Single-photon emission computed tomography imaging with a humanized, apoptosis-inducing antibody targeting human death receptor 5 in pancreas and breast tumor xenografts

Abstract

Purpose: To characterize a novel humanized, apoptosis-inducing, antibody targeting human death receptor 5 (DR5) radiolabeled with Tc-99m by in vitro assays and in vivo imaging in murine pancreatic and breast cancer models. Experimental Design: Cytotoxicity and binding assays were conducted with Tc-99m-labeled humanized TRA-8 (hTRA-8) using human breast (2LMP) and pancreatic (MIA PaCa-2) tumor lines. SPECT/CT studies and biodistribution analyses were performed in nude mice with these xenografted tumors to measure in vivo distribution of Tc-99m-hTRA-8. Planar imaging was conducted in cynomolgus monkeys. Results: The acid-labile and acid-stable binding affinities (Kd) of hTRA-8 for MIA PaCa-2 cells were 3.0 ± 1.0 (mean ± SE) nM and 5.1 ± 1.4 nM, while those for 2LMP cells were 2.5 ± 0.6 nM and 3.3 ± 0.4 nM, respectively. DR5 receptors per MIA PaCa-2 cell averaged 8620 ± 1920; not statistically different from the mean per 2LMP cell at 11170 ± 4170. The distribution of Tc-99m-hTRA-8 within tumors was not uniform, the second 1.5-mm shell from the surface was highest, averaging 6.9 ± 0.4 (MIA PaCa-2) and 6.7 ± 0.2 (2LMP) %ID/g at four hours. At 6, 24 and 48 hours after Tc-99m-hTRA-8 injection, MIA PaCa-2 tumor retentions were 7.6 ± 0.6, 18.4 ± 1.4 and 18.3 ± 1.0 % of injected dose per gram (%ID/g) respectively, while Tc-99m-isotype control antibody averaged 4.9 ± 0.4 %ID/g for MIA PaCa-2 tumors at 24 hours after dosing. The planar monkey images revealed a blood-pool distribution of Tc-99m-hTRA-8, which was confirmed by blood analyses. Conclusions: These studies demonstrate effective delivery and retention of Tc-99m-labeled humanized TRA-8 in human xenograft tumors for the first time, and support further human studies for cancer therapy. ©2007 Landes Bioscience.