Standardization of CDI-mediated DTPA-coupling to IgG and IgG2a antibodies for 113mIn labeling.

A systematic analysis of various factors involved in the CDI-mediated coupling of DTPA to IgG have been carried out, in order to optimize the labeling yield of a monoclonal antibody labeled to high specific activity. Various CDI-to-DTPA ratios were tested, followed by a range of DTPA (activated) to IgG ratios. Specific activities as high as 600 Ci/mmol could be obtained following labeling with 113mIn, when an IgG : DTPA : CDI ratio of 0.01 : 1:20 was used. Other aspects, such as the volume of each of the reactants, the pH and the reaction times, were also standardized to yield a labeled IgG2a (KS1/4) that could be consistently tested for biodistribution and tumor-binding.

[67Ga]desferrioxamine--HSA: synthesis of chelon protein conjugates using carbodiimide as a coupling agent.

Desferrioxamine-human serum albumin conjugate was prepared at pH 4.7 using a water-soluble carbodiimide and at pH 7.8 using glutaraldehyde. The crude conjugate was purified by dialysis and gel filtration. The efficiency of 67Ga labeling of the purified conjugate was greater than 95%, with good in vitro stability. The specific activity ranged from 5 to 50 microCi/mg. Treatment of the conjugate with urea resulted in a 20--30-fold increase in specific activity, suggesting that a number of chelon groups are unavailable for Ga binding due to non-covalent intramolecular cross-linking.

Effect of pH on tumor cell uptake of radiogallium in vitro and in vivo.

When injected at tracer levels into the blood, radiogallium as 67Ga-citrate binds to, and it transported to the site of the tumor by, transferrin. The process by which transferrin-bound Ga is converted to tumor-bound Ga is not fully understood, but may involve the differential physiology of neoplasms compared with normal tissues. Based on the slight acidity known to be exhibited by the extracellular fluid of many animal and human tumors, we have studied the effect of pH on stability and dissociation of the Ga-transferrin complex and on the uptake of Ga by tumor cells in vitro and animal tumors in vivo. When plasma from rabbits injection 67Ga-citrate was dialyzed at pH 6.5-7.5, dissociation of Ga from transferrin showed an inverse pH-dependence. A similar inverse dependence on pH was observed for the uptake of Ga by L1210 leukemia cells and Ehrlich ascites cells incubated with Ga-transferrin complex. Tumor uptake of Ga in rats bearing Walker-256 carcinosarcoma or Murphystum lymphosarcoma whose tumor pH had been further lowered by administration of glucose showed a statistically significant increase over control rats receiving no glucose. These results demonstrate that the stability of the Ga-transferrin complex is pH-dependent and suggest that dissociation of this complex due to decreased pH at the tumor site may be one factor involved in tumor localization and binding of Ga.

Detection of bacterial endocarditis with technetium-99m-labeled antistaphylococcal antibody.

The reliable diagnosis of bacterial endocarditis is an important but difficult clinical problem. The potential ability of technetium-99m-labeled antistaphylococcal antibody to detect infective endocarditis was investigated in a rabbit model. Radiolabeling of the purified antibody was effected by a mild electrolytic procedure, with full retention of immunologic activity. Infective endocarditis was induced in rabbits by placing a catheter through the carotid artery into the left ventricle, followed by i.v. injection of Staphylococcus aureus. The labeled antistaphylococcal antibody was subsequently injected, and its clearance and distribution were studied in the infected rabbits and in normal controls. The ratio of radioactivity on the aortic valve to that in the surrounding heart tissue or blood pool was significantly higher for the infected animals (> 10:1) than for the normals, and should permit visualization of the infection site. This radiolabeled antibody technique may provide a feasible approach to detection of infective endocardial lesions.

Differences in the sites of iodination of proteins following four methods of radioiodination.

