Bifunctional antibody-Renilla luciferase fusion protein for in vivo optical detection of tumors

Suppression of tumour growth from transplanted astrocytoma cells transfected with luciferase in mice by bioluminescence mediated, systemic, photodynamic therapy

BACKGROUND

Grade 4 astrocytomas are usually incurable due to their diffusely infiltrative nature. Photodynamic therapy (PDT) is a promising therapeutic option, but external light delivery is impractical when cancer cells infiltrate unknown areas of normal brain. Hence the search for endogenous sources to generate light at cancer cells. In vitro, astrocytoma cells, transfected with firefly luciferase, can be killed by bioluminescence-mediated PDT (bPDT). This study asks if bPDT can suppress tumour growth In vivo, when all components of treatment are administered systemically.

METHODS

Transfected astrocytoma cells were injected subcutaneously or intra-cranially in athymic CD1 nu/nu mice. bPDT required ip bolus of mTHPC (photosensitiser) and delivery of the d-luciferin substrate over 7 days via an implanted osmotic pump. Control animals had no treatment, photosensitiser only or d-luciferin only. For subcutaneous tumours, size and BLI (light emitted after d-luciferin bolus) were measured before and every 2 days after PDT. For intracranial tumours, monitoring was weekly BLI.

RESULTS

For subcutaneous tumours, there was significant suppression of the tumour growth rate (P<0.05), and absolute tumour size (P<0.01) after bPDT. Proliferation of subcutaneous and intracranial tumours (monitored by BrdU uptake) was significantly reduced in treated mice. (P<0.001) CONCLUSIONS: This study reports bPDT suppression of tumour growth from luciferase transfected astrocytoma cells with all components of treatment given systemically, as required for effective management of recurrent astrocytomas in unknown sites. However, research on systemic bPDT is needed to establish whether effects on non-transfected tumours can be achieved without any unacceptable effects on normal tissues.

Engineered Amber-Emitting Nano Luciferase and Its Use for Immunobioluminescence Imaging In Vivo

The NanoLuc luciferase (NLuc) and its furimazine (FRZ) substrate have revolutionized bioluminescence (BL) assays and imaging. However, the use of the NLuc-FRZ luciferase-luciferin pair for mammalian tissue imaging is hindered by the low tissue penetration of the emitting blue photons. Here, we present the development of an NLuc mutant, QLuc, which catalyzes the oxidation of a synthetic QTZ luciferin for bright and red-shifted emission peaking at ∼585 nm. Compared to other small single-domain NLuc mutants, this amber-light-emitting luciferase exhibited improved performance for imaging deep-tissue targets in live mice. Leveraging this novel bioluminescent reporter, we further pursued in vivo immunobioluminescence imaging (immunoBLI), which used a fusion protein of a single-chain variable antibody fragment (scFv) and QLuc for molecular imaging of tumor-associated antigens in a xenograft mouse model. As one of the most red-shifted NLuc variants, we expect QLuc to find broad applications in noninvasive mammalian imaging. Moreover, the immunoBLI method complements immunofluorescence imaging and immuno-positron emission tomography (immunoPET), serving as a convenient and nonradioactive molecular imaging tool for animal models in basic and preclinical research.

Antibody-Invertase Fusion Protein Enables Quantitative Detection of SARS-CoV-2 Antibodies Using Widely Available Glucometers

Rapid diagnostics that can accurately inform patients of disease risk and protection are critical to mitigating the spread of the current COVID-19 pandemic and future infectious disease outbreaks. To be effective, such diagnostics must rely on simple, cost-effective, and widely available equipment and should be compatible with existing telehealth infrastructure to facilitate data access and remote care. Commercial glucometers are an established detection technology that can overcome the cost, time, and trained personnel requirements of current benchtop-based antibody serology assays when paired with reporter molecules that catalyze glucose conversion. To this end, we developed an enzymatic reporter that, when bound to disease-specific patient antibodies, produces glucose in proportion to the level of antibodies present in the patient sample. Although a straightforward concept, the coupling of enzymatic reporters to secondary antibodies or antigens often results in low yields, indeterminant stoichiometry, reduced target binding, and poor catalytic efficiency. Our enzymatic reporter is a novel fusion protein that comprises an antihuman immunoglobulin G (IgG) antibody genetically fused to two invertase molecules. The resulting fusion protein retains the binding affinity and catalytic activity of the constituent proteins and serves as an accurate reporter for immunoassays. Using this fusion, we demonstrate quantitative glucometer-based measurement of anti-SARS-CoV-2 spike protein antibodies in blinded clinical sample training sets. Our results demonstrate the ability to detect SARS-CoV-2-specific IgGs in patient serum with precise agreement to benchmark commercial immunoassays. Because our fusion protein binds all human IgG isotypes, it represents a versatile tool for detection of disease-specific antibodies in a broad range of biomedical applications.

