Tumor localization of CEA-containing human tumors in nude mice by means of monoclonal anti-CEA antibodies

Discrepancy Between Tumor Antigen Distribution and Radiolabeled Antibody Binding in a Nude Mouse Xenograft Model of Human Melanoma

OBJECTIVES

Biodistribution of antibodies is vital to successful immunoscintigraphy/immunotherapy, and it is assumed to be similar to antigen distribution. We measured and compared the binding pattern of radiolabeled antibody to tissue antigen distribution in a nude mouse xenograft model of human melanoma.

METHODS

We transplanted 10⁷ FEM-XII human melanoma cells into the right flank of five nude mice. For the control, we transplanted 5 × 10⁶ LS174T human colon cancer cells into the left flank. Two weeks later, 10 μCi of ¹³¹I-labeled melanoma-associated 96.5 monoclonal antibody (targeting p97 antigen) was intravenously injected. Three days later, we sacrificed the mice and evaluated 96.5 antibody binding and concentration in the tumors by ex vivo quantitative autoradiography (QAR). Two months later, we incubated adjacent tumor tissue slices in various concentrations of ¹²⁵I-labeled 96.5 MoAb and evaluated the distribution/concentration of p97 antigen by in vitro QAR.

RESULTS

p97 antigen distribution was homogeneous in the tumors (total antigen concentration [Bmax] = 17.36-38.36 pmol/g). In contrast, radiolabeled 96.5 antibody binding was heterogenous between location within the tumor (estimated bound antigen concentration = 0.7-6.6 pmol/g). No quantifiable parameters were found to be related with radiolabeled antibody binding and tumor antigen distribution. Antibody-bound tumor antigen to total antigen ratios ranged between 2% and 38%.

CONCLUSIONS

Heterogeneous features of target antibody binding were observed in contrast to relatively homogenous feature of tumor antigen. We did not identify any correlations between p97 antigen distribution and 96.5 antibody binding in melanoma tissue. Radiolabeled 96.5 antibody binding patterns within melanoma cannot be predicted based on p97 antigen distribution in the tumor, which needs to be further studied with several other methods and more subjects in the future.

76Br-labeled monoclonal anti-CEA antibodies for radioimmuno positron emission tomography

For the application of anti-tumor monoclonal antibodies (MAbs) in positron emission tomography (PET), labeling radionuclides with half-lives allowing a suitable time frame for imaging are required. The anti-CEA MAb 38S1 was labeled with the positron emitting nuclide 76Br (t1/2 16 h) using bromoperoxidase (BPO), and subsequently affinity purified. A procedure was devised to allow reproducible production of MAb-preparations of high immunoreactivity and with acceptable bromination yield. The biological activity of 76Br-38S1 was retained and comparable to that of chloramine-T labeled 125I-38S1, as tested in vitro.

Radioimmunolocalization of hepatic metastases and subcutaneous xenografts from a human colonic cancer in the nude rat. Aspects of tumour implantation site and mode of antibody administration

Antibody localization was analyzed following intraperitoneal (i.p.) or intravenous (i.v.) injection of the 125I-labelled anti-CEA-MAb I-38S1 in 44 nude rats, in order to evaluate the influence of tumour implantation site and the route of MAb administration. The animals were xenografted with a human colonic cancer (LS 174 T), either in the form of hepatic metastases, subcutaneous (s.c.) tumours or both. Tissue measurements, 4 days after MAb injection, showed better uptake for hepatic than for s.c. tumours, irrespective of the route of antibody administration. Antibody accumulation per g liver metastases was not size dependent for noduli weighing between 4 and 1,110 mg. MAb excretion evaluated in 20 animals and blood activity studied in 11 rats were equivalent 24-96 h following i.p. and i.v. injection. Dissimilar autoradiographic patterns were seen in hepatic metastases with predominantly peripherally located clusters following i.p. and more homogeneously distributed grains after i.v. MAb administration. The results indicate that tumour implantation site has a quantitative, and the route of administration at least a qualitative impact on the tumour accretion of anti-CEA MAb I-38S1 in the present xenograft model.

Radiolocalisation and imaging of stably HPLAP-transfected MO4 tumours with monoclonal antibodies and fragments

Immunotargeting of PLAP-expressing tumours was studied for two radioiodinated, highly specific anti-PLAP monoclonal antibodies, 7E8 and 17E3, differing 10-fold in affinity, as well as for 7E8 F(ab')2 fragments. An anti-CEA monoclonal antibody or anti-CD3 F(ab')2 fragments were used as controls. Specific and non-specific targeting was examined in nude mice simultaneously grafted with PLAP-positive tumours derived from MO4 1-4 cells, and CEA-positive tumours, derived from 5583-S cells. Results indicated that (1) MO4 1-4 tumours, with a stable expression of PLAP on the plasma membrane, represent a useful new in vivo model for immunodirected tumour targeting; (2) differences in antibody affinity for PLAP in vitro are not reflected in antibody avidity for tumour cells in vivo; and (3) excellent selective and specific localisation of the PLAP-positive tumours is achieved when 7E8 F(ab')2 fragments are used. The high tumour/blood ratios (10.7 +/- 3.9 at 46 h after injection) were due to a much faster blood clearance of 7E8 F(ab')2 fragments. At this time point, the mean tumour/non-tumour tissue ratio was as high as 34.5, and the mean specific localisation index was 29.0. As expected, the F(ab')2 fragments provided high tumour imaging efficiency on gamma camera recording. These data imply important potentials of the PLAP/anti-PLAP system for immunolocalisation and therapy in patients, but also emphasise that in vitro criteria alone are not reflected in in vivo tumour localisation capacities of antibodies.

