Declaration 2 Publications
2.2 CLASSIFICATION OF AMPHIPHILIC DENDRIMERS
2.3.1 Non-stimuli-responsive (NSR) self-assembling dendrimers
2.3.1.2 Janus dendrimers based NSR delivery systems
assay of the hybrid against porcine kidney epithelial cells (LLC-PK) demonstrated that the PAMAM-PCL-PEG amphiphilic molecule was nontoxic, and that the entrapped drug produced significantly better anticancer effect than the free drug (Wang et al., 2005). These types of core shell assemblies could be employed to non-covalently entrap the drug within the dendrimer structure. The advantage of these systems is that the release process is not chemical dependent, but purely dependent on ‘soft-bonds’, such as the ionic pairing, hydrogen and halogen bonds, which requires lower energy interactions and more subtle conditions, such as shifts in local equilibria, to release the drug (Jain and Asthana, 2007).
encapsulated the antimalarial drugs (CQ and PQ) and fluorescent dye, Rodamine B (rho B). High entrapment efficiency (%EE) was achieved (ca. 50 to 100) for both drugs at a drug/dendritic derivatives ratio of 5:1, with drugs %EE being highest in the Janus dendrimers A and B, whereas the %EE of rhoB was highest in dendritic polymer C and D (Movellan et al., 2014).
Fig 6. Structural representation of two Janus dendrimers (A, B) and two hybrid dendritic-linear- dendritic block copolymers (C and D) (Movellan et al., 2014). Scanning electron microscopy (SEM) studies showed ovoid or spherical morphologies for CQ and PQ encapsulated micelles with a mean long axes/diameters rangie of ca. 170 to 500 nm (Fig. 7). Reproduced with copyright permission from Elsevier
All the rhoB encapsulated nano-carriers were significantly smaller (100 to 300 nm) and more spherical compared to the antimalarial drugs encapsulated micelles. The in vitro activity against P. falciparum revealed intrinsic activity for all dendritic derivatives, and the cell targeting studies by fluorescence microscopy displayed selective targeting of these micelles to plasmodium infected RBCs compared to non-infected RBCs. These results can be accredited to the chemical interplay between the dendritic derivatives, the drug and elongated shape of the micelle (Movellan et al., 2014).
It has been reported that amphiphilic Janus dendrimers can self-assemble into fiber-like aggregates, which further arrange themselves into a supramolecular structure, as displayed in Fig.
8, and can trap water molecules to form hydrogels with outstanding mechanical properties. One such example is Janus dendrimers, which consists of G3 Bis-MPA hydrophilic dendron and alkyl gallate ether with varioius branching pattern [(3,4); (3, 5) and (3,4,5)] joined together via triazole linker (Fig. 9) (Nummelin et al., 2015).
Fig. 7. Scanning electron microscopy analysis of the dendritic derivatives encapsulating chloroquine, primaquine, and rhodamine B (Movellan et al., 2014). Reproduced with copyright permission from Elsevier.
Fig. 8. SEM images of vitrified cold dried 1.0% amphiphilic dendritic hydrogel, prepared from a) (3,4), b) (3,5), and c) (3,4,5). Scale bars 1 mm (Nummelin et al., 2015). Copyright (2015) John Wiley and Sons.
Fig. 9. a) Synthesis of amphiphilic Janus dendrimers through click chemistry, b) Schematic representation of hydrogel formation (Nummelin et al., 2015). Copyright (2015) John Wiley and Sons
The ethanolic injection of these Janus dendrimers into water caused self-assembly of the nanofibers with a thickness of 5-7 nm, which then bundled up and crosslinked with each other to form a three dimensional network (steps shown in Fig. 11b.), and further trap water molecules to form hydrogel. The authors further showed that these hydrogels were able to encapsulate a drug (nadalol), a dipeptide (gondoreline) and an active enzyme (horseradish peroxidase). Studies performed on these hydrogels showed that release of these cargo was by first order release kinetics, which was indicative of sustained release profile.
