ICANCER RESEARCH ISUPPL.) 55. 5786s—5793s.December I. 19951
Departments of Nuclear Medicine fT. M. B.. W. S. B., H. J. B., F. G. W.] andD-9/054 Erlangen, Germans'
Abstract
The goal of this study was to intraindividuaHy compare a complete vei@usa fragmented,directly @fc-labeled,monoclonalanti-carcinoembryonicantigen (CEA) antibody, with reaped to their antigen-targeting kinetics, sensidvity, and diagnostic accuracy in patients with CEA-expressing tumors.
Twenty-fivepatientswere investigatedwiththe @“Tc-labeledanti-CEAIgGiBW431/26and the F(ab')JFab' fragmentmixtureF023C5within7 days.Forquantitative analysis, the region of interest technique was applied to planarscans, whole-body scans, and single photon emission computed tomographyslices 10 min to 48 h postinjection (Pt). Final correlations were performedaccording to the histopatholorj@ after surgery or biopsy. Earliest tumordetection was possible with complete IgGi 4 h P1 (52% of finally positivelesions). Twenty-four- or even 48-h scans were necessary in 48% of finally
positive lesions; tumor detection with fragments was possible in 17% at 1 h P1
and in 94% at 4 h P1. With both monoclonal antibodies, in 35%, singlephoton emission computed tomography was necessary for tumor detection.Absolute antibody uptake in tumor lesions was higher with complete monoclonal antibodies than with fragsnents. The sensitivity of fragments washigher in detecting primary tumors, local recurrences, and lymph nodemetastases. For detection of liver metastases, sensitivity was also higher forfragments than for IgG (87 vei@cus73%), but in scintigraphkally positivelesioits, tumor:background ratios were significanfly lower with fragments
(1.26 ±0.12 versus 1.70 ±0.32; P < 0.01). Therefore, fragments seem to bemore suitable for earlier detection oflesions known for their good vascular
ization, vascular permeability, and antigen accessibility, such as local recurrences, lymph node metastases, and peritoneal carcinomatoses. In liver metastases (high interstitial pressure, low vascular leakage), sensitivity offragments is hiajwr, but their rapid senim and whole-body dearance lead toa lowerabsoluteantibodyuptake,with the consequenceofsignificantly lowertumor:background ratios than with IgG.
Introduction
Immunoscintigraphy has crossed the “research/clinicalbarrier―(1)in oncology to become a new diagnostic procedure; several MAbs3are available with 1231 WIn, and 99mTc labels (2—8).Completeimmunoglobulin molecules have rather slow kinetics with regard totumor uptake and whole-body clearance (for overview, see Ref. 7).This led to the development of fragments with a more rapid clearance(6), especially suitable for labeling with short-lived isotopes, suchas 99mTc
Numerous animal and clinical studies on the kinetics, sensitivities,and diagnostic accuracy of whole and fragmented antibodies labeledwith 1311 1251 1231 II‘In,and 99mTc have been conducted (3—19).There have been studies comparing different cohorts of patients oranimals with different antibody fragmentation grades (20—22),butuntil now, no clinical imaging investigation using two different anti
I Presented at the “Fifth Conference on Radioimmunodetection and Radioimmuno
therapy of Cancer,―October 6—8.1994, Princeton, NJ. Supported in part by Grant DFGBel689/l—lof the Deutsche Forschungsgemeinschaft.
2 To whom requests for reprints should be addressed, at Department of Nuclear
Medicine, University of Gottingen, Robert-Koch-Strasse 40, D-37075 Gottingen, Ocrmany. Phone: ++49/55l/39-8510; Fax: + +49/551/39-8526.
3 The abbreviations used are: MAb, monoclonal antibody; CEA, carcinoembryonic
antigen; P1, postinjection; CT. computed tomography; SPECT, single photon emissionCT;MRI,magneticresonanceimaging.
Surgery [M. W. K., J. R. S.], Friedrich@Alexander-University Erlangen-Nuremberg,
body fragmentation grades in the same patient has been reported. Thisis the field in which animal experiments cannot deliver enoughrelevant information because of the different physicochemical tumorenvironment, which would include vascularization, interstitial pressure, vasculary leakage, and several additional factors affecting antibody targeting (12, 23—26).Furthermore, the kinetic aspect of tumortargeting with complete IgG versus fragments has not been investigated in any of the previous studies.
