Fluorophore:dendrimer ratio impacts cellular uptake and intracellular fluorescence lifetime

Diverse Approaches for the Difunctionalization of PPH Dendrimers, Precise Versus Stochastic: How Does this Influence Catalytic Performance?

Random difunctionalization of dendrimer surfaces, frequently employed in biological applications, provides the advantage of dual functional groups through a synthetic pathway that is simpler compared to precise difunctionalization. However, is the random difunctionalization as efficient as the precise difunctionalization on the surface of dendrimers? This question is unanswered to date because most dendrimer families face challenges in achieving precise functionalization. Polyphosphorhydrazone (PPH) dendrimers present a unique opportunity to obtain precise difunctionalization at each terminal branching point. The work concerning catalysis we report with PPH dendrimers, whether precisely or randomly functionalized, addresses this question. Across PPH dendrimers, from generations 1 to 3, precise functionalization consistently outperforms random functionalization in terms of efficiency. This finding introduces a novel concept in dendrimer science, emphasizing the superiority of precise over random functionalization methodologies. Introducing a groundbreaking concept in the field of dendrimers.

Surface chemistry mediates the tumor entrance of nanoparticles probed using single-molecule dual-imaging nanodots

The active transport of nanoparticles into solid tumors through transcytosis has been recognized as a promising way to enhance tumor accumulation and penetration, but the effect of the physicochemical properties of nanoparticles remains unclear. Herein, we develop a type of single-molecule dual imaging nanodot by divergent growth of perylenediimide (PDI)-dye-cored polylysine dendrimers and internal orthogonal conjugation of Gd(III)-based macrocyclic probes for fluorescence imaging and magnetic resonance imaging (MRI) of surface chemistry-dependent tumor entrance. The MRI and fluorescence imaging show that sixth-generation nanodots with acetylated (G6-Ac) and oligo ethylene glycol (G6-OEG) surfaces exhibit similar high tumor accumulation but different intratumor distribution. Cellular uptake and transport experiments suggest that G6-Ac nanodots have lower lysosomal entrapment (61% vs. 83%) and a higher exocytotic rate (47% vs. 29%) than G6-OEG. Therefore, G6-Ac is more likely to undergo intercellular transport through cell transcytosis, and is able to reach a tumor area distant from blood vessels, while G6-OEG mainly enters the tumor through enhanced permeability and retention (EPR) effect-based passive transport, and is not able to deliver to distant tumor areas. This study suggests that it is possible to boost the tumor entrance of nanoparticles by engineering surface chemistry for active transport.

Divergent Synthesis of Ultrabright and Dendritic Xanthenes for Enhanced Click-Chemistry-Based Bioimaging

Biorthogonal labelling with fluorescent small molecules is an indispensable tool for diagnostic and biomedical applications. In dye-based 5-ethynyl-2'-deoxyuridine (EdU) cell proliferation assays, augmentation of the fluorescent signal entails an overall enhancement in the sensitivity and quality of the method. To this end, a rapid, divergent synthetic procedure that provides ready-to-click pH-insensitive rhodamine dyes exhibiting outstanding brightness was established. Compared to the shortest available synthesis of related high quantum-yielding rhodamines, two fewer synthetic steps are required. In a head-to-head imaging comparison involving copper(I)-catalyzed azide alkyne cycloaddition reactions with in vitro administered EdU, our new 3,3-difluoroazetidine rhodamine azide outperformed the popular 5-TAMRA-azide, making it among the best available choices when it comes to fluorescent imaging of DNA. In a further exploration of the fluorescence properties of these dyes, a set of bis-MPA dendrons carrying multiple fluorescein or rhodamine units was prepared by branching click chemistry. Fluorescence self-quenching of fluorescein- and rhodamine-functionalized dendrons limited the suitability of the dyes as labels in EdU-based experiments but provided new insights into these effects.

