99Tcm-MAG3: problems with radiochemical purity testing

The radiochemical purity (RCP) of 99Tcm-MAG3 was determined using solid-phase extraction (SPE), high-performance liquid chromatography (HPLC) and thin-layer chromatography (TLC). The difference between the HPLC and SPE methods was highly significant (P < 0.001), yielding values for RCP of 94.4 +/- 1.4% and 86.0 +/- 5.1% [corrected] respectively (mean +/- s). Further qualitative analysis of the SPE fractions obtained, was carried out using HPLC and TLC. The unexpected presence of 99Tcm-MAG3 in one of the fractions was observed together with the appearance of hydrophilic impurities in the hydrophobic extract. This lack of specificity may be the reason for the discrepancy between the SPE and HPLC methods. Use of the SPE method leads to an underestimation of the RCP of 99Tcm-MAG3 and, indeed, had we been relied solely on this method of analysis, we would have had to reject most kits we prepared. In a separate study, we compared a TLC method with HPLC. Differences were found to be highly significant (P < 0.001), yielding values of 98.3 +/- 0.6% and 95.8 +/- 0.9% respectively. Comparison of the data points showed that TLC gave consistently higher RCP yield than HPLC. This elevated value was found to be due to the inability of the TLC method to separate 99Tcm-lipophilic impurity, seen on HPLC, from the 99Tcm-MAG3. Therefore, use of this TLC method leads to an overestimation of the RCP of 99Tcm-MAG3.