Impact of Radiometal Chelates on In Vivo Visualization of Immune Checkpoint Protein Using Radiolabeled Affibody Molecules.

Abstract

The immune checkpoint protein B7-H3 (CD276) is overexpressed in various cancers and is an attractive target for the treatment of malignant tumors. Radionuclide molecular imaging of B7-H3 expression using engineered scaffold proteins such as Affibody molecules is a promising strategy for the selection of potential responders to B7-H3-targeted therapy. Feasibility of B7-H3 imaging was demonstrated using two ^99mTc-labeled probes, AC12 and an affinity-matured SYNT179 using a [^99mTc]Tc-GGGC label. This study aimed to evaluate whether the use of a residualizing ¹¹¹In-based label provides better imaging contrast compared with a nonresidualizing label. To do that, SYNT179 and AC12-GGGC Affibody molecules were labeled with ¹¹¹In using (4,10-bis-carboxymethyl-7-{[2-(2,5-dioxo-3-thioxo-pyrrolidin-1-yl)-ethylcarbamoyl]-methyl}-1,4,7,10-tetraaza-cyclododec-1-yl)-acetic acid (maleimide-DOTA) chelator, site-specifically coupled to the C-terminus of Affibody molecules. The binding affinities of the ¹¹¹In-labeled conjugates to B7-H3-expressing living cells were higher compared with the affinities of the ^99mTc-labeled variants. In mice with B7-H3-expressing xenografts, the tumor uptake of ¹¹¹In-labeled proteins (3.6 ± 0.3 and 1.8 ± 0.5%ID/g for [¹¹¹In]In-SYNT179-DOTA and [¹¹¹In]In-AC12-DOTA, respectively) was significantly (p < 0.05, ANOVA) higher than those for ^99mTc-labeled counterparts (1.6 ± 0.2%ID/g and 0.8 ± 0.2%ID/g for [^99mTc]Tc-SYNT179 and [^99mTc]Tc-AC12-GGGC, respectively). The best variant, [¹¹¹In]In-SYNT179-DOTA, provided a tumor-to-blood ratio of 31.1 ± 2.9, which was twice higher than that for [^99mTc]Tc-SYNT179 and 7-fold higher than that for [^99mTc]Tc-AC12-GGGC. Both ¹¹¹In-labeled Affibody molecules had higher renal retention compared with ^99mTc-labeled ones, but the hepatobiliary excretion of ¹¹¹In-labeled proteins was appreciably lower, potentially improving the imaging of abdominal metastases. Overall, [¹¹¹In]In-SYNT179-DOTA is the most promising tracer for visualization of B7-H3 expression.