Cyanine-5-Driven Behaviours of Hyperbranched Polymers Designed for Therapeutic Delivery Are Cell-Type Specific and Correlated with Polar Lipid Distribution in Membranes

The ability to predict the behaviour of polymeric nanomedicines can often be obfuscated by subtle modifications to the corona structure, such as incorporation of fluorophores or other entities. However, these interactions provide an intriguing insight into how selection of molecular components in multifunctional nanomedicines contributes to the overall biological fate of such materials. Here, we detail the internalisation behaviours of polymeric nanomedicines across a suite of cell types and extrapolate data for distinguishing the underlying mechanics of cyanine-5-driven interactions as they pertain to uptake and endosomal escape. By correlating the variance of rate kinetics with endosomal escape efficiency and endogenous lipid polarity, we identify that observed cell-type dependencies correspond with an underlying susceptibility to dye-mediated effects and nanomedicine accumulation within polar vesicles. Further, our results infer that the ability to translocate endosomal membranes may be improved in certain cell types, suggesting a potential role for diagnostic moieties in trafficking of drug-loaded nanocarriers.

Evaluating Endosomal Escape of Caspase-3-Containing Nanomaterials Using Split GFP

The ability for biologics to access intracellular targets hinges on the translocation of active, unmodified proteins. This is often achieved using nanoscale formulations, which enter cells through endocytosis. This uptake mechanism often limits the therapeutic potential of the biologics, as the propensity of the nanocarrier to escape the endosome becomes the key determinant. To appropriately evaluate and compare competing delivery systems of disparate compositions, it is therefore critical to assess endosomal escape efficiencies. Unfortunately, quantitative tools to assess endosomal escape are lacking, and standard approaches often lead to an erroneous interpretation of cytosolic localization. In this study we use a split-complementation endosomal escape (SEE) assay to evaluate levels of cytosolic caspase-3 following delivery by polymer nanogels and mesoporous silica nanoparticles. In particular, we use SEE as a means to enable the systematic investigation of the effect of polymer composition, polymer architecture (random vs block), hydrophobicity, and surface functionality. Although polymer structure had little influence on endosomal escape, nanogel functionalization with cationic and pH-sensitive peptides significantly enhanced endosomal escape levels and, further, significantly increased the amount of nanogel per endosome. This work serves as a guide for developing an optimal caspase-3 delivery system, as this caspase-3 variant can be easily substituted for a therapeutic caspase-3 cargo in any system that results in cytosolic accumulation and cargo release. In addition, these data provide a framework that can be readily applied to a wide variety of protein cargos to assess the independent contributions of both uptake and endosomal escape of a wide range of protein delivery vehicles.

Tracking exogenous intracellular casp-3 using split GFP

Cytosolic protein delivery promises diverse applications from therapeutics, to genetic modification and precision research tools. To achieve effective cellular and subcellular delivery, approaches that allow protein visualization and accurate localization with greater sensitivity are essential. Fluorescently tagging proteins allows detection, tracking and visualization in cellulo. However, undesired consequences from fluorophores or fluorescent protein tags, such as nonspecific interactions and high background or perturbation to native protein's size and structure, are frequently observed, or more troublingly, overlooked. Distinguishing cytosolically released molecules from those that are endosomally entrapped upon cellular uptake is particularly challenging and is often complicated by the inherent pH-sensitive and hydrophobic properties of the fluorophore. Monitoring localization is more complex in delivery of proteins with inherent protein-modifying activities like proteases, transacetylases, kinases, etc. Proteases are among the toughest cargos due to their inherent propensity for self-proteolysis. To implement a reliable, but functionally silent, tagging technology in a protease, we have developed a caspase-3 variant tagged with the 11th strand of GFP that retains both enzymatic activity and structural characteristics of wild-type caspase-3. Only in the presence of cytosolic GFP strands 1-10 will the tagged caspase-3 generate fluorescence to signal a non-endosomal location. This methodology facilitates easy screening of cytosolic vs. endosomally-entrapped proteins due to low probabilities for false positive results, and further, allows tracking of the resultant cargo's translocation. The development of this tagged casp-3 cytosolic reporter lays the foundation to probe caspase therapeutic properties, charge-property relationships governing successful escape, and the precise number of caspases required for apoptotic cell death.

