Synthesis, quality control and dosimetry of the radiopharmaceutical 18F-sodium fluoride produced at the Center for Development of Nuclear Technology - CDTN

ARTICLE

Synthesis, quality control and dosimetry of the radiopharmaceutical ¹⁸F-sodium fluoride produced at the Center for Development of Nuclear Technology - CDTN

Marina Bicalho Silveira^I; Marcella Araugio Soares^I; Eduardo Sarmento Valente^I; Samira Soares Waquil^I; Andréa Vidal Ferreira^II; Raquel Gouvêa dos Santos^I; Juliana Batista da Silva^I,*

^IServiço de Aplicação das Radiações na Saúde, Centro de Desenvolvimento da Tecnologia Nuclear/Comissão Nacional Energia Nuclear CDTN/CNEN

^IIServiço de Reator e Irradiações, Centro de Desenvolvimento da Tecnologia Nuclear/Comissão Nacional Energia Nuclear CDTN/CNEN

ABSTRACT

¹⁸F-Sodium fluoride (Na¹⁸F) is a radiopharmaceutical used for diagnosis in nuclear medicine by positron emission tomography (PET) imaging. Bone scintigraphy is normally performed using ^99mTc-MDP. However, ¹⁸F PET scans promise high quality imaging with increased resolution and improved sensitivity and specificity. In order to make available a tool for more specific studies of tumors and non-oncological diseases of bone tissue, the UPPR/CDTN team undertook the production and quality control of Na¹⁸F injectable solution with the physical-chemical, microbiological and biological characteristics recommended in the U.S. Pharmacopeia. Na¹⁸F radiochemical purity was 96.7 ± 1.3 %, with Rf= 0.026 ± 0.006. The product presented a pH of 5.3 ± 0.6, half life of 109.0 ± 0.8 minutes, endotoxin limit < 5.0 EU.mL^-1 and no microbial contaminants. The biodistribution of Na¹⁸F was similar to that described in the literature, with a clearance of 0.19 mL.min^-1 and distribution volume of 18.76 mL. The highest bone concentration (5.0 ± 0.5 %ID.g^-1) was observed 20 minutes after injection. Na¹⁸F produced at the UPPR presented all the quality assurance requirements of the U.S. Pharmacopeia and can be safely used for clinical bone imaging.

Uniterms: Radiopharmaceuticals/biodistribution. Sodium fluoride ¹⁸F/production. Sodium fluoride ¹⁸F/quality control. Sodium fluoride. ¹⁸F/dosimetry. Bone imaging.

RESUMO

O Fluoreto de sódio ¹⁸F (Na¹⁸F) é um radiofármaco empregado para diagnóstico através da Tomografia por Emissão de Pósitrons (PET). Cintilografias ósseas são normalmente obtidas utilizando-se ^99mTc-MDP. Entretanto, o interesse pelo Na¹⁸F é crescente, principalmente devido à obtanção de imagens de elevada resolução. Com o objetivo de tornar disponível uma ferramenta mais específica para estudos de tumores e doenças não-oncológicas do tecido ósseo, o grupo da UPPR/CDTN implementou a produção e o controle de qualidade da solução injetável de Na¹⁸F com as características físico-química, microbiológica e biológica preconizadas pela farmacopéia. Sua pureza radioquímica foi de 96,7 ± 1,3 %, com Rf= 0,026 ± 0,006. O produto apresentou pH igual a 5,3 ± 0,6, tempo de meia-vida de 109,0 ± 0,8 minutos, limite de endotoxinas < 5,0 EU.mL^-1 e ausência de microrganismos. O perfil de biodistribuição em camundongos foi semelhante ao disponível na literatura, com depuração igual a 0,19 mL.min^-1 e volume de distribuição igual a 18,76 mL. A concentração máxima (5,0 ± 0,5 % DI.g^-1) foi observada no osso 20 minutos após a injeção. O Na¹⁸F produzido na UPPR do CDTN apresentou os parâmetros de qualidade definidos na farmacopéia americana e pode ser usado com segurança para uso clínico em cintilografia óssea.

Unitermos: Radiofármacos/biodistribuição. Fluoreto de sódio. ¹⁸F/produção. Fluoreto de sódio. ¹⁸F/controle de qualidade. Fluoreto de sódio. ¹⁸F/dosimetria. Cintilografia óssea.

