Declaration 2 Publications
2.2 CLASSIFICATION OF AMPHIPHILIC DENDRIMERS
2.3.3. Low molecular weight dendritic amphiphiles in drug delivery
Fig. 15. Structure of dendritic amphiphiles for siRNA delivery obtained from (Malhotra et al., 2012) with permission from American Chemical Society.
The dendritic head group was grown divergently on azide focal point and then condensed to the hydrophobic chain by click chemistry. Thereafter, N-boc glycine was condensed to the hydroxyl terminal by using 4-(dimethylamino)pyridine and l-ethyl-3-[3-(dimethylamino) propyl]carbodiimide hydrochloride (EDCl), and ionized with the help of trifluro acetic acid (TFA) (Scheme. 2).
Scheme. 2. Synthesis of G2 octamine dendritic amphiphile redrawn from (Malhotra et al., 2012) with permission from American Chemical Society.
Physicochemical characterization of all these dendritic amphiphiles by determination of the CMC showed that all four amphiphiles aggregated in a low concentration range of 10 to 60 µM (Fig.
16). CMC values increased from G1 monoamine to G1 tetraamine due to an increase in positive charge that induced more repulsion in the head group of the amphiphiles. All these amphiphiles aggregated in the micelles, with the size range of 7 to 9 nm and a zeta potential values of 40-58 mV. The cationic glycine functional end groups complexed with the DNA and acted as an efficient gene delivery vehicle. Furthermore, the complexation ability of these amphiphiles with DNA was due to the combined effect of the hydrophobic alkyl chain and hydrophilic glycine molecules. This complexation results in the self-assembly of amphiphiles-DNA polyplexes of average size range of 69 to 306 nm. Cytotoxicity and siRNA transfection results confirmed the application of these dendritic amphiphiles as efficient vectors for siRNA transfection and cytotoxicity (Malhotra et al., 2012).
Fig. 16. Determination of CMC of glycine amphiphiles in 0.5 μM DPH (aqueous HEPES saline (pH 7.2, 9.4 mM NaCl) obtained from (Malhotra et al., 2012) with permission from American Chemical Society
Sikwal et al. also reported the synthesis of dendritic amphiphiles based glycerol monostearate (GMS-G2-OH, GMS-G3-OH) and glycerol monooleate derivatives (GMOA-G2-OH, GMOA-G3- OH). These amphiphiles were studied for their applications as both solubility enhancer and stearic stabilizers for nanoparticles (D.R. Sikwal et al., 2017). Biocompatible polyester dendritic amphiphiles were prepared using the OH terminated G2 and G3 Bis-MPA dendrons dendritic head group, and stearic acid or oleic acid as the hydrophobic tail. The synthesis of these dendritic amphiphiles involved simple protection and deprotection steps, where the hydrophilic dendrons were grown divergently with acetonide protected Bis-MPA on a focal point of glycerol monostearate or monooleate. Deprotection of the acetonide group was carried out by using acidic Dowex® H+ resin to produce a focal point for the next generation.
Cytotoxicity studies against MCF 7, Hep G2 and A549 human carcinoma cells using MTT assay indicated a biocompatibility (70% cell viability) for all derivatives and further studies showed that the amphiphiles had HLB values that were greater than 15. This confirmed that the synthesized dendritic amphiphiles could be good solubilizers, as shown by the higher solubilization of the hydrophobic fusidic acid when compared to water and Pluronic F-68. Aggregation behavior showed the formation of ultra-small micelles, with an average size range of 6.48-12.38 nm and narrow polydispersity indices. The reason for higher solubilization was attributed to the spatial arrangement of these dendritic amphiphiles, and chain kink confirmation of unsaturated bond in oleic acid. In-vitro antibacterial activity proved that the GMOA-G2-OH micellar carrier also enhanced the antibacterial potency of fusidic acid.
To prove the stearic stabilizing property of these dendritic amphiphiles, solid lipid nanoparticles (SLNs) were formulated using Compritol 888 ATO as solid lipids, and amphiphiles as surfactants.
The results from the formulation studies proved that dendritic amphiphiles were capable of providing sufficient stability to the SLNs, as confirmed by an optimum particle size in the range of 133 to 291 nm, narrow polydispersity index values (0.296 to 0.485) and higher zeta potential values (-22.33 to -34.10). Overall results of this study indicated that these dendritic amphiphiles could be promising excipients for pharmaceutical application (Dhiraj R. Sikwal et al., 2017).
Table 3. Summary of low molecular weight dendritic amphiphiles based drug delivery systems.
Purpose Structural components of dendritic amphiphile
Payload Important finding Reference
Micellar delivery Non-ionic lower molecular weight dendritic amphiphiles with hydrophilic polyglycerol dendrons (G1-G3) and hydrophobic C11 or C16 alkyl chains joined together by mono or biaryl spacer
Nile red Pyrene
• Dendritic head group influenced supramolecular self-assembly and aggregation number while biaryl spacer influenced transport capacity
• G1 amphiphile formed ring-like or worm-like micelles, G2 and G3 amphiphiles formed spherical micelles with average size of 8 nm.
• High entrapment of hydrophobic molecules was attributed to and 74% of empty space in the micelles composed of 15 amphiphilic molecules.
• Lower critical micellar concentration (CMC) was recorded.
(Trappmann et al., 2010)
Solubilizing agents Glycerol (G2) based amphiphiles with different hydrophobic moieties (C18 chain, C18 chain with naphthyl and biaryl end groups) with single or double aromatic linkers.
Sagopilone • All amphiphiles formed ultra-small micellar structures with size range 7 to 10 nm and polydispersity index of 0.04 to0.2.
• All amphiphilic structure did not show any cytotoxicity up to concentration of 0.01g/ml after 24 h.
• The amphiphiles showed 2 to 3 -fold higher solubilization of sagopilone and greater stabilization of micellar structures than Cremophor® ELP and polysorbate-80.
• G2 amphiphile with diaromatic spacer and C18 chain showed highest solubilization capacity.
(Trappmann et al., 2010)
Gene delivery Oligoglycerol based dendritic amphiphiles with glycine terminals as hydrophilic part and stearic acid as hydrophobic chain.
siRNA/NDA • Amphiphiles formed micellar aggregates in size range of 7-9 nm and zeta potential of 40-58 mV.
• Amphiphiles were efficient vectors for siRNA transfection and cytotoxicity.
• First time in vitro siRNA transfection was achieved using dendritic amphiphiles.
(Malhotra et al., 2012)
Pharmaceutical excipients
Polyester dendritic amphiphiles with G2 and G3 Bis-MPA based dendritic head group and stearic acid and oleic acid as hydrophobic tail.
Fusidic acid Vancomycin
• Biocompatible amphiphilic dendrimers with low CMC values that self-assembled into ultra-small micellar aggregates.
• Micellization of fusidic acid through these amphiphiles enhanced solubility of fusidic acid.
• Amphiphiles also acted as good stearic stabilizers for solid lipid nanoparticles (SLNs) formulations.
(Dhiraj R.
Sikwal et al., 2017)
Photoresponsive delivery
Photoswichable non-ionic dendritic amphiphiles with G2, G3 glycerol based dendrons and C11, C16 tail connected together with dibenzodiazepine connector.
Nile Red • All dendritic amphiphiles formed spherical micelles with size range of 7.2-10.2 nm.
• Amphiphiles undergo trans-cis photoisomerization under UV/Visible leading to disassembly of micelles.
• CMC of these amphiphiles changed effectively under influence of light, rendering structures photoresponsive.
(Kördel, Popeney and Haag, 2011)
Experimental paper Chapter 3