An Assessment of Prolonged Reactivity of Seven Monoclonal Antibodies Against CX-1 Tumor Xenografts Using A Hand-Held Gamma-Detecting Probe

The biodistribution and kinetics of 7 monoclonal antibodies (MAb) with known reactivity against CX-I tumor were examined over 21 days using a hand-held gamma-detecting probe (Neoprobe system). Twenty-eight irnmuno-deprived (athymic) nude mice implanted with human colon adenocarcinoma CX-1 xenografts were injected intraperitoneally with 50 µCi of¹²⁵I-labeled antibodies (4 mice/antibody). Of the 7 monoclonal antibodies, 4 were anti-CEA (MA, MB, MC, and MD), 2 were anti-TAG 72 (B72.3 NCI and B72.3 fermented) and one was anti-colorectal cancer (17-1A). Daily probe counts were recorded in duplicate over the tumor site and the contralateral nontumor site (background), and tumor-to-background (Tu/Bkg) ratios were calculated. Animals were sacrificed on day 21, and blood, heart, liver, spleen, lungs, kidneys, intestine, muscle, and the tumor were removed for gamma well counting. All antibodies identified the tumor as early as 24 h postinjection and speciJic tumor localization improved over time. Patterns of prolonged tumor binding varied considerably from one antibody to another, although all but one (MB) showed continuously increasing TulBkg ratios. These data indicate progressive clearance of the antibodies from the background tissue and a persistence of labeled MAb activity in tumor resulting in improved tumor localizution with increasing postinjection time.     

Localization by hand-held gamma probe of tumor labeled with antibody "cocktail"

Five groups (n = 4) of congenitally athymic female nude mice bearing subcutaneous implants of CX-1 and/or SW-1116 tumor in the hind limbs received iodine-125 radio-labeled monoclonal antibodies (MoAbs) B72.3 (two groups), 17-1A (two groups), and cocktail (one group) (iodogen method, 50 microCi/10 micrograms/mouse). Daily probe counts were made in duplicate with a hand-held detector over each tumor site and the front leg (background) for 21 days. Animals were sacrificed and appropriate well counts were obtained. All the single MoAb preparations localized well in both tumor cell lines. Uptake of monoclonal antibody 17-1A was similar in the two tumor cell lines, with counts initially high and slowly decreasing over the 21-day period. Tumor/background ratios continued to increase over time, indicating that both tumor lines have similar antigenic expression for the monoclonal antibody 17-1A. This was not the case for monoclonal antibody B72.3, which showed a preferential uptake by the CX-1 tumor, with higher initial counts and prolonged binding of the antibody, giving rise to higher tumor/background ratios. The mixture of monoclonal antibodies B72.3 and 17-1A markedly improved the uptake by the CX-1 tumor cell line but not that by the SW-1116 cell line, where the effect was negative when compared to the uptake of the single MoAb preparations. The use of a monoclonal antibody mixture can enhance targeting of some tumor sites. Due to the heterogeneity of tumor cell lines, even within the same animal, different mixtures of monoclonal antibodies are needed to increase the targeting of tumor.     

Management of diabetic midfoot ulcers.

Radioimmunoguided surgery (RIGS) has been employed intraoperatively in cases of colorectal cancer to assess the extent of local tumor spread and metastatic disease. This technique uses radiolabeled monoclonal antibodies (MAbs) directed against tumor-associated antigens, and a hand-held gamma-detection probe to detect the radiolabel fixed to tumor tissue. Recently introduced is an MAb directed against tumor-associated glycoprotein (anti-TAG), CC49. Sixty patients were entered into the initial study. Eighteen of 21 (86%) primary tumors were localized by the CC49 MAb and the gamma-detecting probe. Twenty-nine of 30 (97%) recurrent tumors were localized. Antibody dose did not affect localization. Specimens were divided into tissue types I through IV, based on antibody localization and hematoxylin and eosin (H&E) staining: type I, RIGS (-) and histologically (-); type II, RIGS (-) and histologically (+); type III, RIGS (+) and histologically (-); type IV, RIGS (+) and histologically (+). Type IV tissue were further classified by whether they were grossly apparent, IVa, or grossly inapparent, IVb (occult). Occult tumor found by RIGS and confirmed by H&E staining (type IV) had localization ratios similar to RIGS-positive, histology-negative tissue (type III). Traditionally found cancer (type IV) had significantly higher ratios. In 12 of 24 patients (50%) with primary tumors and 14 of 30 patients (47%) with recurrent tumors, RIGS with CC49 altered the planned operative procedure. Radioimmunoguided surgery with CC49 provides useful, immediate intraoperative information not available by other techniques.     

