Alternative methods for testing radiochemical purity of ¹¹¹In-pentetreotide

Objectives The SepPak^® method for testing ¹¹¹In-pentetreotide radiochemical purity (RCP) involves expensive supplies, excess radiation exposure, and is time consuming. Although there is a published ITLC-SG method, the production of ITLC-SG strips has been discontinued. The purpose of this study was to develop an alternative method using ITLC-SA paper.

Methods Both ITLC-SA and ITLC-SG alternatives were compared with the standard SepPak^® method. Both ITLC methods consisted of 0.7 x 6.0 cm sized chromatography strips, 0.9% NaCl as the mobile phase, and 0.05 M DTPA as the chelator for free ¹¹¹InCl. Kits were reconstituted according to the package insert which requires a RCP minimum of 90%. ¹¹¹InCl was added to create trials with RCP values below the accepted value of 90% purity in order to evaluate the sensitivity of the alternative methods. Assessment of the two alternative methods consisted of estimating their test-retest variability and their agreement of 90% purity with the standard method.

Results A total of 6 experiments were conducted with 5 replications resulting in 30 individual trials of each alternative method. The test-retest variability of the ITLC-SA method was 0.84 and had perfect agreement (100%) with the SepPak^® method in determining if ¹¹¹In-pentetreotide was above or below the acceptable RCP level. For the ITLC-SG method, however, the test-retest variability was 2.95 and the agreement was only 83%. All of the disagreement for the ITLC-SG method was a result of rejecting samples that were deemed acceptable by the SepPak^® method.

Conclusions ITLC-SA chromatography has smaller test-retest variability in comparison to ITLC-SG system. The agreement between SepPak^® and ITLC-SA methods was also higher than that between SepPak^® and ITLC-SG methods, but more trials below the critical 90% purity level are needed. ITLC-SA paper is readily available and could serve as a viable alternative RCP testing method for ¹¹¹In-pentetreotide