A Practical Guide for the Production and PET/CT Imaging of 68Ga-DOTATATE for Neuroendocrine Tumors in Daily Clinical Practice

Summary

Well-differentiated neuroendocrine tumors overexpress somatostatin receptors which can be utilized for diagnostic imaging with the radiolabeled somatostatin analog ⁶⁸Ga-DOTATATE. This protocol details the radiolabeling of ⁶⁸Ga-DOTATATE, quality control, patient preparation, and subsequent PET/CT imaging. Radiation safety and time constrictions due to the short half-life of ⁶⁸Ga are taken into account.

Abstract

Neuroendocrine tumors are a rare form of cancer that arise from neuroendocrine cells and can be present at almost any location throughout the body. Although heterogeneous in presentation, a common denominator among these tumors is the overexpression of somatostatin receptors. ⁶⁸Ga-DOTATATE is a somatostatin analog labeled with the positron emitter gallium-68 (⁶⁸Ga). For well-differentiated neuroendocrine tumors, ⁶⁸Ga-DOTATATE positron emission tomography (PET)/computed tomography (CT) imaging is used for diagnosis, determination of disease burden, and therapy selection.

This protocol details the radiolabeling of ⁶⁸Ga-DOTATATE, quality control, patient preparation, and subsequent PET/CT imaging. Radiolabeling of ⁶⁸Ga-DOTATATE is performed with a fully automated labeling module coupled to a germanium-68 (⁶⁸Ge)/⁶⁸Ga generator. Quality control of the final product evaluates radiochemical purity with instant thin-layer chromatography and solid-phase chromatography, and pH prior to patient injection. Periodic quality control is performed to determine ⁶⁸Ge breakthrough, sterility, and (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) content. Patient preparation includes patient instructions, a protocol for ⁶⁸Ga-DOTATATE during treatment with somatostatin analogs, and intravenous administration of the radiopharmaceutical. For PET/CT imaging, the acquisition and reconstruction settings are described. For each step, radiation safety will be highlighted, as well as time constrictions due to the short half-life of ⁶⁸Ga.

Fully automated in-house production and quality control of ⁶⁸Ga-DOTATATE leads to very high success rates (95%) and produces two to four patient dosages per batch, depending on the yield of the generator. In conclusion, ⁶⁸Ga-DOTATATE PET/CT imaging is a noninvasive and fast method of providing information on the tumor burden of neuroendocrine tumors (NETs) while also assisting in diagnosis and therapy selection.

Introduction

NETs are a heterogeneous group of tumors that arises from neuroendocrine cells. They can occur at almost any location in the body but are most common in the gastrointestinal tract, pancreas, and lung¹. Although NETs are a rare disease, their incidence in the United States has risen from 1.09 per 100,000 people in 1973 to 6.98 per 100,000 people in 2012². For an accurate diagnosis and staging of a NET, ⁶⁸Ga-DOTATATE PET/CT is the standard of care. This protocol describes the production and quality control of ⁶⁸Ga-DOTATATE, as well as patient preparation and the acquisition of PET/CT images.

Well-differentiated NETs are characterized by an overexpression of somatostatin receptors¹. Somatostatin analogs that bind to this receptor can be labeled with a radioactive isotope to allow for nuclear medicine imaging. At first, iodine-123 was used with gamma camera imaging, which was soon replaced by indium-111 (¹¹¹In)^3,4. ¹¹¹In-octreotide scintigraphy was the golden standard for nuclear medicine NET imaging for over a decade⁵. Meanwhile, technical advances were made in PET, which has a higher sensitivity and resolution than gamma camera imaging. For NETs, somatostatin analogs coupled to the positron emitter ⁶⁸Ga, such as ⁶⁸Ga-DOTATATE, were developed⁶.