Bioluminescent Antibodies through Photoconjugation of Protein G-Luciferase Fusion Proteins

Bioluminescent antibodies represent attractive detection agents in both bioanalytical assays and imaging. Currently, their preparation relies on genetic fusion of luciferases to antibodies or nonspecific chemical conjugation strategies. Here, we report a generic method to generate well-defined covalent antibody-luciferase conjugates starting from commercially available monoclonal antibodies. Our approach uses fusion proteins consisting of the bright blue light-emitting luciferase NanoLuc (NL) and an Fc-binding protein domain (Gx) that can be photo-cross-linked to the antibody using UV light illumination. Green and red color variants were constructed by tight fusion of the NanoLuc with a green fluorescent acceptor domain and introduction of Cy3, respectively. To increase the already bright NanoLuc emission, tandem fusions were successfully developed in which the Gx domain is fused to two or three copies of the NanoLuc domain. The Gx-NL fusion proteins can be efficiently photo-cross-linked to all human immunoglobulin G (IgG) isotypes and most mammalian IgG's using 365 nm light, yielding antibodies with either one or two luciferase domains. The bioluminescent antibodies were successfully used in cell immunostaining and bioanalytical assays such as enzyme-linked immunosorbent assay (ELISA) and Western blotting.

Engineering of galectin-3 for glycan-binding optical imaging

Galectin-3 (Gal-3) is a multifunctional glycan-binding protein that participates in many pathophysiological events and has been described as a biomarker and potential therapeutic target for severe disorders, such as cancer. Several probes for Gal-3 or its ligands have been developed, however both the pathophysiological mechanisms and potential biomedical applications of Gal-3 remain not fully assessed. Molecular imaging using bioluminescent probes provides great sensitivity for in vivo and in vitro analysis for both cellular and whole multicellular organism tracking and target detection. Here, we engineered a chimeric molecule consisting of Renilla luciferase fused with mouse Gal-3 (RLuc-mGal-3). RLuc-mGal-3 preparation was highly homogenous, soluble, active, and has molecular mass of 65,870.95 Da. This molecule was able to bind to MKN45 cell surface, property which was inhibited by the reduction of Gal-3 ligands on the cell surface by the overexpression of ST6GalNAc-I. In order to obtain an efficient and stable delivery system, RLuc-mGal-3 was adsorbed to poly-lactic acid nanoparticles, which increased binding to MKN45 cells in vitro. Furthermore, bioluminescence imaging showed that RLuc-mGal-3 was able to indicate the presence of implanted tumor in mice, event drastically inhibited by the presence of lactose. This novel bioluminescent chimeric molecule offers a safe and highly sensitive alternative to fluorescent and radiolabeled probes with potential application in biomedical research for a better understanding of the distribution and fate of Gal-3 and its ligands in vitro and in vivo.

Integration of Nanomaterials and Bioluminescence Resonance Energy Transfer Techniques for Sensing Biomolecules

Bioluminescence resonance energy transfer (BRET) techniques offer a high degree of sensitivity, reliability and ease of use for their application to sensing biomolecules. BRET is a distance dependent, non-radiative energy transfer, which uses a bioluminescent protein to excite an acceptor through the resonance energy transfer. A BRET sensor can quickly detect the change of a target biomolecule quantitatively without an external electromagnetic field, e.g., UV light, which normally can damage tissue. Having been developed quite recently, this technique has evolved rapidly. Here, different bioluminescent proteins have been reviewed. In addition to a multitude of bioluminescent proteins, this manuscript focuses on the recent development of BRET sensors by utilizing quantum dots. The special size-dependent properties of quantum dots have made the BRET sensing technique attractive for the real-time monitoring of the changes of target molecules and bioimaging in vivo. This review offers a look into the basis of the technique, donor/acceptor pairs, experimental applications and prospects.

Zipbodyzyme: Development of new antibody-enzyme fusion proteins

A new antibody-enzyme fusion protein, named Zipbodyzyme, composed of a Fab antibody (i.e., an antigen-binding fragment of an antibody) and an enzyme, has been successfully produced in the cytoplasm of Escherichia coli. Zipbodyzymes have a leucine zipper (LZ) pair at the C-termini of the heavy chain (Hc) and the light chain (Lc) of Fab, to promote the association of the Hc and the Lc in E. coli cytoplasm, adjoining a fused enzyme. A Zipbodyzyme containing mouse-derived anti-E. coli O157 Fab and a luciferase or a green fluorescent protein retained both the antigen-binding and an enzymatic activity/fluorescence. The bifunctional proteins were applicable in direct enzyme-linked immunosorbent assay (ELISA) without the need for a secondary antibody, indicating that the utility of the antibody enzyme bifunctional fusion protein will be expanded.

Near-Infrared Visual Differentiation in Normal and Abnormal Breast Using Hemoglobin Concentrations

Introduction: Near-infrared (NIR) optical imaging is a non-ionizing modality that is emerging as a diagnostic/prognostic tool for breast cancer according to NIR differentiation of hemoglobin (Hb) concentration. Methods: The transmission values of LED-sourced light at 625 nm were measured by power meter to evaluate the optical properties of Hb in breast phantom containing major and minor vessels. For the simulation of blood variations in cancerous breast condition, we prepared 2 concentrations of pre-menopausal Hb and 4 concentrations of post-menopausal Hb and, for comparison with normal tissue, one concentration of Hb injected inside the phantom's vessels. Imaging procedure on the phantom was also conducted by LED source and CCD camera. The images from the experiments were compared with the results obtained from the images analyzed by MATLAB software. Finally, mammography of phantom including various concentration of Hb was prepared. Results: The transmitting intensities of NIR in blood containing 1, 2 and 4 concentrations of Hb in the major vessels were 52.83±2.85, 43.00±3.11 and 31.17±2.27 µW, respectively, and in minor vessels containing similar Hb concentrations were 73.50±2.43, 60.08±5.09 and 42.42±4.86 µW, respectively. The gray-scale levels on the major vessel were about 96, 124, 162 and on the minor vessel about 72, 100, 130 measured for 1, 2 and 4 Hb concentrations, respectively. The sensitivity and specificity of NIR imaging differentiation were 97.4% and 91.3%, respectively. Conclusion: Significant differences in transmitting intensity, optical imaging as well as software analysis of images were observed for 1, 2 and 4 concentrations of Hb in major and minor breast phantom vessels. Differentiation capability of minor vessels was higher than major vessels for Hb concentrations. Despite a good detection for location of vessels by mammography, it could not show differences between vessels with various concentrations. However, NIR optical imaging demonstrated a good image contrast for showing vessels in terms of concentration. This study recommends NIR optical imaging for prescreening breast cancer due to its potential for early diagnosis.

