an electro-amalgamation approach to isolate no-carrier-added 177lu from neutron irradiated yb for...

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Nuclear Medicine and Biology 37 (2010) 811–820www.elsevier.com/locate/nucmedbio

An electro-amalgamation approach to isolate no-carrier-added 177Lu fromneutron irradiated Yb for biomedical applications

Rubel Chakravarty, Tapas Das, Ashutosh Dash⁎, Meera VenkateshRadiopharmaceuticals Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Received 24 November 2009; received in revised form 7 April 2010; accepted 7 April 2010

Abstract

Introduction: A novel two-step separation process for the production of no-carrier-added (NCA) 177Lu from neutron irradiated Yb targetthrough an electrochemical pathway employing mercury-pool cathode has been developed.Methods: A two-cycle electrolysis procedure was adopted for separation of 177Lu from 177Lu/Yb mixture in lithium citrate medium. Theinfluence of different experimental parameters on the separation process was investigated and optimized for the quantitative deposition of Ybin presence of 177Lu. The first electrolysis was performed for 50 min in the 177Lu/Yb feed solution at pH 6 applying a potential of 8 V usingplatinum electrode as anode and mercury as the cathode. The second electrolysis was performed under the same conditions using freshelectrodes. The radionuclidic and chemical purity of 177Lu was determined by using gamma ray spectrometry and atomic absorptionspectrometry. The suitability of 177Lu for biomedical applications was ascertained by labeling 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid D-Phe1-Tyr3-octreotate(DOTA-TATE) with 177Lu.Results: This process could provide NCA 177Lu with N99.99% radionuclidic purity and an overall separation yield of ∼99% was achievedwithin 3–4 h. The Hg content in the product was determined to be b1 ppm. Radiolabeling yield of N98% was obtained with DOTA-TATEunder the optimized reaction conditions.Conclusions: An efficient strategy for the separation of NCA 177Lu, suitable for biomedical applications, has been developed.© 2010 Elsevier Inc. All rights reserved.

Keywords: NCA 177Lu; DOTA-TATE; Electrochemical separation; 177Lu from Yb; Mercury cathode

1. Introduction

In recent years, 177Lu has emerged as a promising short-rangeβ−emitter for targeted radiotherapy owing to its favorablenuclear decay characteristics and straightforward coordinationchemistry [1,2]. 177Lu decays to stable 177Hf with a half-life of6.71 days by emission of β− particles with Emax of 497 keV(78.6%), 384 keV (9.1%) and 176 keV (12.2%) [3]. Thepresence of low-energy gamma rays [Eγ=113 keV (6.4%), 208keV (11%)] [3] helps in imaging the in-vivo localizationwithoutthe use of a surrogate nuclide [1]. The long half-life of 177Luprovides logistic advantage for shipment to places far awayfrom the reactors and also provides ample time for theproduction of 177Lu-based radiopharmaceuticals [1]. Owing tothe large neutron absorption cross-section of 177Lu (σth=2090 b)[3], it can be prepared with adequately high specific activity by

⁎ Corresponding author. Fax: +91 22 2550 5151.E-mail address: [email protected] (A. Dash).

0969-8051/$ – see front matter © 2010 Elsevier Inc. All rights reserved.doi:10.1016/j.nucmedbio.2010.04.082

neutron irradiation of enriched 176Lu target [176Lu(n,γ)177Lu][1]. Although this straight forward method produces 177Lu ofhigh specific activity, it is never free from the presence of 176Lu.Typically 20-30% of Lu atoms are 177Lu when it is produced ina moderate flux reactor (∼1×1014 n.cm-2s-1) after irradiation for∼3 weeks. Though it is reported that this specific activity issuitable for the preparation of receptor-specific radiopharma-ceuticals such as 177Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid D-Phe1-Tyr3-octreotate(177Lu-DOTA-TATE) [4], it is essential to use 177Lu soon after itsproduction. Further, the metastable 177mLu, (T1/2=161 days) isco-produced in the “direct”method. 177mLu being long-lived, isgenerally not desired for application in hospitals owing to theissues related to the storage and disposal of waste arising afterthe use of 177Lu. No-carrier-added (NCA) 177Lu of very highspecific activity could be produced through an alternative pathwherein an isotopically enriched 176Yb target undergoes (n,γ)reaction to produce 177Yb which subsequently decays by β−

emission (T1/2=1.9 h) to yield177Lu. Thismethod also precludes

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812 R. Chakravarty et al. / Nuclear Medicine and Biology 37 (2010) 811–820

the presence of 177mLu as radionuclidic impurity. However, thisroute requires efficient radiochemical separation of 177Lu fromlarge amounts of the Yb target.