⁶⁸Ga-somatostatin receptor (⁶⁸Ga-SRS) PET/CT is the current modality of choice in nuclear medicine imaging of well-differentiated NETS. The superiority of ⁶⁸Ga-SRS PET/CT over ¹¹¹In-octreotide has been demonstrated in several studies^7,8. The reported sensitivity and specificity lie around 90%-95% and 85%-100%, respectively^9,10. A meta-analysis has shown that ⁶⁸Ga-SRS PET/CT leads to a change in management in 44% of cases, even if preceded by ¹¹¹In-octreotide scintigraphy¹¹. In guidelines, ⁶⁸Ga-SRS PET/CT is now recommended over ¹¹¹In-octreotide scintigraphy for NET imaging, and it is also approved by the Food and Drug Administration and European Medicines Agency¹². A guideline for tumor imaging with ⁶⁸Ga-conjugated peptides is also available¹³.

This protocol details the radiolabeling of ⁶⁸Ga-DOTATATE (conform the quality control requirements of the European Pharmacopoeia¹⁴), patient preparation, and subsequent PET/CT imaging. Radiation safety and time constrictions due to the short half-life of ⁶⁸Ga are taken into account.

Protocol

1. General radiation and radiopharmaceutical safety

1. Ensure that radioactive materials are only worked with and handled by trained personnel. The dose to hospital staff, patients, and everyone else present should always be kept as low as reasonably achievable (ALARA).
2. Regarding the preparation of radiopharmaceuticals, adhere to national laws, regulations, and guidelines, such as Good Manufacturing Practice (GMP).
   CAUTION: The following protocol is for the ⁶⁸Ga-DOTATATE PET/CT imaging of adults only and is not suitable for children or pregnant women.

2. Preparations required prior to the labeling of ⁶⁸Ga-DOTATATE

1. Elute the ⁶⁸Ge/⁶⁸Ga generator with hydrochloric acid (HCl) according to the manufacturer’s specifications, between 4 and 24 h prior to the start of labeling ⁶⁸Ga-DOTATATE.

3. Labeling of ⁶⁸Ga-DOTATATE

NOTE: The preparation for and labeling of ⁶⁸Ga-DOTATATE takes 90 min and should be started 2 h prior to patient administration, to allow for quality control. The labeling module should be placed in a lead shielding that can be closed during the labeling process to ensure radiation protection of personnel. If a registered kit is used, then the summary of the product characteristics (SMPC) must be followed or cross-validated locally with the presented protocol.

1. Place the ⁶⁸Ga labeling kit on the labeling module according to the manufacturer’s specifications. Place the three manifolds on the corresponding module units. Attach the solutions provided in the ⁶⁸Ga labeling kit to the manifolds.
2. Prepare the final vial in a sterile environment, such as a downflow unit or laminar flow cabinet.
3. Place a nonvented 0.22 µm filter underneath a vented 0.22 µm filter and attach the nonvented filter to a sterile needle (20 G). Place the needle with the two filters attached in a 30 mL sterile vial. Place a vented 0.2 µm bent filter with a needle (22 G) in the same sterile vial as per step 3.2 to allow venting.
4. Attach the sterile vial with the nonvented filter to the output of the labeling module and place the vial in the lead shielding sufficient for positron emitters.
5. Attach the output of the ⁶⁸Ga/⁶⁸Ge generator to the input of the labeling module.
6. Dissolve 50 µg of HA-DOTATATE (DOTA-3-iodo-Tyr3-octreotate) or 20 µg of DOTATATE (DOTA-0-Tyr3-octreotate) peptide in 1.5 mL of 1.5 M HEPES buffer solution provided in the kit and place it in the reaction vial.
7. Close the lead shielding around the labeling module and start the production of ⁶⁸Ga-DOTATATE via the tablet computer attached to the labeling module.
8. Wait until the synthesis of ⁶⁸Ga-DOTATATE is finished (~36 min).
9. After labeling, remove the needles with filters from the glass vial and close the lead shielding around the vial.
10. Test the integrity of the nonvented 0.22 µm filter as follows.
    1. Fill a syringe (10 mL) with air and place the syringe on top of the filter. Place the needle attached to the filter in a tube filled with water.
    2. Force the air through the filter and needle and determine when bubbles begin to form. The air should be compressed to <20% of the original volume.
11. Measure the activity of ⁶⁸Ga-DOTATATE produced by placing the vial in a dose calibrator and note the activity reference time (ART).
12. In a sterile environment such as a laminar flow cabinet, remove 0.5 mL of ⁶⁸Ga-DOTATATE from the vial for quality control and prepare syringes for patient administration.
    NOTE: Place the solution with ⁶⁸Ga-DOTATATE in lead shielding. In this protocol, the diluted solution does not have to be shielded due to the low levels of activity and short exposure time. However, a radiation risk assessment should be performed prior to performing quality control of the ⁶⁸Ga-DOTATATE.