Engineering of monobody conjugates for human EphA2-specific optical imaging

In a previous study, we developed an E1 monobody specific for the tumor biomarker hEphA2 [PLoS ONE (2015) 10(7): e0132976]. E1 showed potential as a molecular probe for in vitro and in vivo targeting of cancers overexpressing hEphA2. In the present study, we constructed expression vectors for E1 conjugated to optical reporters such as Renilla luciferase variant 8 (Rluc8) or enhanced green fluorescent protein (EGFP) and purified such recombinant proteins by affinity chromatography in E. coli. E1-Rluc8 and E1-EGFP specifically bound to hEphA2 in human prostate cancer PC3 cells but not in human cervical cancer HeLa cells, which express hEphA2 at high and low levels, respectively. These recombinant proteins maintained >40% activity in mouse serum at 24 h. In vivo optical imaging for 24 h did not detect E1-EGFP signals, whereas E1-Rluc8 showed tumor-specific luminescence signals in PC3 but not in HeLa xenograft mice. E1-Rluc8 signals were detected at 4 h, peaked at 12 h, and were undetectable at 24 h. These results suggest the potential of E1-Rluc8 as an EphA2-specific optical imaging agent.

Super RLuc8-sFv; a new luciferase-labeled probe for detection of human CD4+ cells

A probe has been designed, produced and assayed for the detection of T-helper cells. The sFv fragment fused to the C-terminus of super Renilla luciferase 8 (Super RLuc8) and the probe successfully used for detection of human CD4+ cells using luminometer.

Optical imaging probes in oncology

Cancer is a complex disease, characterized by alteration of different physiological molecular processes and cellular features. Keeping this in mind, the possibility of early identification and detection of specific tumor biomarkers by non-invasive approaches could improve early diagnosis and patient management.Different molecular imaging procedures provide powerful tools for detection and non-invasive characterization of oncological lesions. Clinical studies are mainly based on the use of computed tomography, nuclear-based imaging techniques and magnetic resonance imaging. Preclinical imaging in small animal models entails the use of dedicated instruments, and beyond the already cited imaging techniques, it includes also optical imaging studies. Optical imaging strategies are based on the use of luminescent or fluorescent reporter genes or injectable fluorescent or luminescent probes that provide the possibility to study tumor features even by means of fluorescence and luminescence imaging. Currently, most of these probes are used only in animal models, but the possibility of applying some of them also in the clinics is under evaluation.The importance of tumor imaging, the ease of use of optical imaging instruments, the commercial availability of a wide range of probes as well as the continuous description of newly developed probes, demonstrate the significance of these applications. The aim of this review is providing a complete description of the possible optical imaging procedures available for the non-invasive assessment of tumor features in oncological murine models. In particular, the characteristics of both commercially available and newly developed probes will be outlined and discussed.

NanoLuc Luciferase - A Multifunctional Tool for High Throughput Antibody Screening

Based on the recent development of NanoLuc luciferase (Nluc), a small (19 kDa), highly stable, ATP independent, bioluminescent protein, an extremely robust and ultra high sensitivity screening system has been developed whereby primary hits of therapeutic antibodies and antibody fragments could be characterized and quantified without purification. This system is very versatile allowing cellular and solid phase ELISA but also homogeneous BRET based screening assays, relative affinity determinations with competition ELISA and direct Western blotting. The new Nluc protein fusion represents a “swiss army knife solution” for today and future high throughput antibody drug screenings.

Application of enzyme bioluminescence for medical diagnostics

Nowadays luciferases are effectively used as analytical instruments in a great variety of research fields. Of special interest are the studies dealing with elaboration of novel analytical systems for the purposes of medical diagnostics. The ever-expanding spectrum of clinically important analytes accounts for the increasing demand for new techniques for their detection. In this chapter we have made an attempt to summarize the results on applications of luciferases as reporters in binding assays including immunoassay, nucleic acid hybridization assay, and so on. The data over the last 15 years have been analyzed and clearly show that luciferase-based assays, due to extremely high sensitivity, low cost, and the lack of need for skilled personnel, hold much promise for clinical diagnostics.

