Tumor and Organ Uptake of 64Cu-Labeled MORAb-009 (amatuximab), an Anti-Mesothelin Antibody, by PET Imaging and Biodistribution Studies

Objectives: To investigate the effect of the injection dose of MORAb-009 (amatuximab, an anti-mesothelin monoclonal antibody), the tumor size and the level of shed mesothelin on the uptake of the antibody in mesothelin-positive tumor and organs by biodistribution (BD) and positron emission tomography (PET) imaging studies.Methods: 2-S-(4-Isothiocyanatobenzyl)-1,4,7-triazacydononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) was conjugated to amatuximab and labeled with (CuCl2)-Cu-64 in 0.25 M acetate buffer, pH 4.2. The resulting Cu-64-NOTA-amatuximab was purified with a PD 10 column. To investigate the dose effect or the effect of tumor size, the BD was performed in groups of nude mice (n = 5) with mesothelin-expressing A431/H9 tumors (range, 80-300 mm(3)) one day after iv injection of Cu-64-NOTA-amatuximab (10 mu Ci) containing a total amatuximab dose of 2, 30, or 60 mu g. The BD and PET imaging were also investigated 3, 24 and 48 h after injecting a total dose of 30 mu g (10 mu Ci for BD), and 2 or 60 mu g (300 mu Ci for PET), respectively.Results: Comparing the results of the BDs from three different injection doses, the major difference was shown in the uptake (%ID/g) of the radiolabel in tumor, liver and blood. The tumor uptake and blood retention from 30 and 60 mu g doses were greater than those from 2 mu g dose, whereas the liver uptake was smaller. The BD studies also demonstrated a positive correlation between tumor size (or the level of shed mesothelin in blood) and liver uptake. However, there was a negative correlation between tumor size (or the shed mesothelin level) and tumor uptake and between tumor size and blood retention. These findings were confirmed by the PET imaging study, which clearly visualized the tumor uptake with the radiolabel concentrated in the tumor core and produced a tumor to liver ratio of 1.2 at 24 h post-injection with 60 mu g amatuximab, whereas the injection of 2 mu g amatuximab produced a tumor to liver ratio of 0.4 at 24 h post-injection.Conclusion: Our studies using a nude mouse model of A431/H9 tumor demonstrated that the injection of a high amatuximab dose (30 to 60 mu g) could provide a beneficial effect in maximizing tumor uptake while maintaining minimum liver and spleen uptakes of the radiolabel, and in facilitating its penetration into the tumor core. Published by Elsevier Inc.