INTRODUCTION

¹⁸F-Sodium fluoride injection (Na¹⁸F), a positron-emitting radiopharmaceutical containing no carrier-added (NCA), is used for diagnostic purposes in conjunction with positron emission tomography (PET) imaging. This radiopharmaceutical became widely used for bone scintigraphy after its introduction by Blau et al. (1962) in the early 1960s and was approved for clinical use by the U.S. Food and Drug Administration in 1972. The use of ^99mTc-MDP has been brought into question given the advantages of PET imaging and the limited global supply of technetium-99m (^99mTc). Moreover, bone uptake of Na¹⁸F is 2-fold greater than that of ^99mTc-MDP and ¹⁸F-fluoride PET is more accurate than planar imaging or SPECT for localizing and characterizing bone lesions (Blau et al., 1962).

The radiopharmaceutical Na¹⁸F has the desirable pharmacokinetics properties of high and rapid bone uptake coupled with very rapid blood clearance, which results in a high bone-to-background ratio within a short time frame. After intravenous administration, ¹⁸F-fluoride diffuses through the bone capillaries into the bone's extracellular fluid (ECF). From the bone ECF, ¹⁸F-fluoride ions exchange with hydroxyl groups in the hydroxyapatite at the surface of bone crystals forming fluoroapatite mainly at sites of bone remodelling with high turnover. Therefore, uptake of ¹⁸F-fluoride reflects blood flow and osteoblastic activity (Grant et al., 2008).

¹⁸F-fluoride PET is a highly sensitive imaging modality for detection of benign and malignant osseous abnormalities and allows the regional characterization of lesions in metabolic bone diseases. Using hybrid PET/CT systems improves the specificity of ¹⁸F-fluoride PET in cancer patients by accurately differentiating between benign and malignant sites of uptake (Grant et al., 2008). The clinical usefulness of ¹⁸F-fluoride PET has been demonstrated for monitoring the response to therapy against a wide range of clinical indications such as osteoporosis, Paget's disease, compression fractures, marrow lesion, stress injuries and many other abnormal bone activities (Bridges et al., 2007).

Given the applicability of Na¹⁸F for bone imaging has been confirmed, our objective was to obtain a high quality product. The commercial delivery system used for ¹⁸F-FDG can also be used for the efficient delivery of other ¹⁸F-labeled radiopharmaceuticals, including Na¹⁸F. Microbiological, physical-chemical and biological quality control tests were performed, and the Na¹⁸F injectable solution met the quality control requirements contained in the United States Pharmacopeia (USP 31). It is now feasible to perform high-quality Na¹⁸F bone scans in most nuclear medicine departments, contributing to bone disorder and tumour diagnoses. This radiopharmaceutical is available for clinical use in PET/CT imaging.

All drug manufacture and quality control complied with Current Good Manufacturing Practice regulations described in the regulatory requirements and general guidance published by the Brazilian Regulatory Agency for Medicines (ANVISA).

MATERIALS AND METHODS

Na¹⁸F Production

¹⁸F-Fluoride was produced on a 16.5 MeV Cyclotron PETtrace^® (GE Healthcare) by the ¹⁸O (p,n)¹⁸F nuclear reaction. The niobium target (with yield of 215 mCi/μA_Sat) was filled with 2.2 mL of enriched O¹⁸ water, which was irradiated with protons for 10 minutes at an intensity of 25 µA. The solution containing ¹⁸F^- was transferred into an automatic synthesis module, TracerLab^® MX_FDG (GE Healthcare), modified and prepared with reagents kit and accessories for Na¹⁸F production (Figure1). ¹⁸F-fluoride ions were trapped in a SepPak Light Accell, Plus QMA, anion exchange column and were then eluted with NaCl 0.9% solution. Finally, the resulting 15 mL of Na¹⁸F was dispensed into sterile, pyrogen-free vials, through a 0.22 μm filter, in an automated dispensing unit (Theodorico, Comecer^®).

Physical-chemical quality control

The pH value of Na¹⁸F was measured using pH paper (0-14) which displayed a different colour depending on the pH range of the samples. The results were compared with standard pH buffer and the estimated value was registered.

Radiochemical identity and purity of Na¹⁸F were confirmed either by thin layer chromatography (TLC) according to Nandy et al. (2007) or using a high-performance liquid chromatography (HPLC) system.

The TLC stationary phase was silica gel and the mobile phase was acetonitrile : water (95:5% v/v). The radioactivity was determined by scanning the TLC SG plate with a suitable collimated radiation detector (Minigita star beta detector, Raytest^®)_. The main peak was analyzed to define the retention factor (Rf) value and the radioactivity related to the sample, that must lie between 0.0 - 0.12 and be no less than 95%, respectively.

HPLC analyses were performed on a chromatographic system (Agilent^®), connected in serie with a gamma detector (Raytest^®). The anion-exchange column (Shimadzu, IC.A1) (size: 46 mm x 10 cm) temperature was kept constant at 25 ºC. The flow rate was 1.0 mL.min^-1, the sample volume was 20 µL and the solvent system was NaOH 0.1M (pH = 4.0).

