Synthesis and evaluation of a ^99mTc-labelled deuterated tropane derivative as a SPECT probe for dopamine transporter

The dopamine transporter (DAT) is intricately associated with a broad spectrum of neurological disorders including Parkinson's disease and various other neurodegenerative conditions. In vivo imaging DAT with radiolabeled tracers has emerged as a prevalent and highly favored approach. This study aims to develop a novel in vivo stable ^99mTc-labelled probe [^99mTc]Tc-TRODAT-d₄ for single photon emission computed tomography (SPECT) for DAT-related disorders with the reported [^99mTc]Tc-TRODAT-1 as the leading compound.

The deuterated labelling precursor for [^99mTc]Tc-TRODAT-d₄ was synthesized and the ^99mTc-labelled radioligand [^99mTc]Tc-TRODAT-d₄ was successfully obtained with radiochemical yield (RCY) > 90 % and radiochemical purity (RCP) > 90 %. In vivo stability of the deuterated probe was evaluated and compared with the non-deuterated [^99mTc]Tc-TRODAT-1 in rodents with radio-HPLC. The specificity and selectivity of the deuterated probe [^99mTc]Tc-TRODAT-d₄ for DAT were assessed through comparative analyses of in vivo biodistribution, blocking experiments, and autoradiography.

Biological distribution results revealed that the target-to-non-target ratio (striatum/cerebellum, ST/CB) for [^99mTc]Tc-TRODAT-d₄ surpassed that of [^99mTc]Tc-TRODAT-1 at 120 min post-injection (3.95 ± 0.39 vs 3.05 ± 0.61, p < 0.05). In vivo metabolism investigations demonstrated that [^99mTc]Tc-TRODAT-d₄ exhibits enhanced in vivo stability in mice compared with [^99mTc]Tc-TRODAT-1. Ex vivo autoradiographic findings indicated that [^99mTc]Tc-TRODAT-d₄ selectively accumulated in the DAT-abundant striatum and its ST/CB ratio was higher than that of [^99mTc]Tc-TRODAT-1 (4.11 ± 0.25 vs 3.27 ± 0.09, p < 0.01).

A novel in vivo stable ^99mTc-labelled SPECT probe [^99mTc]Tc-TRODAT-d₄ targeting DAT was synthesized by deuteration on [^99mTc]Tc-TRODAT-1 with an RCP of >90 %. The deuterated tracer [^99mTc]Tc-TRODAT-d₄ showed good DAT affinity and specificity. Furthermore, [^99mTc]Tc-TRODAT-d₄ with improved in vivo stability provides a higher target-to-non-target ratio than [^99mTc]Tc-TRODAT-1, giving access to a highly promising ^99mTc-labelled radioligand for SPECT imaging of DAT in the brain.