1052
`
`Bioconjugate Chem. 2003, 14, 1052-1056
`
`Ascorbic Acid: Useful as a Buffer Agent and Radiolytic Stabilizer
`for Metalloradiopharmaceuticals
`
`Shuang Liu,*,† Charlie E. Ellars,‡ and D. Scott Edwards‡
`
`Department of Industrial and Physical Pharmacy, School of Pharmacy, Purdue University,
`575 Stadium Mall Drive, West Lafayette, Indiana 47907-2051 and Discovery R & D, Bristol-Myers Squibb
`Medical Imaging, 331 Treble Cove Road, North Billerica, Massachusetts 01862. Received June 26, 2003;
`Revised Manuscript Received August 1, 2003
`
`The goal of this study is to explore the use of ascorbic acid (AA) as a buffer agent and a radiolytic
`stabilizer for preparation and stabilization of radiolabeled DOTA-biomolecule conjugates. Results
`from a titration experiment show that 0.1 M AA solution has sufficient buffer capacity at pH 5.0
`while 0.5 M AA solution is useful even at pH 6.0. The radiolabeling experiment using TA138, a DOTA-
`conjugated nonpeptide integrin Rv(cid:226)3 receptor antagonist, clearly demonstrates that AA is a good buffer
`agent for pH control and an excellent antioxidant for stabilization of metal-labeled diagnostic (111In)
`and therapeutic (90Y and 177Lu) radiopharmaceuticals if the radiolabeling is performed at pH 5-6.
`There is no need for the additional stabilizer (e.g., gentisic acid) and buffer agent such as ammonium
`acetate. The anaerobic AA formulation described in this study is particularly useful for radiolabeling
`of small biomolecules, which are sensitive to the radiolytic degradation during radiolabeling.
`
`INTRODUCTION
`There has been a great current interest in radiolabeled
`small biomolecules (peptides and nonpeptides) as diag-
`nostic and therapeutic radiopharmaceuticals (1-15).
`Radiopharmaceuticals comprising R- or (cid:226)-emitting ra-
`dionuclides often undergo radiolysis during preparation
`and storage (11). During radiolysis, emissions from the
`radionuclide attack the metal chelate, targeting biomol-
`ecule, and other compounds in proximity, which results
`in decomposition or destruction of the metal chelate or/
`and the biomolecule. Since the tumor uptake is largely
`dependent on the receptor binding of the radiolabeled
`biomolecule, radiolytic degradation may lead to the
`decreased therapeutic efficacy and unwanted radiation
`toxicity to normal organs. Thus, it is important that the
`radionuclide remains linked to the targeting moiety, and
`specificity of the targeting biomolecule is preserved.
`A radiopharmaceutical composition usually contains a
`bifunctional chelator-conjugated biomolecule (BFC-BM),
`a buffer agent for pH control, a weak chelator to prevent
`metal-colloid formation, and a stabilizer to prevent
`radiolytic degradation of the radiolabeled BFC-BM
`conjugate. The pH is critical for the success and repro-
`ducibility of the 90Y- or 111In-labeling. When it is used at
`high concentrations, ammonium acetate can serve as a
`buffer agent and weak chelator. As a matter of fact,
`ammonium acetate has been widely used as the buffer
`agent for the radiolabeling of various DOTA-BM and
`DTPA-BM conjugates (16-25). The stabilizer can be
`added into the reaction mixture before or after radiola-
`beling. However, the combination of ammonium acetate
`and stabilizer often results in high osmolarity of the
`radiopharmaceutical composition. Therefore, there is a
`
`* To whom correspondence should be addressed. Phone:
`765-494-0236
`(S.L.);
`fax
`765-496-3367;
`e-mail:
`lius@
`pharmacy.purdue.edu.
`† Purdue University.
`‡ Bristol-Myers Squibb Medical Imaging.
`
`Figure 1. TA138: a DOTA-conjugated vitronectin receptor
`antagonist.