In this study, we prospectively compared the antibody targetingkinetics, sensitivity, and diagnostic accuracy of two @mTc@labe1edanti-CEA MAbs of different fragmentation grade (IgG1 BW 431/26versus a F(ab')@/Fab'mixture of clone F023C5) intraindividually.Both MAbs belong to different clones, but both recognize a relatedclass III protein antigenic determinant of CEA according to theclassification of Primus et a!. (27), with similar affinity (28, 29) oftheir parent IgG species. Hence, a comparable behavior of both MAbscould be expected, with differences relying only on the differentfragmentation grades.
Patients, Materials, and Methods
Patient Selection. Between November 1992 and February1994, we prospectively investigated 25 patients with CEA-expressing tumors (23 with
recurrent colorectal carcinoma, I endometrial, and 1 lung adenocarcinomawith immunohistologically proven CEA expression; Table 1). All patients hadsuspected recurrent or metastatic disease. CEA levels ranged between 1.0and 1178.0 ng/ml (mean, 72.5 ng/ml; median, 9.4 ng/ml; normal value,
5.0 ng/ml). Within a period of 1 week, patients were studied with both@‘°Tc-labeledantibodies in a randomized fashion.Antibody Preparation. Both murine antibodies, belonging to the IgGi
class, react with a related class III antigenic determinant of the CEAmolecule (27, 28, 30). The IgGI class antibody BW 431/26 (Behringwerke,Marburg, Germany), an extensively studied anti-CEA antibody (3, 4, 12,
31), was directly labeled with 99mTcin a two-step process according toSchwarz and Steinstrhsser (32). F023C5 (Sorin Biomedica, Saluggia, Italy)
has been studied as whole IgGl and F(ab')2 fragments with 131! 1231 and1 1 ‘In-labels (8, 29, 33, 34). ssmTclabeled F(ab')2 fragments were intro
duced recently in clinical investigation (13, 35). These F(ab')2 fragmentswere reduced to deliver free thiol groups for Tc labeling by a conventionalpretinning method (35—37).
Antibody Molecular Species Analysis and Affinity Determination. Foranalysis ofmolecular composition, the labeled antibodies were analyzed by gelfiltration chromatography on a column of Sephadex G-l00 Superfine (0.5 X120 cm), as is described in more detail elsewhere (37). Fractions were collected and analyzed for their activity and protein content. The affinity ofantibodies was determined in a Scatchard plot analysis, essentially as describedby Bosslet et a!. (28).
Antibody Administration, Scintigraphic ProtocoL Seven hundred fortyto 1295 MBq of @‘°Tc-labeledantibody (1.5—1.9mg of IgG, 0.3—0.5mg offragments) were injected i.v. within 30 mm after labeling through an indwelling catheter. Whole-body scans were acquired at 10 mm and 1, 4, and 24 h;planar imaging of pelvis, abdomen, thorax, and head was perfonned 4—6and18—24h P1. In selected cases, 48-h scans were performed on critical regions.
SPECT was performed at 4—6and 18—24h P1. All images wereconcordantly judged by three experienced nuclear medicine physicians,without knowledge of the results of other imaging modalities, and were
5786s
Diagnostic Accuracy and Tumor-targeting Kinetics of Complete versusFragmented @mTc@labeledAnti-Carcinoembryonic AntigenAntibodies: An Intraindividual Comparison1
Thomas M. Behr, Wolfgang S. Becker,2 Michael W. Klein, Hans-J. Bair, Johannes R. Scheele, and Friedrich G. Wolf
SH, M, 5359.4lymph nodelocoregional recurrenceperitonealcarcinomatosis(22)
BT, M. 547.8NSAID inj. site (false positive bonemetastasis)
(23) Si, M, 65 3.7 locoregional recurrence
(24) Mi, M, 76 14.0 inflammatory tissue (false positive
locoregional recurrence)suspected from lymph node
(25) MG, M. 60 5.4 liver metastasis
lymph node
a Time P1 and method (planar/SPECT) that could detect tumorous lesion first.
later compared to conventional procedures (CT, MRI, ultrasonography).
