A comparison of the technetium-labeled myocardial agents DiArs and DMPE to 201Tl in experimental animals

Preparation of 99mTc-labelled conjugates of ouabagenin and their biological evaluation in animal models

BACKGROUND

Ouabagenin and its 1,19-acetonide were conjugated with nitrilotriacetic acid (NTA) and diethylenetriaminepentaacetic acid (DTPA) through their respective anhydrides.

METHODS

The reaction mixtures were exhaustively purified by silica gel column chromatography and preparative high-performance liquid chromatography to furnish the ligands in good purity and moderate yield. These ligands were labelled with 99mTc to produce four chelates in 90-95% yield. Of these chelates the 99mTc-oubagenin-NTA conjugate and the corresponding acetonide exhibited appreciable myocardial uptake with respect to that of other vicinal organs in a guinea-pig model. However, all these 99mTc chelates exhibited poor heart-to-blood ratios, which could be attributed to the absence of a 3beta sugar residue in this molecule.

CONCLUSION

The result is in agreement with that previously reported in connection with radioiodinated digoxin and digoxigenin derivatives.

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.

Biodistribution of two 99mTc-cardiac glycosides with end glucose unit: effect of lipophilicity on their relative myocardial accumulation

In biodistribution experiments with tritium labelled cardiac glycoside it was observed that compounds of low lipophilicity showed a considerably higher affinity towards myocardium with respect to other tissues and organs. A similar trend was also observed with 99mTc-cardiac glycosides except for one compound with glucose residue, which in spite of its lower lipophilicity exhibited an unexpectedly low heart to non-target concentration ratio, thereby indicating a possible influence of carbohydrate residue on biodistribution. To confirm this, in this article we radiolabelled two glucose containing cardiac glycosides (K-strophanthin-beta and K-strophanthoside) with 99mTc and, in biodistribution experiments, less lipophilic 99mTc-K-strophanthoside showed a much better heart to non-target ratio over 99mTc-K-strophanthin-beta. It is thus concluded that, in addition to lipophilicity, the affinity of the carbohydrate residue for non-target organs is also an important consideration in determining the structure-distribution relationship of 99mTc-cardiac glycosides.

Preparation and evaluation of technetium-99m labeled cardiac glycoside derivatives as potential myocardial imaging agent

Three cardiac glycosides, two natural, cymarin and convallotoxin and one synthetic, strophanthidin-beta-D-glucoside were converted to their thiosemicarbazone and subsequently radiolabeled with 99mTc by chelation. The resulting radioactive chelate complexes were evaluated in animals to determine the suitability of this class of compounds for myocardial imaging. It was observed from the animal biodistribution data of the three radioactive compounds, there was a considerable variation in the heart to non-target organ uptake ratio. A possible explanation of this variation was offered in the light of their lipophilic character, protein binding ability and affinity towards non-target receptors. It is anticipated that this study may help to develop a 99mTc-cardiac glycoside complex with better distribution characteristics, and such a compound may offer a suitable alternative to 201Tl, which is at present used for myocardial imaging.

Synthesis, characterization and evaluation of 99Tc/99mTc DIARS and DMPE complexes containing pentadecanoic acid

The synthesis and purification of two new ligands, 15-[bis(3,4-dimethyl arsino)phenyl] pentadecanoic acid (FA-DIARS) and [bis(16,17-dimethyl phosphino)] heptadecanoic acid (FA-DMPE) and the preparation of their corresponding 99Tc/99mTc complexes of the general formula [Tc(III)L2Cl2]+, wherein L is FA-DIARS or FA-DMPE are reported. After careful purification by preparative HPLC, the 99Tc complexes were characterized by spectroscopy, while the 99mTc complexes were used to determine the biodistribution in mice. The results of the organ distribution studies indicated a relatively high uptake in heart (5-7% dose/g). The complexes were, however, found to be very slowly cleared from the blood, probably due to strong binding to serum proteins. Further, the heart to liver and heart to lung ratios were unfavourable. Attempts to improve the efficacy by synthesizing complexes containing one fatty acid per molecule are also described.

Search for 99mTc labeled DTS bifunctional radiopharmaceutical: role of functional groups in myocardial accumulation

Development of 99mTc-bifunctional radiopharmaceutical (BR) is attracting the interest of various research groups. In the present paper, various molecules containing a neutral 99mTc-dithiosemicarbazone (DTS) structure as the technetium chelating site, along with various functional groups (amino, carboxyl or isobutyl group with diverse charge) are tested for their chemical or biological functions. The study on the effect of those functional groups is carried out in vitro and in vivo. The validity of introducing an amino group along with the technetium chelating site DTS for myocardial accumulation is discussed.