Fluorescent Image-Guided Surgery with an Anti-Prostate Stem Cell Antigen (PSCA) Diabody Enables Targeted Resection of Mouse Prostate Cancer Xenografts in Real Time.

PURPOSE: The inability to visualize cancer during prostatectomy contributes to positive margins, cancer recurrence, and surgical side effects. A molecularly targeted fluorescent probe offers the potential for real-time intraoperative imaging. The goal of this study was to develop a probe for image-guided prostate cancer surgery. EXPERIMENTAL DESIGN: An antibody fragment (cys-diabody, cDb) against prostate stem cell antigen (PSCA) was conjugated to a far-red fluorophore, Cy5. The integrity and binding of the probe to PSCA was confirmed by gel electrophoresis, size exclusion, and flow cytometry, respectively. Subcutaneous models of PSCA-expressing xenografts were used to assess the biodistribution and in vivo kinetics, whereas an invasive intramuscular model was utilized to explore the performance of Cy5-cDb-mediated fluorescence guidance in representative surgical scenarios. Finally, a prospective, randomized study comparing surgical resection with and without fluorescent guidance was performed to determine whether this probe could reduce the incidence of positive margins. RESULTS: Cy5-cDb demonstrated excellent purity, stability, and specific binding to PSCA. In vivo imaging showed maximal signal-to-background ratios at 6 hours. In mice carrying PSCA(+) and negative (-) dual xenografts, the mean fluorescence ratio of PSCA(+/-) tumors was 4.4:1. In surgical resection experiments, residual tumors <1 mm that were missed on white light surgery were identified and resected using fluorescence guidance, which reduced the incidence of positive surgical margins (0/8) compared with white light surgery alone (7/7). CONCLUSIONS: Fluorescently labeled cDb enables real-time in vivo imaging of prostate cancer xenografts in mice, and facilitates more complete tumor removal than conventional white light surgery alone.

Microfluidic-based 18F-labeling of biomolecules for immuno-positron emission tomography.

Methods for tagging biomolecules with fluorine 18 as immuno-positron emission tomography (immunoPET) tracers require tedious optimization of radiolabeling conditions and can consume large amounts of scarce biomolecules. We describe an improved method using a digital microfluidic droplet generation (DMDG) chip, which provides computer-controlled metering and mixing of 18F tag, biomolecule, and buffer in defined ratios, allowing rapid scouting of reaction conditions in nanoliter volumes. The identified optimized conditions were then translated to bench-scale 18F labeling of a cancer-specific engineered antibody fragments, enabling microPET imaging of tumors in xenografted mice at 0.5 to 4 hours postinjection.

An affinity matured minibody for PET imaging of prostate stem cell antigen (PSCA)-expressing tumors.

PURPOSE: Prostate stem cell antigen (PSCA), a cell surface glycoprotein expressed in normal human prostate and bladder, is over-expressed in the majority of localized prostate cancer and most bone metastases. We have previously shown that the hu1G8 minibody, a humanized anti-PSCA antibody fragment (single-chain Fv-C(H)3 dimer, 80 kDa), can localize specifically and image PSCA-expressing xenografts at 21 h post-injection. However, the humanization and antibody fragment reformatting decreased its apparent affinity. Here, we sought to evaluate PET imaging contrast with affinity matured minibodies. METHODS: Yeast scFv display, involving four rounds of selection, was used to generate the three affinity matured antibody fragments (A2, A11, and C5) that were reformatted into minibodies. These three affinity matured anti-PSCA minibodies were characterized in vitro, and following radiolabeling with (124)I were evaluated in vivo for microPET imaging of PSCA-expressing tumors. RESULTS: The A2, A11, and C5 minibody variants all demonstrated improved affinity compared to the parental (P) minibody and were ranked as follows: A2 > A11 > C5 > P. The (124)I-labeled A11 minibody demonstrated higher immunoreactivity than the parental minibody and also achieved the best microPET imaging contrast in two xenograft models, LAPC-9 (prostate cancer) and Capan-1 (pancreatic cancer), when evaluated in vivo. CONCLUSION: Of the affinity variant minibodies tested, the A11 minibody that ranked second in affinity was selected as the best immunoPET tracer to image PSCA-expressing xenografts. This candidate is currently under development for evaluation in a pilot clinical imaging study.

Minibodies and Multimodal Chromatography Methods: A Convergence of Challenge and Opportunity.

This case study describes early phase purification process development for a recombinant anticancer minibody produced in mammalian cell culture. The minibody did not bind to protein A. Cation-exchange, anion-exchange, hydrophobic-interaction, and hydroxyapatite (eluted by phosphate gradient) chromatographic methods were scouted, but the minibody coeluted with BSA to a substantial degree on each. Hydroxyapatite eluted with a sodium chloride gradient separated BSA and also removed a dimeric contaminant, but BSA consumed so much binding capacity that this proved impractical as a capture tool. Capto MMC media proved capable of supporting adequate capture and significant dimer removal, although both loading and elution selectivity varied dramatically with the amount of supernatant applied to the column. An anion-exchange step was included to fortify overall virus and DNA removal. These results illustrate the value of multimodal chromatography methods when affinity chromatography methods are lacking and conventional alternatives prove inadequate.

