¹⁸F-Tetrafluoroborate, a PET Probe for Imaging Sodium/Iodide Symporter Expression: Whole-Body Biodistribution, Safety, and Radiation Dosimetry in Thyroid Cancer Patients

Radiopharmaceutical Preparation

Synthesis of the ¹⁸F-TFB IMP (Investigational Medicinal Product) was performed by a method similar to a previously described automated labeling protocol (14) using an Eckert and Ziegler Modular-Lab synthesis unit (Imaging Equipment Ltd.). ¹⁸F-fluoride (46 ± 6 GBq), obtained from proton-irradiated ¹⁸O-water (98 atom% [Rotem Industries Ltd.]; 11-MeV protons from a CTI RDS 112 cyclotron; beam current, 30 μA; irradiation time, 60 min), was trapped in a QMA cartridge (Sep-Pak Light QMA cartridge; Waters) conditioned with 1.0 M sodium hydrogen carbonate (10 mL, Ph Eur; Merck) followed by water for injections (10 mL, BP, Hamelin Pharmaceuticals) and air (10 mL). Hydrochloric acid (1.5 M, 1.2 mL), prepared by dilution of hydrochloric acid fuming 37% (Ph Eur; Merck) with water for injections (BP, Hamelin Pharmaceuticals), was then passed through the QMA cartridge, eluting the ¹⁸F-fluoride into the reactor, which contained sodium tetrafluoroborate (Sigma-Aldrich) (1 mg in 0.1 mL water for injections). The reaction mixture (1.3 mL) was heated to 80°C for 15 min, cooled to 20°C, and passed through a silver ion-loaded cation exchange cartridge (OnGuard II AG [Dionex], conditioned with 10 mL of water and 5 mL of air) to remove chloride ions and raise the pH, and through 2 alumina cartridges (SepPak Light Alumina N [Waters], conditioned with 10 mL of water and 5 mL of air) to remove unreacted ¹⁸F-fluoride. The product was eluted with water for injections (2 mL) into a nitrogen-filled USP type 1 glass sterile vial (N46; GE Healthcare) containing water for injections (2 mL). After production, the vial containing the unfiltered product (5.3 mL) was transferred to a sterile isolator, for sterile filtration and quality control sampling. The process was validated to demonstrate that ¹⁸F-TFB can be consistently manufactured on the Modular-Lab synthesizer to meet the required quality control release specifications. Stability studies were conducted by testing an aliquot from the validation batches for radiochemical purity and pH at 2, 4, and 6 h from the end of synthesis.

Radiopharmaceutical Quality Control

Radiochemical identity, purity, and specific activity of the final product were checked by ion chromatography (930 Compact IC Flex; Metrohm) with in-line conductimetric and γ-detectors (B-FC-3200; LabLogic) using a Shodex IC I-524A column (4.6 × 100 mm) with 2.5 mM phthalic acid (adjusted to pH 4.0 with tris[hydroxymethyl]aminomethane) as eluent. The flow rate was 1.5 mL/min, and column temperature was 40°C. The ion chromatography quality control method was validated following the International Conference on Harmonisation guidelines, and the concentration of ^19/18F-BF₄- in the final product was determined from the ion chromatogram by reference to a calibration curve. Radio–thin-layer chromatography (TLC) was performed using a silica gel stationary phase (Silica gel 60 F₂₅₄, 5 × 10 cm; Merck) with methanol as the mobile phase. Plates were scanned using a radio-TLC scanner (Scan-RAM; LabLogic) with a β⁺ probe (PS Plastic-PM; LabLogic). The radiochemical purity of the product was determined as the radioactivity associated with the tetrafluoroborate peak (Rf = 0.6, c.f. Rf = 0 for fluoride) as a percentage of the total chromatogram radioactivity. Silver content was determined using Quantofix Silver test strips (Macherey-Nagel). The bacterial endotoxin assay was performed using the Endosafe Portable Test System (Charles River) using limulus amebocyte lysate reagent water and licensed Endosafe Portable Test System cartridges (Charles River) with a sensitivity of 0.05–5.0 EU/mL. The integrity of the sterilizing filters was tested after use with the bubble point method, and sterility testing was performed by Wickham Laboratories.

Patients

Patients were recruited in accordance with the European Council Directive. The clinical protocol was approved by the local Research Ethics Committee (reference no. 14/LO/1247, ISRCTN: 75827286). All patients provided written informed consent.

Five subjects (2 men, 3 women; average age, 46 y) with newly diagnosed, cytologically confirmed thyroid cancer were imaged before elective total thyroidectomy and ¹³¹I-NaI thyroid remnant ablation. Pregnant and lactating women were excluded.

A cannula was placed in a vein in the antecubital fossa of each arm, one for injection of ¹⁸F-TFB and the other for withdrawal of venous blood. Electrocardiographic monitoring, pulse, blood pressure, body temperature (using a temperature strip on the patient’s forehead), and respiratory rate were monitored throughout the study.

PET/CT Data Acquisition

Bolus injection of ¹⁸F-TFB (mean ± SD, 185 ± 15 MBq) was performed with the patient positioned on the scanning couch. Imaging comprised a 90-min series of 7 sequential whole-body PET/CT scans (from head to foot), followed by 2 further whole-body PET/CT scans at 120 and 240 min after injection (Fig. 1) with a Discovery 710 PET/CT scanner (GE Healthcare) with an axial field of view of 15.7 cm. Each scan covered the vertex to midthigh, with an 11-slice overlap between bed positions. Three low-dose whole-body CT scans (140 kV, 10 mA, 0.5-s rotation time, 40-mm collimation) were obtained, one before the block of 7 sequential scans and then one before each of the scans at 120 and 240 min, for attenuation correction and anatomic reference. The initial 1 min per bed scan duration was adjusted to compensate for radioactive decay up to a maximum of 3 min per bed step for the final scan at 240 min.

