Comparative stability versus cysteine of mixed ligand ^99mTc complexes containing monothiols of differing nucleophilicity.

Our work over the past few years has been focused on the design and evaluation of novel chelating systems suitable for oxorhenium and/or oxotechnetium that may lead to useful radioagents. Several ^99mTc mixed ligand complexes carrying the SNS/S or the SNN/S donor atom set have shown promising biological properties for future application. The in vivo stability and thereby biological properties of these complexes is greatly dependent on the electronic and lipophilic character of the monothiol.
We report here on the comparative resistance to cysteine substitution of two parallel series of ^99mTcO{Et₂NCH₂CH₂N(CH₂CH₂S)₂. The electronic influence on cysteine substitution rate can thus be investigated.
All above complexes are prepared at tracer level using glucoheptonate as transfer ligand and are purified by organic extraction and HPLC. Their structure has been previously resolved by comparison with authentic samples. The ^99mTc complexes are stable in solution in the absence of thiol for long periods of time. Competition experiments with cysteine are performed in aqueous medium at pH 7.4 and using two different cysteine concentrations, 10 mM and 1 mM, by incubation at 37°C. Aliquots of the incubate withdrawn at 0, 15, 30, 60, 120 min and 24 h time intervals are subjected to HPLC analysis. A RP C18 column is eluted by TEAP buffer pH 7.4/EtOH gradient system, whereby all radioactive ingredients are well separated.
All complexes are attacked by cysteine under the above conditions forming mixtures of intermediates, that with time lead to total loss of the original ^99mTc compounds. Relative resistance to cysteine substitution is directly proportional to monothiol nucleophilicity in both series of complexes: -NH₂ > -H> -NO₂. Since the free thiol content - mainly cysteine and its derivatives - varries in vivo, mixed ligand systems as the above can be used for directing and trapping ^99mTc in tissues of choice by finely monitoring stability via the monothiol.