Heterogeneity in antigenic expression and radiosensitivity in human colon carcinoma cell lines

A panel of human colon carcinoma cell lines were characterized regarding both antigenic heterogeneity and variations in radiosensitivity. Monoclonal antibodies were used to study the expression of carcinoembryonic antigen (CEA), gastrointestinal cancer antigen (GICA or CA 19-9) and carcinoma-associated antigen (CA-50). Radiosensitivity was studied with the clonogenic survival technique. Three cell lines, LS 174T, HCTC, and SW 1116 stained positive for all three antigens. HT-29 was positive for CA 19-9 and CA-50 whereas Caco-2 was positive for CEA and CA 19-9. The cell lines SW 620 and LIM 1215 only stained positive for one of the antigens, CA-50 and CEA, respectively. In nearly all positive cases the stainings were very heterogeneous with mixtures of positive and negative cells. One exception was the HCTC cells which stained homogeneously for the CA 19-9 and CA-50 antigens. The neuroendocrinelike COLO 320 cells were negative in all cases. The radiosensitivity varied strongly between the cell lines with Dq-values between 0.8 and 1.9, extrapolation numbers between 2.0 and 4.7, Do-values between 1.1 and 2.8. The surviving fraction at 2 Gy varied between 0.3 and 0.7 with HCTC as the most radiosensitive and HT-29 as the most radioresistant cell line. Thus, there were differences in antigenic expression and intrinsic radiosensitivity between the cell-lines and antigenic heterogeneities within each cell line. The analyzed panel of cell lines will be valuable in studies of dose-effect relations for monoclonal antibodies labeled with toxic radionuclides simulating both antigenic heterogeneity and variations in radiosensitivity.

Localization of monoclonal antibody G250 and bispecific monoclonal antibody CD3/G250 in human renal-cell carcinoma xenografts: relative effects of size and affinity

A study was made of the relative effects of size and binding strength of various forms of the monoclonal antibody (MAb) G250, reacting with primary and metastatic human renal-cell carcinoma (RCC), on the localization in human RCC xenografts in nude mice. Preferential tumor localization was demonstrated after injection of 125I-labelled intact IgG, with increasing tumor/non-tumor ratios in time. Approximately 27.4% of the injected dose/gram (%ID/g) was localized in the xenograft 24 hr post-injection. A control MAb did not preferentially localize in xenografts. With F(ab')2 fragments, higher tumor/blood ratios were obtained, although a lower percentage of injected dose per gram was bound to the tumor, 24 and 48 hr post-injection. Using a bispecific MAb CD3/G250 F(ab')2 fragment, which reacts with CD3 on human T lymphocytes and binds monovalently to RCC, an enhanced accumulation in tumor tissue was also observed. The %ID/g tumor obtained with bispecific CD3/G250 F(ab')2 was comparable with %ID/g tumor found with G250 F(ab')2. The 10-fold lower binding affinity to RCC compared with intact IgG or F(ab')2 had only marginal effects on %ID/g tumor. These results show that MAb G250 preferentially localizes to RCC xenografts. Because injection of F(ab')2 fragments resulted in higher tumor/non-tumor ratios, G250 F(ab')2 may therefore be more suitable for diagnostic evaluation of RCC in patients. The tumor uptake is more dependent on size than on affinity. Furthermore, the data obtained with bispecific MAb CD3/G250 F(ab')2 support the hypothesis that this bispecific MAb may be able to target cytotoxic T lymphocytes to RCC in humans to mediate destruction of RCC.

Comparison of seven iodine-labelled monoclonal antibodies in nude mice with human colon carcinoma xenografts

The biokinetics of seven 131I-labelled monoclonal antibodies (MAbs), directed against human colon carcinoma and one 125I-labelled unspecific MAb have been examined. The study in nude mice, carrying human colon carcinoma, was intended to be a step in the selection of the most suitable antibody for clinical scintigraphy. The biological half-life in blood was found to be between 1.3 and 7.4 days for the different MAbs. Chromatography of plasma samples showed that the radioiodine was mainly bound to IgG-sized molecules. The (normal tissue)/blood ratios were similar for all the MAbs. The tumour/blood ratio was 0.41 for the unspecific MAb and 0.49-1.1 for the specific MAbs, and the tumour/muscle ratio was between 3.2 and 6.8 for the specific MAbs 6 days after injection. For one MAb tumour/blood and tumour/muscle ratios were 3.9 and 9.8 respectively 9 days after injection. Localization indices were at their highest 2.6 6 days after injection. For at least two of the monoclonal antibodies the tumour/blood and tumour/muscle ratios found are high enough to justify clinical trials regarding their usefulness for scintigraphy of colon cancer in man.

Suppression of tumor vascular activity by radioantibody therapy: implications for multiple cycle treatments

Autoradiographic analysis of the intratumor location of radioiodinated Mu-9 anti-CSAp antibody within 7 days of administration reveals a restricted distribution within 3-6 cell layers surrounding tumor vessels. Within 7-14 days after suboptimal radioantibody treatment (approximately 3000 rads), tumor vessel morphology and physiology are altered. Vessel number is reduced by 60-70%, vessel diameter is reduced, and remaining vessels are surrounded by fibrotic tissue. Vascular volume (VV) is reduced by 75%, blood flow rate (BF) is reduced 65%, and vascular permeability (VP) to an IgG is reduced by 60%. The change in VV is reversible by day 35 but BP and VP remain suppressed. Normal tissue (liver and lung) vasculature experience only small pertubations in physiology. These functional changes in tumor vessels reduce the magnitude of accretion of a second dose of radioantibody in tumor but not in normal tissue.

