Abstract 4400: A novel bispecific antibody with high-affinity to DOTA-metal haptens for pretargeted radioimmunotherapy

Abstract In an effort to reduce background and at the same time maintain hapten retention in the tumor for pretargeted radioimmunotherapy (PRIT) applications, we recently engineered a high-affinity single chain variable fragment (scFv) to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) metal chelates. This scFv displays ∼100 pM affinity to Lu and Y chelates of DOTA, ∼1 nM affinity to In chelates of DOTA-aminobenzene (DOTABn), and ∼10 nM affinity to In chelates of DOTA. A bispecific antibody (bsAb) construct was then engineered as a fusion of this scFv to the C-terminus of an IgG light chain. We present a three-step method for PRIT using the IgG-scFv bsAb, a dextran-based clearing agent, and radiolabeled DOTA. Biodistribution and tumor uptake were evaluated in mice with LS174T carcinoembryonic antigen (CEA)-positive tumors using this anti-CEA bsAb and 177Lu-DOTA, 111In-DOTABn, or 111In-DOTA after pretargeting with the bsAb at time t = −25 h, and clearing of residual bsAb with a dextran-based compound at t = −1 h. The bsAb exhibited tumor uptake of 19 ± 3 %ID/g at 24 h post injection resulting in approximately 10^5 DOTA binding sites per cell. The PRIT approach resulted in 177Lu-DOTA tumor uptake of ∼14 %ID/g at 24 h and 48 h, tumor to kidney ratios of ∼20 at 24 h and 48 h and tumor to blood ratios greater than 300 at 24 h and 48 h. The PRIT system was used to analyze in vivo the effect of small molecule affinity on tumor uptake. 177Lu-DOTA with an affinity of ∼100 pM for the bsAb resulted in 9-fold higher tumor uptake at 24 h than 111In-DOTABn (∼1 nM), and 28-fold higher tumor uptake than 111In-DOTA (∼10 nM). In addition, haptens with higher affinity resulted in higher tumor to kidney and tumor to blood ratios. We present here a PRIT approach with the highest yet reported 48 h tumor to blood and tumor to kidney ratios for CEA targeting. In addition, we report for the first time a systematic study of the effect of small molecule affinity on tumor uptake with affinities spanning a range of two orders of magnitude.24 h biodistribution of DOTA haptens with varying affinities Affinity for bsAb24 h tumor uptake24 h tumor/kidney ratio24 h tumor/blood ratio177Lu-DOTA∼100 pM13.8 ± 2.1 %ID/g18.0 ± 2.2324 ± 56111In-DOTABn∼1 nM1.6 ± 0.3 %ID/g5.6 ± 1.774 ± 4111In-DOTA∼10 nM0.5 ± 0.1 %ID/g1.2 ± 0.4not measurable Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4400.