Pharmaceutical Application of Dendrimers
Dendrimers Drug Delivery: Targeted and Controlled Release Drug Delivery
The high accuracy of control over the drugs distribution is highly effective to eliminate the typical disadvantages of conventional medicine. For this reason, a drug delivery system should be developed to reach the target site preferentially. In Recently improvement of bioavailability, biodistribution and sustained release of the drug molecule to the particular targeted site has been accomplished with polymer controlled drug delivery. Unlike conventional polymers, dendrimers have achieved considerable application in biological and pharmaceutical field due to their high water solubility, precise molecular weight biocompatibility, and polyvalency .
For investigating the use of dendrimers as drug delivery, their bio-permeable behavior across the biological membranes should be explored. Kitchens et al. described cationic PAMAM-NH 2 (G0-G4) dendrimers and found its permeability across the monolayers of Caco-2 cell as a function of generation of dendrimer, concentration and incubation time. Various parameters like Trans epithelial electrical resistance, 14 C-mannitol permeability and leakage of lactate dehydrogenase enzyme were studied and it was suggested that these amine terminated PAMAM dendrimers might cross the biological membranes possibly by endocytosis pathways. In addition, by optimizing the surface charge and size, these dendritic polymers can be developed into oral delivery systems.
Different classes of drugs can be incorporated in these versatile dendrimers carriers. Recently, a dendrimer based prodrug has been formulated for the taxol drug that has improved permeability and transportation of drug across cellular barriers. The highly functional lauryl-modified PAMAM dendrimer-paclitaxel conjugates showed a great stability under different physiological conditions and enhance greater permeability across Caco-2 cell and porcine brain endothelial cells monolayers than paclitaxel alone.
Fig 1: Active Cellular targeting of Dendrimer molecule
The targeted delivery of anticancer drug can reduce the side effects of healthy tissues such as liver, spleen, kidneys and bone marrow significantly associated with traditional therapy. The site specific delivery of the drug could be accomplished by surface modification of dendrimers employing various targeting ligands such as folic acid (FA), peptides, monoclonal antibodies and sugar groups. There are many successful active and passive targeting attempts were achieved by modifying the branching units and surface groups of dendrimers. Patri et al described the conjugation of FA to G5 PAMAM dendrimer for the targeted delivery of the methotrexate and found receptor mediated drug delivery which showed high specificity for KB cells overexpressing folate receptors.
Dendrimer as Solubility Enhancers
Several drugs in pharmaceutical application have good therapeutic activity but due to lack of solubility in suitable solvents, they have not been formulated for therapeutic purposes. Water soluble dendrimers can be utilized for solubilizing small acidic hydrophobic molecules with cer,anticanantifungal or antibacterial properties. Dendrimers have a hydrophobic core and a hydrophilic surface layer which will form unimolecular micelles. The important feature of dendrimer molecule is that they do not have a critical micelle concentration. This property provide extra opportunity to soluble poorly soluble drugs by using encapsulation technique within the dendritic structure at all concentrations of dendrimer. PAMAM interior may have a hydrophilic-hydrophobic core-shell and an exterior long alkane chain which can be used to bind water-soluble anti-tumor drug 5-flurouracil. oral bioavailability of 5-flurouracil can be increased twice by using phospholipid coating of the dendrimer-fatty- acid macromolecule. The poor soluble drug can be modified with the dendrimer-based carriers to enhance the oral bioavailability of problematic drugs. G3 PAMAM dendrimer can be conjugated with Propranolol to enhance the solubility of the drug by over two orders of magnitude. For this reasons, dendrimer Nano carriers explorer the opportunity l to enhance the bioavailability of drugs that are poorly soluble and poorly absorbable.
Cellular Delivery Using Dendrimer Carriers
PAMAM dendrimers (G4-NH2, G3-NH2, G4-OH, PEGlayted G3 [G3-PEG]) and a hyper branched polymer (polyol) can be explored to make the molecules effective to produce an cellular entry into A549 human lung epithelial carcinoma cellsThe through investigative reasearcg revealed that the fast entry of G4-NH2 could be due to the amine surface groups which possess cationic nature. This may conjugate electrostatically with epithelial cells which creates negatively charged atomosphere .As a result the molecule will enter pinocytosis.. Dendrimer-ibuprofen complexes can be formulated to make an entry of the cells rapidly compared with pure drug (1 hr versus>3 hr). This will suggest that dendrimers can efficiently carry conjugated and complexed drug inside cells. surface-engineereing or urgace modification of PAMAM dendrimers with lauryl were chainscan reduce toxicity and as well enhance cellular uptake.
