Quality control of positron emission tomography radiopharmaceuticals: An institutional experience.

PURPOSE OF THE STUDY: To study quality control parameters of routinely prepared positron emission tomography (PET) radiopharmaceuticals.
MATERIALS AND METHODS: Three PET radiopharmaceuticals fluorine-18 fluorodeoxyglucose (F-18 FDG), N-13 ammonia (N-13 NH3), and Ga-68 DOTATATE (n = 25 each), prepared by standardized protocols were used. The radionuclide purity, radiochemical purity, residual solvents, pH, endotoxins, and sterility of these radiopharmaceuticals were determined.
RESULTS: The physical half-life of radionuclide in radiopharmaceuticals, determined by both graphical and formula method, demonstrated purity of radionuclides used. pH of all PET radiopharmaceuticals used was in the range of 5-6.5. No microbial growth was observed in radiopharmaceutical preparations. The residual solvents, chemical impurity, and pyrogens were within the permissible limits.
CONCLUSIONS: All three PET radiopharmaceuticals were safe for intravenous administration.

INTRODUCTION

The positron-emitting radiopharmaceuticals are of particular interest due to the high sensitivity and excellent resolution of the positron emission tomography (PET). These radiopharmaceuticals are formulated from cyclotron-produced or generator-eluted radionuclides and should be subjected to quality control (QC) tests to assure their safety and efficacy before injecting to the patient. Fluorine-18 fluorodeoxyglucose (F-18 FDG), a glucose analog, is the most popular PET radiopharmaceutical. Apart from F-18 FDG, few other radionuclides such as C-11, N-13, O-15, Ga-68, I-124, and Cu-64, and so on are also being used for PET imaging.

F-18 FDG can be synthesized by electrophilic or nucleophilic fluorination reaction. Nucleophilic fluorination is more popular because of higher yield and shorter synthesis time. However, catalysts like kryptofix (Kryptofix 2.2.2.) or tetrabutylammonium (TBA) salts are required for F-18 FDG synthesis.[12] The QC of F-18 FDG has been reported earlier.[34]

Ga-68 is now available as an eluate from 68Ge/68Ga column generator. Various generator systems have been studied.[56] Recently developed “ionic” 68Ge/68Ga generators provide Ga+3 for labeling of molecules.[78] Somatostatin peptide analogues like DOTATATE, DOTANOC, and DOTATOC have been labeled with 68Ga for somatostatin receptor imaging.

N-13 is a short-lived PET radionuclide (T1/2 = 10 min) and is produced in cyclotron. Several methods of N-13 NH3 synthesis have been reported.[91011] The proton irradiation of a natural water target 16O (p, α) N13 is an efficient method. Oxo anions of nitrogen N-13 (N-13-NO-3 and N-13-NO-2) are produced from radiolytic oxidation. With the use of reducing agents such as Devarda's alloy/sodium hydroxide or titanium-chloride/hydroxide, these oxo-anions can be converted to N-13 NH3[1011] QC parameters of N-13 NH3 were studied by various methods.[1213]

The prepared radiopharmaceuticals are administered intravenously; therefore, QC is mandatory, but there is no global consensus in relation to required quality attributes. The three major standards – Ph.Int., Ph. Eur., and USP are available for F-18 FDG.[141516] However, there are differences in F-18FDG quality requirements among USP, EP, and Int Phar.

In this study, the QC parameters of three PET radiopharmaceuticals, F-18 FDG, N-13 NH3, and Ga-68 DOTATATE are studied.

MATERIALS AND METHODS

PET radiopharmaceuticals, thin layer chromatography (TLC) scanner (EZ-SCAN, USA) with multimode radiation detector, (OMNI-RAD) gas chromatography (GC) (Varian 3900) with flame ionization detector (FID), multichannel analyzer (Captus 3000, Capintec), well counter, dose calibrator (Capintec CRC-25PET), Silica gel chromatography paper, pH Paper (Merck), LAL reagents from Charles River Laboratories India Pvt. Ltd.

