Tandem Mass Spectrometry as an Independent Method for Corroborating Fluorine-18 Radioactivity Measurements in Positron Emission Tomography

Positron emission tomography (PET) uses many tracers labeled with fluorine-18 (t _1/2 = 109.8 min; β⁺ 97%) for quantitative imaging of biochemical and physiological processes in animal and human subjects. In PET methodology, the radioactivity in a dose of an ¹⁸F-labeled tracer to be administered to a living subject is measured with a calibrated ionization chamber. This type of detector measures the radioactivity of a sample relative to those of certified amounts of longer-lived surrogate isotopes that are recommended for detector calibration. No alternative means for corroborating widely varying fluorine-18 radioactivity measurements from calibrated ionization chambers has been available. Here, we describe an independent nonradiometric method for this purpose. In this method, highly sensitive liquid chromatography-tandem mass spectrometry (LC-MS/MS) is used to quantify the relative masses of the radioactive isotopologue ([¹⁸F]1) and the accompanying nonradioactive counterpart (carrier 1) in an ¹⁸F-labeled tracer preparation to give the mole ratio of [¹⁸F]1. High-performance liquid chromatography (HPLC) with a mass-calibrated absorbance detection is used alongside to provide a separate measurement of the aggregate mass of all isotopologues. The radioactivity of the radiotracer is then derived in becquerels (Bq) from these two measurements, plus Avogadro's number and the decay constant of fluorine-18. For the chosen example [¹⁸F]LSN3316612, the radioactivity values determined nonradiometrically and with a selected ionization chamber were in fair agreement. In addition, LC-MS/MS alone was found to provide an accurate measure of the half-life of fluorine-18.

The Influence of Specific Activity on the Biodistribution of 18F-rhPSMA-7.3: A Retrospective Analysis of Clinical Positron Emission Tomography Data

We investigated whether the time between synthesis and injection and the resulting decrease in specific activity affects the normal organ and tumor uptake of the PSMA ligand, ¹⁸F-rhPSMA-7.3, in patients with prostate cancer. Methods: The biodistribution of ¹⁸F-rhPSMA-7.3 on PET/CT scans performed with a high specific activity (median = 178.9MBq/µg, n = 42) and a low specific activity (median = 19.3MBq/µg, n = 42) were compared. Results: Tracer uptake by the parotid gland, submandibular gland and spleen was moderately, but significantly lower in the "low specific activity" group than in the "high specific activity" group (median SUVmean 16.7 vs. 19.2; 18.1 vs. 22.3, and 7.8 vs. 9.6, respectively). No other statistically significant differences were found for normal organs or tumor lesions. Conclusion: A 10-fold decrease in specific activity has only minor effects on the biodistribution of ¹⁸F-rhPSMA-7.3. These findings suggest that ¹⁸F-labeled PSMA ligands can be centrally produced and shipped to PET clinics in a similar way to ¹⁸F-fluorodeoxyglucose.

GMP-Automated Purification of Copper-61 Produced in Cyclotron Liquid Targets: Methodological Aspects

BACKGROUND

Expanding the range of metal-based PET radiopharmaceuticals that can be produced by the widely available network of biomedical cyclotrons is a major priority. Copper- 61 is a positron emitter with very favourable physical (61.5% β+, 1.22 MeV max.) and chemical properties, which emerged as a promising PET imaging agent.

OBJECTIVES

This work aimed to develop and optimise a GMP-automated purification method for copper-61 produced in a cyclotron using a natural zinc liquid target.

METHODS

The automated purification process was performed using a commercially available Synthera ® Extension module (IBA, Louvain-la-Neuve, Belgium) using a three-column method: two extraction chromatographic resins and a strong anion exchange resin. The final product was evaluated using HPGe and ICP-MS analysis, to assess the radionuclidic and chemical purity of the final copper- 61 solution.

RESULTS

The automated purification process was completed within 1 h of processing time, with an average yield of 63.0 ± 15.0%, in a maximum volume of 5 mL. The radionuclidic purity of copper- 61 in the final solution was over 95% for 7 h after EOB. ICP-MS analysis revealed 4.8 ± 2.4 μg of natural zinc in the final purified sample, and the copper-61 molar activity was 230.5 ± 139.3 GBq/μmol.