Improved radioimmunotherapy of colorectal cancer xenografts using antibody mixtures against carcinoembryonic antigen and colon-specific antigen-p

Radioimmunotherapy of GW-39 human colonic tumor xenografts grown in the hamster cheek pouch with 131I-labeled NP-4 anti-(carcinoembryonic antigen) (CEA) and 131I-labeled Mu-9 anti-(color-specific antigen-p) (CSAp) murine monoclonal antibodies, administered in combination, was more effective than using either antibody alone for tumor masses less than 0.5 cm3 in size. The antibody mixture had no therapeutic advantage for larger tumors. Therapeutic efficacy was determined by measuring the change in tumor size over time, quantifying the absolute number of tumors responding to radioantibody therapy, and determining the percentage growth inhibition of each treatment at various times after radioantibody administration. Several mechanisms are discussed to explain the improved tumoricidal effect of the antibody mixture noted in this model system, such as (a) the possibility that an antibody mixture could target a greater number of tumor cells, (b) the potential for antibody mixtures to provide better tumor distribution and (c) the possibility that antibodies administered in combination can increase the magnitude of tumor uptake of individual radioantibodies, thereby resulting in a greater radiation dose delivered to the tumor.

Influence of animal host and tumor implantation site on radio-antibody uptake in the GW-39 human colonic cancer xenograft

The magnitude and kinetics of tumor uptake of a monoclonal antibody (MAb) directed against carcinoembryonic antigen (CEA) in the GW-39 human colorectal cancer xenograft differ according to the animal used (nude mouse or hamster) and the site of implantation of the tumor within the animal (cheek pouch, leg muscle, subcutaneous or liver). Several physiological factors have been evaluated in an attempt to explain these differences in radio-antibody accumulation. The following observations have been made: (1) The animal host with the slower blood clearance of radio-antibody and the higher non-tumor tissue uptake has the higher tumor uptake; (2) the xenografts with a higher blood-flow rate, vascular volume and/or vascular permeability have a higher specific radio-antibody targeting; (3) the smaller, more viable tumors take up more radio-antibody per gram than the larger tumors; and (4) tumors with higher specific antigen content accrete more radio-antibody. These results are discussed in terms of the feasibility of clinical tumor imaging and therapy with radiolabelled antibodies.

Homogeneous penetration but heterogeneous binding of antibodies to carcinoembryonic antigen in human colon carcinoma HT-29 spheroids

The monoclonal antibodies 38S1, directed against the carcinoembryonic antigen (CEA), were tested for penetration and binding in human colon carcinoma HT-29 spheroids. Penetration was studied with a method which has not previously been used in immunological investigations. The method, which allows unbound substances to be visualized, is based on freeze drying, vapour fixation, dry sectioning and dry autoradiography. The antibodies penetrated easily and all parts of the HT-29 spheroids seemed to be reached within 15 min. The penetration was even faster than in control glioma U-118MG spheroids that did not express CEA. Binding of the 38S1 antibodies was demonstrated after processing with conventional histology and autoradiography. The binding in the HT-29 spheroids was, after a 1-h incubation period, extremely heterogeneous and occurred mainly in the peripheral parts. More cells were binding the antibodies after 8-h and 32-h incubations and these cells were arranged in peripheral clusters. No binding at all was seen in the CEA-negative glioma spheroids. The distribution of CEA antigens in monolayers and in frozen sections of spheroids of HT-29 cells was analysed with immunohistochemical staining using polyclonal CEA antibodies. The CEA antigens were heterogeneously distributed in both spheroids and monolayers and were as heterogenous as the binding of the monoclonal antibodies in the living spheroids. Thus, the heterogeneous binding in the living spheroids was not due to penetration barriers, but instead to the heterogeneity in the CEA antigen expression.

The spatial distribution of parenterally administered monoclonal antibodies against CEA in a human colorectal tumour xenograft

A recently developed experimental model consisting of athymic rats carrying human colonic tumours from the cell line LS 174 T in both hind legs was used. 125I-labelled anti-carcinoembryonic (anti-CEA) monoclonal antibodies were injected either intra-arterially after a bolus injection of mannitol, or intra-peritoneally with or without mannitol. On the fourth day the rats were killed and pieces from the tumours and various organs were measured in a well scintillation counter. Tumour pieces were then submitted to autoradiography and immunohistochemistry for examination of the antibody distribution at the cellular level. In all examined tumours injected with anti-CEA antibodies, most of the antibodies were located in the periphery close to fibrovascular septa. It appears, in addition to the specificity of the antibody for the CEA, that the tumour vascular permeability and anatomy are of utmost importance for tumour targeting in this experimental model with the particular antibody used.

Tumor imaging of colo-rectal carcinoma with an anti-CEA monoclonal antibody

A 131I-labelled anti-CEA monoclonal antibody (MAb) (FO23C5) was used in a preclinical in vivo study in an animal model and in a clinical study in patients with colo-rectal carcinoma. Preliminary screening on different tissue was performed with immunoperoxidase staining, which showed a good specificity of the antibody for gastric and colo-rectal carcinoma. Initially immunoscintigraphy was performed as a pilot study at the National Cancer Institute of Milan on 51 patients with 64 localizations of colo-rectal cancer. Serial scans were performed after injection of 2 radioactive reagents, namely 131I and 111In. In vivo pharmacokinetics of the compound were studied. High sensitivity (80%) and high specificity were obtained in detecting local recurrences and abdominal metastases whereas a low positivity rate was obtained in detecting liver metastases. Our data were confirmed by preliminary results of the multicentric trial organized by the Italian National Research Council on 300 patients with about 700 localizations of CEA producing tumors.

