In vitro and in vivo studies on the development of the alpha-emitting radionuclide bismuth 212 for intraperitoneal use against microscopic ovarian carcinoma.

OBJECTIVE: Our objective was to develop the alpha-emitting radionuclide bismuth 212 for possible intraperitoneal use against microscopic ovarian cancer. STUDY DESIGN: The radiobiologic effectiveness of bismuth 212 was compared in vitro to x rays and chromic phosphate phosphorus 32). The distribution, toxicity, and maximum tolerated dose of bismuth 212 were determined after intraperitoneal administration in animal models. Dose estimates in animals and humans were made. RESULTS: In in vitro studies bismuth 212 was three times more effective in eradicating tumor cells grown in monolayers and in 800 microm spheroids. In in vivo studies bismuth 212 was distributed uniformly after intraperitoneal administration. The maximum tolerated dose in rabbits was 60 mCi. There was reversible hematologic toxicity with minimal organ damage. Bismuth 212 prolonged survival and cured up to 40% of animals inoculated with Ehrlich carcinoma cells. Dose estimates made from these studies indicated that dosages administered were effective in eradicating tumor cells and were within the radiotolerance of normal human tissue. CONCLUSION: Bismuth 212 appears to be a suitable candidate for intraperitoneal use against microscopic ovarian cancer.

Morphological, biochemical, and molecular changes in endothelial cells after alpha-particle irradiation.

The response of cultured bovine aortic endothelial (BAE) cells after exposure to alpha-particle radiation from chelated 212Bi has been evaluated. The results suggest that even relatively high doses of alpha-particle radiation from 212Bi (20-72 Gy) cause only minor acute changes in the morphology of BAE cells (light and electron microscopy) under conditions of confluent monolayer growth. Significant morphological changes can be detected in cells that detach from the monolayer, though it is unclear whether these changes represent a genuine response to irradiation or reflect the causes or effects of monolayer detachment with the consequent loss of intercellular biochemical communication. After alpha-particle irradiation (20-40 Gy) angiotensin-converting-enzyme activity was not detectable in the monolayer culture medium but was significantly decreased within the cell monolayer. Neutral-elution-assay data demonstrated that DNA double-strand-break (DSB) damage occurred in these cells and that about 35% of the DSBs were repairable.

G2M arrest and apoptosis in murine T lymphoma cells following exposure to 212Bi alpha particle irradiation.

Asynchronous exponentially growing EL4 murine T lymphoma cells were exposed either to high LET alpha-radiation from 212Bi-DTPA or to gamma-radiation from a 137Cs source. Radiation-induced cell cycle perturbation was studied by flow cytometry. Alpha irradiation, like gamma, transiently arrested cells in the G2M phase in a dose-dependent manner. The maximum percentages of cells accumulated in G2M 18 h after alpha- and gamma-irradiation were comparable, though the dose-response relationships differed. The "RBE" value for G2M block for alpha- versus gamma-radiation was approx. 4. Electron microscopic studies of the cell samples where a large proportion of cells were arrested in G2M showed subcellular changes in nuclear membrane and the presence of morphologically apoptotic cells. Biochemical analysis of DNA from irradiated cells by agarose gel electrophoresis revealed more extensive DNA fragmentation for alpha- vs gamma-irradiation, even at relatively low total doses. We conclude that the high LET radiation is more efficient in inducing G2M block and apoptosis in EL4 lymphoma cells. The overall radiosensitivity of some high and low grade malignant lymphoma cells to radiation may correlate with these processes. The clinical implications of 212Bi-induced G2M delay may be particularly important for biologically targeted high LET radiopharmaceutical therapy.

MRL/lpr-->severe combined immunodeficiency mouse allografts produce autoantibodies, acute graft-versus-host disease or a wasting syndrome depending on the source of cells.

