Declaration 2 Publications
2.2 CLASSIFICATION OF AMPHIPHILIC DENDRIMERS
2.3.2 Stimuli-responsive (SR) self-assembling dendrimers
2.3.2.3 Facially amphiphilic dendrimer based SR delivery systems
Application of Discussions involving facially amphiphilic dendrimer based SR delivery systems will be discussed in this subsection. Overexpression of proteins and enzymes has been frequently implicated in the diseased state of cells. Disruption of hydrophilic-lipophilic balance, using an external stimulus, could lead to disassembly of the aggregates, which can be utilized to cause an actuation event, such as guest molecule release (Raghupathi et al., 2014). Azagarsamy et. al.
demonstrated enzyme-induced disassembly of amphiphilic nanocontainers based on dendrimers, with the system consisting of biaryl dendrimers composed of a hexyl ester as the lipophilic unit and PEG as a hydrophilic unit. The enzyme-induced disassembly tested using porcine liver esterase revealed that there was a systematic decrease with time in the size of the self-assembled nanoparticles (Azagarsamy et al., 2009).
In another study, enzyme responsive facially amphiphilic dendritic systems consisting of (i) a polyglycerol dendrimer core, (ii) a dipeptide Phe–Lys attached self-immolative enzyme responsive para amino benzyloxy carbonyl group, and (iii) the tetrapeptide Ala–Phe–Lys–Lys, to which either doxorubicin and methotrexate were attached. Size-exclusion chromatograms, after incubation with the cathepsin B enzyme, showed the individual mass of the conjugated drugs, indicating an effective release of the drugs after cleavage by the enzyme. The dendritic drug conjugates appeared to be biosafe after being evaluated on human tumor cell lines MDA-MB-231 and AsPCl (Calderón et al., 2009). In the above studies, the efficacy and safety of these enzyme responsive systems were mostly performed using in vitro study models. However, studies on enzyme triggered release and pharmacokinetic evaluation in animal models will add value and open the door for further exploitation of this system, due to their ability to address difficulties in managing disease conditions and offer more efficient ways to deliver drugs.
Advancements in synthetic chemistry, and the development of technology for analysis and chemical characterization, have led to the design of elaborate dendritic systems that can respond to various disease biomarkers, including reactive oxygen species (ROS), which are often elevated in cancer cells. Using this biomarker in the pathogenesis of cancer cells, Fernandes, and Malkoch
synthesized a family of dendrimers with internally queued disulfide bridges that selectively rupture into a set of monomeric mercaptans in the presence of ROS. Their composition was dictated by three dendritic regions: (i) the symmetrical trithiol of the core (C3), (ii) the interior-asymmetric trithiols (CD2), and (iii) the periphery-asymmetric monothiols (DB2) (Fig. 14). In the dendritic system, sulfide bridges were specifically selected as they can undergo selective redox cleavage in a single step, and are involved in biological functions, such as the thioredoxin or glutaredoxin redox systems. To prove their concept of disassembly, the synthesized multi-stimuli responsive amphiphilic layered dendrimers-were evaluated in human lung carcinoma A549 cells to establish the effect of the ROS. Analysis by MALDI-TOF-MS showed that the mass fragmented dendrimers building blocks were isolated after incubation, and that there was a significant increase of ROS inside the cancer cells exposed to the dendrimers (Andrén et al., 2017). These dendrimer scaffolds can be considered as next generation precision polymers in the field of nanomedicines.
Fig 14. Synthetic strategy of rupturing amphiphilic dendrimers from (Andrén et al., 2017) and with permission from American Chemical Society.
Table 2 Stimuli-responsive self-assembling dendrimers for drug delivery.
Dendrimer Stimuli Drug/model molecule Main findings Reference
Amphiphilic layered dendrimers Poly (propy1eneimine) (PPI) dendrimers with 64 amino ends protected with t- butoxycarbonyl (t BOC)- protected phenylalanine groups to afford a dendrimer with a “sterically closed” shell
pH Rose Bengal • Box-like dendrimer stearic entrapment of hydrophobic guest molecule witnessed.
• t-butoxycarbonyl (t- BOC)-protected phenylalanine attached dendrimers formed sterically closed shell, while acid hydrolysis of amine end groups regenerated open-shell form after exposure to formic acid.
• Sterically mediated release mechanism provided unique method for stimuli responsive and controlled drug delivery.
(Jansen, Meijer and de Brabander-van den Berg, 1995)
Hyperbranched poly(ethylene imine) cores and different shells which contain aliphatic chains and poly(ethylene glycol) chains
pH Congo red • Higher capacities for polar dyes and drugs to be encapsulated and extracted from dendrimer observed.
• pH labile shells were cleaved in 5–6 pH environments.
(Xu, Krämer and Haag, 2006)
Amphiphilic layered dendrimers (DAB-PN) with hydrophobic diaminobutane poly(propyleneimine) core and with hydrophilic polyphosphazene outer segments
Salts (Sodium chloride)
Pyrene • Dendrimers formed unimolecular micelles and sequestered hydrophobic pyrene molecules within nonpolar interiors.
• Increase in pyrene solubility was observed as generations of DAB- PN increased.