`
`need for a new agent, which can serve both as a buffer
`agent for pH control and as a stabilizer to stabilize the
`radiolabeled BFC-BM conjugate.
`TA138 (Figure 1) is a DOTA-conjugated nonpeptide
`integrin Rv(cid:226)3 receptor antagonist that binds with high
`affinity and specificity to integrin Rv(cid:226)3 receptors overex-
`pressed on endothelial cells of tumor neovascularture and
`tumor cells (26-30). 90Y-TA138 has demonstrated sig-
`nificant therapeutic effects in several preclinical tumor
`models, including c-neu Oncomouse, HCT116, and HT460
`xenografts (31). Since 90Y is a pure (cid:226)-emitter, 111In-TA138
`was chosen as the imaging surrogate for biodistribution
`and dosimetry determination. To support the clinical
`studies, it is necessary to develop a robust formulation
`for routine preparation of 90Y-TA138 and 111In-TA138.
`In our previous contribution (38), we reported synthesis
`of complexes 90Y-TA138 and 177Lu-TA138. Through a
`series of radiolabeling experiments, we developed an
`anaerobic formulation for routine preparation of 90Y-
`TA138 and 177Lu-TA138. It was found that 90Y-TA138 and
`177Lu-TA138 are very sensitive to radiolytic degradation,
`and exclusion of oxygen is necessary during the radiola-
`beling. Using the anaerobic formulation, 90Y-TA138 and
`177Lu-TA138 can be prepared in high yield and high
`specific activity. We also found that Tris ((cid:24)0.1 M, pH )
`6.0-8.0) and ascorbic acid (AA: (cid:24)0.1 M, pH ) 5.0-7.0)
`can also be used as buffer agents to replace ammonium
`acetate in the anaerobic formulation. Since AA is a known
`
`© 2003 American Chemical Society
`10.1021/bc034109i CCC: $25.00
`Published on Web 08/19/2003
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`AAA, Ex. 2004
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`Page 1 of 5
`
`

`

`radiolytic stabilizer and has the buffer capacity at pH
`4-6, there is no need for gentisic acid and ammonium
`acetate in the formulation matrix if the radiolabeling is
`performed at pH 5-6. That led us to explore the pos-
`sibility of using AA as a buffer agent for pH control and
`a radiolytic stabilizer for stabilization of the radiophar-
`maceutical during or/and after radiolabeling. In this
`report, we present an anaerobic formulation for routine
`preparation of radiometal-labeled small biomolecule ra-
`diopharmaceuticals. This formulation is particularly use-
`ful for small biomolecules sensitive to radiolytic degra-
`dation during radiolabeling.
`
`EXPERIMENTAL SECTION
`Materials. Ammonium acetate, ascorbic acid (sodium
`salt), and diethylene-triaminepentaacetic acid (DTPA)
`were purchased from Sigma/Aldrich Chemical Co. and
`were used as received. 90YCl3 and 111InCl3 (in 0.05 N HCl)
`were purchased from PerkinElmer Life Sciences, N.
`Billerica, MA. High specific activity 177LuCl3 was obtained
`from University of Missouri Research Reactor, Columbia,
`MO. Synthesis of TA138, 3-sulfon-N-[[4,7,10-tris(car-
`boxymethyl)1,4,7,10-tetraaza-cyclododec-1-yl]acetyl]-L-
`alanyl-N-[2-[4-[[[(1S)-1-carboxy-2[[[1,4-dihydro-7-[(1H-
`imidazol-2-ylamino]methyl]-1-methyl-4-oxo-3-quino-
`linyl]carbonyl]amino]ethyl]amino]sulfonyl]-3,5-di-
`methylphenoxy]-1-oxobutyl]amino]ethyl]-3-sulfo-L-alanin-
`amide, has been reported in our previous communication
`(25).