MRL/lpr (lpr) mice spontaneously develop a lupus-like illness as well as massive lymphadenopathy. Attempts to transfer autoimmunity by adoptive transfer or radiation bone marrow chimeras have been unsuccessful. Since severe combined immunodeficiency (SCID) mice have been engrafted with human and rat xenografts without apparent graft-versus-host disease (GVHD), we subjected SCID mice to low-dose irradiation and reconstituted the mice with spleen cells from young or old lpr mice or with lpr bone marrow. Fourteen out of twenty (70%) of SCID mice engrafted with spleen cells from old lpr mice produced autoantibodies (anti-DNA and anti-Sm) without evidence of the severe lymphoid atrophy previously described for lpr spleen-->+/+ chimeras. SCID mice engrafted with spleen cells from young lpr mice developed acute GVHD and 5/6 (83%) died within 4 weeks post-transfer. Although 8/11 (73%) of lpr-->SCID bone marrow allografts survived for at least 4 months, these mice developed a wasting disease characterized by lymphoid atrophy and fibrosis without the production of autoantibodies. None of the lpr-->SCID grafts resulted in the transfer of double negative T cells or the lymphoproliferative syndrome characteristic of MRL/lpr mice. These findings indicate that SCID mice can be engrafted with splenocytes from old MRL/lpr mice and that B cells continue to secrete autoantibodies for several months in the SCID recipients. This study also demonstrates that, unlike i.p. transplant of xenogeneic cells, acute GVHD is a consistent feature of i.p. transplants of normal allogeneic mononuclear cells into SCID mice.

Cellular kinetics, dosimetry, and radiobiology of alpha-particle radioimmunotherapy: induction of apoptosis.

Though clinical results for radioimmunoconjugate therapy of most common epithelial tumors have been disappointing, dramatic responses have been observed repeatedly in the treatment of high- and low-grade malignant lymphomas. This high clinical responsiveness after radioimmunoconjugate therapy sometimes appears to be out of proportion to the calculated radiation dose absorbed by the lymphoma tissue. Here we describe some key aspects of the kinetics, dosimetry, and cellular radiobiology of murine lymphoma cells exposed to 212Bi-radiolabeled alpha-particle-emitting immunoconjugates specific for the differentiation antigen Thy 1.2. Approximately 25 cell-bound alpha-particle-emitting immunoconjugates per target cell were required to reduce clonogenic survival by 90% (the radiobiological D10). Serial kinetic analyses of the antibody and radioisotope components of the immunoconjugates revealed significant levels of dechelation and up to 7.5% cellular internalization of the isotope. Cellular radiation dosimetry performed by Monte Carlo computer simulation of alpha-particle energy deposition patterns based on the observed radiopharmacokinetics showed that the D10 resulted from approximately four alpha-particle traversals through the nucleus, corresponding to an absorbed radiation dose of approximately 0.95 Gy to the cell nucleus. Electron micrographs and DNA gel studies of murine lymphoma cells undergoing radioimmunoconjugate therapy in vivo and in vitro demonstrated bizarre blebbing patterns, condensation of chromosomal material, and internucleosomal DNA fragmentation patterns characteristic of programmed cell death (apoptosis). We conjecture that the efficacy of radioimmunoconjugates against responsive cell types may be the result of passive DNA damage by ionizing radiation and the initiation of apoptosis in response to radioimmunotherapy.

Long-term biodistribution in tumored mice of murine and chimeric B72.3-IgG antibody radiolabeled with 114mIn via both DTPA and a macrocyclic chelator.

To assess the possibilities of using 114mIn as a therapeutic agent, the long-term biodistribution of 114mIn was studied, in tumor-bearing nude mice, after injection of labeled monoclonal antibody (MoAb) B72.3 IgG, either DTPA-coupled murine, DTPA-coupled chimeric, or macrocycle-coupled chimeric antibody. Although the biodistributions in all cases were similar, there were important differences. The use of DTPA-coupled chimeric antibody led to higher concentrations of radioactivity in tumor, and to lower concentrations in liver and bone, as compared to DTPA-coupled murine antibody. The use of macrocycle-coupled chimeric antibody led to higher concentrations of radioactivity in the liver and in bone as compared to the DTPA-coupled chimeric antibody. However, in this case there were no significant differences in tumor uptake or clearance. Radiation doses were calculated based on the organ retention and by neglecting source-to-target contributions. Radiation dose distribution was marginally favorable for therapy in the group injected with DTPA-coupled chimeric antibody.

