cently described “double-dilution―technique, which uses amethylene-blue/phosphate buffer solution to stabilize the99mTc@,exame@imepreparation (1). This enabled evaluationof in vivo stability and demonstration of the normal biodistribution and pharmacokinetics of leukocytes labeled withstabilized 99@―Tc-exametazimein healthy individuals. Radiation dosimetry estimates for critical organs and the wholebody have been calculated. Clinical evaluation of imageswas performed to determine any visual difference in thedistribution of activity using the stabilized preparation of

@Tc-exametazime compared with nonstabilized, particularly in the gastrointestinal and renal tracts, because nonspecific bowel activity can be problematic on delayed images(2). It is possible that with stabilized @°‘Tc-exametazimethere is less in vivo elution of secondary complexes from thegranulocytes, resulting in reduced renal and bowel activity.

A practical advantage of using stabilized 99mTc..exametar..ime to label leukocytes, compared with the previous nonstabilized formulation, is increased in vitro stability (up to 4—6h) and preparation with higher reconstituted activities ispossible (up to 7400 MBq) (1). In addition, multiple doses ofstabilized @“Tc-exametazimecan be made from 1 kitpreparation and used for leukocyte-labeling procedures aswell as other uses, such as cerebral perfusion imaging, withassociated cost savings (1).

MATERIALSAND METHODS

SubjectsScans were performed on 10 healthy volunteers (6 men, 4

women; age range, 19—32y; mean, 25 y) who gave negativehistories for renal, hepatic, pulmonary, cardiovascular, and gastrointestinal pathology. Imaging was delayed in those with evidenceof current inflammatory or infectious disease, including respiratorytract infections or antibiotic use in the preceding week. All subjectswere interviewed and underwent physical examination. Eachvolunteer's medical records were reviewed to exclude a significantmedical or surgical history. Pregnant or breast-feeding femaleswere excluded, and females ofchildbearing potential ages requireda negative serum pregnancy test within 48 h of the procedure. Ninevolunteers were Caucasian and I was African-American. Twosubjects had histories of moderate tobacco use. All subjectsreceived compensation as determined by an internal committee,

Labeling leukocytes with @“Tc-exametazimeis a validated technique for imaging infection and inflammation. Anew radiolabelingtechnique has recently been described that enables leukocytelabelingwith a morestable form of @“Tc-exametazime.A normalvalue study of stabilized @Tc-exametazime-IabeIedleukocyteshas been performid, including biodistribution and dosimetryestimates in normal subjects. Methods: Ten volunteers wereinjected with stabilized @“Tc-exametazime—labeledautologousleukocytesto study labeled leukocyte kinetics and dosimetry innormalsubjects.Serial whole-bodyimagingand blood samplingwere performedup to 24 h after injection.Cell-labelingefficiencyand in vivo viability,organ dosimetry,and clearancecalculationswereobtainedfromthebloodsamplesandimagingdataaswellas urineandstoolcollectionupto 36 h afterinjection.Results:Cell-labelingefficiencyof 87.5% ±5.1% was achieved,which issimilar to or better than that reportedwith the standardpreparation of @“Tc-exametazime.In vivo stability of the radiolabeledleukocytes was also similar to in vitro results with stabilized

@“Tc-exametazimeand better than previously reported in vivostability for nonstabilized somTc@exametazime@@labeledleukocytes.Organdosimetryandradiationabsorbeddosesweresimilar with a whole-body absorbed dose of 1.3 x 10@ mGy/MBq. Urinary and fecal excretion of actMty was minimal, andvisualassessmentofthe imagesshowedlittle renalparenchymalactivity and no bowel activity up to 2 h after injection. Conclusion: Cell labelingand in vivostabilityappearimprovedcornparedwith the leukocyteslabeledwith the nonstabilizedpreparation of @“Tc-exametazime.There are advantagesin more costeffectivepreparationof the stabilized @“Tc-exametazimeand anextended window for clinical usage, with good visualization ofabdominalstructureson early images.No significantincreaseinspecific organ and whole-body dosimetry estimates was notedcompared with previous estimates using nonstabilized @“Tcexametazime—labeledleukocytes.

