3targeting
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1
Surface Modification of Nanoparticles for
Biomedical Applications
1
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• Consequently a multi-component nanomedical system can be constructed in reverse order of controlling events, namely from the inside out. The outer components are the first to be used. The inner components are the last.
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lipids, polimers (biocompatible-biodegradable materials)Also water is available (liposomes)
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The drug can be inserted into the core
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Ligands for targeting
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Cell Targeting
A. antibodiesB. PeptidesC. AptamersD. Other ligands
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Antibodies• Antibodies directed against tissue-specific
antigens.
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Examples
Receptors: Vascular endothelial growth factor (VEGF); folate (highly expressed in tumours); Transferrin, opiod peptides (Brain), Apolipoproteins (ApoE, Brain) , Human epidermal growth factor (EGF)
αvβ3 IntegrinMatrix metalloproteinases
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APTAMERS
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Aptamers are oligonucleic acid molecules that bind a specific target molecule. Aptamers are usually created by selecting them from a large random sequence pool,
but natural aptamers also exist in riboswitches. Aptamers can be used for both basic research and
clinical purposes as macromolecular drugs.
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• aptamers offer advantages over antibodies as they can be engineered completely in a test tube, are readily produced by chemical synthesis, possess desirable storage properties, and elicit little or no immunogenicity in therapeutic applications.
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Aptamer target protein or molecule ApplicationPSMA Prostate cancer diagnosis and therapyWT1 Understanding Wilm's tumor pathogenesis4,4 -methylenedianiline′ Detecting DNA-damaging compoundsVEGF Inhibiting angiogenesisRET Inhibition of pro-growth signalingHER-3 Reducing drug resistance in HER-2+ cancersTCF-1 Colon cancer growth inhibitionTenascin-C Glioblastoma (brain cancer) detectionMUC1 Breast, pancreatic, ovarian cancers; targeting demonstratedPDGF/PDGFR Improving transport to tumors and targeting brain cancersNF-κB Targeting a transcription factor implicated in many diseasesPhosphatidylcholine:cholesterol liposomes Triggering liposome degradationRaf-1 Inhibiting pro-growth signalingαvβ3 integrin Targeting tumor-associated vasculatureHuman keratinocyte growth factor Inhibiting pro-growth signali
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Properties of aptamers versus antibodies
Aptamers
Binding affinity nanomolar to picomolar
Selection is a chemical process carried out in vitro and can therefore target any protein
Can select for ligands under a variety of conditions for in vitro diagnostics
Uniform activity regardless of batch synthesis
PK parameters can be changed on demand
Investigator determines target site of protein
Wide variety of chemical modifications to molecule for diverse functions of molecule
Return to original conformation after temperature insult
Unlimited shelf-life
No evidence of immunogenicity
AntibodiesBinding affinity nanomolar to picomolarSelection requires a biological system, thus it is difficult to raise antibodies to toxins (not tolerated by animal) or non-immunogenic targets.Limited to physiologic conditions for diagnosticsScreening monoclonal antibodies time consuming and expensiveActivity of antibodies vary from batch to batchDifficult to modify PK parametersImmune system determines target site of proteinTemperature sensitive and undergo irreversible denaturationLimited shelf-lifeSignificant immunogenicity
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PEPTIDES
• Peptide sequences recognized by receptors responsible of binding can be identified and synthesized.
• Examples are peptide sequences derived from ApoE apolipoprotein that are recognized by LDL receptor on cell membranes
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Peptides aptamers• Peptide aptamers consist of a variable peptide loop attached at both
ends to a protein scaffold. This double structural constraint greatly increases the binding affinity of the peptide aptamer to levels comparable to an antibody's (nanomolar range).The variable loop length is typically comprised of 10 to 20 amino acids, and the scaffold may be any protein which has good solubility and compacity properties. Currently, the bacterial protein Thioredoxin-A is the most used scaffold protein, the variable loop being inserted within the reducing active site, which is a -Cys-Gly-Pro-Cys- loop in the wild protein, the two Cysteines lateral chains being able to form a disulfide bridge.Peptide aptamer selection can be made using different systems, but the most used is currently the yeast two-hybrid system.
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OTHER LIGANDS
• Natural ligands for receptors can be employed.Examples:Folate ApoETrasferrin
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+
Via succinimide
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biotin
biotin
streptavidin
nanoparticlenanoparticle
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streptavidin
antibody
nanoparticle
biotin
antibody
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LNA• A locked nucleic acid (LNA), often
referred to as inaccessible RNA, is a modified RNA nucleotide. The ribose moiety is modified with an extra bridge connecting the 2' oxygen and 4' carbon. LNA nucleotides can be mixed with DNA or RNA residues in the oligonucleotide whenever desired. Such oligomers are commercially available. The locked ribose conformation enhances base stacking and backbone pre-organization. This significantly increases the hybridization properties (melting temperature) of oligonucleotides.[1]
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cystein