Continuous, real-time bioimaging of chemical bioavailability and toxicology using autonomously bioluminescent human cell lines

Bioluminescent imaging is an emerging biomedical surveillance strategy that uses external cameras to detect in vivo light generated in small animal models of human physiology or in vitro light generated in tissue culture or tissue scaffold mimics of human anatomy. The most widely utilized of reporters is the firefly luciferase (luc) gene; however, it generates light only upon addition of a chemical substrate, thus only generating intermittent single time point data snapshots. To overcome this disadvantage, we have demonstrated substrate-independent bioluminescent imaging using an optimized bacterial bioluminescence (lux) system. The lux reporter produces bioluminescence autonomously using components found naturally within the cell, thereby allowing imaging to occur continuously and in real-time over the lifetime of the host. We have validated this technology in human cells with demonstrated chemical toxicological profiling against exotoxin exposures at signal strengths comparable to existing luc systems (~1.33 × 10⁷ photons/second). As a proof-in-principle demonstration, we have engineered breast carcinoma cells to express bioluminescence for real-time screening of endocrine disrupting chemicals and validated detection of 17β-estradiol (EC₅₀ = ~ 10 pM). These and other applications of this new reporter technology will be discussed as potential new pathways towards improved models of target chemical bioavailability, toxicology, efficacy, and human safety.

Expression of an antibody-enzyme complex by the L-chain fusion method

In this report, we describe a novel method for directly preparing enzyme-labeled antibodies harvested from IgM-producing hybridoma cells. We constructed expression vectors for antibody light (L) chain-enzyme fusion proteins by linking either the genes for the murine lambda L chain or its constant region (C(L)) with one of two proteins, either the secreted placental alkaline phosphatase or Gaussia luciferase (Gluc). When the vectors were transfected into anti-NP (4-hydroxy-3-nitrophacetyl) IgM-producing myeloma cells, secretion of the IgM-enzyme complex from the gene-transfected cells was confirmed by a direct enzyme-linked immunosorbent assay with an immobilized antigen. Furthermore, when human hybridoma HF10B4, a cell line that produces anti-human lung cancer IgM, was transfected with the vector containing L-Gluc, a significantly stronger signal was obtained for the human lung carcinoma SBC-1 cells than for cervical HeLa cells. Because successful production of an active IgM-enzyme complex containing a heterologous L chain-enzyme fusion was observed, the L-chain fusion method will be a generally applicable method for preparing various IgM-enzyme complexes.

Intelligent design of nano-scale molecular imaging agents

Visual representation and quantification of biological processes at the cellular and subcellular levels within living subjects are gaining great interest in life science to address frontier issues in pathology and physiology. As intact living subjects do not emit any optical signature, visual representation usually exploits nano-scale imaging agents as the source of image contrast. Many imaging agents have been developed for this purpose, some of which exert nonspecific, passive, and physical interaction with a target. Current research interest in molecular imaging has mainly shifted to fabrication of smartly integrated, specific, and versatile agents that emit fluorescence or luminescence as an optical readout. These agents include luminescent quantum dots (QDs), biofunctional antibodies, and multifunctional nanoparticles. Furthermore, genetically encoded nano-imaging agents embedding fluorescent proteins or luciferases are now gaining popularity. These agents are generated by integrative design of the components, such as luciferase, flexible linker, and receptor to exert a specific on-off switching in the complex context of living subjects. In the present review, we provide an overview of the basic concepts, smart design, and practical contribution of recent nano-scale imaging agents, especially with respect to genetically encoded imaging agents.

Irradiation induced fluorescence enhancement in PEGylated cyanine-based NIR nano- and mesoscale GUMBOS

We report on the synthesis and characterization of a PEGylated IR786 GUMBOS (Group of Uniform Materials Based on Organic Salts). The synthesis of this material was accomplished using a three step protocol: (1) substitution of chloride on the cyclohexenyl ring in the heptamethine chain of IR786 by 6-aminohexanoic acid, (2) grafting of methoxy polyethylene glycol (MeOPEG) onto the 6-aminohexanoic acid via an esterification reaction, and (3) anion exchange between [PEG786][I] and lithium bis(trifluoromethylsulfonyl)imide (LiNTf(2)) or sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in order to obtain PEG786 GUMBOS. Examination of spectroscopic data for this PEG786 GUMBOS indicates a large stokes shift (122 nm). It was observed that this PEG786 GUMBOS associates in aqueous solution to form nano- and mesoscale self-assemblies with sizes ranging from 100 to 220 nm. These nano- and mesoscale GUMBOS are also able to resist nonspecific binding to proteins. PEGylation of the original IR786 leads to reduced cytotoxicity. In addition, it was noted that anions, such as NTf(2) and AOT, play a significant role in improving the photostability of PEG786 GUMBOS. Irradiation-induced J-aggregation in [PEG786][NTf(2)] and to some extent in [PEG786][AOT] produced enhanced photostability. This observation was supported by use of both steady state and time-resolved fluorescence measurements.

Efficient method for site-specific 18F-labeling of biomolecules using the rapid condensation reaction between 2-cyanobenzothiazole and cysteine

An efficient method based on a rapid condensation reaction between 2-cyanobenzothiazole (CBT) and cysteine has been developed for (18)F-labeling of N-terminal cysteine-bearing peptides and proteins. An (18)F-labeled dimeric cRGD ([(18)F]CBTRGD(2)) has been synthesized with an excellent radiochemical yield (92% based on radio-HPLC conversion, 80% decay-corrected, and isolated yield) and radiochemical purity (>99%) under mild conditions using (18)F-CBT, and shown good in vivo tumor targeting efficiency for PET imaging. The labeling strategy was also applied to the site-specific (18)F-labeling of a protein, Renilla lucifierase (RLuc8) with a cysteine residue at its N-terminus. The protein labeling was achieved with 12% of decay-corrected radiochemical yield and more than 99% radiochemical purity. This strategy should provide a general approach for efficient and site-specific (18)F-labeling of various peptides and proteins for in vivo molecular imaging applications.