The rate of deiodination of radioiodinated proteins varies with the method of iodination. To elucidate differences in the iodinated protein labeled by various methods, we have hydrolyzed fibrinogen and several small peptides iodinated by the iodine monochloride, chloramine-T, electrolytic and enzymatic methods. Under conditions of either acidic or basic proteolysis, extensive deiodination occurred and the major product was I-. When a protease of Streptomyces griseus was used, radio-iodinated fibrinogen and other polypeptides were degraded to single iodinated amino acid residues and only a small yield of I-. The iodinated amino acids resulting from proteolysis were separated by ion-exchange chromatography. The iodine monochloride and enzymatic methods yielded largely iodotyrosine with small amounts of other iodinated amino acids. The chloramine-T product spectrum varied with the chloramine-T:protein ratio, whereas the electrolytic method yield was a complex function of the reaction conditions. The different methods of iodination lead to some differences in the site of iodination which correlate with stability of the protein-iodine bond.

Radioiodinated plasminogen: an imaging agent for pre-existing thrombi.

We have reinvestigated radioiodinated plasminogen as an agent for localizing preformed thrombi. Canine plasminogen was isolated from fresh plasma by the affinity chromatography technique on a lysine-sepharose 4B column and tagged with I-123 or I-131, at less than one iodine atom per molecule of enzyme, by the conventional ICI method. When injected into dogs more than 2 days after thrombus induction, radioiodinated plasminogen produced thrombus-to-blood activity ratios of 7.8 +/- 2.4. Thrombi as old as 6 days can be visualized in 80% of the cases. Both the weight of the thrombus and the thrombus-to-blood ratio are more variable for 1-day-old thrombi; this may be associated with plasminogen release accompanying thrombus retraction. The results suggest that radioiodinated plasminogen has potential as an imaging agent for pre-existing thrombi.

Splenic imaging with 99mTc-labeled erythrocytes: a comparative study of cell-damaging methods.

Several methods of damaging red blood cells (RBCs) for splenic imaging were compared to determine the optimum approach. The RBCs from donor animals were labeled with 99mTcO4- and damaged by heat, excess acid citrate dextrose (ACD), excess Sn(II) ion, or the sulfhydryl inhibitors N-ethylmaleimide (NEM) or p-hydroxymercuribenzoate (PMB). The organ distributions of undamaged and damaged RBCs were determined in rats, and splenic imaging studies were performed in rabbits. Splenic deposition and spleen-to-liver ratios with heat- or sulfhydryl-damaged 99mTc-RBCs were significantly greater (p less than 0.001) than the values obtained using ACD or Sn(II) ion. Heat-damaging produces good splenic localization of 99mTc-RBCs but requires rigidly controlled incubation conditions. NEM-damaging provides an excellent and predictable alternative approach.

Serial radionuclide determinations of the electron fraction with 99mTc-labeled red blood cells.

The reproducibility of area/length ejection fraction determinations was evaluated in 21 subjects using 99mTc-labeled red blood cells (99mTc-RBCs). The ejection fraction was determined within 30 minutes and 3-5 hours after a single injection of 99mTc-RBCs. The mean absolute difference between the initial and subsequent ejection fractions was 7% (maximum, 15%) (r = 0.90). 99mTc-RBCs offer a reproducible method of determining the ejection fraction which is suitable for serial noninvasive evaluations of left ventricular function.

Solid phase bovine thrombin. Preparation and properties.

A new solid-phase thrombin (EC 3.4.21.5) was prepared through conjugation of the enzyme under mild conditions to a glass support bearing an active ester of N-hydroxysuccinimide. The immobilized enzyme retained 50 +/- 10% of the specific esterase activity of the parent soluble enzyme. The Km (apparent) for the esterase activity of the immobilized enzyme has a value of 5 mM, identical of the Km value of the parent-soluble enzyme. Only 6 +/- 1% of the specific proteolytic activity was retained and a higher Km (apparent) value of 67 muM was obtained for the insoluble enzyme compared to Km value of 12.5 muM for the parent soluble thrombin. Solid-phase thrombin prepared by the diazocoupling technique was previously reported to retain only 3% of the specific proteolytic activity. The observed loss of specific proteolytic activity can be attributed to steric interference, a change in charge characteristics, or both. Nevertheless, the present method of preparation has the advantages of rapidity and simplicity. It can readily be adapted to use for studying the fate of various complexes of fibrinogen, fibrin and their degradation products. It should also be useful for preparing radiolabeled autologous soluble fibrin for thrombus detection in patients undergoing active thrombosis.

Preparation and use of 123I-labeled highly iodinated fibrinogen for imaging deep-vein thrombi.

