Radioimmunoscintigraphy using an anti-prostate monoclonal antibody (E4): a dosimetric evaluation.

The aim of this study was to evaluate different strategies to increase the tumour radiation dose for experimental radioimmunotherapy using 125I-labelled monoclonal antibody (MAb) E4 in a nude mice model xenografted with DU-145 tumours. The effects from a single injection of the 125I-labelled MAb E4, the same total amount of radiolabelled MAb E4 divided into three repeated injections, and the effect of pre-targeting with non-labelled MAb E4 for reducing the amount of shed antigen were investigated. Based on repetitive quantitative radioimmunoscintigraphies, calculation of the tumour radiation dose delivered from the 125I-nuclide was performed for each strategy. The single injection strategy without pretargeting rendered the highest mean tumour radiation dose, i.e. 0.23 Gy/MBq. Pretargeting with non-labelled MAb E4 before a single injection of [125I]E4 resulted in a slightly lower mean tumour radiation dose, i.e. 0.19 Gy/MBq, compared to the single injection alone. An even lower mean tumour radiation dose, i.e. 0.14 Gy/MBq, was obtained when the same total administered amount of activity was divided into three separate injections given in 10-day intervals. We concluded that the single injection strategy is the most efficient when using MAb E4 in this tumour model. The tumour radiation doses were not increased by dividing the same amount of activity into three injections or by pretargeting with non-labelled MAb E4.

Idiotypic-anti-idiotypic antibody interactions in experimental radioimmunotargeting.

Idiotypic-anti-idiotypic antibody interactions can be used in vivo to regulate the serum levels of specific radiolabeled antibodies. Anti-idiotypic antibodies can also be used as clearing agents for radiolabeled antibodies in radioimmunolocalization and radioimmunotherapy. The present study describes the immunochemical interactions between the monoclonal idiotype (H7) and three generated monoclonal anti-idiotypic antibodies (alphaH7:1, alphaH7:35, and alphaH7:38). An unexpected variability in complex formation could be demonstrated in vitro, revealing three different stable complex patterns, i.e., low molecular weight 1:1 complexes, ladder formation with oligomeric, consecutively added constituents, and large linear polymeric complexes of high molecular weight. Within 24 h, the anti-idiotypes were able to cause a significant decrease in total body radioactivity, and the antibody generating a ladder formation (alphaH7:38) was found to be the most efficient at removing radiolabeled idiotypes from the circulation. It is concluded that monoclonal anti-idiotypic antibodies may be valuable tools in improving radioimmunolocalization and radioimmunotargeting.

Radioimmunotherapy of DU-145 tumours in nude mice--a pilot study with E4, a novel monoclonal antibody against prostate cancer.

The anti-tumour effect of the 131I-labelled antiprostate monoclonal antibody (MAb) E4 was studied in an experimental model with 41 nude mice, subcutaneously xenografted with a human prostate cancer cell line (DU-145). The mice were divided into four study groups, i.e. one receiving single and another repeated injections of the radiolabelled MAb. A third group was injected with non-labelled MAb, and the fourth served as an untreated control group. The tumour volumes increased similarly in all groups during the 27-day observation period. The tumour tissue was morphologically disintegrated in the group that received repeated radioimmunotherapy (RIT). The tumours from this group contained large fluid-filled cystic parts and demonstrated pronounced cellular and subcellular polymorphism in the remaining viable tumour tissue. The untreated control tumours and single therapy tumours remained solid. The proportion of the total tumour volume that consisted of viable tumour cells, as determined by morphometric techniques, was significantly lower in the 131I-E4-treated groups. The use of 131I-labelled E4 MAb has thus demonstrated a promising therapeutic potential.

Radioimmunoscintigraphy with a novel monoclonal antiprostate antibody (E4): an experimental study in nude mice.

BACKGROUND: Prostate cancer is one of the leading causes of death among men, despite achievements in diagnosis and therapy. Radioimmunolocalization and radioimmunotherapy of malignant tumors have demonstrated increasing potential and may become useful tools in the management of prostate cancer. METHODS: Nude mice were inoculated subcutaneously with cells from the poorly differentiated human prostate cancer cell line DU-145. The intact monoclonal antibody (MoAb) E4 and an intact anticytokeratin-8 MoAb, TS1, used for comparison were labeled with 125I and injected intraperitoneally (i.p.) in the mice. Repetitive quantitative scintigraphic recordings were performed during 1 month. The mice were killed at Day 29 after injection of the radiolabeled MoAb. The tumors and the organs were dissected and weighed. The remaining activity was measured in a gamma well counter. One part of the tumor was immediately fixed in Bouin's solution for autoradiography and the other in formaldehyde for microscopy. RESULTS: The study demonstrated significant radioimmunolocalization of the MoAb E4 into the DU-145 prostate tumor tissue in the animal model, with an average radiation dose of 0.08 Gy/MBq in the tumor. TS1 localized preferentially in necrotic parts of the tumor, yielding a tumor dose of 0.02 Gy/MBq. CONCLUSIONS: The MoAb E4 is a promising radiotracer for prostate cancer and may be used in radioimmunotherapy. As in earlier studies, TS1 shows significant radioimmunolocalization into necrotic tumor tissue, which also exists in prostate cancer.

Endogenous anti-tumor antibody responses in nude mice.

BACKGROUND: Nude mice with xenografted human tumors is the most exploited animal model used to elucidate the efficacy of experimental radioimmunolocalization and radioimmunotherapy. These animals accept transplants and are generally considered immunologically inert with regard to cell-mediated and humoral immune responses against the tumors. METHODS: Nude control mice and mice carrying human HeLa Hep-2 tumor xenografts were studied for appearance of endogenous antibodies following inoculation with tumor cells. The titers of these antibodies were investigated by isotype-specific enzyme-linked immunosorbent assay (ELISA) technologies, fluorescence-activated cell sorter analysis (FACS), BIAcore (Pharmacia Biosensor AB, Uppsala, Sweden) technology, and immunofluorescence. RESULTS: The HeLa Hep-2 cell line was found to be immunogenic in all investigated animals by means of ELISA, FACS, and BIAcore evaluations as well as by immunofluorescence against both tested antigens, placental alkaline phosphatase and cytokeratin 8. Predominantly immunoglobulin M antibodies were induced, but immunoglobulin G isotypes could also be identified. Sera from these tumor-bearing mice were used for immunohistochemistry of the tumor cells. The antibodies seemed to be of low affinity and may be displaced by high-affinity monoclonal antibodies used in radioimmunotargeting. CONCLUSIONS: Nude mice bearing tumor xenografts produce significant amounts of antibodies against these two human tumor-derived antigens. These endogenous antibodies may influence targeting of radiolabeled antibodies. They also have the potential to interfere with the pharmacokinetics of labeled or nonlabeled idiotypic antibodies during experimental immunolocalization.

Dosimetry of fractionated administration of 125I-labeled antibody at experimental radioimmunotargeting.

BACKGROUND: Radiotherapy of solid tumors is preferably performed in fractionated doses. Conversely, radioimmunotherapy with nuclide-carrying antibodies delivers a continuously decreasing low dose rate during a longer time period after a single injection. In the current study, the same total amount of 125I-labeled anticytokeratin monoclonal antibody (MoAb) was administrated in one, three, or ten injections and the dosimetry was evaluated. METHODS: Three groups of nude mice (10 mice each) with HeLa Hep 2 xenografts were injected with 1 x 100 microg/22.2 megabecquerel (MBq), 3 x 33 microg/7.4 MBq, and 10 x 10 microg/2.22 MBq 125I-labeled TS1 MoAb, respectively. The mice were examined scintigraphically over a 54-day period (total number of radio immunoscintigraphies (RISs) = approximately 700) and doses to tumor and normal tissues were estimated according to the medical internal radiation dose formalism. RESULTS: A single bolus injection caused higher tumor uptake, tumor dose, and tumor to nontumor dose ratio than administration of the same total dose of antibody and radioactivity in three or ten separate injections. The single bolus injection caused a tenfold higher tumor uptake (% injected dose, or ID) compared with the group receiving ten injections. This caused a tumor dose of 17 gray to the group receiving a single bolus injection. CONCLUSIONS: In this antigen target system, a single injection of a large amount of antibody was found to be more efficient than the same antibody dose subdivided into three or ten fractions. It was concluded that not only the radioactivity but also the amount of antibody per fraction should be considered when determining optimal fractionated radioimmunotherapy.

Dosimetry of fractionated experimental radioimmunotargeting with idiotypic and anti-idiotypic anticytokeratin antibodies.

BACKGROUND: Repeated injections of iodine-125 (125I)-labeled tumor targeting anticytokeratin monoclonal antibody (TS1) and a nonlabeled antiidiotypic monoclonal antibody against TS1 (alphaTS1) were compared with a single injection of the radiolabeled TS1 in experimental radioimmunotargeting. Anti-TS1 was used to remove nontargeting TS1. METHODS: Nude mice were inoculated with HeLa Hep2 cells. The animals in Group A received a single injection of 13 MBq 125I-labeled TS1. The animals in Group B received four injections of 125I-labeled TS1 (8-13 MBq) followed by alphaTS1 24 hours later, at 2-week intervals. The mean absorbed doses were calculated according to the Medical Internal Radiation Dose Committee criteria based on repetitive radioimmunoscintigraphies during an observation period of 59 days. RESULTS: A 11 gray (Gy) mean dose to the tumor and 2 Gy to the whole body was achieved in Group A. Mean peak tumor uptake of 5% of the injected dose (ID), corresponding to 14% ID/g, was observed on Day 17 after a single injection of the labeled monoclonal antibody. A mean peak tumor uptake of the same order of magnitude was seen in Group B. An absolute increase in the tumor uptake was observed in Group B during the entire observation period. The mean absorbed dose to the tumors was 11 Gy at the end of the observation period, whereas the whole body dose was only 2.5 Gy in Group B. Autoradiography of the tumors at the end of the observation period confirmed an intensive heterogeneous accumulation of activity in the entire tumor. CONCLUSIONS: The fractionated strategy can contribute to a significant accumulation of radiolabeled TS1 in the tumors. Furthermore, the use of alphaTS1 makes it possible to increase the tumor-to-nontumor dose ratio and maintain a prolonged high activity accumulation in the tumor.

Experimental radioimmunotargeting combining nonlabeled, iodine-125-labeled, and anti-idiotypic anticytokeratin monoclonal antibodies: a dosimetric evaluation.

BACKGROUND: Preinjection of a nonlabeled tumor targeting anticytokeratin monoclonal antibody (TS1) and postinjection of an anti-idiotypic anticytokeratin monoclonal antibody (alphaTS1) were evaluated separately and in combination to investigate their effects on the accumulation of iodine-125 (125I)-labeled TS1 in experimental radioimmunotargeting. TS1 targets deposited extracellular cytokeratin 8 from necrotic tumor cells. METHODS: Nude mice were inoculated with HeLa Hep 2 cells. Four different groups were followed with 504 repetitive quantitative radioimmunoscintigraphic recordings during a 78-day observation period. The absorbed doses were calculated according to criteria of the Medical International Radiation Dose Committee. RESULTS: As much as 2% of the injected dose (ID) of 125I-labeled TS1 accumulated in the tumor, and the peak tumor uptake was recorded as late as Day 30 after the injection of 125I-labeled TS1. Anti-TS1 caused a rapid decrease in the whole body activity. The highest tumor-to-nontumor activity ratios were obtained when a pre-injection of nonlabeled TS1 was combined with a postinjection of alphaTS1. The mean absorbed dose in tumor per unit activity administered was 0.44 gray/megabecquerel (Gy/MBq) and in nontumor tissues 0.15 Gy/MBq after a single injection of 125I-TS1. The efficacy was 0.34 Gy/MBq in tumor and 0.1 Gy/MBq in nontumor tissues after a combination of preinjection of nonlabeled TS1 and postinjection of nonlabeled alphaTS1. This indicates a 20% increase in tumor doses compared with a single injection of labeled TS1. CONCLUSIONS: This study confirms an extensive accumulation of TS1 in the tumor, with peak values as late as 30 days after injection of labeled TS1. Furthermore, both preinjection of nonlabeled TS1 and postinjection of alphaTS1 can improve radioimmunotargeting.

Syngeneic anti-idiotypic antibodies eliminate excess radiolabeled idiotypes at experimental radioimmunolocalization.

Significant improvements in tumor/nontumor ratio can be achieved by injections of nonlabeled anti-idiotypic monoclonal antibodies (MAbs) during radioimmunolocalization and radioimmunotherapy using MAbs to target experimental tumors. The in vivo effects of an anti-idiotypic MAb (alpha H7) against a radioiodinated, high affinity, low dissociation rate, monoclonal antiplacental alkaline phosphatase antibody (H7) was investigated. Following in vivo injection of the anti-idiotypic MAb, the radioactivity in experimental tumors was found to decrease only 25% while the reduction of corresponding radioactivity in nontumor tissues amounted to 65-85%, compared to the group receiving no anti-idiotypic MAbs. These results indicate that it is possible to partially clear the circulation and nontumor tissues from excess of radiolabeled idiotypic antibody, without significant decrease in specific tumor localization, increasing the tumor/nontumor ratio three- to fourfold. Circulating nontumor targeting radiolabeled antibodies is one of the major limiting factors in radioimmunotherapy today. Injection of anti-idiotypic MAbs could selectively significantly reduce the radiation dose to radio-sensitive tissues, i.e., bone marrow and intestine, thus improving efficiency in radioimmunoscintigraphy and radioimmunotherapy.