Technetium biodistribution

A greater tendency of rhenium complexes (compared to technetium analogues) to expand their coordination numbers has been invoked to rationalize the stronger interaction of the perrhenate ion with carboxylate ligands. This association has been suggested as a possible cause of the different quantitative biodistribution and excretion characteristics of pertechnetate and perrhenate perrhenate is accumulated in thyroid to a lesser extent and renally excreted more rapidly than pertechnetate [6]. 

Radiopharmaceutical formulations based on "mTc may be divided into two major classes.635 The Class I formulations include what may be described as technetium tagged materials. These may be particles such as cells or colloidal substances, or they may be molecular species such as proteins or other large biological molecules. The essential feature of these agents is that their biodistribution behaviour is determined by the nature of the substrate material. Thus the attachment of a "mTc label should have little or no effect on their biodistribution. However, it cannot necessarily be assumed that this will be the case, especially when relatively low molecular weight substrates are involved. 

The Class II formulations are the so called technetium essential radiopharmaceuticals whose biodistribution depends not only on the nature of the substrate to be labelled, but also on the presence of "mTc itself. Some examples of the major types of "mTc radiopharmaceutical agents in use, or under investigation, are summarized in Table 22. The nature of the "mTc binding site... 

Those in which the labeled compound retains essentially the same biodistribution as the untagged molecule. Most agents in this category involve cells, colloids or macromolecules so that the presence of the technetium represents only a small perturbation. 

Agents in which the biodistribution depends on the properties of the complex itself and on the nature of the particular technetium ion present. This class encompasses nearly all of the smaller technetium coordination complexes used for diagnostic purposes. 

Mitra A, Nan A, Ghandehari H, et al. Technetium-99m-labeled N-(2-hydroxypropyl) methacrylamide copolymers synthesis, characterization, and in vivo biodistribution. Pharm Res 2004 21 1153-1159. 

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