour area and in the homolateral hemisphere. Immunohistochemistry of serum proteins, i.e. total serum protein, albumin and IgG reveals impairment of the blood-brain barrier after one week, reaching its maximum after two and three weeks. The oedematous changes decrease dramatically after four and six weeks, when most of the serum proteins are reabsorbed by cellular activities in the tumour scar. The vasogenic brain oedema in this xenogeneic glioma transplantation model may be enhanced by the immunological reactions in the brain.

Immunological and biochemical strategies for the identification of brain tumor-associated antigens

Various strategies have been used to identify and characterize the antigens associated with human brain tumors. These approaches have included the raising of polyclonal and monoclonal antibodies against tumor antigens and, more recently, efforts toward the direct biochemical identification of such proteins. This review summarizes the progress made in this area, suggests reasons for the broad antigenic cross-reactivity and heterogeneity revealed by these studies, and proposes additional methods for deciphering the complex antigenic composition of human brain tumors.

The localisation of radiolabelled murine monoclonal antibody 81C6 and its Fab fragment in human glioma xenografts in athymic mice

The localisation of the radioiodinated Fab fragment of monoclonal antibody (Mab) 81C6, reactive with a glioma-associated extracellular matrix antigen, was studied in athymic mice bearing subcutaneous and intracranial xenografts of D-54 MG glioma cells. In vitro 81C6 Fab showed a marked loss of immunoreactivity and affinity for antigen compared to intact Mab 81C6. In vivo, the plasma half-life of 81C6 Fab was 7.0 hours compared to 2.1 days for 81C6. 81C6 Fab levels in tumours peaked at 2.6-3.8% injected dose/g in 2-6 h; Mab 81C6 reached 33.9% dose/g at 48 h. Localisation indices and tumour:tissue ratios were superior for Mab 81C6. Estimated radiation doses to tumour and normal tissues were lower for 131I-81C6 Fab than 131I-81C6. To realise the theoretical benefits of fragments as localising agents, Fab fragments of higher immunoreactivity and affinity, or bivalent F(ab')2 fragments are required.

Monoclonal antibodies against human astrocytomas and their reactivity pattern

The establishment of hybridomas after fusion of X63-Ag8.653 mouse myeloma cells and splenocytes from BALB/c mice hyperimmunized against human astrocytomas is presented. The animals were primed with 5 X 10(6) chemically modified uncultured or cultured glioma cells. Six weeks after the last immunization step an intrasplenal booster injection was administrated and 3 days later the spleen cells were prepared for fusion experiments. According to the specificity analysis of the generated antibodies 7 hybridoma products (MUC 7-22, MUC 8-22, MUC 10-22, MUC 11-22, MUC 14-22, MUC 15-22 and MUC 2-63) react with gliomas, neuroblastomas and melanomas as well as with embryonic and fetal cells but do not recognize non-neurogenic tumors. The selected monoclonal antibodies (McAbs) of IgG1 and IgG2a isotypes are not extensively characterized but these antibodies have been demonstrated to be reactive with a panel of glioma cell lines with varying patterns of antigen distribution. Using the McAbs described above and a series of cryosections of glioma biopsies and paraffin sections of the same material as well as glioma cultures established from these, variable antigenic profiles among glioma cell populations could be demonstrated. From these results it is evident that there is not only a distinct degree of antigenic heterogeneity among and within brain tumors, but also that the pattern of antigenic expression can change continuously. Some of the glioma associated antigens recognized by the selected antibodies persist after fixation with methanol/acetone and Karnovsky's fixative and probably are oncoembryonic/oncofetal antigen(s). The data suggest that the use of McAbs recognizing tumor associated oncofetal antigens in immunohistochemistry facilitates objective typing of intracranial malignancies and precise analysis of fine needle brain/tumor biopsies in a sensitive and reproducible manner.

Imaging of a glioma using peripheral benzodiazepine receptor ligands

Two types of benzodiazepine receptors have been demonstrated in mammalian tissues, one which is localized on neuronal elements in brain and the other, on glial cells and in peripheral tissues such as kidney. In vivo administration of 3H-labeled PK 11195 [1-(2-chlorophenyl-N-methyl-N-(1-methylpropyl)-3-isoquinoline carboxamide] or [3H]flunitrazepam with 5 mg of clonazepam per kg to rats with intracranial C6 gliomas resulted in high levels of tritiated-drug binding to the tumor as shown by quantitative autoradiography. Pharmacological studies indicated that the bound drugs labeled the peripheral benzodiazepine binding site. Binding to the peripheral benzodiazepine site was confirmed primarily to malignant cells with little binding to adjacent normal brain tissue or to necrotic tissue. Tumor cell binding was completely inhibited by preadministration of the peripheral benzodiazepine blocking agent PK 11195 at 5 mg/kg. The centrally selective benzodiazepine ligand clonazepam had no effect on PK 11195 binding to the tumor cells. When binding to other tumor cell lines grown in nude mice and nude athymic rats was evaluated, little or no peripheral benzodiazepine binding was detected on human pheochromocytoma (RN1) and neuroblastoma (SK-N-MC, SK-N-SH) tumor cells, respectively. However, high densities of peripheral benzodiazepine binding sites were observed on tumors derived from a human glioma cell line (ATCC HTB 14, U-87 MG). The presence of high concentrations of specific peripheral benzodiazepine receptors on glial tumors suggests that human primary central nervous system tumors could be imaged and diagnosed using peripheral benzodiazepine ligands labeled with positron- or gamma-emitting isotopes.