4. Quality control of ⁶⁸Ga-DOTATATE prior to patient administration

NOTE: The quality control of ⁶⁸Ga-DOTATATE takes 30 min and should be started 30 min prior to patient administration. The described dilutions for stock solutions lead to <5% dead time in the measurement equipment. This can vary between different equipment and should be tested prior to performing quality control of the ⁶⁸Ga-DOTATATE. The European Pharmacopoeia describes the quality control of gallium edotreotide injections based on the following release criteria: appearance = clear and colorless; pH = 4.0–8.0; sterility; endotoxins <175 IU per administered volume; ethanol <10% v/v; radionuclide purity >99.9% of total activity; radiochemical purity >91% of total activity; absence of other impurities; HEPES <200 µg per administered volume¹⁴. All tests have been evaluated during the validation of the preparation method. For routine quality control, a selected subset of tests (based on trend monitoring) is performed and described below. The solid-phase extraction in this protocol has been cross-validated with and obtains the same results as the high-performance liquid chromatography method described in the European Pharmacopoeia. This was performed based on GMP.

1. Prepare the following solutions in advance.
   1. Prepare a 1 M ammonium acetate solution by dissolving 3.9 g of ammonium acetate in 50 mL of water. The solution can be stored at room temperature for up to 2 weeks.
   2. Prepare 5 mM ethylenediaminetetraacetic acid (EDTA) by dissolving 0.1 g of EDTA in 50 mL of water. The solution can be stored at room temperature for up to 1 year.
   3. Prepare 50:50 methanol:1 M ammonium acetate. The solution can be stored at room temperature for up to 24 h.
2. Visually inspect the final ⁶⁸Ga-DOTATATE product to ensure that it is a colorless liquid without any particles.
3. Measure the pH of the ⁶⁸Ga-DOTATATE solution with a pH indicator strip. The pH should lie between 6.5 and 7.5.
4. Measure ⁶⁸Ga colloids through instant thin-layer chromatography (ITLC).
   1. Add 500 µL of water and 20 µL of ⁶⁸Ga-DOTATATE to prepare a stock solution and homogenize (⁶⁸Ga colloids [GC]) by carefully shaking the vial.
   2. Cut a strip of ITLC- silica gel (SG) glass fiber paper of at least 7 cm long and 1 cm wide.
      NOTE: Only use clean ITLC-SG paper without damages. If the paper is damaged, the components traveling with the solvent can be hindered and the results will be inaccurate.
   3. Add 5 µL of GC 1.5 cm from the bottom of the ITLC-SG paper and place it in a tube containing 2 mL of 50:50 methanol:1 M ammonium acetate. Ensure that the ⁶⁸Ga-DOTATATE does not come into contact with the liquid. Close the tubes to prevent evaporation.
   4. Wait several minutes until the solvent has traveled at least 5 cm above where the ⁶⁸Ga-DOTATATE was applied. Cut the paper in half and place the bottom and upper halves in separate tubes (BH1 and UH1).
   5. Place the UH1 and BH1 tubes in a well counter and determine the number of counts in each vial in the 400–600 keV energy window for 30 s, to determine the colloid percentage (see the calculations in steps 4.4.7–4.4.9).
   6. Repeat steps 4.4.2–4.4.5 to acquire BH2 and UH2..
   7. Perform a background measurement of an empty well counter and determine the number of counts in the 400–600 keV energy window for 30 s.
   8. Correct the counts in the UH1, UH2, BH1 and BH2 for decay and background (determined in step 4.4.7) to obtain UH1cor, UH2cor, BH1cor, BH2cor. Δt is the time difference between the measurement of the sample and the ART (determined in step 3.11) in minutes.
      
   9. Calculate the number of ⁶⁸Ga colloids with the following formula; note that this should be less than 3%. UH1cor, UH2cor, BH1cor, BH2cor are the corrected values obtained in step 4.4.8.
      