The F-18 was produced by 18O (p, n) 18F reaction using 16.5 MeV on-site cyclotron (PET TRACE-4, GE Healthcare, Milwaukee, USA) by proton irradiation for 45 min at 40μA current using O-18 enriched water. At the end of bombardment, the fluoride was transferred directly to the automatic synthesizer module (TRACER lab MXFDG, GE Healthcare, Milwaukee, USA) by helium pressure. The ready to use sterilized cassettes and a “reagent kit” (ABX, Germany) that contained all chemicals required for the nucleophilic radiosynthesis were used. The cassette was opened and fitted 15 before the start of the synthesis. The automatic synthesis was achieved in 26 min by nucleophilic substitution using mannose triflate. The labeled precursor was hydrolyzed under basic medium to eliminate the protecting group and sterilized by 0.22 μm Millipore filter. The F-18 FDG (16 mL) was collected in a sterile glass vial and QC was performed.

Ga-68 DOTATATE was prepared using Ge-68/Ga-68 generator and IQs® Fluidic Labeling Module with in-built heater, both from ITG Germany. Ethanol (30%) was used for elution of prepared peptide (Ga-68 DOTATATE) from C-18 cartridge. Eluted product was passed through a 0.22 μm low protein binding filter and collected in a sterile vial.

N-13NH3 was produced by 16O (p, α) 13N irradiation reaction using on-site cyclotron. Disposable cassette was used in the production of N-13 NH3. Oxoanions of nitrogen (N-13 NO-3, and N-13 NO-2) were produced from radiolytic oxidation. Oxoanions were converted to N-13 NH3 by the use of Devarda's alloy, a reducing agent, in sodium hydroxide. N-13 NH3 vapors were collected in normal saline in a sterilized vial after passing through 0.22 μm filter.

pH

A total of 25 samples each of F-18 FDG and Ga-68 DOTATATE and N-13 NH3 were randomly selected. pH was assessed using a narrow range pH paper. A drop of test sample was placed on a pH strip. Developed color intensity was matched with given reference colors.

Radionuclide purity

Radionuclide purity was assessed by half-life of radionuclides (T1/2) and was measured by two methods, well counter and formula method. Small aliquot of test samples were taken. The initial radioactivity (A0) and activity of test samples (F-18 FDG, Ga-68 DOTATATE) after every 10 min (A10) was recorded. T1/2 was calculated from the measured values as per the formula:

Where, T and t are in minutes.

Counts/activity of F-18 FDG sample at 10 s interval was also takeical">n to determine the impurity of N-13 in F-18 FDG. In case of N-13 NH3, the counts were taken after every 2 min.

A time activity curve was plotted on semilog graph and half-life (T) was takeical">n as the time at which the total activity was decayed to half.

Gamma ray spectra

For a gamma spectrum of PET radionuclides, a point source was prepared and placed in well counter. The spectrum was acquired using multichannel analyzer with open window. A repeat spectrum of Ga-68 point source was also obtained after 24 and 48 h to determine the Ge-68 breakthrough. The visualization of peak corresponding to Ga-68 was taken as Ge-68 breakthrough.

Radiochemical purity

Radiochemical purity was assessed by silica gel instant thin layer chromatography (SG-ITLC). A drop (4-5 μL) of test sample was placed on a SG-ITLC strip (10*1 cm for F-18 FDG, 8*1 for Ga-68DOTATATE and N-13 NH3) and air dried. Acetonitrile: Water (95:5, v/v) and sodium citrate was used as mobile phase for F-18 FDG and Ga-68 DOTATATE, respectively. Water: acetone: acetic acid (3:2:1) saturated with sodium chloride (NaCl) was used as mobile phase for N-13-NH3. The radioactivity profile and Rf were recorded by TLC radioactivity scanner.

Residual solvent

The amount of organic solvents present in the test samples was estimated by GC using FID detector. The temperature of detector, oven, and injector were kept constant for all PET radiopharmaceuticals. Hydrogen, air flow, and nitrogen flow rate were also kept constant for all samples. Reference standard of ethanol (4000 ppm) and acetonitrile, (400 ppm) and samples (2-3 μL) were injected into the GC column and an analysis report was generated. The area under the peak of standard and test samples was noted. The concentration of ethanol and acetonitrile in the sample was calculated by the following formula:

where C1 and C2 are the concentration of standard and sample.

