Avidin-biotin technology in targeted therapy

Site-specific derivatization of avidin using microbial transglutaminase

Avidin conjugates have several important applications in biotechnology and medicine. In this work, we investigated the possibility to produce site-specific derivatives of avidin using microbial transglutaminase (TGase). TGase allows the modification of proteins at the level of Gln or Lys residues using as substrate an alkyl-amine or a Gln-mimicking moiety, respectively. The reaction is site-specific, since Gln and Lys derivatization occurs preferentially at residues embedded in flexible regions of protein substrates. An analysis of the X-ray structure of avidin allowed us to predict Gln126 and Lys127 as potential sites of TGase's attack, because these residues are located in the flexible/unfolded C-terminal region of the protein. Surprisingly, incubation of avidin with TGase in the presence of alkylamine containing substrates (dansylcadaverine, 5-hydroxytryptamine) revealed a very low level of derivatization of the Gln126 residue. Analysis of the TGase reaction on synthetic peptide analogues of the C-terminal portion of avidin indicated that the lack of reactivity of Gln126 was likely due to the fact that this residue is proximal to negatively charged carboxylate groups, thus hampering the interaction of the substrate at the negatively charged active site of TGase. On the other hand, incubation of avidin with TGase in the presence of carbobenzoxy-l-glutaminyl-glycine in order to derivatize Lys residue(s) resulted in a clean and high yield production of an avidin derivative, retaining the biotin binding properties and the quaternary structure of the native protein. Proteolytic digestion of the modified protein, followed by mass spectrometry, allowed us to identify Lys127 as the major site of reaction, together with a minor modification of Lys58. By using TGase, avidin was also conjugated via a Lys-Gln isopeptide bond to a protein containing a single reactive Gln residue, namely, Gln126 of granulocyte-macrophage colony-stimulating factor. TGase can thus be exploited for the site-specific derivatization of avidin with small molecules or proteins.

In vivo fate of avidin-nucleic acid nanoassemblies as multifunctional diagnostic tools

This study describes the formulation optimization and body-cell distribution and clearance in mice of a dually fluorescent biodegradable poly avidin nanoassembly based on the novel Avidin-Nucleic-Acid-Nano-ASsembly (ANANAS) platform as a potential advancement of classic avidin/biotin-based targeted delivery. The nanoformulation circulates freely in the bloodstream; it is slowly captured by filter organs; it is efficiently cleared within 24-48 h, and it is poorly immunogenic. The system displays more favorable properties than its parent monomeric avidin and it is a promising tool for diagnostic purposes for future translational aims, for which free circulation in the bloodstream, safety, multifunctionality and high composition definition are all necessary requirements. In addition, the assembly shows a time-dependent cell penetration capability, suggesting it may also function as a NP-dependent drug delivery tool. The ease of preparation together with the possibility to fine-tune the surface composition makes it also an ideal candidate to understand if and how nanoparticle composition affects its localization.

The potential and hurdles of targeted alpha therapy - clinical trials and beyond

This article presents a general discussion on what has been achieved so far and on the possible future developments of targeted alpha (α)-particle therapy (TAT). Clinical applications and potential benefits of TAT are addressed as well as the drawbacks, such as the limited availability of relevant radionuclides. Alpha-particles have a particular advantage in targeted therapy because of their high potency and specificity. These features are due to their densely ionizing track structure and short path length. The most important consequence, and the major difference compared with the more widely used β⁻-particle emitters, is that single targeted cancer cells can be killed by self-irradiation with α-particles. Several clinical trials on TAT have been reported, completed, or are on-going: four using ²¹³Bi, two with ²¹¹At, two with ²²⁵Ac, and one with ²¹²Pb/²¹²Bi. Important and conceptual proof-of-principle of the therapeutic advantages of α-particle therapy has come from clinical studies with ²²³Ra-dichloride therapy, showing clear benefits in castration-resistant prostate cancer.

Current status and future prospects of drug delivery systems

This is an overview of the current drug delivery systems (DDS) starting with various routes of drug administration. Various drug formulations are then described as well as devices used for drug delivery and targeted drug delivery. There has been a considerable increase in the number of new biotechnology-based therapeutics. Most of these are proteins and peptides, and their delivery presents special challenges. Cell and gene therapies are sophisticated methods of delivery of therapeutics. Nanoparticles are considered to be important in refining drug delivery; they can be pharmaceuticals as well as diagnostics. Refinements in drug delivery will facilitate the development of personalized medicine in which targeted drug delivery will play an important role. There is discussion about the ideal DDS, commercial aspects, challenges, and future prospects.

