Chapter 8
`Synthesis of PET Radiopharmaceuticals
`
`Introduction
`
`PET radiopharmaceuticals are uniquely different from SPECT radiopharmaceuti-
`cals in that the former have radionuclides that are positron emitters and the majority
`of them have short physical half-lives. The most common PET radionuclides are
`11C, 15O, 13N, 18F, and 82Rb, which are short-lived (see Table 7.2) and put
`limitations on the synthesis time for PET radiopharmaceuticals and their clinical
`use. The attractive advantage of PET radiopharmaceuticals, however, is that the
`ligands used in radiopharmaceuticals are common analogs of biological molecules
`and, therefore, often depict a true representation of biological processes after
`in vivo administration. For example, 18F-fluorodeoxyglucose (FDG) is an analog
`of glucose used for cellular metabolism and H15
`2 O for cerebral perfusion.
`
`Automated Synthesis Device
`
`Conventional manual methods of synthesis of radiopharmaceuticals using a high
`level of radioactivity are likely to subject the personnel involved in the synthesis to
`high radiation exposure. This is particularly true with short-lived positron emitters
`such as 11C, 13N, 15O, and 18F, because the quantity of these radionuclides handled
`in the synthesis is very high. To minimize the level of exposure, automated modules
`have been devised for the synthesis of PET radiopharmaceuticals.
`The automated synthesis device, often called the black box, is a unit controlled
`by microprocessors and software programs to carry out the sequential physical and
`chemical steps to accomplish the entire synthesis of a radiolabeled product. The
`unit consists of templates or vials prefilled with required chemicals attached to the
`apparatus via tubings that are connected to solenoid valves to switch on and off as
`needed. Most black boxes are small enough to be placed in a space of
`
`© Springer International Publishing Switzerland 2016
`G.B. Saha, Basics of PET Imaging, DOI 10.1007/978-3-319-16423-6_8
`
`161
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`8 Synthesis of PET Radiopharmaceuticals
`
`20  20  20 in. and are capable of self-cleaning. In some units, disposable
`
`cassettes (cartridges) are employed so that new cassettes can be used for each
`new synthesis, thus minimizing contamination and radiation exposure. Various
`parameters for synthesis such as time, pressure, volume, and other requisites are
`all controlled by a remote computer using appropriate software. The unit has a
`graphic display showing the status of the ongoing process. After the synthesis, a
`report with the date and start and end time of the radiosynthesis and the calculated
`yield is printed out. Technologists can operate these units very easily. Automated
`synthesis modules for 18F-FDG, 13N-NH3, 11C-CH3I, 11C-HCN, 11C-acetate, and a
`few other PET tracers are commercially available. Versatile automated modules are
`commercially available to use for the synthesis of a variety of PET tracers in the
`single module. This is accomplished by simple exchange or modification of various
`segments inside the unit to suit the specific product synthesis. To minimize radia-
`tion exposure, often the synthesis box is placed inside a minicell (see Chap. 7).
`After each synthesis, the product is passed through a high-performance liquid
`chromatography (HPLC) described later to achieve a high-purity finished product.
`A schematic diagram of a black box for 18F-FDG synthesis is shown in Fig. 8.1.
`Often the synthesis box is placed inside a minicell to minimize radiation exposure.
`An automated multi-synthesis module (FASTlab2) for the synthesis of different
`PET radiopharmaceuticals marketed by GE Healthcare is shown in Fig. 8.2. Other
`vendors include Siemens Medical Solutions, Inc. (Explora), IBA (Synthera), and
`Eckert & Ziegler (FDG-Plus).
`
`Fig. 8.1 A schematic block diagram showing different components in the 18F-FDG synthesis box
`(Reproduced with kind permission of Kluwer Academic Publishers from Crouzel C et al. (1993)
`Radiochemistry automation PET. In: St€ocklin G, Pike VW (eds) Radiopharmaceuticals for
`positron emission tomography, Kluwer Academic, Dordrecht, the Netherlands, p 64. Fig. 9)
`
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`PET Radiopharmaceuticals
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`
`Fig. 8.2 Automated synthesis box, FASTlab2, from GE Healthcare (Courtesy of GE Healthcare)
`
`PET Radiopharmaceuticals
`
`Many radiopharmaceuticals have been used for PET imaging; however, only a few
`are routinely utilized for clinical purposes. Almost all of them are labeled with one
`of the four common positron emitters: 11C, 13N, 15O, and 18F. Of the four, 18F is
`¼ 110 min) that allows
`preferred most, since it has a relatively longer half-life (t1/2
`its supply to relatively remote places. In all cases, a suitable synthesis method is
`adopted to provide a stable product with good labeling yield, high specific activity,
`high purity, and, most importantly, high in vivo tissue selectivity. The following is a
`description of the syntheses of the common clinically used PET radiopharmaceu-
`ticals and a few with potential for future use.
