Synthesis, plasma clearance, and in vitro stability of protein containing a conjugated indium-111 chelate

Preparation and protein conjugation of a divinyl sulphone derivatized bifunctional chelating agent

A new bifunctional chelating agent with a novel linking arm, 2-[p-¿N-benzyl-N-(2-vinylsulfoethyl)¿- (aminobenzyl)¿-1,3-propane-diamine-N,N,N',N'-tetraacetic acid (VS-PDTA) was synthesized and was conjugated to protein for the purpose of attaching radiometals to monoclonal antibodies (MAbs). The effect of various parameters such as ligand concentration, protein concentration, pH, temperature and reaction period on the conjugation have been examined using chromatographic (SE and TLC) analysis after labeling with 111In. The parameters and chemical variables studied have significant effects on the efficiency and rate of protein conjugation.

Synthesis of a monoclonal antibody-indium-111-porphyrin conjugate

Antibodies were labelled with indium-111 with a view to their use in the radio-immunodetection of cancers. The covalent coupling between indium-111 porphyrin and monoclonal antibodies (IgG and F(ab')2 fragment) was achieved using the ester activated method [N-hydroxy-succinimide/1-ethyl-3-(3-dimethylaminopropyl)carbodiimide]. After purification, this provided conjugated with specific activities of 6 muCi/micrograms Mab (9.3 molecules per Mab) or 1 muCi/microgram (F(ab')2 fragment (1.5 molecule per F(ab')2). ELISA procedures suggested the full retention of immunoreactivity by the radiolabelled antibodies.

Use of paramagnetic chelated metal derivatives of polysaccharides and spin-labeled polysaccharides as contrast agents in magnetic resonance imaging

Soluble and insoluble polysaccharides were derivatized with diethylenetriaminepentaacetic acid (DTPA) and/or spin-labeled with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). Polysaccharides derivatized with DTPA were prepared via cyanogen bromide activation, coupling to a diamine linker, and to DTPA anhydride. Spin-labeled polysaccharides were also prepared via cyanogen bromide activation. The extent of derivatization for dextran (18 kDa) was about 120 glucose units per DTPA, and for cellulose and starch about 15-30 units per DTPA. For spin-labeled polysaccharides, the average loading ranged from 1 nitroxide per 16 glucose units for starch to 181 for dextran (82 kDa). These derivatized paramagnetic polysaccharides were shown to be more effective relaxants than the small paramagnetic molecules alone. Both soluble and insoluble polysaccharide-linker-DTPA-Gd(III) complexes were effectively cleared from the body (rats) after oral administration. After intravenous administration, the biodistribution of dextran-linker-DTPA-Gd(III) complexes differed significantly from that of GdDTPA. Reduction of the nitroxide by ascorbic acid was retarded in the polysaccharide derivatives, particularly in starch derivatized with both nitroxide and linker-DTPA-Cu(II). These agents showed contrast enhancement in the gastrointestinal tract of rabbits.

Differential cellular catabolism of 111In, 90Y and 125I radiolabeled T101 anti-CD5 monoclonal antibody

We evaluated the differences in catabolism of 125I, 111In and 90Y-T101 monoclonal antibody (anti-CD5) by peripheral blood mononuclear cells (PBMNC), HUT 102, CCRF-CEM and MOLT-4 cells. All cells showed higher retention of 111In than of 125I. PBMNC showed similar retention of 90Y and 111In. The release of 125I was reduced by using metabolic inhibitors, F(ab')2 of T101, pure lymphocytes or incubating at 4 degrees C. Our findings suggest differences in the intracellular catabolism of the radionuclides.

