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 89Zr-B3 were dose-independent when 3 doses, 2, 15, and 60 μg B3, were compared at 24 h after injection. In contrast,
tumor and organ uptakes of 89Zr-amatuximab were dose-dependent, whereby a high dose (60 μg) was needed to achieve
tumor targeting comparable to the low dose (2 μg) of 89Zr-B3, suggesting that shed
mesothelin may affect
amatuximab tumor targeting as well as serum half-life. The autoradiography analysis showed that the distribution of 89Zr-B3 was nonuniform with the radioactivity primarily localized at the
tumor periphery independent of the B3 dose. However, the autoradiography analysis for 89Zr-amatuximab showed dose-dependent distribution profiles of the radiolabel; at 10 μg dose, the radiolabel penetrated toward the
tumor core with its activity comparable to that at the
tumor periphery, whereas at 60 μg dose, the distribution profile became similar to those of 89Zr-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.