Tumour imaging using an improved method of DTPA-coupled monoclonal antibodies radiolabelled with metallic radionuclides

Dendrimers as Innovative Radiopharmaceuticals in Cancer Radionanotherapy

Radiotherapy is one of the most commonly used cancer treatments, with an estimate of 40% success that could be improved further if more efficient targeting and retention of radiation at the tumor site were achieved. This review focuses on the use of dendrimers in radionanotherapy, an emerging technology aimed to improve the efficiency of radiotherapy by implementing nanovectorization, an already established praxis in drug delivery and diagnosis. The labeling of dendrimers with radionuclides also aims to reduce the dose of radiolabeled materials and, hence, their toxicity and tumor resistance. Examples of radiolabeled dendrimers with alpha, beta, and Auger electron emitters are commented, along with the use of dendrimers in boron neutron capture therapy (BNCT). The conjugation of radiolabeled dendrimers to monoclonal antibodies for a more efficient targeting and the application of dendrimers in gene delivery radiotherapy are also covered.

Peptides and proteins as drugs

Relatively small modifications of clinically useful endogenous compounds have been shown to have therapeutically beneficial effects on their pharmacodynamic and pharmacokinetic properties. These effects include increased potency and effect selectivity, and prolonged duration of action. In addition, these modifications have resulted in compounds that can be administered orally where only parenteral administration was previously possible. One type of modification resulting in distinctive properties is exemplified by the hybrid interferons produced by the recombination of segments of genes coding for different molecular species. Chemical modification has also resulted in many examples of analogs of natural peptides that are more potent, more selective and more stable than the endogenous compounds. Conjugation to peptide or protein carriers is a third method used to selectively modify the properties of an endogenous compound. The carriers that have been used include synthetic polypeptides, endogenous proteins, toxins and monoclonal antibodies. The effect that covalent attachment to a carrier has on the properties of a ligand is highly dependent upon the carrier, the ligand and the linkage between them.

Recombinant versus natural human 111In-beta2-microglobulin for scintigraphic detection of Abeta2m amyloid in dialysis patients

BACKGROUND

We previously introduced scintigraphy with 131I-labeled beta2-microglobulin (beta2m), purified from uremic hemofiltrate, that is, "natural" beta2m, to specifically detect beta2m-associated amyloidosis (Abeta2m) in hemodialysis (HD) patients.

METHODS

To improve the safety and resolution of the scan, we covalently bound the chelator diethylenetriaminepentaacetic acid to natural beta2m to allow radiolabeling with 111In. In a second step, we generated and evaluated the usage of recombinant human beta2m (rhbeta2m) for scintigraphy.

RESULTS

Using natural 111In-labeled beta2m, eight patients on HD for 0 to 17 years, without evidence of Abeta2m, were scanned. Whole-body scintigraphy at 48 to 72 hours postinjection revealed no significant tracer accumulation over joint regions. In contrast, nine patients on HD for 10 to 21 years with clinical, radiological, or histologic (N = 4) evidence of Abeta2m showed selective tracer uptake over various joint regions. Tracer accumulation in visceral organs, which could not be related to tracer elimination or metabolism, was not detected. Compared with the previous 131I beta2m scan, scintigraphy with 111In-labeled beta2m offered highly improved image contrast, increased sensitivity, and a 50 to 70% reduction of the radiation exposure. Scanning with 111In-labeled recombinant human beta2m was performed in six patients: No significant tracer accumulation was observed over joint regions in two patients on short-term HD without evidence of Abeta2m; in contrast, local tracer accumulations similar to those observed with natural beta2m could be demonstrated in four long-term (10 to 27 years) HD patients with clinical, radiological, and histologic (N = 1) evidence of Abeta2m.

CONCLUSION

Scintigraphy for Abeta2m with 111In-labeled rhbeta2m provides a homogenous and safe recombinant protein source and leads to enhanced sensitivity and lower radiation exposure.

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.

Labelling of monoclonal antibodies 10B, 8C, and M2A with indium-111

Monoclonal antibodies 10B, 8C and M2A were conjugated with cyclic DTPA anhydride to a substitution level of 1-2 mol DTPA/mol antibody. The DTPA-conjugated antibodies were purified by size-exclusion mini-column chromatography and labelled with [111In]In acetate to a specific activity of 37-74 kBq/micrograms (1-2 microCi/micrograms). The radiochemical purity of the antibody preparation was 76-81%. The immunoreactive fraction of the radiolabelled antibodies ranged from 0.17 to 0.31, significantly lower than that of their radioiodinated analogs.

