Specific and cooperative interactions between oximes and PAMAM dendrimers as demonstrated by (1)H NMR study

Dendrimer-based posaconazole nanoplatform for antifungal therapy

We examined formulating a new antifungal agent, posaconazole (POS) and its derivatives, with different molecular vehicles. Several combinations of drug and carrier molecules were synthesized, and their antifungal activities were evaluated against Aspergillus fumigatus. Posaconazole and four of its derivatives were conjugated to either generation 5 (G5) dendrimers or partially modified G5 dendrimers. The in vitro antifungal activities of these compounds suggest that conjugates with specific chemical linkages showed better fungistatic activity than direct conjugates to POS. In particular, a polyethylene glycol (PEG)-imidazole modified G5 dendrimer demonstrated improved antifungal efficacy relative to the parent G5 molecule. Further studies were then conducted with POS derived molecules coupled to PEG-imidazole modified G5 dendrimers to achieve a highly soluble and active conjugate of POS. This conjugated macromolecule averaged 23 POS molecules per G5 and had a high solubility with 50 mg/mL, which improved the molar solubility of POS from less than 0.03 mg/mL to as high as 16 mg/mL in water. The primary release profile of the drug in human plasma was extended to over 72 h, which is reflected in the in vitro inhibition of A. fumigatus growth of over 96 h. These POS–polymer conjugates appear to be novel and efficient antifungal agents.

Oximes: Novel Therapeutics with Anticancer and Anti-Inflammatory Potential

Oximes have been studied for decades because of their significant roles as acetylcholinesterase reactivators. Over the last twenty years, a large number of oximes have been reported with useful pharmaceutical properties, including compounds with antibacterial, anticancer, anti-arthritis, and anti-stroke activities. Many oximes are kinase inhibitors and have been shown to inhibit over 40 different kinases, including AMP-activated protein kinase (AMPK), phosphatidylinositol 3-kinase (PI3K), cyclin-dependent kinase (CDK), serine/threonine kinases glycogen synthase kinase 3 α/β (GSK-3α/β), Aurora A, B-Raf, Chk1, death-associated protein-kinase-related 2 (DRAK2), phosphorylase kinase (PhK), serum and glucocorticoid-regulated kinase (SGK), Janus tyrosine kinase (JAK), and multiple receptor and non-receptor tyrosine kinases. Some oximes are inhibitors of lipoxygenase 5, human neutrophil elastase, and proteinase 3. The oxime group contains two H-bond acceptors (nitrogen and oxygen atoms) and one H-bond donor (OH group), versus only one H-bond acceptor present in carbonyl groups. This feature, together with the high polarity of oxime groups, may lead to a significantly different mode of interaction with receptor binding sites compared to corresponding carbonyl compounds, despite small changes in the total size and shape of the compound. In addition, oximes can generate nitric oxide. This review is focused on oximes as kinase inhibitors with anticancer and anti-inflammatory activities. Oximes with non-kinase targets or mechanisms of anti-inflammatory activity are also discussed.

New Insights into the Binding Site and Affinity of the Interaction between Biotin and PAMAMs-NH2 via NMR Studies

Biotin-dendrimer conjugates (such as biotin-PAMAMs-NH₂) are important macromolecules in the field of host-guest chemistry and widely used systems for delivery. The similar chemical structures of the inner and outer layers of PAMAM-NH₂ make it difficult to illuminate the interaction and the binding affinity of biotin-PAMAMs-NH₂. By utilizing NMR techniques including ¹H NMR titration, CSSF-TOCSY, STDD methods, and 2D DOSY analysis, we demonstrate a method to sort out these interactions. The methylene protons of the inner and outer layers of PAMAM-NH₂ are successfully identified and accurately positioned so that the carboxylic acid groups of biotins are having ionic interactions with the outermost amine groups of PAMAM-NH₂. The inner PAMAM-NH₂ is protonated when reaching the isoelectric point of PAMAM-NH₂, increasing the hydrodynamic radius. On the basis of the NMR experiments, a model is proposed, where the carboxylic acid groups and heterocyclic skeleton of biotin arched over the outer layers of PAMAM-NH₂ like a bridge. Furthermore, using STDD epitope mapping, the binding affinity between biotin and PAMAM-NH₂ was quantified. The diffusion behavior of biotin-G5 PAMAM-NH₂ complex is more complicated than that of biotin-G3 PAMAM-NH₂ complex due to steric hindrance. The results provide a theoretical basis for understanding these complicated drug delivery systems.

