Despite their proven ability for precise and targeted release, nanoplatform systems for photocontrolled delivery often face formidable synthetic challenges, in part due to the paucity of advanced linker strategies. Here, we report on a novel linker strategy using a thioacetal ortho-nitrobenzaldehyde (TNB) cage, demonstrating its application for delivery of doxorubicin (Dox) in two nanoscale systems. This photocleavable linker, TNB(OH), which presents two identical arms, each terminated with a hydroxyl functionality, was prepared in a single step from 6-nitroveratraldehyde. TNB(OH) was used to cross-link Dox to a folate receptor (FAR)-targeting poly(amidoamine) dendrimer conjugate G5(FA)n=5.4(Dox)m=5.1, and also used to prepare an upconversion nanocrystal (UCN) conjugate, UCN–PPIX@(Dox)(G5FA), a larger core/shell nanostructure. In this core/shell nanostructure, the UCN core emits UV and visible light luminescence upon near-infrared (NIR) excitation, allowing for the photocleavage of the TNB linker as well as the photostimulation of protoporphyrin IX (PPIX) coupled as a cytotoxic photosensitizer. Drug-release experiments performed in aqueous solutions with long-wavelength ultraviolet A (UVA) light showed that Dox release occurred rapidly from its TNB linked form or from its dendrimer conjugated form with comparable decay kinetics. Cellular toxicity studies in FAR-overexpressing KB carcinoma cells demonstrated that each nanoconjugate lacked intrinsic cytotoxicity until exposed to UVA or NIR (980 nm) (for the UCN nanoconjugate), which resulted in induction of potent cytotoxicity. In summary, this new TNB strategy offers synthetic convenience in drug conjugation chemistry with the ability for the temporal control of drug activation at the delivery site.

We report an active delivery mechanism targeted specifically to Gram(−) bacteria based on the photochemical release of photocaged ciprofloxacin carried by a cell wall-targeted dendrimer nanoconjugate.

We report on the practicality of a heteromultivalent design strategy for a nanoplatform that targets lipopolysaccharide molecules (LPS) present on the surface of Gram-negative bacteria. This design is based on the conjugation of a poly(amido amine) (PAMAM) dendrimer with two types of ligands, each having distinct affinities: (i) polymyxin B (PMB) as a primary high affinity ligand; (ii) a PMB-mimicking dendritic branch as an auxiliary low affinity ligand. Co-conjugation of these two ligands maximizes the efficiency of the primary ligand even when the primary ligand is present at a low valency on the nanoplatform (mean nPMB ≈ 1). By performing surface plasmon resonance studies using a LPS-immobilized cell wall model, we identified an ethanolamine (EA)-terminated branch as the auxiliary ligand that promotes binding avidity via heteromultivalent association. PMB conjugation of the dendrimer with excess EA branches led to LPS avidity two orders of magnitude greater than unconjugated PMB. Such tight binding observed by SPR corresponded well with adsorption to E. coli cells and with potent bactericidal activity in vitro.

Despite extensive studies on drug delivery using multivalent complexation systems, the biophysical basis for release kinetics remains poorly defined. The present study addresses this aspect involved in the complexation of a fifth generation poly(amidoamine) (PAMAM) dendrimer with atropine, an essential antidote used for treating organophosphate poisoning. First, we designed 1H NMR titration studies for determining the molecular basis of the drug complexation with a glutarate-modified anionic dendrimer. These provide evidence pointing to a combination of electrostatic and hydrophobic interactions as the driving forces for dendrimer complexation with the alkaloid drug molecule. Second, using LC–MS/MS spectrometry, we determined the dissociation constants (KD) at steady state and also measured the drug release kinetics of atropine complexes with four negatively charged dendrimer types. Each of these dendrimers has a high payload capacity for up to ∼100 atropine molecules. However, the affinity of the atropine to the carrier was highly dependent on the drug to dendrimer ratio. Thus, a complex made at a lower loading ratio (≤0.1) displayed greater atropine affinity (KD ≈ μM) than other complexes prepared at higher ratios (>10), which showed only mM affinity. This negative cooperative variation in affinity is tightly associated with the nonlinear release kinetics observed for each complex in which drug release occurs more slowly at the later time phase at a lower loading ratio. In summary, the present study provides novel insights on the cooperativity as the mechanistic basis for nonlinear release kinetics observed in multivalent carrier systems.