A1 and A2 are the area under the curve of standard and sample.

Chemical purity

The chemical purity was calculated for F-18 FDG as kryptofix 2.2.2 (amino polyether) may be present in the final product of F-18 FDG. The permissible limit for kryptofix 2.2.2 is ≤0.22 mg/mL. A total of 4-5μL of F-18 FDG test sample and the reference standard of kryptofix 2.2.2 (0.22 mg/mL) were developed in the mobile phase composed of methanol: Ammonia (9:1, V/V). The developed strips, after drying, were exposed to iodine vapors in a closed container of Iodine crystals. The color intensity of test samples was compared with the color of reference standard.

Sterility

All the samples were checked for sterility in the department of microbiology. The samples were decayed to negligible radioactivity level and sent to microbiology laboratory. Robertson Cooked Meat Medium broth (RCMB) was used for cultivation of aerobes and anaerobes, especially pathogenic clostridia. Small inoculation was made near the bottom of the tube and incubated at 37°C for 7 days. The samples were observed every day for 7 days to observe the turbidity or bubble formation by organisms.

Bacterial endotoxin test

The widely used and accepted test for assessing the presence of bacterial endotoxin in a radiopharmaceutical preparation is limulus amebocyte lysate (LAL) test. We followed the gel-clot technique. The estimation of bacterial endotoxin test is based on the formation of gel clot in the presence of endotoxin. This test was performed in randomly selected samples for each PET radiopharmaceutical (n = 25) in a thermally regulated and vibration-free environment. The gel formation was inspected manually. The permissible limit of endotoxin in injectable formulation is 175 EU/V according to USP and per mLaccording to Ph. Eur. LAL test can detect even 0.125 EU of endotoxin concentration. The endotoxin concentration in positive water control (PWC) was 0.25 and 0.5 EU/ mL. Each sample was diluted at two different maximum valid dilutions (MVDs, i.e., 350 and 700 times to match with the above-mentioned dilutions of endotoxin. LAL reagent water was used as negative control. After adding reagents in appropriate volume and concentration according to the protocol, the test vials were incubated for 60 min at 37° ± 1°C and were inverted to observe the formation of gel. The gel formation indicated the presence of endotoxins.

RESULTS

The product of F-18 FDG, Ga-68 DOTATATE, and N-13 NH3, were clear, colorless, and free from particulate matter. The pH of all the samples of F-18 FDG, N-13 NH3, and Ga-68 DOTATATE was in the range of 5.0-6.5.

The half-life of radionuclide of F-18 FDG, N-13 NH3, and Ga-68 DOTATATE samples with graph were 110, 10, and 69 min, respectively [Figures 1a–c]. The calculated T1/2 (mean) from the measured values using the above-mentioned formula was 115.5, 10.8, and 67.5 min, respectively. Single peak corresponding to photon energy of 511 keV was observed in the gamma ray spectrum of F-18 FDG and N-13 NH3 [Figures 2a and b]. In gamma ray spectra of Ga-68, one prominent peak corresponding to photon energy of 511 keV and a small sum peak corresponding to 1022 keV were observed [Figure 3a]. No peak was observed when the spectrum was recorded with Ga-68 point source after 24 and 48 h [Figure 3b]. Based on half-life measurement and gamma spectra, the radionuclide purity was almost 100%. The retardation factor (Rf) of F-18 FDG, Ga-68 DOTATAE, and N-13 NH3 were in the range of 0.39-0.48, 0.16-0.23, and 0.64-0.73 [Figures 4a–c]. Single peaks were observed in each sample of all three PET radiopharmaceuticals used which demonstrated the radiochemical purity more than 99%.

