Biomedical Engineering Reference
In-Depth Information
Endocytosis
Endosomal
escape
(1) Tumor
localization
(2) Penetration
(3) Cellular uptake
(4) Intracellular release
Injection
Figure 24.2
(See companion CD for color igure.)
.Schematic.of.delivery.barriers.to.a.tumor..After.administration.into.blood.
circulation,.the.siRNA-loaded.LNPs.(siRNA-LNPs).must.reach.the.tumor.through.an.extravasation.process.(1),.
which.can.be.facilitated.by.the.enhanced.permeability.and.retention.(EPR).effect..Then,.siRNA-LNPs.need.to.
further.redistribute.within.tumor.interstitium.either.by.convection.or.by.diffusion.(2),.followed.by.binding.to.
the.target.cell.membrane.and.uptake.by.endocytosis.(3)..Inside.the.cells,.the.siRNA.has.to.escape.from.the.endo-
some.(4).to.reach.the.cytoplasm.to.exert.bioactivity.
in. serum,. rapid. excretion. by. renal. iltration,. and. clearance. by. the. reticuloendothelial.
system. (RES).. Second,. the. siRNAs. need. to. reach. tumor. cells. following. extravasation..
Third,.the.siRNAs.must.be.eficiently.taken.up.by.the.target.cells,.typically.via.an.endo-
cytic.process..Finally,.they.must.be.released.from.the.endosome.to.gain.access.to.the.RNAi.
machinery. located. in. the. cytoplasm. (Juliano. et. al.. 2009).. An. effective. delivery. strategy.
must.take.into.account.the.need.to.overcome.all.of.these.barriers,.as.well.as.to.avoid.intro-
ducing.toxicity.and.stimulating.undesirable.cytokine.response.
24.4 Lipid Nanoparticles for siRNA Delivery
Lipid.nanoparticles.(LNPs).have.been.recognized.as.one.of.the.most.promising.delivery.
systems.mainly.due.to.their.biocompatibility.and.relatively.ease.of.large-scale.production..
Moreover,.early.clinical.trials.have.demonstrated.the.potential.safety.and.eficacy.of.these.
systems.
Synthetic. cationic. lipids. are. the. building. blocks. for. LNPs.. Cationic. LNPs. have. been.
widely.used.as.nonviral.delivery.systems.for.nucleic.acids.ever.since.the.successful.dem-
onstration. of. 1,2-di-
O
-octadecenyl-3-trimethylammonium. propane. (DOTMA). for. DNA.
transfection. in. vitro. by. Felgner. et. al.. in. 1987.. Most. cationic. LNPs. involve. formulations.
composed. of. both. cationic. and. neutral. lipids.. The. role. of. the. cationic. lipid. is. to. enable.
formation.of.complexes.between.the.lipid.components.and.the.negatively.charged.siRNAs.
by.electrostatic.interaction..At.an.optimal.lipid.nitrogen.to.nucleic.acid.phosphate.(N/P).
ratio,. the. resulting. complexes,. known. as. lipoplexes,. have. a. positively. charged. surface..
This.enables.interaction.with.the.negatively.charged.cell.membrane,.and.therefore.facili-
tates.internalization.by.endocytosis..Furthermore,.cationic.lipids.can.form.ion.pairs.with.
anionic.phospholipids.within.the.endosomal.membrane,.thus.destabilizing.the.membrane.
by.promoting.bilayer.transition.to.an.inverted.micelle.or.hexagonal.(H
II
).phase,.a.trans-
formation.expected.to.trigger.endosomal.escape.and.cytoplasmic.localization.of.the.cargo.