Fig 2: Electrostatic interaction of dendrimer molecules with cells
Dendrimers as Nano-Drugs
Poly(lysine) dendrimers modified with sulfonated naphthyl groups have been shown to be effective as antiviral drugs against the herpes simplex virus can potentially prevent/reduce transmission of HIV and other sexually transmitted diseases (STDs).In previous studies, it was investigated that PAMAM dendrimers covalently conjugated with naphthyl sulfonate residue molecules on the surface and exhibited antiviral activity against HIV infection. This dendrimer-based nano-drug also inhibited virus/cell adsorption in their early stage and aswellas later stage viral replication by interfering with reverse transcriptase and integrase enzyme activities. PPI based dendrimers conjugated with tertiary alkyl ammonium groups attached to the surface have been found to be effective antibacterial biocides against Gram positive and Gram negative bacteria. Poly (lysine) dendrimers with mannosyl surface groups are potent inhibitors of the adhesion of E. coli to horse blood cells in a haemagglutination assay, making these combination promising antibacterial agents. Chitosan-dendrimer hybrids have been found to be effective as antibacterial agents, carriers in drug delivery systems, and in other biomedical applications.
Dendrimers in Photodynamic Therapy
5-aminolevulinic acid based dendrimer which is used as the photosensitizer has been attached to the surface of the dendrimers and is used as a PDT agent for of tumorigenic keratinocytes .Photo sensitive dyes can be also incorporated into dendrimers and utilized in PDT devices. Light- activated photosensitizing drug can be used for the cancer treatment after the administration of that selectively concentrates in diseased tissue. Appropriate un-functionalization of their periphery molecule improves the properties of dendrimers through dendrimers promising carriers for photosensitizers. ALA is a natural precursor agent of the photosensitizer protoporphyrin IX , and its accumulation is known to increase cellular concentrations of protoporphyrin .
Fig 3: Dendrimer in formation of photosensitizing complex drug
Dendrimers in Gene delivery
Dendrimer based Gene therapy has garnered a huge attentions over the latter two decades since it is capable of provides promising method for treating cancers as well as genetic disorders. In particular, RNAi based dendrimer become more popular for gene targeting and gene silencing both in research and as clinical therapeutic due to its high efficacy and specificity. Although, identification of safe vectors to deliver siRNA for successful RNA entrapment in body tissues remains the major challenge, both in vitro and in vivo. Considering the safety issues and other limitation of viral approach, the nonviral vectors for siRNA delivery are increasingly replacing the alternatives of viral ones. Among nonconventional non-viral vectors, dendrimer-based siRNA carriers have been gradually ong the scientists become popular am since their performance in DNA delivery was approved. Literature survey finds that, there are lots of dendritic molecules developed to deliver siRNA, while PAMAM dendrimers are most used dendrimer due to the advantages of synthesis and modification and the commercial availability. There are basically three kind to optimize the dendrimer structure for lowering the cytotoxicity as well as improving the delivering efficiency: (1) Formulation of new dendritic structure or explore new core unit for dendrimer preparation, (2) synthesis of functional derivatives of the interior or exterior part of dendrimer compounds, and (3) Complexing and conjugating of other biocompatible/bioactive molecules to form effective complexes with dendrimers. As different cell lines have different approach for the delivery system, scientists should have a careful approach, when studying dendrimer-based siRNA delivery, to choose the proper dendrimer structure, the optimizing method and the delivery condition depending on the type of targeting gene as well as the cells to be transferred. Recently, many scientists are working on dendrimers for efficient delivery of siRNA. With a combination of research data from extensive work and the experience in commercializing DNA delivery reagent, we can envisage the prospective of dendrimer-base practical product for siRNA delivery in future.
