Dendrimer Backbone Research Articles

PAMAM dendrimers: blood-brain barrier transport and neuronal uptake after focal brain ischemia

Drug delivery to the central nervous system is restricted by the blood-brain barrier (BBB). However, with the onset of stroke, the BBB becomes leaky, providing a window of opportunity to passively target the brain. Here, cationic poly(amido amine) (PAMAM) dendrimers of different generations were functionalized with poly(ethylene glycol) (PEG) to reduce cytotoxicity and prolong blood circulation half-life, aiming for a safe in vivo drug delivery system in a stroke scenario. Rhodamine B isothiocyanate (RITC) was covalently tethered to the dendrimer backbone and used as a small surrogate drug as well as for tracking purposes.The biocompatibility of PAMAM was markedly increased by PEGylation as a function of dendrimer generation and degree of functionalization. The PEGylated RITC-modified dendrimers did not affect the integrity of an in vitro BBB model. Additionally, the functionalized dendrimers remained safe when in contact with the bEnd.3 cells and rat primary astrocytes composing the in vitro BBB model after hypoxia induced by oxygen-glucose deprivation. Modification with PEG also decreased the interaction and uptake by endothelial cells of PAMAM, indicating that the transport across a leaky BBB due to focal brain ischemia would be facilitated. Next, the functionalized dendrimers were tested in contact with red blood cells showing no haemolysis for the PEGylated PAMAM, in contrast to the unmodified dendrimer. Interestingly, the PEG-modified dendrimers reduced blood clotting, which may be an added beneficial function in the context of stroke.The optimized PAMAM formulation was intravenously administered in mice after inducing permanent focal brain ischemia. Twenty-four hours after administration, dendrimers could be detected in the brain, including in neurons of the ischemic cortex.Our results suggest that the proposed formulation has the potential for becoming a successful delivery vector for therapeutic application to the injured brain after stroke reaching the ischemic neurons.

Nano-sized metabolic precursors for heterogeneous tumor-targeting strategy using bioorthogonal click chemistry in vivo

Herein, we developed nano-sized metabolic precursors (Nano-MPs) for new tumor-targeting strategy to overcome the intrinsic limitations of biological ligands such as the limited number of biological receptors and the heterogeneity in tumor tissues. We conjugated the azide group-containing metabolic precursors, triacetylated N-azidoacetyl-d-mannosamine to generation 4 poly(amidoamine) dendrimer backbone. The nano-sized dendrimer of Nano-MPs could generate azide groups on the surface of tumor cells homogeneously regardless of cell types via metabolic glycoengineering. Importantly, these exogenously generated ‘artificial chemical receptors’ containing azide groups could be used for bioorthogonal click chemistry, regardless of phenotypes of different tumor cells. Furthermore, in tumor-bearing mice models, Nano-MPs could be mainly localized at the target tumor tissues by the enhanced permeation and retention (EPR) effect, and they successfully generated azide groups on tumor cells in vivo after an intravenous injection. Finally, we showed that these azide groups on tumor tissues could be used as ‘artificial chemical receptors’ that were conjugated to bioorthogonal chemical group-containing liposomes via in vivo click chemistry in heterogeneous tumor-bearing mice. Therefore, overall results demonstrated that our nano-sized metabolic precursors could be extensively applied to new alternative tumor-targeting technique for molecular imaging and drug delivery system, regardless of the phenotype of heterogeneous tumor cells.

Benzophenone and zinc(II) phthalocyanine dichromophores-labeled poly (aryl ether) dendrimer: synthesis, characterization and photoinduced energy transfer

A series of benzophenone chromospheres and zinc(II) phthalocyanine dichromophores labeled poly (aryl benzyl ether) dendrimer (Gn-DZnPc(BP)8n, n = 1−2) were synthesized. Their structures were characterized by elemental analysis, 1H NMR, IR, UV–vis and matrix-assisted laser desorption/ionization time-of-flight spectrometry (MALDI-TOF MS). Their photophysical properties were examined by steady-state and time-resolved fluorescence methods. Both the poly (aryl benzyl ether) dendrimer and BP terminal chromophores had a significant effect on photophysical properties of the zinc(II) phthalocyanine core. Time-resolved spectroscopic measurements indicated that the lifetime of benzophenone (donor) chromophore was longer than that of the zinc(II) phthalocyanine (acceptor). The fluorescence of the peripheral benzophenone chromophores was quenched by the phthalocyanine group attached to the focal point. All of these observations suggest that an intramolecular singlet energy transfer occurs in Gn-DZnPc(BP)8n molecules. The light-harvesting abilities of these molecules increased with generations due to an increase in the number of benzophenone chromophores. The energy transfer efficiencies were ca. 0.49 and 0.68 for generations 1 and 2, respectively, and the rate constants of the singlet-singlet energy transfer were ca. 108 s−1. The rate constants changed inconspicuously with increase of dendron generations. The intramolecular singlet-singlet energy transfer is proposed to proceed mainly via a Förster-type interaction mechanism involving the dendrimer backbone as a scaffold to hold the peripheral benzophenone chromophores and the phthalocyanine core together. This dendrimer was an effective new energy transmission complex with high efficiency and could be used as a potential light-harvesting system.