Anti-P autoantibody production requires P1/P2 as immunogens but is not driven by exogenous self-antigen in MRL mice.

Considerable evidence supports the idea that autoantibody production in human and murine SLE is Ag driven. To determine whether Ag (the ribosomal P proteins) could initiate autoantibody production in lupus mice, 34 MRL/lpr mice were immunized with mouse riboosomal proteins in Freund's adjuvant. Neither intact ribosomes, denatured total mouse ribosomal proteins, nor the purified mouse ribosomal proteins, P1 and P2, induced the production of anti-P autoantibodies in the MRL/lpr mice. In contrast to these negative findings, MRL/lpr mice immunized with Artemia salina ribosomes produced anti-P antibodies as well as anti-P autoantibodies. Although the induced anti-P autoantibodies bound exclusively to the carboxyl terminus, these anti-P antibodies differed from spontaneously occurring anti-P autoantibodies in their predominant binding to mouse P0 on immunoblots and their preferential reactivity against A. salina synthetic peptides by ELISA. Induction of anti-P antibodies required the presence of P1 and P2 on the ribosome because ribosomal cores devoid of P1 and P2 dimers did not induce anti-P. Despite the presence of approximately 80 ribosomal proteins, autoantibodies to other mouse ribosomal proteins were rarely observed. Immunization of MRL/+ mice and a normal H-2-matched strain of mice, C3H, also resulted in anti-P antibodies reactive with the A. salina P proteins and mouse P0. Whereas anti-P levels gradually declined in C3H mice, anti-P levels either remained elevated (MRL/lpr) or showed a secondary rise (MRL/+) at the onset of autoimmunity. These observations indicate that: i) high levels of autologous Ag are not sufficient to drive antiribosomal autoantibody production in MRL mice, ii) multivalency of the P proteins may explain their potent immunogenicity and ability to break tolerance, and iii) immunized MRL mice show an abnormal persistence of high level anti-P production presumably reflecting T cell activation of presensitized B cells.

Detection and quantification of human anti-Sm antibodies using synthetic peptide and recombinant SmB antigens.

Anti-Sm-positive sera from patients with systemic lupus erythematosus (SLE) recognize a major epitope located within the carboxyl-terminal 27 amino acids of a recombinant SmB fusion protein. To determine whether a synthetic peptide corresponding to this region could be used as an antigen to detect anti-Sm antibodies, sera were typed as anti-Sm positive or anti-Sm negative by counterimmunoelectrophoresis (CIE). Twenty-three SLE sera that were anti-Sm positive by CIE, 22 that were anti-Sm negative by CIE, and 42 sera from patients with other autoimmune diseases were tested for anti-Sm antibodies by enzyme-linked immunosorbent assay (ELISA), using either the synthetic peptide (C27) or a recombinant SmB (rSmB) fusion protein as the antigen. More than 90% of the sera that were anti-Sm positive by CIE were also positive by both the C27 and rSmB ELISAs, and an additional 2 SLE sera originally typed as anti-Sm negative were found to be positive (1 by the C27 ELISA, 1 by the rSmB ELISA), due to the greater sensitivity of the ELISAs. In the rSmB ELISA, anti-Sm antibodies were not detected in any of the sera from patients with other autoimmune diseases, whereas 3 patients with anti-U1 RNP antibodies (1 each with polymyositis, scleroderma, and mixed connective tissue disease) had a positive result in the C27 ELISA. These results indicate that both the C27 synthetic peptide and rSmB are excellent antigens for use in ELISAs to quantify anti-Sm antibodies.

Iodine-125-NRLU-10 kinetic studies and bismuth-212-NRLU-10 toxicity in LS174T multicell spheroids.