Intraoperative localization of colorectal cancers using radiolabelled monoclonal antibodies

Radiation detectors may allow the intraoperative localization of small cancer deposits following administration of radiolabelled tumour-associated antibodies. This technique was evaluated in 16 patients with colorectal tumours (14 cancers, one adenoma, one lipoma) with the 111In-labelled monoclonal antibody (MAb) ICR2 which recognizes the tumour-associated epithelial membrane antigen (EMA). At operation counting was carried out (3 x 20 s per site) using a hand-held radiation probe over the primary lesions and any palpable lymph nodes in the mesocolon. The tumour to normal colon (T/NC) ratio of counts recorded at operation was more than 1.5:1 in eight of the 14 patients with cancer (mean(s.d.), 1.54(0.41):1) and 0.91:1 and 1.06:1 respectively in the two patients with benign tumours. Node to normal colon ratios were higher in lymph nodes containing metastases. The uptake of radiolabelled antibody (T/NC ratio) was higher in EMA-expressing cancers than in those not expressing the target antigen (mean(s.d.), 2.45(0.65):1 versus 1.40(0.20):1, P = 0.019). An abdominal tumour model was also developed. Radioactively filled containers of 0.5-10 ml representing tumour deposits were suspended in a tank of 111In solution representing the background activity found in normal tissues. The ratio of radioactivity in the 'tumour' to that of background varied from 2:1 to 8:1. The 'tumour' was considered to be detectable if the mean counts recorded over the 'tumour' exceeded the mean of counts recorded over background by three standard deviations. At a ratio of 2:1 only 'tumours' greater than 5 ml could be detected with a sodium iodide probe and those over 10 ml could be detected with a cadmium telluride (CdTe) probe. At a ratio of 8:1, 'tumours' of 0.5 ml could be detected with either probe. At all ratios and counting periods the NaI probe was more sensitive than the CdTe.     

Radioimmunoguided surgery using monoclonal antibody

The potential proficiency of radioimmunoguided surgery in the intraoperative detection of tumors was assessed using labeled monoclonal antibody B72.3 in 66 patients with tissue-proved tumor. Monoclonal antibody B72.3 was injected 5 to 42 days preoperatively, and the hand-held gamma-detecting probe was used intraoperatively to detect the presence of tumor. Intraoperative probe counts of less than 20 every 2 seconds, or tumor-to-adjacent normal tissue ratios less than 2:1 were considered negative (system failure). Positive probe counts were detected in 5 of 6 patients with primary colon cancer (83 percent), in 31 of 39 patients with recurrent colon cancer (79 percent), in 4 of 5 patients with gastric cancer (80 percent), in 3 of 8 patients with breast cancer (37.5 percent), and in 4 of 8 patients with ovarian cancer (50 percent) undergoing second-look procedures. Additional patients in each group were scored as borderline positive. Overall, radioimmunoguided surgery using B72.3 identified tumors in 47 patients (71.2 percent), bordered on positive in 6 patients (9.1 percent), and failed to identify tumor in 13 patients (19.7 percent). Improved selection of patients for antigen-positive tumors, the use of higher affinity second-generation antibodies, alternate routes of antibody administration, alternate radionuclides, and more sophisticatedly bioengineered antibodies and antibody combinations should all lead to improvements in radioimmunoguided surgery.     

