To investigate the improved clearance effect of avidin(Av) and streptavidin-gal (SA-gal) for blood radiolabeled biotinylated antibody in pretargeting radioimmuno therapy (RIT) of colon carcinoma xenograft models. Biotinylated antibody radiolabeled with 131 I was injected into the nude mice bearing the colon carcinoma via the tail vein. 24h later

in 2 test groups

SA-gal or Av at a 10-fold molar excess of antibody was intraperitoneally injected into the animals whereas no any chase agents were given to the control animals. Animals were killed for biodistribution study at 30h after 131 I-labeled biotinylated antibody administration. Results showed that Av and SA-gal took the effect of chase very fast. At 6h after injection

the blood level of radioactivity decreased very fast. The tumor/blood raios of control group

Av chase group

SA-gal chase group were 0.42

2.09

5.23 respectively

P <0.05 and P <0.01 for the latter two groups as compared other control groups. Compared with Av

SA-gal showed better chase effect

its T/B ratios was 5.23

significantly higher than 2.09 of Av ( P <0.01). In the tissue biodistributions

relatively high non-specific radioactivity uptakes in non-liver organs were seen in Av chase group. Utilized in pretargeting RIT

both Av and SA-gal could make the blood level of radioactivity decrease quickly and considerably improves tumor T/NT ratios. The chase effect of SA-gal was superior to that of avidin.