Interposition of different chemical linkages between antibody and 111In-DTPA to accelerate clearance from non-target organs and blood

Two chemically labile linkages, disulfide and diester, and two stable linkages, thioether and hydrocarbon, were introduced between antibody and 111In-DTPA in order to modify their biodistributions. The biodistributions of the new linkages were evaluated in rats with target antigens localized in lungs. For a comparison purpose, the antibody-DTPA conjugate with a peptide linkage was used as a control conjugate. The antibody conjugates with the stable linkages produced the biodistributions similar to that of the peptide linked conjugate during a 48 h period. The disulfide and diester conjugates, however, cleared from blood much faster and are retained in normal organs much lower than the peptide conjugate. The disulfide and the diester conjugate amplified the lung (target) to blood ratio by 15 and 6 times, respectively at 48 h, as compared to the corresponding target to blood ratio of the control conjugate. Compared to the control conjugate, a 3 times higher target to liver ratio was also obtained by the disulfide conjugate and a 4 times higher target to kidney ratio was obtained by the diester conjugate at 48 h.

Rapid miniaturized chromatography for 111In labeled monoclonal antibodies: comparison to size exclusion high performance liquid chromatography

Our laboratory investigated the use of a rapid miniaturized chromatography system, ITLC-SG with 0.9% NaCl, to assess the radiochemical purity of 111In labeled monoclonal antibodies (MoAbs). Radiochemical analysis was performed on numerous 111In labeled antibody preparations with labeling efficiencies ranging from 40 to greater than 95% and the results compared to those obtained with size exclusion high performance liquid chromatography (HPLC). The chromatographic procedure involved challenging radiolabeled antibodies with 0.05 M DTPA to chelate unbound and/or non-specific bound 111In, spotting on miniaturized instant thin layer-silica gel chromatography strips, developing in 0.9% NaCl, and counting appropriate segments for radioactivity. Results of the study demonstrated that the miniaturized chromatography procedure was rapid, taking less than 4 min to complete, and accurate in assessing the amount of unbound or non-specific bound 111In in 111In labeled monoclonal antibodies, when compared to size exclusion HPLC.

DTPA-coupled proteins--procedures and precautions

While demonstrating that the cyclic anhydride of diethylenetriaminepentaacetic acid (DTPA) may be used to covalently attach this chelator to proteins, it was necessary to develop several new procedures and techniques. We were able to show that coupled proteins may be labeled with indium-111 (111In) simply by transcomplexation from a weaker complex such as the acetate to avoid subjecting the protein, even momentarily, to the acidity of the chloride (however, we have also observed that too high a concentration of acetate and other complexing agents such as citrate will interfere with protein labeling by competing with DTPA for the radioactivity). In agreement with other investigators using bifunctional chelate methodology we have observed that trace metals may interfere with the labeling. Accordingly, we developed a simple paper chromatographic assay which may be used to establish whether trace metals are present at interfering concentrations and to identify their sources. We have also employed a hydrolyzed control assay to determine whether and to what degree the radioactivity is bound to protein other than at the DTPA groups. We have developed a simple method of measuring the average number of DTPA groups per protein molecule which involves labeling the products of coupling with 111In. Procedures used and precautions observed in the radiolabeling of proteins by these methods are discussed.

Radioimmunodetection of cutaneous T-cell lymphoma with 111In-labeled T101 monoclonal antibody

T101 monoclonal antibody recognizes a pan-T-cell antigen present on normal T cells and also found in high concentrations in cutaneous T-cell lymphoma. We used this antibody, radiolabeled with 111In, in gamma-camera imaging to detect sites of metastatic cutaneous T-cell lymphoma in 11 patients with advanced disease. In all patients, [111In]T101 concentrated in pathologically or clinically detected nodes, including those in several previously unsuspected nodal regions. Concentrations (per gram of tissue) ranged from 0.01 to 0.03 percent of the injected dose and were consistently 10 to 100 times higher than previously reported on radioimmunodetection. Focal uptake was seen in skin tumors and heavily infiltrated erythroderma but not in skin plaques. The specificity of tumor targeting was documented by control studies with [111In]chloride or [111In]9.2.27 (anti-melanoma) monoclonal antibody. Increasing the T101 dose (1 to 50 mg) altered distribution in nontumor tissues. These studies suggest that imaging with [111In]T101 may be of value in identifying sites of cutaneous T-cell lymphoma. In contrast to the targeting of solid tumors, the mechanism of localization appears to be related to binding to T cells, which can then carry the radioactivity to involved sites.

