Technetium-99m amino acid chelates: correlation of their physico-chemical and physiological parameters. Part I

Technetium-99m radiolabeled ouabagenin-cysteine conjugate: biological evaluation in animal models

Two ouabagenin-cysteine conjugates were synthesized by condensing 3-beta monochloroacetyl and 3-beta, 11-alpha dichloroacetyl ouabagenin with cysteine. The resulting ligands were radiolabeled with technetium-99m (99mTc) to furnish a single homogenous 99mTc chelate in each case with good stability. The animal experiments with these 99mTc-labeled conjugates established the superiority of guinea pig over rat and rabbit as an animal model, as previously observed for other tritiated or radioidinated cardiac glycosides or aglycones. In biodistribution experiments in guinea pig, these 99mTc chelates showed a favorable heart to liver (and other nontarget organ) uptake ratio, comparable to that of recently reported 125I-digoxigenin iodohistamine-3-oxime. The low heart to blood ratio in animal experiments with ouabagenin derivatives could be attributed to the absence of 3-beta sugar residues in these molecules, which is in agreement with the previous observation reported in connection with radioiodinated digoxin and digoxigenin derivatives.

Evaluation of the biodistribution and in vivo biochemistry of 99mTc-cysteine and 99/99mTc-cysteine complexes--a potential renal imaging agent

Cysteine was chelated with 99mTc and/or 99Tc in a freeze-dried kit containing Tin(II) ions, and the yellow 99m/99Tc-cysteine complex (complex I) was separated to study the biodistribution. Comparison of the distribution of 99m/99Tc-cysteine and 99mTc-cysteine complexes was made in rats and mice. The renal excretion patterns were studied in rats in the presence and absence of the renal tubular transport inhibitor 2,4-dinitrophenol. The carrier of Tc-cysteine complex in the blood and also the radioactive compounds in the urine were studied by HPGFC and SDS-electrophoresis. The kidney was confirmed as the target organ; serum albumin serves as a carrier for transport of Tc-cysteine complex to the kidney. The Tc-cysteine complex was the primary form in excreta, and glomerular filtration was the dominant excretory pathway.

Cysteine, a chelating moiety for synthesis of technetium-99m radiopharmaceuticals--Part IV. Benzyl cysteine and derivatives

To explore the possibility of utilizing cysteine derivatives for technetium-99m radiopharmaceutical preparation with clinical potential, we synthesized two benzyl substituted cysteine compounds, namely, S-benzyl cysteine 1 and cysteine benzyl ester 3. It was expected, from our previous studies on benzoyl cysteines, that the above two ligands after chelation with 99mTc would be excreted by the hepatobiliary pathway. Although for 99mTc-3 the above expectation was realized, 99mTc-1 behaved in a most unexpected way by affixing itself with kidney and selecting the renal tubular secretory pathway for its excretion. It is anticipated that the affinity of 99mTc-1 for kidney is due to its interaction with the kidney sulphhydryl group and it also formed an adduct with other sulphydryl containing compounds like thiophenol. In terms of the kidney-to-background ratio, 99mTc-1 showed some superiority over other kidney structure agents, like 99mTc-dimercaptosuccinic acid and 99mTc-glucoheptanoic acid and, therefore, the chelate (99mTc-1) may have the potential to replace the above two radiopharmaceuticals in clinical use.