Development of streptavidin-based nanocomplex for siRNA delivery

In our previous study, we have identified a PCBP2 siRNA that exhibits antifibrotic activity in rat hepatic stellate cells (HSCs) by inhibition of αCP2, a protein responsible for stabilization of the collagen α1 (I) mRNA in alcoholic liver fibrosis. This study aims to develop a streptavidin-based nanocomplex that can efficiently deliver the PCBP2 siRNA to HSCs. Biotin-siRNA and biotin-cholesterol were mixed with streptavidin to form the streptavidin-biotin complex, which was further condensed electrostatically with positively charged protamine to form the final multicomponent siRNA nanocomplex in the size range of 150-250 nm. The siRNA nanocomplex does not induce cytotoxicity in rat HSCs as compared to commercially available transfection agents. The cellular uptake efficiency of the siRNA nanocomplex is higher in rat HSCs than other cell lines, such as Caco-2 and PC-3, indicating that receptor-mediated endocytosis mainly contributes to the cellular uptake of the siRNA nanocomplex. The siRNA nanocomplex exhibits more than 85% silencing effect on the PCBP2 mRNA in HSCs. Stability study indicates that the nanocomplex can efficiently protect siRNA from degradation in the serum. The streptavidin-based multicomponent siRNA nanocomplex provides a promising strategy to deliver the PCBP2 siRNA to HSCs. Moreover, the nanocomplex can be used as a platform for other diseases by changing the siRNA sequence and targeting ligand.

A versatile drug delivery system using streptavidin-tagged pegylated liposomes and biotinylated biomaterials

Here we have developed a versatile liposome-mediated drug delivery system (DDS) allowing a strong bridge between the streptavidin-tagged liposome (SAL) and biotin (Bi)-tagged biomaterials which has strong affinity to surface proteins expressed in restricted cell lineages. This DDS was effective and specific for many leukemia cells in vitro and in vivo. When examining 6 human leukemia cell lines using calcein-encapsulated SALs in combination with Bi-granulocyte colony-stimulating factor (G-CSF), Bi-anti-CD33 monoclonal antibody (MAb) or Bi-anti-CD7 MAb, the fluorescent positive rate of each cell line was in almost proportion to degree of G-CSF receptor, CD33 or CD7 expression, respectively. More importantly, the binding ability was shown to be well maintained in a mouse xenograft model. Furthermore the cytosine arabinoside (AraC)-encapsulated SALs could kill the corresponding cells much more effectively in combination with Bi-biomaterials than free AraC, as expected. These findings strongly indicate that our SAL/Bi-biomaterial system could allow various types of medical agents to be delivered reliably and stably to the cells targeted.

Trans-resveratrol loaded chitosan nanoparticles modified with biotin and avidin to target hepatic carcinoma

Conventional liver targeted system focuses on delivering drugs to liver, bringing toxicity on hepatic normal tissues. The purpose of this study is to construct a new system capable of specially targeting to hepatic carcinoma instead of the whole liver. Based on the fact that nanoparticles (NPs) bound with either biotin or avidin tend to accumulate in tumors and avidin-attached reagents were quickly eliminated from blood circulation and assembled in liver, trans-resveratrol loaded chitosan nanoparticles (CS-NPs), CS-NPs with the surface modified either by biotin (B-CS-NPs) or by both biotin and avidin (A-B-CS-NPs) were prepared and their physiochemical properties were investigated. The in vitro release profiles of the three NPs all conformed to bioexponential equation. Pharmacokinetic experiment indicated that A-B-CS-NPs rapidly assembled in liver after injection, with the highest liver targeting index of 2.70, while the modification of biotin attenuated the liver targeting ability of NPs. Inhibitory study on HepG2 cells declared that compared to trans-resveratrol solution and CS-NPs, both B-CS-NPs and A-B-CS-NPs significantly improved the anticancer activity. When incubated with HepG2 cells at high concentration for longer time, A-B-CS-NPs exhibited superior cytotoxicity than B-CS-NPs. This study exclaims that A-B-CS-NPs may be a potent drug delivery vector specially targeting to hepatic carcinoma.

Structural investigation of the interactions of biotinylruthenocene with avidin

The crystal structure of avidin, a protein from hen egg white, was determined in the form of a complex with biotinylruthenocene. This biotin-derived organometallic ligand is a potential anticancer agent for targeted therapy based upon avidin-biotin technology. Isothermal titration calorimetry experiments, involving avidin complexes with biotin (vitamin H or B7) derivatives, show differences in their affinity to the protein in comparison to its avidin-biotin complex, the strongest known biochemical interaction in Nature. The crystal structure of the first complex of avidin with biotinylruthenocene, determined at 2.5Å resolution (PDB: 4I60), shows unique interactions of the ruthenocene moiety with avidin. Biotin derivatives exhibit weaker affinity to avidin then biotin, which allows their wider use in biotechnology. The specific properties of biotinylruthenocene and the knowledge of its interactions with avidin may be useful in biochemical, medical, and nanotechnological applications.