Putative riboflavin receptors are considered as biomarkers due to their overexpression in breast and prostate cancers. Hence, these receptors can be potentially exploited for use in targeted drug delivery systems where dendrimer nanoparticles with multivalent ligand attachments can lead to greater specificity in cellular interactions. In this study, the single molecule force spectroscopy technique was used to assess the physical strength of multivalent interactions by employing a riboflavin (RF)-conjugated generation 5 PAMAM dendrimer G5(RF)n nanoparticle. By varying the average RF ligand valency (n = 0, 3, 5), the rupture force was measured between G5(RF)n and the riboflavin binding protein (RFBP). The rupture force increased when the valency of RF increased. We observed at the higher valency (n = 5) three binding events that increased in rupture force with increasing loading rate. Assuming a single energy barrier, the Bell–Evans model was used to determine the kinetic off-rate and barrier width for all binding interactions. The analysis of our results appears to indicate that multivalent interactions are resulting in changes to rupture force and kinetic off-rates.

Pyridinium aldoxime (PAM) molecules constitute a group of small molecule antidotes essential for the treatment of reactive organophosphate (OP) poisoning. Their therapeutic efficacy stems from a combination of two activities: (i) reactivation of acetylcholine esterase inhibited by OP; (ii) scavenging of free OP. Here, we report the design, synthesis and in vitro functional characterization of fifth generation poly(amidoamine)dendrimer conjugates, each tethered with PAM or hydroxamate, as OP scavenging macromolecules. We chose paraoxon (POX) as the model OP, and performed extensive reaction kinetic studies in aqueous media to determine the activities and mechanisms of these dendrimer conjugates in POX hydrolysis with 1H NMR, UV-vis and LCMS/MS methods. Quantitative kinetic analysis suggests that the conjugate hydrolyzed POX through a catalytic mechanism as effective as that of the unconjugated molecules. In summary, we report the first class of dendrimer conjugates which are stable in the plasma and functioning as OP-responsive catalytic scavengers.

Poly(amido amine) (PAMAM) dendrimers constitute an important class of nonviral, cationic vectors in gene delivery. Here we report on a new concept for dendrimer vector design based on the incorporation of dual binding motifs: DNA intercalation, and receptor recognition for targeted delivery. We prepared a series of dendrimer conjugates derived from a fifth generation (G5) PAMAM dendrimer, each conjugated with multiple folate (FA) or riboflavin (RF) ligands for cell receptor targeting, and with 3,8-diamino-6-phenylphenanthridinium (“DAPP”)-derived ligands for anchoring a DNA payload. Polyplexes of each dendrimer with calf thymus dsDNA were made and characterized by surface plasmon resonance (SPR) spectroscopy, dynamic light scattering (DLS) and zeta potential measurement. These studies provided evidence supporting polyplex formation based on the observation of tight DNA-dendrimer adhesion, and changes in particle size and surface charge upon coincubation. Further SPR studies to investigate the adhesion of the polyplex to a model surface immobilized with folate binding protein (FBP), demonstrated that the DNA payload has only a minimal effect on the receptor binding activity of the polyplex: KD = 0.22 nM for G5(FA)(DAPP) versus 0.98 nM for its polyplex. Finally, we performed in vitro transfection assays to determine the efficiency of conjugate mediated delivery of a luciferase-encoding plasmid into the KB cancer cell line and showed that RF-conjugated dendrimers were 1 to 2 orders of magnitude more effective in enhancing luciferase gene transfection than a plasmid only control. In summary, this study serves as a proof of concept for DNA-ligand intercalation as a motif in the design of multivalent dendrimer vectors for targeted gene delivery.

Riboflavin receptors are overexpressed in malignant cells from certain human breast and prostate cancers, and they constitute a group of potential surface markers important for cancer targeted delivery of therapeutic agents and imaging molecules. Here we report on the fabrication and atomic force microscopy (AFM) characterization of a core–shell nanocomposite consisting of a gold nanoparticle (AuNP) coated with riboflavin receptor-targeting poly(amido amine) dendrimer. We designed this nanocomposite for potential applications such as a cancer targeted imaging material based on its surface plasmon resonance properties conferred by AuNP. We employed AFM as a technique for probing the binding interaction between the nanocomposite and riboflavin binding protein (RfBP) in solution. AFM enabled precise measurement of the AuNP height distribution before (13.5 nm) and after chemisorption of riboflavin-conjugated dendrimer (AuNP–dendrimer; 20.5 nm). Binding of RfBP to the AuNP–dendrimer caused a height increase to 26.7 nm, which decreased to 22.8 nm when coincubated with riboflavin as a competitive ligand, supporting interaction of AuNP–dendrimer and its target protein. In summary, physical determination of size distribution by AFM imaging can serve as a quantitative approach to monitor and characterize the nanoscale interaction between a dendrimer-covered AuNP and target protein molecules in vitro.