`
`18F-Sodium Fluoride
`¼ 110 min)
`is produced by irradiation of 18O-water with
`Fluorine-18 (t1/2
`10–18 MeV protons in a cyclotron and recovered as 18F-sodium fluoride by passing
`the irradiated water target mixture through a carbonate-type anion-exchange resin
`
`column. The water is forced out of the column with neon gas, whereas 18F
`is
`retained on the column, which is recovered by elution with potassium carbonate
`solution. Its pH should be between 4.5 and 8.0. While 18F-sodium fluoride is most
`commonly used for the synthesis of FDG, it is also used for other 18F-labeled PET
`radiopharmaceuticals.
`The US FDA has approved it for bone scintigraphy, since it localizes in bone by
`ion in the hydroxyapatite crystal.
`exchanging with PO
`
` 4
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`8 Synthesis of PET Radiopharmaceuticals
`
`18F-Fluorodeoxyglucose
`
`18F-2-fluoro-2-deoxyglucose (2-FDG) is normally produced in places where a
`18FO5 with molecular
`cyclotron is locally available. Its molecular formula is C8H11
`weight of 181.3 Da. 18F-2-FDG can be produced by electrophilic substitution with
`18F-fluorine gas or nucleophilic displacement with 18F-fluoride ions. The radio-
`chemical yield is low with the electrophilic substitution, so the nucleophilic dis-
`placement reaction has become the method of choice for 18F-FDG synthesis.
`Deoxyglucose is labeled with 18F- by nucleophilic displacement reaction of an
`acetylated sugar derivative followed by hydrolysis (Hamacher et al. 1986).
`In nucleophilic substitution, a fluoride ion reacts to fluorinate the sugar derivative. A
`solution of 1,3,4,6-tetra-O-acetyl-2-O-trifluoromethane-sulfonyl-β-D-mannopyranose
`in anhydrous acetonitrile is added to a dry residue of 18F-fluoride containing
`aminopolyether (Kryptofix 2.2.2) and potassium carbonate (Fig. 8.3). Kryptofix
`2.2.2 is used as a catalyst to enhance the reactivity of the fluoride ions. The mixture
`is heated under reflux for about 5 min. The solution is then passed through a C-18
`Sep-Pak column, and acetylated carbohydrates are eluted with tetrahydrofuran (THF),
`
`C for 15 min.
`which are then hydrolyzed by refluxing in hydrochloric acid at 130
`18F-2-fluoro-2-deoxyglucose (2-FDG) is obtained by passing the hydrolysate through
`a C-18 Sep-Pak column. The yield can be as high as 60%, and the preparation time is
`approximately 50 min. The final solution is filtered through a 0.22-μm filter and diluted
`with saline, as needed. According to USP specifications, it should have pH of 4.5–7.5
`and a specific activity of more than 1 Ci (37 GBq)/μmol. The chemical purity is limited
`to 50 μg/mL of Kryptofix 2.2.2 and 1 mg of 2-chloro-2-deoxy-D-glucose per total
`volume. Radiochemical purity should be >90%, as determined by the TLC method
`
`Fig. 8.3 Schematic synthesis of 18F-2-fluoro-2-deoxyglucose (FDG) (Reprinted with the permis-
`©
`sion of the Cleveland Clinic Center for Medical Art and Photography
`2009. All rights reserved)
`
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`PET Radiopharmaceuticals
`
`165
`
`using activated silica gel as the solid phase and a mixture of acetonitrile and water
`(95:5) as the liquid phase.
`Since Kryptofix 2.2.2 is toxic causing apnea and convulsions, modifications have
`been made to substitute it with tetrabutylammonium hydroxide or bicarbonate,
`which have been adopted by many commercial vendors. Also, in some other
`methods, the C-18 Sep-Pak column separation has been eliminated so as to carry
`out the acidic hydrolysis in the same vessel. In methods where Kryptofix 2.2.2 is
`still used, several Sep-Pak columns are used to separate Kryptofix 2.2.2 and reduce
`it to practically a negligible quantity.