Synthesis of biotin labelled cap analogue--incorporable into mRNA transcripts and promoting cap-dependent translation

Analogues of the eukaryotic messenger RNA 5' end (m(7)G cap) are useful tools for studying mRNA fate and serve as reagents for in vitro preparation of 5' capped mRNAs. We designed a biotin-labeled dinucleotide cap analogue that can be incorporated into transcripts to produce 5'-capped and biotinylated mRNAs which retain their biological functionality and may be employed for biotin-(strept)avidin technologies.

C2-streptavidin mediates the delivery of biotin-conjugated tumor suppressor protein p53 into tumor cells

We have previously generated a recombinant C2-streptavidin fusion protein for the delivery of biotin-labeled molecules of low molecular weight into the cytosol of mammalian cells. A nontoxic moiety of Clostridium botulinum C2 toxin mediates the cellular uptake, whereas the streptavidin unit serves as a binding platform for biotin-labeled cargo molecules. In the present study, we used the C2-streptavidin transporter to introduce biotin-conjugated p53 protein into various mammalian cell lines. The p53 tumor suppressor protein is inactivated in many human cancers by multiple mechanisms and therefore the restoration of its activity in tumor cells is of great therapeutic interest. Recombinant p53 was expressed in insect cells and biotin-labeled. Biotin-p53 retained its specific high-affinity DNA-binding as revealed by gel-shift analysis. Successful conjugation of biotin-p53 to the C2-streptavidin transporter was monitored by an overlay blot technique and confirmed by real-time surface plasmon resonance, providing a KD-value in the low nM range. C2-streptavidin significantly enhanced the uptake of biotin-p53 into African Green Monkey (Vero) epithelial cells as shown by flow cytometry. Using cell fractionation, the cytosolic translocation of biotin-p53 was detected in Vero cells as well as in HeLa cervix carcinoma cells. In line with this finding, confocal microscopy displayed cytoplasmic staining of biotin-p53 in HeLa and HL60 leukemia cells. Internalized biotin-p53 partially colocalized with early endosomes, as confirmed by confocal microscopy. In conclusion, our results demonstrate the successful conjugation of biotin-p53 to C2-streptavidin and its subsequent receptor-mediated endocytosis into different human tumor cell lines.

Production of [(211)At]-astatinated radiopharmaceuticals and applications in targeted α-particle therapy

(211)At is a promising radionuclide for α-particle therapy of cancers. Its physical characteristics make this radionuclide particularly interesting to consider when bound to cancer-targeting biomolecules for the treatment of microscopic tumors. (211)At is produced by cyclotron irradiation of (209)Bi with α-particles accelerated at ~28 MeV and can be obtained in high radionuclidic purity after isolation from the target. Its chemistry resembles iodine, but there is also a tendency to behave as a metalloid. However, the chemical behavior of astatine has not yet been clearly established, primarily due to the lack of any stable isotopes of this element, which precludes the use of conventional analytical techniques for its characterization. There are also only a limited number of research centers that have been able to produce this element in sufficient amounts to carry out extensive investigations. Despite these difficulties, chemical reactions typically used with iodine can be performed, and a number of biomolecules of interest have been labeled with (211)At. However, most of these compounds exhibit unacceptable instability in vivo due to the weakness of the astatine-biomolecule bond. Nonetheless, several compounds have shown high potential for the treatment of cancers in vitro and in several animal models, thus providing a promising basis that has allowed initiation of the first two clinical studies.

A one-pot three-component radiochemical reaction for rapid assembly of 125I-labeled molecular probes

Nuclear imaging in conjunction with radioactive tracers enables noninvasive measurements of biochemical events in vivo. However, access to tracers remains limited due to the lack of methods for rapid assembly of radiolabeled molecules with the prerequisite biological activity. Herein, we report a one-pot, three-component, copper(II)-mediated reaction of azides, alkynes, and [(125)I]iodide to yield 5-[(125)I]iodo-1,2,3-triazoles. Using a selection of azides and alkynes in a combinatorial approach, we have synthesized a library of structurally diverse (125)I-labeled triazoles functionalized with bioconjugation groups, fluorescent dyes, and biomolecules. Our preliminary biological evaluation suggests that 5-[(125)I]iodo-1,2,3-triazoles are resistant to deiodination in vivo, both as small molecular probes and as antibody conjugates. The ability to incorporate radioactive iodide into triazoles directly from the parent azides and alkynes makes the method broadly applicable and offers the potential to rapidly assemble molecular probes from an array of structurally diverse, and readily available, building blocks.