We report the design of a water-soluble, quaternized tamoxifen photoprobe and demonstrate its application in light-controlled induction of green fluorescent protein expression via a Cre-ER recombinase system.

We investigated two types of generation 5 polyamidoamine (PAMAM) dendrimers, each conjugated stochastically with a mean number of 5 or 10 methotrexate (MTX) ligands per dendrimer (G5-MTX5, G5-MTX10), for their binding to surface-immobilized folate binding protein (FBP) as a function of receptor density. The binding study was performed under flow by surface plasmon resonance spectroscopy. Two multivalent models were examined to simulate binding of the dendrimer to the receptor surface, showing that at relatively high receptor density, both dendrimer conjugates exhibit high avidity. However, upon reducing the receptor density by a factor of 3 and 13 relative to the high density level, the avidity of the lower-valent G5-MTX5 decreases by up to several orders of magnitude (KD = nM to μM), whereas the avidity of G5-MTX10 remains largely unaffected regardless of the density variation. Notably, on the 13-fold reduced FBP surface, G5-MTX5 displays binding kinetics similar to that of monovalent methotrexate, which is patently different from the still tight binding of the higher-valent G5-MTX10. Thus, the binding analysis demonstrates that avidity displayed by multivalent MTX conjugates varies in response to the receptor density and can be modulated for achieving tighter, more specific binding to the higher receptor density by modulation of ligand valency. We believe this study provides experimental evidence supportive of the mechanistic hypothesis of multivalent NP uptake to a cancer cell over a healthy cell where the diseased cell expresses the folate receptor at higher density.

Vancomycin represents the preferred ligand for bacteria-targeting nanosystems. However, it is inefficient for emerging vancomycin-resistant species because of its poor affinity to the reprogrammed cell wall structure. This study demonstrates the use of a multivalent strategy as an effective way for overcoming such an affinity limitation in bacteria targeting. We designed a series of fifth generation (G5) poly(amidoamine) (PAMAM) dendrimers tethered with vancomycin at the C-terminus at different valencies. We performed surface plasmon resonance (SPR) studies to determine their binding avidity to two cell wall models, each made with either a vancomycin-susceptible (d)-Ala-(d)-Ala or vancomycin-resistant (d)-Ala-(d)-Lac cell wall precursor. These conjugates showed remarkable enhancement in avidity in the cell wall models tested, including the vancomycin-resistant model, which had an increase in avidity of four to five orders of magnitude greater than free vancomycin. The tight adsorption of the conjugate to the model surface corresponded with its ability to bind vancomycin-susceptible Staphylococcus aureus bacterial cells in vitro as imaged by confocal fluorescent microscopy. This vancomycin platform was then used to fabricate the surface of iron oxide nanoparticles by coating them with the dendrimer conjugates, and the resulting dendrimer-covered magnetic nanoparticles were demonstrated to rapidly sequester bacterial cells. In summary, this article investigates the biophysical basis of the tight, multivalent association of dendrimer-based vancomycin conjugates to the bacterial cell wall, and proposes a potential new use of this nanoplatform in targeting Gram-positive bacteria.Keywords: bacterial cell wall; iron oxide nanoparticle; poly(amidoamine) dendrimer; surface plasmon resonance spectroscopy; vancomycin

Cancer-targeting drug delivery can be based on the rational design of a therapeutic platform. This approach is typically achieved by the functionalization of a nanoparticle with two distinct types of molecules, a targeting ligand specific for a cancer cell, and a cytotoxic molecule to kill the cell. The present study aims to evaluate the validity of an alternative simplified approach in the design of cancer-targeting nanotherapeutics: conjugating a single type of molecule with dual activities to nanoparticles, instead of coupling a pair of orthogonal molecules. Herein we investigate whether this strategy can be validated by its application to methotrexate, a dual-acting small molecule that shows cytotoxicity because of its potent inhibitory activity against dihydrofolate reductase and that binds folic acid receptor, a tumor biomarker frequently upregulated on the cancer cell surface. This article describes a series of dendrimer conjugates derived from a generation 5 polyamidoamine (G5 PAMAM) presenting a multivalent array of methotrexate and also demonstrates their dual biological activities by surface plasmon resonance spectroscopy, a cell-free enzyme assay, and cell-based experiments with KB cancer cells.Nanotechnology platform for cancer-targeting drug delivery is designed by employing methotrexate (MTX) as a dual-acting cytotoxic molecule that targets folic acid receptor (FAR), a cancer surface biomarker.Highlights► MTX-presenting multivalent conjugates were prepared with G5 PAMAM dendrimer. ► Their avidity to the folate receptor was enhanced by up to ∼10,000-fold over MTX. ► MTX conjugate displayed the folate receptor-targeted uptake by a KB cancer cell.