Radioimmunodetection of cancer with monoclonal antibodies: current status, problems, and future directions

Early studies of immunoscintography with affinity-purified 131I-labeled polyclonal antibodies reactive against oncofetal antigens such as carcinoembryonic antigen (CEA) were moderately successful in detecting metastatic colorectal carcinoma. However, because of low tumor to background ratios of isotope, background subtraction techniques using 99Tc-labeled albumin were required to visualize small lesions. Antisera were often of low titer and lacked specificity. These problems could be overcome for the most part following the development of highly specific monoclonal antibodies (MoAb) against a variety of tumor-associated antigens. A number of clinical trials using 131I- or 111In-labeled MoAb to image tumors have demonstrated successful localization without the use of subtraction techniques. Variables limiting the usefulness of murine MoAb for diagnosis have included increased localization in liver and spleen, tumor vascularity and heterogeneity of antigen expression, and development of human antimurine globulins. Methods to overcome some of these problems are discussed. Radiolabeled MoAb appear useful as an adjunct to conventional diagnostic techniques both as a means to predict which antibodies might be useful for treatment and, in select patients, as a basis for treatment decisions.

Development and characterization of a monoclonal antibody to human embryonal carcinoma

A monoclonal anti-testicular carcinoma antibody was obtained via the somatic cell fusion technique by immunization of BALB/c mice with freshly prepared single cell suspension from a patient with testicular embryonal carcinoma with choriocarcinoma components. The hybridoma supernates were screened against the testicular carcinoma cells used in the immunization as well as normal mononuclear white blood cells isolated from the same patient. An antibody (5F9) was selected which bound to fresh tumor cells from two patients with embryonal testicular carcinoma and failed to bind to fresh tumor cells from 24 patients (2 seminoma, 2 melanoma, 3 neck, 2 esophageal, 1 ovarian, 3 colon, 1 prostate, 2 breast, 1 liposarcoma, 3 endometrial, 1 kidney, 1 adrenal, 1 larynx and 1 bladder tumors) or cell suspensions prepared from normal liver, lung, spleen, ovary, testes, kidney, red blood cells or white blood cells. The antibody was tested for its binding to several well established cancer cell lines, and was found to bind to the BeWo human choriocarcinoma and two human embryonal carcinoma cell lines. The antibody did not react with 22 other cell lines or with hCG. The antibody was labeled with 131I and injected into nude mice bearing BeWo tumors and evaluated for tumor localization by performing whole body scans with a gamma camera 5 days later. Six mice injected with the antibody showed positive tumor localization without the need for background subtraction while six mice injected with MOPC-21, a murine myeloma immunoglobulin, demonstrated much less tumor localization. Tissue distribution studies performed after scanning showed specific tumor localization (8:1 tumor: muscle) for the monoclonal antibody and no specific localization for MOPC-21. This antibody thus has selective reactivity with the surface of tumor cells from embryonal carcinoma (testicle) and choriocarcinoma both in vitro and in vivo.

Antibody protein dose and radioimmunodetection of GW-39 human colon tumor xenografts

This study investigates the influence of antibody protein dose on the radioimmunodetection of a CEA-producing, human colonic tumor xenograft (GW-39) using affinity-purified goat anti-carcinoembryonic antigen (CEA) antibody and a murine monoclonal anti-CEA antibody (NP-2). Hamsters bearing GW-39 tumors were given an equal mixture of 131I-labelled antibody and 125I-labelled irrelevant IgG at doses varying from 0.01 to 1.0 mg (0.1 to 10 mg/kg body weight) for each antibody preparation. No differences were found in the percentage of antibody in the tumor or the clearance rate from the blood as the antibody dose was increased. The concentration of antibody in the tumor increased proportionally with the antibody dose and maintained a linear relationship with increasing dose, indicating that antigenic sites in the tumor were not saturated with antibody. The concentration of irrelevant IgG in the tumor also increased proportionally as the dose was increased, but the concentration of irrelevant IgG in the tumor was less than the antibody and was removed more rapidly from the tumor than the antibody. By considering the amount of irrelevant IgG in the tumor as a measure of the amount of antibody non-specifically bound, we determined that there was no change in the amount of antibody in the tumor between days 3 and 7, and the amount of antibody increased uniformly for both the tumor and non-tumor tissues, resulting in no improvement in the tumor/non-tumor ratios with increasing antibody dose. External scintigraphy verified that the dose of the antibody did not influence tumor imaging. Thus, tumor accretion of antibody in this model is not dependent principally on the antibody protein dose, and other factors such as accessibility to antigen and antibody metabolism may play important roles in regulating the uptake of antibody in tumor.

An experimental model for pharmacokinetic studies of monoclonal antibodies in human colonic cancer

An experimental model consisting of athymic rats carrying human colonic tumours from cell line LS 174T in both hind legs was used. 125I-labelled anti-carcinoembryonic antigen (anti-CEA) monoclonal antibodies were injected intra-arterially (i.a.), either alone (21 rats) or together with degradable starch microspheres (6 rats). As a control, an irrelevant antibody was injected i.a., alone (6 rats) or together with microspheres (3 rats). An intra-arterial injection was given on the side bearing one tumour in each rat, while the contralateral tumour served as an 'intravenous' control. The rats were submitted to external gamma measurements daily for four days. On the fourth day they were killed and pieces from the tumours and from various organs were examined by in vitro measurements. The results indicate strong expression of CEA in LS 174T cells grafted to athymic rats. No lasting enhancement of the tumour uptake was achieved by intra-arterial injection of antibodies as compared with the control tumours.