Targeting prostate cancer cells with PSMA inhibitor-guided gold nanoparticles

Prostate-specific membrane antigen (PSMA) is a notable biomarker for diagnostic and therapeutic applications in prostate cancer. Gold nanoparticles (AuNPs) provide an attractive nanomaterial platform for combining a variety of targeting, imaging, and cytotoxic agents into a unified device for biomedical research. In this study, we present the generation and evaluation of the first AuNP system functionalized with a small molecule phosphoramidate peptidomimetic inhibitor for the targeted delivery to PSMA-expressing prostate cancer cells. The general approach involved the conjugation of streptavidin-coated AuNPs with a biotin-linked PSMA inhibitor (CTT54) to generate PSMA-targeted AuNPs. In vitro evaluations of these targeted AuNPs were conducted to determine PSMA-mediated and time-dependent binding to PSMA-positive LNCaP cells. The PSMA-targeted AuNPs exhibited significantly higher and selective binding to LNCaP cells compared to control non-targeted AuNPs, thus demonstrating the feasibility of this approach.

Radioprotective effects produced by the condensation of plasmid DNA with avidin and biotinylated gold nanoparticles

The treatment of aqueous solutions of plasmid DNA with the protein avidin results in significant changes in physical, chemical, and biochemical properties. These effects include increased light scattering, formation of micron-sized particles containing both DNA and protein, and plasmid protection against thermal denaturation, radical attack, and nuclease digestion. All of these changes are consistent with condensation of the plasmid by avidin. Avidin can be displaced from the plasmid at higher ionic strengths. Avidin is not displaced from the plasmid by an excess of a tetra-arginine ligand, nor by the presence of biotin. Therefore, this system offers the opportunity to reversibly bind biotin-labeled species to a condensed DNA-protein complex. An example application is the use of biotinylated gold nanoparticles. This system offers the ability to examine in better detail the chemical mechanisms involved in important radiobiological effects. Examples include protein modulation of radiation damage to DNA, and radiosensitization by gold nanoparticles.

Direct biomolecule binding on nonfouling surfaces via newly discovered supramolecular self-assembly of lysozyme under physiological conditions

When lysozyme is dissolved in a neutral HEPES buffer solution (pH = 7.4) with 0.001-0.050 M TCEP added, a fast phase transition process occurs and the resulting novel fiber-like hierarchical supramolecular assemblies made by primary spherical-particle aggregation can function as a "superglue" that binds strongly and quickly onto non-fouling coatings. This binding is highly selective towards lysozyme, and excludes synthetic, chemical/physical activation/deactivation (blocking) steps. By using biotinylated lysozyme, such a phase transition quickly creates a perfect biotinylated surface on non-fouling surfaces for avidin binding, showing great potential for the development of low-cost and practical biochips.

Pre-targeting and direct immunotargeting of liposomal drug carriers to ovarian carcinoma

Background

Epidermal growth factor receptor (EGFR) is overexpressed in many solid tumor types, such as ovarian carcinoma. Immunoliposome based drug targeting has shown promising results in drug delivery to the tumors. However, the ratio of tumor-to-normal tissue concentrations should be increased to minimize the adverse effects of cytostatic drugs.

Methodology/Principal Findings

We studied the EGFR-targeted doxorubicin immunoliposomes using pre-targeting and local intraperitoneal (i.p.) administration of the liposomes. This approach was used to increase drug delivery to tumors as compared to direct intravenous (i.v.) administration of liposomes. EGFR antibodies were attached on the surface of PEG coated liposomes using biotin-neutravidin binding. Receptor mediated cellular uptake and cytotoxic efficacy of EGFR-targeted liposomes were investigated in human ovarian adenocarcinoma (SKOV-3 and SKOV3.ip1) cells. In vivo distribution of the liposomes in mice was explored using direct and pre-targeting approaches and SPECT/CT imaging. Targeted liposomes showed efficient and specific receptor-mediated binding to ovarian carcinoma cells in vitro, but the difference in cytotoxicity between targeted and non-targeted liposomes remained small. The relatively low cytotoxic efficacy is probably due to insufficient doxorubicin release from the liposomes rather than lack of target binding. Tumor uptake of targeted liposomes in vivo was comparable to that of non-targeted liposomes after both direct and pre-targeting administration. For both EGFR-targeted and non-targeted liposomes, the i.p. administration increased liposome accumulation to the tumors compared to i.v. injections.

Conclusions/Significance

Intraperitoneal administration of liposomes may be a beneficial approach to treat the tumors in the abdominal cavity. The i.p. pre-targeting method warrants further studies as a potential approach in cancer therapy.