Photochemistry provides a unique mechanism that enables the active control of drug release in cancer-targeting drug delivery. This study investigates the light-mediated release of methotrexate, an anticancer drug, using a photocleavable linker strategy based on o-nitrobenzyl protection. We evaluated two types of the o-nitrobenzyl-linked methotrexate for the drug release study and further extended the study to a fifth-generation poly(amidoamine) dendrimer carrier covalently conjugated with methotrexate via the o-nitrobenzyl linker. We performed the drug release studies by using a combination of three standard analytical methods that include UV/vis spectrometry, 1H NMR spectroscopy, and anal. HPLC. This article reports that methotrexate is released by the photochemical mechanism in an actively controlled manner. The rate of the drug release varies in response to multiple control parameters, including linker design, light wavelength, exposure time, and the pH of the medium where the drug release occurs.

Nanoparticle (NP)-based targeted drug delivery involves cell-specific targeting followed by a subsequent therapeutic action from the therapeutic carried by the NP system. NPs conjugated with methotrexate (MTX), a potent inhibitor of dihydrofolate reductase (DHFR) localized in cytosol, have been under investigation as a delivery system to target cancer cells to enhance the therapeutic index of methotrexate, which is otherwise non-selectively cytotoxic. Despite improved therapeutic activity from MTX-conjugated NPs in vitro and in vivo, the therapeutic action of these conjugates following cellular entry is poorly understood; in particular it is unclear whether the therapeutic activity requires release of the MTX. This study investigates whether MTX must be released from a nanoparticle in order to achieve the therapeutic activity. We report herein light-controlled release of methotrexate from a dendrimer-based conjugate and provide evidence suggesting that MTX still attached to the nanoconjugate system is fully able to inhibit the activity of its enzyme target and the growth of cancer cells.

Oximes are important in the treatment of organophosphate (OP) poisoning, but have limited biological half-lives. Complexing these drugs with a macromolecule, such as a dendrimer, could improve their pharmacokinetics. The present study investigates the intermolecular interactions that drive the complexation of oxime-based drug molecules with fifth generation poly(amidoamine) (PAMAM) dendrimers. We performed steady-state binding studies of two molecules, pralidoxime and obidoxime, employing multiple NMR methods, including 1D titration, 1H–1H 2D spectroscopy (COSY, NOESY), and 1H diffusion-ordered spectroscopy (DOSY). Several important insights were gained in understanding the host–guest interactions occurring between the drug molecules and the polymer. First, the guest molecules bind to the dendrimer macromolecule through a specific interaction rather than through random, hydrophobic encapsulation. Second, this specificity is driven primarily by the electrostatic or H-bond interaction of the oxime at a dendrimer amine site. Also, the average strength for each drug and dendrimer interaction is affected by the surface modification of the polymer. Third, individual binding events between oximes and a dendrimer have a negative cooperative effect on subsequent oxime binding. In summary, this report provides a novel perspective important for designing host systems for drug delivery.

The present study describes the biophysical characterization of generation-five poly(amidoamine) (PAMAM) dendrimers conjugated with riboflavin (RF) as a cancer-targeting platform. Two new series of dendrimers were designed, each presenting the riboflavin ligand attached at a different site (isoalloxazine at N-3 and d-ribose at N-10) and at varying ligand valency. Isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC) were used to determine the binding activity for riboflavin binding protein (RfBP) in a cell-free solution. The ITC data shows dendrimer conjugates have KD values of ≥465 nM on a riboflavin basis, an affinity ∼93-fold lower than that of free riboflavin. The N-3 series showed greater binding affinity in comparison with the N-10 series. Notably, the affinity is inversely correlated with ligand valency. These findings are also corroborated by DSC, where greater protein–conjugate stability is achieved with the N-3 series and at lower ligand valency.