`Analytical Methods. The radio-HPLC method used
`a HP-1100 HPLC system with a UV/visible detector ((cid:236) )
`230 nm), an IN-US radio-detector, and a Zorbax C18
`column (4.6 mm (cid:2) 250 mm, 80 Å pore size). The flow
`rate was 1 mL/min with a gradient mobile phase starting
`from 92% solvent A (0.025 M ammonium acetate buffer,
`pH 6.8) and 8% solvent B (acetonitrile) to 90% solvent A
`and 10% solvent B at 18 min. The mobile phase was
`isocratic using 40% of solvent A and 60% solvent B from
`19 to 25 min. The isocratic condition was used to make
`sure that more lipophilic radioimpurities were washed
`out from the column. The TLC method used the C18
`reverse phase glass plates and a mobile phase containing
`methanol, acetone, and saline (2:1:1 ) v:v:v). By this
`method, the radiolabeled DOTA-conjugate migrates to
`solvent front while unchelated radiometal (metal-colloid
`and metal-acetate complex) remain at the origin. The
`corrected radiochemical purity (RCP) was calculated by
`subtracting the percentage of unchelated radiometal
`obtained by TLC from that obtained by radio-HPLC.
`General Procedure for the Synthesis of 90Y-
`TA138. To a 5 mL vial containing 50 (cid:237)g of TA138 was
`added 0.5 mL of the AA buffer (0.1 or 0.5 M; pH ) 5.0-
`7.0). The solution was degassed under vacuum (<1
`mmHg) for (cid:24)2 min. Upon addition of 10-15 (cid:237)L of 90YCl3
`((cid:24)10 mCi) in 0.05 N HCl, the reaction mixture was
`heated at 50 °C or 95 °C for 5 or 35 min. After
`radiolabeling, a sample of the resulting solution was
`added to a 2 mLHPLC autosampler vial containing a
`mixture of 0.5 mL of AA and 0.5 mL of 2 mM DTPA
`solution and was then analyzed by radio-HPLC and TLC.
`Each sample was run twice, and the RCP data are
`presented as an average of two independent measure-
`ments.
`Synthesis of 177Lu-TA138. To a clean sealed 5 mL
`vial was added 1.0 mL of 0.1 M AA buffer (pH ) 5.0)
`containing 100 (cid:237)g of TA138. The solution was degassed
`under vacuum (<1 mmHg) for (cid:24) 2 min. Upon addition
`of 25 (cid:237)L of 177LuCl3 solution ((cid:24)20 mCi) in 0.05 N HCl,
`
`Bioconjugate Chem., Vol. 14, No. 5, 2003 1053
`
`Figure 2. The pH titration curves for 0.1 M (top) and 0.5 M
`(bottom) AA solutions.
`
`the reaction mixture was heated at 95 °C for 30 min.
`After being cooled to room temperature, a sample of the
`resulting solution was diluted 2-fold with 2 mM DTPA
`solution, analyzed by HPLC and TLC. The mixture was
`kept at -78 °C for 5 days, and then reanalyzed using
`the same HPLC and TLC methods.
`Synthesis of 111In-TA138. TA138 (40-100 (cid:237)g) was
`dissolved in 1.5 mL of AA buffer (0.1 M at pH 5.0 or 0.5
`M at pH ) 6.0). The solution was immediately degassed
`under vacuum (<1 mmHg) for (cid:24)2 min. Upon addition of
`111InCl3 solution (2-2.5 mCi) in 0.05 N HCl, the reaction
`mixture was heated at 95 °C for 30 min. After being
`cooled to room temperature, a sample of the resulting
`solution was analyzed by radio-HPLC and TLC. The
`resulting mixture was kept at room temperature for 24
`h and was then reanalyzed using the same HPLC and
`TLC methods.
`
`RESULTS
`The pH Titration Experiment. In this experiment,
`sodium ascorbate was used as the starting material to
`prepare 0.1 M (20 mg/mL) and 0.5 M (100 mg/L) AA
`solutions. The original pH in both solutions was (cid:24)7.6.