Helical Conjugated Polyelectrolyte Aggregation Induced by Biotin-Avidin Interaction

Fluorescence correlation spectroscopy (FCS) is applied to demonstrate avidin-induced cross-linking in a system consisting of a helical anionic conjugated polyelectrolyte (P1) and a biotin-tetramethylrhodamine (TMR) conjugate (2). In a previous study, we used fluorescence spectroscopy to demonstrate that 2 binds to P1 via intercalation of the TMR chromophore into the P1 helix. Addition of avidin to the P1/2 complex induces little change in the fluorescence of the system; however, FCS reveals a remarkable increase in the diffusion time of the P1/2 complex in the presence of avidin. This change is attributed to supramolecular polymer aggregates produced by cross-link formation between the biotin unit of intercalated 2 and avidin. Atomic force microscopy imaging provides evidence supporting the existence of these aggregates. The highly sensitive FCS method is used to develop a novel sensor for the biotin-avidin interaction, with a detection limit of <100 pM for avidin.

Targeting prostate cancer cells with a multivalent PSMA inhibitor-guided streptavidin conjugate

Prostate-specific membrane antigen (PSMA), a type II membrane glycoprotein, its high expression is associated with prostate cancer progression, and has been becoming an active target for imaging or therapeutic applications for prostate cancer. On the other hand, streptavidin-biotin system has been successfully employed in pretargeting therapy towards multiple cancers. Herein, we describe the synthesis of bifunctional ligands (biotin-CTT54, biotin-PEG(4)-CTT54, and biotin-PEG(12)-CTT54) possessing two functional motifs separated by a length-varied polyethylene glycol (PEG) spacer: one (CTT54) binds tumor-marker PSMA and the other (biotin) binds streptavidin or avidin. All three compounds exhibited high potencies (IC(50) values: 1.21, 2.53, and 10nM, respectively) and irreversibility; but only biotin-PEG(12)-CTT54 demonstrated specifically labeling PSMA-positive prostate cancer cells in a two-step pretargeting procedure. Additionally, the pre-formulated complex between biotin-PEG(12)-CTT54 and Cy5-streptavidin displayed the improved inhibitory potency (IC(50)=1.86 nM) and irreversibility against PSMA and rapid uptake of streptavidin conjugate into PSMA-positive prostate cancer cells through PSMA-associated internalization. Together, all these results supported a proof-concept that combination of streptavidin and PSMA's biotinylated inhibitor may lead to development of a novel strategy of tumor-targeting imaging or drug delivery towards prostate cancer.

Structure of bradavidin-C-terminal residues act as intrinsic ligands

Bradavidin is a homotetrameric biotin-binding protein from Bradyrhizobium japonicum, a nitrogen fixing and root nodule-forming symbiotic bacterium of the soybean. Wild-type (wt) bradavidin has 138 amino acid residues, whereas the C-terminally truncated core-bradavidin has only 118 residues. We have solved the X-ray structure of wt bradavidin and found that the C-terminal amino acids of each subunit were uniquely bound to the biotin-binding pocket of an adjacent subunit. The biotin-binding pocket occupying peptide (SEKLSNTK) was named “Brad-tag” and it serves as an intrinsic stabilizing ligand in wt bradavidin. The binding of Brad-tag to core-bradavidin was analysed by isothermal titration calorimetry and a binding affinity of ∼25 µM was measured. In order to study the potential of Brad-tag, a green fluorescent protein tagged with Brad-tag was prepared and successfully concentrated from a bacterial cell lysate using core-bradavidin-functionalized Sepharose resin.

Conditional expression of Lodavin, an avidin-tagged LDL receptor, for biotin-mediated applications in vivo

Lodavin represents an engineered fusion protein that consists of a cytoplasmic and a transmembrane domain of the human low-density lipoprotein receptor coupled to an extracellular avidin monomer. Biotinylated compounds have been successfully targeted to Lodavin-expressing cells that have been transduced by a Lodavin-containing virus, and the targeting is based on the high affinity between biotin and avidin. We engineered a Rosa26 (R26R) knock-in Lodavin mouse to develop biotin-based applications such as targeted drug delivery, cell purification, and tissue imaging in vivo. A cDNA encoding Lodavin was inserted downstream of a floxed βgeo resistance gene in the R26R locus in embryonic stem cells, and a germ line-derived R26RLodavin mouse line was generated. Efficient removal of the floxed βgeo cassette and conditional activation of Lodavin expression was achieved as a result of crossing the R26RLodavin mice with HoxB7-Cre, Wnt4-Cre, or Tie1-Cre mice. In summary, the R26RLodavin mouse line may provide a useful tool for testing and developing applications with the aid of avidin and biotin interaction.

Single reporter for targeted multimodal in vivo imaging

We have developed a multifaceted, highly specific reporter for multimodal in vivo imaging and applied it for detection of brain tumors. A metabolically biotinylated, membrane-bound form of Gaussia luciferase was synthesized, termed mbGluc-biotin. We engineered glioma cells to express this reporter and showed that brain tumor formation can be temporally imaged by bioluminescence following systemic administration of coelenterazine. Brain tumors expressing this reporter had high sensitivity for detection by magnetic resonance and fluorescence tomographic imaging upon injection of streptavidin conjugated to magnetic nanoparticles or fluorophore, respectively. Moreover, single photon emission computed tomography showed enhanced imaging of these tumors upon injection with streptavidin complexed to (111)In-DTPA-biotin. This work shows for the first time a single small reporter (∼40 kDa) which can be monitored with most available molecular imaging modalities and can be extended for single cell imaging using intravital microscopy, allowing real-time tracking of any cell expressing it in vivo.