KeyWords:stabilized@“Tc-exametazime;radiolabeledleukocytes;dosimetry;biodistribution;normalvaluestudy

J NucIMed2000;41:934—940

tudies were performed on 10 normal subjects using@“Tc-exametazime—labeledleukocytes prepared by a re

Received Mar. 8, 1999; revisIon accepted Aug. 9, 1999.For correspondence or reprints contact: Joseph C. Hung, PhD, Mayo Clinic,

200FirstSt.SW,Rochester,MN55905.

934 THE JOURNALOF NUCLEARMEDICINE •Vol. 41 •No. 5 •May 2000

Biodistribution and Radiation Dosimetry ofStabilized 99mTc@Exametazime_LabeledLeukocytes in Normal SubjectsPeter D. Robins, Isabel Salazar, Lee A. Forstrom, Brian P. Mullan, and Joseph C. Hung

Nuclear Medicine Department ofDiagnostic Radiology, Mayo Clinic, Rochester, Minnesota; and Faculdade De CiênciasMédicas,Universidade Nova De Lisboa, Lisbon, Portugal

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30mm1.7±0.517.6±4.21h1 .5 ±0.41 6.3 ±3.82hI .3 ±0.314.2 ±3.34h0.9 ±0.411 .4 ±2.68h0.9 ±0.18.9 ±2.024h0.6 ±0.25.3 ±1.3

Study Normalvalue

and the Mayo institutional review board approval of the studyprotocol was obtained before volunteer selection.

Preparationof StabIlIzed @Tc-ExametazIme—LabeIedLeukocytes

Stabilized @“Tc-exametazimewas prepared according to themanufacturer's instructions, using methylene-blue combined witha phosphate buffer solution and fresh eluate from a @MoP@mTcgenerator that had been eluted within the previous 24 h (3). Eachsubject had a freshly reconstituted kit, because studies wereperformed on separate days. The Ceretec kit (Medi-Physics, Inc.,Amersham Healthcare, Arlington Heights, IL) was reconstitutedwith 7400 MBq @Tcactivity in 2 mL, and the methylene-bluestabilizing solution was then immediately added to the reconstituted Ceretec vial. The radiochemical purity of the stabilized

@“Tc-exametazimepreparation was determined with a single-strippaper chromatography method (4,5).

A total of 80 mL whole blood was drawn from each volunteerand, after separation of a mixed leukocyte preparation, the cellswere labeled with stabilized @°‘Tc-exametazimeusing the doubledilution technique recently described by Hung et al. (1). An averageof 1058 MBq stabilized @°‘Tc-exametazimewas initially incubatedwith the isolated mixed leukocytes for 15 mm, and then thepreparation went through the double-dilution process to isolate thefinal radiolabeled leukocytes (1). The final dose of stabilized

@Tc-exametazime-labeledleukocyteswas measured to be 743.7 ±40.7MBq at re-injection.Quality controlon eachdoseincludedinvitro assessment of cell viability by trypan-blue staining, determination ofcell labeling efficiency, and complete blood count and celldifferential.

SampleCollectionBlood samples (5 mL each) were obtained at 30 mm and I, 2,4,

8,and24hafterinjectionofstabilized@“Tc-exametazime—labeledleukocytes to determine leukocyte recovery and blood clearance.For each sample, 1 mL whole blood, plasma, and packed cells wereeach counted to obtain an estimate of the activity retained withinthe cells, which was then decay-corrected to give a percentage ofthe original injected dose. Stool and urine collections in 12-haliquots were collected beginning immediately after injection andfor 36 h (i.e., 12, 24, and 36 h) to enable estimation of excretion ofunbound @“@Tc-exametazimeand free @“Tc-pertechnetateand,therefore, in vivo labeling stability. Representative samples ofurine (5 mL each) and the entire stool sample were counted in adose calibrator, and these values were then also decay-corrected toobtain a percentage of the injected dose.

ImagingProtocolVolunteers were injected in the supine position under a ‘ycamera

equipped with a general-purpose, low-energy collimator (GeneralElectric 500A; General Electric, Milwaukee, WI), and 60 30-simages over the chest were stored on a dedicated computer in 64 X64 word matrix (Pinnacle System, Medasys, Ann Arbor, MI).Whole-body images at 1, 2, 4, 8, and 24 h after injection were

performed using a whole-body, dual-head -y camera (SiemensBodyscan, Des Plaines, IL) equipped with high-resolution collimators. The whole-body anterior and posterior images were acquiredfor 20 mm per pass and stored on the computer in 1,024 X 256word mode matrix. A known standard activity was included in theinitial whole-body pass at 1 h to facilitate accurate dosimetryestimates from the images. Pulmonary transit times were calculatedas well as determination of counts in regions of interest (ROIs) over

TABLE1Plasma and Whole Blood Activity as Percentage of Injected

DoseoverTime

Time after injection Plasma activity (%) Whole blood (%)

liver, spleen, kidney, bone marrow, bladder, lungs, bowel, andbrain. Three experienced nuclear medicine physicians evaluatedthe series of whole-body images, and the organ distribution ofactivity at each time-point was arbitrarily graded relative to theliver.