Positive liver metastasis delineation was defined as either (a) warm or hot
liver lesions (i.e., a tumor:background ratio 1. 10, or (b) a cold lesion withscintigraphically hot rim [“rimsign― (12)] if compared to normal liver
parenchyma. Lesions that appeared as (c) scintigraphically isointense (i.e.,tumor:background ratio of 0.90—1.09) or (d) cold (tumor:background ratio
0.90), when compared to normal liver parenchyma, were interpreted asnegative. Also read as negative were lesions that had some diffuse antibodyuptake but remained below physiological liver activity during the imagingcourse.
Camera Equipment and Imaging Technique. A double-headedrotating.y camera (Rota-lI) interfaced with a Microdelta Plus Computer (Siemens
Gammasonics, Erlangen, Germany) was used. Images were acquired in a 128x 128 word-modematrix(energywindow, 140 keV ±10%;planarscans at1 and 4 h after antibody administration with 500,000 counts/view and at 24 and
48 h with 200,000 counts/View). For SPECT, 60 planar projections over 360°
in step-and-shoot technique (4—6 h P1, 30 s/view; 24 h P1, 40 s/view) were
acquired in a 64 x 64 matrix (filtered back projection: Butterworth filter,Nyquist frequency, 0.4—0.5).
Additional Imaging Techniques and Laboratory Examinations. All pa
tients had a chest radiograph, abdominal and chest CT, and/or (if clinicallyindicated) MRI. All patients had routine baseline blood chemistry measurements before antibody administration. In all patients, CEA serum levels weredetermined by a standard enzyme-linked immunoassay procedure (Abbott
Whole-Body and Serum Clearance Determinations. For determinationof the whole-body clearance and organ uptake kinetics, region of interesttechnique was applied onto whole-body scans and SPECT slices at 10 mm and1,4, and 24 h after antibody administration (calculation of the geometric mean,physical half-life correction, expression in percentage of injected dose byreferring to whole-body counts at 10 mm after MAb injection). One-ml serumsamples, taken 10 mm and 1, 4, and 24 h after antibody injection, were
measured in a well counter for determination of serum clearance; molecular
Table 2 NornwI antibody distribution and organ uptake kineticsof IgGI BW431i26 andfragments F023C5 (allvalues in % injecteddose/organ)OrganIgOFragmentsI
COMPLETE VERSUS FRAGMENTED w@@Fc.LABELED ANTIBODIES
A B
C@ D
Fig. 1. Normal antibody distribution (patient 23) at 4 and 24 h P1. A and B, IgG I at 4 and 24 h P1. Prominent liver uptake; high residual blood pool activity after 24 h; in dorsal
view, locoregional recurrence faintly visible (arrows). C and D. fragments at 4 and 24 h P1. Renal uptake predominant; already after 4 h more “interstitialactivity―(outside the bloodvessels) than with IgG; almost no residual blood pool activity after 24 h; in dorsal view, locoregional recurrence faintly visible (arrow; in this whole-body scan at 4 h obscured bybladder activity).
weight species composition was analyzed by gel filtration chromatography,
essentially as described for native MAbs (see “Antibodymolecular speciesanalysis and Affinity Determination―), and is published elsewhere in moredetail (37).
Surgical Protocol and Histopathological Evaluation. After immunoscintigraphic assessment, surgery and/or biopsy of all lesions were done
within a period of 4 weeks. Final correlation as true-positive/negative orfalse-positive/negative was performed according to the histopathologicalresult.
Results
Molecular Composition, Antigen Affinity, Serum and Whole-BodyClearance, and Organ Distribution of Antibodies
IgGl BW 431/26 could be shown by SDS-PAGE and gel filtrationchromatography to consist of a single protein molecule of about Mr
150,000 that contained more than 97% of the total Tc label, consistentwith a Tc-labeled intact IgGI molecule. In contrast, F(ab')2 F023C5was demonstrated to contain two molecular weight species after Tclabeling, presumably due to reductive disulfide bond fission duringthe pretinning procedure (37, 38—40).Twenty-one % of protein hada Mr of about 100,000 (79% had a Mr of 50,000), compatible withF(ab')2 and Fab' fragments, respectively. The affinity constant of99rnTclabeled IgG BW 431/26 was determined by Scatchard plotanalysis as 5 X iO@M 1 in good accordance with the literature (28);the affinity of 99―@Tc-labeledfragments F023C5 was found as8 X 108 M1.