`The FDA has approved 18F-2-FDG for many clinical uses such as the metabo-
`lism in the brain and heart and the detection of epilepsy and various tumors. In
`metabolism, 18F-2-FDG is phosphorylated by hexokinase to 2-FDG-6-phosphate
`which is not metabolized further. It should be noted that 3-fluorodeoxyglucose
`(3-FDG) is not phosphorylated and hence is not trapped and essentially eliminated
`rapidly from the cell. This is why 3-FDG is not used for metabolic studies. Detailed
`protocols of 18F-FDG usage in humans are given in Chap. 13.
`Because of the relatively longer half-life of 18F among the PET radionuclides,
`commercial and institutional facilities having cyclotrons produce 18F-FDG in bulk
`quantities and supply to nearby clinics and hospitals as needed. Supply can be made
`as far as 200 miles away with a loss of activity, which can be compensated by
`adding more activity. The details of 18F-FDG distribution is given in Chap. 10.
`
`6-18F-L-Fluorodopa
`
`Like 18F-2-FDG, 6-18F-L-fluorodopa is also produced in places where a cyclotron is
`available locally. There are several methods of synthesizing 6-18F-fluoro-3,4-
`dihydroxyphenylalanine (6-18F-L-fluorodopa), of which the method of fluorodeme-
`tallation using electrophilic fluorinating agents is most widely used. Electrophilic
`reactions involve the reaction of fluorine in the form of F+ with other molecules. Only
`the L-isomer of dopa is important, because the enzymes that convert dopa to dopa-
`mine, which is targeted by the radiopharmaceutical, are selective for this isomer.
`Initially, a suitably protected organomercury precursor (N-[trifluoroacetyl]-3,4-
`dimethoxy-6-trifluoroacetoxymercuriophenylalanine ethyl ester) of dopa is prepared.
`[18F]-labeled acetylhypofluorite prepared in the gas phase is then allowed to react
`with the mercury precursor in chloroform or acetonitrile at room temperature. Other
`precursors using metals such as tin, silicon, selenium, and germanium have been
`reported. Acid hydrolysis with 47% HBr provides a relatively high yield (10–12%) of
`6-18F-L-fluorodopa (Luxen et al. 1992) compared with other available methods.
`Substitution at position 6 is most desirable, because this does not alter the behavior
`of dopa, whereas substitutions at 2 and 5 do. It is sterilized by filtering through a
`0.22-μm membrane filter and is supplied at pH between 6 and 7. Normally EDTA and
`ascorbic acid are added to the final preparation for stability. Its specific activity
`should be more than 100 mCi (3.7 GBq)/mmol and radiochemical purity >95% as
`
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`166
`
`8 Synthesis of PET Radiopharmaceuticals
`
`b
`
`d
`
`f
`
`O
`
`O
`
`N
`
`N
`
`N
`
`O
`
`11CH3
`
`F
`
`H3
`
`11C
`
`COOH
`
`SCH2CH2CH
`NH2
`
`N
`
`N
`
`O
`
`N
`
`O
`
`[18]F
`
`O
`
`H
`
`a
`
`c
`
`e
`
`g
`
`HN
`
`H2N
`
`COOH
`
`H
`
`18F
`
`HO
`
`HO
`
`F
`
`O
`CC
`
`(CH2)3
`
`N
`
`11CH3
`
`N
`
`O
`
`N
`
`O
`CH2NHC
`N
`CH2CH3
`
`HO
`
`11CH3
`O
`Cl
`
`Cl
`
`O O O
`
`18F
`
`N
`
`(D)Phe Cys Tyr....(D) Trp
`
`Thr(OL)
`
`Cys
`
`Thr...Lys
`
`N H
`
`COO
`
`O
`C
`
`N
`
`N
`
`N
`
`N
`
`68Ga
`
`h
`
`OOC
`
`OOC
`
`Fig. 8.4 Molecular structures of (a) 6-18F-L-fluorodopa, (b) 11C-flumazenil, (c) 11C-methylspiperone,
`(d) 11C-L-methionine, (e) 11C-raclopride, (f) 18F-fluoromisonidazole, (g) 18F-florbetapir, and
`(h) 68Ga-DOTATOC (Reprinted with the permission of the Cleveland Clinic Center for Medical Art
`©
`2009. All rights reserved)
`and Photography
`
`impurity (toxic) that
`determined by HPLC. Mercury is the major chemical
`originates from organomercuric precursor used in the synthesis, and its USP limit
`is 0.5 μg/mL of L-dopa solution. The molecular structure of 6-18F-L-fluorodopa is
`shown in Fig. 8.4a.