New insights into the blue intrinsic fluorescence of oxidized PAMAM dendrimers considering their use as bionanomaterials

Like other bionanomaterials, dendrimers are usually labelled with fluorescent compounds in order to be optically detected within cells. However, this process can interfere with their biological properties, so it is crucial to find other solutions for their traceability. Here, the blue intrinsic fluorescence of amine-terminated poly(amidoamine) (PAMAM) dendrimers was enhanced using oxidative treatment with ammonium persulfate (APS). The effects of dendrimer generation (G3, G4, and G5) and pH on the spectroscopic behavior of both pristine and APS-treated PAMAM dendrimers were studied in aqueous solution. Overall, the results pointed out that there are at least two types of emitting electron-rich hetero-atomic sub-luminophores (HASLs) confined within the dendrimer scaffold that have very close maximum emission wavelengths and whose emission properties strongly depend on pH. The APS treatment significantly enhanced the fluorescence intensity by leading to the protonation of the interior of the dendrimer. However, fluorescence intensity was not only dependent on the number of HASLs in the dendrimer scaffold (i.e., on dendrimer generation), but also on the rigidification suffered by the dendrimer due to the acidic environment (at low pH values, APS-treated G4 was indeed the most emissive species). Moreover, photoluminescence studies with lyophilized samples were also conducted, which confirmed the coexistence of more than one type of HASLs emitting in the dendrimer structure. The APS treatment affected these HASLs to a different extent. Time-resolved fluorescence experiments always showed higher average lifetimes of HASLs for APS-treated dendrimers than for pristine ones, in accordance with the fluorescence intensity results. On the other hand, the fraction and lifetimes of HASLs in APS-treated dendrimers were similar in solution and the lyophilized form. This behaviour was different for the pristine dendrimers that presented increased luminescence upon aggregation. Finally, the highly emissive oxidized dendrimers were shown not only to be much less cytotoxic and hemotoxic than pristine dendrimers but also to be detectable inside cells upon excitation with UV light.

Tuning the Brightness and Photostability of Organic Dots for Multivalent Targeted Cancer Imaging and Surgery

Specific labeling of biomarkers with bright and high photostable fluorophores is vital in fluorescent imaging applications. Here, we report a general strategy to develop single-molecule dendritic nanodots with finely tunable optical properties for in vivo fluorescent imaging. The well-defined nanodots are based on the divergent growth of biodegradable polylysine dendrimers with a fluorophore as the core. By tuning the size and surface chemistry, we obtained fluorescent nanodots with excellent brightness and photostability, favorable pharmacokinetics, and multivalent tumor-targeting capability. The nanodots provided robust, stable, long-lasting, and specific fluorescence enhancement in tumor tissue with an in situ tumor-to-normal ratio (TNR) of ∼3 and lasting over 5 days and an ex vivo TNR up to ∼17, holding considerable promise for cancer imaging and image-guided surgery. This strategy significantly improves the in vivo performance of fluorophores and can be applied to other modality imaging probes.

New insights into the blue intrinsic fluorescence of oxidized PAMAM dendrimers considering their use as bionanomaterials

The fluorescence intensity of oxidized PAMAM dendrimers is not only dependent on the number of HASLs in the dendrimer scaffold (i.e., on dendrimer generation), but also on the rigidification suffered by the dendrimer due to the acidic environment.

New insights into the blue intrinsic fluorescence of oxidized PAMAM dendrimers considering their use as bionanomaterials

The fluorescence intensity of oxidized PAMAM dendrimers is not only dependent on the number of HASLs in the dendrimer scaffold (i.e., on dendrimer generation), but also on the rigidification suffered by the dendrimer due to the acidic environment.

