DEVELOPMENT OF A STREPTAVIDIN-ANTI-CARCINOEMBRYONIC ANTIGEN-ANTIBODY,RADIOLABELED BIOTIN PRETARGETING METHOD FOR RADIOIMMUNOTHERAPY OF COLORECTAL-CANCER - STUDIES IN A HUMAN COLON-CANCER XENOGRAFT MODEL

Pretargeting methodologies can produce high tumor:blood ratios, but th eir role in cancer radioimmunotherapy (RAIT) is uncertain. A pretarget ing method was developed using a streptavidin (StAv) conjugate of MN-1 4 IgG, an anti-carcinoembryonic antigen (CEA) murine monoclonal antibo dy (mab) as the primary targeting agent, an anti-idiotype antibody (WI 2 IgG) as a clearing agent, and DTPA- or DOTA-conjugated biotin as the radiolabeled targeting agent. A variety of reagents and conditions we re examined to optimize this method. At 3 h, In-111-DTPA-peptide-bioti n tumor uptake was 3.9 +/- 0.8 % per gram and tumor:blood ratios were >11:1. By 24 h, this ratio was 178:1, but tumor accretion declined in accordance with the gradual loss of StAv-MN-14 from the tumor. Tissue retention was highest in the liver and kidneys, but their tumor:organ ratios were >2:1. Dosimetry predicted that radiolabeled MN-14 alone wo uld deliver higher tumor doses than this pretargeting method. Increasi ng the specific activity and using DOTA-biotin in place of DTPA increa sed tumor uptake nearly 2-fold, but analysis of StAv-MN-14's biotin-bi nding capacity indicated over 90% of the initial biotin-binding sites were blocked within 24 h. Animals fed a biotin-deficient diet had 2-fo ld higher In-111-DOTA-biotin uptake in the tumor, but higher uptake al so was observed in all normal tissues. Although exceptionally adept at achieving high tumor:blood ratios rapidly, the tumor uptake of radiol abeled biotin with this pretargeting method is significantly (p < 0.00 01) lower than that with a radiolabeled antibody. Endogenous biotin an d enhanced liver and kidney uptake may limit the application of this m ethod to RAIT, especially when evaluating the method in animals, but w ith strategies to overcome these limitations, this pretargeting method could be an effective therapeutic alternative.