Figure 1

Graphical demonstration of time-activity curve on semilogthermic graph for determination of T1/2 of (a) F-18, (b) N-13, and (c) Ga-68

Figure 2

Spectra obtained by using MCA (a) F-18 FDG (b) N-13 NH3

Figure 3

Spectra of Ga-68 obtained by using MCA (a) After elution, (b) 24 h after elution

Figure 4

Instant thin layer chromatograms of (a) fluorine-18 fluorodeoxyglucose, (b) Ga-68 DOTATATE, and (c) N-13 NH3

The concentration of ethanol and acetonitrile of F-18 FDG sample was in the range of 20-1695ppm and 3-57 ppm, respectively [Figure 5a,b]. However, the concentration of ethanol in Ga-68 DOTATATE was in the range of 294-1147 ppm [Figure 6a,b]. GC was not performed for N-13 NH3 samples as organic solvents were neither used nor formed during the synthesis of N-13 NH3. The violet-brown color was developed in a standard Kryptofix strip when exposed to iodine vapors. However, no color band was visible when F-18 FDG sample strips were exposed to iodine vapors [Figure 7].

Figure 5

Gas chromatograms of (a) ethanol and acetonitrile standard and (b) fluorine-18 fluorodeoxyglucosesample

Figure 6

Gas chromatograms of (a) ethanol standard and (b) Ga-68 DOTATATE sample

Figure 7

Showing chromatograms of (a) Kryptofix standard (0.22 mg/ml), (b) F-18 FDG samples, developed in methanol: ammonia (9:1, V/V) and exposed to iodine vapors; (c) Iodine chamber

No microbial growth was observed in the samples of F-18 FDG, N-13 NH3, and Ga-68 DOTATATE. It showed that the samples were free from microorganisms and were safe for injection. Gel formation was not observed in the samples of F-18 FDG, N-13 NH3, Ga-68 DOTATATE, and in negative water control. However, gel was formed in PWC and positive product control at an endotoxin concentration of 0.25 and 0.50 EU/mL.

All the results are summarized in Table 1.

Table 1

Summary of quality control parameters of positron emission tomographyradiopharmaceuticals

DISCUSSION

The visual inspection per batch of in-house synthesized PET radiopharmaceutical is important and mandatory prior to use in the patient and is also a measure of process performance and validation. Presence of particulate matter in a test sample indicates possible failure at some stage of radiopharmaceutical synthesis. This may include the failure of the purification, sterilizing filtration, and inadequate environmental control during assembly of reagents in the preinstallation stage. However, a slight yellow color in the F-18 FDG preparation is acceptable according to Ph. Int.[16]

Due to short half-life of PET radionuclides, there is a practical need to release the product as soon as possible. A precisely measured count at two points within a 10 min interval is sufficient to determine the physical half-life of F-18 and Ga-68. However, the counts were also taken at 10 min interval up to 2 h and half-life was calculated at which the activity/counts were reduced to half. In our tests, the gamma spectrum of a test sample demonstrated a major peak at 511 keV and a sum peak at 1022 keV. But the presence of a 511or 1022 keV peak in the g ray spectrum is not sufficient to determine radionuclidic identity. Impurities such as 13N (arising from 16O impurity in the 18O-water) or other positron emitters may not be detected, as gamma peaks with an energy of 511 and 1022 keV sum peak are a common feature of positron emitters. A combination of gamma spectrum and the half-life measurement provides the best assurance of the identity and purity of the radionuclide. The measured half-life will be lower if N-13 impurity is present in FDG. As the half-life of N-13 is only 10 min therefore, activity as a measure of counts at 2 min interval is important.

The Ga-68 sample prepared for gamma spectra was kept for 48 h in order to decay all Ga-68 present to a level that permits the detection of long lived Ge-68 impurity. The gamma photons resulting from the decay of Ge-68 will have energy of 511 keV and a sum peak of 1022 keV may also be observed. In our study, we could not find any energy peak at 24 and 48 h. These results demonstrated that Ge-68 is not present at detectable range. The availability of nonmetallic silica based column generator eliminates the additional purification step for removal of metallic ions by cation exchange column. This also improves the radiochemical yield of Ga-68 DOTATATE.