As expected, serum clearance of the fragment mixture was significantly higher (P < 0.001) than that of complete antibody [biologicalserum t½Of IgG approximately 36 h, t½Of F(ab'), 16 h, Fab' 4 h}.
Comparison of organ distribution and uptake of both MAbs revealed
a Per definition (whole-body scan before first voiding of urinary bladder).
b Determined from blood samples as described in “Patients, Materials, and Methods.―
The negative lesions were indistinguishable from normal liver parenchyma.
b Both lesions appeared as “rimsigns― (i.e., cold with hot surrounding).
C One lesion scintigraphically cold, the other isointense when compared to the normal
liver parenchyma.
COMPLETE VERSUS FRAGMENTED w@@Fc@LABELEDANTIBODIES
IgG1, these lesions were indistinguishable from normal liver parenchyma (interpreted as negative; see “Patients,Materials, and Methods―).Large metastases appeared as scintigraphically isointense orcold when using fragmented MAbs, whereas with whole IgGI theydeveloped a hot rim [rim sign (12)] within 24 h. These larger lesionstook up fragmented antibody protein more homogenously and dif
fusely, remaining as a whole “colder―than normal liver parenchyma.
The overall sensitivity in liver metastasis detection was, therefore,higher for fragments (13 of 15 versus 11 of 15), but tumor:Iiverbackground ratios of scintigraphically positive lesions were found tobe significantly higher with IgG than with fragments [1.70 ±0.32versus 1.26 ±0. 12 (P < 0.01); Figs. 2 and 3]. (Values at time ofmaximal tumor:background ratio, usually 24 h P1 with IgGl and 4h P1 with fragments). The cutoff for positive liver lesion delineation was defined at 1.10 (see “Patients,Materials, and Methods―),but the unequivocal blinded interpretation as positive, withoutknowledge of the CT scan result, was possible at tumor:background ratios above 1.25. Only 7 of 13 “positive―liver metastasesfulfilled this requirement with fragmented MAb, compared to 11 of11 with IgGl . Also, as a consequence of these lower tumor:background ratios, only one liver lesion was seen in planar scanswith fragments, whereas with IgGi , 6 of 15 liver metastases wereseen in planar scans 24 h after antibody administration. A tumor:background ratio of over 1.55 (calculated from SPECT) was foundto be the approximate cutoff for detectability of peripherallylocated metastases on planar scans (Fig. 3).
A comparison of typical liver metastasis targeting kinetics with IgGversus fragments is shown in Fig. 2D. A fundamental differencebetween complete IgG and fragments becomes obvious. The initial
absolute antibody uptake during the first hour after antibody injectionin the liver and antigen-expressing tumor tissue is very similar withIgG and fragments (presumably, mostly blood pool). With IgG, continuous uptake occurs over 24 h in the normal liver parenchyma, aswell as in the antigen-expressing metastatic lesions, leading slowly toincreasing tumor:background ratios. With fragments, uptake is mostlyconfined to the first hour after antibody injection. Afterward, washoutpredominates but is more rapid out of normal liver parenchyma thanout of antigen-specific binding. Through this rapid washout, diagnostically significant tumor:background ratios are achieved earlier, allowing diagnosis mostly after 4 h (with IgG, often not before 24 h;compare Table 4). The absolute antibody uptake of fragments, however, is only about 40% of the value of complete IgG.
Lymph Node Metastases. Lymph node metastasis detection waspossible with fragmented MAb earlier after injection and with highersensitivity when compared to complete antibodies (Table 5). Theoverall sensitivities of fragments versus whole IgG1 were 9 of 9(100%) versus 6 of 9 (67%), respectively. Positive lesion detectionwas possible earlier with fragments; in a higher percentage, planarscans were sufficient. When examining antibody uptake kinetics in
Table 4 Liver metastases
clear differences (Fig. I, Table 2). The most striking differences werethe high whole-body clearance rate of fragments and, with respect toorgan targeting, with whole IgG1 liver uptake is predominant,whereas there is a marked renal antibody uptake of the fragments.