5. Determine the ⁶⁸Ga ions through column separation.
   1. Make a stock solution by diluting 20 µL of ⁶⁸Ga-DOTATATE in 1 mL of 5 mM EDTA.
   2. Prepare a C-18 cartridge by slowly flushing it with 1 mL of 100% ethanol, followed by 1 mL of sterile water.
   3. Prepare a sample (S) by diluting 10 µL of the stock solution in 1 mL of sterile water, placing it in a well counter, and determining the number of counts in the 400–600 keV energy window for 30 s.
   4. Flush the sample slowly (5–10 mL/min) through the C-18 cartridge with a syringe and collect the remaining solution (C). Rinse the sample tube with 1 mL of water and flush this through the column in the collection tube.
   5. Place the collecting tube (C), the empty sample tube (E), and the syringe (Sy) in a well counter and determine the number of counts in each of them in the 400–600 keV energy window for 30 s. Use this to estimate the ion percentage (see the calculations in steps 4.5.7 and 4.5.8).
   6. Repeat steps 4.5.2‒4.5.5.
   7. Correct the counts for decay and background (determined in step 4.4.7) in C, S, E, and Sy to determine C’, S’, E’ and Sy’, to determine the number of counts at the ART with the following formula, in which Δt is the time difference between the measured sample and the ART in minutes.
      
   8. Calculate the ⁶⁸Ga ion percentage with the following formula; note that this should be less than 2%.
      
6. Determine the radiopharmaceutical purity by calculating the total amount of ⁶⁸Ga-DOTATATE with the following formula, which should be at least 91%.
   

5. Periodical quality control of ⁶⁸Ga-DOTATATE after patient administration

NOTE: This should be performed >48 h after the preparation of ⁶⁸Ga-DOTATATE to allow for the decay of ⁶⁸Ga.

1. Prepare the following solutions in advance.
   1. Prepare HEPES reference solution by dissolving 20 mg of HEPES in 50 mL of sterile water. The solution can be stored at room temperature for up to 6 months.
      NOTE: This is based on the maximum recommended HEPES dose of 200 µg per administered volume.
   2. Prepare 25:75 v/v water:acetonitrile.
2. Determine HEPES concentration in the final product (weekly).
   1. Transfer 3 µL of ⁶⁸Ga-DOTATATE solution in steps of 1 µL to an ITLC-SG F₂₅₄ paper of at least 8 cm length. Use a blow dryer to dry the paper in between the 1 µL applications.
   2. Repeat step 5.2.1 with the HEPES reference solution.
   3. Place the strips in a solvent of 25:75 water:acetonitrile. Ensure that the applied solutions do not come into contact with the liquid.
   4. Wait several minutes until the solvent has traveled to at least 2/3 of the paper length.
   5. Develop the paper for at least 4 min in a closed chamber with iodine crystals.
   6. Visually assess the outcome; a yellow spot should appear. The spot on the paper with ⁶⁸Ga-DOTATATE should be less intense than that of the reference solution.
3. Determine ⁶⁸Ge breakthrough in the final product (monthly).
   1. Prepare a sample with 200 µL of ⁶⁸Ga-DOTATATE solution (G). Place it in a well counter and determine the number of counts in each in the 400–600 keV energy window for 30 s.
   2. Repeat step 5.3.1.
   3. Correct the counts in G to determine G’ for decay, to determine the number of counts at the ART with the following formula, in which Δt is the time difference between the measured sample and the ART in minutes.
      
   4. Calculate the ⁶⁸Ge breakthrough (MBq/MBq), which should be less than 0.001%, with the following formula.
      
4. Determine the sterility of the final product (monthly).
   1. Add the remaining ⁶⁸Ga-DOTATATE solution to a tryptic soy broth (TSB) medium. Incubate for 14 d at 30–35 °C.
   2. Check that the TSB medium is a clear liquid after the incubation period.

6. Patient preparation and administration of ⁶⁸Ga-DOTATATE

NOTE: The injected activity in this protocol provides good quality images with the PET/CT system available and the imaging protocol as described in section 7. With other imaging systems and protocols, the injected activity should be optimized.