Intraoperative probe-directed immunodetection using a monoclonal antibody

To assess monoclonal antibody (MAb) 17-1A and its F(ab')2 fragment in intraoperative radioimmunodetection and to evaluate further the clinical usefulness of a hand-held gamma-detecting probe (GDP), we injected radiolabeled monoclonal antibody 17-1A three to six days preoperatively or its F(ab')2 fragment two to three days preoperatively into 18 patients with colorectal cancer. Intraoperative GDP counts with tumor-tissue ratios of 1.5:1 or greater were obtained from 15 (75%) of 20 tumor sites, with ratios averaging 2.3:1 for fragments and 3.4:1 for whole antibody. The GDP counts contributed to intraoperative decision making in three patients, either by localization of tumor not identified by inspection or palpation or by mapping margins of resection with histologic confirmation of a local/regional recurrence. These preliminary data demonstrate that probe-directed, intraoperative radioimmunodetection can assist the surgeon in detecting subclinical tumor deposits and thus better evaluate the extent of primary or recurrent colorectal cancers intraoperatively.     

Efficacy of monoclonal antibody 131I-B72.3 immunoscintigraphy of pancreatic adenocarcinoma xenografts in nude mice.

OBJECTIVE: To assess the efficacy of monoclonal antibody (MoAb) B72.3 for in vivo-immunoscintigraphy of pancreatic carcinoma in nude mice. DESIGN: Experimental controlled animal study. SETTING: University hospital, The Netherlands. SUBJECTS: 11 nude mice with subcutaneously xenografted human pancreatic carcinoma. INTERVENTIONS: Specific MoAb B72.3 and non-specific MoAb MOPC21 were iodinated with 131I and injected intraperitoneally in nude mice. Scintigrams were taken on days 1-10 and tumour:non-tumour ratios of the regions of interest (tumour, thorax, abdomen, background) were calculated. The mice were then killed for in vitro tissue counts. MAIN OUTCOME MEASURES: Tumour:non-tumour ratios in vivo and in vitro. RESULTS: Results of immunoscintigraphy on days 1, 2, and 6 were compared. In the B72.3-group all ratios were only moderately raised, the tumour:background ratio being the highest (2.35 (SD 0.67)) on day 6. There were no obvious differences between the ratios of the B72.3-group and the MOPC21-group. The results of tissue counts done at the end of the study, showed that tumour:non-tumour ratios were twice as high in the B72.3-group, suggesting some specificity of this MoAb. CONCLUSION: The results of our study suggest that MoAb B72.3 is not powerful enough for in vivo detection of pancreatic cancer as assessed in this xenograft model in nude mice.     

Targeting epitopes in prostate-specific membrane antigen for antibody therapy of prostate cancer

Prostate-specific membrane antigen (PSMA) is a target for immunotherapy of prostate cancer. It has been shown that antibodies against PSMA inhibited the in vivo growth of LNCaP tumor. In the present study, monoclonal antibodies against four epitopes in PSMA were raised. MAb 24.4E6 (IgG1), specific for the epitope (residues 638-657) in PSMA, significantly reduced the growth rate of established LNCaP tumor in SCID mice. Mouse IgG was detected in the tumor of mice treated with 24.4E6, but not with an unrelated MAb. These results suggest that this epitope may be the main target in PSMA for antibody therapy of prostate cancer.     

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.     

Immunolocalization of anti-placental alkaline phosphatase monoclonal antibody in mice with testicular tumors and lymph node metastasis

To evaluate the ability of an anti-placental alkaline phosphatase (PLAP) monoclonal antibody (MAb) to localize to PLAP-expressing tumors, we established a model of testicular tumor with metastasis to lymph nodes and liver in severe combined immunodeficient (SCID) mice. ¹³¹I-labeled or ¹²⁵I-labeled MAb was simultaneously administered via the intravenous or lymphatic route, respectively. Preferential accumulation of MAb in PLAP-expressing tumors at primary as well as metastatic sites was demonstrated. The percentage of the injected dose of MAb found in the tumor was generally higher when MAb was administered intravenously. Identical tumor/blood ratios were found with the two routes of administration. These data suggest that intravenous administration of a radiolabeled MAb is superior to lymphatic administration for tumor imaging and radioimmunotherapy. Received: 31 December 1996 / Accepted: 18 September 1997     

