In vitro stabilization of a low-tin bone-imaging agent (99mTc-Sn-HEDP) by ascorbic acid

Abstract

The presence of oxidants in the 99mTc-pertechnetate and of oxygen in diagnostic kits containing low concentrations of Sn(II) has a detrimental effect upon in vitro and in vivo stability. Maintaining a nitrogen atmosphere or increasing the Sn(II) concentration inhibits the formation of 99mTcO4-. However, the latter remedy is likely to cause uptake in the reticuloendothelial system and has been associated with false positive or negative brain scans. We used ascorbic acid (an antioxidant) to ensure the in vitro stability with the low-Sn(II) bone agent disodium etidronate. In vitro stability studies by instant thin-layer chromatography, using high-acitivity generators and "instant pertechnetate," yielded less than 2% free pertechnetate at 24 hr after preparation. Distribution studies in guinea pigs show neither altered distribution of the bone agent nor abnormal distribution of ascorbic acid, suggesting its sole function as a noncomplexing stabilizer.