Characterization of multifunctional nanosystems based on the avidin-nucleic acid interaction as signal enhancers in immuno-detection

The Avidin-Nucleic-Acids-Nano-Assembly (ANANAS) is a kind of soft poly avidin nanoparticle originating from the high affinity interaction between avidin and the nucleic acids. In this work we investigated the possibility of transforming ANANAS cores into stoichiometrically controlled multifunctional nanoparticles through a "one-pot" procedure, and we measured in a quantitative way their ability to work as reagents for enhanced immunodiagnostic detection. Initially, we measured the ANANAS loading capability for biotinylated proteins of different nature. About 200 molecules of biotin-horseradish-peroxidase (40KDa b-HRP) and 60 molecules of biotin-immunoglobulin-G (150KDa b-IgG) could be accommodated onto each nanoparticle, showing that steric limitations dictate the number of loadable entities. Stoichiometrically controlled functional assemblies were generated by mixing core particles with subsaturating amounts of b-HRP and b-IgG. When applied as detection reagents in an Enzyme-Linked-ImmunoSorbed-Assay (ELISA), these assemblies were up to two-orders of magnitude more sensitive than commercial HRP-based reagents. Assemblies of different composition displayed different efficacy, indicating that the system functionality can be fine-tuned. Within-assay variability (CV%), measured to assess if the assembly procedure is reproducible, was within 10%. Stability experiments demonstrated that the functionalyzed assemblies are stable in solution for more than one week. In principle, any biotinylated function can be loaded onto the core particle, whose high loading capacity and tunability may open the way toward further application in biomedicine.

Macromolecule functionalization of disulfide-bonded polymer hydrogel capsules and cancer cell targeting

We present a generic and versatile method for functionalization of disulfide-stabilized PMA hydrogel capsules (HCs) with macromolecules, including a number of specific antibodies to cancer cells. Functionalization was achieved by reversible addition-fragmentation chain transfer (RAFT) polymerization of poly(N-vinyl pyrrolidone) (PVPON), which introduced biorelevant heterotelechelic end groups (thiol and amine) to the polymer chain. The PVPON with heterotelechelic end groups was conjugated to the outermost layer of PMA HCs through the thiol groups and reacted with biotin via the amine groups to generate PMA/PVPON(biotin) HCs. On the basis of the high specific interaction and high affinity between biotin and avidin, and its derivates, such as NeutrAvidin (NAv), we functionalized the PMA HCs with biotinylated antibodies. We demonstrate significantly enhanced cellular binding and internalization of the antibody (Ab)-functionalized capsules compared with control human immunoglobulin (IgG)-functionalized capsules, suggesting these capsules can specifically interact with cells through antibody/antigen recognition. We anticipate that the versatility of the functionalization approach reported in this study will assist in targeted therapeutic delivery applications.

Preclinical evaluation of a 68Ga-labeled biotin analogue for applications in islet transplantation

INTRODUCTION

Islet transplantation is a promising treatment for type 1 diabetes mellitus, but the fate of the cells after intraportal infusion is unclear. It is therefore imperative to develop novel techniques for noninvasive imaging and quantification of events following islet transplantation.

METHODS

Small islet-like microbeads, avidin-covered agarose resins (AARs), were used as a model system for islet transplantation. Capability for specific [(68)Ga]Ga-DOTA-(PEG)(2)-biotin uptake and retention for either AARs or human islets conjugated with avidin by means of a heparin scaffold was studied in vitro. Biodistribution of the novel positron emission tomography (PET) tracer [(68)Ga]Ga-DOTA-(PEG)(2)-biotin was evaluated in mice treated by intraportal transplantation of AARs by μPET/computed tomography and ex vivo organ distribution and compared with control mice.

RESULTS

AARs had high capability to bind [(68)Ga]Ga-DOTA-(PEG)(2)-biotin, close to 50% of administrated tracer/μl in vitro (>0.25 MBq/μl). Avidin-tagged human islets could bind on average 2.2% of administered tracer/μl. Specificity (>90%) and retention (>90% after 1 h) were high for both AARs and avidin-tagged islets. Hepatic tracer uptake and retention were increased in mice transplanted with AARs [standardized uptake value (SUV)=2.6] compared to the untreated group (SUV=1.4). In vivo uptake of tracer to AARs was blocked by preadministration of unlabeled biotin.