Biodistribution and radioimmunoscintigraphy studies of renal cell carcinoma using tumor-preferential monoclonal antibodies and F(ab')2 fragments

The in vivo localization of renal cell carcinoma-preferential monoclonal antibodies A6H, D5D, and C5H was evaluated and the biodistribution of F(ab')2 antibody fragments of A6H and the intact Mab were compared in over 100 nude mice. A6H localized well to most renal cell carcinoma xenografts studied; the median tumor to blood ratios ranged from 6.4 to 11.5 for various xenografts. C5H also localized well to most renal cell carcinoma xenografts tested. However, D5D did not localize well to renal cell carcinoma xenografts in vivo despite its highly restrictive in vitro reactivity. The F(ab')2 fragments of A6H produced higher tumor to blood ratios, which probably resulted from fast clearance of the fragments from the circulation. Preliminary results showed that indium-111 labeling may further improve imaging.     

Radioimmunoguided surgery using iodine 125 B72.3 in patients with colorectal cancer

Preliminary data using B72.3 murine monoclonal antibody labeled with iodine 125 suggested that both clinically apparent as well as occult sites of colorectal cancer could be identified intraoperatively using a hand-held gamma detecting probe. We report the preliminary data of a multicenter trial of this approach in patients with primary or recurrent colorectal cancer. One hundred four patients with primary, suspected, or known recurrent colorectal cancer received an intravenous infusion of 1 mg of B72.3 monoclonal antibody radiolabeled with 7.4 x 10 Bq of iodine 125. Twenty-six patients with primary colorectal cancer and 72 patients with recurrent colorectal cancer were examined. Using the gamma detecting probe, 78% of the patients had localization of the antibody in their tumor; this included 75% of primary tumor sites and 63% of all recurrent tumor sites; 9.2% of all tumor sites identified represented occult sites detected only with the gamma detecting probe. The overall sensitivity was 77% and a predictive value of a positive detection was 78%. A total of 30 occult sites in 26 patients were identified. In patients with recurrent cancer, the antibody study provided unique data that precluded resection in 10 patients, and in another eight patients it extended the potentially curative procedure.     

Intraportal injection of specific monoclonal antibody in nude mice with liver metastasis

To assess the importance of monoclonal antibody route of administration, we compared the selective (intraportal) and systemic injection of specific radiolabeled monoclonal antibody, using a murine model of hepatic metastases from a human colorectal carcinoma. Tumor uptake was studied over time after injection of 0.1, 1.0 or 2.0 micrograms of a specific antibody (HT29-15) or an isotype-matched control (BL-3). Significantly higher tumor uptake and tumor/liver or tumor/blood uptake ratios were seen in animals receiving intraportal injection for all tested doses. Intraportal injection of specific monoclonal antibody resulted in significant improvement in metastases uptake; these findings could be applied to the diagnosis and treatment of hepatic metastases from colorectal cancer using radiolabeled monoclonal antibodies.     

Radiolocalization of human prostate tumor in a mouse subrenal capsule model by monoclonal antibody TURP-27

The subrenal capsule assay was used to determine if 125I-labeled anti-prostate monoclonal antibody TURP-27 could target human prostate tumor fragments implanted under the renal capsule of normal immunocompetent C57 BL/6 mice. Maximal binding and optimal tumor to non-tumor tissue ratios occurred within 24-48 hours postadministration of 125I-TURP-27. No significant localization was observed in mice bearing TURP-27 antigen-negative human colon tumor tissue implants or with an isotype-matched control monoclonal antibody. These preclinical data suggest that TURP-27 may have clinical application for imaging metastatic prostate tumors and further application in immunoconjugate and/or radiotherapy of prostate cancer.     