The present study screened riboflavin mimicking small molecules to determine their binding activity for the riboflavin binding protein. We performed thermodynamic and kinetic binding studies of these molecules using a combination of two analytical approaches: isothermal titration calorimetry and surface plasmon resonance spectroscopy. Screening of a biased set of nonriboflavin-based small molecules by microcalorimetry led to the discovery of two known drug molecules, quinacrine and chloroquine, as favorable ligands for the riboflavin receptor with KD values of 264 and 2100 nM, respectively. We further demonstrated that quinacrine is a competitive ligand for the receptor as measured by surface plasmon resonance. Thus, this study describes the identification of a novel class of dual-acting riboflavin antagonists that target riboflavin receptor for cellular uptake and display multifunctional activities upon cellular entry.Keywords: isothermal titration calorimetry; riboflavin antagonist; surface plasmon resonance spectroscopy; Targeted delivery

We report the synthesis and in vitro evaluation of folate receptor-targeted nanoconjugate that releases its therapeutic payload via a photochemical mechanism.

•Photoactive nanomaterials display ecotoxicity mediated by reactive oxygen species (ROS) or photothermal heating.•ROS phototoxicity is determined by its production rate, particle contact area and uptake.•Photothermal toxicity occurs significantly by high aspect nano-Au among others.Photoactive nanomaterials have unique ability for generating functional activities in response to light. These nanomaterials have been increasingly used as novel research tools for various biomedical applications and active ingredients in certain consumer products. Given their emergent roles and wider uses, it would be of considerable significance to analyze their potential impact on human health and the surrounding ecosystems. In this review article, these photoactive nanomaterials are identified and classified on the basis of two primary mechanisms of action, spanning nanomaterials that are: (i) producers of reactive oxygen species (ROS), (ii) generators of plasmonic photothermal heat. Exposure to specific light plays an important role in each of these mechanisms as it triggers the activation of these nanomaterials as well as causes spatiotemporal variation of such activity. These mechanisms are suggested as the basis that determines the light-induced toxicity and/or genotoxicity in model cells and organisms by these nanomaterials. It is hoped that this review offers fundamental insights into the light activation properties of photoactive nanomaterials and their potential implications in ecotoxicology.Download high-res image (82KB)Download full-size image

We report on the practicality of a heteromultivalent design strategy for a nanoplatform that targets lipopolysaccharide molecules (LPS) present on the surface of Gram-negative bacteria. This design is based on the conjugation of a poly(amido amine) (PAMAM) dendrimer with two types of ligands, each having distinct affinities: (i) polymyxin B (PMB) as a primary high affinity ligand; (ii) a PMB-mimicking dendritic branch as an auxiliary low affinity ligand. Co-conjugation of these two ligands maximizes the efficiency of the primary ligand even when the primary ligand is present at a low valency on the nanoplatform (mean nPMB ≈ 1). By performing surface plasmon resonance studies using a LPS-immobilized cell wall model, we identified an ethanolamine (EA)-terminated branch as the auxiliary ligand that promotes binding avidity via heteromultivalent association. PMB conjugation of the dendrimer with excess EA branches led to LPS avidity two orders of magnitude greater than unconjugated PMB. Such tight binding observed by SPR corresponded well with adsorption to E. coli cells and with potent bactericidal activity in vitro.

We report the design of a water-soluble, quaternized tamoxifen photoprobe and demonstrate its application in light-controlled induction of green fluorescent protein expression via a Cre-ER recombinase system.

Pyridinium aldoxime (PAM) molecules constitute a group of small molecule antidotes essential for the treatment of reactive organophosphate (OP) poisoning. Their therapeutic efficacy stems from a combination of two activities: (i) reactivation of acetylcholine esterase inhibited by OP; (ii) scavenging of free OP. Here, we report the design, synthesis and in vitro functional characterization of fifth generation poly(amidoamine)dendrimer conjugates, each tethered with PAM or hydroxamate, as OP scavenging macromolecules. We chose paraoxon (POX) as the model OP, and performed extensive reaction kinetic studies in aqueous media to determine the activities and mechanisms of these dendrimer conjugates in POX hydrolysis with 1H NMR, UV-vis and LCMS/MS methods. Quantitative kinetic analysis suggests that the conjugate hydrolyzed POX through a catalytic mechanism as effective as that of the unconjugated molecules. In summary, we report the first class of dendrimer conjugates which are stable in the plasma and functioning as OP-responsive catalytic scavengers.

We report the synthesis and in vitro evaluation of folate receptor-targeted nanoconjugate that releases its therapeutic payload via a photochemical mechanism.

We report an active delivery mechanism targeted specifically to Gram(−) bacteria based on the photochemical release of photocaged ciprofloxacin carried by a cell wall-targeted dendrimer nanoconjugate.