Diverse characteristics of 111In labelled anti-CEA monoclonal antibodies for tumour immunoscintigraphy: radiolabelling, biodistribution and imaging studies in mice with human tumour xenografts

Three monoclonal anti-CEA antibodies, designated 161, 198 (both IgG1) and 228 (IgG2a) have been labelled with 111In via DTPA chelation and assessed for localization in human gastro-intestinal carcinomas as xenografts in athymic nude mice. Following reaction of the antibodies with DTPA anhydride, efficiency of chelation of 111In varied between the antibodies with mean values of 30%, 52% and 62% with 161, 198 and 228 respectively. Gel filtration chromatography with all three labelled antibodies showed radiolabel predominantly coincident with IgG with little radioactivity in either high molecular weight form or as free 111In. However, the efficiency of binding of radiolabelled antibodies to CEA producing tumour cells varied, with maxima of 42%, 65% and 20% for 161, 198 and 228. In vivo, in mice, 111In was excreted at virtually identical rates (half times approx. 12 days) with all three preparations and this was similar to the clearance of indium injected as 111In-indium chloride, but 111In-DTPA was rapidly eliminated (half time approximately 5 h). After injection into mice with CEA producing xenografts of colon carcinoma HT29 and LS174T and gastric carcinoma MKN 45, circulating radiolabel was still predominantly in the form of labelled antibody with little or no detectable immune complexes or 111In labelled transferrin. Tumour localization of all three antibodies was visualized by gamma camera imaging with target: non target ratios of up to 5:1. Dissection of mice with MKN45 gastric carcinoma xenograft showed 16%, 19.5% and 13% of the injected dose of 111In from 161, 198 and 228 antibodies in each g of tumour tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Epitope specificity and cross-reactivity pattern of a large series of monoclonal antibodies to carcinoembryonic antigen

Monoclonal antibodies (MAbs) were produced against purified carcinoembryonic antigen (CEA) from liver metastases of colo-rectal and lung adenocarcinoma. Three and eight anti-CEA MAbs from the two groups were analyzed in detail. All antibodies were IgG1. With one exception they recognized epitopes present on all eight individual CEA preparations investigated irrespective of whether they were from colo-rectal or lung carcinoma. The exceptional MAb reacted with an epitope present on most but not all CEA preparations. With two, or possibly three, exceptions the MAbs recognized conformation dependent epitopes in the peptide moiety of CEA. One MAb reacted strongly with reduced and carboxymethylated CEA but only weakly with native CEA. Four MAbs appeared to be CEA-specific in that they did not react with any of the known CEA-cross-reactive substances including nonspecific cross-reactive antigen of 160,000 mol. wt (NCA-160). A total of nine different epitopes were detected in native CEA using this and our previous series [Hedin A., Hammarström S. and Larsson A. (1982) Molec. Immun. 19, 1641] of anti-CEA MAbs. With one exception practically all molecules (70-90%) in purified CEA preparations contained these epitopes.

Selective tumor localization of radiolabeled anti-human melanoma monoclonal antibody fragment demonstrated in the nude mouse model

Monoclonal antibodies (Mab) directed against distinct epitopes of the human 240 kD melanoma-associated antigen have been evaluated for their capacity to localize in human melanoma grafted into nude mice. A favorable tumor to normal tissue ratio of 13 was obtained with intact 131I-labeled MAb Me1-14. This ratio was further increased to 43 and 23 by the use of F(ab')2 and Fab fragments, respectively. The specificity of tumor localization was demonstrated by the simultaneous injection of F(ab')2 fragments from MAb Me1-14 and anti-CEA MAb 35, each labeled with a different iodine isotope, into nude mice grafted with a melanoma and colon carcinoma. The fragments from both MAb localized with perfect selectivity in their relevant tumor as shown by differential whole body scanning and by direct measurement of the two isotopes in tumors and normal tissues. These in vivo experimental results suggest that the F(ab')2 fragment from MAb Me1-14 is suitable for melanoma detection by immunoscintigraphy in patients.

The effect of tumor CEA content and tumor size on tissue uptake of indium 111-labeled anti-CEA monoclonal antibody

This study was undertaken to determine the effect of tumor size and tumor carcinoembryonic antigen (CEA) content on the uptake of indium 111 (111In)-labeled anti-CEA monoclonal antibody in nude mice bearing xenografts. The tumor cell lines were WiDr, SW403, and LS174T, human colon cancer derivatives. The murine breast carcinoma cell line EMT-6 was used as a control. Tumor CEA levels (ng/g of tumor +/- standard error of the mean [SEM], measured by enzyme immunoassay (EIA) were: EMT-6, 0; WiDr, 105 +/- 5.7; LS174T, 2052 +/- 198; SW403, 17,575 +/- 1,785. The 111In-labeled monoclonal antibody was injected intravenously into mice bearing a single tumor. At 48 hours postinjection, scintiscan was performed, and the mice were killed so that biodistribution studies could be performed. The uptake of the monoclonal antibody was expressed as percent injected counts per minute per gram of tissue +/- SEM. The non-CEA-producing tumor, EMT-6, showed the lowest tumor uptake (1.4 +/- 0.3). WiDr, an intermediate CEA-producing tumor, showed some tumor uptake (16.4 +/- 1.5). The high CEA-producing tumors, SW403 and LS174T, had high tumor uptake (29.5 +/- 5.0 and 51.1 +/- 6.1, respectively). Biodistribution and scintiscan quality were closely related. Although LS174T had the best tumor uptake, SW403 had the highest CEA tumor content, indicating tumor CEA content cannot entirely predict scintiscan and biodistribution results. Tumor-to-blood (T/B), tumor-to-liver (T/L), and liver-to-blood (L/B) ratios were calculated for each animal and compared with tumor size. It was found that T/L had a negative correlation with tumor size (r = -0.72) and L/B had a positive correlation with tumor size (r = 0.94). These ratios may be useful clinically to follow response to therapy.