`Since 90YCl3, 111InCl3 and 177LuCl3 are all dissolved in 0.05
`N HCl solution, we used 0.05 N HCl to titrate both AA
`solutions. Figure 2 shows the titration curves (pH versus
`added 0.05 M HCl/mL AA) for 0.1 M (top) and 0.5 M
`(bottom) AA solutions.
`Radiolabeling Experiment. This experiment was
`designed to explore the possibility of using AA as a buffer
`agent for pH control and as a radiolytic stabilizer for
`stabilization of 90Y-TA138. We used 100 (cid:237)g of TA138 for
`20 mCi of 90Y to make sure that TA138 was in large
`excess. We also fixed the 90Y:TA138 ratio (10 mCi/50 (cid:237)g/
`mL) to explore other factors influencing the radiolabeling
`yield of 90Y-TA138. Four factors were considered in the
`experimental design. These include pH value (5, 6, and
`7), heating time (5 and 35 min), AA level (20 and 100
`mg/mL), and temperature (50 °C and 95 °C). There were
`20 conditions (Table 1) for the radiolabeling experiment.
`Each condition contains 2-3 vials unless specified. The
`
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`1054 Bioconjugate Chem., Vol. 14, No. 5, 2003
`Table 1. Radiolabeling Results (at (cid:24)10 mCi Level)
`heating time
`temp
`AA level
`average RCP (%)
`(min)
`(°C)
`(mg/mL)
`for 90Y-TA138
`96.2 ( 0.8 (n ) 2)
`35
`95
`100
`98.5 ( 0.3 (n ) 6)
`35
`95
`20
`95.3 ( 1.3 (n ) 3)
`5
`95
`100
`98.1 ( 0.4 (n ) 6)
`5
`95
`20
`97.8 ( 0.8 (n ) 6)
`35
`95
`100
`98.5 ( 0.5 (n ) 3)
`35
`95
`20
`97.5 ( 0.7 (n ) 2)
`5
`95
`100
`98.7 ( 0.3 (n ) 3)
`5
`95
`20
`97.3 ( 0.6 (n ) 2)
`35
`95
`100
`98.4 ( 0.3 (n ) 3)
`35
`95
`20
`98.0 ( 0.5 (n ) 3)
`5
`95
`100
`98.5 ( 0.4 (n ) 2)
`5
`95
`20
`17.5 ( 15.3 (n ) 3)
`5
`50
`20
`74.4 ( 7.5 (n ) 3)
`35
`50
`100
`36.2 ( 9.4 (n ) 2)
`35
`50
`20
`38.4 ( 8.6 (n ) 3)
`5
`50
`100
`78.5 ( 2.3 (n ) 2)
`5
`50
`100
`80.6 ( 3.5 (n ) 2)
`5
`50
`20
`91.2 ( 1.5 (n ) 3)
`35
`50
`100
`87.1 ( 2.7 (n ) 2)
`35
`50
`20
`
`pH
`5.0
`5.0
`5.0
`5.0
`6.0
`6.0
`6.0
`6.0
`7.0
`7.0
`7.0
`7.0
`5.0
`5.0
`5.0
`6.0
`7.0
`7.0
`7.0
`7.0
`
`Table 2. RCP Performance of the AA Formulation for
`90Y-TA138 (at 100 mCi level)
`TLC RCP(%)
`buffer concn
`buffer pH HPLC RCP (%)
`99.7 ( 0.2/T ) 0
`0.1 M (20 mg/mL) 6.0 (n ) 3) 98.4 ( 0.6/T ) 0
`0.1 M (20 mg/mL) 6.0 (n ) 3) 97.8 ( 0.2/T ) 3 d 99.5 ( 0.4/T ) 3 d
`0.5 M (100 mg/mL) 5.0 (n ) 3) 98.9 ( 0.5/T ) 0
`99.2 ( 0.1/T ) 0
`0.5 M (100 mg/mL) 5.0 (n ) 3) 98.3 ( 0.6/T ) 3 d 99.4 ( 0.2/T ) 3 d
`activity level in each vial was (cid:24)10 mCi. The RCP data
`for the radiolabeled vials are summarized in Table 1.
