MR Immuno-Imaging Study Using Avidin-Biotin Pretargeting System On Nude Mice Grafted With Human Colorectal Carcinoma

Purposes: Magnetic resonance immunoimaging (MRII) possesses a characteristic of tissue-specific enhancement, however, the small amount of gadolinium located on tumor providing low signal intensity is one of the major obstacles in magnetic resonance imaging with gadolinium labeled monoclonal antibodies. Radioimrnunoimaging has proved that the three step pretargeting technique using avidin-biotin system can magnify the tumor signal and increase tumor-to-background uptake ratio in animal experiment and clinical application. The principles of MR are basically the same as those of radioimmunoimaging except that in the fonner the MR contrast media is used and in the latter the radioisotope ions used. To further improve the amount of gadolinium located on tumour, a gadolinium chelate enhanced magnetic resonance imaging pretargeting with avidin-biotin system technique was adopted and the enhancing characteristics of difference of signal intensity at various scan timing were investigated in our experiment. Materials and Methods: 1) Twenty BALB/C nude mice were grafted with human colon carcinoma by subcutaneous injection of LoVo cells. 2) Anti- CEA antibody CL3 were biotinylated in a mixture with antibody to NHS-LS- biotin with a molar ratio of 1:30-50. 3) After the reaction of GdC13 and DTPA- B, the unconjugated gadolinium was removed by chromatography on G-10 column. 4) Steps for pretargeting tumor: First step, McAb-B was injected intravenously into nude mice on the first day. Second step, Av and SA were 4 injected intraperitoneally 24 hours later. Third step, Gd-DTPA-B was injected intravenously 48 hours after the first injection. MRI was performed with plane scans, enhanced scans at 2Ominutes, 2 hours, 8 hours and 24 hours after the third step. Signal intensities of tumor and muscles were measured. The pretargeting effect was compared with those of McAb-Gd-DTPA and Gd- DTPA. Results: 1) Each Monoclonal antibody conjugated with 11-23 biotin and the immunuactivity of biotinylated antibody with 12 biotinlantibody was 95%. 2) The enhancing effect of pretargeting approach was tumor specificity. Contrarily that of Gd-DTPA was not. The enhancing effect of the pretargeting decreased slowly, however it was still perceptible even 24 hours after. The enhancing rate of signal intensity specificity of pretargeting approach was 43%, while that of McAb-Gd-DTPA was 17.9% only, so the enhancing ratio was 2.4.The pretargeting approach had a high contrast to noise ratio comparing with the approach of Gd3~ directly labeled McAb. Conclusions: Pretargeting approach using avidin-biotin system improves the amounts of gadolinium locating on tumors and yields a specific enhancing effect. It is a promising modality, which promotes the ability of Gd labeled magnetic resonance immunoimaging in the detection of colon cancer and its recurrence. A couple of questions related to the determination of optimal dosing schedule, injection timing, metabolic rate of antibody and so forth with the pretargeting approach are still needed to further solved.