The aim of this work was to develop an efficient andviable technique for the separation of NCA 177Lu frommacroscopic amounts of the Yb target. In the recent years,numerous methods for the separation of NCA 177Lu fromlarge amounts of Yb have been reported [5–12]. Separationof Lu from irradiated Yb, using common methods such asion-exchange chromatography and solvent extraction isdifficult, though not impossible. In this context, electro-chemical method appeared to be an attractive and feasibleproposition and, hence, pursued. Utilization of the differencein the electrode potentials of Yb and Lu from ionic state tometallic state was precluded owing to the deeply negativereduction potentials (more negative than hydrogen dis-charge) of lanthanides and due to the difficulty in controllingtheir electrolytic deposition onto the solid cathode. Hence, anelectrochemical separation based on the selective reductionof ytterbium to the divalent state and its preferential transferon to a mercury cathode was considered as a possiblesolution owing to the high over-potential of hydrogen onmercury together with the ability of Yb to form amalgam.This approach was investigated.

The applicability of the electrochemical method usingmercury cathode for the separation of Yb from otherlanthanides has been well known since the original workof Marsh [13–16]. It has been reported that Lu has noamalgam forming property whereas Yb forms an amalgamreadily [15] and this property could be satisfactorilyexploited to achieve their separation. Two such successfulpreparative-scale separations of Lu from Yb for clinicalapplications have been reported in the literature by Lebedevet al. [10] and Bilewicz et al. [12]. However, the reportedprocedures are complex in nature and require severalpurification steps to achieve satisfactory radionuclidic purityof 177Lu.

Herein we report our method of separation of 177Lu (up to18.5 GBq, 500 mCi) from the bulk quantity of irradiated Ybconsisting of two consecutive electrolytic separation steps. Inthe first step, the bulk of Yb target is removed and in thesecond step tracer quantities of Yb present in the 177Lu areremoved to obtain no carrier added grade 177Lu. AvailingNCA 177Lu from Yb, through two-step electrolytic processwithout the inclusion of ion-exchange purification step is theattractive feature of the present work. The feasibility of themethod, both in terms of yield and the purity of the 177Lu forpreparation of radiopharmaceuticals, has been demonstratedand evaluated.

Fig. 1. Experimental arrangement for the dissolution of irradiated target.

2. Materials

Natural Yb2O3 was procured from Indian Rare Earths,Mumbai, Maharastra, India. Reagents such as hydrochloricacid, ammonium hydroxide, lithium citrate, ammonium

acetate and ethyl alcohol were of analytical grade and wereprocured from S.D. Fine Chemicals, Mumbai, Maharashtra,India. DOTA-TATE was obtained from PiChem, Austria.Platinum metal wires of high purity were procured fromHindustan Platinum Limited, Mumbai, Maharashtra, India.Polarographic grade high purity mercury was obtained fromE. Merck, Drastadt, Germany. Paper chromatography (PC)strips were purchased from Whatman, Kent, UK. Highperformance liquid chromatography (HPLC) grade waterwas procured from E. Merck, Dramstadt, Germany.

HPGe detector coupled with a multichannel analyzer(MCA) (Canberra Eurisys, Lingolsheim, France) with a 1.5keV resolution at 1333 keV and range from 1.8 to 2 MeVwas used for analysis of 177Lu in the presence of Yb. Theefficiency of this instrument was estimated using a standard152Eu source. A D.C. power supply with 100 V compliancewith a maximum current of 2 A and current resolution of 1.2nA was used for electrochemical studies. A Perkin-Elmeratomic absorption spectrometer (Model 1100B) equippedwith an HGA-700 graphite furnace and an AS-70 auto-sampler was used for the estimation of trace levels of Hgcontamination present in NCA 177Lu.