A molecular imaging primer: modalities, imaging agents, and applications

Molecular imaging is revolutionizing the way we study the inner workings of the human body, diagnose diseases, approach drug design, and assess therapies. The field as a whole is making possible the visualization of complex biochemical processes involved in normal physiology and disease states, in real time, in living cells, tissues, and intact subjects. In this review, we focus specifically on molecular imaging of intact living subjects. We provide a basic primer for those who are new to molecular imaging, and a resource for those involved in the field. We begin by describing classical molecular imaging techniques together with their key strengths and limitations, after which we introduce some of the latest emerging imaging modalities. We provide an overview of the main classes of molecular imaging agents (i.e., small molecules, peptides, aptamers, engineered proteins, and nanoparticles) and cite examples of how molecular imaging is being applied in oncology, neuroscience, cardiology, gene therapy, cell tracking, and theranostics (therapy combined with diagnostics). A step-by-step guide to answering biological and/or clinical questions using the tools of molecular imaging is also provided. We conclude by discussing the grand challenges of the field, its future directions, and enormous potential for further impacting how we approach research and medicine.

Recent trends in antibody-based oncologic imaging

Antibodies, with their unmatched ability for selective binding to any target, are considered as potentially the most specific probes for imaging. Their clinical utility, however, has been limited chiefly due to their slow clearance from the circulation, longer retention in non-targeted tissues and the extensive optimization required for each antibody-tracer. The development of newer contrast agents, combined with improved conjugation strategies and novel engineered forms of antibodies (diabodies, minibodies, single chain variable fragments, and nanobodies), have triggered a new wave of antibody-based imaging approaches. Apart from their conventional use with nuclear imaging probes, antibodies and their modified forms are increasingly being employed with non-radioisotopic contrast agents (MRI and ultrasound) as well as newer imaging modalities, such as quantum dots, near infra red (NIR) probes, nanoshells and surface enhanced Raman spectroscopy (SERS). The review article discusses new developments in the usage of antibodies and their modified forms in conjunction with probes of various imaging modalities such as nuclear imaging, optical imaging, ultrasound, MRI, SERS and nanoshells in preclinical and clinical studies on the diagnosis, prognosis and therapeutic responses of cancer.

In vivo bioluminescent imaging (BLI): noninvasive visualization and interrogation of biological processes in living animals

In vivo bioluminescent imaging (BLI) is increasingly being utilized as a method for modern biological research. This process, which involves the noninvasive interrogation of living animals using light emitted from luciferase-expressing bioreporter cells, has been applied to study a wide range of biomolecular functions such as gene function, drug discovery and development, cellular trafficking, protein-protein interactions, and especially tumorigenesis, cancer treatment, and disease progression. This article will review the various bioreporter/biosensor integrations of BLI and discuss how BLI is being applied towards a new visual understanding of biological processes within the living organism.

Tuning the serum persistence of human serum albumin domain III:diabody fusion proteins

The long circulation persistence of human serum albumin (HSA) is enabled by its domain III (DIII) interaction with the neonatal Fc receptor (FcRn). A protein scaffold based on HSA DIII was designed. To modify the serum half life of the scaffold, residues H535, H510, and H464 were individually mutated to alanine. HSA DIII wild type (WT) and variants were fused to the anti-carcinoembryonic antigen (CEA) T84.66 diabody (Db), radiolabeled with (124)I and injected into xenografted athymic mice for serial PET/CT imaging. All proteins targeted the CEA-positive tumor. The mean residence times (MRT) of the proteins, calculated by quantifying blood activity from the PET images, were: Db-DIII WT (56.7 h), H535A (25 h), H510A (20 h), H464A (17 h), compared with Db (2.9 h). Biodistribution confirmed the order of blood clearance from slow to fast: Db-DIII WT > H535A > H510A > H464A > Db with 4.0, 2.0, 1.8, 1.6 and 0.08 %ID/g of remaining blood activity at 51 h, respectively. This study demonstrates that attenuating the DIII-FcRn interaction provides a way of controlling the pharmacokinetics of the entire Db-DIII fusion protein without compromising tumor targeting. H464 appears to be most crucial for FcRn binding (greatest reduction in MRT), followed by H510 and H535. By mutating the DIII scaffold, we can dial serum kinetics for imaging or therapy applications.

Expression of antibody variable region-human alkaline phosphatase fusion proteins in mammalian cells

Antibody fragments and their fusion proteins are indispensable tools as immunoassay reagents in diagnostics and molecular/cellular biotechnology. However, bacterial expression of cloned antibody genes with correct tertiary structure is not always guaranteed because of the lack of proper folding machinery and/or post-translational modifications. In addition, frequently used bacterial alkaline phosphatase as a fusion partner generally shows lower specific activity than the mammalian enzyme, which hampers its wider use as a detection reagent. Here we tried to express the fusion proteins of antibody variable region(s) and secreted human placental alkaline phosphatase (SEAP) using mammalian cell culture. As a result, functional V(H)-SEAP and single-chain Fv-SEAP fusion proteins were successfully obtained from COS-1 cells, which was confirmed by ELISA and Western blotting. This system will be applicable to the rapid production of various antibody-enzyme fusions suitable for ELISA and open-sandwich ELISA that utilizes antigen-dependent V(H)/V(L) interaction for antigen quantitation.