• Cationization of ethyleneoxy moieties by Na+ ions increased charge density causing expansion of ethyleneoxy coils, resulting in release of entrapped cargo.
• Enhanced solubility and stimuli-dependent controlled release of hydrophobic molecules observed.
(Cho and Allcock, 2007)
Janus dendrimers Polyester dendrimer functionalized with acetal shells (JDs)
pH DOX • Hydrolysis of acetal function at acidic pH resulted in release of entrapped DOX.
• Hydrolysis at acidic pH caused disruption of the micelles and larger aggregates due to rearrangement.
• Localization of DOX in intracellular organelles achieved.
(Gillies and Fréchet, 2005)
Janus dendrimers prepared by coupling of G3
PAMAM dendron containing diazonaphthoquinone (DNQ) end groups and PAMAM dendron decorated with lactose groups.
Light [ultra violet light (UV) and near infra- red light (NIR)]
DOX • DOX loaded micelle with average size of 59 to 70 nm and loading capacity of 8.3 to 15.6 wt% formed.
• Wolff rearrangement of hydrophobic DNQ ends to hydrophilic 3- indencarboxylic acid due to NIR and UV light resulted in destabilization of micellar structure and faster drug release.
• Faster DOX release was observed after irradiation of micelles for 30 min with an 808 nm laser or 365 nm high pressure mercury lamp.
• On-demand spatiotemporal delivery achieved for anticancer drug.
(Sun et al., 2012)
Janus dendrimers consisting of G2 hydrophilic PAMAM dendron and two hydrophobic C18 alkyl chains bridged together via click chemistry.
pH DOX • Ultra-small micelles with average size of 10 nm with narrow PDI
and DOX loading up to 42% formulated.
• High drug loading was attributed to large void spaces within inner cores of the micellar structure.
• pH dependent DOX release observed.
(Wei et al., 2015)
mPEG-b-PAMAM-DOX Amphiphilic Janus dendrimer consisting of
methoxypoly(ethylene glycol) (MPEG)-b poly(amidoamine) (PAMAM)-DOX prodrug
pH 10-hydroxycamptothecin
(HCPT) and DOX
• Janus amphiphilic dendrimers with mPEG and PAMAM dendritic polymer attached to DOX through pH liable hydrazone linkages synthesized.
• Self-assembly to formed nano-aggregates with size range of 49.0±5.4 and 59.1±7.8nm.
• Encapsulation of HCPT was 19.2 to 21.6% and of DOX was 22.0 to 41.2%.
• Degradation of hydrazone linkage occurred within acidic rage (pH 4.5 to 5.5), releasing both drugs concurrently.
• Co-delivery systems with pH responsive controlled release and enhanced anticancer activity drug delivery system was reported.
(Zhang et al., 2013)
Janus type amphiphilic linear dendritic block copolymer, semi polyamidoamine-b poly(d,l-lactic acid) (PALA)
pH Docetaxel (DTX) • pH faster release of DTX loaded micelles in acidic microenvironment was observed.
• In vivo studied in Sprague-Dawley (SD) rats model showed increased AUC and prolonged clearance of DTX compared to conventional DTX
(Qiao et al., 2013)
Polyester dendrimers UV light Nile red and Fluorescein
• polyester dendrimers self-assembled to form dendrimersomes
• UV light triggered release of both hydrophilic and hydrophobic payloads from the system.
(Nazemi and Gillies, 2014)
PAMAM-co oligo(ethylene glycol) (PAG)
Temperature Methotrexate • Unimolecular and multimolecular aggregates with particle sizes of 8 and 200 nm were achieved.
(Guo et al., 2014)
• PAG exhibited only 10% release after 8 h at 37oC, while at 48oC faster release of 55% 1h observed.
Amphiphilic peptide dendrimer
Enzyme (Papain)
DOX • DOX conjugated mPEGylated dendron with
glycylphenylalanylleucylglycine tetra-peptide (GFLG) as enzyme sensitive linker was synthesized.
• mPEGylated-GFLG-DOX dendritic conjugate self-assembled into nanoparticles with average size of 80 nm.
• Incubation with papain triggered 50% release of DOX after 6h.
• System showed effective killing of cancer cells in vitro when compared to conventional DOX.
• No significant side effects to normal organs that amphiphilic dendrimer was exposed to.
(Li et al., 2014)
Facially amphiphilic dendrimers Facially amphiphilic biaryl dendrimers with hexyl ester as hydrophobic moiety and pentaethylene glycol as hydrophilic group.
Enzyme [porcine liver esterase (PLE)]
Pyrene • Dendrimer-based amphiphilic assemblies with 100 nm size that could noncovalently sequester hydrophobic guest molecules were formulated.
• Hydrolysis of ester moieties in hydrophobic part in presence of PLE lead to destabilization and subsequent cargo release.
(Azagarsamy, Sokkalingam and Thayumanavan, 2009)
Amphiphilic pentaethylene glycol unit and coumarin derivative based dendrimer
Enzyme (PLE)
1,1’-dioctadecyl-3,3,3’3’- tetramethylindo
carbocyanine perchlorate
• PLE enzyme triggered the release of the lipophilic fluorophore from the dendritic backbone.
(Raghupathi, Azagarsamy and Thayumanavan, 2011)