Radiolabeled monoclonal antibodies (mAbs) can target tumors selectively. Sustained activity levels in nontarget tissues limit their application. Pretargeting approaches using bispecific mAbs (bsmAbs) or the biotin-avidin interaction have been proposed to improve tumor:nontumor ratios. Pretargeting a tumor and subsequently administering the radioactivity as a low molecular weight ligand fundamentally changes the pharmacokinetics of the radiolabel. In previous studies, we have shown successful radioimmunotargeting of diethylenetriaminepentaacetic acid (DTPA) labeled with indium-111 to renal cell carcinoma (RCC) after pretargeting in nude mice. In this study, we aimed to optimize further a pretargeting strategy in nude mice with RCC xenografts based on a bispecific anti-RCC × anti-DTPA mAb.

Using this two-step approach, we studied whether the use of a bivalent chelate (¹¹¹In-diDTPA) could improve radioimmunotargeting. The ¹¹¹In-diDTPA dose greatly affected the uptake of the radiolabeled chelate in the tumor. At a low ¹¹¹In-diDTPA dose (≤7 pmol), tumor uptake of ¹¹¹In-diDTPA was very high [>50% injected dose (ID)/g, 1 h postinjection (p.i.)], whereas at higher doses (≥20 pmol), tumor uptake of ¹¹¹In-diDTPA decreased (<30% ID/g). With monovalent ¹¹¹In-DTPA uptake of the radiolabel in the tumor was much lower (<10% ID/g, 1 h p.i.). Furthermore, the bivalent chelate accreted rapidly in the tumor (78% ID/g, 4 h p.i.) and was virtually completely retained in the tumor during several days p.i. (92% ID/g, 72 h p.i.). Clearance of the ¹¹¹In-diDTPA from the blood and kidneys was rapid and complete without the need to clear the bsmAb from the blood, probably due to the relative lability of the univalent bsmAb-diDTPA complexes in the blood. As a result, with this two-step pretargeting approach tumor:blood ratios increased up to values as high as 3500 at 72 h p.i. High doses of diDTPA could be targeted preferentially to the tumor, indicating that this approach could also be used for radioimmunotherapy. Tumors could be imaged up to 1 week p.i. of 50 μCi of ¹¹¹In-diDTPA. Quantitative analysis of the images confirmed the biodistribution data and indicated that, at 20 h p.i., 50 ± 15% of the whole-body activity was localized in the tumor.

In conclusion, these studies indicate that the use of bivalent chelates can very effectively optimize two-step targeting of tumors with bsmAbs. Our data indicate that this approach could optimize radioimmunotherapy.