Antibody Targeting of Tumors
Overall, 2 primary tumors, 11 local recurrences, 9 patients withlymph node metastases, 8 patients suffering from liver metastases, 1kidney and 1 brain metastasis patient, and 4 peritoneal carcinomatoseswere investigated (Table 1). With whole IgG BW 43 1/26 antibodies,the earliest possible tumor detection was at 4 h P1 (52% of lesionsbecoming finally positive), whereas in 48%, 24- or even 48-h scanswere necessary. In 52%, SPECT was necessary for earlier detection oflesions than was possible from the planar scans, and 35% of thelesions were visualized only by SPECT. With fragments, however,tumor detection was possible in 17% of finally positive lesions at 1 hP1, and at 4 h, 94% of the lesions were detected. For the earliestpossible tumor visualization, in 60%, planar scans were sufficient,whereas in 40%, early diagnosis required SPECT; 34% of the lesionscould be detected only by SPECT.
Overall Diagnostic Accuracy
In the 25 double studies, three false-positive lesions were observed;in 1 patient, strong antibody accumulation in the cecal pole occurred24 h P1 with both MAbs and was interpreted as a colonic primary. No
adequate lesions (not even inflammation) could be verified by endoscopy. The second was a nonspecific antibody accumulation at the siteof i.m. injections of nonsteroidal anti-inflammatory drugs in thegluteal muscle, taken false-positively as a bone metastasis. The thirdhad a retrovesical soft tissue mass strongly suspected as a locoregionalrecurrence by CT, and was interpreted as positive with the completeMAb, whereas it was doubtful with fragments; histology revealedonly some nonspecifically inflamed scar tissue.
Analysis by Tumor Sites
Locoregional Recurrence. A total of 11 patients suffering fromlocal recurrence was investigated (Tables 1 and 3; Fig. 1). Overall, the
sensitivity was higher with fragments, and in a higher percentage,planar scans were sufficient. Whereas 10 of 11 fragment scans werepositive after 4 h, in 2 of 11 patients, 48-h scans were necessary withIgG. Six of the 11 local recurrences (all histologically confirmed)were detected only by immunoscintigraphy, whereas CT scan wasinterpreted as normal (5 of 11 cases) or had not been performed(1 case).
Liver Metastases. When comparing both MAbs in their targetingof liver metastases, clear differences in sensitivities in relationship tolesion size became evident (Table 4); small liver lesions (<2 cm)could be detected with similar sensitivity as scintigraphically hot withboth MAbs. Medium-sized lesions (2—4cm in diameter) were detected with higher sensitivity when using fragments. With whole
COMPLETE VERSUS FRAGMENTED @@‘°Fc-LABELEDANTIBODtES
A
C
Fig. 2. Liver metastases in a patient with polycystic liver and kidney disease (patient 2), where neither D' nor MRS could reliably differentiate between cystic and metastatic lesions.A, SPED' 4 h P1. Top, IgG [two clearly hot metastases (arrowheads)]; bottom, fragments [three metastases appear as “warm―(arrowheads)]. B, CT slice in the same plane as A, Multiple“round―liver lesions, most of them being cysts; only the three lesions positive in CEA scan were confirmed to be metastatic. C, histology (H&E stain, X25) confirmed all three lesionsas metastatic. D, tumor uptake kinetics and tumor:background (tu/backgr) ratios for the three lesions with IgO and fragments. ID, injected dose; •,liver metastatis V; I, liver metastasisVII; •,liver metastasis VNII; Ls,liver parenchyma.
I I.....@@@‘..@ . . . . . . . s:@@@@ .@
.‘,.....•....•-@•.(@ :@
lymph node metastases, similar to liver metastases, a clear trend wasobservable; the absolute protein uptake is lower with fragments, butrapid background clearance leads to faster tumor delineation. Mostuptake occurs in the first 4 h after antibody administration of fragments, whereas uptake is continuous over 24 h with IgG1 (1 h P1, 0.5± 0.2 versus 0.6 ± 0.2; 4 h P1, 0.7 ± 0.4 versus 0.7 ± 0.2; 24 h P1,
1.1 ±0.6 versus 0.6 ±0.3; values in percentage of injected dose/lOg for IgGl versus fragments; n = 3; no statistical significance givenbecause of low numbers available). Fig. 4 shows the time course oftumor:background ratios of lymph node metastases. Up to 4 h P1,
tumor:background ratios were higher with fragments after 24 h thanwith IgG (washout of fragments versus continuous uptake of IgG).