Drug localisation and growth inhibition studies of vindesine-monoclonal anti-CEA conjugates in a human tumour xenograft

The distribution of tritiated vindesine (3H-VDS) was studied in the tissues and tumours of athymic mice bearing a human colorectal tumour xenograft. Selective tumour localisation was obtained when 3H-VDS was injected as a conjugate with a monoclonal anti-CEA antibody (11.285.14) but not as a conjugate with a non-binding monoclonal IgGl (Ag8) or as free succinoyl-VDS. The amounts of VDS that localised in the tumour following injections of 3H-VDS-11.285.14 increased in proportion to the amount injected, over a wide dose range. Conjugates prepared using the Fab fragments of 11.285.14 showed no evidence of selective tumour uptake in comparison with normal tissues. Various dose levels of VDS-11.285.14 conjugate and free VDS were studied for effects on the growth of the tumour xenograft. A growth inhibition of 50% was obtained at 1.5 mg/kg with free VDS and at 2.5 mg/kg with conjugated VDS. The conjugate was, however, considerably less toxic.

Effect of dose escalation of a monoclonal anti-CEA IgG on tumour localisation and tissue distribution in nude mice xenografted with human colon carcinoma

A monoclonal anti-CEA antibody (1H12) has been examined for the effect of dosage on tumour localisation in immunodeprived mice xenografted with human colon carcinoma. Increased doses produced a linear rise in the absolute concentration found in the tumour but this was found to depend on tumour size, with the smaller tumours (mean weight 44 mg) accumulating significantly more antibody compared to larger tumours (mean weight 146 mg). With the smallest tumour (18 mg), in which saturation was achieved, a 500 micrograms dose produced a concentration in tumour of 60 micrograms/g. In the larger tumours a dose of 256 micrograms produced a mean concentration of 5.2 micrograms/g. Prolonged retention of 1H12 by tumour up to 8 days, observed at doses of 4, 128 and 256 micrograms, indicated that the dynamics of localisation is unaffected by dosage. Increased doses of 125I-1H12 caused an increase in the levels of radioactivity associated with all normal tissues studied. Thus at 8 days after injection an increase from 4 to 128 micrograms produced 50% and 42% declines in the tumour to blood and liver ratios, respectively. Cumulative localisation of 1H12 in tumour, from 4 h to 8 days, relative to normal tissue clearance was diminished on increasing dosage. This study shows that attempted therapy with escalated amounts of intact antibody is likely to be limited by a protracted excretory process and measures aimed at accelerating circulatory clearance are necessary.

A short computer program for calculating tissue distributions in tumour-bearing nude mice after administration of radiolabelled specific and non-specific antibodies

The short computer program described provides for rapid calculation of tumour and normal tissue distribution data for radiolabelled specific antibody and non-specific immunoglobulin in the nude mouse xenograft model. Results are displayed in convenient tabular form and also as a bar graph. Facilities for providing a hard copy of all results are installed within the program.

Comparative tumour localization of antibody fragments and intact IgG in nude mice bearing a CEA-producing human colon tumour xenograft

The dynamics of distribution of radiolabelled F(ab')2 fragments of a monoclonal anti-CEA antibody have been studied in the nude mouse bearing a CEA-producing human colon carcinoma. Our results showed that the fragment was rapidly cleared from all normal organs reaching about 1% of that injected by 24 hr. Specific tumour localization occurred as early as 2 hr after injection and was complete by 4 hr. The amount of fragment localized in tumour was 4% of the injected dose, equivalent to that obtained with the intact antibody. Greatly improved tumour:normal tissue ratios were obtained with the fragment compared to intact IgG. However, the residence time of the fragment was much shorter (24 hr) than that of intact antibody (more than 3 days). Tumour localization indices suggested that fragments were superior to intact IgG at locating tumour specifically. The specificity indices based on lung, spleen and liver were much higher than those for intact antibody, reflecting the lack of Fc-receptor binding of fragments and their reduced excretion by these organs. The 'fragment index' enabled tumour:normal tissue ratios for the fragment and intact IgG to be compared. Together with the distribution study at different time points, this simplifies the task of defining a 'time window' in which tumour imaging and therapy might be optimal.

