Grafting protein ligand monolayers onto the surface of microparticles for probing the accessibility of cell surface receptors

Designing the nanoparticle-biomolecule interface for "targeting and therapeutic delivery"

The endogenous transport mechanisms which occur in living organisms have evolved to allow selective transport and processing operate on a scale of tens of nanometers. This presents the possibility of unprecedented access for engineered nanoscale materials to organs and sub-cellular locations, materials which may in principle be targeted to precise locations for diagnostic or therapeutic gain. For this reason, nano-architectures could represent a truly radical departure as delivery agents for drugs, genes and therapies to treat a host of diseases. Thus, for active targeting, unlike the case of small molecular drugs where molecular structure has evolved to promote higher physiochemical affinity to specific sites, one aims to exploit these energy dependant endogenous processes. Many active targeting strategies have been developed, but despite this truly remarkable potential, in applications they have met with mixed success to date. This situation may have more to do with our current understanding and integration of knowledge across disciplines, than any intrinsic limitation on the vision itself. In this review article we suggest that much more fundamental and detailed control of the nanoparticle-biomolecule interface is required for sustained and general success in this field. In the simplest manifestation, pristine nanoparticles in biological fluids act as a scaffold for biomolecules, which adsorb rapidly to the nanoparticles' surface, conferring a new biological identity to the nanoparticles. It is this nanoparticle-biomolecule interface that is 'read' and acted upon by the cellular machinery. Moreover, where targeting moieties are grafted onto nanoparticles, they may not retain their function as a result of poor orientation, and structural or conformational disruption. Further surface adsorption of biomolecules from the surrounding environment i.e. the formation of a biomolecule corona may also obscure specific surface recognition. To transfer the remarkable possibilities of nanoscale interactions in biology into therapeutics one may need a more focused and dedicated approach to the understanding of the in situ (in vivo) interface between engineered nanomaedicines and their targets.

Supersensitive Time-resolved Immunofluorometric Assay of Free Prostate-specific Antigen with Nanoparticle Label Technology

Background: The extreme specific activity of the long-lifetime fluorescent europium(III) chelate nanoparticles and the enhanced monovalent binding affinity of multivalent nanoparticle-antibody bioconjugates are attractive for noncompetitive immunoassay.

Methods: We used a noncompetitive, two-step immunoassay design to measure free prostate-specific antigen (PSA). Europium(III) chelate nanoparticles (107 nm in diameter) were coated with a monoclonal anti-PSA antibody (intrinsic affinity, 6 × 109 L/mol). The nanoparticle-antibody bioconjugates had an average of 214 active binding sites per particle and a monovalent binding affinity of 7 × 1010 L/mol. The assay was performed in a low-fluorescence microtitration well passively coated with an another monoclonal anti-PSA antibody (affinity, 2 × 1010 L/mol), and the europium(III) fluorescence was measured directly from the bottom of the well by a standard time-resolved microtitration plate fluorometer.

Results: The detection limit (mean + 2 SD) was 0.040 ng/L (7.3 × 105 molecules/mL), and the dynamic detection range covered four orders of magnitude in a 3-h total assay time. The imprecision (CV) over the whole assay range was 2–10%. The detection limit of the assay was limited by the fractional nonspecific binding of the bioconjugate to the solid phase (0.05%), which was higher than the nonspecific binding of the original antibody (<0.01%).

Conclusions: The sensitivity of the new assay is equal to that of the ambient-analyte, microspot immunoassay and will be improved by use of optimized, high binding-site density nanoparticle-antibody bioconjugates with reduced nonspecific binding and improved monovalent binding affinity.