We have used a series of bifunctional chelating agents to prepare 206Bilabeled monoclonal antibody and have assessed the in vivo stability and tumor targeting of these conjugates in the Rauscher murine erythroleukemia model. Several derivatives of diethylenetriaminepentaacetic acid [the dicyclic dianhydride of diethylenetriaminepentaacetic acid (ca-DTPA), 2-(p-isothiocyanatobenzyl)diethylenetriaminepentaacetic acid (SCNBzDTPA), and 2-(p-isothiocyanatobenzyl)-5(6)-methyl-diethylenetriaminepentaacetic acid (MxDTPA)], as well as a macrocyclic polyazacycloalkane-N-acetic acid [2-(p-isothiocyanatobenzyl)-1,4,7,10-tetraazacyclododecane-N,N′,N″,N‴-tetraacetic acid (DOTA)], were conjugated to monoclonal antibody 103A, which is specific for gp70 expressed on Rauscher virus-infected cells. The stability in vivo of 206Bi chelate-103A conjugates was first evaluated in normal mice by determining the levels of 206Bi in blood and kidney, since these were the organs in which free 206Bi, 206Bi-caDTPA-103A, and 35S-103A accumulated. The biodistribution of 206Bi administered as a chelate of caDTPA-103A was virtually indistinguishable from that of free 206Bi, indicating a low degree of in vivo stability of this bismuth chelate when compared to biosynthetically labeled 35S-103A. There was a progressive increase in the 206Bi levels observed in blood when the series of 103A conjugates prepared using SCNBzDTPA, MxDTPA, and DOTA was compared to 206Bi administered free or as a caDTPA-103A chelate. At 1 h after injection into normal mice, the blood level of 206Bi-DOTA-103A was 25-fold greater than that observed for 206Bi-caDTPA-103A and the level in kidney was 6-fold less, values that did not differ significantly from those observed for 35S-103A. Targeting to leukemic spleen was increased by 10-fold when the DOTA conjugate was used; the tumor level was 90% injected dose/g for DOTA, as compared to only 9% injected dose/g for caDTPA-103A at 1 h after injection. Use of the DOTA chelator also reduced by 7-fold the level of uptake by the kidney in the leukemic animals. We, therefore, conclude that the chelator DOTA is a promising reagent for the delivery of 212Bi-antibody conjugates to vascularized tumors under conditions that require targeting via the circulatory system.

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This research was supported in part by Public Health Service Research Grant CA-33470 from the NIH.

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