Polymer design and component selection contribute to uptake, distribution & trafficking behaviours of polyethylene glycol hyperbranched polymers in live MDA-MB-468 breast cancer cells

As polymeric nanomedicines grow increasingly complex in design, an effective therapeutic release is often inherently tied to localisation to specific intracellular compartments or microenvironments. The inclusion of environmentally-sensitive moieties links the functionality of such materials to the trafficking behaviours exhibited once materials have obtained access to the cellular milieu. In order to perform their designed function, such materials often need to encounter specific biological cues or stimuli. As such, there is an increased need to improve our understanding of how the physicochemical properties of nanomaterials influence post-internalisation behaviours. Amongst the unknown factors that may contribute to the trafficking behaviours and distribution of polymers within the cellular environment, is the influence of the components selected in the development of such materials. To examine whether composition and arrangement of components within small polymeric nanomaterials contribute to their ability to navigate the intracellular space, here we utilise fluorophores to model component selection, varying the fluorescent handle selected and its method of incorporation. We explore the intracellular behaviours of well-characterised hyperbranched polymers in live MDA-MB-468 breast cancer cells in vitro. Changes in distribution as a function of both fluorophore selection and placement are reported, and our data suggest that the individual components used to produce potential nanomedicines are critical to their overall functioning and efficacy. Further to this, through the use of a novel non-conjugated targeting ligand, we demonstrate that there is inherent competition between component-directing factors and cellular influences on the ultimate fate of the polymers. The behaviours reported here suggest that not only does component selection contribute to intracellular processing, but these factors could potentially be harnessed when designing polymers to ensure improved functionality of future materials for therapeutic delivery.

Use of Polyamidoamine Dendrimers in Brain Diseases

Polyamidoamine (PAMAM) dendrimers are one of the smallest and most precise nanomolecules available today, which have promising applications for the treatment of brain diseases. Each aspect of the dendrimer (core, size or generation, size of cavities, and surface functional groups) can be precisely modulated to yield a variety of nanocarriers for delivery of drugs and genes to brain cells in vitro or in vivo. Two of the most important criteria to consider when using PAMAM dendrimers for neuroscience applications is their safety profile and their potential to be prepared in a reproducible manner. Based on these criteria, features of PAMAM dendrimers are described to help the neuroscience researcher to judiciously choose the right type of dendrimer and the appropriate method for loading the drug to form a safe and effective delivery system to the brain.

Distributions: The Importance of the Chemist's Molecular View for Biological Materials

Characterization of materials with biological applications and assessment of physiological effects of therapeutic interventions are critical for translating research to the clinic and preventing adverse reactions. Analytical techniques typically used to characterize targeted nanomaterials and tissues rely on bulk measurement. Therefore, the resulting data represent an average structure of the sample, masking stochastic (randomly generated) distributions that are commonly present. In this Perspective, we examine almost 20 years of work our group has done in different fields to characterize and control distributions. We discuss the analytical techniques and statistical methods we use and illustrate how we leverage them in tandem with other bulk techniques. We also discuss the challenges and time investment associated with taking such a detailed view of distributions as well as the risks of not fully appreciating the extent of heterogeneity present in many systems. Through three case studies showcasing our research on conjugated polymers for drug delivery, collagen in bone, and endogenous protein nanoparticles, we discuss how identification and characterization of distributions, i.e., a molecular view of the system, was critical for understanding the observed biological effects. In all three cases, data would have been misinterpreted and insights missed if we had only relied upon spatially averaged data. Finally, we discuss how new techniques are starting to bridge the gap between bulk and molecular level analysis, bringing more opportunity and capacity to the research community to address the challenges of distributions and their roles in biology, chemistry, and the translation of science and engineering to societal challenges.

