Some observations on the instability of 99mTc-complexes of propylene amine oxime (PnAO)

When eluates from expired 99mMo/99mTc generators were used to form the neutral lipophilic complexes of PnAO and its derivatives, degradation of these chelates occurred on standing to form secondary 99mTc-species. It was determined that the conversion of the primary complexes to secondary complexes was due to the impurities present in the 99mTcO4- eluate. The stability was found to be improved by extraction of the complexes into CHCl3 and reconstituting in buffer after evaporation of CHCl3, indicating that the impurities are non extractable into CHCl3. Purification of the 99mTcO4- eluate from the generator by MEK extraction and its use for the preparation of the PnAO complex produced products which showed practically no degradation for a 6 h period of storage. All the secondary 99mTc complexes of PnAO are converted back to primary complex, when the solution was heated in a boiling water bath for 10-20 min.