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DENDRIMERS By T.Saritha M.Pharm (II-sem.) Pharmaceutics U.C.P.SC, K.U. OUTLINE Historical background Introduction Structure&components of a dendrimer Types of dendrimers Synthesis of dendrimers Properties of dendrimers Methods for characterization of dendrimers Applications Conclusion References HISTORY First discovered in the early 1980’s by Donald Tomalia & coworkers The term dendrimers originates from the two Greek words dendron meaning tree & meros meaning part & relates to the symmetrical branch-like structure of these polymers At the same time Newkome's group prepared similar macromolecules & called them arborols Arborols stems from the latin term for tree The term cascade molecule is also used INTRODUCTION Dendrimers are a novel class of spheroid/globular nanoscaled macromolecules Characterized by highly branched tree like structers that provides a high degree of surface functionality &versatility Dendrimers are also referred to as the polymers of 21st century Due to their multivalent & monodisperse character,dendrimers have stimulated wide interest in the field of chemistry & biology, especially in applications like drug delivery, gene therapy & chemotherapy DEVELOPMENT OF DENDRIMERS STRUCTURE&COMPONENTS OF A DENDRIMER A typical dendrimer is comprised of 3 different parts a focal core Building blocks with several interior layers (generations) composed of repeating units, radically attached to the interior core Multiple peripheral functional groups (end groups/terminal groups) attached to the outermost interior generations DENDRIMER STRUCTURE TYPES OF DENDRIMERS Pamam Dendrimer Poly (amido amine) dendrimers (PAMAM) are synthesized by the divergent method starting from ammonia or ethylenediamine initiator core reagents. PAMAM dendrimers are commercially available, usually as methanol solutions. Starburst dendrimers is applied as a trademark name for a subclass of PAMAM dendrimers based on a tris-aminoethyleneimine core. The name refers to the star like pattern observed when looking at the structure of the high-generation dendrimers of this type in two-dimensions. Pamamos Dendrimer Radially layered poly(amidoamine-organosilicon) dendrimers (PAMAMOS) are inverted unimolecular micelles that consist of hydrophilic, nucleophilic polyamidoamine (PAMAM) interiors and hydrophobic organosilicon (OS) exteriors. These dendrimers are exceptionally useful precursors for the preparation of honeycomb-like networks with nanoscopic PAMAM and OS domains. PPI Dendrimer PPI-dendrimers stand for “Poly (Propylene Imine)” These dendrimers are generally poly-alkyl amines having primary amines as end groups, the dendrimer interior consists of numerous of tertiary tris-propylene amines. PPI dendrimers are commercially available up to G5, and has found widespread applications in material science as well as in biology. As an alternative name to PPI, POPAM is sometimes used to describe this class of dendrimers. POPAM stands for Poly (Propylene Amine) In addition, these dendrimers are also sometimes denoted “DAB-dendrimers” where DAB refers to the core structure, which is usually based on Diamino butane. Multilingual Dendrimers In these dendrimers, the surface contains multiple copies of a particular functional group. Hybrid Dendrimers Linear Polymers These are hybrids (block or graft polymers) of dendritic and linear polymers. Amphiphilic Dendrimers They are built with two segregated sites of chain end, one half is electron donating and the other half is electron withdrawing. Micellar Dendrimers These are unimolecular micelles of water soluble hyper branched polyphenylenes Multiple Antigen Peptide Dendrimers It is a dendron-like molecular construct based upon a polylysine skeleton. Lysine with its alkyl amino side-chain serves as a good monomer for the introduction of numerous of branching points. It has predominantly found its use in biological applications, e.g. vaccine and diagnostic research. Fréchet-Type Dendrimers It is a more recent type of dendrimer developed by Hawker and Fréchet,based on poly-benzyl ether hyper branched skeleton These dendrimers usually have carboxylic acid groups as surface groups, serving as a good anchoring point for further surface functionalisation, and as polar surface groups to increase the solubility of this hydrophobic dendrimer type in polar solvents or aqueous media. Tecto dendrimer These are composed of a core dendrimer, surrounded by dendrimers of several steps (each type design) to perform a function necessary for a s therapeutic nanodevice. Different compounds perform varied functions ranging from diseased cell recognition, diagnosis of disease state drug delivery. DENDIMER SYNTHESIS The synthesis used for dendrimer preparation permit almost entire control over the critical molecular design parameters such as size, shape, surface/interior chemistry, flexibility, and topology. Many dendrimer synthesis rely upon traditional reactions, such as the Michael reaction or the Williamson ether synthesis whilst others involve the use of modern techniques and chemistry, such as solid-phase synthesis, organo-transition-metal chemistry, organosilicon chemistry, organo-phosphorus chemistry. Two major synthetic strategies used are Divergent method Convergent method DIVERGENT METHOD Proposed by Tomalia & newkomes in the early 1980s Initiates growth at core Expands from in to out The first synthesized dendrimers were polyamidoamines (PAMAMs).They are also known as starbust dendrimers .Ammonia is used as the core molecule & In the presence of methanol, it reacts with methylacrylate and then ethylenediamine is added NH3 + 3CH2CHCOOCH3 N(CH2 CH2COOCH3)3 3 NH2 CH2 CH2NH2 N(CH2 CH2CONHCH2 CH2NH2)3 + 3CH3OH At the end of each branch there is a free amino group that can react with 2 methyl acrylate monomers and 2 ethylenediamine molecules. Each complete reaction sequence results in a new dendrimer generation. The number of reactive surface sites is doubled with every generation the mass increases more than twice Generation mw measured diameter surface groups 0 517 1.5 4 1 1430 2.2 8 2 3256 2.9 16 3 6909 3.6 32 4 14215 4.5 64 CONVERGENT METHOD Proposed by Hawker & Frechet in 1989 Initiates growth at exterior Progress inward with coupling reactions After enough couplings, the dendron can be attached to a poly functional core Advantages Divergent Production of large quantities Achieve large molecular dendrimers Convergent Easy to purify desired product Occurrence of defects is minimized Possible to introduce subtle engineering Disadvantages Divergent Large excess of reagents Difficulties in purification Possible incomplete reactions with terminal groups Incomplete reactions increases exponentially Convergent Does not allow the formation of high generation dendrimer because steric problems occur in the reactions of the dendrons and the core molecule Properties of dendrimers Because of their molecular architecture,dendrimers show some significantly improved physical and chemical properties when compared to traditional linear polymers. Well defined monodisperse macromolecules (compare polymers) &have a uniform molecular weight. Nanoscale objects with a surface & interior. The classical polymerization process which results in linear polymers is usually random in nature &produces molecules of different sizes, whereas size &molecular mass of dendrimers can be specially controlled during synthesis. Large dendrimers can host small molecules/nano particles but don’t have an cmc. The solubility& reactivity of dendrimer is strongly influenced by the nature of surface groups. Dendrimers terminated in hydrophilic groups are soluble in polar solvents. Dendrimers having hydrophobic end groups are soluble in non-polar solvents. Dendrimer solutions have significantly lower viscosity than linear polymers because entanglement or interpenetration of dendrimers is unfavorable due to their densely packed surface. In large dendrimers,the surface is highly congested,where as a substantial amount of free space is encapsulated in the interior part, which allows for a wide range of applications such as site specific pockets for the accommodation of a variety of guest molecules. Biological properties Biological properties of dendrimers are crucial because of the growing interest in using them in biomedical applications. Size is a key determinant of dendrimer cytotoxicity for both PAMAM and PPI dendrimers. Cytotoxicity of PAMAM dendrimers increases with generation for both full generation cationic dendrimers (G2–G4) and the “half-generation” anionic intermediates (G2.5, G3.5). The nature and density of charged groups are other factors that determine dendrimer toxicity Cationic (surface) charges are in general more toxic but details depend on the specific groups involved, that is, for amines it has been proposed that primary amines are relatively more toxic than secondary or tertiary amines. A concentration dependent tendency to cause haemolysis and changes in erythrocyte morphology has been linked to the presence of –NH2 groups Anionic dendrimers,bearing a carboxylate surface,are not cytotoxic over a broad concentration range Properties of dendrimers and linear polymers structure Dr-compact,globular synthesis Dr –careful &stepwise growth Lr -noncompact Lr –single step poly condensation Solubility Dr –aq, np-high Lr –aq, np-low Viscosity Dr –nonlinear relationship with MW Lr –linear relationship with MW Reactivity Dr –high Lr -low Compressibility Dr –low Lr -high Poly dispersity Dr –mono disperse Lr –poly disperse Shape Dr –spherical Lr –random coil Crystallinity Dr –noncrystalline,low glasstemperatures Lr – semicrystaline/crystalline, high glass temperatures Architecture Dr –regular Lr -irregular METHODS FOR CHARACTERIZATION OF DENDRITIC POLYMERS Following methods can be used for characterization of dendritic polymers . 1.Spectroscopy and spectrometry methods like Nuclear Magnetic Resonance (NMR), Infra-red (IR) and Raman, Ultraviolet-visible (UV-VIS), Fluorescence, Chirality, Optical rotation, Circular dichroism (CD), X-ray diffraction, and Mass spectrometry 2.Scattering techniques like Small angle X-ray scattering (SAXS), Small angle neutron scattering (SANS), and Laser light scattering (LLS) 3.Electrical techniques like Electron paramagnetic resonance (EPR), Electrochemistry, and Electrophoresis 4.Size exclusion chromatography (SEC) 5.Microscopy like Transmission electron microscopy, Scanning electron microscopy and atomic force microscopy 6.Rheology, physical properties like intrinsic viscosity, Differential Scanning Calorimetry (DSC), and Dielectric spectroscopy (DS) 7.Miscellaneous like X-ray Photoelectron Spectroscopy (XPS), measurements of dipole moments, titrimetry, etc APPLICATIONS OF DENDRIMERS In gene delivery In boron neutron capture therapy As MRI contrast agents In drug delivery As solubility enhancers DENDRIMERS IN GENE DELIVERY Dendrimers are used as non viral gene carriers due to there regularity and multivalent properties. MECHANISM OF