Alpha emitter-labeled antibodies (Abs) are of considerable interest in cancer therapy. Alpha particles are densely ionizing and therefore have a high radiobiologic effectiveness, and the cell killing produced is influenced very little by dose rate or hypoxic conditions. LS174T human colon adenocarcinoma spheroids were used in this study to evaluate the efficacy of alpha emitter-labeled Abs in a three-dimensional model. NRLU-10, an IgG2b Ab to a pancarcinoma antigen, and its Fab fragment were used. Initial kinetic studies using 125I-NRLU-10 revealed that a large number of binding sites/cell and high Ab affinity led to slow Ab penetration. This effect could be overcome by increasing the Ab concentration ten-fold for Fab but not for intact Ab. Bismuth-212-NRLU-10 therapy was very effective in killing single cells (over 3 log reduction in surviving fraction) but was ineffective in spheroids (less than 1 log reduction). This was likely due to inadequate penetration into the spheroids before the 212Bi decayed. The use of higher Ab concentrations, tumors with fewer antigenic sites/cell for the Ab being used, lower affinity Abs, alpha emitters with longer half-lives, and pretargeting with bifunctional Ab are all potential ways of increasing the efficacy of alpha emitter-labeled Abs for cancer therapy.

Epitope mapping of recombinant HeLa SmB and B' peptides obtained by the polymerase chain reaction.

SmB and SmB' are the major antigenic proteins contained within small nuclear RNP particles that are recognized by both human SLE and MRL mouse anti-Sm sera. We amplified cDNA obtained from HeLa cells by using the polymerase chain reaction and identified two clones, U2 and L13, that encode SmB and SmB', respectively. The nucleotide sequences of these two clones were identical except for the insertion of a 145-bp sequence in U2 that contained an early in frame termination codon and a potential 3' consensus splice site. The predicted amino acid sequences of HeLa SmB and B' proteins were therefore identical except for the COOH terminal 2 (U2) and 11 (L13) amino acids. U2, L13, and four subclones of U2 (F-B, B-R, F-X, and X-B) were ligated to pATH vectors and expressed as trpE fusion proteins. Epitope mapping with 12 human SLE and 12 MRL/lpr mouse anti-SmB/B' sera revealed that antibodies directed against the X-B peptide accounted for most (65.5 +/- 15.4 and 63.2 +/- 25.3%), B-R intermediate levels (51.5 +/- 30.8 and 18 +/- 19.6%), and F-X none of the anti-SmB activity in human and mouse sera, respectively. Ten human and two mouse sera contained antibodies that cross-reacted with epitopes located within the proline-rich, COOH-terminal, 27-residue peptide encoded by B-R and the NH2-proximal F-B peptide. These observations suggest that a) the polymerase chain reaction is a powerful ancillary method to synthesize autoantigens, b) SmB and B' in HeLa cells are derived from alternative splicing of a common RNA transcript, and c) both SLE and MRL anti-SmB/B' sera recognize multiple epitopes (some shared and some unique) on these proteins.

Alpha particle radio-immunotherapy: animal models and clinical prospects.

Short-lived isotopes that emit alpha particles have a number of physical characteristics which make them attractive candidates for radioimmunotherapy. Among these characteristics are high linear energy transfer and correspondingly high cytotoxicity; particle range limited to several cell diameters from the parent atom; low potential for repair of alpha-induced DNA damage; and low dependence on dose rate and oxygen enhancement effects. This report reviews the synthesis, testing and use in animal models of an alpha particle emitting radioimmunoconjugate constructed via the noncovalent chelation of Bismuth-212 to a monoclonal IgM antibody specific for the murine T cells/neuroectodermal surface antigen, Thy 1.2. These 212Bi-anti-Thy 1.2 immunoconjugates are capable of extraordinary cytotoxicity in vitro, requiring approximately three 212Bi-labeled conjugates per target cell to suppress 3H-thymidine incorporation to background levels. The antigen specificity afforded by the monoclonal antibody contributes a factor of approximately 40 to the radiotoxicity of the immunoconjugate. Animals inoculated with a Thy 1.2+ malignant ascites were cured of their tumor in an antigen-specific fashion by intraperitoneal doses of approximately 200 microCi per mouse. Alpha particle emitting radioimmunoconjugates show great potential for regional and intracavitary molecular radiotherapy.

212Bismuth linked to an antipancreatic carcinoma antibody: model for alpha-particle-emitter radioimmunotherapy.

