Nanoparticles: take only pictures, leave only footprints

Multivalent Probes in Molecular Imaging: Reality or Future?

The rapidly developing field of molecular medical imaging focuses on specific visualization of (patho)physiological processes through the application of imaging agents (IAs) in multiple clinical modalities. Although our understanding of the principles underlying efficient IAs design has increased tremendously, many IAs still show poor in vivo imaging performance because of low binding affinity and/or specificity. These limitations can be addressed by taking advantage of multivalency, in which multiple copies of a ligand are employed to strengthen the interaction. We critically address specific challenges associated with the application of multivalent compounds in molecular imaging, and we give directions for a stepwise approach to the design of multivalent imaging probes to improve their target binding and pharmacokinetics (PK) for improved diagnostic potential.

Colloidal capsules: nano- and microcapsules with colloidal particle shells

This review provides a comprehensive overview of the synthesis strategies and the progress made so far of bringing colloidal capsules closer to technical and biomedical applications.

Gram-scale synthesis of coordination polymer nanodots with renal clearance properties for cancer theranostic applications

An ultrasmall hydrodynamic diameter is a critical factor for the renal clearance of nanoparticles from the body within a reasonable timescale. However, the integration of diagnostic and therapeutic components into a single ultrasmall nanoparticle remains challenging. In this study, pH-activated nanodots (termed Fe-CPNDs) composed of coordination polymers were synthesized via a simple and scalable method based on coordination reactions among Fe³⁺, gallic acid and poly(vinylpyrrolidone) at ambient conditions. The Fe-CPNDs exhibited ultrasmall (5.3 nm) hydrodynamic diameters and electrically neutral surfaces. The Fe-CPNDs also exhibited pH-activatable magnetic resonance imaging contrast and outstanding photothermal performance. The features of Fe-CPNDs greatly increased the tumour-imaging sensitivity and facilitated renal clearance after injection in animal models in vivo. Magnetic resonance imaging-guided photothermal therapy using Fe-CPNDs completely suppressed tumour growth. These findings demonstrate that Fe-CPNDs constitute a new class of renal clearable nanomedicine for photothermal therapy and molecular imaging.

Coordination polymers are promising drug delivery nanomaterials as their structural properties can be easily controlled. Here, Wang et al. prepare coordination nanodots that integrate cancer imaging and therapeutic functions with beneficial renal clearance.

Positron emission tomography imaging using radiolabeled inorganic nanomaterials

CONSPECTUS: Positron emission tomography (PET) is a radionuclide imaging technology that plays an important role in preclinical and clinical research. With administration of a small amount of radiotracer, PET imaging can provide a noninvasive, highly sensitive, and quantitative readout of its organ/tissue targeting efficiency and pharmacokinetics. Various radiotracers have been designed to target specific molecular events. Compared with antibodies, proteins, peptides, and other biologically relevant molecules, nanoparticles represent a new frontier in molecular imaging probe design, enabling the attachment of different imaging modalities, targeting ligands, and therapeutic payloads in a single vector. We introduce the radiolabeled nanoparticle platforms that we and others have developed. Due to the fundamental differences in the various nanoparticles and radioisotopes, most radiolabeling methods are designed case-by-case. We focus on some general rules about selecting appropriate isotopes for given types of nanoparticles, as well as adjusting the labeling strategies according to specific applications. We classified these radiolabeling methods into four categories: (1) complexation reaction of radiometal ions with chelators via coordination chemistry; (2) direct bombardment of nanoparticles via hadronic projectiles; (3) synthesis of nanoparticles using a mixture of radioactive and nonradioactive precursors; (4) chelator-free postsynthetic radiolabeling. Method 1 is generally applicable to different nanomaterials as long as the surface chemistry is well-designed. However, the addition of chelators brings concerns of possible changes to the physicochemical properties of nanomaterials and detachment of the radiometal. Methods 2 and 3 have improved radiochemical stability. The applications are, however, limited by the possible damage to the nanocomponent caused by the proton beams (method 2) and harsh synthetic conditions (method 3). Method 4 is still in its infancy. Although being fast and specific, only a few combinations of isotopes and nanoparticles have been explored. Since the applications of radiolabeled nanoparticles are based on the premise that the radioisotopes are stably attached to the nanomaterials, stability (colloidal and radiochemical) assessment of radiolabeled nanoparticles is also highlighted. Despite the fact that thousands of nanomaterials have been developed for clinical research, only very few have moved to humans. One major reason is the lack of understanding of the biological behavior of nanomaterials. We discuss specific examples of using PET imaging to monitor the in vivo fate of radiolabeled nanoparticles, emphasizing the importance of labeling strategies and caution in interpreting PET data. Design considerations for radiolabeled nanoplatforms for multimodal molecular imaging are also illustrated, with a focus on strategies to combine the strengths of different imaging modalities and to prolong the circulation time.