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FIGURE 1.

Imaging scan schedule for whole-body dynamic PET/CT study (not to scale). Dots above scan positions show timings of venous blood sampling, and squares represent nominal urine sampling times.

Quantification of Blood and Urine Data

Venous blood samples (5 mL each) were acquired at 0, 1, 2, 5, 10, 20, 40, 60, 90, 120, and 240 min after injection. Three 0.2-mL aliquots from each blood sample were weighed and counted on a 10-sample well counter (2470 WIZARD2; PerkinElmer), previously cross-calibrated with ¹⁸F to the PET scanner using a standard calibration technique subject to daily quality control. Whole-blood samples were centrifuged for 5 min (6,000 rpm), and three 0.2-mL samples of plasma from each were then counted in the well-counter. Following the method of Namias et al. (18), the distribution ratio, R(t), between blood cells and plasma can be calculated from experimental measurements of the whole-blood and plasma activity concentrations (C_WB and C_P, respectively) at each time point using the equation: Eq. 1where the blood cell activity concentration, C_BC, can be calculated from: Eq. 2The value H represents hematocrit (assumed for each patient as H = 0.39). Before application of Equations 1 and 2, C_BC and C_P were expressed in Bq/kg of water, assuming that plasma contains 0.94 kg water/L and erythrocytes contain 0.73 kg/L (19).

Urine was collected as voided in the intervals between imaging (i.e., between 90–120 min and 140–260 min) and weighed for total excreted volume, and 3 aliquots (0.2 mL each) of each void were used to determine the total excreted activity of ¹⁸F-TFB.

Image Reconstruction

Each whole-body PET image was attenuation-corrected using its corresponding CT image and included standard scanner-based corrections for decay, scatter, randoms, and dead-time. Each emission scan was reconstructed with a time-of-flight ordered-subset expectation maximization algorithm (2 iterations, 24 subsets), into a 256 × 256 matrix. A 4 mm in full width at half maximum gaussian postreconstruction smoothing filter was applied. The accuracy of the activity in the PET images was verified by summing the activity in the first whole-body PET scan and comparing with the decayed injected activity.

Absorbed Dose Calculations

Whole-body CT images from the 120- and 240-min PET/CT scans were rigidly registered to the 0- to 90-min CT using the PFUS package of PMOD (version 3.7; PMOD Technologies). The resulting transformations were then applied to the corresponding PET images. All PET frames were then merged to a single 4-dimensional series decay-corrected to a common time point. Organs of interest were identified and manually delineated by an experienced clinician using a fusion of both the corresponding CT and the PET images. A software correction was included to account for the imaging time of each bed position in the whole-body sweeps. Visible organs with high uptake or easily identified on CT images were the brain, thyroid, liver, spleen, stomach, heart, kidneys, lungs, testes (male only), and urinary bladder contents. Organ volumes were transferred to all frames, and independent volumes were adjusted where necessary to account for any movement of organs between different frames. All organs with the exception of the bladder were assumed to have a constant volume over time. The volume of the bladder was quantified on each PET scan using a 25% of maximum isocontour. PMOD software allows the automatic naming and standardized volume determination of each organ relating to the 70-kg Cristy–Eckerman adult anthropomorphic phantom used by the OLINDA/EXM dose calculation software, a software package widely used to obtain radiation dose estimates in radiopharmaceutical studies (20). Time–activity curves were produced for each organ, and using the PKIN package of PMOD (version 3.7). Organ normalized cumulated activities (NCA: numerically equivalent to the older terminology of residence time measured in MBq⋅h/MBq) were automatically calculated using the net injected activity and organ volume. Time–activity curves were fitted to biexponential functions where possible, assuming radionuclide decay (i.e., no further excretion) on reaching the last imaging point. Mean blood activity concentration was generated in calibrated kBq/mL from the average value of the 3 samples. Total blood activity was calculated assuming that 5.4% of the reference phantom’s mass (70 kg) was blood. The blood time–activity curve was fitted with an exponential curve, the NCA determined, and this value used as the NCA for the red marrow in OLINDA calculations.

Summed activities of the urinary bladder and voided urine over the course of the study were fitted with a 1-phase exponential curve using the formula: Eq. 3where U(t), U_max, and T_1/2 represent the accumulated urine activity at time t, maximum accumulated urine activity, and biologic half-life, respectively. From the fitting parameters, U_max and T_1/2 were determined and the NCA of the urinary bladder contents was calculated using a dynamic urinary bladder model in the OLINDA/EXM (version 1.1) software. A 3.5-h voiding interval was used as recommended by the International Commission on Radiological Protection (21). The remainder of body was assigned by activity not accounted for by organ delineation or excretion. Effective doses according to the tissue-weighting factors of International Commission on Radiological Protection publication 60 in each organ were calculated by inputting calculated NCAs into OLINDA/EXM. Absorbed doses to the 24 organs specified by the MIRD scheme in the target organs of the Cristy–Eckerman adult hermaphrodite male and female phantoms were determined, and sex-specific absorbed doses were averaged. Gonad-absorbed dose was taken to be the mean of the absorbed doses to the testes and ovaries. Absorbed doses to the salivary glands, which are not assigned organs in the OLINDA/EXM scheme, were also investigated. The submandibular and parotid glands were assigned a mass of 9.9 and 18 g, respectively, with a density of 1 g/cm³ (values previously used for salivary gland dosimetric purposes (22)). NCAs for each gland from imaging were inputted into the Spheres module of the OLINDA/EXM software, and a curve of the form was fitted to the resulting dose (mGy/MBq) to mass (g) relationship to determine the dose at the mass of the gland under investigation.