`It is particularly useful for the detection of Parkinson’s disease.
`
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`PET Radiopharmaceuticals
`
`18F-Fluorothymidine
`
`167
`
`18F-Fluorothymidine (FLT) is prepared by nucleophilic reaction between 18F-sodium
`0
`0
`0
`-O-benzoyl-2
`-anhydro-5
`-deoxythymidine, which is
`fluoride and a precursor, 2, 3
`prepared by standard organic synthesis (Machula et al. 2000). 18F-Sodium fluoride is
`added to a mixture of Kryptofix 2.2.2 and potassium carbonate in acetonitrile, and the
`
`C for 5 min. The precursor in dimethyl
`mixture is dried to a residue by heating at 120
`
`C for 10 min.
`sulfoxide (DMSO) is added to the dried residue and heated at 160
`0
`-0-protecting group is performed with sodium hydroxide.
`Hydrolysis of the 5
`18F-FLT is isolated by passing through alumina Sep-Pak and further purified by
`using high-performance liquid chromatography (HPLC). The overall yield is about
`45% and the radiochemical purity is more than 95%. The synthesis time is about
`60 min.
`Since thymidine is incorporated into DNA and provides a measure of cell
`proliferation, 18F-FLT is commonly used for in vivo diagnosis and characterization
`of tumors in humans.
`
`18F-O-(2-Fluoroethyl)-L-Tyrosine
`
`The synthesis of 18F-O-(2-fluoroethyl)-L-tyrosine (FET) is carried out in two steps
`(Wester et al. 1999): First, ethylene glycol-1,2-ditosylate in acetonitrile is reacted
`
`with dry 18F-containing Kryptofix 2.2.2 and potassium bicarbonate at 90
`C for
`10 min. The product is purified by absorbing it on a polystyrene cartridge and then
`eluting with dimethyl sulfoxide. Second, the eluate is mixed with dipotassium
`
`C for 10 min. The mixture is purified
`sodium salt of L-tyrosine and heated at 90
`by HPLC and cation exchange to obtain 18F-FET. The radiochemical yield is about
`40–45% with purity between 97 and 99%.
`18F-FET is used as a PET tracer for the detection of a variety of tumors.
`
`18F-Fluoromisonidazole
`
`18F-Fluoromisonidazole (FMISO) is synthesized in one-step reaction between the
`0
`0
`-imidazolyl)-2-O-tetrahydropyranyl-3-O-tolue-
`-nitro-1
`protected precursor, 1-(2
`nesulfonyl-propanediol (NITTP) and 18F-containing Kryptofix 2.2.2 in acetonitrile
`solution (Kamarainen et al. 2004). The labeled product is hydrolyzed with acid to
`give 18F-FMISO, which is further purified by column chromatography using a
`Sep-Pak cartridge. From automated synthesis, the radiochemical yield is 34%
`at EOS after a synthesis time of 50 min. HPLC shows a radiochemical purity of
`97%. The molecular structure of 18F-FMISO is shown in Fig. 8.4f.
`18F-FMISO is a specific tracer used for detection of hypoxic tissues by PET.
`
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`168
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`8 Synthesis of PET Radiopharmaceuticals
`
`18F-1-(5-Fluoro-5-Deoxy-α-Arabinofuranosyl)-2-
`Nitroimidazole
`
`18F-1-(5-Fluoro-5-deoxy-α-arabinofuranosyl)-2-nitroimidazole (FAZA) is synthe-
`sized by nucleophilic substitution reaction between the precursor 1-(2,3-di-O-
`acetyl-5-O-tosyl-α-D-arabinofuranosyl)-2-nitroimidazole in DMSO solution and
`18F-fluoride containing mixture of Kryptofix 2.2.2 and potassium carbonate at
`
`C for 5 min (Reischl et al. 2005). The product is hydrolyzed with NaOH
`100
`
`C for 2 min, and the solution is neutralized with NaH2PO4. It is
`solution at 20
`further purified by HPLC and sterilized by 0.22-μm filtration. The radiochemical
`yield is about 61%.
`18F-FAZA is used for the detection of tissue hypoxia by PET.