Immunohistochemical localization and molecular characteristics of three monoclonal antibody-defined epitopes detectable on carcinoembryonic antigen (CEA)

Three murine monoclonal antibodies (MAbs) reactive to different epitopes on CEA were selected according to their ability to bind to various human tissue sections. The most selective MAb, BW 431/31, bound to the majority of colon carcinomas but only faintly to normal colon mucosa. In addition to the tissues stained by MAb BW 431/31, MAb BW 250/183 reacted with granulocytes, colon mucosa and faintly with pancreatic ducts. The third MAb, BW 374/14, reacted with granulocytes, colon mucosa, strongly with pancreatic ducts and with alveolar and bronchial epithelium. The antigenic determinants recognized by the 3 MAbs in human tissue sections were resistant to formaldehyde fixation and paraffin embedding as well as to periodic acid oxidation and neuraminidase treatment. The last two treatments suggest that the epitopes are protein in nature. Using MAb affinity chromatography, 3 antigen preparations were isolated from a human colon carcinoma xenograft with an approximate molecular weight of 180 kd. These preparations were shown to bear the epitopes from each of the MAbs and from a polyclonal antiserum specific for purified CEA. Furthermore, the ability of MAb BW 431/31 to localize its antigenic determinant in vivo on a human colon carcinoma xenograft is evaluated and its possible application in the patient is suggested.

Localization of an anti-CEA monoclonal antibody in colo-rectal carcinoma xenografts

A mouse monoclonal anti-CEA antibody (11.285.14) has been examined for tumour localization potential by assessing its distribution in immunodeprived mice with xenografts of human colon carcinoma cell lines HCT-8, HRT-18, HT-29, and LS174T and a xenograft (HRVB) established from a primary rectal carcinoma. With four carcinomas (HCT-8, HT-29, LS174T, and HRVB) preferential tumour localization of 125I anti-CEA was seen. Compared with 131I normal IgG1 localization indices of up to 4.4:1 were achieved. Up to 10% of the injected dose of 125I anti-CEA was present/g of tumour tissue and with the largest xenografts examined (3-4 g) up to 40% of the total body reactivity was localized in tumour tissue. The tumour localization of 131I labelled antibody was visualized by external gamma camera imaging. Overall antibody localization correlated with the CEA content of the xenografts and the fourth colon carcinoma xenograft (HRT-18) and an osteogenic sarcoma xenograft (791T), both with very low CEA levels, showed no localization of the monoclonal antibody.

Human monoclonal antibodies: methods of production and some aspects of their application in oncology

The currently available methods for the production of human monoclonal antibodies (Mabs) are: mouse-human hybrids, human-human hybrids, EBV transformation, and fusion of EBV-transformed specific B-lymphocytes with a malignant cell line. Each method has its advantages and drawbacks, but they are all inferior to an optimal functioning mouse hybridoma system. A significant obstacle is the lack of a reproducible method to obtain high amounts of antigen-specific B-lymphocytes. However, it seems that newly developed in vitro antigen-priming methods may be a significant aid to the solution of this problem. Human Mabs have already been obtained against some types of human cancer, and the reactivity pattern has provided some biological information that seems not to be obtainable with non-human Mabs. It furthermore seems that the technology for production of human Mabs now has progressed to the stage where it permits a realistic hope that they soon can be applied in the diagnosis and treatment of human cancer.

Stable quantitative differences of antigen expression in human melanoma cells isolated by flow cytometric cell sorting

The stability of expression of a membrane antigen on human FME melanoma cells was investigated by means of flow cytometric cell sorting and analysis. The melanoma-associated 250 kd antigen was strongly expressed on all cells, as recognized by binding of the monoclonal antibody 9.2.27. By flow cytometric cell sorting, cells of high and low antigen expression were isolated, and the difference in antigen expression between the two populations was examined as a function of time in culture. Immediately after sorting, the median fluorescence intensities of the two populations differed by a factor of 2.7. After the first few days in culture, much of the range in antigen expression of the parent population was regenerated. However, a lasting difference in antigen expression was established, corresponding to 50% higher density of antigen on the cells sorted for high fluorescence intensity, compared to those sorted for low intensity. After trypsin treatment, which removed the antigen from the cell surface, normal antigen expression was regained after 2-3 days in culture, with the same difference between the two populations as before the trypsin treatment. The stability of the established difference in antigen expression between the two sorted subpopulations indicates that expression of this antigen is a precisely controlled, heritable characteristic of the FME melanoma cells.

Portable gamma probe for radioimmune localization of experimental colon tumor xenografts

Tumor radioimmune detection as presently practiced utilizes a gamma scintillation camera to image tumors. A major clinical limitation is the inability to detect tumors smaller than 2 cm. This limitation is due in part to the inverse square law which states: the number of detected radioactive counts is inversely proportional to the square of the distance separating a radioactive source from the detecting device. A hand-held gamma-detecting probe (GDP) suitable for intraoperative use has been developed. The GDP can be placed near radioactive tumors and take advantage of the inverse square law in a way not possible with external scanning cameras. The use of radiolabeled baboon carcinoembryonic antigen (CEA)-specific antisera produced increased tumor isotope localization in CEA-producing tumors compared to the injection of nonspecific antisera. Tumor isotope-antisera localization was not influenced by tumor volume or time since tumor implantation. The GDP probe counts demonstrated a high degree of correlation with gamma well tissue counts. The probe was able to detect preferential tumor localization in doses lower than could be detected with external scintillation cameras.

A gamma-detecting probe for radioimmune detection of CEA-producing tumors. Successful experimental use and clinical case report

The detection of tumors with radiolabeled antibodies against CEA is possible; however, current nuclear medicine scanning cameras rarely detect tumors smaller than 2 cm in diameter. One of the limitations to tumor detection is the inability to place a detecting camera near a deeply seated intra-abdominal tumor. A hand-held gamma-detecting probe, suitable for intraoperative use, was designed to locate radioactive tumors. Experimental work with CEA-producing colon tumor xenografts in nude mice suggests this probe is more sensitive than external scanners in detecting small tumors. A case report documents the clinical use of this new intraoperative probe.