For comparison of cytotoxicity from alpha-particle irradiation with that from conventional x-irradiation, 212Bi, an alpha-emitting radionuclide, was attached to a monoclonal antibody that recognizes a cell surface antigen on human pancreatic carcinoma cells. For a given level of survival, the 212Bi-antibody complex was found to be approximately 20 times more efficient in cell killing than x-irradiation and 5 times more cytotoxic when compared with the cytotoxicity of an antigen-negative cell line or an isotype-matched control antibody. High linear energy transfer radioimmunotherapy using alpha emitters linked to monoclonal antibodies may be useful in vivo and in vitro for selectively killing target cell populations, especially those resistant to other forms of treatment.

Radioimmunotherapy with alpha-particle-emitting immunoconjugates.

Alpha particles are energetic short-range ions whose higher linear energy transfer produces extreme cytotoxicity. An alpha-particle-emitting radioimmunoconjugate consisting of a bismuth-212-labeled monoclonal immunoglobulin M specific for the murine T cell/neuroectodermal surface antigen Thy 1.2 was prepared. Analysis in vitro showed that the radioimmunoconjugate was selectively cytotoxic to a Thy 1.2+ EL-4 murine tumor cell line. Approximately three bismuth-212-labeled immunoconjugates per target cell reduced the uptake of [3H]thymidine by the EL-4 target cells to background levels. Mice inoculated intraperitoneally with EL-4 cells were cured of their ascites after intraperitoneal injection of 150 microcuries of the antigen-specific radioimmunoconjugate, suggesting a possible role for such conjugates in intracavitary cancer therapy.

An improved generator for the production of 212Pb and 212Bi from 224Ra.

We have developed an improved generator for the production of the alpha emitting radionuclide 212Bi and its parent, 212Pb. These radionuclides are well suited to use as radiotherapeutic agents due to their relatively short half lives and appropriate particle particle emissions. The parent, 224Ra, is available from a long-lived parent and can be isolated on a generator which produces the daughters in good yield and low breakthrough. The 212Bi can be eluted by itself or in equilibrium with its parent.

Bismuth-212-labeled anti-Tac monoclonal antibody: alpha-particle-emitting radionuclides as modalities for radioimmunotherapy.

Anti-Tac, a monoclonal antibody directed to the human interleukin 2 (IL-2) receptor, has been successfully conjugated to the alpha-particle-emitting radionuclide bismuth-212 by use of a bifunctional ligand, the isobutylcarboxycarbonic anhydride of diethylenetriaminepentaacetic acid. The physical properties of 212Bi are appropriate for radioimmunotherapy in that it has a short half-life, deposits its high energy over a short distance, and can be obtained in large quantities from a radium generator. Antibody specific activities of 1-40 microCi/microgram (1 Ci = 37 GBq) were achieved. Specificity of the 212Bi-labeled anti-Tac was demonstrated for the IL-2 receptor-positive adult T-cell leukemia line HUT-102B2 by protein synthesis inhibition and clonogenic assays. Activity levels of 0.5 microCi or the equivalent of 12 rad/ml of alpha radiation targeted by anti-Tac eliminated greater than 98% the proliferative capabilities of HUT-102B2 cells with more modest effects on IL-2 receptor-negative cell lines. Specific cytotoxicity was blocked by excess unlabeled anti-Tac but not by human IgG. In addition, an irrelevant control monoclonal antibody of the same isotype labeled with 212Bi was unable to target alpha radiation to cell lines. Therefore, 212Bi-labeled anti-Tac is a potentially effective and specific immunocytotoxic reagent for the elimination of IL-2 receptor-positive cells. These experiments thus provide the scientific basis for use of alpha-particle-emitting radionuclides in immunotherapy.

Disruption of the purine nucleotide cycle by inhibition of adenylosuccinate lyase produces skeletal muscle dysfunction.