Dosimetry CalculationThe final injected dose was determined by subtracting the

residual activity in the syringe after injection from the dispenseddose (both radioactivities were measured in a dose calibrator). Weused this value to compare with the blood, urine, and stool sampleactivity, which was also measured in the dose calibrator.

The percentage distribution of activity in various organs wasestimated from the whole-body images at different time pointsallowing for appropriate decay correction. The dosimetry programused for calculation allowed the selection of set organ ROIs, whichwere then applied to the series of whole-body images. The relativeactivity in the various organ systems compared with the injecteddose was calculated at the specific imaging times. This techniquewas semiautomated and increased objectivity regarding ROI selection. The data were incorporated into the MIRDOSE 3 softwareprogram to determine residence times and relevant organ andwhole-body absorbed dose estimates (6).

RESULTS

The change in plasma and whole-blood activity over timeas a percentage of the injected dose is presented in Table I.The mean cell-labeling efficiency was 87.5% ±5.1%, andthe distribution of cellular elements in the final preparationof mixed leukocytes is given in Table 2. The lung half

TABLE 2Normal Value Study of 10 Patients with Stabilized

@Tc-Exametazime—LabeledLeukocytes

Labelingefficiency(%)Injecteddose(MBq)TotalWBCsin injecteddose(x 10@)Total PMNs in injected dose (x 106)Cell% in injecteddose

WBCs(%)RBCs(%)PLTs(%)

WBCs = white bloodcells; PMNs = polyrnorphonuclearleukocytes RBCs= redbloodcells PLTs= platelets.

87.5 ±5.1743.7 ±40.7179.1±48.9106.6 ±42.6

13.6±3.364.7 ±6.821.7 ±6.0

NORMAL VALUES OF STABILIZED 99MTc@HMPAO@Labeled Leukocytes •Robins et al. 935

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OrganPercentagedistributionof injectedcellsovertime(%)lh2h

4h8h24hLiver24.3

±6.320.1 ±5.4 17.6±3.716.0 ±3.615.2 ±2.9Spleen18.5±5.018.2 ±3.8 15.4 ±4.212.5 ±2.39.3 ±4.3Spine3.6±1.14.3 ±1.1 4.5 ±0.94.1 ±0.93.8 ±1.1Bladder3.2±0.84.3 ±1.6 4.1 ±3.23.4 ±2.72.2 ±0.8SI00.1

±0.2 0.1 ±0.300ULI0.7±0.90.7 ±1.0 0.8 ±1.11.4 ±1.12.3 ±2.0LLI0.2±0.40.2 ±0.5 0.3 ±0.50.2 ±0.30.5 ±0.5Brain0.4±0.10.4 ±0.2 0.4 ±0.20.4 ±0.20.3 ±0.1Kidneys00

00.1 ±0.10.1 ±0.2Lungs7.6±1.15.8 ±0.7 5.1 ±0.64.3 ±0.53.3 ±0.5Blood16.3±3.814.2 ±3.3 11.4±2.68.9 ±2.05.3 ±1.3Remainder26.1±0.830.8 ±2.5 35.5±3.632.4 ±3.033.1 ±7.6SI

= smallintestine;ULI= upperlargeintestine;LLI= lowerlargeintestine.

OrganRadiation absorbeddose(mGyIMBq)Spleen0.0851

±0.0170Redmarrow0.0203 ±0.0044Liver0.0111

±0.0038Bonesurfaces0.0099 ±0.0021Urinary

bladder0.0090 ±0.0036Lungs0.0056±0.0014ULI

wall0.0027 ±0.0022LLIwall0.0010 ±0.0014Small

intestine0.0002 ±0.0002Kidneys0.0003±0.0004Breasts0.0001±0.0001Thyroid0.0007±0.0017Ovaries0.0001±0.0001Testes0.0001±0.0002Totalbody0.0013±0.0002Effective

dose (body)0.0070 ±0.0009mSv/MBqULI

= upperlargeintestine;LLI= lowerlargeintestine.