CONCLUSION

The described purification process allows for the production of a highly radionuclidically and chemically pure, GMP compliant copper-61 solution, ready to be used for the development of copper-61 based radiopharmaceuticals for routine clinical use.

Fluorine-18 Labeled Fluorofuranylnorprogesterone ([18F]FFNP) and Dihydrotestosterone ([18F]FDHT) Prepared by "Fluorination on Sep-Pak" Method

To further explore the scope of our recently developed “fluorination on Sep-Pak” method, we prepared two well-known positron emission tomography (PET) tracers 21-[¹⁸F]fluoro-16α,17α-[(R)-(1′-α-furylmethylidene)dioxy]-19-norpregn-4-ene-3,20-dione furanyl norprogesterone ([¹⁸F]FFNP) and 16β-[¹⁸F]fluoro-5α-dihydrotestosterone ([¹⁸F]FDHT). Following the “fluorination on Sep-Pak” method, over 70% elution efficiency was observed with 3 mg of triflate precursor of [¹⁸F]FFNP. The overall yield of [¹⁸F]FFNP was 64–72% (decay corrected) in 40 min synthesis time with a molar activity of 37–81 GBq/µmol (1000–2200 Ci/mmol). Slightly lower elution efficiency (~55%) was observed with the triflate precursor of [¹⁸F]FDHT. Fluorine-18 labeling, reduction, and deprotection to prepare [¹⁸F]FDHT were performed on Sep-Pak cartridges (PS-HCO₃ and Sep-Pak plus C-18). The overall yield of [¹⁸F]FDHT was 25–32% (decay corrected) in 70 min. The molar activity determined by using mass spectrometry was 63–148 GBq/µmol (1700–4000 Ci/mmol). Applying this quantitative measure of molar activity to in vitro assays [¹⁸F]FDHT exhibited high-affinity binding to androgen receptors (K_d~2.5 nM) providing biological validation of this method.

Impact of 18F-PSMA-1007 Uptake in Prostate Cancer Using Different Peptide Concentrations: Preclinical PET/CT Study on Mice

PET radioligands with low molar activity (MA) may underestimate the quantity of the target of interest because of competitive binding of the target with unlabeled ligand. The aim of this study was to evaluate the change in the whole-body distribution of ¹⁸F-PSMA-1007 targeting prostate-specific membrane antigen (PSMA) when solutions with different peptide concentrations are used. Methods: Mouse xenograft models of LNCaP (PSMA-positive prostate cancer) (n = 18) were prepared and divided into 3 groups according to the peptide concentration injected: a high-MA group (1,013 ± 146 GBq/μmol; n = 6), a medium-MA group (100.7 ± 23.1 GBq/μmol; n = 6), and a low-MA group (10.80 ± 2.84 GBq/μmol; n = 6). Static PET scans were performed 1 h after injection (scan duration, 10 min). SUV_mean in tumor and normal organs was compared by the multiple-comparison test. Immunohistochemical staining and Western blot analysis were performed to confirm expression of PSMA in tumor, salivary gland, and kidney. Results: The low-MA group (SUV_mean, 1.12 ± 0.30) showed significantly lower uptake of ¹⁸F-PSMA-1007 in tumor than did the high-MA group (1.97 ± 0.77) and the medium-MA group (1.81 ± 0.57). On the other hand, in salivary gland, both the low-MA group (SUV_mean, 0.24 ± 0.04) and the medium-MA group (0.57 ± 0.08) showed significantly lower uptake than the high MA group (1.27 ± 0.28). The tumor-to-salivary gland SUV_mean ratio was 1.73 ± 0.55 in the high-MA group, 3.16 ± 0.86 in the medium-MA group, and 4.78 ± 1.29 in the low-MA group. The immunohistochemical staining and Western blot analysis revealed significant overexpression of PSMA in tumor and low expression in salivary gland and kidney. Conclusion: A decrease in the MA level of the injected ¹⁸F-PSMA-1007 solution resulted in decreased uptake in tumor and, to a greater degree, in normal salivary gland. Thus, there is a possibility of minimizing the adverse effects in salivary gland by setting an appropriate MA level in PSMA-targeting therapy.