Specific localization and imaging of amyloid deposits in vivo using 123I-labeled serum amyloid P component

Highly specific, high-resolution scintigraphic images of amyloid-laden organs in mice with experimentally induced amyloid A protein (AA) amyloidosis were obtained after intravenous injection of 123I-labeled serum amyloid P component (SAP). Interestingly, a much higher proportion (up to 40%) of the injected dose of heterologous human SAP localized to amyloid and was retained there than was the case with isologous mouse SAP, indicating that human SAP binds more avidly to mouse AA fibrils than does mouse SAP. Specificity of SAP localization was established by the failure of the related proteins, human C-reactive protein and Limulus C-reactive protein, to deposit significantly in amyloid and by the absence of human SAP deposition in nonamyloidotic organs. However, only partial correlations were observed between the quantity of SAP localized and two independent estimates, histology and RIA for AA of the amount of amyloid in particular organs. It is not clear which of the three methods used reflects better the extent or clinical significance of the amyloid deposits but in vivo localization of radiolabeled SAP, detectable and quantifiable by gamma camera imaging, is apparently extremely sensitive. These findings establish the use of labeled SAP as a noninvasive in vivo diagnostic probe in experimental amyloidosis, potentially capable of revealing the natural history of the condition, and suggest that it may also be applicable generally as a specific targeting agent for diagnostic and even therapeutic purposes in clinical amyloidosis.

Radioimmunodetection of cancer with monoclonal antibodies: current status, problems, and future directions

Early studies of immunoscintography with affinity-purified 131I-labeled polyclonal antibodies reactive against oncofetal antigens such as carcinoembryonic antigen (CEA) were moderately successful in detecting metastatic colorectal carcinoma. However, because of low tumor to background ratios of isotope, background subtraction techniques using 99Tc-labeled albumin were required to visualize small lesions. Antisera were often of low titer and lacked specificity. These problems could be overcome for the most part following the development of highly specific monoclonal antibodies (MoAb) against a variety of tumor-associated antigens. A number of clinical trials using 131I- or 111In-labeled MoAb to image tumors have demonstrated successful localization without the use of subtraction techniques. Variables limiting the usefulness of murine MoAb for diagnosis have included increased localization in liver and spleen, tumor vascularity and heterogeneity of antigen expression, and development of human antimurine globulins. Methods to overcome some of these problems are discussed. Radiolabeled MoAb appear useful as an adjunct to conventional diagnostic techniques both as a means to predict which antibodies might be useful for treatment and, in select patients, as a basis for treatment decisions.

Monoclonal antibody modification with chelate-linked high-molecular-weight polymers: major increases in polyvalent cation binding without loss of antigen binding

Polyethyleneimine or polylysines of differing molecular sizes were substituted with either EDTA or DTPA and then with succinic acid groups. These polymers were then reacted with the amino groups on myosin-specific monoclonal antibody or its Fab using a water soluble carbodiimide. The polymer-antibody complexes were capable of binding up to 150 di- or trivalent ions per mole (Mn++, Gd , or 111In ) without attendant loss of antigen binding. The polylysine derivatives of the intact antibody were rapidly cleared and sequestered in the liver, whereas the polylysine 14-kilodalton (kd) derivative of Fab was cleared from the circulation with minimal hepatic and kidney sequestration. This differed from the biodistribution of intact antimyosin or its Fab labeled with 111In via direct attachment of DTPA to the epsilon amino group of the lysyl residues. Applications in magnetic resonance and nuclear imaging are envisioned.

Indium-111 labelled monoclonal antibody to placental alkaline phosphatase in in the detection of neoplasms of testis, ovary, and cervix

A monoclonal antibody (H17E2) against placental alkaline phosphatase (PLAP) and testicular placental-like alkaline phosphatase was labelled with indium-111 and used in radioimmunoscintigraphy of 15 patients known or suspected to have germ-cell carcinoma of the testis or carcinoma of the ovary or cervix. Good images of neoplastic lesions were obtained in most patients with active disease. In 1 patient with testicular teratoma and elevated human chorionic gonadotropin who had a normal computed tomography scan, the labelled antibody located microscopic disease in a lymph node, which was then removed. No false positive localisation was seen in patients with PLAP-negative tumours or sites of inflammation. This method may be helpful in the diagnosis, staging, and monitoring of PLAP-positive neoplasms of testicular, ovarian, and cervical origin.