`
`18F-Florbetapir
`
`18F-Florbetapir (brand name 18F-AV-45 or Amyvid) has the molecular structure
`(E)-4-(2-(6-(2-(2-(2-([18F]-fluoroethoxy)ethoxy)ethoxy)pyridin-3-yl)vinyl)-N-
`of
`methylbenzenamine. It is prepared by nucleophilic substitution reaction between
`18F-containing mixture of Kryptofix 2.2.2 and potassium carbonate in acetonitrile
`solution and the precursor
`(E)-2-(2-(2-(2-tosyloxyethoxy)ethoxy)ethoxy)-5-
`(4-(tertbutoxycarbonyl (methyl)amino)styryl)pyridine (-OTs derivative) in DMSO
`
`C for 10 min (Liu et al. 2010). The product is hydrolyzed by
`solution at 115
`hydrochloric acid solution followed by neutralization with a solution of NaOH and
`ammonium acetate solution. The solution is loaded on a Sep-Pak C18 column, and
`
`is removed by rinsing with deionized water, and 18F-florbetapir is
`unreacted 18F
`eluted with acetonitrile. 18F-Florbetapir is purified by HPLC and finally taken up in
`ethyl alcohol for clinical use. The radiochemical yield is 34% and the radiochem-
`ical purity is nearly 95%.
`18F-florbetapir binds to amyloid plaques in Alzheimer’s disease (AD) and other
`dementia-related conditions and has been approved by the FDA for the detection of
`these diseases in patients. The molecular structure of 18F-florbetapir is shown in
`Fig. 8.4g.
`
`15O-Water
`
`¼ 2 min) is produced in the cyclotron by the 15N(p, n)15O reaction,
`15O-Oxygen (t1/2
`or the 14N(d, n)15O reaction, and the irradiated gas is transferred to a [15O] water
`generator in which 15O is mixed with hydrogen and passed over a palladium/
`
`C (Meyer et al. 1986; Welch and Kilbourn 1985). The
`charcoal catalyst at 170
`15O vapor is trapped in saline, and the saline solution is filtered through a 0.22-μm
`H2
`
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`

`PET Radiopharmaceuticals
`
`169
`
`membrane filter. The sample is then passed through a radiation detector for
`radioassay and ultimately injected online into a patient in a very short time.
`15O is commonly used for myocardial and cerebral perfusion studies.
`H2
`
`n-15O-Butanol
`
`is prepared by the reaction of 15O-oxygen, produced by the
`n-15O-Butanol
`15N(p, n)15O reaction, with tri-n-butyl borane loaded onto an alumina Sep-Pak
`cartridge (Kabalka et al. 1985). Carrier oxygen at a concentration of about 0.5% is
`added to the 15N target in order to recover 15O. After the reaction, n-15O-butanol is
`eluted from the cartridge with water. It is further purified by passing through C-18
`Sep-Pak and eluting with ethanol–water.
`n-15O-Butanol is used for blood flow measurement in the brain and other organs.
`It is a better perfusion agent than 15O-water, because its partition coefficient is
`nearly 1.0 compared to 0.9 for water.
`
`13N-Ammonia
`Nitrogen-13-labeled ammonia (t1/2 ¼ 10 min)
`is produced by reduction of
`13N-labeled nitrates and nitrites that are produced by proton irradiation of water
`in a cyclotron. The reduction is carried out with titanium chloride in alkaline
`medium. 13N-NH3 is then distilled and finally trapped in acidic saline solution.
`Wieland et al. (1991) has used a pressurized target of aqueous solution of acetic
`acid and ethanol, in which ethanol acts as a hydroxyl free radical scavenger to
`improve the yield of 13N-NH3. The mixture is passed through an anion-exchange
`resin to remove all anion impurities. It is filtered through a 0.22-μm membrane filter
`and its pH should be between 4.5 and 7.5. The radiochemical purity is greater than
`95% as determined by HPLC.
`The US FDA has approved it for measurement of myocardial and cerebral
`perfusion.
`
`11C-Sodium Acetate
`
`11C-Sodium acetate is produced by the reaction of
`the Grignard reagent,
`methylmagnesium bromide in diethyl ether, with cyclotron-produced 11C-carbon
`dioxide at 15
`
`C (Oberdorfer et al. 1996). After reaction, the product is allowed to
`react with O-phthaloyl dichloride to produce 11C-acetyl chloride, which is then
`hydrolyzed to 11C-acetate with saline. The solution is filtered through a 0.22-μm
`
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`170
`
`8 Synthesis of PET Radiopharmaceuticals
`
`membrane filter. 11C-Acetate has been found to be stable at pH between 4.5 and 8.5
`for up to 2 h at room temperature. The overall yield is about 10–50%.