Only single peak was observed in TLC scans of F-18 FDG, Ga-68 DOTATATE, and N-13 NH3. The Rf of the principal peak corresponded with the studied radiopharmaceuticals [Table 1]. More than 99% of radioactivity corresponds to the respective PET radiopharmaceuticals. This test was completed on per batch basis before injection to the patient. Radio-TLC provides an easy and reliable means to determine radiochemical identity and purity of radiopharmaceuticals. Narrow range of Rf values of the principal spot in the test samples confirmed the radiochemical identity of F-18 FDG, Ga-68 DOTATATE, and N-13 NH3.

Radioactivity was measured in a dose calibrator after calibration with an appropriate reference standard, Cs-137 (662 keV). The total radioactivity, volume (MBq/mL or mCi/mL) and reference time of radiopharmaceuticals were specified on product vial label. On the basis of this information, radioactivity per patient was dispensed and exposure to the worker was also reduced.

The pH of F-18 FDG, Ga-68 DOTATATE, and N-13 NH3 fall within the range, that is, 4.5-8.5. This test was completed on every day/batch prior to patient use. The allowed range of acceptable pH for radiopharmaceuticals is broad; hence, pH meter was not used. A narrow range pH strip was used and the results were within the reference pH range. The pH check of N-13 NH3 is very important as the radio-labeled nitrates may be carried over during the reduction and distillation process. This may contribute to radiochemical impurities and increase the pH of N-13 NH3.

Acetonitrile/ethanol is used during synthesis for reagent preparations and conditioning of the purification cartridges. Traces of these organic solvents may potentially contaminate F-18 FDG and Ga-68 DOTATATE and therefore should be controlled. The permissible limit of acetonitrile is 400 (0.04%) and of ethanol is 4000 ppm (0.4%). These impurities may be present in F-18 FDG samples. Our results indicated presence of these residual solvents within the permissible limit. Ga-68 DOTATATE was eluted in ethanol, but the concentration of ethanol in random samples was within the permissible limit [Table 1]. Organic solvent was not used in the preparation protocol of N-13 NH3 synthesis and thus there was no requirement of GC. No color band/spot was visualized in the chromatogram of F-18 FDG when exposed to iodine vapors after developed in Methanol: Ammonia (1:1). These results demonstrated that kryptofix was not present at detectable range in F-18 FDG samples. Kryptofix, a cyclic crown ether, acts as catalyst and binds the potassium ion to prevent the formation of F-18 KF, therefore, enhances the reactivity of (F-18) ion. Kryptofix may cause apnea and convulsions; therefore, it must be within appropriate limits (0.22 mg/mL) and conformance with these limits is to be demonstrated. Currently available automatic synthesis modules have multiple removal steps to reduce the amount of kryptofix to negligible level.

We have used RCMB as it has the ability to initiate growth of bacteria from very small inocula and also maintain the viability of cultures for longer duration. Mixed cultures of bacteria also survive in RCMB without displacing the slow growing organisms. Aerobes grow at the top and the anaerobes grow deeper in the medium. Moreover, RCMB is routinely used in the department of microbiology, so there was no need for growing reference bacteria. The bacterial endotoxin test was performed in all randomly selected samples. The tests were performed on undiluted samples but done quantitatively to ensure accurate results. The gel was not formed in any of the samples indicated the absence of endotoxin in F-18 FDG, Ga-68 DOTATATE, and N-13 NH3 test samples. All positive controls demonstrated the gel formation even with the endotoxin concentration of 87.5EU/mL. However, negative control and samples did not show any gel formation.

F-18 FDG, Ga-68 DOTATATE, and N-13 NH3 comply with requirements for intravenous administrations. Additionally, no adverse effects were observed or reported after intravenous injection to patients. F-18 FDG preparations fulfill all three major standards available for F-18 FDG.[141516] Ga-68 DOTATATE also fulfills the standard available for Ga-68 labeled somatostatin peptides.

CONCLUSION

F-18 FDG, Ga-68 DOTATATE, and N-13 NH3 conformed to various quality attributes of purity; efficacy and safety in the samples included in this study and comply with all requirements for injectable radiopharmaceutical products.