Radionuclidic purity was evaluated by gamma-ray spectrometry (Camberra Multichannel Analyzer). Half-life of ¹⁸F was calculated after measuring the radioactivity decay of the sample over a 20-minute period in a radioisotope dose calibrator (Capintec CRC^®-25R). The equation used is showed below:

Where: A₀= initial activity; A = activity measured after 20 minutes; t = time interval (in minutes) between the two measures (t_A-t_Ao); t_½ = half-life.

Microbiological quality control

Bacterial endotoxins were quantified by the chromogenic method, using an Endosafe® Portable Test System - PTS. This device includes a pumping system, a portable spectrophotometer and internal software to calculate sample data. Na¹⁸F samples in duplicate (previously diluted) were applied inside cartridges in parallel with positive control testing. The product was considered apyrogenic when the endotoxins level was no more than 11.6 EU.mL^-1

The sterility of the Na¹⁸F solution was assayed performing a test by direct inoculation of Na¹⁸F solution in trypticase soy broth and fluid thioglycollate medium. The test was performed in duplicate and a negative control test was also performed. The culture media were incubated at 25 ºC and 37 ºC, respectively, and verified daily over a fourteen-day period. The product was considered sterile when there was no evidence of microbiological growth.

Physiological quality control

The Na¹⁸F physiological quality control was done by biodistribution studies in animals. Female Swiss mice with a body weight of around 25 g purchased from the Animal House Center (CEBIO) of the Biological Sciences Institute (ICB/UFMG). Mice were maintained on a light-dark cycle (12-h light, 12-h dark) at a room temperature of 25 ºC and given food and water ad libitum. Animal experiments were performed in compliance with the Institute of Laboratory Animal Resources - Commission on Life Sciences - National Research Council, Washington, D.C. and the Brazilian society in science of Laboratory Animals (SBCAL).

Doses of 70 kBq of Na¹⁸F were injected by intravenous route into Swiss mice. After different time intervals (2.5 - 60 minutes), the animals were sacrificed. Blood samples were taken, and the thyroid, heart, lungs, liver, spleen, pancreas, kidneys, stomach, intestine, bone, muscle, brain and bladder removed and weighed. Their respective radioactivity was measured in an automatic gamma spectrometer (1480 Wizard 3''- Wallac - Counting efficiency for 0.511MeV: 48%). The biodistribution was evaluated after calculating the percentage uptake of injected dose per gram of organ (% DI.g^-1).

Pharmacokinetics parameters were estimated by the Biexp Pharmacokinetics program (Murphy,R., Inaoe tonantzindra, Puebla, México 1991).

Radiation dosimetry

The absorbed radiation dose for each mouse organ was calculated applying MIRD formalism (Snyder et al., 1978) to the biodistribution results, according to the relations:

Where: Ã_his the cumulated activity in the target organ; S(r_k←r_h) is the mean absorbed dose in the target organ per unit of cumulated activity in the source organ; D_i s the mean energy emitted; ϕ_i(r_k←r_h) is the absorbed energy fraction; m is the target tissue mass.

A value of ϕ = 1 (non-penetrating radiation) was adopted for the ¹⁸F positron emission radionuclide. The experimental results of the cumulated activity in the mice organs were extrapolated to humans, assuming a similar concentration ratio among various tissues between mouse and patient. This extrapolation converted mouse %ID.g^-1 data by adjusting organ mass difference between mouse and patient (Kirschner et al., 1975 and Shen et al., 2005):

Where: [A₀]_HO is the activity in the human organ; [A₀]_MO is the activity in the mouse organ; M_HO is the human organ mass; M_H is the human mass; m_M is the mouse mass; m_MO is the mouse organ mass.

This extrapolation was based on the organ masses of mouse used in the experiments and the adult human organ masses of the Cristy-Eckerman phantom (Cristy, Eckerman, 1987).

RESULTS AND DISCUSSION

Na¹⁸F production

Enriched water H₂¹⁸O was successfully irradiated on the Cyclotron. ¹⁸F^- was produced by the nuclear reaction ¹⁸0 (p,n) ¹⁸F, by irradiation with protons for 10 minutes at the intensity of 25 µA. Na¹⁸F was produced in high yields of 87.3 ± 6.1%. Samples of the final product were sent to the quality control laboratories in appropriately shielded containers.

Physical-chemical and microbiological quality control

Na¹⁸F physical-chemical and microbiological characteristics were evaluated. The analyses described earlier determined its radiochemical identity and purity, radionuclidic identity and purity, pH, bacterial endotoxins and sterility.