3. Experimental

3.1. Production of 177Lu/Yb

Natural Yb2O3 (20 mg) was irradiated in the Dhruvareactor of our institute at a flux of ∼7×1013 n.cm−2.s−1 for7 days, in 10 different batches. The irradiated target wasallowed to decay for 2 days and then dissolved in 5 ml of 2 NHCl with gentle warming. The radioactive solution obtainedwas evaporated to near dryness and then reconstituted with5 ml of HPLC grade water. Subsequently, the solution wasremotely pumped into the transfer vessel using vacuum fromwhich it was collected in a beaker (Fig. 1). The 177Lu/Ybactivity produced was measured in a HPGe detector bymonitoring the 208 keV γ-ray peak of 177Lu after

813R. Chakravarty et al. / Nuclear Medicine and Biology 37 (2010) 811–820

appropriate dilution of the sample. In order to scale up theprocess, 200 mg of natural Yb2O3 was irradiated andprocessed under the same conditions as described above.

3.2. Electrolysis setup

The electrochemical separation involved selectiveamalgamation of Yb from 177Lu/Yb mixture into mercu-ry-pool cathode. High purity platinum plate (4×1 cm) wasused as anode and mercury (5 ml) was used as thecathode. The electrochemical cell consisted of a waterjacketed glass cell (34×70 mm, 30 mm internal diameter)for the circulation of cold water to keep the cell cool. Theglass cell was fitted with a stop-cock at the bottom for theremoval of mercury after electrolysis. The mercury- poolcathode was connected to the negative potential of the

Fig. 2. Schematic diagram of th

power supply via a platinum wire. A provision wasprovided for passing gas through a glass tube, dipped intothe electrolysis solution. Argon gas was continuouslypurged through the electrolyte during the course ofelectrolysis. A schematic diagram of the electrochemicalcell is given in Fig. 2.

3.3. Optimization of electrochemical parameters forseparation of 177Lu from 177Lu/Yb mixture

Parameters, such as the applied potential, pH of theelectrolyte, and time of electro-deposition which influencethe electrolytic deposition of Yb were varied to optimizethe process and achieve effective separation of 177Lu fromYb. A mixture of 15 ml lithium citrate solution (0.15 M)and 37 MBq (1 mCi) 177Lu (as 177Lu/Yb mixture containing

e electrochemical set-up.


∼0.35 mg Yb) was used as the electrolyte. Initially, theeffect of applied potential on selective amalgamation of Ybwas studied by carrying out the electrolysis at pH 6–7 andvarying the applied potential. Subsequently, the electro-chemical amalgamation of Yb was studied as a function ofpH at the optimal potential. Experiments were thenperformed to estimate the minimum time required forquantitative amalgamation of Yb at the optimum pH andapplied potential.

3.4. Process demonstration

3.4.1. Step 1: Separation stepThe process was demonstrated by taking 1.85 GBq (50

mCi) of 177Lu containing ∼17.5 mg Yb (as 177Lu/Ybmixture) in 15 ml of 0.15 M lithium citrate solution. The pHof this solution was adjusted to 6–7 by dropwise addition of3% ammonium hydroxide. Electrolysis was carried out byapplying a constant potential of 8 V (current 500 mA) for30 min. After the electrolysis, the power supply wasswitched off and the mercury cathode was drained offfrom the cell through the stop-cock. The aqueous electrolytecontaining 177Lu was collected separately by passingthrough a Whatman filter paper.

3.4.2. Step 2: Purification stepIn order to remove any trace amount of Yb that may be

present in the 177Lu electrolyte, Step 1 was repeated usingfresh mercury cathode and platinum anode in a newelectrolytic cell. Subsequently, the aqueous electrolytesolution containing NCA 177Lu was separated and collectedin a beaker. The radioactive solution containing NCA 177Luwas concentrated by evaporating it to near dryness, and thenreconstituting to 2 ml with HPLC grade water.

3.5. Determination of radionuclidic purity of 177Lu

The radionuclidic purity of the separated 177Lu wasmeasured in an aliquot by γ-spectrometry, using a calibratedMCA coupled HPGe detector. The activity of 177Lu wasquantified by measuring the 208 keV γ-ray peak (11%) andthe amount of Yb present in it was determined by measuringthe 196 keV (35.9%) and 396 keV (6.5%) γ-ray peaks,emitted from the 169Yb and 175Yb isotopes, respectively.Owing to its short half-life, 177Yb (T1/2=1.9 h), would havedecayed during cooling and could not be detected inthe samples.