CONCLUSIONS

Avidin-tagged islet-like objects can be tracked in hepatic volume after intraportal transplantation by using [(68)Ga]Ga-DOTA-(PEG)(2)-biotin and PET.

Retargeting of viruses to generate oncolytic agents

Oncolytic virus therapy is based on the ability of viruses to effectively infect and kill tumor cells without destroying the normal tissues. While some viruses seem to have a natural preference for tumor cells, most viruses require the modification of their tropism to specifically enter and replicate in such cells. This review aims to describe the transductional targeting strategies currently employed to specifically redirect viruses towards surface receptors on tumor cells. Three major strategies can be distinguished; they involve (i) the incorporation of new targeting specificity into a viral surface protein, (ii) the incorporation of a scaffold into a viral surface protein to allow the attachment of targeting moieties, and (iii) the use of bispecific adapters to mediate targeting of a virus to a specified moiety on a tumor cell. Of each strategy key features, advantages and limitations are discussed and examples are given. Because of their potential to cause sustained, multiround infection—a desirable characteristic for eradicating tumors—particular attention is given to viruses engineered to become self-targeted by the genomic expression of a bispecific adapter protein.

Magnetically enhanced nucleic acid delivery. Ten years of magnetofection-progress and prospects

Nucleic acids carry the building plans of living systems. As such, they can be exploited to make cells produce a desired protein, or to shut down the expression of endogenous genes or even to repair defective genes. Hence, nucleic acids are unique substances for research and therapy. To exploit their potential, they need to be delivered into cells which can be a challenging task in many respects. During the last decade, nanomagnetic methods for delivering and targeting nucleic acids have been developed, methods which are often referred to as magnetofection. In this review we summarize the progress and achievements in this field of research. We discuss magnetic formulations of vectors for nucleic acid delivery and their characterization, mechanisms of magnetofection, and the application of magnetofection in viral and nonviral nucleic acid delivery in cell culture and in animal models. We summarize results that have been obtained with using magnetofection in basic research and in preclinical animal models. Finally, we describe some of our recent work and end with some conclusions and perspectives.

Synthesis of phosphine and antibody-azide probes for in vivo Staudinger ligation in a pretargeted imaging and therapy approach

The application of intact monoclonal antibodies (mAbs) as targeting agents in nuclear imaging and radioimmunotherapy is hampered by the slow pharmacokinetics of these molecules. Pretargeting with mAbs could be beneficial to reduce the radiation burden to the patient, while using the excellent targeting capacity of the mAbs. In this study, we evaluated the applicability of the Staudinger ligation as pretargeting strategy using an antibody-azide conjugate as tumor-targeting molecule in combination with a small phosphine-containing imaging/therapeutic probe. Up to 8 triazide molecules were attached to the antibody without seriously affecting its immunoreactivity, pharmacokinetics, and tumor uptake in tumor bearing nude mice. In addition, two (89)Zr- and (67/68)Ga-labeled desferrioxamine (DFO)-phosphines, a (177)Lu-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-phosphine and a (123)I-cubyl phosphine probe were synthesized and characterized for their pharmacokinetic behavior in nude mice. With respect to the phosphine probes, blood levels at 30 min after injection were <5% injected dose per gram tissue, indicating rapid blood clearance. In vitro Staudinger ligation of 3.33 μM antibody-azide conjugate with 1 equiv of radiolabeled phosphine, relative to the azide, in aqueous solution resulted in 20-25% efficiency after 2 h. The presence of 37% human serum resulted in a reduced ligation efficiency (reduction max. 30% at 2 h), while the phosphines were still >80% intact. No in vivo Staudinger ligation was observed in a mouse model after injection of 500 μg antibody-azide, followed by 68 μg DFO-phosphine at t = 2 h, and evaluation in blood at t = 7 h. To explain negative results in mice, Staudinger ligation was performed in vitro in mouse serum. Under these conditions, a side product with the phosphine was formed and ligation efficiency was severely reduced. It is concluded that in vivo application of the Staudinger ligation in a pretargeting approach in mice is not feasible, since this ligation reaction is not bioorthogonal and efficient enough. Slow reaction kinetics will also severely restrict the applicability of Staudinger ligation in humans.

The application of nanoparticles in gene therapy and magnetic resonance imaging

The combination of nanoparticles, gene therapy, and medical imaging has given rise to a new field known as gene theranostics, in which a nanobioconjugate is used to diagnose and treat the disease. The process generally involves binding between a vector carrying the genetic information and a nanoparticle, which provides the signal for imaging. The synthesis of this probe generates a synergic effect, enhancing the efficiency of gene transduction and imaging contrast. We discuss the latest approaches in the synthesis of nanoparticles for magnetic resonance imaging, gene therapy strategies, and their conjugation and in vivo application.