The quality requirements used for Na¹⁸F were in accordance with those found in the United States Pharmacopeia (USP 31), supplemented with literature data (Nandy et al.., 2007) (Table I).

Radiochemical purity of the Na¹⁸F solution was assessed by thin-layer chromatography (TLC). The product migration profile was determined by scanning the chromatogram plate with a suitable collimated radiation detector (the radiochromatogram is represented in Figure 2).

The main peak on the radiochromatogram obtained using HPLC for the Na¹⁸F test solution had approximately the same retention time (about 2 minutes) as the peak for ¹⁸F described in literature data (Olberg et al., 2008). More than 99% of the radioactivity was related to ¹⁸F peak (Figure 3).

The final product, analyzed by gamma spectrometry, presented high radionuclidic purity. The spectrum obtained (Figure 4) shows one main peak at 0.511 MeV, an usual energy level for a positron annihilation product.

Energy

The results of the tests performed on the PTS were in compliance with the requirements established by the USP 31, showing that the samples contained less than 5.0 EU.mL^-1.

All the samples assayed for sterility were observed for 14 days and none showed evidence of microbiological growth. Therefore, the Na¹⁸F solutions were considered sterile.

Biological quality control

The biological quality control of Na¹⁸F was carried out through biodistribution studies in Swiss female mice. 70 kBq of Na¹⁸F were injected in mice via the intravenous route and, after different time intervals (2.5 - 60 minutes), the animals were sacrificed and radiopharmaceutical distribution in the organs was calculated as percentage uptake of injected dose per gram of organ (%ID.g¹).

The biodistribution profile is shown in Figure 5 and the results indicated that the blood concentration of the radiopharmaceutical was quickly reduced in the first hour post injection (4.50 ± 0.35 % ID.mL^-1) (Figure 6a). ¹⁸F-Sodium fluoride presented biexponential blood clearance with half-life of 5.1 minutes in the fast distribution phase and 87 minutes in the slow elimination phase. The Na¹⁸F clearance was 0.19 mL.min^-1 and the distribution volume was 18.76 mL.

The Na¹⁸F kinetics in the main vital organs followed blood kinetics, and the radiopharmaceutical was rapidly cleared out. The only exception was bone kinetics, where the Na¹⁸F concentration increased during the time period, reaching a peak (5.0 ± 0.5 % ID.g^-1) 20 minutes after the injection (Figure 6b).

Radiation dosimetry

Dosimetric studies aimed to evaluate the safety of the ¹⁸F-Sodium fluoride produced in the UPPR/CDTN for bone scintigraphy. The results (Table II) demonstrated that the highest absorbed doses in the mice were in bladder (11.5 mGy.70 KBq^-1) and bone (7.1 mGy.70 KBq^-1). Absorbed doses for the other organs were significantly lower, ranging from 1.3 - 4.0 mGy.70 KBq^-1. The radiation doses extrapolated to patients indicated that radiation doses in bone and bladder were 13.6 and 21.9 mGy.370 MBq^-1, respectively. These results are quite similar to those published by ICPR53 and show that Na¹⁸F produced at the UPPR/CDTN can be safely used for clinical bone imaging.

CONCLUSIONS

The automated synthesis of Na¹⁸F and its availability was successfully accomplished using the commercial ¹⁸FDG synthesizer TracerLab^® MX_FDG. The final solution presented high quality in accordance with the requirements of the USP 31. The biodistribution of Na¹⁸F was similar to that described in the literature and absorbed doses in organs were lower than radioprotection limits.

Considering the clinical importance of ¹⁸F-Sodium fluoride and the high resolution PET images provided by it, the UPPR group wish to start Na¹⁸F production and make it widely available. This study showed that the product presents good quality and meet the needs of hospitals, clinics, and research facilities. The radiopharmaceutical Na¹⁸F was produced according to the regulatory requirements and general guidance of Good Manufacturing Practices, published by the Brazilian Regulatory Agency for Medicines (ANVISA).

ACKNOWLEDGEMENTS

The authors wish to thank the whole team at the UPPR/ CDTN for technical support on production and quality control of Na¹⁸F. In addition, the authors are grateful to PRO da Silva, LM Gabriel and FAS Vilas Boas for technical assistance on biodistribution experiment. This research was supported by Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN-MG), INCT de Medicina Molecular and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq).

Received for publication on 17^th September 2009

Accepted for publication on 05^th March 2010

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Publication Dates

• Publication in this collection
  12 Nov 2010
• Date of issue
  Sept 2010

History

• Accepted
  05 Mar 2010
• Received
  17 Sept 2009