Peritoneal Carcinomatosis. The typical feature of diffuse peritoneal carcinomatosis was seen in four patients. Diffuse delineation ofthe whole peritoneal cavity and especially the peritoneal wall, forminga typical “u―-likeactivity accumulation, was clearly discernible withfragments in all cases, beginning 4 h after antibody administration,and unequivocally positive after 24 h. Using complete IgG, 24-h scansbecame positive but not as impressive as with fragments (higher
COMPLETE VERSUS FRAGMENTED °@°1@c-LABELEDANTIBODIES
tumor/backgroundratio
S
I
S
lopontine region measured only 0.4 cm. Both MAbs were positive forthe larger metastasis but negative for the smaller one (fragmentspositive in planar scans, IgG only in SPECT).
Previously Unsuspected Lesions. In 56% (14 patients) of the 25cases, previously unsuspected lesions were found (compare Table 2).No significant differences in frequency of detection of previously
unsuspected lesions were noticed between both MAbs; two patientswith previously unknown liver metastases, six locoregional recurrences, six patients with lymph node metastases, and four peritonealcarcinomatoses were detected.
Discussion
To our knowledge, this is the first time that tumor targeting kinetics, sensitivity, and diagnostic accuracy have been investigated usingMAbs of different fragmentation sizes in the same patients underidentical imaging conditions. The study of different lesion sites withtheir complex and different pathophysiological characteristics onlyseems possible in the human situation. Animal models cannot mimicthese individual unique conditions (i.e. , vascularization, vascular permeability, interstitial pressure, etc.; Refs. 23—25).Both antibodies areprobably directed against different epitopes (27—29)on the CEAmolecule because they derive from different hybridoma clones. Webelieve that the results are nevertheless valid; the antigen affinity ofthe parent MAb (both IgGl subclass) of both could be demonstratedto be similar (the affinity of fragments being about 15% of a completeIgG has been reported; Ref. 41), and both are directed against a relatedclass III peptide determinant of carcinoembryonic antigen (27).Hence, no fundamental differences, with the exception of molecularweight, should be involved.
background ratio, 1. 10) liver metastases with IgG and fragments. neg., negative. Points,mean; bars, SD.
Individual Cases: Primary Tumors, and Kidney, Lung, Bone,and Brain Metastases. Table 6 summarizesimaging resultsof mdividual cases. One colonic tumor primary could be detected exclusively by fragments (patient 1, 4 h P1, planar and SPECT), whereascomplete IgGi was negative until 24 h P1 (48-h imaging not performed). A kidney metastasis was seen with fragments as a cold holein the kidney parenchyma; with complete IgG, however, the 4-h
SPECT was clearly positive. Patient 19 had a lung metastasis (2 cm indiameter) in the right lower lobe (Fig. 5), which was negative withfragments, whereas it became faintly positive in SPECT with complete IgGi after 24 h P1 (in planar scans it was not visible, despite itslocation near the anterior chest wall). One bone metastasis (patient 4with a spine lesion) was delineated as positive with both MAbs, withthe fragmented antibody as early as 1 h after injection, and with IgGnot before 4 h P1 (SPECT).
One of the patients (patient 7) suffered from two brain metastasesdue to occult non-small cell lung cancer, with diffuse mediastinallymph node involvement as well. One metastasis in the right parietooccipital lobe measured 2.5 cm in diameter; the other in the cerebel
tumor/backgroundratio
BW43I/26
I.50
time after MAb injection
Fig. 4. Tumor:background ratios of lymph node metastases in the time course with IgOand fragments (n = 3). Points, mean; bars, SD.