3.6. Estimation of Hg contamination in NCA 177Lu

In order to estimate the trace amount of Hg that may bepresent in NCA 177Lu obtained, the sample was allowed todecay for 2.5 months (N10 half-lives of 177Lu). The Hg levelin the decayed samples was quantified by cold vaporgeneration graphite furnace atomic absorption spectrometrytechnique. The calibration curve for Hg was obtained byusing standard solutions of Hg and the curve was fitted bylinear regression.

3.7. Scaling up the production of NCA 177Lu

Based on the procedure standardized at the 1.85 GBq(50 mCi) level, the process was scaled up to 18.5 GBq(500 mCi) using the optimized conditions. After the twoelectrolysis steps, the radioactive solution containing NCA177Lu was concentrated to 2 ml following the proceduredescribed in Step 3.4.2.

3.8. Labeling efficacy of NCA 177Lu with DOTA-TATE

In order to evaluate the suitability of NCA 177Lu foruse in the preparation of radiopharmaceuticals, 177Lu-DOTA-TATE was prepared following the reported proce-dure [4]. A simulated solution equivalent to 1.85 GBq(50 mCi) of NCA 177Lu was prepared by adding 0.4 μg ofLu carrier in 200 μl (∼185 MBq, 5 mCi) of NCA 177Lusolution, obtained from step 3.4. For the radiolabelingstudies, 25 μl of DOTA-TATE solution in HPLC gradewater (concentration 1 μg/μl) was mixed with 275 μl of0.1 M ammonium acetate buffer (pH ∼5.5) to which 200μl of NCA 177Lu solution (equivalent to 1.85 GBq,50 mCi) was added. The resulting mixture was incubatedat 80°C for 45 min after carefully adjusting its pH to ∼5.The complexation yield was determined by PC using 50%acetonitrile in water as the eluting solvent [4]. In order todemonstrate the labeling efficacy at higher level ofactivity, 200 μl (∼1.85 GBq, 50 mCi) of NCA 177Luobtained from step 3.7 was used for the preparation of177Lu-DOTA-TATE under the same conditions.

3.9. Simulated study on the recovery of 177Lu from a Lu/Ybcarrier-added solution

The effects of macroscopic amounts of Yb on theelectrochemical separation of NCA 177Lu were investigatedby conducting experiments using Lu/Yb mixtures containinginactive Lu and Yb carriers equivalent to ∼37 GBq (1 Ci)177Lu (∼10 μg) and 400 mg of Yb. The mixture was spikedwith 37 MBq (1 mCi) of 177Lu (as 177Lu/Yb mixture), the pHof the solution was adjusted to ∼6–7 and the electrolyticseparation was carried out as described above. The activitycontent of the recovered 177Lu was measured to determinethe electrochemical separation yield.

3.10. Recovery of amalgamated Yb from mercury cathode

After the isolation of 177Lu, the Yb amalgam wasallowed to decay for 2 days. It was subsequentlytransferred into a separating funnel, washed with ethylalcohol and etched with 20 ml of 6 M HCl for 30 min atroom temperature in order to retrieve the Yb. The Hg wascollected and the process was repeated for four successivetimes. The HCl solution containing Yb was evaporated todryness and then reconstituted with 20 ml of de-ionizedwater. Subsequently, this solution was treated with 20 mlof 1 N oxalic acid solution to precipitate Yb as oxalate.


The precipitate was converted to Yb2O3 by calcination at850°C in a furnace [17].

Fig. 3. Effect of pH of on separation of 177Lu from Yb.

4. Results

4.1. Production of 177Lu/Yb

Typically, in one batch about 1.85 GBq (50 mCi) of 177Luwas produced when 20 mg of natural Yb2O3 powder wasirradiated at a thermal neutron flux of 7×1013 n.cm−2.s−1 for7 days.

