Biological background subtraction improves immunoscintigraphy by subsequent injection of antigen

We have developed an 111In-labeled antibody for in vivo use directed against tissue plasminogen activator demonstrating focal fibrinolytic activity. However, a major problem in immunoscintigraphy is the low signal-to-noise ratio due to circulating antibody. The hepatic clearance of t-PA is very rapid. The effect of a subsequent injection of a small amount of t-PA shortly after the antibody administration to increase the blood clearance rate of the formed antigen-antibody complexes was examined in six rabbits. More than 99% of the circulating antigen-antibody complexes were eliminated by the liver within 10 min. This technique could make immunoscintigraphy a first line diagnostic tool in acute medicine including imaging of thromboembolic lesions in organs with high blood volumes such as the lungs, the heart, and the brain.

Three-step monoclonal antibody tumor targeting in carcinoembryonic antigen-positive patients

We describe a method to postlabel, in vivo, biotinylated monoclonal antibodies pretargeted onto tumor deposits when most of the non-tumor-bound antibodies have already been cleared as avidin-bound complexes. The application of this principle to tumor detection by immunoscintigraphy was tested in 20 patients with histologically documented cancer and increased circulating carcinoembryonic antigen levels. One mg of biotinylated anti-carcinoembryonic antigen monoclonal antibody (FO23C5) was administered i.v. (first step). After 3 days, 4-6 mg of cold avidin were injected i.v. (second step), followed 48 h later by 0.2-0.3 mg of a biotin derivative labeled with 111In (2-3 mCi) (third step). No evidence of toxicity was observed. Whole body radioactivity distribution was measured in five patients at various intervals postinjection by the conjugate counting technique. Tumors and metastases were detected in 18 of 19 patients (the remaining patient was a true negative) within 3 h after administration of 111In-biotin by planar or single photon emission tomography imaging. At the time of imaging, tumor/blood pool ratio was 5.5 +/- 3.2, and tumor/liver ratio was 6.7 +/- 3.9. Blood clearance of 111In-biotin was multiexponential, with the fast component having a t1/2 of 5 +/- 3 min. Urinary excretion of radioactivity over 3 h was 63.5 +/- 4.9% of the injected dose. Radioactivity at 3 h was 6.5 +/- 1.8% in blood, 1.6 +/- 0.3% in the kidney, and 2.4 +/- 0.6% in the liver. This approach represents an improvement in immunoscintigraphic techniques for tumor localization. The potential use for radioimmunotherapy is discussed.

Improvement by affinity chromatography on antiidiotypic monoclonal antibodies (MAbs) of immunoreactivity and in vivo targeting of radiolabelled anti-HMW-MAA MAb TP61.5 in nude mice bearing human melanoma lesions

The human high-molecular-weight melanoma-associated antigen (HMW-MAA) represents a useful marker for immunoscintigraphy in patients with melanoma. Since injection of a radiolabelled anti-HMW-MAA monoclonal antibody (MAb) visualizes only about 60% of melanoma lesions, approaches are being developed to increase the sensitivity of immunoscintigraphy. One of them aims at improving the immunoreactivity of radiolabelled anti-HMW-MAA MAbs, since this approach may improve the targeting of radiolabelled MAbs to melanoma lesions. We have previously shown that affinity chromatography on insolubilized anti-idiotypic MAbs is a useful method for purifying immunoreactive anti-HMW-MAA MAb TP61.5 from 125I-labelled MAb preparations and that not all the anti-idiotypic MAbs are useful for this purpose. Since the increasing number of available anti-idiotypic MAbs is likely to facilitate the application of this procedure in many antigenic systems, we have now tested criteria to select anti-idiotypic MAbs suitable for the purification procedure. Furthermore, we have investigated the effect of the increase in immunoreactivity of 125I-MAb TP61.5 on its in vivo targeting to human melanoma lesions transplanted into nude mice. Among the 3 anti-idiotypic MAbs tested, the most effective in purifying immunoreactive MAb TP61.5 molecules following radiolabelling is MAb TK7-110, with which 125I-MAb TP61.5 displays an immunoreactivity similar to that displayed with melanoma cells. This parameter may represent a useful criterion to identify anti-idiotypic MAbs suitable for the purification procedure, if the present results are confirmed with a large number of anti-idiotypic MAbs in different antigenic systems. We have also shown that an incubation time for up to 4 hr of 125I-MAb TP61.5 with insolubilized MAb TK7-110 is the most effective in increasing immunoreactivity and in recovering immunoreactive MAb applied to the affinity matrix. The increase in the immunoreactive fraction of 125I-MAb TP61.5 significantly increases its specific localization in human melanoma lesions transplanted into nude mice. These results suggest that purification of radiolabelled immunoreactive anti-HMW-MAA MAb TP61.5 by affinity chromatography using anti-idiotypic MAb TK7-110 represents a useful approach to increasing the sensitivity of immunoscintigraphy in patients with melanoma.