Organ Residencetime(h)

TABLE 3Percentage @°Tc-Exametazime—LabeledLeukocytes in Various Body Compartments over Time

clearance time was performed on the initial dynamic imageseries of the chest to assess cell integrity, and the meanhalf-clearance time was 16.0 ±6.0 mm.

Urine activity collected during the 36 h after injectionranged from 17%—26%of the injected dose. Stool activitywas mmnimal, and only 2 volunteers had detectable excretedactivity in the 36 h after injection. In 1 volunteer this was2.6% of injected activity at 24 h, and in the other volunteerthiswas 1.5% at 36h.

The percentage of administered activity of stabilized99mTc,@exame@ime_labeled leukocytes in various body compartments over time is presented in Table 3. This confirmsminimal activity in the renal and gastrointestinal tracts. Therelative activity in the liver and spleen progressively decreases over time, whereas activity in the urinary bladderand bowel remains stable up to 4—8h. Low-grade activityover the region of the brain likely represents blood-poolactivity in the scalp. Table 4 lists the residence times for

various organs used in dosimetry calculations with the

TABLE 4Residence Times ofActivity in Source Organs

MIRDOSE 3 program. The residence time can be thought ofas the “average―or “effective―life of administered activityin a source organ and accounts for physical decay as well asbiological accumulation and removal. In most situations thiscannot be represented by a single exponential, and theresidence time is not the retention half-time of activity in asource organ. The residence time is longest in the redmarrow, followed by liver and spleen. The radiation absorbed dose estimates for critical organs are listed in Table 5.The highest dose is to the spleen, followed by red marrow,liver, bone, and urinary bladder wall. The estimated wholebody absorbed dose is 0.0013 mGyIMBq, and the calculatedeffective dose is 0.007 mSv/MBq (6, 7).

On clinical evaluation of the whole-body images, thespleen was the organ of most intense uptake visually,

TABLE 5Radiation Absorbed Dose Estimates for Critical Organs

4.19 ±0.951.96±0.471.56 ±0.360.55 ±0.130.36 ±0.13

00.13 ±0.110.03 ±0.040.01 ±0.010.01 ±0.010.80 ±1.01

RedmarrowLiverSpleenLungsUrinarybladderSIULILLIKidneysBrain

Remainder

SI = smallintestine;ULI= upperlargeintestine;LLI= lowerlargeintestine.

936 THE JOURNALOF NUCLEARMEDICINE •Vol. 41 •No. 5 •May 2000

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followed by the liver and urinary bladder. Bone marrow wasseen well in all images as expected. The brain and thyroidgland were never visualized. The renal parenchyma wasfaintly evident on all images, although urinary activitywithin the renal pelvis and, especially, the bladder was seenin most subjects. Lung activity was most evident in the first 2h, consistent with blood-pool activity, and decreased significantly thereafter, with virtually absent lung activity at 24 h in6 of 10 volunteersandminimalactivityin the remainder.The 2 volunteers who smoked did not show elevation oftheir lung half-clearance time or significant delayed lungactivity.

Two volunteers had no visualized small bowel activity atany stage. No bowel activity was seen on the 30-mm or 1-himages in any volunteer (Fig. 1). A trace of small bowelactivity was first seen in 4 volunteers at 2 h and in 4 others at

4 h. This activity appeared to pass through into the largebowel in the majority of volunteers, with persistent smallbowel activity in only 3 subjects at 8 h after injection. Thelarge bowel was first seen at 4 h in half the volunteers andthe remainder at 8 h after injection (Fig. 2). All volunteershad some activity in the large bowel at 24 h. The gallbladderwas seen in only 1 volunteer on the 1- to 8-h images.

DISCUSSION

There are many reported studies using II11n-oxine—and99mTc..exametarime.4abeled leukocytes to image inflammation and infection. Initial studies using 99mTc@exametazime_labeled leukocytes were reported by Peters et al. (8) in 1986,

and ‘ttlnoxine labeling techniques in humans were described as early as 1977 (9). The neutral and lipid solubleI I ‘In-oxine penetrates cell membranes, and indium is then

displaced from oxine by intracellular components with

higher binding affinities for the indium (10). The highlabeling efficiency and stability 11‘In-oxine—labeledleuko

cytes make it a useful imaging agent that has been usedclinically with good diagnostic accuracy for various skeletaland soft tissue pathologies. The imaging characteristics of‘@In,however, are not ideal, and a 99mTc@basedlabelingagent was sought.