4.2. Optimization of electrochemical parameters

4.2.1. Optimization of applied potentialThe applied potential is an important factor that

influences the separation of Lu from Yb. Accordingly, theexperiments on Yb amalgamation were performed over awide range of applied potential at room temperature,maintaining the electrolyte at pH 6–7. The results aredepicted in Table 1, and it can be seen that the percentage ofamalgamated Yb increased with increasing potential, reach-ing the maximum value at ∼8 V.

4.2.2. Optimization of pH of electrolyteThe pH of the electrolytic solution plays a crucial role in

any electrochemical separation. The influence of pH on theamalgamation yield of Yb as well as Lu was thereforestudied. As seen in Fig. 3, the amalgamation of Yb reaches amaximum (N99.9%) at pH 6, and it remained constant onfurther increase in pH, up to a value of 7. However, Luremained unamalgamated under this condition. On increasein pH beyond a value of 7, the amalgamation yield of Ybremained constant whereas there was a ∼5% decrease inthe yield of 177Lu. This may be due to the formation of Lu(OH)3 precipitate under basic pH conditions, which getssorbed on to Hg. Therefore, a pH range ∼6–7 was found tobe most suitable for the separation. The pH of the electrolyteincreased slightly during the course of electrolysis and it wasadjusted to a value between 6 and 7 by adding HCl solution.

4.2.3. Optimization of time of electrolysisThe amalgamation yield of Yb as a function of time is

illustrated in Fig. 4, which shows the need to run theelectrodeposition for at least 20 min. It was observed that atthe end of 20 min of electrolysis at the applied potential of

Table 1Yields of Yb amalgamation at various applied potentials (pH ∼6)

Applied potential (V) % Yb amalgamated

2 2 (1)4 12 (2)6 60 (1)8 99.5 (0.5)10 99.8 (0.1)

% 177Lu amalgamated was undetectable at all the applied potentials studied.Figures in the parentheses indicate standard deviation (n=3).

8 V, the current dropped from about 500 mA to 200 mA,indicating the sharp decline in transport of ions, which inturn could be interpreted as the completion of Yb+3 iontransport to the cathode.

Bubbling of argon as an inert gas through the solutionbefore and during electrolysis was essential to vent the gasesas well as to keep the solution in dynamic form.

4.3. Process demonstration

4.3.1. Step 1: Separation stepOn measurement of 177Lu activity, it was observed that

there was negligible loss of 177Lu activity during the firstelectrolysis step. After the completion of electrolysis,mercury-aqueous interface could be easily identified byvisual inspection. The Yb-amalgam containing 99.9% of Ybcould be removed efficiently. Although majority of the 177Lu

Fig. 4. Effect of time of electrolysis on separation of 177Lu from Yb.

able 2eparation of 18.5 GBq (500 mCi) 177Lu

otopes produced ActivityGBq (Ci)

Activity in the electrolyteafter 2 step electrolysis


was retained in the aqueous phase, it was associated with0.1% of Yb activity. Further purification of the 177Lusolution was therefore warranted for removing Yb impurity,and this was achieved by second electrolysis step.

GBq (Ci)77Lu 18.5 (0.5) 18.5 (0.5)69Yb 7.4 (0.2) 5.9×10−5 (1.6×10−6)75Yb 614.2 (16.6) 7.0×10−4 (1.9×10−5)

4.3.2. Step 2: Purification stepA second electrolysis run was carried out using a new

mercury electrode to remove trace quantities of Yb present inNCA 177Lu. The results from this operation indicated that thedecay corrected yield of 177Lu obtained was (99.3±0.3) %with ∼10−4% of Yb contamination present in it. The resultobtained was the mean of 10 different batches. It wasobserved that the developed separation procedure wascapable of providing high purity NCA 177Lu in N99%yield consistently.

After the purification step, ∼1.85 GBq (50 mCi) of 177Luactivity was obtained in ∼10 ml solution. After electrolysis,the volume of the aqueous electrolyte decreased from 15 to∼10 ml. After evaporation and reconstitution, ∼1.85 GBq

Fig. 5. γ-Ray spectra of (A) 177Lu/Yb mixture and (B) separated 177Lu.

TS

Is

1

1

1

(50 mCi) of 177Lu activity was obtained in a volume of 2 ml(at a radioactive concentration of∼0.92 GBq/ml, 25mCi/ml).