a Diagnosis of peritoneal involvement usually made from planar scans (NA, not applicable). See “Results― for discussion of 245 and h
COMPLE'I'E VERSUS FRAGMENTED °°°1@c-LABELEDANTIBODIES
Both MAbs were directly labeled with @mTc.Whole IgGi BW431/26 was still a homogenous Mr 150,000 protein after labeling, andthe F(ab')2 fragments of F023C5 are further fragmented, essentially toFab', by the reductive step of the labeling procedure, which is a wellknown phenomenon inherent to conventional direct @mTclabeling
procedures (36—40).As expected from earlier studies (4—8,16), the overall sensitivities
were higher with fragments than with complete IgG. Positive tumordetection was possible in 17% as early as 1 h P1 of fragments, whereasfor IgG 24-h images were essential, and some lesions were notvisualized before 48 h P1. Such late imaging is very difficult withradionuclides having a short physical half-life, such as @“@Tc.Theirhigher sensitivity at earlier time points probably results from the
known (7, 23—25) ability of fragments to diffuse and penetrate more
easily in tissue and to be cleared faster from blood and nonspecificbackground, yielding higher tumor:background ratios more rapidly.However, the absolute antibody uptake was significantly less thanwith whole IgG. In early scans (10 mm to 1 h), some lesions (e.g.,locoregional recurrences) showed a marked blood pool, leading to asomewhat faint delineation at this early time point. It was of interestthat those regions that can be visualized early and with high sensitivityare known for their good perfusion, delivering enough antibody molecules to the tumor sites.
Definition of positive liver metastasis detection in immunoscintig
raphy has been controversial in the literature (3, 12, 31). Some authors(31) interpret every liver abnormality in an antibody scan as metastasis (scintigraphically cold, warm, or with hot rim). We do not agreewith this view; cold lesions could be cysts, benign liver tumors, ormetastases from non-CEA-expressing tumors as well (12). Hence,
only tumor:background ratios above 1.10 (either homogenously or rimsigns) were interpreted as positive. Several studies have shown lowsensitivities for liver metastasis detection with whole antibodies (12)because of slow diffusion of large molecules into tissue of high
interstitial pressure and with poor blood supply (23—26).The reportedhigher sensitivities of fragments (6, 35, 42) were proven in this studyas well. However, the absolute antibody uptake and the tumor:back
ground ratios of fragments were significantly lower. In our view, thereare several reasons for this: (a) the blood clearance of fragments is sofast that only a short time exists for diffusion of molecules into thelesions with high interstitial pressure (23, 24). Whole IgG has a longerresidence time in blood so that more antibody is exposed to theantigen-expressing cells, reaching a significantly higher protein uptake per tumor weight; and (b) the affinity of Fab' fragments issignificantly lower than that of IgGl (41). Hence, despite the highersensitivity of fragments, neither IgG nor Fab' seem to be the idealagents for liver lesion imaging. Perhaps stable F(ab')2 could meet allrequirements: (a) less nonspecific liver parenchymal uptake and morerapid kinetics than whole IgG; (b) a longer residence time in the bloodthan Fab' such that slow diffusion kinetics are possible (24); and (c)a similar antigen affinity as complete IgG (41).
Two completely different principles of tumor visualization becameobvious when comparing IgG and fragments. Whereas IgG shows acontinuous uptake in antigen-expressing lesions over the whole observation period of 24—48 h, leading, together with a very slowbackground clearance later than with fragments, to diagnosticallyuseful tumor:background ratios, fragments possess a very limited time
frame of specific uptake (only 1—4h). However, the rapid background
clearance delivers early (1—4h P1) tumor:background ratios that aresufficiently high for diagnostic purposes.
In summary, our comparative study shows that for rapid andreliable diagnosis of most tumor sites (e.g. , locoregional recurrence, lymph node metastases, and peritoneal carcinomatoses; i.e.,sites with low interstitial pressure and good perfusion), fragments
5792s
A
:@_@Fig. 5. Lung metastasis (patient 19) in the right lower lobe (2 cm in diameter). A, top,
IgO SPED' 24 h P1 [faintly positive delineation of the lung lesion (arrowhead)]; bottom,fragment SPED' 4 h P1 [lesion not visible toward the blood pool background (remainingnegative also at 24 h)]. B, corresponding CT scan.
COMPLETE VERSUS FRAGMENThD @@°‘Tc-LABELEDANTIBODIES
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