Efficacy of type-specific and cross-reactive murine monoclonal antibodies directed against endotoxin during experimental sepsis

To study the role of antibodies in promoting survival during gram-negative bacterial sepsis, we have developed several murine monoclonal antibodies (MAbs). One MAb (5B10) reacted in an enzyme-linked immunosorbent assay with only a single organism (Escherichia coli 0111:B4), while the other (8A1) reacted to all gram-negative whole-cell and lipopolysaccharide (LPS) antigens examined. Either 5B10 MAb, 8A1 MAb, or sterile saline solution was administered intravenously to outbred male Swiss-Webster mice immediately before one of three challenges: (1) viable bacteria intravenously, (2) viable bacteria with hemoglobin intraperitoneally, or (3) intravenous actinomycin D plus LPS. 5B10 MAb provided significant protection against either an E. coli 0111:B4 bacterial or LPS challenge but not against any other organism or type of LPS. 8A1 MAb provided protection against several challenge bacteria (intravenously or intraperitoneally) and against all types of LPS studied except Pseudomonas aeruginosa LPS. A higher dose (2 mg) of cross-reactive antibody (8A1 MAb) was required to produce protection when compared with the type-specific protection produced with 5B10 MAb (0.1 mg). Although ideal antibody therapy would consist of directing a specific MAb against a single microorganism, the acute nature of the disease process and time required to prepare reagents may preclude the use of type-specific MAbs. We believe that the cross-reactive and cross-protective capacity of 8A1 MAb or a similar MAb may be useful in averting the lethal effects of clinical gram-negative bacterial sepsis and warrants testing in the clinical setting.     

A novel method to localize antibody-targeted cancer deposits intraoperatively using handheld PET beta and gamma probes

Background Assessing cancer margins, lymph nodes, and small cancer deposits intraoperatively can be challenging. A new device has become available that allows the detection of positron emission tomography (PET) radiotracers through both high-energy gamma and short-range beta emissions. These PET probes are handheld, allowing for real-time evaluation of cancer using a tool that provides surgeons with better intraoperative assessment of tumor sites. Methods Within the context of two institutional review board (IRB)-approved protocols investigating new applications of antibody-labeled PET scanning, ¹²⁴I-labeled humanized monoclonal antibodies specific for colorectal cancer (huA33) and renal tumors (cG250) were constructed. Patients underwent preoperative PET scans, approximately seven days post-tracer infusion, when tumor-to-nontumor ratios were high. Suspected tumor deposits were evaluated intraoperatively with handheld beta and gamma PET probes. Results Handheld PET probes detected emissions from all tumors. Count rates from the gamma probe on tumor ranged from 48 to 306 cps, and for the beta probe ranged from 18 to 190 cps. Gamma and beta emissions exhibited a strong positive correlation. The ratio of gamma and beta counts was at least twice that of the background counts for all tumors evaluated. Conclusions This study is the first to demonstrate the utility of beta probes for the intraoperative detection of radiolabeled antibodies targeting cancer. Importantly, the recorded beta count rates from the beta probe correlate with the count rates from the high-energy gamma probe. Furthermore, the beta probe may offer superior specificity for real-time localization of small tumor deposits, compared to gamma probes. The intraoperative portable PET probe may prove a valuable bridge to combining tumor biology and PET technology to guide surgical therapy.     

Detection of primary colorectal cancer with indium 111 monoclonal antibody B72.3

B72.3 is a murine monoclonal antibody of the immunoglobulin subclass IgG1 directed against TAG-72, a cell surface antigen present on colorectal carcinoma cells. We investigated the utility of scanning with indium 111-labeled B72.3 in 16 patients with a high clinical suspicion of or biopsy-proven primary colorectal cancer. Each patient received 1 or 2 mg of B72.3 monoclonal antibody labeled with 152 MBq of indium 111. Patients underwent scanning 2 to 3 days and 7 days after infusion by planar and emission computed tomography. Nineteen lesions were confirmed in 12 patients. Three patients with benign polyps had true-negative monoclonal antibody scans. Indium 111-labeled imaging of B72.3 detected nine of 19 lesions. Unsuspected tumor sites were identified by monoclonal antibody scan in three patients. By detection of additional abdominal disease and extra-abdominal spread, indium 111-labeled scanning of B72.3 directly affected treatment in 18% of patients.     

