BACKGROUND: The reliable method for determination of
identity and radiochemical purity (RCP) is of great importance
in radiopharmaceutical development. This is especially relevant
when more than one form of radiometal/ligand complex can be
formed during radiolabelling, such as complexes of 99mTc or 188Re
with meso-2,3-dimercaptosuccinic acid (DMSA), where depending
on the pH, metal can occur either at +3 or +5 oxidation state.
The aim of our study was to evaluate possibilities for optimization
of chromatographic systems leading to specific and reliable
analytical method for determination of the identity and RCP of
DMSA complexes with 99mTc or 188Re.

MATERIAL AND METHODS: The commercial DMSA kits
(POLATOM) were used for preparation of technetium-99m (III) and (V) complexes with DMSA. 99mTc(V)-DMSA complexes
were prepared by addition of NaHCO3 to the kit vial prior to
99mTc-eluate to obtain pH ~8. 188Re(V)-DMSA was prepared either
directly or using intermediate 188Re(III)-EDTA complex added
to DMSA. RCP was evaluated by TLC using: ITLC-SG developed
in methylethylketon, SG60 coated plates developed in:
n-BuOH/H2O/CH3COOH and n-PrOH/H2O/CH3COOH systems,
and in H2O. Comparative biodistribution studies were performed
in normal Wistar rats.

RESULTS: Using silica gel plates and n-PrOH, H2O and acetic
acid in the developing solution, we observed that 99mTc/188Re(III)-
DMSA and 99mTc/188Re(V)-DMSA complexes could be well
separated from each other and from the impurities in the form
of free pertechnetate/perrhenate. In vivo studies showed quite
different biodistribution of 99mTc(III)- and 99mTc(V)-DMSA. The
trivalent complex accumulated mainly in kidneys (>40%ID),
while 99mTc(V)-DMSA revealed high excretion with urine and
relatively high concentration in osseous tissue (ca. 2 %ID/g).
Accumulation of this complex in kidneys was very low (ca.
2.5 %ID). Biodistribution pattern of 188Re(V)-DMSA prepared
directly was almost identical to that of 99mTc(V)-DMSA. Biodistribution
results of the 188Re preparation obtained using 188Re(III)-
EDTA intermediate indicated that the preparation contained the
mixture of penta- and trivalent 188Re complexes. The quite high
accumulation of radioactivity in kidneys (23 %ID) gave evidence
of the presence of 188Re(III)-DMSA in this preparation, what was
also confirmed by the results of TLC analysis performed using
silica gel plate and n-propanol/water/acetic acid as developing
system.

CONCLUSIONS: Based on our study, we have made recommendation
on the suitable methods for investigations of RCP of
DMSA complexes, i.e.: SG60 plates developed in the mixture
of n-propanol/water/acetic acid, which enable determination of the tri- and pentavalent DMSA complexes, as well as, the
pertechnetate/perrhenate impurity, and developed in water for
determination of the colloidal residue.