Antibody vectors for imaging

Noninvasive molecular imaging approaches include nuclear, optical, magnetic resonance imaging, computed tomography, ultrasound, and photoacoustic imaging, which require accumulation of a signal delivered by a probe at the target site. Monoclonal antibodies are high affinity molecules that can be used for specific, high signal delivery to cell surface molecules. However, their long circulation time in blood makes them unsuitable as imaging probes. Efforts to improve antibodies pharmacokinetics without compromising affinity and specificity have been made through protein engineering. Antibody variants that differ in antigen binding sites and size have been generated and evaluated as imaging probes to target tissues of interest. Fast clearing fragments, such as single-chain variable fragment (scFv; 25 kDa), with 1 antigen-binding site (monovalent) demonstrated low accumulation in tumors because of the low exposure time to the target. Using scFv as building block to produce larger, bivalent fragments, such as scFv dimers (diabodies, 50 kDa) and scFv-fusion proteins (80 kDa minibodies and 105 kDa scFv-Fc), resulted in higher tumor accumulation because of their longer residence time in blood. Imaging studies with these fragments after radiolabeling have demonstrated excellent, high-contrast images in gamma cameras and positron emission tomography scanners. Several studies have also investigated antibody fragments conjugated to fluorescence (near infrared dyes), bioluminescence (luciferases), and quantum dots for optical imaging and iron oxides nanoparticles for magnetic resonance imaging. However, these studies indicate that there are several factors that influence successful targeting and imaging. These include stability of the antibody fragment, the labeling chemistry (direct or indirect), whether critical residues are modified, the number of antigen expressed on the cell, and whether the target has a rapid recycling rate or internalizes upon binding. The preclinical data presented are compelling, and it is evident that antibody-based molecular imaging tracers will play an important future role in the diagnosis and management of cancer and other diseases.

A novel molecule integrating therapeutic and diagnostic activities reveals multiple aspects of stem cell-based therapy

Stem cells are promising therapeutic delivery vehicles; however pre-clinical and clinical applications of stem cell-based therapy would benefit significantly from the ability to simultaneously determine therapeutic efficacy and pharmacokinetics of therapies delivered by engineered stem cells. In this study, we engineered and screened numerous fusion variants that contained therapeutic (TRAIL) and diagnostic (luciferase) domains designed to allow simultaneous investigation of multiple events in stem cell-based therapy in vivo. When various stem cell lines were engineered with the optimized molecule, SRL(O)L(2)TR, diagnostic imaging showed marked differences in the levels and duration of secretion between stem cell lines, while the therapeutic activity of the molecule showed the different secretion levels translated to significant variability in tumor cell killing. In vivo, simultaneous diagnostic and therapeutic monitoring revealed that stem cell-based delivery significantly improved pharmacokinetics and anti-tumor effectiveness of the therapy compared to intravenous or intratumoral delivery. As treatment for highly malignant brain tumor xenografts, tracking SRL(O)L(2)TR showed stable stem cell-mediated delivery significantly regressed peripheral and intracranial tumors. Together, the integrated diagnostic and therapeutic properties of SRL(O)L(2)TR answer critical questions necessary for successful utilization of stem cells as novel therapeutic vehicles.

Cell-free production of Gaussia princeps luciferase--antibody fragment bioconjugates for ex vivo detection of tumor cells

Antibody fragments (scFvs) fused to luciferase reporter proteins have been used as highly sensitive optical imaging probes. Gaussia princeps luciferase (GLuc) is an attractive choice for a reporter protein because it is small and bright and does not require ATP to stimulate bioluminescence-producing reactions. Both GLuc and scFv proteins contain multiple disulfide bonds, and consequently the production of active and properly folded GLuc-scFv fusions is challenging. We therefore produced both proteins individually in active form, followed by covalent coupling to produce the intended conjugate. We used an Escherichia coli-based cell-free protein synthesis (CFPS) platform to produce GLuc and scFv proteins containing non-natural amino acids (nnAAs) for subsequent conjugation by azide-alkyne click chemistry. GLuc mutants with exposed alkyne reactive groups were produced by global replacement of methionine residues in CFPS. Antibody fragment scFvs contained a single exposed azide group using a scheme for site-specific incorporation of tyrosine analogs. Incorporation of tyrosine analogs at specific sites in proteins was performed using an engineered orthogonal tRNA-tRNA synthetase pair from an archaebacterium. The unique azide and alkyne side chains in GLuc and the antibody fragment scFv facilitated conjugation by click chemistry. GLuc-scFv conjugates were shown to differentiate between cells expressing a surface target of the scFv and cells that did not carry this marker.