125I标记抗胃癌单克隆抗体3H11在胃癌放射免疫导向手术中的应用

目的　探讨12 5I标记抗胃癌单克隆抗体MAb 3H11在胃癌放射免疫导向手术 (RIGS)中的应用价值。　方法　胃癌患者 35例 ,在胃镜直视下将12 5I标记的MAb 3H11注射于癌周围粘膜下 ,4～ 11d后手术 ;术中使用手持式γ探测仪对靶部位 (T)及对照本底 (NT)进行放射性计数 ,T/NT比值≥ 3 5为判别胃癌及癌浸润胃壁的标准 ,T/NT≥ 3 0为判定转移淋巴结的标准 ;对RIGS阳性、而术后常规病理检查阴性的 19组淋巴结 ,采用免疫组织化学方法检测微转移癌。　结果　本组病例中 ,33例RIGS获得成功 ,RIGS判别切缘癌浸润的总特异性和准确率分别为 98 5 %和 97 0 % ;注射标记抗体后 6～ 8d手术的患者 (n =2 5 ) ,RIGS判别区域淋巴结转移的敏感度、特异性及准确率分别为 83 6 %、95 0 %和 91 3% ;免疫组织化学染色从 10组 (5 2 6 % )RIGS阳性 ;而术后常规病理检查阴性的淋巴结中检出胃癌微转移灶。　结论　应用12 5I标记抗胃癌单克隆抗体MAb 3H11进行胃癌RIGS ,可有效判别胃癌浸润范围及区域淋巴结转移程度。     

Tumor localization of human brain malignant glioma xenograft in nude mice with a radiolabeled monoclonal antibody

A murine monoclonal antibody designated as MAb 3H9 (IgG1 subclass immunoglobulin, kappa light chain) expressed specific antibody binding activity to a human brain malignant glioma cell line (GBM8401/TSGH,NDMC) and many formalin-fixed and paraffin-embedded malignant gliomas that have been produced in our laboratory by hybridoma technology. The immunohistochemical indirect immunofluorescence, peroxidase-antiperoxidase assays, and specific electron microscopic immunogold staining revealed that 3H9 probably recognized a distinct glioma-associated surface antigen on the GBM8401 cultured cells. In vivo radioimmunolocalization of GBM8401 xenografts in nude mice by external scintigraphy with radiolabeled 3H9 has been performed to evaluate potential clinical application as diagnostic or therapeutic reagents. On the 3rd day after an intravenous injection of 15 microCi, the 125I- or 131I-radiolabeled 3H9 was successful in immunolocalization of a human brain GBM8401 xenograft in the nude mouse. In large xenografts, the radioactivity ratios of tumor to brain and tumor to blood were 11.0 and 2.4, respectively. In small xenografts, the tumor to brain and tumor to blood ratios were 14.0 and 2.9, respectively. The clearance of radiolabeled 3H9 in the bloodstream of the nude mouse was not affected by the presence of a GBM8401 xenograft. This preliminary experiment reveals that human brain GBM8401 xenografts in nude mice can be detected in vivo by radiolabeled 3H9.     

Localization of radioiodinated monoclonal antibody in colorectal cancer. Initial dosimetry results

HT-29-15 is an IgG1 monoclonal antibody reacting with a neuraminidase-sensitive determinant on a cell-surface antigen (molecular weight, 200,000 daltons) present on the colon cancer cell line HT-29. HT-29-15 was selected for a tumor localization study because the antigen was shown to be present, by immunohistochemical staining, in a high percentage of primary and metastatic colorectal cancers. HT-29-15 labeled with iodine 131 was given intravenously over a dose range of 0.2 to 10.0 mg to 23 patients with colorectal cancer. No significant toxicity was seen. Imaging of hepatic metastases was successful from days 5 to 7. Analysis of tissue radioactivity by biopsy showed that the tumor-liver ratio increased from day 1 to day 7, suggesting more rapid clearance of antibody from normal tissue than from tumor. Thus, tissue biopsy specimens and scintigraphy have shown that imaging of metastatic colorectal cancer is possible with monoclonal antibody HT-29-15. Tissue biopsy specimens are essential for demonstrating specificity of localization. Scans alone provide insufficient evidence of specific localization by monoclonal antibodies. Simultaneous infusion of a nonreactive control antibody would be necessary for specific localization to be demonstrated unequivocally.