A method for producing protein-iodination-grade 123I suitable for use with a compact bio-medical cyclotron is reported. The preparation of highly iodinated fibrinogen (25 123I atoms per molecule) is described, and its successful use as a thrombus-imaging agent in experimental animals is reported. This new agent clears from the blood faster than conventional radioiodinated fibrinogen and gives higher thrombus-to-blood activity ratios. Thus, the detection of deep-vein thrombi in areas of large blood pool is enhanced, and images can be obtained sooner after administration of the radiopharmaceutical. Induced 4--8-hr-old femoral-vien thrombi in dogs can be well visualized with a scintillation camera as early as 4 hr and as late as 15 hr after administration of 1 mCi of 123I-labeled highly iodinated fibrinogen.

In vivo behavior of 99mTc-fibrinogen and its potential as a thrombus-imaging agent.

We have investigated the in vivo behavior of 99mTc-fibrinogen, prepared by a mild and efficient electrolytic method employing tin electrodes. The clearance mechanisms of this agent were studied, and its efficacy for imaging deep-vein thrombi in dogs with an Anger camera was determined. The 99mTc-fibrinogen preparations, which are stable in vitro, undergo partial rapid exchange of the technetium with other plasma proteins and with anions of the blood buffer system in vivo, resulting in an early drop in the percent of radioactivity associated with clottable protein. However, very little or no oxidation to pertechnetate occurs. The nonclottable material is much more rapidly cleared from the blood than the remaining 99mTc-fibrinogen, and the proportion of clottable protein activity increases with time. The fraction of 99mTc-fibrinogen that remains intact in vivo is biologically active and will incorporate into thrombi. Higher thrombus-to-blood activity ratios are obtained with 99mTc-fibrinogen than with radioidinated fibrinogen when both agents are injected into dogs 4 hr after induction of femoral vein thrombosis. Clearly delineated images of the thrombi are obtained, beginning about 2.5 hr after injection. Thus, 99mTc-fibrinogen may be of clinical use as a thrombus-imaging agent in patients under-going active thrombosis, especially in regions of high blood pool.

Highly iodinated fibrinogen: a new thrombus-localizing agent.

We have examined radioiodinated fibrinogen prepared at high levels of iodination as an agent for improved in vivo thrombus detection. Fibrinogen containing 25, 50, and 100 atoms of iodine per molecule is prepared by an electrolytic procedure and is compared with conventional radiolabeled fibrinogen (less than 0.5 iodine atom per molecule) prepared by the iodine monochloride method. The level of iodination has little effect on the isotopic clottability of the product, but its degree of aggregation and its rate of blood clearance in experimental animals is strongly dependent on iodination level. Isotopic thrombus: blood ratios obtained in recently induced thrombi with the 25 atom per molecule preparation average about 50:1, twice as high as the ratios obtained with conventionally labeled fibrinogen.

Radioiodinated soluble canine fibrin. Preparation and evaluation as a thrombus localizing agent in the dog.

To develop a thrombus localizing tracer which has characteristics superior to labeled fibrinogen for external detection, we evaluated radioiodinated soluble fibrin. Labeled soluble fibrin was prepared by clotting and dissolving radioiodinated (131I) canine fibrinogen under specified conditions. Biological clearance studies revealed rapid clearance of the labeled soluble fibrin from the blood with a half-life of 5 hours. The accumulation of labeled soluble fibrin and fibrinogen in induced venous thrombi, coronary artery thrombi, and the myocardium was compared in dogs. In venous thrombi, soluble fibrin and fibrinogen exhibited maximum thrombus-blood ratios when they were injected 4 hours after thrombus induction; the thrombus-blood ratio was greater for soluble fibrin than it was for fibrinogen when these agents were injected 4, 8, or 24 hours after thrombosis induction. In induced coronary artery thrombi, soluble fibrin and fibrinogen accumulated to the same extent. Since the blood clearance of soluble fibrin is faster than that of fibrinogen, a higher thrombus-blood ratio was obtained with soluble fibrin in coronary artery thrombi. The thrombus-infarcted myocardium, thrombus-normal myocardium, and infarcted myocardium-normal myocardium ratios obtained with soluble fibrin were slightly higher than those obtained with fibrinogen. Thus, soluble fibrin offers some advantages when it is compared with fibrinogen as a thrombus detecting agent.