Development, validation and regulatory acceptance of improved purification and simplified quality control of [13N] Ammonia

BACKGROUND

[13N]Ammonia is a cyclotron produced myocardial perfusion imaging agent. With the development of high-yielding [13N]ammonia cyclotron targets using a solution of 5 mM ethanol in water, there was a need to develop and validate an automated purification and formulation system for [13N]ammonia to be in a physiological compatible formulation of 0.9% sodium chloride since there is no widely available commercial system at this time. Due to its short half-life of 10 min, FDA and USP regulations allow [13N]ammonia to be tested in quality control (QC) sub-batches with limited quality control testing performed on the sub-batches for patient use. The current EP and the original USP method for the determination of the radiochemical purity and identity of [13N]ammonia depended on an HPLC method using a conductivity detector and a solvent free of other salts. This HPLC method created issues in a modern cGMP high volume PET manufacturing facility where the HPLC is used with salt containing mobile phase buffers for quality control analysis of other PET radiopharmaceuticals. Flushing of the HPLC system of residual salt buffers which may interfere with the [13N]ammonia assay can take several hours of instrument time. Since there are no mass limits on [13N]ammonia, a simplified TLC assay to determine radiochemical identity and purity could be developed to simplify and streamline QC.

RESULTS

We have developed and validated a streamlined automated synthesis for [13N]ammonia which provides the drug product in 8 mL of 0.9% sodium chloride for injection. A novel radio-TLC method was developed and validated to demonstrate feasibility to quantitate [13N]ammonia and separate it from all known radiochemical impurities.

CONCLUSIONS

The process for automated synthesis of [13N]ammonia simplifies and automates the purification and formulation of [13N]ammonia in a cGMP compliant manner needed for high-throughput manufacture of [13N]ammonia. The novel radio-TLC method has simplified [13N]ammonia quality control (QC) and now enables it to be tested using the same QC equipment as [18F]fludeoxyglucose (FDA/USP recognized name for 2-[18F]fluoro-2-deoxy-D-glucose). Both the streamlined automated synthesis of [13N]ammonia and the novel radio-TLC method have been accepted and approved by the US Food and Drug Administration (FDA) for the cGMP manufacture of [13N]ammonia.

New 99mTc Radiotracers for Myocardial Perfusion Imaging by SPECT

OBJECTIVE

Myocardial Perfusion Imaging (MPI) with radiotracers is an integral component in evaluation of the patients with known or suspected coronary artery diseases (CAD). 99mTc-Sestamibi and 99mTc-Tetrofosmin are commercial radiopharmaceuticals for MPI by single photon-emission computed tomography (SPECT). Despite their widespread clinical applications, they do not meet the requirements of an ideal perfusion imaging agent due to their inability to linearly track the regional myocardial blood flow rate at >2.5 mL/min/g. With tremendous development of CZT-based SPECT cameras over the past several years, the nuclear cardiology community has been calling for better perfusion radiotracers with improved extraction and biodistribution properties.

METHODS

This review will summarize recent research efforts on new cationic and neutral 99mTc radiotracers for SPECT MPI. The goal of these efforts is to develop a 99mTc radiotracer that can be used to detect perfusion defects at rest or under stress, determine the regional myocardial blood flow, and measure the perfusion and left ventricular function.

RESULTS

The advantage of cationic radiotracers (e.g. 99mTc-Sestamibi) is their long myocardial retention because of the positive molecular charge and fast liver clearance kinetics. 99mTc-Teboroxime derivatives have a high initial heart uptake (high first-pass extraction fraction) due to their neutrality. 99mTc- 3SPboroxime is the most promising radiotracer for future clinical translation considering its initial heart uptake, myocardial retention time, liver clearance kinetics, heart/liver ratios and SPECT image quality.

CONCLUSION

99mTc-3SPboroximine is an excellent example of perfusion radiotracers, the heart uptake of which is largely relies on the regional blood flow. It is possible to use 99mTc-3SPboroximine for detection of perfusion defect(s), accurate quantification and determination of regional blood flow rate. Development of such a 99mTc radiotracer is of great clinical benefit for accurate diagnosis of CAD and assessing the risk of future hard events (e.g. heart attack and sudden death) in cardiac patients.

Novel 99mTc(III)-azide complexes [99mTc(N3)(CDO)(CDOH)2B-R] (CDOH2=cyclohexanedione dioxime) as potential radiotracers for heart imaging

INTRODUCTION

In this study, novel ^99mTc(III)-azide complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R] (^99mTc-ISboroxime-N₃: R=IS; ^99mTc-MPboroxime-N₃: R=MP; ^99mTc-PAboroxime-N₃: R=PA; ^99mTc-PYboroxime-N₃: R=PY; and ^99mTc-Uboroxime-N₃: R=5U) were evaluated as heart imaging agents.

METHODS

Complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R] (R=IS, MP, PA, PY and 5U) were prepared by ligand exchange between NaN₃ and [^99mTcCl(CDO)(CDOH)₂B-R]. Biodistribution and imaging studies were carried out in Sprague-Dawley rats. Image quantification was performed to compare their initial heart uptake and myocardial retention.

RESULTS

^99mTc-ISboroxime-N₃, ^99mTc-PYboroxime-N₃ and ^99mTc-Uboroxime-N₃ were prepared with high RCP (93-98%) while the RCP of ^99mTc-MPboroxime-N₃ and ^99mTc-PAboroxime-N₃ was 80-85%. The myocardial retention curves of ^99mTc-ISboroxime-N₃, ^99mTc-PYboroxime-N₃ and ^99mTc-Uboroxime-N₃ were best fitted to the bi-exponential decay function. The half-time of the fast component was 1.6±0.4min for ^99mTc-ISboroxime-N₃, 0.7±0.1min for ^99mTc-PYboroxime-N₃ and 0.9±0.4min for ^99mTc-Uboroxime-N₃. The 2-min heart uptake from biodistribution studies followed the ranking order of ^99mTc-ISboroxime-N₃ (3.60±0.68%ID/g)>^99mTc-PYboroxime-N₃ (2.35±0.37%ID/g)≫^99mTc-Uboroxime-N₃ (1.29±0.06%ID/g). ^99mTc-ISboroxime-N₃ had the highest 2-min heart uptake among ^99mTc radiotracers revaluated in SD rats. High quality SPECT images were obtained with the right and left ventricular walls being clearly delineated. The best image acquisition window was 0-5min for ^99mTc-ISboroxime-N₃.

CONCLUSION

Both azide coligand and boronate caps had significant impact on the heart uptake and myocardial retention of complexes [^99mTc(N₃)(CDO)(CDOH)₂B-R]. Among the radiotracers evaluated in SD rats, ^99mTc-ISboroxime-N₃ has the highest initial heart uptake with the heart retention comparable to that of ^99mTc-Teboroxime. ^99mTc-ISboroxime-N₃ is a promising alternative to ^99mTc-Teboroxime for SPECT MPI.

Advanced tracers in PET imaging of cardiovascular disease

Cardiovascular disease is the leading cause of death worldwide. Molecular imaging with targeted tracers by positron emission tomography (PET) allows for the noninvasive detection and characterization of biological changes at the molecular level, leading to earlier disease detection, objective monitoring of therapies, and better prognostication of cardiovascular diseases progression. Here we review, the current role of PET in cardiovascular disease, with emphasize on tracers developed for PET imaging of cardiovascular diseases.