Label stability in serum of four radionuclides on DTPA-coupled antibodies--an evaluation

Although DTPA forms strong chelates with many metals, the harsh chemical environment of the serum may nevertheless dissociate metallic radionuclides from DTPA attached to proteins. We have investigated the stability in 37 degrees C serum of 111In, 99mTc, 90Y and 153Gd chelated to DTPA-coupled antibodies. Stability was evaluated primarily by affinity chromatography and HPLC analysis of serum incubates; however, the stability of 111In was determined in vivo in patient studies. Analysis of patient urine showed no evidence for dissociation of DTPA from antibody whereas analysis of serum demonstrated exchange rates of 111In to transferrin of about 9%/day. When labeled by the described method, 99mTc on DTPA coupled antibodies shows instability in serum but superior stability than 99mTc on antibodies without the attached DTPA. Both 90Y and 153Gd dissociate in serum at rates which are comparable to that of 111In.

Synthesis and evaluation of a new bifunctional chelating agent for 99mTc labeling proteins: p-carboxyethylphenylglyoxal-di(N-methylthiosemicarbazone)

A new bifunctional chelating agent, p-carboxyethylphenylglyoxal-di(N-methylthiosemicarbazone) (CE-DTS), containing a di(N-methylthiosemicarbazone) as the technetium coordinating site and an aralkyl carboxylate site for the protein conjugation was synthesized. Coupling to human serum albumin (HSA), selected as a model protein was carried out by the phosphorylazide method using diphenylphosphoryl azide (DPPA). The conjugation level of CE-DTS to HSA played a critical role in its biological evaluation. A 99mTc-CE-DTS-HSA with high in vivo stability was obtained when CE-DTS was coupled to HSA at 1:1 molar ratio. This compound showed similar in vivo stability to 131I labeled HSA in mice and rabbits.

An efficient method for labelling antibodies with 111In

Using a new method, rabbit IgG and a monoclonal antibody have been conjugated with the chelating agent DTPA. This was accomplished with reaction conditions that should entail lower antibody damage than existing methods. Gel filtration of the 111In-labelled antibody conjugate indicated minimal damage to the antibody and radioimmunoassay showed no significant change in its immunological activity.

The preparation of DTPA-coupled antibodies radiolabeled with metallic radionuclides: an improved method

Isotopes of iodine are often employed as radiolabels for antibodies used in radioimmunodetection studies in which tumor localization is determined by external imaging. Because of drawbacks associated with the use of these isotopes, alternative labeling methods have been considered; such as covalently attaching strong chelators so that the coupled protein may be radiolabeled with metallic radionuclides by chelation. We have developed a method of coupling the strong chelator diethylenetriaminepentaacetic acid (DTPA) which is simple, efficient, and superior to reported methods. Using the cyclic anhydride, coupling to IgG antibody is about 75% efficient and is completed in less than 1 min at neutral pH. Because the concentration of hydrolytic products is small, the coupled protein is rapidly purified for use or storage. Labeling of the protein is also accomplished rapidly and the labeled product has been shown to be stable both in vitro and in vivo.

Tumor imaging with radioactive metal chelates conjugated to monoclonal antibodies

High-resolution gamma camera images of mouse erythroid tumors were obtained by use of leukemia cell-specific monoclonal antibodies labeled with bifunctional radioactive metal chelates. Small tumors (200 to 300 milligrams) were visible without subtraction or enhancement 1 to 5 hours after injection of antibody. Chelate-derivitized monoclonal antibodies permit targeting of a broad spectrum of radioisotopes, including those that are optimum for agamma for gamma camera imaging or positron tomography, as well as those that are tumoricidal.