Imaging and therapy of gastrointestinal cancers with radiolabeled antibodies

Radiolabeled antibodies are gaining in application for the imaging and therapy of cancers. Much progress has been made in the use of these new biological reagents for tumors of the gastrointestinal system, and the current status, problems and prospects are reviewed in this paper. It is apparent that murine monoclonal antibodies against cancer-associated antigens are safe and useful for revealing a high percentage of sites of tumor, including some missed by conventional radiological methods. Tumors as small as 0.5 cm can be disclosed, even in organs that are difficult for CT scan interpretation. Major advances in cancer imaging have involved the use of antibody fragments and simple, direct labeling methods for 99mTc, the most widely used radionuclide in nuclear medicine. The use of radiolabeled antibodies for cancer therapy has been a more difficult endeavor, particularly because of the delivery of inadequate rad doses to tumor. Advances include humanization of monoclonal antibodies and the improved attachment of therapeutic radionuclides, such as 90Y and 186Re or 188Re.

Comparison of 99Tcm-labelled monoclonal anti-granulocyte antibody and 111In-labelled IgG for the detection of focal sites of infection in rats

The abilities of 99Tcm-labelled monoclonal anti-human granulocyte antibody (AGAb) and 111In-labelled nonspecific polyclonal human immunoglobulin (IgG) to localize at focal sites of inflammation were compared in rats with deep thigh infection due to E. coli. The radiolabelled antibodies were coadministered followed 4-6 and 24 h later by imaging and biodistribution studies. At 4-6 h after injection, the target to background ratio (T/B, lesion to contralateral leg) and percentage residual activity (% RA, counts in the lesion/total body counts) were nearly identical for both antibody preparations. At 24 h, T/B and % RA increased significantly (P less than 0.001) for both proteins but differences between the agents were not significant. In vitro analysis of the binding of AGAb and human polyclonal IgG to rat granulocytes showed a low level of binding with both agents. These results suggest that the primary mechanism of localization, by either antibody preparation in this model, is not antigen related. 111In-labelled nonspecific human IgG and 99Tcm-AGAb are equivalent reagents for the detection of focal sites of infection in the rat.

Imaging of systemic Candida albicans infections with a radioiodinated monoclonal antibody: experimental study in the guinea pig

Guinea pigs intravenously infected with Candida albicans were scanned to evaluate the use of radioiodinated monoclonal antibodies (MAb) to fungal antigens for detecting tissue infection sites. A total of 18 infected and 8 uninfected animals were used. MAb and F(ab')2 fragments directed against cell wall glycoproteins of C. albicans were labeled with 131I. Another MAb directed against a Schistosoma mansoni glycoprotein was labeled with 125I and used as a nonspecific control. Radiolabeled MAbs were injected at a dose of 12.5 micrograms (500 kBq) per animal. Images were acquired 24 h later. Animals were then killed and the dissected organs were separately gamma-counted. The number of C. albicans colony forming units (cfu) per gram was determined in each organ. A clear relationship was found between the anatomic distributions of C. albicans and 131I. The biodistribution of 131I radioactivity associated with anti-Candida MAb was greater in infected animals than in healthy animals and increased with the number of cfu per g in each organ. The distribution was highly specific in animals with Candida endophthalmitis, a pathognomic feature of organ involvement during hematogenous dissemination. In contrast, the distribution of 125I radioactivity associated with the nonspecific MAb was similar in healthy and infected animals. In infected animals, it was totally independent of the intensity of fungal infection.

