Pitfalls in the Standard Radiochemical Purity Testing for Technetium-99m-Exametazime

Abstract

Objective: The purpose of this study was to examine discrepancies in the standard method for determining the radiochemical purity (RCP) of ^99mTc-exametazime. The package insert for ^99mTc-exametazime indicates that the entire RCP testing procedure requires approximately 15 min to complete, although the solvent-developing time for the three-paper chromatography system is described to be ≤100 sec. According to the package insert, all solvent-migrated radiochemical components moved up to relative front (R_f) 0.8–1.0; however, the suggested cut lines for three paper strips are all well below the stated R_f value.

Methods: Radiochromatogram and autoradiography techniques were used to determine and evaluate the radioactivity distribution of different radiochemical species on the strips. The times for solvent migration and the entire RCP testing procedure were measured.

Results: The mean times to run the three standard paper chromatography systems, namely, ITLC-SG/MEK, ITLC-SG/saline and Whatman/50% CH₃CN strips were 130.4 ± 9.0 sec, 86.7 ± 9.4 sec and 123.1 ± 6.1 sec, respectively (n = 55). Although our solvent-developing time was longer than the suggested ≤100 sec, the entire RCP analysis procedure (i.e., sample spotting, solvent migration, radioactivity measurement and RCP calculation) can be completed within 5–7 min. Both ITLC-SG/MEK and Whatman/50% CH₃CN systems showed significant streaking of radioactivity on the strips (i.e., 0.5–1.0 and 0.6–1.0, respectively) which does not agree with the recommended R_f 0.8–1.0.

Conclusion: The quality control information for ^99mTc-exametazime should be modified in order to provide accurate information for the time required to perform the entire RCP procedure, the solvent-developing times for three strips and the R_f values for three chromatography systems.