`Solution Stability of 90Y-TA138. In this experiment,
`we prepared six 90Y-TA138 vials at a 100 mCi level using
`0.1 M AA (pH ) 5.0: n ) 3) or 0.5 M AA (pH ) 6.0: n )
`3), and studied the solution stability of 90Y-TA138 at -78
`°C over 3 days. The TA138 concentration was 100 (cid:237)g/
`mL, and the activity concentration was 20 mCi/mL.
`Radiolabeling was readily accomplished by adding 100
`mCi of 90YCl3 (in 0.05 N HCl) into the degassed AA
`solution containing TA138 and heating the reaction
`mixture at 95 °C for 30 min. After the radiolabeling, a
`sample of the resulting solution was analyzed by radio-
`HPLC and TLC. Vials containing 90Y-TA138 were then
`placed in a lead pig and stored at -78 °C for 3 days.
`Frozen vials were allowed to thaw for 30-40 min at room
`temperature. Samples were analyzed by radio-HPLC and
`TLC. The RCP data at t ) 0 and t ) 3 days postlabeling
`are listed in Table 2. As an example, Figure 3 shows the
`typical radio-HPLC chromatograms of 90Y-TA138 at 0
`and 72 h post-labeling.
`90Y-Labeling Efficiency of TA138 Using the AA
`Formulation. We studied the 90Y-labeling efficiency of
`TA138 by determining the minimal amount of TA138
`required to achieve 95% RCP for 90Y-TA138. We prepared
`90Y-TA138 using 10, 20, 50, and 100 (cid:237)g of TA138 for 20
`mCi of 90YCl3 in 1.0 mL of 0.5 M AA buffer (pH ) 6.0).
`The heating temperature was 95 °C, and the heating time
`was 30 min. Figure 4 shows the effect of TA138 concen-
`tration on the RCP for 90Y-TA138. At pH 6.0, the minimal
`amount of TA138 required to achieve 95% RCP for 90Y-
`TA138 is about 20 (cid:237)g for 20 mCi of 90YCl3 corresponding
`to a TA138:90Y ratio of (cid:24)30:1. In all cases, the formation
`of [90Y]colloid was minimal.
`AA Formulation for 177Lu-TA138. We also used the
`AA formulation to prepare 177Lu-TA138. Radiolabeling
`was easily accomplished by adding 177LuCl3 solution ((cid:24)20
`mCi) into 1.0 mL of the degassed AA solution (0.5 M at
`pH ) 6) containing 100 (cid:237)g of TA138 and heating the
`
`Figure 3. Radio-HPLC chromatograms of 90Y-TA138 at -78
`°C.
`
`Figure 4. The effect of TA138 concentration on RCP of 90Y-
`TA138 and 177Lu-TA138.
`
`reaction mixture at 95 °C for 30 min. The RCP for 177Lu-
`TA138 was >95%. 177Lu-TA138 remains stable at -78
`°C for at least 5 days. Under optimized conditions (0.1-
`0.5 M, pH ) 5.0-6.0), the minimal amount of TA138
`required to achieve 95% RCP for 177Lu-TA138 is (cid:24)20 (cid:237)g
`for 20 mCi of 177LuCl3 (Figure 4) even though the specific
`activity of 177Lu is much higher than that of 90Y.
