Problems associated with stannous 99 mTc- radiopharmaceuticals

Abstract

Possible reasons for anomalous in vivo behavior of 99 m Tc radiopharmaceuticals are evaluated. The complicated and poorly understood solution chemistry of technetium as well as tin (II), the most widely used reducing agent, is shown to contribute to problems. Unreliable performance of stannous kits is deduced to be partly due to initial oxidation and hydrolysis of tin (II) as a result of poor formulation, and also due to low stoichiometric ratios of complexing agent to tin. The kits could fail in two ways: (1) only a fraction of the original tin may be available in the desired form at reconstitution; (2) undesirable side reactions of tin and technetium may occur. Evaluation of generator as well as instant technetium has occasionally revealed situations where the carrier content of 99 m Tc solutions could exceed the reductive capability of the stannous ion; this becomes critical with kits containing a very small quantity of Sn (II) in the “usable” form. Parameters for effective performance of tin (II) containing kits are examined, and a titration method allowing for in vitro evaluation of available kits is presented. A model procedure for preparing stable Sn (II) kits has been developed.