The different step starting from the dendrimer -gene conjugation is shown in
Fig4.Targeting of gene via dendrimer conjugation
Dendrimers can act as vectors, in gene therapy. PAMAM dendrimers have been tested as genetic material carriers. Numerous reports have been published describing the use of amino-terminated PAMAM or PPI dendrimers as non-viral gene transfer agents, enhancing the transfection of DNA by endocytosis and, ultimately, into the cell nucleus . A transfection reagent called SuperFect TM consisting of activated dendrimers is commercially available. Activated dendrimers can carry a larger amount of genetic material than viruses. SuperFect-DNA complexes are characterized by high stability and provide more efficient transport of DNA into the nucleus than liposomes. The high transfection efficiency of dendrimers may not only be due to their well-defined shape but may also be caused by the low pK of the amines (3.9 and 6.9). The low pK permit the dendrimer to buffer the pH change in the endosome compartment. PAMAM dendrimers functionalized with cyclodextrin showed luciferase gene expression about 100 times higher than for unfunctionalized PAMAM or for non-covalent mixtures of PAMAM and cyclodextrin . It should be noted that dendrimers of high structural flexibility and partially degraded high-generation dendrimers (i.e., hyper branched architectures) appear to be better suited for certain gene delivery operations than intact high-generation symmetrical Dendrimers.
Figure 5: Dendrimer involved in gene transfection
Paramagnetic metal chelates such as Gd(III)- N,N',N'',N"'-tetracarboxymethyl-1,4,7,10-tetraazacy-clododecane (Gd(III)-DOTA), Gd(III)-diethylenetri- amine pentaacetic acid (Gd(III)-DTPA), and their derivatives used as contrast agents for magnetic resonance imaging (MRI) (14) . The (Gd(III)-DTPA) conjugate (MagnevistR) (Schering AG) and is a widely used MRI contrast agent. In another approach, the conjugation of (Gd(III)-DOTA) to poly(l-glutamic acid) (molecular weight 50 kDa) via the biodegradable disulfide spacer cystamine was studied to find a safe and effective macromolecular MRI contrast agent. Consequently, dendrimer-based Gd(III) chelates consisting of generations 2 and 6 PAMAM dendrimers with 12 and 192 terminal surface amines conjugated to the chelating ligand 2-(4-isothiocyanatobenzyl)-6- ethyldiethylenetriamine- pentaacetic acid through a thiourea linkage were synthesized .These contrast agents exhibited excellent MRI images of blood vessels upon intravenous injection. These dendrimer polychelates were exploited for high-quality MR angiography (MRA) images up to 60 min post injection. DNA-dendrimers, are constructed for routine, use in high-throughput functional genomic analysis, and as biosensors for the rapid diagnosis have genetic, and pathogenesis. Radiolabelled monoclonal antibodies with high specific activity have been Prepared by attachment of PAMAM dendrimers loaded with111In or153Gd complexes In vivo oxygen imaging is a strategy that offers the potential for diagnosing complications from diabetes and peripheral vascular diseases, as well as the detection of tumors and the design of their therapeutic treatment
Dendritic Catalysts / Enzymes
The combination of high surface area and high solubility makes dendrimers useful as nanoscale catalysts. Dendrimers have a multifunctional surface and all catalytic sites are always exposed towards the reaction mixture. They can be recovered from the reaction mixture by easy ultra-filtration technology .Dendritic core can be utilized to form a microenvironment sitable for catalysis or make shielding effects for functional groups at the dendritic core. Due to their 'pseudo'-spherical nature and their resultant conformations the metal sites in these well-defined polymeric catalysts should be easily accessible for substrate molecules and reagents, and therefore show characteristics- fast kinetics, specificity and solubility .
2. Catalysis with phosphine-based dendrimers
3.Catalysis with (metallo)dendrimers containing chiral ligands
4.Non-metal based dendrimers
Dendrimers can incorporate insoluble materials, such as metals, and transport them into a solvent within their interior core. Cooper and co-workers produced fluorinated dendrimers, which are highly soluble in supercritical CO2 and can be used to extract strongly hydrophilic compounds from water into liquid CO2. This may help develop Technologies in which hazardous organic solvents are replaced by liquid CO2.
Figure 6: Dendrimer used as transmitting catalyst