The labeling efficiency of radiolabeled leukocytes that weused in our study was determined to be 87.5% ±5.1%. Thislabeling yield is comparable with the ones obtained in ourprevious study (1) and by Sampson et al. (II) and Ozker etal. (12, 13). Although the yield is not as high as the labelingefficiency of II11n-oxine—labeledleukocytes, it certainly is asignificant improvement compared with the 50%—60%yieldrange associated with leukocytes radiolabeled with nonstabilized 99mTc@exametazime(8, 14—16).The kinetics and biodistribution of99mTc@exametazime_labeled leukocytes are simmlar to those of II11n-labeled granulocytes with similar lungtransit times and splenic, bone marrow, and hepatic uptakes

as described in several studies (17—19).Reticuloendothelialactivity up to 24 h is similar in both labeled-cell preparations. With nonstabilized 99mTc@exametazime,however, significantly more nonspecific activity is seen in the kidneys,urine, gallbladder, and bowel. The bowel is often seen asearly as 1—2h after injection and is seen consistently in the

FIGURE1. No significantbowelactivitywaspresentat 1hafterinjection.Prominentblood-pool activity persists in heart andlarge vessels and to lesser extent the lungswith marrow uptake visible. Urinary activityis present in bladder and right renal peMs.

NORMALVALUESOF STABILIZED99MTc@HMPAO@Labeled Leukocytes •Robins et al. 937

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FIGURE 2. Backgroundand blood-poolactivity has decreased at 8 h. Prominentlarge-bowelactivity is present in ascendingcolon, and some small bowel activity ispresent in midabdomen.Lung activity hasdecreased,whereasnormalleukocyteactivity in liver, spleen, and marrow persists.

mmnimize confusion with normal gallbladder and bowelactivity (17).

Previous studies have been performed to determine thenormal biodistribution and dosimetry of nonstabilized 99mTc@exametazime—labeled leukocytes in healthy subjects, andresults have been similar (21). The package insert for

Ceretec was changed after Food and Drug Administrationapproval in April 1995 and now describes a method tostabilize 99mTcexamet@ime with methylene-blue and sodium phosphate buffer (3). The in vitro stability has beenincreased from 30 mm to 4—6h after preparation (3).However, the manufacturer currently recommends the stabilized compound only for brain imaging and not for leukocyte labeling, because of the difficulty in separating out theleukocyte button in a dark blue medium (3). A recent study

from our laboratory describes a double-dilution techniquethat enables a simple and safe leukocyte label that shows ahigher labeling efficiency and much greater in vitro stabilityof radiolabeled leukocytes (1).

Our previous study also demonstrated that Ceretec can bereconstituted with a higher @“@Tcactivity and still maintainacceptable purity 6 h after preparation, allowing multipledoses of stabilized 99mTc@exametazime to be obtained from asingle kit preparation (1). This results in cost-effective usageof cold kits of Ceretec. With the combined usage of thestabilized @‘@‘Tc-exametazimeand the double-dilution technique for labeling leukocytes, a higher yield and lower invitro elution of the radiolabeled leukocytes can be easilyachieved. If this is also the case in vivo, one would expect

colon by 4 h after injection (15, 17,19). This nonspecificactivity is a disadvantage of leukocytes radiolabeled withnonstabilized @Tc-exametazimein interpretation of abdominal images.

The lipophilic character of 99mTc..examet@ime enables itto diffuse through the cell membrane, where it is thought toform a hydrophilic complex that is trapped within the cell(20). There is, however, variable elution of activity from thelabeled cells with subsequent renal and biliary excretion ofsecondary complexes, but this process remains poorlyunderstood. Unbound lipophilic @“Tc-exametazimeor 99mTc@pertechnetate is minimal, because no significant thyroid orbrain uptake is usually identified (17). Details of thekidneys, collecting system, bladder, and gastrointestinaltract are often obscured by this excreted activity, which is asignificant disadvantage in certain abdominal infectious orinflammatory disease processes when using nonstabilized

