The GMP certified [68Ge]/[68Ga] generator is self-lead shielded and easy to operate with a manual and automatic labeling modules. Ultra-pure HCl is used to separate [68Ga] from the [68Ge]/[68Ga] generator and sodium acetate buffer which is obtained from Sigma-Aldrich used for peptide labeling. The PSMA−11 peptide was obtained from ABX Pharmaceuticals. Disposable labeling kits were procured from Huayi Isotopes Co.(China).
The radiochemical purity of [68Ga]Cl3 precursor obtained from the generator and of the synthesized radiotracer were analyzed by a radio-HPLC system (Scintomics 8100 radio-HPLC system).
It is known that metallic impurities in radionuclide precursors may reduce the labeling yield in radiopharmaceutical preparation. For this reason, qualitative and quantitative analyzes of metal pollution (Ge, Zn, Al, Mn, Fe, Pb, Co and Ni) in the composition of both the radionuclide precursor and the radiotracers were made at the parts per billion (ppb) level using inductive coupled plasma-optical emission spectrometry (ICP-OES) device in our university Advanced Technology Application and Research Center. Prior to analysis, eluates were incubated during with were incubated for ten half-lives in vials according to the Nuclear Regulatory Agency's regulations for handling radioactive materials.
The generator was always eluted 24 hours before labeling in order to remove the [68Ga] degradation product Zn accumulated and other metal impurities. Labeling was performed with PSMA−11 peptide. For this purpose, 25µg PSMA−11 peptide was labeled in the synthesis unit (automated module, Lindach, Fürstenfeldbruck, Germany) at 95oC for 10 minutes at pH ≈ 4.1–4.4 by sodium acetate buffer. The labeled elution was passed through a C18 cartridge and recovered with ethyl alcohol/water (1:1) and filtered through a 0.22 µm filter for sterilization prior to quality control. The pH value was adjusted with phosphate buffer at the final stage of production of PET radiotracer [68Ga]GaPSMA−11. The whole process was performed under aseptic conditions. After measuring the product radioactivity in the dose calibrator, the pH test of the product was performed by pH indicator test strips.
Before being injected into the patient, quality control analyzes of the radiotracers were performed with radio-HPLC. The radiochemical purity of the obtained PET radiotracer was analyzed by the method described in our previous study [11].
The study protocol was approved by the Non-Interventional Clinical Research Ethics Committee of our University, Faculty of Medicine (approval number 04.07.2022-E.227606), and the study was performed in accordance with the World Medical Association Declaration of Helsinki. Biodistribution of [68Ga]-radiotracer by PET-CT was evaluated retrospectively.
Patients’ Characteristics
A total 21 patients with prostate cancer who agreed to participate in our study were performed PET-CT imaging after [68Ga]GaPSMA−11 injection. Normal distribution and tumoral lesions were evaluated from PET-CT images of these 21 patients. All patients were informed verbally and in writing before [68Ga]GaPSMA−11 PET-CT imaging and their written consent were obtained. Ten out of 21 patients were referred to our department for staging and 11 for evaluation of treatment response. Of the 11 patients referred for evaluation of response to treatment, one had undergone radical prostatectomy and the other ten were on hormone therapy. Gleason scores were 6(3 + 3) in 5 patients, 7(4 + 3) in 4 patients, 7(3 + 4) in 4 patients, 8(4 + 4) in one patient, 9(4 + 5) in 2 patients, 9(5 + 4) in 2 patients and 10(5 + 5) in one patient. Since the diagnosis of two patients was made from bone metastases, no prostate biopsy has performed.
Image Acquisition
An average of 185 MBq (range 125–317 MBq) [68Ga]GaPSMA−11 was injected per patient and imaging was performed 45–60 minutes after injection. The patients were imaged in the supine position. PET-CT images were acquired using Gemini TF TOF PET-CT (Philips, Cleveland, OH, USA). Intravenous contrast was not used during emission scans. Sections were taken from the apex of the skull to the tip of the feet. CT images were acquired using a low-dose CT protocol (50–120 mA, 90–140 kV, 5 mm slice thickness). Attenuation correction was performed for PET images using CT features and ordered subsets-expectancy maximization algorithm (33 subsets, three iterations). PET images were created by iterative reconstruction method. Transverse, sagittal and coronal sections (slice thickness 5 mm) were created from PET/CT fusion images and evaluated using Philips Fusion Viewer software (version 2.1; Philips Healthcare, Best, The Netherlands).
Visual and quantitative image analyzes were performed as described by Prasad et al. and Hofman et al.[12, 13]. Normal uptake values and visual whole body normal distribution scheme by Prasad et al. for 101 patients were taken as reference [12].
The organs and tissues evaluated for normal distribution areas of [68Ga]GaPSMA−11 are as follows: lacrimal gland, nasal mucosal structures, parotid gland, submandibular gland, liver, spleen, kidney, duodenum, pancreas (head, body, tail), colon, blood pool (aorta), adrenal glands, bone marrow (over the iliac bone), lymph nodes with fatty hilum and prostate gland [12, 13].