`AA Formulation for 111In-TA138. We also carried out
`a radiolabeling study using the AA formulation for
`preparation of 111In-TA138. Radiolabeling was accom-
`plished by simply adding 111InCl3 ((cid:24)2.0 mCi) into 1.5 mL
`of the degassed AA solution (0.5 M at pH ) 6.0 or 0.1 M
`at pH ) 5.0) containing TA138 (67 (cid:237)g/mL) and heating
`the mixture at 95-100 °C for 30 min. The RCP data for
`111In-TA138 are summarized in Table 3. The TA-138
`concentration can be varied from 40 (cid:237)g to 100 (cid:237)g for 2.0-
`
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`

`Table 3. RCP Performance of the AA Formulation for
`RP748 (at (cid:24) 2.5 MCi Level)
`111In source
`buffer pH RCP (%) HPLC
`NEN (n ) 4)
`97.5 ( 0.4
`6.0
`Indiclor (n ) 3)
`97.2 ( 0.6
`6.0
`NEN (n ) 3)
`98.5 ( 0.2
`5.0
`Indiclor (n ) 3)
`97.2 ( 0.5
`5.0
`
`TLC RCP(%)
`99.6 ( 0.3
`99.5 ( 0.3
`99.6 ( 0.2
`99.7 ( 0.2
`
`2.5 mCi of 111InCl3. Apparently, the 111In-labeling ef-
`ficiency was not as high as that for 90Y and 177Lu,
`probably due to the presence of trace metal contaminants
`from the 111InCl3 source (11).
`
`DISCUSSION
`A radiolytic stabilizer is a radical scavenging antioxi-
`dant, which reacts readily with hydroxyl and superoxide
`radicals (32). In general, the ideal stabilizer possesses
`the following characteristics:
`low or no toxicity, no
`interference with the receptor binding of the radiolabeled
`compound, and the ability to stabilize the radiopharma-
`ceutical composition for a reasonable period of time
`during preparation, release, storage, and transportation.
`AA is a known antioxidant and has been used for
`stabilization of both diagnostic and therapeutic radio-
`pharmaceuticals (23, 24, 33-37). It has a pKa of 4.2 with
`high buffer capacity at pH 3.5-5.5. At higher concentra-
`tions (>50 mg/mL or 0.25 M), it may also have sufficient
`buffer capacity at the pH range 5.5-6.0. Since AA
`contains two hydroxyl groups, one of which is deproton-
`able at pH > 4.2, it can also be used as a weak chelator
`to prevent the formation of radiometal colloid. If AA is
`used at pH 4.0-6.0, there is no need for extra buffer
`agent in the formulation matrix. In doing so, it will
`eliminate the use of ammonium acetate and reduce the
`osmolarity of the radiopharmaceutical composition.
`To demonstrate the utility of AA as a buffer agent, we
`performed a titration experiment, in which 0.1 M (20 mg/
`mL) and 0.5 M (100 mg/L) AA solutions were titrated
`with the 0.05 N HCl. Figure 2 shows the titration curves
`for 0.1 and 0.5 M AA solutions. For an ideal buffer agent,
`the pH change should not exceed 0.2 units after addition
`of 20 (cid:237)L of 0.05 N HCl solution and during radiolabeling.
`If one assumes that the activity concentration for 90YCl3
`and 177LuCl3 is about 1.0 mCi/(cid:237)L, the volume of the 90YCl3
`or 177LuCl3 stock solution for a 20 mCi activity in 1.0 mL
`of the AA buffer solution is about 20 (cid:237)L. Obviously, 0.1
`M AA solution has sufficient buffer capacity at pH 5.0
`while 0.5 M AA solution has the buffer capacity even at
`pH 6.0.
`The radiolabeling experiment was designed to explore
`the optimal conditions for routine preparation of 90Y-
`TA138. Since 90Y-TA138 is sensitive to radiolytic degra-
`dation, exclusion of oxygen is required during radiola-
`beling. Results from the radiolabeling experiments show
`that heating temperature is the most dominant factor.
`Heating the reaction mixture at 95 °C for 5-35 min is
`required to achieve high yield radiolabeling (RCP > 95%).
`It is interesting to note that the pH shows little effect on
`the RCP at pH ) 5.0-7.0 if the heating temperature is
`95 °C. At pH < 6.0, lower AA level seems to give a slightly
`better RCP for 90Y-TA138 while it does not have a
`significant effect on the RCP of 90Y-TA138 at pH ) 7.0.
