Bioconjugation in Drug Delivery: Practical Perspectives and Future Perceptions

For the past three decades, pharmaceutical research has been mainly converging to novel carrier systems and nanoparticulate colloidal technologies for drug delivery, such as nanoparticles, nanospheres, vesicular systems, liposomes, or nanocapsules to impart novel functions and targeting abilities. Such technologies opened the gate towards more sophisticated and effective multi-acting platform(s) which can offer site-targeting, imaging, and treatment using a single multifunctional system. Unfortunately, such technologies faced major intrinsic hurdles including high cost, low stability profile, short shelf-life, and poor reproducibility across and within production batches leading to harsh bench-to-bedside transformation.Currently, pharmaceutical industry along with academic research is investing heavily in bioconjugate structures as an appealing and advantageous alternative to nanoparticulate delivery systems with all its flexible benefits when it comes to custom design and tailor grafting along with avoiding most of its shortcomings. Bioconjugation is a ubiquitous technique that finds a multitude of applications in different branches of life sciences, including drug and gene delivery applications, biological assays, imaging, and biosensing.Bioconjugation is simple, easy, and generally a one-step drug (active pharmaceutical ingredient) conjugation, using various smart biocompatible, bioreducible, or biodegradable linkers, to targeting agents, PEG layer, or another drug. In this chapter, the different types of bioconjugates, the techniques used throughout the course of their synthesis and characterization, as well as the well-established synthetic approaches used for their formulation are presented. In addition, some exemplary representatives are outlined with greater emphasis on the practical tips and tricks of the most prominent techniques such as click chemistry, carbodiimide coupling, and avidin-biotin system.

Assay of Single-Cell Apoptosis by Ensemble and Single-Molecule Fluorescence Methods: Annexin-V/Polyethylene Glycol-Functionalized Quantum Dots as Probes

Apoptosis plays a critical role in many biological processes and the etiology of various diseases of the immune system. The study of apoptosis would allow both improving the diagnosis of certain diseases and serving as a target of drug screening. In this paper, we developed a sensitive assay of single-cell apoptosis using semiconductor quantum dots (QDs) as fluorescent-labeling probes. The principle of this assay is based on the detection of phosphatidylserine (PS) exposed on the plasma membrane during the drug-induced apoptosis. The QD-labeled annexin V (AV) was prepared to specifically target PS on the membrane of apoptotic cells, and PS was detected by fluorescent imaging, flow cytometer, and single-molecule fluorescence correlation spectroscopy (FCS). We developed the procedures for conjugation of QDs to AV and for purification of their conjugates by gel chromatography. The obtained conjugates were characterized by FCS, capillary electrophoresis, and zeta potential analyzer. We studied the nonspecific adsorption of cells to different surface-modified QDs and found that the nonspecific adsorption effects were significantly reduced by modification of QDs with polyethylene glycol in the detection of apoptosis. In this assay, the results obtained by flow cytometry were consistent with the commercial test kit. Furthermore, a home-built single-molecule FCS system was developed for in situ study the drug-induced apoptosis. We observed the significant change in the diffusion coefficients of QDs on cells during the progress of cell apoptosis. Compared with conventional methods, the fluorescent methods represented here possess high sensitivity because of the use of high photo stability and brightness QDs as labeling probes and provide the temp-spatial information on a single apoptotic cell.

Upconversion nanoparticle bioconjugates characterized by capillary electrophoresis

Upconversion nanoparticles (UCNPs) are an emerging class of optical materials with high potential in bioimaging due to practically no background signal and high penetration depth. Their excellent optical properties and easy surface functionalization make them perfect for conjugation with targeting ligands. In this work, capillary electrophoretic (CE) method with laser-induced fluorescence detection was used to investigate the behavior of carboxyl-silica-coated UCNPs. Folic acid, targeting folate receptor overexpressed by wide variety of cancer cells, was used for illustrative purposes and assessed by CE under optimized conditions. Peptide-mediated bioconjugation of antibodies to UCNPs was also investigated. Despite the numerous advantages of CE, this is the first time that CE was employed for characterization of UCNPs and their bioconjugates. The separation conditions were optimized including the background electrolyte concentration and pH. The optimized electrolyte was 20 mM borate buffer with pH 8.

Fluorescence and Scattering Light Cross Correlation Spectroscopy and Its Applications in Homogeneous Immunoassay

In this work, we propose fluorescence and scattering light cross-correlation spectroscopy (FSCCS) based on laser confocal configuration using silver nanoparticle (SNPs) and Alexa Fluor 488 (Alexa) as probe pairs. FSCCS is a single molecule (particle) method, and its principle is similar to that of fluorescence cross-correlation spectroscopy (FCCS). We established the setup of FSCCS using single wavelength laser and developed an immunoassay model of FSCCS. The reliability and adaptability of FSCCS method were evaluated by homogeneous sandwich immunoassay mode. In the study, liver cancer biomarker alpha-fetoprotein (AFP) was used as an assay model, two different antibodies were labeled with SNPs and fluorophore Alexa Fluor 488, respectively. In the optimal conditions, the linear range of AFP covers 5 pM to 580 pM and the detection limit is 3.1 pM. This method was successfully applied for direct determination of AFP levels in human serum samples, and the obtained results were in good agreement with data obtained via ELISAs. The advantage of this method lies in its simplicity, attractive SNPs probes, high sensitivity and selectivity and high efficiency. We believe that FSCCS method exhibits promising potential applications in homogeneous bioassays and study on the molecular interaction and nanoparticle-molecule interaction.

Asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry for the quantification of quantum dots bioconjugation efficiency

Hyphenation of asymmetric flow field-flow fractionation (AF4) to an on-line elemental detection (inductively coupled plasma-mass spectrometry, ICP-MS) is proposed as a powerful diagnostic tool for quantum dots bioconjugation studies. In particular, conjugation effectiveness between a "model" monoclonal IgG antibody (Ab) and CdSe/ZnS core-shell Quantum Dots (QDs), surface-coated with an amphiphilic polymer, has been monitored here by such hybrid AF4-ICP-MS technique. Experimental conditions have been optimized searching for a proper separation between the sought bioconjugates from the eventual free reagents excesses employed during the bioconjugation (QDs and antibodies). Composition and pH of the carrier have been found to be critical parameters to ensure an efficient separation while ensuring high species recovery from the AF4 channel. An ICP-MS equipped with a triple quadropole was selected as elemental detector to enable sensitive and reliable simultaneous quantification of the elemental constituents, including sulfur, of the nanoparticulated species and the antibody. The hyphenated technique used provided nanoparticle size-based separation, elemental detection, and composition analysis capabilities that turned out to be instrumental in order to investigate in depth the Ab-QDs bioconjugation process. Moreover, the analytical strategy here proposed allowed us not only to clearly identify the bioconjugation reaction products but also to quantify nanoparticle:antibodies bioconjugation efficiency. This is a key issue in future development of analytical and bioanalytical photoluminescent QDs applications.