In vivo far-red luminescence imaging of a biomarker based on BRET from Cypridina bioluminescence to an organic dye

We aimed to develop a far-red luminescence imaging technology for visualization of disease specific antigens on cell surfaces in a living body. First, we conjugated a far-red fluorescent indocyanine derivative to biotinylated Cypridina luciferase. This conjugate produced a bimodal spectrum that has long-wavelength bioluminescence emission in the far-red region as a result of bioluminescence resonance energy transfer. To generate a far-red luminescent probe with targeting and imaging capabilities of tumors, we then linked this conjugate to an anti-human Dlk-1 monoclonal antibody via the biotin-avidin interaction. This far-red luminescent probe enabled us to obtain high-resolution microscopic images of live, Dlk-1-expressing Huh-7 cells without an external light source, and to monitor the accumulation of this probe in tumor-bearing mice. Thus this far-red luminescent probe is a convenient analytical tool for the evaluations of monoclonal antibody localization in a living body.

Method of bioluminescence imaging for molecular imaging of physiological and pathological processes

Molecular imaging has emerged as a powerful tool in basic, pre-clinical and clinical research for monitoring a variety of molecular and cellular processes in living organisms. Optical imaging techniques, mainly bioluminescence imaging, have extensively been used to study biological processes because of their exquisite sensitivity and high signal-to noise ratio. However, current applications have mainly been limited to small animals due to attenuation and scattering of light by tissues but efforts are ongoing to overcome these hurdles. Here, we focus on bioluminescence imaging by giving a brief overview of recent advances in instrumentation, current available reporter gene-reporter probe systems and applications such as cell trafficking, protein-protein interactions and imaging endogenous processes.

In Vivo Cellular Imaging for Translational Medical Research

Personalized treatment using stem, modified or genetically engineered, cells is becoming a reality in the field of medicine, in which allogenic or autologous cells can be used for treatment and possibly for early diagnosis of diseases. Hematopoietic, stromal and organ specific stem cells are under evaluation for cell-based therapies for cardiac, neurological, autoimmune and other disorders. Cytotoxic or genetically altered T-cells are under clinical trial for the treatment of hematopoietic or other malignant diseases. Before using stem cells in clinical trials, translational research in experimental animal models are essential, with a critical emphasis on developing noninvasive methods for tracking the temporal and spatial homing of these cells to target tissues. Moreover, it is necessary to determine the transplanted cell's engraftment efficiency and functional capability. Various in vivo imaging modalities are in use to track the movement and incorporation of administered cells. Tagging cells with reporter genes, fluorescent dyes or different contrast agents transforms them into cellular probes or imaging agents. Recent reports have shown that magnetically labeled cells can be used as cellular magnetic resonance imaging (MRI) probes, demonstrating the cell trafficking to target tissues. In this review, we will discuss the methods to transform cells into probes for in vivo imaging, along with their advantages and disadvantages as well as the future clinical applicability of cellular imaging method and corresponding imaging modality.

Site-specific, thiol-mediated conjugation of fluorescent probes to cysteine-modified diabodies targeting CD20 or HER2

Small, engineered antibody fragments such as diabodies (50 kDa noncovalent dimers of single-chain Fv fragments) are useful alternatives to their larger antibody counterparts. However, due to their size, they are more susceptible to disruption of their antigen binding sites when modified using random conjugation techniques. Previous work has demonstrated the utility of a C-terminal cysteine modification for site-specific radiolabeling of an anti-CEA diabody, resulting in the creation of a cys-diabody (CysDb). In the present work, the adaptability of the CysDb system was explored by creating two additional CysDbs: one specific for CD20 and one for HER2. Purified CysDbs of both specificities demonstrated behavior consistent with stable, covalent dimers harboring a readily reducible disulfide bond. Each CysDb was site-specifically conjugated to three different fluorophores for optical detection: the large fluorescent proteins phycoerythrin (PE) and allophycocyanin (APC), and the small fluorescent molecule Alexa Fluor488. Fluorophore-conjugated CysDbs bound specifically to their targets in both antigen systems and with each different fluorescent tag as determined by flow cytometry. In vitro specific antigen binding was observed in the presence of a mixture of specific and nonspecifically conjugated CysDbs. Conjugates retained both specificity and fluorescence, demonstrating the successful expansion of the CysDb repertoire to new targets and to new site-specific conjugation possibilities.

Radionuclide-Based Cancer Imaging Targeting the Carcinoembryonic Antigen

Carcinoembryonic antigen (CEA), highly expressed in many cancer types, is an important target for cancer diagnosis and therapy. Radionuclide-based imaging techniques (gamma camera, single photon emission computed tomography [SPECT] and positron emission tomography [PET]) have been extensively explored for CEA-targeted cancer imaging both preclinically and clinically. Briefly, these studies can be divided into three major categories: antibody-based, antibody fragment-based and pretargeted imaging. Radiolabeled anti-CEA antibodies, reported the earliest among the three categories, typically gave suboptimal tumor contrast due to the prolonged circulation life time of intact antibodies. Subsequently, a number of engineered anti-CEA antibody fragments (e.g. Fab’, scFv, minibody, diabody and scFv-Fc) have been labeled with a variety of radioisotopes for CEA imaging, many of which have entered clinical investigation. CEA-Scan (a ^99mTc-labeled anti-CEA Fab’ fragment) has already been approved by the United States Food and Drug Administration for cancer imaging. Meanwhile, pretargeting strategies have also been developed for CEA imaging which can give much better tumor contrast than the other two methods, if the system is designed properly. In this review article, we will summarize the current state-of-the-art of radionuclide-based cancer imaging targeting CEA. Generally, isotopes with short half-lives (e.g. ¹⁸F and ^99mTc) are more suitable for labeling small engineered antibody fragments while the isotopes with longer half-lives (e.g. ¹²³I and ¹¹¹In) are needed for antibody labeling to match its relatively long circulation half-life. With further improvement in tumor targeting efficacy and radiolabeling strategies, novel CEA-targeted agents may play an important role in cancer patient management, paving the way to “personalized medicine”.