Controversy exists as to whether the purine nucleotide cycle is important in normal skeletal muscle function. Patients with disruption of the cycle from a deficiency of AMP deaminase exhibit variable degrees of muscle dysfunction. An animal model was used to examine the effect of inhibition of the purine nucleotide cycle on muscle function. When the compound 5-amino-4-imidazolecarboxamide riboside (AICAriboside) is phosphorylated to the riboside monophosphate in the myocyte it is an inhibitor of adenylosuccinate lyase, one of the enzymes of the purine nucleotide cycle. AICAriboside was infused in 28 mice, and 22 mice received saline. Gastrocnemius muscle function was assessed in situ by recording isometric tension developed during stimulation. The purine nucleotide content of the muscle was measured before and after stimulation. Disruption of the purine nucleotide cycle during muscle stimulation was evidenced by a greater accumulation of adenylosuccinate, the substrate for adenylosuccinate lyase, in the animals receiving AICAriboside (0.60 +/- 0.10 vs. 0.05 +/- 0.01 nmol/mumol total creatine, P less than 0.0001). There was also a larger accumulation of inosine monophosphate in the AICAriboside vs. saline-treated animals at end stimulation (73 +/- 6 vs. 56 +/- 5 nmol/mumol total creatine, P less than 0.03). Inhibition of flux through the cycle was accompanied by muscle dysfunction during stimulation. Total developed tension in the AICAriboside group was 40% less than in the saline group (3,023 +/- 1,170 vs. 5,090 +/- 450 g . s, P less than 0.002). An index of energy production can be obtained by comparing the change in total phosphagen content per unit of developed tension in the two groups. This index indicates that less high energy phosphate compounds were generated in the AICAriboside group, suggesting that interruption of the purine nucleotide cycle interfered with energy production in the muscle. We conclude from these studies that defective energy generation is one mechanism whereby disruption of the purine nucleotide cycle produces muscle dysfunction.

Repetitive episodes of brief ischaemia (12 min) do not produce a cumulative depletion of high energy phosphate compounds.

During myocardial ischaemia the purine (ATP, GTP) and pyrimidine (CTP, UTP) nucleotide content of the myocyte falls. When the ischaemic episode resolves, many hours or even days are required for restoration of nucleotide pools. These observations suggest that repetitive episodes of ischaemia might produce progressive depletion of nucleotide pools. In order to determine the effect of repetitive episodes of brief ischaemia on nucleotide pools, open-chest dogs underwent three 12 min periods of occlusion of the left anterior descending coronary artery, with each occlusion followed by 10 min of reperfusion. During the first occlusion nucleotide pools decreased by 30% (ATP); 36% (GTP), 52% (CTP), and 48% (UTP). The subsequent two occlusions produced no further decrease in nucleotide pools. The myocardial content of adenine nucleotide catabolites (adenosine + inosine + hypoxanthine) tended to be greater during the first occlusion than during the subsequent occlusions, and substrate delivery (ie regional myocardial blood flow) was similar during each of the periods of ischaemia. These results indicate that a decrease in the rate of nucleotide degradation, rather than an increase in nucleotide synthesis, accounts for the maintenance of nucleotide content during subsequent ischaemic episodes after the initial ischaemic period. Thus repetitive episodes of regional ischaemia do not produce a cumulative decrease in the high energy phosphate content of the myocardium.

A comparison of methods for quantitation of metabolites in skeletal muscle.

Quantitation of gastrocnemius muscle metabolites in mice was evaluated using three commonly employed descriptors: wet weight, total protein, and total creatine. A fourth previously unreported descriptor, NAD+, was also evaluated. In resting muscle the coefficients of variation were similar when the metabolite data were normalized by any of the four descriptors. However, in tetanically stimulated muscle, normalization by wet weight yielded metabolite levels which were 23-25% lower than the results obtained when the data were normalized by total protein, total creatine, or NAD+.

Accelerated repletion of ATP and GTP pools in postischemic canine myocardium using a precursor of purine de novo synthesis.

During ischemia, the myocardial content of the purine nucleotides ATP and GTP falls and remains depressed for hours to days. Prolonged depletion of ATP in the postischemic state is accompanied by functional and ultrastructural abnormalities. This report describes the successful use of the purine precursor 5-aminoimidazole-4-carboxamide riboside to selectively enhance the rate of repletion of the ATP and GTP pools in postischemic myocardium.