Quantitative evaluation of the localization of a monoclonal antibody (791T/36) in human osteogenic sarcoma xenografts

Monoclonal antibody 791T /36 (IgG2b) directed against human osteogenic sarcoma-associated antigens has been used to examine quantitative aspects of in vivo tumour localization into osteogenic sarcoma xenografts. With small doses of antibody (less than 40 micrograms) the extent of localization consistently correlated with xenograft size, an average of 36% of the total body burden of radiolabelled antibody being present/g tumour after 2 or 3 days. Parallel studies with labelled control IgG2b showed a similar correlation, but only 7% of the total body radiolabel was localized/g tumour. With the 791T /36 antibody, the tumour: blood ratio did not increase with tumour size except with the largest xenografts (800-900 mg), where up to 30-40% of the total body radioactivity was localized within the xenografts and this was sufficient to produce a measurable decrease in its blood level. There was also a significant correlation between the dose of injected radiolabelled antibody and the amount localized in xenografts but only up to a dose of about 100 micrograms, beyond which the amount localized was not directly proportional to the injected dose. The maximum level of radiolabelled antibody localized in xenografts was 70 micrograms antibody/g tumour tissue. It was not possible to displace radiolabelled antibody already localized in xenografts by systemic administration of a large dose of unlabelled antibody. The rate of localization of antibody was such that maximum uptake, measured as the absolute amount of localized antibody, was seen 2-4 days after injection. After this time the absolute amount of antibody in the tumours declined, but not as rapidly as the blood and whole body levels, so that the tumour: blood ratio continued to increase with time.

Tumour and cellular localization by use of monoclonal and polyclonal antibodies to placental alkaline phosphatase

Monoclonal and polyclonal antibodies against placental alkaline phosphatase (PLAP) were evaluated for tumour immunolocalization of human PLAP-producing Hep 2 tumours in nude mice. The antibodies were labelled with 125I and injected i.p. in mice with developing Hep 2 tumours. The distribution of 125I-anti PLAP in various tissues showed that the labelled antibody was enriched in the tumour, the mean concentration ratio being 7.1 and 6.8 for polyclonal and monoclonal antibodies, respectively. A PLAP negative tumour (RD) showed a mean ratio of 1.2. There was a positive correlation between PLAP content and uptake of labelled antibody in the tumours. Hep 2 tumour cells in tissue culture showed 100% positivity for PLAP, while imprints of the tumour after passage in nude mice showed 40-50% positivity. PLAP offers potential as a useful marker for localizing tumours in humans.

Radioimmunodetection and radioimmunotherapy

The development of monoclonal antibodies that recognize tumor-associated antigens has led to significantly greater practical possibilities for producing highly specific radiolabeled antibodies for diagnosis and therapy of human tumors. A number of problems remain before this technique will be ready for routine clinical application however. Achieving the high target to background ratio that are predicted on theoretical grounds is a major challenge in cancer investigation.

Nuclear imaging techniques with radiolabelled antibodies

The behaviour of radioimmunolocalisation with respect to neoplasia is reviewed. The present limitations are outlined and several new approaches to improve its clinical utility are discussed.

Radiolabeled fragments of monoclonal antibodies against carcinoembryonic antigen for localization of human colon carcinoma grafted into nude mice

Four monoclonal antibodies against carcinoembryonic antigen (CEA) have been selected from 32 hybrids that produce antibodies against this antigen, by the criteria of high affinity for CEA and low cross-reactivity with granulocyte glycoprotein(s). The specificity of tumor localization in vivo of the four MAb, and their F(ab')2 and Fab fragments was compared in nude mice bearing grafts of a serially transplanted, CEA-producing, human colon carcinoma. The distribution of radiolabeled MAb and their fragments after intravenous injection was analyzed by direct measurement of radioactivity in tumor and normal organs, as well as by whole-body scanning and by autoradiography of tumor sections. Paired labeling experiments, in which 131I-labeled antibody or fragments and 125I-labeled control IgG are injected simultaneously, were undertaken to determine the relative tumor uptakes of each labeled protein. The tumor antibody uptake divided by that of control IgG defines the specificity index of localization. Tumor antibody uptakes (as compared with the whole mouse), ranging between 7 and 15, and specificity indices ranging between 3.4 and 6.8, were obtained with the four intact MAb at day 4-5 after injection. With F(ab')2 fragments of the four MAb, at day 3, the tumor antibody uptakes ranged between 12 and 24 and the specificity indices between 5.3 and 8.2. With the Fab fragments prepared from the two most promising MAb, the antibody uptakes reached values of 34 and 82 at day 2-3 and the specificity indices were as high as 12 and 19. The scanning results paralleled those obtained by direct measurement of radioactivity. With intact MAb, tumor grafts of 0.5-1 g gave very contrasted positive scans 3 d after injection. Using MAb fragments, tumors of smaller size were detectable earlier. The best results were obtained with Fab fragments of MAb 35, which gave clear detections of tumors weighing only 0.1 g as early as 48 h after injection. Autoradiographs of tumor sections from mice injected with 125I-labeled MAb demonstrated that the radioactivity was localized in the tumor tissues and not in the stromal connective tissue of mouse origin. The highest radioactivity concentration was localized in areas known to contain CEA such as the pseudolumen of glands and the apical side of carcinoma cells. The penetration of radioactivity in the central part of tumor nodules and the pseudolumen appeared to be increased with the use of MAb fragments.