Nanomaterial-Enabled Sensors and Therapeutic Platforms for Reactive Organophosphates

Unintended exposure to harmful reactive organophosphates (OP), which comprise a group of nerve agents and agricultural pesticides, continues to pose a serious threat to human health and ecosystems due to their toxicity and prolonged stability. This underscores an unmet need for developing technologies that will allow sensitive OP detection, rapid decontamination and effective treatment of OP intoxication. Here, this article aims to review the status and prospect of emerging nanotechnologies and multifunctional nanomaterials that have shown considerable potential in advancing detection methods and treatment modalities. It begins with a brief introduction to OP types and their biochemical basis of toxicity followed by nanomaterial applications in two topical areas of primary interest. One topic relates to nanomaterial-based sensors which are applicable for OP detection and quantitative analysis by electrochemical, fluorescent, luminescent and spectrophotometric methods. The other topic is directed on nanotherapeutic platforms developed as OP remedies, which comprise nanocarriers for antidote drug delivery and nanoscavengers for OP inactivation and decontamination. In summary, this article addresses OP-responsive nanomaterials, their design concepts and growing impact on advancing our capability in the development of OP sensors, decontaminants and therapies.

Shielded α-Nucleophile Nanoreactor for Topical Decontamination of Reactive Organophosphate

Here, we describe a nanoscale reactor strategy with a topical application in the therapeutic decontamination of reactive organophosphates (OPs) as chemical threat agents. It involves functionalization of poly(amidoamine) dendrimer through a combination of its partial PEG shielding and exhaustive conjugation with an OP-reactive α-nucleophile moiety at its peripheral branches. We prepared a 16-member library composed of two α-nucleophile classes (oxime, hydroxamic acid), each varying in its reactor valency (43-176 reactive units per nanoparticle), and linker framework for α-nucleophile tethering. Their mechanism for OP inactivation occurred via nucleophilic catalysis as verified against P-O and P-S bonded OPs including paraoxon-ethyl (POX), malaoxon, and omethoate by ¹H NMR spectroscopy. Screening their reactivity for POX inactivation was performed under pH- and temperature-controlled conditions, which resulted in identifying 13 conjugates, each showing shorter POX half-life up to 2 times as compared to a reference Dekon 139 at pH 10.5, 37 °C. Of these, 10 conjugates were further confirmed for greater efficacy in POX decontamination experiments performed in two skin models, porcine skin and an artificial human microtissue. Finally, a few lead conjugates were selected and demonstrated for their biocompatibility in vitro as evident with lack of skin absorption, no inhibition of acetylcholinesterase (AChE), and no cytotoxicity in human neuroblastoma cells. In summary, this study presents a novel nanoreactor library, its screening methods, and identification of potent lead conjugates with potential for therapeutic OP decontamination.

Characterization of zwitterion-modified poly(amidoamine) dendrimers in aqueous solution via a thorough NMR investigation

Zwitterions are a class of unique molecules that can be modified onto nanomaterials to render them with antifouling properties. Here we report a thorough NMR investigation of dendrimers modified with zwitterions in terms of their structure, hydrodynamic size, and diffusion time in aqueous solution. In this present work, poly(amidoamine) (PAMAM) dendrimers of generation 5 (G5) were partially decorated with carboxybetaine acrylamide (CBAA), 2-methacryloyloxyethyl phosphorylcholine (MPC), and 1,3-propane sultone (1,3-PS), respectively with different modification degrees. The formed zwitterion-modified G5 dendrimers were characterized using NMR techniques. We show that the zwitterion modification leads to increased G5 dendrimer size in aqueous solution, suggesting that the modified zwitterions can form a hydration layer on the surface of G5 dendrimers. In addition, the hydrodynamic sizes of G5 dendrimers modified with different zwitterions but with the same degree of surface modification are discrepant depending on the type of zwitterions. The present study provides a new physical insight into the structure of zwitterion-modified G5 dendrimers by NMR techniques, which is beneficial for further design of different biomedical applications.

Reactivity and mechanism of α-nucleophile scaffolds as catalytic organophosphate scavengers

Despite their unique benefits imparted by their structure and reactivity, certain α-nucleophile molecules remain underexplored as chemical inactivators for the topical decontamination of reactive organophosphates (OPs). Here, we present a library of thirty α-nucleophile scaffolds, each designed with either a pyridinium aldoxime (PAM) or hydroxamic acid (HA) α-nucleophile core tethered to a polar or charged scaffold for optimized physicochemical properties and reactivity. These library compounds were screened for their abilities to catalyze the hydrolysis of a model OP, paraoxon (POX), in kinetic assays. These screening experiments led to the identification of multiple lead compounds with the ability to inactivate POX two- to four-times more rapidly than Dekon 139-the active ingredient currently used for skin decontamination of OPs. Our mechanistic studies, performed under variable pH and temperature conditions suggested that the differences in the reactivity and activation energy of these compounds are fundamentally attributable to the core nucleophilicity and pKa. Following their screening and mechanistic studies, select lead compounds were further evaluated and demonstrated greater efficacy than Dekon 139 in the topical decontamination of POX in an ex vivo porcine skin model. In addition to OP reactivity, several compounds in the PAM class displayed a dual mode of activity, as they retained the ability to reactivate POX-inhibited acetylcholine esterase (AChE). In summary, this report describes a rationale for the hydrophilic scaffold design of α-nucleophiles, and it offers advanced insights into their chemical reactivity, mechanism, and practical utility as OP decontaminants.

Hydrophilic scaffolds of oxime as the potent catalytic inactivator of reactive organophosphate

Despite its efficacy as a skin decontaminant of reactive organophosphates (OP), Dekon 139-a potassium salt of 2,3-butanedione monooxime (DAM)-is associated with adverse events related to percutaneous absorption largely due to its small size and lipophilicity. In order to address this physicochemical issue, we synthesized and evaluated the activity of a focused library of 14 hydrophilic oxime compounds, each designed with either a DAM or monoisonitrosoacetone (MINA) oxime tethered to a polar or charged scaffold in order to optimize the size, hydrophilicity, and oxime acidity. High-throughput colorimetric assays were performed with paraoxon (POX) as a model OP to determine the kinetics of POX inactivation by these compounds under various pH and temperature conditions. This primary screening led to the identification of 6 lead compounds, predominantly in the MINA series, which displayed superb catalytic activity by reducing the POX half-life (t_1/2) by 2-3 fold relative to Dekon 139. Our mechanistic studies show that POX inactivation by the oxime compounds occurred faster at a higher temperature and in a pH-dependent manner in which the negatively charged oximate species is ≥ 10-fold more effective than the neutral oxime species. Lastly, using one of the lead compounds, we demonstrated its promising efficacy for POX decontamination in porcine skin ex vivo, and showed its potent ability to protect acetylcholine esterase (AChE) through POX inactivation. In summary, we report the rational design and chemical biological validation of novel hydrophilic oximes which address an unmet need in therapeutic OP decontamination.

Measuring the Adhesion Forces for the Multivalent Binding of Vancomycin-Conjugated Dendrimer to Bacterial Cell-Wall Peptide

Multivalent ligand-receptor interaction provides the fundamental basis for the hypothetical notion that high binding avidity relates to the strong force of adhesion. Despite its increasing importance in the design of targeted nanoconjugates, an understanding of the physical forces underlying the multivalent interaction remains a subject of urgent investigation. In this study, we designed three vancomycin (Van)-conjugated dendrimers G5(Van) _n ( n = mean valency = 0, 1, 4) for bacterial targeting with generation 5 (G5) poly(amidoamine) dendrimer as a multivalent scaffold and evaluated both their binding avidity and physical force of adhesion to a bacterial model surface by employing surface plasmon resonance (SPR) spectroscopy and atomic force microscopy. The SPR experiment for these conjugates was performed in a biosensor chip surface immobilized with a bacterial cell-wall peptide Lys-d-Ala-d-Ala. Of these, G5(Van)₄ bound most tightly with a K_D of 0.34 nM, which represents an increase in avidity by 2 or 3 orders of magnitude relative to a monovalent conjugate G5(Van)₁ or free vancomycin, respectively. By single-molecule force spectroscopy, we measured the adhesion force between G5(Van) _n and the same cell-wall peptide immobilized on the surface. The distribution of adhesion forces increased in proportion to vancomycin valency with the mean force of 134 pN at n = 4 greater than 96 pN at n = 1 at a loading rate of 5200 pN/s. In summary, our results are strongly supportive of the positive correlation between the avidity and adhesion force in the multivalent interaction of vancomycin nanoconjugates.

Different self-assemblies and absorption–emission properties of the picrate salts of aromatic amine or heterocycle linked oximes

Different sub-assemblies and fluorescence quenching in picrate salts of an aromatic amine and of three different heterocycle tethered aldoximes are described.

Spectroscopic and calorimetric studies on the interaction between PAMAM G4-OH and 5-fluorouracil in aqueous solutions

The results of spectroscopic measurements (an increase in solubility, equilibrium dialysis, ¹H NMR titration) and calorimetric measurements (isothermal titration ITC) indicate spontaneous (ΔG<0) binding of 5-fluorouracil molecules by PAMAM G4-OH dendrimer with terminal hydroxyl groups in an aqueous solution. PAMAM G4-OH dendrimer bonds about n=8±1 molecules of the drug with an equilibrium constant of K=70±10. The process of saturating the dendrimer active sites by the drug molecules is exothermal (ΔH<0) and is accompanied by an advantageous change in entropy (ΔS>0). The parameters of binding 5-fluorouracil by PAMAM G4-OH dendrimer were compared with those of binding this drug by the macromolecules of PAMAM G3-OH and G5-OH.

Oxime synthons in the salts and cocrystals of quinoline-4-carbaldoxime for non-covalent synthesis

Synthons in the cocrystals and salts of quinoline-4-carbaldoxime with acids are discussed.

Spectroscopic and calorimetric studies of formation of the supramolecular complexes of PAMAM G5-NH₂ and G5-OH dendrimers with 5-fluorouracil in aqueous solution

The results of spectroscopic measurements (increase in solubility, equilibrium dialysis, (1)H NMR titration) and calorimetric measurements (isothermal titration ITC) indicate exothermic (ΔH<0) and spontaneous (ΔG < 0) combination of an antitumor drug, 5-fluorouracil, by both cationic PAMAM G5-NH2 dendrimer and its hydroxyl analog PAMAM G5-OH in aqueous solutions at room temperature. PAMAM G5-NH2 dendrimer combines about 70 molecules of the drug with equilibrium constant K ≅ 300, which is accompanied by an increase in the system order (ΔS < 0). Hydroxyl dendrimer, PAMAM G5-OH, combines about 14 molecules of 5-fluorouracil with equilibrium constant K ≅ 100. This process is accompanied by an increase in the system disorder (ΔS > 0).

Mechanisms and implications of dual-acting methotrexate in folate-targeted nanotherapeutic delivery

The rational design of a nanoplatform in drug delivery plays a crucial role in determining its targeting specificity and efficacy in vivo. A conventional approach relies on the surface conjugation of a nanometer-sized particle with two functionally distinct types of molecules, one as a targeting ligand, and the other as a therapeutic agent to be delivered to the diseased cell. However, an alternative simplified approach can be used, in which a single type of molecule displaying dual function as both a targeting ligand and therapeutic agent is conjugated to the nanoparticle. In this review, we evaluate the validity of this new strategy by using methotrexate, which displays multifunctional mechanisms of action. Methotrexate binds to the folate receptor, a surface biomarker frequently overexpressed in tumor cells, and also inhibits dihydrofolate reductase, an enzyme critical for cell survival and division. Thus we describe a series of fifth generation poly(amido amine) dendrimers conjugated with methotrexate, and discuss several lines of evidence supporting the efficacy of this new platform strategy based on surface plasmon resonance spectroscopy, enzyme activity assays, and cell-based studies with folate receptor (+) KB cancer cells.

A lipopolysaccharide binding heteromultivalent dendrimer nanoplatform for Gram negative cell targeting

Heteromultivalent design of PAMAM dendrimer by conjugation with polymyxin B (PMB) ligand and excess auxiliary ethanolamine (EA) branches led to lipopolysaccharide (LPS) avidity two orders of magnitude greater than free PMB.

Deswelling and Electrolyte Dissipation in Free Diffusion of Charged PAMAM Dendrimers

The diffusion coefficient of charged PAMAM dendrimers was measured by fluorescence correlation spectroscopy in aqueous solution at submicromolar concentrations. The solution pH was varied for conditions ranging from a fully charged to neutral charge dendrimer to infer about electrostatic swelling in the dilute regime. The diffusion coefficient of generation G4 increases by as much as 20% between high and low charge conditions due to the combined effects of polyelectrolyte deswelling and loss of electrolyte dissipation. By taking into account the electrolyte dissipation in the friction factor, we have found that the observed deswelling corresponds to a change of hydrodynamic radius between 7-13% for generation G4 and about 12% for generation G7. Simulations of molecular dynamics of dendrimer G4 show that counterion uptake by the dendrimer structure upon full protonation induces a 16% increase of its radius of gyration. The change in dendrimer size is slightly larger than that previously reported from neutron scattering techniques, thereby suggesting that electrostatic swelling is more pronounced at dilute dendrimer concentration and low ionic strength. It is confirmed that even higher generations, which have more congested molecular structures, can experience some degree of conformational change in response to a change of the dendrimer charge density.

Atomic force microscopy probing of receptor-nanoparticle interactions for riboflavin receptor targeted gold-dendrimer nanocomposites

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.

Study of the complexation of oxacillin in 1-(4-carbomethoxypyrrolidone)-terminated PAMAM dendrimers

The complexation of oxacillin to three generations of 1-(4-carbomethoxypyrrolidone)-terminated PAMAM dendrimers was studied with NMR in CD3OD and CDCl3. The stochiometries, which were determined from Job plots, were found to be both solvent- and generation-dependent. The dissociation constants (K(d)) and Gibbs energies for complexation of oxacillin into the 1-(4-carbomethoxypyrrolidone)-terminated PAMAM dendrimer hosts were determined by (1)H NMR titrations and showed weaker binding of oxacillin upon increasing the size (generation) of the dendrimer.

DNA condensation by partially acetylated poly(amido amine) dendrimers: effects of dendrimer charge density on complex formation

The ability of poly(amido amine) (or PAMAM) dendrimers to condense semiflexible dsDNA and penetrate cell membranes gives them great potential in gene therapy and drug delivery but their high positive surface charge makes them cytotoxic. Here, we describe the effects of partial neutralization by acetylation on DNA condensation using light scattering, circular dichroism, and single molecule imaging of dendrimer-DNA complexes combed onto surfaces and tethered to those surfaces under flow. We find that DNA can be condensed by generation-five (G5) dendrimers even when the surface charges are more than 65% neutralized, but that such dendrimers bind negligibly when an end-tethered DNA is stretched in flow. We also find that when fully charged dendrimers are introduced by flow to end-tethered DNA, all DNA molecules become equally highly coated with dendrimers at a rate that becomes very fast at high dendrimer concentration, and that dendrimers remain bound during subsequent flow of dendrimer-free buffer. These results suggest that the presence of dendrimer-free DNA coexisting with dendrimer-bound DNA after bulk mixing of the two in solution may result from diffusion-limited irreversible dendrimer-DNA binding, rather than, or in addition to, the previously proposed cooperative binding mechanism of dendrimers to DNA.