1. In advance, send the appointment and patient folder with information about ⁶⁸Ga-DOTATATE PET/CT by mail to each patient. Confirm every appointment by telephone 1 day in advance.
2. Food and drinks are not restricted prior to ⁶⁸Ga-DOTATATE PET/CT imaging. Advise patients to drink an extra 1 L of water in the 2 h prior to imaging. It is also recommended patients do not bring children or pregnant women with them to the nuclear medicine department.
3. On the day of the ⁶⁸Ga-DOTATATE PET/CT, have the patients check in at the department of nuclear medicine 60 min prior to the imaging. Take a short medical history.
4. Inquire about the date of the last somatostatin analog administration. This is not a contraindication for ⁶⁸Ga-DOTATATE PET/CT but should be noted.
5. Place an intravenous cannula in the arm and flush it with saline to verify the placement of the cannula.
6. Inject 100 MBq of ⁶⁸Ga-DOTATATE 45 min prior to the PET/CT imaging.

7. PET/CT imaging

1. Place patient with the arms above their head on the PET/CT scanner. Instruct the patient to remain still throughout the exam.
2. Acquire a survey image and select the scan area from the pituitary gland to the mid-thigh, unless otherwise specified due to clinical indications.
3. Perform a low-dose CT scan with 40 mAs, 140 keV, and 5 mm slices for attenuation correction and anatomical correlation.
4. Perform a PET scan with 150 s per bed position, starting at the head of the patient.
5. Reconstruct the CT images with filtered back projection and 5 mm slices.
6. Reconstruct the PET images with BLOB-OS-TF, with three iterations and 33 subsets with a voxel size of 4 mm x 4 mm x 4 mm.
7. Send all images to the local picture archiving and communication system (PACS).

Results

Making use of an automated labeling system, 357 batches of ⁶⁸Ga-DOTATATE were produced between December 2014 and October 2018. Of the 357 produced, 17 batches failed and 340 batches were released, leading to an overall success rate of 95.2%. Of the failed batches, 11 were caused by a technical failure, whilst in six cases, the produced ⁶⁸Ga-DOTATATE did not meet specifications. Figure 1 shows a flow chart of produced batches and the numb...

Discussion

This protocol describes the production and subsequent PET/CT imaging of ⁶⁸Ga-DOTATATE. In order for the efficient use of each produced batch of ⁶⁸Ga-DOTATATE, an optimal workflow with strict timing is required. Since the half-life of ⁶⁸Ga is 68 min, a relatively small time delay of 15 min leads to a 15% loss of radioactivity. This requires active communication between the production facility, the personnel administrating the dose to the patient, and the PET/CT technician. Also, patients s...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetonitrile	Biosolve	012007	> 99.9 %
Ammonium acetate	Merck	101116	≥ 98 %
Aqua / Water for injections	Braun
Automated labeling system	Scintomics		GRP 3V
C-18 cartridge	Waters	WAT023501	Sep-Pak C18 Plus Light
Dose calibrator	Veenstra Instruments		VIK-202-5051
EDTA	Merck	324503
Ethanol	Sigma Aldrich	32221-M	≥ 99.8 %
Ga-68 labeling kit	ABX	SC-01
Ge-68/Ga-68 generator	Eckert & Ziegler		1850 MBq
HA-DOTATATE	Scintomics	GRPC/R-000095
HCl 0.1M for elution	ABX	HCl-03
HEPES	Sigma Aldrich	H3375	≥ 99.5 %
Iodine	Sigma Aldrich	207772	≥ 99.8 %, solid
ITLC-SG F254 plates	Merck	105735	TLC Silica gel 60 F254
ITLC-SG paper	Agilent	SGI0001	Glass fiber
Methanol	Sigma Aldrich	32213-M	≥ 99.8 %, Ph. Eur.
Non-vented filter	Merck	SLMPL25SS	Millex-MP filter 0.22 µm
PET/CT	Philips		Gemini TOF
pH indicator strips	Merck	109584	MColorpHast (pH2.0-9.0)
Tryptic soy broth medium	Biotrading	K111F010QK
Vented filter	Merck	SLGV0250S	Cathivex GV 0.22 µm
Well counter	Canberra (now Mirion)		Osprey Digital Tube Base MCA Detector 76 BP76/3M-X