`It is used for the measurement of oxygen consumption (oxidative metabolism) in
`the heart, since acetyl CoA synthetase converts 11C-acetate to acetyl coenzyme A
`after myocardial uptake, which is metabolized to 11C-CO2 in the tricarboxylic acid
`cycle.
`
`11C-Flumazenil
`
`11C-Flumazenil is commonly labeled at the N-methyl position by N-methylation
`with 11C-iodomethane, which is prepared from 11C-CO2, and using the freshly
`prepared Grignard reagent, methylmagnesium bromide (Maziere et al. 1984). The
`specific activity is very important for this product and therefore is purified by HPLC
`to give an optimum value between 0.5 and 2 Ci/μmol (18.5–74 GBq/μmol). It
`remains stable for up to 3 h at room temperature at pH 7.0. The molecular structure
`of 11C-flumazenil is shown in Fig. 8.4b.
`Since it is a benzodiazepine receptor ligand, 11C-flumazenil is primarily used for
`the neuroreceptor characterization in humans.
`
`11C-Methylspiperone
`
`11C-Methylspiperone (MSP) is prepared by N-methylation of commercially available
`spiperone with 11C-methyl
`iodide in the presence of Grignard reagent,
`methylmagnesium bromide, using different solvents and bases (Mazie`re et al. 1992).
`Since spiperones are sensitive to bases and to radiolysis at high level of activity,
`the yield of 11C-MSP has been variable for different investigators. Cold spiperone
`present in the preparation reduces its specific activity and should be controlled.
`Specific activity should be around 10–50 GBq/mol (270–1350 mCi/mol). High
`specific activity 11C-MSP undergoes autodecomposition in saline due to radiation,
`and a hydroxyl radical scavenger (e.g., ethanol) is added to prevent it. The final
`preparation is filtered through a 0.22-μm membrane filter and its pH is adjusted to
`7 1 with a suitable buffer. The molecular structure of 11C-methylspiperone is shown
`
`in Fig. 8.4c.
`11C-Methylspiperone is primarily used to determine the dopamine-2 receptor
`density in patients with neurological disorders, because of its high affinity for D-2
`receptors in the brain.
`
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`

`PET Radiopharmaceuticals
`
`171
`
`11C-L-Methionine
`11C-L-Methionine has 11C at its methyl position and has two forms: L-[1 11C]
`methionine and L-[S-methyl-11C] methionine. The former is obtained by the reac-
`tion between 11C-CO2 precursor and carbanion produced by a strong base added to
`the respective isonitrile, followed by hydrolysis with an acid. The latter is the
`common method and obtained by alkylation of the sulfide anion of L-homocysteine
`with 11C-iodomethane or 11C-methyl triflate (La˚ngstr€om et al. 1987). The product is
`purified by HPLC yielding a purity of >98% and further filtered through a 0.22-μm
`membrane filter. The pH should be between 6.0 and 8.0 and it is stable for 2 h at
`room temperature. The molecular structure of 11C-L-methionine is shown in
`Fig. 8.4d.
`This compound is used for the detection of different types of malignancies,
`reflecting the amino acid utilization (transport, protein synthesis, transmethylation,
`etc.).
`
`11C-Raclopride
`
`Raclopride is labeled with 11C either by N-ethylation with [1-11C] iodoethane or by
`O-methylation with [11C] iodomethane, although the latter is more suitable for
`routine synthesis. Production of 11C-labeled iodoethane and iodomethane are
`described in Chap. 7. A recent efficient method of synthesis of 11C-raclopride
`utilizes a loop chemistry in which the precursor (raclopride) is dissolved in a
`small quantity of ethanol and loaded in an HPLC loop. 11C-Methy triflate is
`then passed through the HPLC loop for 3 min, whereby reaction occurs to produce
`11C-raclopride (Shao et al. 2013). The product is purified by HPLC giving a purity
`of greater than 98%. The specific activity should be in the range of 0.5–2Ci/μmol
`(18.5–74GBq/μmol). The product at pH between 4.5 and 8.5 remains stable for
`more than 1 h at room temperature. The molecular structure of 11C-raclopride is
`shown in Fig. 8.4e.
`11C-Raclopride is primarily used to detect various neurological and psychiatric
`disorders, such as schizophrenia, Parkinson’s disease, etc.