GENE TRANSFECTION Complex formation between the negatively charged DNA and polycationic dendrimers Association of complex with the cellular membrane and internalization into the intracellular compartment through an endocytic path way Diagram For effective gene expression the DNA should be able to efficiently escape from the endosomal compartment to the cytosal & then be transferred to the nuclues. PAMAM and PPI dendrimers are used as gene carriers due to there relatively low cytotoxity & high affinity to negatively charged genes. Poly (L-lysine), Polyphenylene dendrimers are also used for gene delivery. PHOTOCHEMICAL GENE DELIVERY Formation of a DNA –poly cation-dendrimer ternary complex by first mixing with cationic peptide molecules containing a nuclear localization signal sequence & subsequently adding the anionic phthalocyanine dendrimer(DpcZn) Irradiating this ternary complex with light results in significantly enhanced gene expression efficacy Mechanism The ternary complex internalizes into an endosome & releases the DpcZn from ternary complex due to the protonation of the peripheral carboxyl groups of DpcZn under acidic conditions in the endosome. Interaction of released DpcZn with the endosomal membrane& disrupts the membrane under light irradiation due to the generation of ROS Finally, the plasmid DNA escapes to the cytosol &is transported to the nucleus. Indeed, the light induced ternary complexes exhibit a more than 100-fold enhancement of in vitro gene expression with minimal cytotoxicity Dendrimers for boron neutron capture therapy(BNCT) BNCT is commonly used approach to cancer treatment A patient is injected with a non radioactive pharmaceutical which selectively migrates to cancer cells(that contain a stable isotope of boron B 10 Next the patient is irradiated by a neutral beam of low energy/thermal neutrons. The neutrons react with the boron in the tumour leaving normal cells unaffected To achieve desired effects of BNCT, the local concentration of B10 in tumor should 109 atoms per cells Dendrimers used as boron carriers due to their well defined structure and multivalency Eg; boron containing PAMAM dendrimers Dendrimers as MRI contrast agents MRI is one of the prominent non-invasive diagnostic tools for disease detection Producing anatomical images of organs & blood vessels Is based on the inhomogeneous relaxation time of protons in different tissues Placing a patient in a well defined generated magnetic field results in the nuclear resonance signal of water which is assigned to its place of origin and converted into pictures Addition of paramagnetic metal cations as contrast agents improves the sensitivity and specificity of the method Gadolinium salt of diethylenetriaminepentaacetic acid (DTPA) is used but it diffuses into the extravenous area due to its low mw So the use of dendrimers as a new class of high mw MRI conrast agents as has been developed eg: Gd(iii)-DTPA-based PAMAM dendrimers DENDRIMERS IN DRUG DELIVERY 2 methods of dendrimer drug delivery are encapsulation of drugs and dendrimer –drug conjugates Encapsulation of Drugs / Host –Guest Relation : Encapsulation drugs uses either the bulk of the exterior of the dendrimer or Interactions between the dendrimer and drug to trap the drug inside the dendrimer Maciejewski introduced the concept of encapsulating guest molecules into special, egg-shell-like structures . Such a system can be used to encapsulate drugs and provide controlled delivery. Initial studies of dendrimers as potential delivery systems focused on their use as unimolecular micelles and ‘dendritic boxes’ for the noncovalent encapsulation of drug molecules The anti cancer drug 5-fluorouracil encapsulated into G=4 PAMAM dendrimers with carboxy methyl PEG5000 surface chains revealed reasonable drug loading &reduced release rate& haemolytic toxicity compared to the non-pegylated dendrimer. Dendrimer drug conjugates In this the drug is attached through a covalent bond either directly or via a spacer to the surface groups of a dendrimer Eg.conjugates of cisplatin to PAMAM dendrimers-increased solubility,decreased toxicity and selective accumulation in tumours. Dendrimers as solubility enhancers: There are many substances,which have a strong therapeutic activity but due to their lack of solubility in pharmaceutically acceptable solvents have not been used for therapeutic purposes Water soluble dendrimers are capable of binding and solubilizing small acidic hydrophobic molecules with antifungal or antibacteral properties Dendrimers having hydrophobic core and a hydrophillic surface layer-unimolecular micells Unlike traditional micells dendrimers don’t have a cmc CONCULSION: The unique architectural design of dendrimers, high degree of branching, multivalency, globular architecture and well-defined molecular weight, provides dendrimers are unique and optimum nanocarriers in medical applications such as drug delivery, gene transfection, tumor therapy, diagnostics, etc. The bioactive agents can be easily encapsulated into the interior of the dendrimers or chemically attached i.e. conjugated or physically adsorbed onto the dendrimer surface, serving the desired properties of the carrier to the specific needs of the active material and its therapeutic applications. REFERENCES Encyclopedia of pharmaceutical technology(vol-II) by james swarbrick Dendrimers from wikipedia,the free encyclopedia www.pharmainfo.net www.dendritech.com www.sciencedirect.com www.pubmed.com www.ninger.com www.elsevier.com Thank you………..
