Simplified in-house kit formulation of 99mTc-TRODAT-1 with high radiolabeling efficiency for routine clinical use. Westmead Hospital Experience

Abstract

Objective: The tropane derivative, TRODAT-1 when labelled with 99mTc has been shown to bind dopamine transporter (DAT) with high affinity and selectivity. In this study, we have modified the in-house formulation of TRODAT-1 cold kit. We investigated if the labeling procedure could be improved by using a block heater and thus be more efficient and consistent than using autoclaving or boiling water-bath methods described elsewhere. Methods: Briefly, 1mg of TRODAT-1 was dissolved in 100µl of ethanolic-HCl. To this solution 500µg SnCl2, 160mg sodium glucoheptonate, 19mg EDTA were added and mixed. The bulk solution containing TRODAT-1 was aseptically dispensed in 1.0mL aliquots and stored at -80°C. TRODAT-1 was radiolabelled by adding 1.4GBq of 99mTc-pertechneate in 3ml saline and heated for 30min at 100oC using a block heater enclosed with a lid. The radiochemical purity (RCP) was analysed using Instant Thin Layer Chromatography-Silica Gel (ITLC-SG) with 0.9% Sodium Chloride as the solvent and High Performance Liquid Chromatography (HPLC) was performed using 0.1% Trifluoroacetic acid (TFA) / Water (H2O) & Acetonitrile (ACN). As proof of concept, biodistribution of 99mTc-TRODAT-1 was studied in a rabbit model. Animals were injected with 30MBq of 99mTc-TRODAT-1 and images were acquired at 60 min post-injection with a Single-photon emission computed tomography (SPECT), Siemens Symbia camera. Results: Previous studies have used autoclaving method or a boiling water bath for radiolabeling TRODAT-1 with 99mTc. These methods have some limitation including the evaporating of water during boiling, uncontrollable temperatures, variable yield and increased possibility of contamination. To ensure consistent temperature and eliminate the possibility of contamination we used a block heater. The percentage of RCP of 99mTc-TRODAT-1 by ITLC-SG was >99% and reproducible. The retention time by HPLC for labeled product was 16.18min and for free 99mTc was 2.5min. Animal biodistribution studies showed brain hippocampus uptake of 0.32±0.08 %ID/g at 60min post-injection. Conclusions: This study describes a simple formulation and method of preparation for 99mTc-TRODAT-1, which is reproducible with radiochemical purity (RCP >99%).The pre-clinical results in rabbits indicated specific binding to dopamine receptors and therefore has the potential for use in DAT imaging in patients.