`
`11C-Choline
`
`11C-Choline is prepared by the reaction between the precursor 11C-methyl iodide
`
`and dimethylaminoethanol at 130
`C for 5 min (Hara and Yuasa 1999). The reaction
`mixture is evaporated to remove the precursors leaving behind the residue of
`11C-choline, which is dissolved in water and further purified by the cation exchange
`
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`

`172
`
`8 Synthesis of PET Radiopharmaceuticals
`
`method. The final product is available as 11C-choline chloride with a radiochemical
`yield of ~43% at the end of synthesis.
`11C-Choline is a highly specific PET tracer used for the detection of various
`tumors, specifically prostate cancer, and has been approved by the FDA for clinical
`use in some specific prostate cancer patients.
`
`62Cu-Pyruvaldehyde-Bis(N4-Methylthiosemicarbazonato)
`Copper(II)
`
`62Cu-Pyruvaldehyde-bis(N4-methylthiosemicarbazonato) copper(II)
`is
`(PTSM)
`prepared by mixing H2(PTSM) in ethanol and 62Cu acetate solution at room
`temperature for 2–3 min (Green et al. 1990). The mixture is passed through C18
`Sep-Pak column, and finally 62Cu-PTSM is eluted from the column with ethanol
`followed by filtration through a 0.2-μm filter. The radiochemical yield is nearly
`50% (without decay correction).
`62Cu-PTSM is used for measurement of myocardial perfusion by PET.
`
`68Ga-DOTA-Peptides
`
`68Ga-labeled
`octreotate
`and
`(DOTATOC)
`octreotide
`as
`such
`peptides
`(DOTATATE) are obtained by mixing the DOTA-peptides in 0.01 M acetic acid
`solution and 68GaCl3 solution with 1.25 M Na-acetate in a small volume (Breeman
`et al. 2005). The reaction is allowed in a temperature-controlled heating block at a
`pH of 4. HPLC is employed to purify the labeled peptide, which is then filtered
`through a 0.22 μm Millipore filter for sterilization. The molecular structure is
`shown in Fig. 8.4g.
`68Ga-DOTATOCand 68Ga-DOTATATE are used for PET imaging of neuroen-
`docrine tumors and more advantageous than 111In-octreotide because imaging time
`with the former is only a few hours compared to 2–3 days with the latter. The FDA
`has recently approved 68Ga-DOTATOC as an orphan drug for specific use in
`neuroendocrine tumor imaging. An orphan drug is stipulated in relatively rare
`diseases that affect less than 200,000 people or for limited clinical use. This will
`ultimately lead to final approval for routine clinical use.
`
`82Rb-Rubidium Chloride
`
`82Rb-Rubidium chloride is available from the 82Sr–82Rb generator, which is
`manufactured and supplied monthly by Bracco Diagnostics, Inc. The activity in
`
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`

`Quality Control of PET Radiopharmaceuticals
`
`173
`
`the column is typically 90–150 mCi (3.33–5.55 GBq) 82Sr at calibration time. 82Rb
`is eluted with saline and must be checked for 82Sr and 85Sr breakthrough daily
`before the start of its use for patient studies. The allowable NRC limit for 82Sr is
`0.02 μCi/mCi or 0.02 kBq/MBq of 82Rb and the limit for 85Sr is 0.2 μCi/mCi or
`0.2 kBq/MBq of 82Rb. Measurement of 82S and 85Sr breakthrough is described later
`in Chap. 14. Since 82Rb has a short half-life of 75 s, it is administered to the patient
`by an infusion pump (see Chap. 14). The administered activity is the integrated
`activity infused at a certain flow rate over a set time, which is provided on the
`printout from a printer. The generator is supplied with calibrated activity enough for
`a month’s or 6 weeks’ study with 82Rb. Because of the long half-life of 82Sr, the
`generator can be shipped to remote places.
`The FDA approved it in 1989 for clinical use. Now 82Rb is routinely used for
`myocardial perfusion imaging to delineate ischemia from infarction. A detailed
`protocol for its clinical use in patients in the USA has been included in Chap. 13.
`
`Quality Control of PET Radiopharmaceuticals
`
`The FDA mandates that synthesis of all PET radiopharmaceuticals including those
`described above for human administration must be carried out in sterile and clean
`environment, following recommendations given in FDA 21CFR212, which is
`described in detail in Chap. 9. All PET radiopharmaceuticals must undergo a set
`of quality control tests prior to human administration, which are described below.