4.4. Determination of radionuclidic purity of 177Lu

In order to ascertain the radionuclidic purity of theseparated NCA 177Lu, it was subjected to gamma rayspectrometry. The principal radionuclidic impurity thatcould be detected in the recovered 177Lu was 175Yb. It wasobserved that the levels of 175Yb present in the sample afterthe purification step were below 10−4% in all the 10 differentbatches. No other gamma emitters were detected in theanalysed samples. The γ-ray spectra of the 177Lu/Yb mixtureand the separated NCA 177Lu are shown in Fig. 5.

4.5. Estimation of Hg contamination in 177Lu

The Hg content in the NCA 177Lu obtained from sixdifferent batches was found to be b1 ppm.

4.6. Scaling up the production of NCA 177Lu

In order to establish the suitability of the developedmethod for producing sufficient quantity of 177Lu fortherapeutic applications, 200 mg of natural Yb2O3 wasirradiated under the same flux conditions and 18.5 GBq (500mCi) of 177Lu was produced by the decay of 177Yb. Theresults of the separation of NCA 177Lu from this batch areshown in Table 2. Separation of up to 18.5 GBq (500 mCi) ofclinical grade 177Lu activity in 2 ml solution (radioactiveconcentration ∼9.25 GBq/ml, 250 mCi/ml) was achievedwith N99% yield.

4.7. Labeling efficacy of NCA 177Lu with DOTA-TATE

The suitability of NCA 177Lu for biomedical applicationswas tested by labeling DOTA-TATE. The complexationyield was determined by PC, where in 177Lu-DOTA-TATEmoved towards the solvent front (Rf=0.6–0.7) (Fig. 6A),while unlabeled 177Lu3+ remained at the point of application(Rf=0) (Fig. 6B).

Since 177Lu is of NCA grade, it is expected that even verylow amounts of peptide would be adequate for nearquantitative complexation with 177Lu. It was observed that25 μg of DOTA-TATE (17.41 nmol) was sufficient forlabeling ∼1.85 GBq (50 mCi) of 177Lu with N98%complexation yield. Here the ligand to metal ratio is 6.7:1.

able 3xtraction of amalgamated Yb with 6 N HCl solution

xtraction no. % Extraction yield

39 (9)30 (10)20 (6)5 (2)2 (1)

et yield N 95%

igures in the parentheses indicate standard deviation (n=3).


4.8. Simulated study on the recovery of 177Lu from a Lu/Ybcarrier-added solution

The separation efficiency and kinetics of recovery of NCA177Lu from the feed solution simulated to represent 37 GBq(1 Ci) of 177Lu (in the form Lu/Ybmixture) was found to be asgood as at lower strengths of Lu/Ybmixture.On analysis of the177Lu product, no peak corresponding to Yb was observed inthe HPGe detector confirming the purity of the product. Yieldsof N99% of 177Lu were observed after the initial electrolysisstep as well as after the second electrolysis step. Therefore, itmay be inferred that the separation method developed iscapable of yielding reliable predictive results even at higherlevels of activity and Yb3+ concentrations.

4.9. Recovery of amalgamated Yb from mercury cathode

In order to scale up the process for producing higherquantity of 177Lu activity, enriched 176Yb target needs to be

Fig. 6. Paper chromatographic patterns of (A) 177Lu-DOTA-TATE and (B)177Lu3+ (blank) in 50% acetonitrile in water.

TE

E

12345N

F

used. However, since enriched 176Yb is expensive, thisprocess would be economically viable if the amalgamatedYb could be recovered and reused. Therefore, an attempt wasbeen made to recover the Yb from the amalgamated Yb andthe results are shown in Table 3. It is clear from the table that∼90% of amalgamated Yb could be recovered by a three-step solvent extraction with 6 N HCl solutions. This step ofextraction of Yb additionally helps in safe disposal of Hgwhich is also an issue of concern. The entire methodologyfor production of NCA grade 177Lu is briefly summarizedin Fig. 7.

