Applications of ImmunoPET: Using 124I-Anti-PSCA A11 Minibody for Imaging Disease Progression and Response to Therapy in Mouse Xenograft Models of Prostate Cancer

Abstract

Purpose Prostate stem cell antigen (PSCA) is highly expressed in local prostate cancers and prostate cancer bone metastases and its expression correlates with androgen receptor activation and a poor prognosis. In this study, we investigate the potential clinical applications of immunoPET with the anti-PSCA A11 minibody, an antibody fragment optimized for use as an imaging agent. We compare A11 minibody immunoPET to 18F-Fluoride PET bone scans for detecting prostate cancer bone tumors and evaluate the ability of the A11 minibody to image tumor response to androgen deprivation. Experimental Design Osteoblastic, PSCA expressing, LAPC-9 intratibial xenografts were imaged with serial 124I-anti-PSCA A11 minibody immunoPET and 18F-Fluoride bone scans. Mice bearing LAPC-9 subcutaneous xenografts were treated with either vehicle or MDV-3100 and imaged with A11 minibody immunoPET/CT scans pre- and post-treatment. Ex vivo flow cytometry measured the change in PSCA expression in response to androgen deprivation. Results A11 minibody demonstrated improved sensitivity and specificity over 18F-Fluoride bone scans for detecting LAPC-9 intratibial xenografts at all time points. LAPC-9 subcutaneous xenografts showed downregulation of PSCA when treated with MDV-3100 which A11 minibody immunoPET was able to detect in vivo. Conclusions A11 minibody immunoPET has the potential to improve the sensitivity and specificity of clinical prostate cancer metastasis detection over bone scans, which are the current clinical standard of care. A11 minibody immunoPET additionally has the potential to image the activity of the androgen signaling axis in vivo which may help evaluate the clinical response to androgen deprivation and the development of castration resistance.

Antibody Fragment and Affibody ImmunoPET Imaging Agents: Radiolabelling Strategies and Applications

“…Such radioiodinations cannot however be used to label specific phenolic or tyrosine groups within the protein. Several minibodies for the imagingo fP CSA and CD20 have been developed by Wu and co-workers, [46][47][48][49] wherein they compared the performance of 124 Ii odinated fragments to that of 89 Zr and 64 Cu versions.I odine labelled fragments are more susceptible to degradation and deiodinationi nv ivo, releasing free [ 124 I]iodideo r[ 124 I]iodotyrosine. This can lead to loss of signal in the region of interesto vert ime.…”

In vivo imaging with antibodies and engineered fragments

“…However, it is non-specific: other biological processes (fractures, arthritis, inflammation) also generate positive signals, and soft tissue lesions are not addressable. Knowles et al recently compared PET imaging using 18 F-NaF bone scans to a tumor-specific 124 I-PSCA minibody for detecting intratibial prostate cancer in a mouse model (Knowles et al, 2014a), illustrating the significant advantages of targeting a tumor-specific marker in this setting (Figure 6). …”

“…PET imaging MET using a 89 Zr-DFO-labeled MET-specific human minibody clearly visualized the overexpression of MET in the gefitinib-resistant tumors. (Knowles et al, 2014a; Li et al, 2014)…”

Aligning physics and physiology: Engineering antibodies for radionuclide delivery

“…The minibody is a bivalent dimer comprising an scFv with a human gamma 1 hinge and C H 3 (~80 kDa) ( Figure 1C) with a serum half-life of 5 to 12 hours, and the size directs clearance to the liver. Radiolabeled minibodies have been effective as diagnostic PET imaging tracers targeting CEA, 69,70 CD8 T cells, 71 CD20 B Cells, PSCA, 72,73 and PSMA, 74 and high tumor-to-blood ratios can typically be achieved by 24 to 48 hours post-injection. 75 Small immunoproteins (SIPs) are recombinant proteins similar to the minibody composed of scFv dimerized by the C H 4 domain of IgE heavy chains, and they have demonstrated success as RIT agents.…”