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

Abstract

Pretargeting methodologies can produce high tumor:blood ratios, but their role in cancer radioimmunotherapy (RAIT) is uncertain. A pretargeting method was developed using a streptavidin (StAv) conjugate of MN-14 IgG, an anti-carcinoembryonic antigen (CEA) murine monoclonal antibody (mab) as the primary targeting agent, an anti-idiotype antibody (WI2 IgG) as a clearing agent, and DTPA- or DOTA-conjugated biotin as the radiolabeled targeting agent. A variety of reagents and conditions were examined to optimize this method. At 3 h, 111In-DTPA-peptide-biotin 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 would deliver higher tumor doses than this pretargeting method. Increasing the specific activity and using DOTA-biotin in place of DTPA increased tumor uptake nearly 2-fold, but analysis of StAv-MN-14's biotin-binding capacity indicated over 90% of the initial biotin-binding sites were blocked within 24 h. Animals fed a biotin-deficient diet had 2-fold higher 111In-DOTA-biotin uptake in the tumor, but higher uptake also was observed in all normal tissues. Although exceptionally adept at achieving high tumor:blood ratios rapidly, the tumor uptake of radiolabeled biotin with this pretargeting method is significantly (p < 0.0001) lower than that with a radiolabeled antibody. Endogenous biotin and enhanced liver and kidney uptake may limit the application of this method to RAIT, especially when evaluating the method in animals, but with strategies to overcome these limitations, this pretargeting method could be an effective therapeutic alternative.

Improving the Delivery of Radionuclides for Imaging and Therapy of Cancer Using Pretargeting Methods

“…Biotin is quite abundant in normal tissues, because it is an essential cofactor for several enzymes (22); thus, a streptavidin-IgG conjugate can be predisposed to the binding of endogenous biotin before the radiolabeled biotin is given (23). Fortunately, levels of free biotin in humans have been reported to be lower than in many other animals (24); thus, the level of biotin binding by a streptavidin conjugate does not seem to be compromised in patients (23,25); however, mice have substantial stores of assessable biotin, making it necessary when examining streptavidin-IgG conjugates to place mice on a biotin-deficient diet for several days before administering the conjugate to optimize tumor localization of the radiolabeled biotin (26).…”

Improved Tumor Targeting and Decreased Normal Tissue Accumulation through Extracorporeal Affinity Adsorption in a Two-Step Pretargeting Strategy

“…These results indicate that there is no advantage in eliminating 111 In-DOTA-streptavidin using a second ECAT step. Timing, scheduling, and dosing are important factors in pretargeting strategies (14). The ECAT system is probably less dependent on dosing and scheduling of the pretargeting immunoconjugate and radioactivity vector because it is possible to eliminate the excess of immunoconjugate before injecting the radioactivity vector.…”

“…There are some disadvantages associated with these strategies, such as clinical complexity about optimization of each of the injections, degradation of mAbs due to the long interval between the injection of mAbs and subsequent targeting of molecules, the immunogenicity of streptavidin, and the high absorbed dose in the reticuloendothelial system (14). By using ECAT to remove biotinylated antibodies followed by the injection of radiolabeled DOTAstreptavidin, some of these problems could be circumvented or reduced.…”

Improved Targeting of Pancreatic Cancer

“…Due to its small size, the radiolabeled material is rapidly cleared from the blood and other normal tissues, whereas at the same time it is bound at the tumor by the second recognition site of the antibody construct. Two major approaches have been reported, the first using an antibodyavidin (or streptavidin) conjugate to pretarget the tumor, with radiolabeled-biotin (or biotin containing material) as the imaging agent (41)(42)(43), and the second, a bispecific antibody (bsMAb) construct, with one arm directed to tumor antigen and the other arm able to bind a subsequently administered radiolabeled-hapten (37,38).…”