Detection of hepatic metastases from colorectal carcinoma using indium-111 (111In) labeled monoclonal antibody (mAb): MSKCC experience with mAb 111In-C110

Sixteen patients with colorectal carcinoma and a rising serum carcinoembryonic antigen (CEA) level and no evidence of extra-abdominal disease were administered 5 mg of an anti-CEA monoclonal antibody (mAb), C110, labeled with approx. 5 mCi of 111In. All patients subsequently underwent exploratory laparotomy, and samples of tumor and normal tissue were obtained. Hepatic lesions (confirmed by histopathology) were visualized as areas of increased radiotracer uptake in 13 of 16 patients. Single photon emission computed tomography (SPECT) considerably aided detection, being positive in two patients with normal planar images. Ten of the 16 patients had positive x-ray computed tomographic (CT) images. The radioimmunodiagnostic study was falsely negative in 3 of 16 patients with subsequently proven hepatic disease, in one of whom CT was also normal. The antibody study was positive in 80% of lesions, thus being, in this small series, significantly more sensitive (P less than 0.01) than CT. 111In-C110 is a promising monoclonal antibody for the detection of hepatic metastases from colorectal carcinoma; this is the first study to show consistently greater concentration of 111In-labeled antibody in hepatic lesions than in surrounding normal hepatic parenchyma.

New indium-111 labeled biotin derivatives for improved immunotargeting

Investigations into the use of streptavidin-conjugated antibodies and labeled biotin to improve radioimmunotargeting have shown background levels drastically reduced over the conventional approach. Nevertheless, accumulation of 111In-biotin in normal tissue as well as streptavidin-independent accumulation in tumor, was observed. In this work, the effect of altering the biotin molecule to reduce this nonspecific uptake without decreasing specific localization has been investigated. Three EDTA and DTPA derivatives of biotin have been synthesized and investigated along with a commercial biotin derivative (DTPA-B2). The labeled biotin chelates were administered i.p. to normal mice implanted with avidin beads in one thigh. A wide variation in biodistribution was seen among the biotin derivatives. The most favorable results were obtained with biotinyl-hydrazino-EDTA (EDTA-B1), which showed the lowest accumulation in normal tissues but equivalent uptake in the target with respect to the other compounds. Averaged over 8 tissues sampled, the target-to-nontarget ratio was 140 vs 9 for EDTA-B1 vs DTPA-B2 (N = 6) at 24 h post administration. Similar observations have been made in culture with two tumor cell lines: positive accumulation of both DTPA-B2 and EDTA-B1 was measured in tumor cells independent of streptavidin-antibody conjugate, however in the case of the latter derivative, this accumulation was 3-5 fold lower. These studies show that modification of the biotin species can alter accumulation in normal tissues as well as the antibody-streptavidin independent accumulation in tumor tissue.

Second antibody for improvement of antibody imaging: liposome-entrapped and free preparations in animal and human studies

When anti-tumour antibodies are given systematically for tumour imaging or therapy, second antibody directed against the first (anti-tumour) antibody can be used to accelerate clearance of first antibody, thus improving discrimination between tumour and normal tissues. Liposome-entrapped, and free second antibodies (LESA and FSA, respectively) have been compared in an animal tumour model system and in patients with cancer. Nude mice bearing xenografts of human colon carcinoma were given goat antibody to carcino-embryonic antigen (CEA) as first antibody and horse anti-goat second antibody. Patients with gastrointestinal carcinomas received i.v. 131I-labelled goat anti-CEA or mouse monoclonal 17-1A first antibody and unlabelled horse angi-goat or rabbit anti-mouse second antibody, respectively. Antibody distribution was studied by serial gamma camera imaging. The effectiveness of LESA and FSA depended on dose. Tumour-to-blood ratios were increased up to eight-fold by either method in animals. Tumour imaging was enhanced among 15 patients with gastrointestinal cancer and tumour was correctly identified at five sites where it was not seen by a background subtraction method. No significant toxicity occurred in patients nor in rabbits studied for evidence of immune complex mediated disease. LESA and FSA appear to be equally effective.