Chelating agents for uranium(VI): 2. Efficacy and toxicity of tetradentate catecholate and hydroxypyridinonate ligands in mice.

Uranium(VI) (UO2(2+), uranyl) is nephrotoxic. Depending on isotopic composition and dosage, U(VI) is also chemically toxic and carcinogenic in bone. Several ligands containing two, three, or four bidentate catecholate or hydroxypyridinonate metal binding groups, developed for in vivo chelation of other actinides, were found, on evaluation in mice, to be effective for in vivo chelation of U(VI). The most promising ligands contained two bidentate groups per chelator molecule (tetradentate) attached to linear 4- or 5-carbon backbones (4-LI, butylene; 5-LI, pentylene; 5-LIO, diethyl ether). New ligands were then prepared to optimize ligand affinity for U(VI) in vivo and low acute toxicity. Five bidentate binding groups--sulfocatechol [CAM(S)], carboxycatechol [CAM(C)], methylterephthalamide (MeTAM), 1,2-hydroxypyridinone (1,2-HOPO), or 3,2-hydroxypyridinone (Me-3,2-HOPO)--were each attached to two linear backbones (4-LI and 5-LI or 5-LIO). Those ten tetradentate ligands and octadentate 3,4,3-LI(1,2-HOPO), an effective actinide chelator, were evaluated in mice for in vivo chelation of 233U(VI) (injection at 3 min, 1 h, or 24 h or oral administration at 3 min after intravenous injection of 233UO2Cl2) and for acute toxicity (100 micromol kg(-1) injected daily for 10 d). The combined efficacy and toxicity screening identified 5-LIO(Me-3,2-HOPO) and 5-LICAM(S) as the most effective low-toxicity agents. They chelate circulating U(VI) efficiently at ligand:uranium molar ratios > or = 20, remove useful amounts of newly deposited U(VI) from kidney and bone at molar ratios > or = 100, and reduce kidney U(VI) levels significantly when given orally at molar ratios > or = 100. 5-LIO(Me-3,2-HOPO) has greater affinity for kidney U(VI) while 5-LICAM(S) has greater affinity for bone U(VI), and a 1:1 mixture (total molar ratio = 91) reduced kidney and bone U(VI) to 15 and 58% of control, respectively--more than an equimolar amount of either ligand alone.

Residualizing and non-residualizing analogues of low-density lipoprotein as iodine-123 radiopharmaceuticals for imaging LDL catabolism.

Low-density lipoprotein (LDL) labeled via direct iodination or via the radioiodinated residualizing moiety tyramine-cellobiose (TC) were compared in rabbits as potential 123I radiopharmaceuticals for imaging sites of LDL catabolism. The tissue deposition of 131I-TC-LDL after 24 h as determined by dissection was in the major catabolic organs (liver, adrenals, spleen), and its plasma clearance was slower in rabbits with dietary hypercholesterolemia than in normals. 131I-LDL was unsuitable as a metabolic tracer due to redistribution of catabolites and/or loss of the label before protein degradation, which resulted in little accumulation of radioactivity in catabolic organs and high thyroid uptake. The plasma clearance half-time was similar (ca 22 h) for the two compounds in normal rabbits, but was increased to about 36 h for 131I-TC-LDL and decreased to approximately 9 h for 131I-LDL in hypercholesterolemic animals. The were similar with dynamic imaging of control and hypercholesterolemic rabbits using 123I-labeled analogues. 123I-TC-LDL rapidly localized in the liver, with low thyroid accumulation of radioactivity. The hepatic uptake of 123I-LDL was about half that of 123I-TC-LDL, and thyroid sequestration of radioactivity was significant for 123I-LDL but not 123I-TC-LDL. These data suggest that whereas the residualizing 123I-TC-LDL has a pharmacokinetic profile representative of lipoprotein metabolism, the biodistribution of the activity from injected 123I-LDL is complicated by processes other than protein degradation. The results are discussed with regard to nuclear medicine applications in evaluating lipoprotein catabolism in man.

Gallium-68 lipophilic complexes for labeling platelets.

Generator produced 68Ga-labeled platelets could be useful for positron emission tomographic (PET) studies of thrombosis or atherosclerosis. To label platelets with 68Ga, we have studied the effects of trace metals in elutions of 68Ga from 68Ge. Studies were conducted on the formation of lipophilic 68Ga complexes 8-hydroxyquinoline, tropolone, and mercaptopyridine-N-oxide (MPO). Parameters such as pH, buffers, concentration of ligand, and stability with time were investigated. High performance liquid chromatography and instant thin layer chromatography were used to quantitate formation of the 68Ga complex. Platelets from human, dog, and rabbit plasma were incubated with the 68Ga complexes and the percent labeling determined. Accumulation of platelets in the catheter scraped aorta of the rabbit was determined by PET imaging, tissue counting, and autoradiography. Gallium-68 MPO gave 40-60% labeling of rabbit platelets with higher accumulation in the scraped aorta compared to the normal.

Radioimmunoassay of erythropoietin: circulating levels in normal and polycythemic human beings.

Techniques are described in detail for the RIA of human Ep in unextracted plasma or serum. With 100 microliters of sample, the essay is sensitive at an Ep concentration of approximately 4 mU/ml, and when required, the sensitivity can be increased to 0.4 mU/ml, a range considerably less than the concentration observed in normal human beings. This is approximately 100 times more sensitive than existing in vivo bioassays for this hormone. Studies concerned with the validation of the Ep RIA show a high degree of correlation with the polycythemic mouse bioassay. Dilutions of a variety of human serum samples show a parallel relationship with the standard reference preparation for Ep. Validation of the RIA is further confirmed by observations of appropriate increases or decreases in circulating Ep levels in physiological and clinical conditions known to be associated with stimulation of suppression of Ep secretion. Significantly different mean serum concentrations of 17.2 mU/ml for normal male subjects and 18.8 mU/ml for normal female subjects were observed. Mean plasma Ep concentrations in patients with polycythemia vera are significantly decreased, and those of patients with secondary polycythemia are significantly increased as compared to plasma levels in normal subjects. These results demonstrate an initial practical value of the Ep RIA inthe hematology clinic, which will most certainly be expanded with its more extensive use.