One important observation about self-assembly behavior of the Janus amphiphilic dendrimers is they can form uniform bilayered vesicular structure called dendrimersomes. Due to their unique properties, such as uniform size, tailored structure, higher vesicular stability, improved mechanical strength and easy functionalization, dendrimersomes are proposed to be more advantageous and stable nanostructures than liposomes and polymersomes (Percec et al., 2004). Percec et. al. studied the efficiency of these dendrimersomes to encapsulate drug molecules using the DOX as a model drug, and were formulated by a simple injection method, where the Janus dendrimer solution in ethanol was injected into water. The DOX was encapsulated in these structures with a film hydration technique, its release being found to be pH dependent due to the cleavage of the aromatic ester functional group present in the Janus dendrimers, which suggested their application for intracellular targeted delivery. Incorporating a pore forming protein (melittin) was also studied, with the results indicating that pore forming proteins could be successfully embedded in their bilayered structures, thus, confirming that the formed dendrimersomes mimicked the natural lipid bilayer cell membrane (Percec, Daniela A. Wilson, et al., 2010).
In another study, Zhang et al. reported the synthesis of isomeric amide containing ’Single-Single’
Janus dendrimers (SS-JD), with a single hydrophilic and a single hydrophobic group compared with twin dendrons in conventional Percec type Janus dendrimers (Percec et al., 2004; S. Zhang et al., 2014). The simple direct or reverse injection of a SS-JD solution into organic solvents, such as tetrahydrofuran, acetone, acetonitrile and 1, 4-dioxane to Milli-Q water or phosphate and HEPES buffer, produces onion-like dendrimersomes. By controlling the final concentration of SS-JD in the solution, the size and numbers of the onion-like dendrimersomes can be tailored. These onion- like dendrimersomes can encapsulate different hydrophilic and hydrophobic cargoes within multiple layers. Time dependent release pattern can be achieved by encapsulating them in different layers of these onion-like dendrimersomes and can thus be regarded as “peeling of one onion-layer at a time” (S. Zhang et al., 2014). Although the authors stated that “these onion-like dendrimersomes can be used to encapsulate both hydrophilic and hydrophobic cargoes”, there was no such cargo encapsulation study reported in the paper. Studies are needed regarding encapsulating drugs or drug-like molecules and evaluating their time dependent release pattern to pave the way for the introduction of onion-like dendrimersomes as smart carriers of both hydrophilic and lipophilic actives.
Table-1. Non-stimuli responsive self-assembling amphiphilic dendrimers as drug carriers.
Dendrimer Structures formed Drug/Payload Key Findings and/or Conclusions Reference
Amphiphilic layered dendrimers 3, 5-dihydroxybenzyl
alcohol based amphiphilic layered dendrimer with hydrophobic 3, 5- benzylic polyether core and
hydrophilic carboxylate end groups
Unimolecular micelles Pyrene Anthracene 1,4 diaminoan- thraquinon 2,3,6,7-
tetranitrofluorenone
• Enhanced solubility of pyrene, anthracene, 1,4-
diaminoanthraquinon, 2,3,6,7-tetranitrofluorenone by 120, 58, 56 and 258 -fold respectively compared to their water solubility.
• High solubilization was attributed to stabilizing π- π interactions between dendrimers and hydrophobic guest molecules.
(Hawker, Wooley and Frechet, 1993)
G3 & G4 poly(glycerol- succinic acid) based polyester amphiphilic layered dendrimers (PGLSA) with carboxylate end groups as hydrophilic corona
Unimolecular micelles Richard’s dye Camptothecins
• PGLSA dendrimers formed unimolecular micelles with average size of 7 nm.
• The hydrodynamic diameter decreased to 4 nm after encapsulating Richard’s dye.
• Solubility of Richard’s dye and camptothecins was enhanced by 2000 and 10 -folds respectively compared to their solubility in water.
(Morgan et al., 2003, 2006)
Folate functionalized amphiphilic dendrimer-like star polymer (DLSP) from polyester dendrons
Unimolecular micelles None • Unimolecular micelles with mean particle size of about 18 nm formed
• Increased cellular uptake of the folate- DLSP hybrid through overexpressed folate-receptor on KB cells
• Folate-DLSP hybrid showed potential as a carrier for targeted drug delivery.
(Cao et al., 2010)
Amphiphilic dendrimer-like star polymers (DLSPs)
Unimolecular micelles Doxorubicin (DOX) • Dendrimers had solubility of 10-25 mg/ml in water.