Cys-diabody quantum dot conjugates (immunoQdots) for cancer marker detection.

The present work demonstrates the use of small bivalent engineered antibody fragments, cys-diabodies, for biological modification of nanoscale particles such as quantum dots (Qdots) for detection of target antigens. Novel bioconjugated quantum dots known as immunoQdots (iQdots) were developed by thiol-specific oriented coupling of tumor specific cys-diabodies, at a position away from the antigen binding site to amino PEG CdSe/ZnS Qdots. Initially, amino PEG Qdot 655 were coupled with reduced anti-HER2 cys-diabody by amine-sulfhydryl-reactive linker [N-ε-maleimidocaproyloxy] succinimide ester (EMCS) to produce anti-HER2 iQdot 655. Spectral characterization of the conjugate revealed that the spectrum was symmetrical and essentially identical to unconjugated Qdot. Specific receptor binding activity of anti-HER2 iQdot 655 was confirmed by flow cytometry on HER2 positive and negative cells. Immunofluorescence results showed homogeneous surface labeling of the cell membrane with Qdot 655 conjugate. In addition, cys-diabodies specific for HER2, as well as prostate stem cell antigen (PSCA), were conjugated successfully with amino PEG Qdot 800. All of these iQdots retain the photoluminescence properties of the unconjugated Qdot 800 as well as the antigen binding specificity of the cys-diabody as demonstrated by flow cytometry. Simultaneous detection of two tumor antigens on LNCaP/PSCA prostate cancer cells (which express PSCA and HER2) in culture was possible using two iQdots, anti-HER2 iQdot 655 and anti-PSCA iQdot 800. Thus, these iQdots are potentially useful as optical probes for sensitive, multiplexed detection of surface markers on tumor cells. The present thiol-specific conjugation method demonstrates a general approach for site-specific oriented coupling of cys-diabodies to a wide variety of nanoparticles without disturbing the antigen binding site and maintaining small size compared to intact antibody.

Diabodies targeting epithelial membrane protein 2 reduce tumorigenicity of human endometrial cancer cell lines.

PURPOSE: Endometrial cancer is the most common gynecologic malignancy. One promising biomarker is epithelial membrane protein 2 (EMP2), and its expression is an independent prognostic indicator for tumors with poor clinical outcome expression. The present study assesses the suitability of EMP2 as a therapeutic target. EXPERIMENTAL DESIGN: Human monovalent anti-EMP2 antibody fragments were isolated from a human phage display library and engineered as bivalent antibody fragments (diabodies) with specificity and avidity to both EMP2 peptides and native cell-surface EMP2 protein. Diabodies were assessed using cell death and apoptosis assays. In addition, the efficacy of EMP2 diabodies on endometrial cancer tumors was determined using mouse xenograft models. RESULTS: Treatment of human endometrial adenocarcinoma cell lines with anti-EMP2 diabodies induced significant cell death and caspase-3 cleavage in vitro. These responses correlated with cellular EMP2 expression and were augmented by progesterone, which physiologically induces EMP2 expression. In vivo, treatment of subcutaneous human xenografts of HEC-1A cell lines with anti-EMP2 diabodies suppressed tumor growth and induced cell death in the xenograft. CONCLUSIONS: These findings suggest that EMP2 may be a potential pharmacologic target for human endometrial cancer.

Humanized radioiodinated minibody for imaging of prostate stem cell antigen-expressing tumors.

PURPOSE: Prostate stem cell antigen (PSCA) is a cell surface glycoprotein that is overexpressed in prostate cancer, including hormone refractory disease. Previous preclinical studies showed the intact anti-PSCA antibodies, 1G8 and hu1G8, localized specifically to PSCA-expressing xenografts. Optimal micro positron emission tomography (microPET) imaging using hu1G8, however, required a delay of 168 hours postinjection. In this study, the 2B3 minibody (an 80-kDa engineered antibody fragment) has been produced for rapid targeting and imaging. EXPERIMENTAL DESIGN: A gene encoding a PSCA-specific minibody, V(L)-linker-V(H)-hinge-huIgG1 C(H)3, was assembled. The minibody was expressed by secretion from mammalian cells and purified by cation exchange chromatography. Relative affinity and specificity were determined by competition ELISA and flow cytometry. Serial microPET imaging using a 124I-labeled minibody was conducted at 4 and 21 hours in mice bearing LAPC-9 AD, LAPC-9 AI, PC-3, and LNCaP-PSCA human prostate cancer xenografts. Tumor and tissue biodistribution was determined, and region of interest analysis of the images was conducted. RESULTS: Yields of 20 mg/L purified 2B3 minibody were obtained that showed specific binding to LNCaP-PSCA cells. Purified 2B3 minibody showed specific binding to LNCaP-PSCA cells with an apparent affinity of 46 nmol/L. Radioiodinated 2B3 minibody showed rapid nontarget tissue and blood clearance kinetics (t1/2beta = 11.2 hours). MicroPET scanning using the 124I-2B3 minibody showed both androgen-dependent and -independent tumors as early as 4 hours and excellent high contrast images at 21 hours postinjection. CONCLUSIONS: Imaging PSCA-positive prostate cancer is feasible using an intermediate size antibody fragment at 21 hours.