@“Tc-exametazimeinstead of@@ ‘In-oxine.@ has advantages over @In-oxine, how

ever, with respect to availability, radiation dosimetry, and -yenergy. A larger dose can be given using 99mTc..exametarime,and earlier imaging is performed with shorter acquisitiontimes, resulting in improved image quality. 99mTc@exametaz@ime—labeledleukocytes have become the investigation matenat of choice in certain instances, such as inflammatorybowel disease and acute bacterial soft tissue infection.Unlike ‘‘‘In-labeledleukocyte studies for abdominal pathology, however, when using 99mTc@exametazime it is necessaryto perform early images (within 1—2h after injection) to

938 THEJOURNALOFNUCLEARMEDICINE•Vol. 41 •No. 5 •May 2000

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less unbound background activity and reduced renal andbiliary excretion of eluted activity. Other stabilizing techniques have been described, including cobalt chloride hexahydrate, but the labeling efficiency and radiochemical purityappear unsatisfactory. None have been recognized by themanufacturer or listed in the package insert (22,23).

The results in Table 1 suggest that the release of unboundactivity eluted from the cells is very low. Average celllabeling efficiency of 87.5% is higher than the previousstudies with nonstabilized 99mTc@exametazime (47%—71%)and similar to results with ‘I‘In-oxine—labeledgranulocytesin normal volunteers (8, 15, 17,21,24). The in vivo stability ofleukocytes labeled with the stabilized 99mTc@exametazime,as assessed by the free plasma activity, and excreted urinaryactivity was similar to the results obtained in vitro (1). Thedistribution of cellular elements in the final preparation(Table 2) is also similar to the results with the nonstabilized99mTc..exametarime4abeled leukocyte preparation (22).

The half-clearance time from the lungs ranged from8.0—28.5mm, with a mean value of 16.0 mm, compared withthe results of Becker et al. (17) with 7 mm and Brown et al.(21) with 9.8 mm. There is, however,no clear cutoff valuefor lung clearance at which cellular damage is indicated. We

did not have any visual evidence of prolonged lung retention, and, in fact, residence times and dosimetry values forthe lungs are less compared with those reported previouslywith nonstabilized @“Tc-exametazime—labeledleukocytes(Tables 3, 4, and 5) (21). There was no appreciable activitywithin the lungs at 24 h in any of our subjects on visualassessment.

The whole-blood activity levels measured at various timepoints and listed in Table 1 represent the granulocyte

recovery from the circulating granulocyte pool over time.Because we have used a mixed leukocyte preparation, therewill be a very small contribution from red cell and plateletactivity. Our value of 17.6% ±4.2% (range, 12.3%—28.l%)at 30 mm appears less than values previously obtained for99mTc..exametarimelabeled leukocytes in humans of 29%—37%(15,21).Thecauseforthisisnotclearbutmayindicatesome increased sequestration from the circulating bloodpool. This may explain the slightly greater liver activity inour study (Table 4) compared with previous values obtainedin our department (24.3% versus 17.4% at 1 h, respectively)using nonstabilized 99mTc@exametazime_1abeled leukocytes(21). One author of an editorial review suggestedthat thecirculating granulocyte-associated activity after equilibration (at 30—40mm) was a sensitive variable for in vitroactivation (25). Therefore, the slightly lower recovery mayindicate some labeling-induced changes in the leukocytes,although this is difficult to prove. Even if this were the case,there is no reason to suggest that this would significantlycompromise the sensitivity of the test.

We did not detect thyroid or stomach activity on anyimages, indicating there was no significant circulating free99mTcpe@echnetate Likewise, the brain was never visual

ized, indicating absence of unbound lipophilic 99mTc@

exametazime. Visually there was minimal renal parenchymal activity in all subjects, and this corresponds with the lowrenal dosimetry values seen with previous studies usingnonstabilized 99mTc@exametazime (21). Urinary activitywithin the bladder was seen in all subjects on the earliestimages. Excreted urinary activity represented approximately22% of injected dose after 36 h. This is significantly lessthan the 54% reported in the previous study from ourdepartment using nonstabilized 99mTc..exametarimelabeledleukocytes but is similar to the 17% reported by Peters et al.(19,21).