5. Discussion

In vivo targeted therapy using radiopharmaceuticals isfast growing and 177Lu is among the prominent isotopes foruse in such therapy [4,18,19]. The convenient availability ofNCA 177Lu would facilitate research on the preparation ofnovel targeted therapy agents. The present investigation aimsat the production of NCA 177Lu suitable for development oftarget-specific radiotherapeutic agents. In the last few years,several papers dealing with the production of NCA 177Luwere reported [2,5–12]. Although solvent extraction [11]and column chromatography [5–9] strategies based onorganic materials have been reported to be effective for theseparation of NCA 177Lu from Yb, their susceptibility toradiolytic damage limits their applicability and acceptabilityfor large-scale operations. Only two electrochemical sepa-ration methods [10,12] were found to be capable ofseparating NCA 177Lu from Yb for such applications. Themethod reported by Lebedev et al. [10] involves eightcementation cycles. Even after the cementation cycles, theseparated 177Lu contained 10 μg Yb(III) from 200 mg ofneutron irradiated Yb2O3 and had to be subjected to an ion-exchange process for purification. The procedure reported byBilewicz et al. [12] is based on the reduction of Yb(III) to Yb(II) with sodium amalgam followed by selective precipita-tion of Yb(II) as sulphate. 177Lu solution obtained afterprecipitation step contained 1 mg Yb(III) from 50 mg ofneutron irradiated Yb2O3 and was therefore subjected topurification by an ion-exchange process. Both the multistepseparation procedures have limitations due to their com-plexity and requirement of a purification step to achievesatisfactory purity. Moreover, none of these reported pro-

Fig. 7. Production flow chart.


cedures has demonstrated the suitability of NCA 177Lu forthe preparation of radiolabeled biomolecules. In view ofthese difficulties, development of a viable process for theseparation of NCA 177Lu for biomedical applications stillremains an interesting challenge.

Our aim was to examine the feasibility of producing 177Luthrough the 176Yb(n,γ) →177Lu route using the electro-chemical separation method based on mercury-pool cathode,from natural Yb2O3 target, irradiated at a moderate thermalneutron flux. The simplicity in the operation of theelectrochemical cell having mercury-pool cathode, itsperformance to achieve good Lu/Yb separation andadaptability for remote operations in shielded systems werethe impetus driving development of this process.

An examination of the redox potentials of the Yb andLu indicated the possibility of Yb existing in bivalent state[20]. However, in the case of Lu, a stable bivalent state isunknown. Further, Yb2+ is known to form amalgam whileLu can not amalgamate [15]. Therefore, Lu is difficult todeposit on Hg cathode from aqueous electrolytes. Incontrast, Yb3+ can be electrolytically reduced to Yb2+ andamalgamated. The process in itself is simple and easy toperform and the condition of electrolysis ensures that Yb+2

stays as amalgam and does not get reoxidized. It alsoensures no re-oxidation of Yb2+, easy handling anddeposition of Yb on Hg. Additionally, the high hydrogenover-voltage of Hg electrode mitigates the disturbance byhydrogen ion reduction, which enables effective reductionof Yb in a mildly acidic solution. Therefore, it was evidentthat Yb could be preferentially amalgamated in presenceof Lu so that high purity Lu would be left behind inthe electrolyte.

In order to achieve an effective separation and quantita-tive deposition of 177Yb at the mercury cathode, the mainexperimental parameters are the pH of the solution, appliedpotential and the time of electrolysis. The optimal current of500 mA corresponded to a potential of 8 V, which is wellabove the potential necessary for the amalgam formation ofthe Yb. The reaction proceeded readily and generated littleheat. The rate of deposition of a metal decreased as electro-lysis proceeded, owing to depletion of the electrolyte andaccumulation of Yb in the mercury. Purging of theelectrolyte with argon not only removed dissolved gasesfrom the electrolysis bath but also provided an agitatingaction during electrolysis.

In order to obtain high specific activity 177Lu, it isadvisable to separate 177Lu from the Yb target as soon aspossible after the end of bombardment, because 175Luformed by the decay of 175Yb is likely to lower the specificactivity. It was difficult to accomplish the near completeremoval of Yb component from 177Lu in a single electrolysisstep. However, it was observed that inclusion of anadditional electrolysis step would help to achieve thisobjective. Therefore, the separation was followed by apurification electrolysis step to provide 177Lu of acceptablequality for clinical applications.