`US Pharmacopeia (USP) 35 Chapter <823> provides the recommended methods
`for all these tests to ensure drug identity, strength, quality, purity, and patient safety.
`Because of the short half-lives of positron emitters used in PET imaging, some
`quality control tests have been grouped into two categories: those with nuclides
`with t1/2 > 20 min and those with nuclides with t1/2 < 20 min. In the case of the
`former, each production on a given day is considered a batch and all quality control
`tests must be performed for the batch; while in the latter group, a batch is defined as
`all related subbatches of PET radiopharmaceuticals compounded on a given day.
`< 20 min) is designated as the initial quality control
`The first subbatch (for t1/2
`subbatch for the day, which is considered good for all quality control tests for all
`subsequent subbatches.
`
`(a) The following quality control tests must be performed for all PET radiophar-
`maceuticals prior to release for human use:
`
`• Visual inspection.
`• pH.
`• Radionuclidic purity.
`• Radiochemical purity.
`• Chemical purity.
`• Specific activity.
`•
`Isotonicity.
`
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`

`174
`
`8 Synthesis of PET Radiopharmaceuticals
`
`• Limits for residual solvents or toxic chemicals.
`• A bubble point test or filter integrity test to check the structural integrity of
`the membrane filter immediately after filtration but prior to release of the
`product. (This is done by applying air pressure through the filter until
`the validated bubble point is reached. Filter integrity is indicated by the
`absence of a steady stream of bubbles. Maximum pressure to be applied is
`specified on the filter.)
`• A 20-min endotoxin test on each batch (t1/2 > 20 min) or quality control
`subbatch (t1/2 < 20 min).
`• Sterility tests on each individual batch (t1/2 > 20 min) or quality control
`subbatch (t1/2 < 20 min) initiated within 24 h (USP 35 Chapter <823>) or
`30 h (FDA 21CFR 212) of sterility filtration.
`• Toxicity.
`
`(b) Written procedures for each quality control test must be established.
`(c) Verification testing of all equipment (e.g., dose calibrator) and procedures
`must be carried out to comply with the acceptance limit. The results must be
`recorded, signed, and dated by the individual carrying out the test.
`
`Methods of Quality Control
`
`In this section, the methods of quality control tests for these products are briefly
`described below. Since PET radiopharmaceuticals are short-lived, some lengthy
`tests cannot be performed prior to release for human use and are performed within a
`short time after the release.
`The quality control tests can be divided into two categories: physicochemical
`tests and biological tests. Refer to Saha (2010) for detailed description of these
`methods. These tests are briefly outlined below.
`
`Physicochemical tests. Physicochemical tests include the tests for the physical and
`chemical parameters of a PET radiopharmaceutical, namely, physical appearance,
`isotonicity, pH, radionuclidic purity, chemical purity, and radiochemical purity.
`
`Physical appearance. Physical appearance relates to the color, clarity, or turbidity
`of a PET radiopharmaceutical and should be checked by visual inspection of the
`sample.
`
`pH. The pH of a PET radiopharmaceutical for human administration should be
`ideally 7.4, but both slightly acidic and basic pH values are tolerated due to the
`buffer capacity of the blood. The pH can be adjusted by adding appropriate buffer to
`the solution.
`
`Isotonicity. Isotonicity is the ionic strength of a solution, which is mainly adjusted
`by adding appropriate electrolytes. Normally PET radiopharmaceuticals have
`appropriate isotonicity for human administration.
`
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`

`Quality Control of PET Radiopharmaceuticals
`
`175
`
`Radionuclidic purity. The radionuclidic purity of a radiopharmaceutical is the
`fraction of total activity in the form of the desired radionuclide in the sample.
`These impurities primarily arise from the radionuclides produced by various
`nuclear reactions in a target as well as the impurities in the target material. Using
`a multichannel spectrometer, one can determine the level of impurities in a sample
`of a positron-emitting radionuclide produced by a specific nuclear reaction in a
`cyclotron. Using highly pure target material and appropriate chemical separation
`techniques, the radionuclidic impurity can be minimized to an acceptable level.
`Short-lived radionuclides can be allowed to decay to have a pure long-lived
`radionuclide in question. Even though the impurities in the routine preparations
`of PET radionuclides do not vary significantly from batch to batch, periodic
`checkup is recommended to validate the integrity of the method of production.
`The radi