Layer-by-layer nanoparticles with a pH-sheddable layer for in vivo targeting of tumor hypoxia

Inspired by the simplicity and versatility of layer-by-layer (LbL) assembly, we applied multilayered polyelectrolyte assemblies on nanoparticles to create viable systemic delivery systems. Focusing on tumor-specific delivery, LbL nanoparticles that exhibit a pH-sensitive outer stealth layer are demonstrated to target and be retained in hypoxic tumor regions. The neutral layers shed in response to acidity to reveal a charged nanoparticle surface that is readily taken up by tumor cells. The first in vivo demonstration of this mechanism of targeting is presented, as well as an initial examination of the mechanism of uptake of the nanoparticles. We further demonstrate that this concept for tumor targeting is potentially valid for a broad range of cancers, with applicability for therapies that target hypoxic tumor tissue.

Synthesis of a new class of ribose functionalized dinucleotide cap analogues for biophysical studies on interaction of cap-binding proteins with the 5' end of mRNA

mRNAs of primitive eukaryotes such as Caenorhabditis elegans and Ascaris summ possess two different caps at their 5' terminus. They have either a typical cap which consists of 7-methylguanosine linked via a 5',5'-triphosphate bridge to the first transcribed nucleotide (MMG cap) or an atypical hypermethylated form with two additional methyl groups at the N2 position (TMG cap). Studies on interaction between the 5' end of mRNA and proteins that specifically recognize its structure have been carried out for several years and they often require chemically modified cap analogues. Here, we present the synthesis of five novel dinucleotide MMG and TMG cap analogues designed for binding studies using biophysical methods such as electron spin resonance (ESR) and surface plasmon resonance (SPR). New analogues were prepared by derivatization of the 2',3'-cis diol of the second nucleotide in the cap structure with levulinic acid, and coupling of the obtained acetal through its carboxylic group with 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino TEMPO), ethylenediamine (EDA) or (+)-biotinyl-3,6,9-trioxaundecanediamine (amine-PEO(3)-biotin).

Construction of chimeric dual-chain avidin by tandem fusion of the related avidins

Background

Avidin is a chicken egg-white protein with high affinity to vitamin H, also known as D-biotin. Many applications in life science research are based on this strong interaction. Avidin is a homotetrameric protein, which promotes its modification to symmetrical entities. Dual-chain avidin, a genetically engineered avidin form, has two circularly permuted chicken avidin monomers that are tandem-fused into one polypeptide chain. This form of avidin enables independent modification of the two domains, including the two biotin-binding pockets; however, decreased yields in protein production, compared to wt avidin, and complicated genetic manipulation of two highly similar DNA sequences in the tandem gene have limited the use of dual-chain avidin in biotechnological applications.

Principal Findings

To overcome challenges associated with the original dual-chain avidin, we developed chimeric dual-chain avidin, which is a tandem fusion of avidin and avidin-related protein 4 (AVR4), another member of the chicken avidin gene family. We observed an increase in protein production and better thermal stability, compared with the original dual-chain avidin. Additionally, PCR amplification of the hybrid gene was more efficient, thus enabling more convenient and straightforward modification of the dual-chain avidin. When studied closer, the generated chimeric dual-chain avidin showed biphasic biotin dissociation.

Significance

The improved dual-chain avidin introduced here increases its potential for future applications. This molecule offers a valuable base for developing bi-functional avidin tools for bioseparation, carrier proteins, and nanoscale adapters. Additionally, this strategy could be helpful when generating hetero-oligomers from other oligomeric proteins with high structural similarity.

Multianalyte on-chip native Western blotting

We introduce and characterize multiplexed native Western blotting in an automated and unified microfluidic format. While slab gel Western blotting is slow and laborious, conventional multiplexed blotting ("reblotting": probing one sample with multiple antibodies) requires even more resources. Here we detail three key advances that enable an automated and rapid microfluidic alternative to slab gel reblotting. First, we introduce both assay and microdevice designs that integrate protein blotting against multiple antibody blotting regions with native polyacrylamide gel electrophoresis. This microfluidic integration strategy overcomes nonspecific material losses inherent to harsh antibody stripping steps typically needed for conventional reblotting; said conditions can severely limit analyte quantitation. Second, to inform rational design of the multiplexed microfluidic device we develop an analytical model for analyte capture on the blotting regions. Comparison to empirical observations is reported, with capture efficiencies of >85%. Third, we introduce label free detection that makes simultaneous and quantitative multiplexed measurements possible without the need for prelabeling of sample. Assay linear dynamic range spans 8-800 nM with assay completion in 5 min. Owing to the speed, automation, enhanced quantitation capability, and the difficulty of conventional slab gel Western reblotting, microfluidic multiplexed native Western blotting should find use in systems biology, in particular in analyses of protein isoforms and multimeric protein complexes.