Transcript DENDRIMERS - Pharmawiki.in

DENDRIMERS
By
T.Saritha
M.Pharm (II-sem.)
Pharmaceutics
U.C.P.SC, K.U.
OUTLINE
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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
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First discovered in the early 1980’s by Donald Tomalia &
coworkers
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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
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At the same time Newkome's group prepared similar
macromolecules & called them arborols
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Arborols stems from the latin term for tree
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The term cascade molecule is also used
INTRODUCTION
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Dendrimers are a novel class of spheroid/globular nanoscaled
macromolecules
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Characterized by highly branched tree like structers that
provides a high degree of surface functionality &versatility
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Dendrimers are also referred to as the polymers of 21st century
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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
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Radially layered poly(amidoamine-organosilicon) dendrimers
(PAMAMOS) are inverted unimolecular micelles that consist of
hydrophilic, nucleophilic polyamidoamine (PAMAM) interiors
and hydrophobic organosilicon (OS) exteriors.
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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)”
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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
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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
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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
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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
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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
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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
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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
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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.
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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.
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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.
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Biological properties
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Biological properties of dendrimers are crucial because of the
growing interest in using them in biomedical applications.
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Size is a key determinant of dendrimer cytotoxicity for both
PAMAM and PPI dendrimers.
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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).
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The nature and density of charged groups are other factors that
determine dendrimer toxicity
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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.
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A concentration dependent tendency to cause haemolysis and
changes in erythrocyte morphology has been linked to the
presence of –NH2 groups
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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
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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
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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.
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MECHANISM OF GENE TRANSFECTION
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Complex formation between the negatively charged DNA and
polycationic dendrimers
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Association of complex with the cellular membrane and
internalization into the intracellular compartment through an
endocytic path way
Diagram
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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.
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PAMAM and PPI dendrimers are used as gene carriers due to
there relatively low cytotoxity & high affinity to negatively
charged genes.
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Poly (L-lysine), Polyphenylene dendrimers are also used for
gene delivery.
PHOTOCHEMICAL GENE DELIVERY
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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)
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Irradiating this ternary complex with light results in significantly
enhanced gene expression efficacy
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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
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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
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Dendrimers for boron neutron capture
therapy(BNCT)
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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
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To achieve desired effects of BNCT, the local concentration of
B10 in tumor should 109 atoms per cells
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Dendrimers used as boron carriers due to their well defined
structure and multivalency
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Eg; boron containing PAMAM dendrimers
Dendrimers as MRI contrast agents
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MRI is one of the prominent non-invasive diagnostic tools for
disease detection
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Producing anatomical images of organs & blood vessels
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Is based on the inhomogeneous relaxation time of protons in
different tissues
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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
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Addition of paramagnetic metal cations as contrast agents
improves the sensitivity and specificity of the method
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Gadolinium salt of diethylenetriaminepentaacetic acid (DTPA)
is used but it diffuses into the extravenous area due to its low
mw
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So the use of dendrimers as a new class of high mw MRI
conrast agents as has been developed
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eg: Gd(iii)-DTPA-based PAMAM dendrimers
DENDRIMERS IN DRUG DELIVERY
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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
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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
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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.
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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:
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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.
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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
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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………..