Noninvasive imaging of cell-mediated therapy for treatment of cancer

Cell-mediated therapy (immunotherapy) for the treatment of cancer is an active area of investigation in animal models and clinical trials. Despite many advances, objective responses to immunotherapy are observed in a small number of cases, for certain tumor types. To better understand differences in outcomes, it is critical to develop assays for tracking effector cell localization and function in situ. The fairly recent use of molecular imaging techniques to track cell populations has presented researchers and clinicians with a powerful diagnostic tool for determining the efficacy of cell-mediated therapy for the treatment of cancer. This review highlights the application of whole-body noninvasive radioisotopic, magnetic, and optical imaging methods for monitoring effector cells in vivo. Issues that affect sensitivity of detection, such as methods of cell marking, efficiency of cell labeling, toxicity, and limits of detection of imaging modalities, are discussed.

Noninvasive imaging of molecular events with bioluminescent reporter genes in living subjects

Bioluminescence imaging has become a very popular tool for noninvasive monitoring of fundamental biological and molecular processes in small living subjects. Luciferases are light-emitting enzymes that can generate light (known as bioluminescence) after reacting with specific substrates. The emitted light is used as a detection system for luciferase activity, which acts as a "reporter" for the activity of any regulatory elements that control its expression. These enzymes are isolated from various organisms, conveniently modified for expression in mammalian cells, and are extensively used in molecular biology and cell culture experiments. Recent advances in optical technology have opened a new dimension for in vivo application of luciferase enzymes in biomedical research. The most commonly utilized luciferases for in vivo bioluminescence are isolated from two very different sources: firefly luciferase (or beetle luciferase) and renilla luciferase (isolated from sea pansy). Although both these luciferases can produce light following interaction with the substrates, structurally and biochemically they are very different. Here we describe the methods and applications of firefly and renilla luciferases in molecular imaging using small animals.

Fusion of Gaussia luciferase to an engineered anti-carcinoembryonic antigen (CEA) antibody for in vivo optical imaging

The bioluminescent protein Gaussia luciferase (GLuc) was fused to an anti-carcinoembryonic antigen (CEA) antibody fragment, the diabody, for in vivo optical tumor imaging. A 15-amino acid N-terminal truncation (GLDelta15) resulted in a brighter protein. Fusions of the anti-CEA diabody to full-length GLuc and GLDelta15 retained high affinity for the antigen, emitted light, and exhibited excellent enzymatic stability. In vivo optical imaging of tumor-bearing mice demonstrated specific targeting of diabody-GLDelta15 to CEA-positive xenografts, with a tumor/background ratio of 3.8 +/- 0.4 at four hours after tail-vein injection, compared to antigen-negative tumors at 1.3 +/- 0.1 (p = 0.001). MicroPET imaging using (124)I-diabody-GLDelta15 demonstrated specific uptake in the CEA-positive tumor (2.6% ID [injected dose]/g) compared to the CEA-negative tumor (0.4% ID/g) at 21 hours. Although further optimization of this fusion protein may be needed to improve in vivo performance, the diabody-GLDelta15 is a promising optical imaging probe for tumor detection in vivo.

Crystal structures of the luciferase and green fluorescent protein from Renilla reniformis

Due to its ability to emit light, the luciferase from Renilla reniformis (RLuc) is widely employed in molecular biology as a reporter gene in cell culture experiments and small animal imaging. To accomplish this bioluminescence, the 37-kDa enzyme catalyzes the degradation of its substrate coelenterazine in the presence of molecular oxygen, resulting in the product coelenteramide, carbon dioxide, and the desired photon of light. We successfully crystallized a stabilized variant of this important protein (RLuc8) and herein present the first structures for any coelenterazine-using luciferase. These structures are based on high-resolution data measured to 1.4 A and demonstrate a classic alpha/beta-hydrolase fold. We also present data of a coelenteramide-bound luciferase and reason that this structure represents a secondary conformational form following shift of the product out of the primary active site. During the course of this work, the structure of the luciferase's accessory green fluorescent protein (RrGFP) was also determined and shown to be highly similar to that of Aequorea victoria GFP.

Red-shifted Renilla reniformis luciferase variants for imaging in living subjects

The use of R. reniformis luciferase (RLuc) as a reporter gene in small-animal imaging has been hampered by its 481 nm peaked emission spectrum, as blue wavelengths are strongly attenuated in biological tissues. To overcome this, we generated variants of RLuc with bathochromic (red) shifts of up to 66 nm (547 nm peak) that also had greater stability and higher light emission than native RLuc.

Engineering antibodies for clinical applications

Molecular engineering has contributed immensely to the clinical success of antibodies in recent years. The modular structure of antibodies has permitted their modification in numerous ways, to meet various clinical requirements. With the help of antibody engineering, it has been possible to modify the molecular size, pharmacokinetics, immunogenicity, binding affinity, specificity and effector function of antibodies. In addition, fusion proteins of antibodies with various proteins and peptides have yielded targeted biological modifiers, toxins and imaging agents. This review focuses on the recent trends in antibody engineering for improving their clinical utility.