The gallbladder was seen in only 1 of our subjects, whichis comparable with the 10% gallbladder visualization described by other authors such as Roddie et al. (15) and lowerthan the rate of gallbladder visualization reported by Becker(1 7). Visualization of bowel, which can often hinder diagnosis of intra-abdominal pathology, also appears similar orpossibly reduced compared with previous studies. Becker etal. (17) detected activity in small and large bowel at 1 h in allpatients; Costa et al. (18) detected activity in small bowel at30—60mm in 16% of their studies; and Brown et al. (21)detected bowel activity at 2 h in all but 1 patient. One studyby Peters et al. (19) reported that significant bowel activitywas never seen before 4 h after injection. In our study groupno bowel activity was seen before 2 h. In 4 volunteers a traceof small bowel activity was first seen at 2 h, with anadditional 4 volunteers showing activity at 4 h and beyond.Small bowel activity was never visualized in 2 volunteers.Activity in small bowel in our study appears less than thatencountered in several of the studies cited above, and theminimal activity seen at 2 h in 4 subjects was consideredunlikely to present significant diagnostic confusion for thereviewing physicians (1 7,18,21). The appearance of nonspecific large bowel activity followed that of small bowelactivity and was present in all subjects at 24 h. No subjects,

however, had activity in the colon before 4 h after injection,which is similar to the experience of Peters et al. (17) andbetter than results reported by Becker et al. (19).

The measured fecal excretion of activity also appearssimilar to other studies, although only 2 volunteers returnedstool samples that contained activity during 36 h. Our resultof 2.3% administered activity excreted in stool suffers froma very small sample size but is comparable with the 1.5%and 6% reported in other studies (19,21).

Radiation absorbed doses and residence times will differslightly depending on the technique used to calculate them.We have used a semiautomated technique, which improvesreproducibility of ROl selection and objectivity, and wehave used the published MIRDOSE data to enable mdividual organ and whole-body dosimetry estimation (6). Theindividual organ dosimetry results obtained and whole-bodyabsorbed dose estimate of 0.0013 mGy/MBq were slightlyless than those reported from our department using nonstabilized 99mTcexame@ime_labeled leukocytes and less thanthose listed in the package insert for Ceretec (3,21). Thewhole-body absorbed dose per study was therefore approxi

NORMAL VALUES OF STABILIZED 99MTc@HMPAO.@Labeled Leukocytes •Robins et al. 939

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mately 1.0 mGy. There was no evidence that there was anincreased radiation dose using the new preparation oflabeled cells, despite the improved stability and retention of

@“Tc-exametazimewithin the leukocytes.

CONCLUSION

Proposed benefits of using this technique include morecost-effective preparation of stabilized @“Tc-exametazimein the nuclear pharmacy with greater flexibility in theclinical use of stabilized @mTc@exametazime because of

prolonged (4—6h) in vitro stability. Our findings also suggestthere is less in vivo elution from the leukocytes, which mayimprove imaging characteristics compared with the use ofnonstabilized 99mTcexametarjme_labeled leukocytes. Thismay be advantageous in imaging the kidney and gastrointestinal tract and may expand the clinical applications of

@“Tc-labeledleukocytes. There is no increase in patientabsorbed dose using the stabilized preparation comparedwith the nonstabilized preparation of @@uTc@exametazime@labeled leukocytes. These results confirm that clinical trialsusing stabilized @Tc-exametazime—labeledleukocytes inselect patient populations can be undertaken safely and costefficiently.

ACKNOWLEDGMENTS

We thank Sushital Chowdry and Barbara S. Hollar fortechnical assistance, Douglas W. Mahoney for statisticaladvice, and William F. Dunn for valuable help in dosimetryestimation using the MIRDOSE 3 computer software program. We also thank the study coordinator, Terry L. Brinkman, for her aid in organizing the clinical studies, and VickiS. Krage and Rose M. Busta, for their assistance in thepreparation and submission of this article. This paper waspresented in a poster session at the 1999 Congress of theEuropean Association of Nuclear Medicine held in Barcelona, Spain, on October 10, 1999.

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940 THEJOURNALOFNUCLEARMEDICINE•Vol. 41 •No. 5 •May 2000

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2000;41:934-940.J Nucl Med.   Peter D. Robins, Isabel Salazar, Lee A. Forstrom, Brian P. Mullan and Joseph C. Hung  Leukocytes in Normal Subjects

Tc-Exametazime-Labeled99mBiodistribution and Radiation Dosimetry of Stabilized

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