• Unimolecular micelles of 14-28 nm size and larger sized (205- 344 nm) aggregates were formed.
• DOX loading was found to be 11.5 wt%
• DLSPs showed potential as candidates for controlled delivery of hydrophobic drugs.
(Cao and Zhu, 2011a)
Folate functionalized amphiphilic dendrimer-like polymer
Micelles DOX • Unimolecular micelles with mean particle size of .15 nm
formed.
• DOX was released in a controlled sustained manner from the micelles.
• Unimolecular micelles could be promising nanosize anticancer drug carrier with excellent targeting property.
(Cao et al., 2011)
Janus dendrimers
11 distinct libraries of Janus dendrimers from six hydrophilic segments derived from oligoethylene oxide, dimethylolpropionic acid, glycerol, thioglycerol,
Various complex
architectures such as vesicles, cubosomes, dendrimersomes, tubular vesicles, disks and helical ribbons
DOX • Bilayer dendrimersomes with varying size range of 33 to 732 nm were formed by ethanolic injection of Janus dendrimers in water.
• Bilayer structure thickness of 5 to 8 nm that could be imbedded in pore-forming proteins.
• System had high stability for 244 days.
(Percec, Daniela A Wilson, et al., 2010)
tert butylcarbamate, and quaternary ammonium salts and with hydrophobic segments such as aliphatic and mixed aliphatic aromatic
• DOX was encapsulated in dendrimersomes using film hydration method.
• System had high stability, mechanical strength, uniformity of size of particles formed, and easy chemical functionalization of the structures.
Amphiphilic Janus Glycodendrimers with D- mannose and D-galactose hydrophilic groups and n- alkyl hydrophobic chains
Bilayered
glycodendrimersomes
None • Glycodendrimersomes with average size of 114 to 126 nm uniformly assembled.
• System offers possibility of lysine-mediated delivery of drugs, genes and imaging agents.
(Percec et al., 2013)
Single–single” amphiphilic Janus dendrimers with polyethylene glycol and aliphatic hydrophobic chains
Onion like
dendrimersomes
None • Formation of multi layered onion-like dendrimersomes with size of 99 to 169 nm, and narrow size distribution achieved.
• Transformation in number of layers was realized by changing concentration of Janus dendrimers.
• Structures could offer time dependent multi layered delivery systems with multiple cargo loading
(Shaodong Zhang et al., 2014)
Janus dendrimers with alkyl gallate
ether dendron as hydrophobic part and hydroxyl terminated bis-MPA as hydrophilic dendron.
Supramolecular hydrogels Nadolol Gonadorelin
Horseradish peroxidase
• Supramolecular ribbon-like hydrogel with outstanding mechanical strength formulated.
• Various drugs, active enzymes and peptide encapsulated in the hydrogel.
• Hydrogel offers the potential for a sustained drug release drug delivery system.
(Nummelin et al., 2015)
Bis-MPA based Janus dendrimer with amine groups as hydrophilic part and aliphatic hydrocarbon chains
Nanomicelles Chloroquine
Primaquine
• Spherical shaped nanomicelles with high entrapment efficiency for both chloroquine and primaquine were formulated.
• Nanmicelles were non-toxic towards mammalian cells.
• in vitro Studies showed intrinsic activity against P.
falciparum.
• Dendrimeric mediated transport of payloads was selectively achieved in plasmodium infected but not non-infected RBCs.
(Movellan et al., 2014)
Facially amphiphilic dendrimers Amphiphilic dendrimers
with repeating orthogonally placed biaryl units with hydrophilic (carboxylic acid) and hydrophobic (decyl chain) substituents and 3, 5- dihydroxy benzylic group as a backbone
Micelles in polar solvents and inverted micelles in non-polar solvents
Reichardt’s dye (pyridinium-N- phenoxide betaine) as hydrophobic guest, Proflavin dye as hydrophilic guest
• Formed micelles and inverted micelles depending on nature of solvent used: unimolecular micelles with average size of 2-4 nm in non-polar solvents; micellar aggregates with average size of 20-40 nm in polar solvents.
• Dendrimers ability to sequester guests was generation dependent.
• Formed structures were capable of encapsulating hydrophobic or hydrophilic guest molecules.
(Vutukuri, Basu and Thayumanavan, 2004)