Decomposition of poly(amidoamine) (PAMAM) dendrimers under O2 plasma conditions

Results of FTIR and XPS measurements of bulk and γ-Al2O3-supported fourth generation hydroxyl-terminated (G4OH) poly(amidoamine) (PAMAM) dendrimers during O2 plasma treatment at room temperature indicate that such treatment can effectively decompose the structure of the dendrimer. No new functionalities appear in the dendrimer structure during such treatment, but instead fragmentation of this structure is taking place. The dendrimer backbone likely disintegrates into small molecules (i.e., NOx, CO, CO2) which in the case of the supported dendrimers interact with the γ-Al2O3 support and form surface nitrate and carboxy species. Subsequent treatment in H2 at 300 °C leads to the complete removal of the nitrogen-containing species from the surface of γ-Al2O3. However, carbon-containing species still remain on the surface in relatively small amounts. Finally, XRD measurements further show that the crystallinity of the support is not affected substantially by the O2 plasma treatment.

The role of the dendritic support in the catalytic performance of peripheral pincer Pd-complexes

To investigate the effects of the dendrimer backbone on catalysis, a series of monomeric and dendritic SCS-pincer Pd-complexes was synthesized and tested in two different Pd(II)-catalyzed reactions. To this end, the three novel polar PAMAM dendrimer-immobilized SCS-pincer Pd-complexes 3, 4, and 5, and the two apolar carbosilane dendrimer-immobilized complexes 7 and 8 were compared to three monomeric analogues 1, 2 and 6. These complexes were investigated in the cross-coupling reaction between vinyl epoxide and styrylboronic acid and the auto-tandem reaction of cinnamyl chloride, hexamethylditin, and 4-nitrobenzaldehyde. The differences in catalytic rate and product selectivity for these complexes are described and discussed. For the cross coupling reaction, the PAMAM dendrimer-immobilized complexes were found to give a similar reaction rate, but a higher product selectivity than their monomeric counterparts. The carbosilane complexes showed a lower reaction rate and similar product selectivity. These observations are explained in view of dendrimer aggregation and peripheral group backfolding.

Synthesis and characterization of poly(propylene imine) dendrimer – Polypyrrole conducting star copolymer

A star copolymer based on poly(propylene imine) (PPI) dendrimer core (generations 1–4) and polypyrrole (PPy) shell was prepared. The synthesis procedure involved a condensation reaction between PPI surface primary amine and 2-pyrrole aldehyde to give the pyrrole-functionalized PPI dendrimer (PPI-2Py). The pyrrole units on the dendrimer backbone were polymerized chemically using ammonium persulfate as an oxidizing agent and electrochemically on a coated platinum electrode with PPI-2Py using cyclic voltammetry. The resulting star copolymer, called poly(propylene imine)-co-polypyrrole (PPI-co-PPy) was characterized using nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), X-ray diffraction (X-RD) and electrochemical impedance spectroscopy (EIS). NMR of PPI-2Py gave a new chemical shift at 8.1 ppm for N CH, which confirmed the incorporation of 2-pyrrole aldehyde into the PPI dendrimer structure. Strong FT-IR bands appeared at 1634 cm −1 for ν(N C) in the dendrimer moiety, and at 729 cm −1 for ν(C H) at the α-position of the PPy ring. The PPI-co-PPy exhibited improved thermal stability than PPI-2Py and ionic conductivity over pristine polypyrrole as interpreted from the TGA and EIS profiles respectively. Hall effect measurements showed that the star copolymer is a semiconductor with an electronic conductivity of 0.7 S cm −1.

Synthesis and characterization of a triazine dendrimer that sequesters iron(III) using 12 desferrioxamine B groups

The synthesis of a third generation triazine dendrimer, 1, containing multiple, iron-sequestering desferrioxamine B (DFO) groups is described. Benzoylation of the hydroxamic acid groups of DFO and formation of a reactive dichlorotriazine provide the intermediate for reaction with the second generation dendrimer displaying twelve amines. This strategy further generalizes the ‘functional monomer’ approach to generate biologically active triazine dendrimers. Dendrimer 1 is prepared in seven steps in 35% overall yield and displays 12 DFO groups making it 56% drug by weight. Spectrophotometric titrations (UV–vis) show that 1 sequesters iron(III) atoms with neither cooperativity nor significant interference from the dendrimer backbone. Evidence from NMR spectroscopy and mass spectrometry reveals a limitation to this functional monomer approach: trace amounts of O-to-N acyl migration from the protected hydroxamic acids to the amine-terminated dendrimer occurs during the coupling step leading to N-benzoylated dendrimers displaying fewer than 12 DFO groups.

Shape-Persistent Oligothienylene-Ethynylene-Based Dendrimers: Synthesis, Spectroscopy and Electrochemical Characterization

A series of novel structurally well-defined oligothienylene-ethynylene-based dendritic macromolecules up to the 3rd generation (G3) were successfully synthesized by a combination of Pd-catalyzed Sonogashira-type cross-coupling and oxidative homocoupling steps. Oxidative homocoupling of dendrons successfully afforded dendrimers up to the 2nd generation (G2). In contrast, the G3 dendrimer was effectively prepared by a four-fold Sonogashira-type cross-coupling reaction. All compounds showed broad and structureless absorption and emission spectra arising from the presence of different pi-conjugated chromophores. With increasing generation, a bathochromic shift of the pi-pi* absorption band and an increase of the absorption coefficient were observed. The insertion of ethynylene groups into the conjugated dendrimer backbone resulted in a hypsochromic shift compared to all-thiophene dendrimers reported earlier by our group. All dendritic compounds are fluorescent and showed moderate quantum efficiencies due to an effective intramolecular charge-transfer (ICT) process. Cyclic voltammetry measurements also revealed the presence of multiple pi-conjugative pathways that show very broad oxidation waves for higher generations. HOMO-LUMO energy levels of these dendrons and dendrimers were estimated from optical and redox measurements and the calculated band gaps were within the range of 3.3 to 2.4 eV, typical for oligo- and polythiophenes. Electrochemical polymerizations of several desilylated compounds were performed and characterization of the films is reported. Preliminary bulk heterojunction solar cells that utilise these ethynylated dendrimers as the donor and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM[60]) as the acceptor showed moderate efficiencies ranging from 0.18-0.64%.

Lactotriaose-containing carbosilane dendrimers: Syntheses and lectin-binding activities

Carbosilane dendrimers periphery-functionalized with lactotriaose (GlcNAcβ1–3Galβ1–4Glc) with valencies of three, four, six, and twelve were prepared for use in a lectin-binding assay. A lactotriaose derivative was prepared from d-glucosamine and d-lactose derivatives. The N-Troc-protected glucosamine glycosyl donor and 3′-O-unprotected lactose glycosyl acceptor were condensed in the presence of silver trifluoromethanesulfonate and methylsulfenyl bromide to provide corresponding trisaccharide with new β-1-3 linkages in 92% yield. The protection group of the trisaccharide was transformed into an acetyl group. The 4-pentenyl glycoside was prepared from the acetate via glycosyl bromide. The alkene was converted into acetyl sulfide by addition of thioacetic acid under radical conditions. The lactotriaose unit was linked with carbosilane dendrimers to afford acetyl-protected glycodendrimers. De-O-acetylation of the dendrimers was carried out in the presence of sodium methoxide and then aq NaOH to give the desired lactotriaose clusters using a carbosilane dendrimer backbone. Their biological activities toward WGA were evaluated by fluorescence methods. The binding constants of free lactotriaose and trivalent, tetravalent, hexavalent, and dodecavalent glycodendrimers to WGA were determined to be 1.1 × 10 3, 4.4 × 10 4, 5.1 × 10 4, 2.8 × 10 6, and 1.3 × 10 6 M −1, respectively. The hexavalent glycodendrimer showed a 2500-fold larger binding effect than that of free lactotriaose.

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (G n-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in G n-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10 10 s −1. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.

Energy Transfer in Unsymmetrical Phenylene Ethynylene Dendrimers

We have applied the fluorescence upconversion technique to explore the electronic excitation energy transfer in unsymmetrical phenylene ethynylene dendrimers. Steady-state emission spectra show that the energy transfer from the dendrons to the core is highly efficient. Ultrafast time-resolved fluorescence measurements are performed at various excitation wavelengths to explore the possibility of assigning absorption band structures to exciton localizations. We propose a kinetic model to describe the time-resolved data. Independent of the excitation wavelength, a typical rise-time value of 500 fs is measured for the fluorescence in the dendrimer without an energy trap, indicating initial delocalized excitation. While absorption is into delocalized exciton states, emission occurs from localized states. When an energy trap such as perylene is introduced on the dendrimer, varying the excitation wavelength yields different energy-transfer rates, and the excitation energy migrates to the trap through two channels. The interaction energy between the dendrimer backbone and the trap is estimated to be 75 cm(-1). This value is small compared to the vibronic bandwidth of the dendrimer, indicating that the monodendrons and the energy trap are weakly coupled.

Photophysical properties of pheophorbide- a-substituted diaminobutane poly-propylene-imine dendrimer

Dendrimers are very promising molecules as hosts for guest embedding as well as for multiple covalent coupling of dye molecules to their peripheral groups. The use of such dendrimer–dye conjugates as part of a modular carrier system for photodynamic therapy has recently been discussed. Using covalent coupling we accomplished an average loading of 12–13 molecules of pheophorbide a to one diaminobutane poly-propylene-imine dendrimer molecule of the third generation (DAB dendrimer) having a total number of 16 binding sites. This was confirmed with various photophysical measurements and MALDI-mass spectrometry. As long as the dye molecules are covalently bound to the dendrimer, their photosensitized generation of singlet oxygen is reduced dramatically due to interactions between different dye molecules. The main interaction is supposed to be a Förster-energy transfer along the surface of the dendrimer–dye complex. Nevertheless the remaining photosensitized generated singlet oxygen leads to a destruction of the dendrimer backbone under illumination. The photophysical properties of the covalently linked dye–dendrimer complexes and especially the remarkable influence of illumination on these properties are presented in this paper.

Exciton localization hierarchy and directed energy transfer in conjugated linear aromatic chains and dendrimeric supermolecules

A unique class of phenylacetylene dendrimer supermolecules have been studied in terms of their electronic absorption and energy-transfer characteristics. Discrete exciton localization on isolated absorbing units within the dendrimer backbone is observed. Due to the branching structure, the vibrationless ground-state absorption is energetically pinned, showing complete independence of absorption energy with changes in molecular size. A simple linear-chain interacting oscillator model is used to calculate the band-edge energies of three isolated linear ring systems: poly(p-phenylenevinylene) (PPV). polytolans, and the dendrimer series. These calculations compare favorably with the experimental data, further illustrating the tight excitation localization.

Targeting Dendrimer-Chelates to Tumors and Tumor Cells Expressing the High-Affinity Folate Receptor

The authors developed a new method for delivering contrast agents to tumors and tumor cells. Gadolinium complexes of folate-conjugated dendrimer-chelates increased the longitudinal relaxation rate of tumor cells expressing the high-affinity folate receptor, hFR. The coupling of folate to polymeric chelates, composed of a dendrimer backbone, targets these chelates to endogenous folate binding proteins. These proteins exist in both the serum of patients with cancer and on the cell surface of many human cancers of epithelial origin. The authors attached folic acid to a generation four ammonia core polyamidoamine dendrimer. The folate-dendrimer was reacted with 2-(4-isothiocyanatobenzyl)-6-methyl-diethylenetriaminepentaacetic acid to form the polymeric chelate f-PAMAM-TU-DTPA. For fluorescent studies, the generation four dendrimer was reacted with fluorescein-5-isothiocyanate and carboxytetramethylrhodamine succinimidyl ester, followed by capping the remaining amines with succinic anhydride. The study results show that cells accumulate the folate-conjugated dendrimer in a receptor specific manner. Tumor cells expressing the high-affinity folate receptor showed a 650% increase in the mean fluorescence. This increase occurred with a rapid rise to 325%, followed by a slow increase to 650%. It required both the expression of the hFR and the coupling of folic acid to the dendrimer. Excess free folic acid inhibited the binding of the folate conjugated polymer. Fluorescent microscopic study showed that the folate-conjugated dendrimer binds to the cell surface and is accumulated within the cells. Treatment of tumor cells that express the hFR with gadolinium complexes of the folate-conjugated polymeric chelate increases the longitudinal relaxation rate by 110%. This increase was inhibited by an excess of free folic acid. These data demonstrate that folate-conjugated magnetic resonance imaging contrast agents represent a promising new approach to tumor targeting.

Directed Energy Transfer Funnels in Dendrimeric Antenna Supermolecules

We have studied the dynamics of directed, multistep energy transport in a class of fractal-like dendrimeric molecules. For particular forms of these highly branched phenylacetylene dendrimers, both theory and experiment put the lowest excitation energy at the center (locus) of the supermolecule. This results in a structurally symmetric and ordered exciton funnel, with a well-directed energy gradient. We have designed and synthesized a derivative of these dendrimers with a perylene moiety at the locus, which acts as an energy trap for the directed exciton funnel. Spectroscopic evidence indicates transfer efficiency of 98% from the photoabsorbing dendrimer backbone to the perylenic trap.