PET Imaging of Prostate Cancer Xenografts with a Highly Specific Antibody against the Prostate-Specific Membrane Antigen

Abstract

Prostate-specific membrane antigen (PSMA), a transmembrane glycoprotein, is highly expressed by virtually all prostate cancers and is currently the focus of several diagnostic and therapeutic strategies. We have previously reported on the generation of several monoclonal antibodies (mAb) and antibody fragments that recognize and bind with high affinity to the extracellular domain of cell-adherent PSMA. This article reports the in vivo behavior and tumor uptake of the radiolabeled anti-PSMA mAb 3/A12 and its potential as a tracer for PET. Methods: The mAb 3/A12 was conjugated with the chelating agent 1,4,7,10-tetraazacyclododecane-N,N9,N$,N9$-tetraacetic acid (DOTA) and radiolabeled with 64 Cu. Severe combined immunodeficient mice bearing PSMA-positive C4-2 prostate carcinoma xenografts were used for small-animal PET imaging. Mice with PSMA-negative DU 145 tumors served as controls. For PET studies, each animal received 20-30 mg of radiolabeled mAb corresponding to an activity of 7.6-11.5 MBq. Imaging was performed 3, 24, and 48 h after injection. After the last scan, the mice were sacrificed and tracer in vivo biodistribution was measured by g-counting. Results: Binding of the mAb 3/A12 on PSMA-expressing C4-2 cells was only minimally influenced by DOTA conjugation. The labeling efficiency using 64 Cu and DOTA-3/A12 was 95.3% 6 0.3%. The specific activity after 64 Cu labeling was between 327 and 567 MBq/mg. After tracer injection, static small-animal PET images of mice with PSMA-positive tumors revealed a tumor-to-background ratio of 3.3 6 1.3 at 3 h, 7.8 6 1.4 at 24 h, and 9.6 6 2.7 at 48 h. In contrast, no significant tracer uptake occurred in the PSMA-negative DU 145 tumors. These results were confirmed by direct counting of tissues after the final imaging. Conclusion: Because of the high and specific uptake of 64 Cu-labeled mAb 3/A12 in PSMA-positive tumors, this ligand represents an excellent candidate for prostate cancer imaging and potentially for radioimmunotherapy.

In Vivo Evaluation of 11C-DASB for Quantitative SERT Imaging in Rats and Mice

“…The rat brains were etched and fragmented into different brain regions and measured in a g-counting device. This could explain the differences from our data; g-counting measurements result in higher activity outcomes than in vivo PET measurements because of the partialvolume effects in PET experiments (37).…”

Optimization of Labeling PSMAHBED with Ethanol-Postprocessed 68Ga and Its Quality Control Systems

“…Because there is no monograph of PSMA HBED listed in the pharmacopeia, the radiopharmaceutical has to be purified of HEPES and an additional quality control step is necessary to determine the residual in the final formulation (as described as part of the monograph for 68 Ga-DOTATOC). To circumvent additional purification steps, 1 M NH 4 OAc solution was used as a buffer medium as part of this study.…”

“…Because PSMA expression is restricted to the prostate and the cell surface at all stages of disease, it holds promise as a target for specific imaging and therapy of prostate cancer and neovasculature (4)(5)(6). Studies have recently shown that low-molecular-weight peptidomimetic radiopharmaceuticals are clinically attractive because prostate cancer lesions can be imaged with high contrast and higher sensitivity than is possible with 18 F-choline-PET/CT (7)(8)(9)(10).…”

Positive Progress in ImmunoPET—Not Just a Coincidence

“…In the first to-date PSMA PET imaging study, Elsä sser-Beile et al 27 labeled a DOTA-conjugated intact anti-PSMA antibody (3/A12) with 64 Cu for evaluation in mice bearing PSMA-positive prostate carcinoma xenografts. Images were acquired at 3, 24, and 48 hours postinjection, and mice were sacrificed after the final scan for ex vivo g-counting of organs.…”