Chemical Principles of Radiolabelling
The basic chemical principle of 99mTc-labelling which occurs during the incubation of the labelling kit solution involves reduction of 99mTc (available in the generator eluate in the chemical form of sodium pertechnetate: Na99mTcO4), to a lower oxidation state that will bind to a chelating molecule of interest (the diphosphonate in MDP kit):
- in the pertechnetate ion, 99mTcO4-, -Tc has the oxidation state 7
- the stannous ions (Sn2+ ) (in solution in the kit once the eluate has been added) will reduce technetium from oxidation state 7 to a lower oxidation state, presumably 4
2 Tc7 3 Sn2 → 2 Tc4 3 Sn4
It is only in this lower oxidation state that 99mTc can form a complex with the diphosphonate molecules.
Complex formation proceeds through the formation of bonds between Tc and hydroxyl groups of medronate (the bond of the oxygen atom with hydrogen is replaced by a bond of the oxygen atom with technetium).
99mTc4 MDP → 99mTc-MDP
The exact nature of this specific radiopharmaceutical 99mTc-MDP is not known, thus, it still is somewhat speculative. It has been shown that the preparation after the complex formation contains several radiolabelled species, and it is assumed that these are monomers and oligomers of 99mTc-MDP complexes in which one or more Tc atoms are bound by the oxygen atoms of 2 or more diphosphonate molecules. In an oligomer, one Tc atom is bound to two medronate molecules and two hydroxyl groups. In a dimer, two Tc-atoms (each also bound to two hydroxyl groups) to form a chain together with three medronate molecules. This chain can also be longer (e.g. trimer, hexamer).
The figure below shows the assumed (3-dimensional) structure of one of these compounds in which 1 Tc-atom is bound by two MDP molecules: