Enhanced antitumor activity of combined pretargeted radioimmunotherapy and paclitaxel in medullary thyroid cancer xenograft.

A significant antitumor effect associated with moderate toxicity was obtained previously with anticarcinoembryonic antigen x antidiethylene-triaminepentaacetic acid (DTPA)-indium F6-734 bispecific antibody and iodine-131-labeled DTPA-indium bivalent hapten in an animal model of medullary thyroid cancer (MTC). The purpose of this study was to determine whether the cytotoxic agents doxorubicin and paclitaxel, also known as radiosensitizers, improve efficacy of pretargeted radioimmunotherapy (RIT) in experimental MTC. Nude mice bearing TT MTC xenograft were treated with F6-734 and iodine-131-labeled DTPA-indium bivalent hapten injected 48 h apart with or without doxorubicin or paclitaxel. The maximum tolerated dose (MTD) of RIT was 92.5 MBq (as determined previously) and that of doxorubicin and paclitaxel 200 and 1000 micrograms, respectively. A control group received no treatment. Animal weight, hematotoxicity, tumor volume, and serum calcitonin were monitored for 5 months. Tumor growth inhibition induced by drugs alone, RIT alone, or combined therapy was characterized by measuring relative tumor volume 20, 40, and 60 days after treatment to detect additivity or synergism. Mean tumor volume doubling time (MTVDT) was 13 +/- 4 days in the control group, 15 +/- 8 days in the group treated with the MTD of doxorubicin, and 32 +/- 13 days in the group treated with the MTD of paclitaxel. After RIT alone at 92.5 MBq, MTVDT was 86 +/- 22 days. After RIT at 74 MBq (80% of MTD), MTVDT was 56 +/- 10 days. MTVDT was not significantly different from this value after RIT plus doxorubicin, 60 +/- 16 days (65 and 100% of the respective single-agent MTDs). Combination of RIT with paclitaxel (65 and 100% of the respective single-agent MTDs) prolonged the suppression of tumor growth. One complete response was observed, and MTVDT was 114 +/- 44 days. This value was significantly longer than the value obtained with RIT alone at 74 MBq (P < 0.05) or with RIT combined with doxorubicin (P < 0.02). The change in serum calcitonin levels paralleled those in tumor volume. Analysis of dose-response curves at days 20 and 40 showed additivity between RIT and paclitaxel, and analysis at day 60 suggested a synergistic effect. In conclusion, addition of doxorubicin did not improve RIT efficacy, whereas paclitaxel improved RIT efficacy significantly without increasing toxicity.

The intraportal injection model for liver metastasis: advantages of associated bioluminescence to assess tumor growth and influences on tumor uptake of radiolabeled anti-carcinoembryonic antigen antibody.

BACKGROUND: Radioimmunotherapy is emerging as a new tool for adjuvant therapy of colorectal cancer. The liver remains the main site for metastases, carrying a high mortality rate. Many animal models are available but none associates easy, reliable implantation and in-vivo follow-up for experimental therapeutic studies. The aims of this study were to develop a reliable hepatic metastatic colonic cancer model in mice using the intraportal route for injection, with follow-up by bioluminescence (BLI) and to evaluate the impact of tumor location on tumor antigen direct targeting using radiolabeled anti-CEA (carcinoembryonic antigen) antibodies. METHODS: Ls-174T Luc+ is a colon carcinoma cell line strongly expressing CEA, transfected with the luciferase gene for BLI. Isolated or aggregated cells (1×10(6)) were injected through the portal vein. The tumor burden was investigated using BLI to assess hepatic implantation and growth kinetics. The biodistribution of the 125I anti-CEA antibody fragment (F6) was studied in this model and was compared with subcutaneous implantation. RESULTS: The tumor implantation rate was 100% using aggregated cells compared with 26.6% of isolated cells. Photons emitted by 1×10(6) cells were detected by BLI immediately after injection and allowed visual confirmation of hepatic distribution. The tumor growth was assessed over time to select homogeneous groups of animals. Radiolabeled anti-CEA antibody biodistributions showed a significantly higher uptake in hepatic than in subcutaneous tumors. CONCLUSION: The association of hepatic tumor graft through the portal route and BLI provides a reliable animal model and permits sensitive in-vivo detection and follow-up of hepatic metastases. The hepatic model seems to more closely reproduce colon cancer metastases compared with subcutaneous metastasis. The hepatic model is of particular interest for studying radioimmunotherapy.

Improved pretargeted delivery of radiolabelled hapten to human tumour xenograft in mice by avidin chase of circulating bispecific antibody.

PURPOSE: Pretargeted therapy with radiolabelled bivalent haptens and bispecific antibodies has shown promising results, but blood clearance of the activity-carrying haptens under conditions designed for radioimmunotherapy is relatively slow. Thus, the chase of excess circulating bispecific antibody by biotinylation of the bispecific antibody and injection of avidin before hapten administration was tested with a view to increasing tumour-to-blood activity ratios. METHODS: The anti-carcinoembryonic antigen (CEA) x anti-diethylene triamine penta-acetic acid-indium (di-DTPA-indium) bispecific antibody (hMN-14x734) was derivatised with NHS-LC-biotin and injected into LS-174T tumour-bearing nude mice at a dose of 3.5 nmol, followed by avidin and finally by the 125I-labelled di-DTPA-indium hapten (1 nmol). Blood samples were collected, animals sacrificed and tumours and normal tissues counted. RESULTS: Avidin chased up to 72% of the circulating antibody in the liver and the spleen within 30 min. When the labelled hapten was injected 3 h after avidin, tumour to blood ratios measured 3 and 24 h after hapten injection were significantly improved by the chase (3.5-fold), whereas tumour uptake was not significantly reduced. Uptake in normal tissues was unchanged (liver, kidney) or decreased (muscle), with the exception of spleen, in which uptake of both antibody and hapten was increased by the avidin chase. CONCLUSION: The chase strategy reduces hapten concentration in blood and thus should reduce bone marrow exposure. The use of two different recognition systems limits possible interference between the chase and targeting steps.