Insights into the failure of the potential, neutral myocardial imaging agent TcN-NOET: physicochemical identification of by-products and degradation species

Abstract 

Introduction

The neutral complex [^99mTc(N)(NOEt)₂], often referred to as TcN-NOET [NOEt=N-ethoxy,N-ethyldithiocarbamate(1−)], was proposed several years ago as a myocardial imaging agent. Despite some favorable clinical properties evidenced during phase I and phase II studies, the overall results of the European and American phase III clinical studies have been judged insufficient for a successful approval process by the regulatory agencies.

Methods

Non-carrier-added and carrier-added experiments using short-lived ^99mTc and long-lived ^99gTc have been utilized to prepare a series of bis-substituted [Tc(N)(DTC)₂] complexes [DTC=dithiocarbamate(1−)]. They have been purified by means of chromatographic techniques (high-performance liquid chromatography and thin-layer chromatography) and identified via double detection (UV-vis and radiometry) by comparison with authenticated samples of ^99gTc compounds prepared by conventional coordination chemistry procedures.

Results

The molecular structure of the lipophilic, neutral complex cis-[Tc(N)(NOEt)₂] has been assigned by comparison with similar nitrido-Tc(V) complexes already reported in the literature. Novel bis-substituted nitrido-Tc complexes containing hydrolyzed portions of coordinated NOEt, namely, N-ethyldithiocarbamate [NHEt(1−)] and N-hydroxy, N-ethyldithiocarbamate [NOHEt(1−)], have been prepared and characterized by means of multinuclear nuclear magnetic resonance spectroscopy and mass spectrometry.

Conclusions

Despite the identification of these “hydrolyzed” species, it is still unclear whether the failure to reach the clinical goal of the perfusion tracer [^99mTc(N)(NOEt)₂] is related to the degradation processes evidenced in this study or is the result of the mediocre imaging properties of the tracer.

Keywords: Technetium radiopharmaceuticals, Coordination chemistry, Mass spectrometry, Myocardial imaging