Radiolabeling studies were carried out to evaluate theefficacy of the separated 177Lu to form labeled compoundsin nanomolar concentrations. 177Lu separated by thismethod could be successfully used for preparation ofhigh specific activity 177Lu-DOTA-TATE, an agentpresently being used for treating in-operable neuro-endocrine tumors over-expressing somatostatin receptors[4,18,19]. It was observed that the 177Lu obtained by this


technique is suitable for the preparation of targeted therapyagents, with adequate purity as well as specific activity forradiopharmaceuticals preparation.

Ten different batches of 177Lu were produced in order toverify the consistency and reproducibility of the describedprocedure. The production process turned out to be reliablethroughout several runs resulting in an overall yield of 99.3±0.3% of 177Lu in these batches. We felt that for long termsustainability, the process needs to be economically viableand hence were keen in recovering the enriched Yb target.We could achieve ∼90% recovery of Yb in three-stepextraction process.

In order to evaluate the usability, it was necessary to scaleup the process in real situation. We hence irradiated 200 mgof natural Yb2O3 target expecting ∼18.5 GBq (500mCi) of177Lu. The process also leads to the coproduction of multi-curie level of 175Yb as impurity. Though the 169Yb and175Yb impurities could be efficiently removed by theseparation process, this would give unnecessary radiationdose to the working personnel. Owing to the radiation doseassociated with 175Yb impurity, the separation was carriedout in a lead shielded facility to provide radiation protection.The level of impurities in the irradiated target and in the finalproduct can be minimized to a great extent by the use ofenriched Yb2O3 target.

Although NCA 177Lu is commercially available (MDSNordion), the high cost of the product has been a deterrentfor its wider use. According to Knapp [2] accessing 177Luthrough 176Yb(n,γ) →177Lu route, although attractive,suffers from lower yields (∼250-fold lower) of 177Lu incomparison with direct neutron capture route. The attributessuch as simplicity of the process, inexpensive target used andpossibility of production in a moderate flux reactor, maymake (n,γ) production of 177Lu an attractive method.However, the newly developed procedure reported by uscould pave the way for availing indigenous source of NCA177Lu needed for developing targeted radiotherapeuticagents, using moderate flux reactors.

Our approach is more appealing than the other reportedmethods, in terms of yield, specificity and reproducibility.Enhanced selectivity of the electrochemical procedure offersthe benefits of a higher purity product while reducing overallprocessing times. The method is inexpensive, not susceptibleto radiolytic damage, generates minimum radioactive wasteand provides sufficient purity of 177Lu for nuclear medicineapplications. The experimental setup is robust, compact,easy to operate and amenable for making automated systems.We envision that this technique can serve as an option foraccessing NCA 177Lu using medium neutron flux nuclearresearch reactors, and could act as a positive factor in thegrowth of 177Lu-based radiopharmaceuticals.

6. Conclusions

The potential utility of electrochemical method based onan in-house-prepared electrochemical cell containing mer-

cury-pool cathode for the separation of NCA 177Lu from Ybwas demonstrated. The simplicity of this technique coupledwith the ease of using liquid mercury cathode makes it anattractive method. Several batches of NCA 177Lu weresuccessfully produced. The suitability of the product in thepreparation of 177Lu labeled formulations such as 177Lu-DOTA-TATE was evaluated and found to be satisfactory.The developed technique could be used for the routineproduction of NCA 177Lu, especially in the developingcountries with limited reactor facilities, to explore 177Lu as atherapeutic radionuclide.

Acknowledgments

The authors are grateful to Dr. V. Venugopal, Director,Radiochemistry and Isotope Group for his constant encour-agement and continued support. Thanks are due to Dr. M. R.A. Pillai, Technical Officer, Industrial Applications andChemistry Section, Division of Physical and ChemicalSciences, IAEA, Vienna, Austria for his encouragement andvaluable suggestions during the course of the work. Theauthors express their sincere thanks to Mr. SubramaniMoorthy of this Division for the fabrication of theelectrochemical cell. Our sincere thanks are due to Dr. S.V. Thakare and Mr. K.C. Jagadeesan of this Division forarranging neutron irradiations of Yb targets. The authorswish to express their sincere thanks to Mr. Pritam Bansodefor making the drawings used in this manuscript.

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an electro-amalgamation approach to isolate no-carrier-added 177lu from neutron irradiated yb for...

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