`Longer heating time gives slightly better RCP at 95 °C,
`particularly at pH ) 5.0.
`It should be noted that the radiolabeling experiment
`was performed at (cid:24)10 mCi level of 90Y. There is often a
`relatively large variability (5-15%) between vials heated
`at 50 °C. There is also a large variability between
`different batches of 90YCl3. Similar results have been
`
`Bioconjugate Chem., Vol. 14, No. 5, 2003 1055
`reported for the 90Y-labeling of DOTA-conjugated anti-
`bodies (16-20). Thus, the RCP difference of (cid:24)2.0%
`between different vials may not be significant within the
`experimental error. Results from the radiolabeling ex-
`periment suggest that 90Y-TA138 can be prepared in high
`yield (RCP > 95%) under the following conditions: 100
`(cid:237)g TA138 for 20 mCi of 90Y in 1 mL of degassed AA buffer
`solution (0.1-0.5 M at pH 5.0; 0.5 M at pH 6.0), and
`heating the reaction mixture at 95 °C for 30-35 min.
`To further validate the anaerobic AA formulation, we
`prepared six 90Y-TA138 vials at 100 mCi level using 0.1
`M AA (pH) 5.0: n ) 3) or 0.5 M AA (pH ) 6.0: n ) 3).
`We studied the solution stability of 90Y-TA138 at -78 °C
`over 72 h. The radio-HPLC and TLC data clearly dem-
`onstrated that 90Y-TA138 can be prepared in high yield
`(RCP > 95%) using the anaerobic AA formulation (0.1-
`0.5 M, pH ) 5-6), and it remains stable at -78 °C for at
`least 72 h. Very high specific activity can be achieved
`for 90Y-TA138. At pH 6.0, the minimal amount of TA138
`required to achieve 95% RCP for 90Y-TA138 is (cid:24)20 (cid:237)g
`for 20 mCi of 90YCl3 (Figure 4) corresponding to a TA138:
`90Y ratio of (cid:24)30:1. The anaerobic AA formulation is also
`good for routine preparation of 177Lu-TA138 and 111In-
`TA138. Very high 177Lu-labeling efficiency has been
`achieved even though the specific activity of 177Lu is much
`higher than that of 90YCl3. The source of 111InCl3 has no
`significant effect on the RCP of 111In-TA138 (Table 3).
`
`CONCLUSION
`This study clearly shows that ascorbic acid is a good
`buffer agent for pH control and an excellent antioxidant
`for stabilization of metal-labeled diagnostic (111In) and
`therapeutic (90Y and 177Lu) radiopharmaceuticals. If the
`radiolabeling is performed at pH 5-6, there is no need
`for an additional stabilizer or buffer agent in the formu-
`lation matrix. The anaerobic AA formulation described
`in this study is simple and is particularly useful for
`radiolabeling of small biomolecules sensitive to the
`radiolytic degradation during radiolabeling.
`Acknowledgment is made to Dr. Thomas D. Harris
`for the synthesis of TA138 (3-sulfon-N-[[4,7,10-tris(car-
`boxymethyl)-1,4,7,10-tetraaza-cyclododec-1-yl]acetyl]-L-
`alanyl-N-[2-[4-[[[(1S)-1-carboxy-2-[[[1,4-dihydro-7-[(1H-
`imidazol-2-ylamino]methyl]-1-methyl-4-oxo-3-
`quinolinyl]carbonyl]amino]ethyl]amino]sulfonyl]-3,5-
`dimethylphenoxy]-1-oxobutyl]amino]ethyl]-3-sulfo-L-
`alaninamide).
`
`LITERATURE CITED
`
`(1) Anderson, C. J., and Welch, M. J. (1999) Radiolabeled agents
`(non-technetium) for diagnostic imaging. Chem. Rev. 99,
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