Increase in Dye:Dendrimer Ratio Decreases Cellular Uptake of Neutral Dendrimers in RAW Cells

Neutral generation 3 poly(amidoamine) dendrimers were labeled with Oregon Green 488 (G3-OG_n) to obtain materials with controlled fluorophore:dendrimer ratios (n = 1-2), a mixture containing mostly 3 dyes per dendrimer, a mixture containing primarily 4 or more dyes per dendrimer (n = 4+), and a stochastic mixture (n = 4_avg). The UV absorbance of the dye conjugates increased linearly as n increased and the fluorescence emission decreased linearly as n increased. Cellular uptake was studied in RAW cells and HEK 293A cells as a function of the fluorophore:dendrimer ratio (n). The cellular uptake of G3-OG _n (n = 3, 4+, 4_avg) into RAW cells was significantly lower than G3-OG _n (n = 1, 2). The uptake of G3-OG _n (n = 3, 4+, 4_avg) into HEK 293A cells was not significantly different from G3-OG₁. Thus, the fluorophore:dendrimer ratio was observed to change the extent of uptake in the macrophage uptake mechanism but not in the HEK 293A cell. This difference in endocytosis indicates the presence of a pathway in the macrophage that is sensitive to hydrophobicity of the particle.

Generation 3 PAMAM dendrimer TAMRA conjugates containing precise dye/dendrimer ratios

The synthesis, isolation, and characterization of generation 3 poly(amidoamine) (G3 PAMAM) dendrimer containing precise ratios of 5-carboxytetramethylrhodamine succinimidyl ester (TAMRA) dye (n = 1-3) per polymer particle are reported. Stochastic conjugation of TAMRA dye to the dendrimer was followed by separation into precise dye-polymer ratios using rp-HPLC. The isolated materials were characterized by rp-UPLC, MALDI-TOF-MS, and 1H NMR spectroscopy, UV-vis, and fluorescence spectroscopies.

Quantitative Measurement of Cationic Polymer Vector and Polymer-pDNA Polyplex Intercalation into the Cell Plasma Membrane

Cationic gene delivery agents (vectors) are important for delivering nucleotides, but are also responsible for cytotoxicity. Cationic polymers (L-PEI, jetPEI, and G5 PAMAM) at 1× to 100× the concentrations required for translational activity (protein expression) induced the same increase in plasma membrane current of HEK 293A cells (30-50 nA) as measured by whole cell patch-clamp. This indicates saturation of the cell membrane by the cationic polymers. The increased currents induced by the polymers are not reversible for over 15 min. Irreversibility on this time scale is consistent with a polymer-supported pore or carpet model and indicates that the cell is unable to clear the polymer from the membrane. For polyplexes, although the charge concentration was the same (at N/P ratio of 10:1), G5 PAMAM and jetPEI polyplexes induced a much larger current increase (40-50 nA) than L-PEI polyplexes (<20 nA). Both free cationic lipid and lipid polyplexes induced a lower increase in current than cationic polymers (<20 nA). To quantify the membrane bound material, partition constants were measured for both free vectors and polyplexes into the HEK 293A cell membrane using a dye influx assay. The partition constants of free vectors increased with charge density of the vectors. Polyplex partition constants did not show such a trend. The long lasting cell plasma permeability induced by exposure to the polymer vectors or the polyplexes provides a plausible mechanism for the toxicity and inflammatory response induced by exposure to these materials.

Molecular probes for fluorescence lifetime imaging

Visualization of biological processes and pathologic conditions at the cellular and tissue levels largely relies on the use of fluorescence intensity signals from fluorophores or their bioconjugates. To overcome the concentration dependency of intensity measurements, evaluate subtle molecular interactions, and determine biochemical status of intracellular or extracellular microenvironments, fluorescence lifetime (FLT) imaging has emerged as a reliable imaging method complementary to intensity measurements. Driven by a wide variety of dyes exhibiting stable or environment-responsive FLTs, information multiplexing can be readily accomplished without the need for ratiometric spectral imaging. With knowledge of the fluorescent states of the molecules, it is entirely possible to predict the functional status of biomolecules or microevironment of cells. Whereas the use of FLT spectroscopy and microscopy in biological studies is now well-established, in vivo imaging of biological processes based on FLT imaging techniques is still evolving. This review summarizes recent advances in the application of the FLT of molecular probes for imaging cells and small animal models of human diseases. It also highlights some challenges that continue to limit the full realization of the potential of using FLT molecular probes to address diverse biological problems and outlines areas of potential high impact in the future.