Tumor-Shed Antigen Affects Antibody Tumor Targeting: Comparison of Two Zr-89-Labeled Antibodies Directed against Shed or Nonshed Antigens

We investigated the effect of shed antigen mesothelin on the tumor uptake of amatuximab, a therapeutic anti-mesothelin mAb clinically tested in mesothelioma patients. The B3 mAb targeting a nonshed antigen was also analyzed for comparison. The mouse model implanted with A431/H9 tumor, which expresses both shed mesothelin and nonshed Lewis-Y antigen, provided an ideal system to compare the biodistribution and PET imaging profiles of the two mAbs. Our study demonstrated that the tumor and organ uptakes of Zr-89-B3 were dose-independent when 3 doses, 2, 15, and 60 mu g B3, were compared at 24 h after injection. In contrast, tumor and organ uptakes of Zr-89-amatuximab were dose-dependent, whereby a high dose (60 mu g) was needed to achieve tumor targeting comparable to the low dose (2 mu g) of Zr-89-B3, suggesting that shed mesothelin may affect amatuximab tumor targeting as well as serum half-life. The autoradiography analysis showed that the distribution of Zr-89-B3 was nonuniform with the radioactivity primarily localized at the tumor periphery independent of the B3 dose. However, the autoradiography analysis for Zr-89-amatuximab showed dose-dependent distribution profiles of the radiolabel; at 10 mu g dose, the radiolabel penetrated toward the tumor core with its activity comparable to that at the tumor periphery, whereas at 60 mu g dose, the distribution profile became similar to those of Zr-89-B3. These results suggest that shed antigen in blood may act as a decoy requiring higher doses of mAb to improve serum half-life as well as tumor targeting. Systemic mAb concentration should be at a severalfold molar excess to the shed Ag in blood to overcome the hepatic processing of mAb-Ag complexes. On the other hand, mAb concentration should remain lower than the shed Ag concentration in the tumor ECS to maximize tumor penetration by passing binding site barriers.

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