Biomedical Engineering Reference
In-Depth Information
higher.speciicity,.LNPs.can.be.surface.modiied.with.ligands.that.speciically.recognize.
receptors.on.tumor.cells..Combining.passive.and.active.targeting.in.a.single.platform.may.
further.improve.the.therapeutic.index.of.LNP.delivered.siRNAs.
This.chapter.is.focused.on.LNPs.for.systemic.delivery.of.siRNA.in.vivo..Decades.of.pub-
lished.work.on.delivery.of.plasmid.DNA.and.antisense.ODN.(AS-ODN).has.provided.a.
framework.for.designing.siRNA.delivery.vehicles..This.chapter.will.provide.an.overview.
of.the.various.types.of.LNPs.for.siRNA.delivery,.highlighting.structure-function.relation-
ship.studies.on.cationic.lipids.for.siRNA.delivery,.and.the.issue.of.vehicle-related.toxicity.
24.2 Mechanism of RNAi
RNA.interference.(RNAi).is.a.potent.and.speciic.gene.silencing.mechanism..Endogenous.
small.RNAs,.called.microRNAs.(miRNA),.which.function.at.the.translational.level,.have.
been. shown. to. regulate. important. genes. associated. with. cell. development,. differentia-
tion,.and.death.(Aagaard.and.Rossi.2007,.Kim.and.Rossi.2007,.Pecot.et.al..2011)..In.contrast,.
siRNAs. are. typically. custom. designed,. synthetic. molecules. that. mediate. gene. silencing.
at. the. transcription. level.. siRNA. is. double-stranded. RNA. chain. 19-23.nt. in. length. (Fire.
et. al.. 1998,. Aagaard. and. Rossi. 2007).. In. siRNA-mediated. RNAi,. siRNA. irst. interacts.
with. Argonaute-2. (Ago-2). to. form. RNA-induced. silencing. complexes. (RISCs).. The. sense.
strand. of. the. siRNAs. is. then. cleaved. and. the. antisense. strand. seeks. out. and. selectively.
degrades. mRNAs. with. the. complementary. sequence,. thus. preventing. translation. of. the.
target. mRNA. into. protein,. i.e.,. “silencing”. the. gene. (Whitehead. et. al.. 2009).. This. RISC-
based. mechanism. is. highly. eficient.. In. theory,. siRNAs. can. be. designed. to. inhibit. any.
gene. target,. including. those. that. are. dificult. to. modulate. selectively. with. traditional.
small. molecules. or. with. antibodies.. In. addition,. a. relatively. low. dosage. is. required. for.
siRNA-mediated.gene.silencing.due.to.its.high.potency..For.these.reasons,.siRNA-based.
RNAi. has. been. widely. utilized. for. gene-function.analysis. and. drug-target. validation. in.
drug.development.(Kim.and.Rossi.2007,.Pecot.et.al..2011).
24.3 Barriers to Delivery of siRNA In Vivo
As. polyanionic. macromolecules,. siRNAs. face. multiple. obstacles. in. reaching. their.
intracellular. site. of. action.. The. therapeutic. application. of. siRNAs. is. hindered. by. their.
limited.chemical.stability.in.serum,.rapid.blood.clearance,.and.poor.cellular.uptake.(Behlke.
2006,.Juliano.et.al..2008,.de.Martimprey.et.al..2009,.Whitehead.et.al..2009,.Yu.et.al..2009)..
The. chemical. stability. of. siRNAs. can. be. improved. by. backbone. modiications. (Kurreck.
2003,.Aboul-Fadl.2005,.De.Paula.et.al..2007,.de.Fougerolles.et.al..2007)..In.the.absence.of.
a.delivery.vehicle,.it.is.likely.that.free.siRNAs.can.only.be.given.by.local.administration..
Local.administration.of.siRNA.appears.effective.for.siRNA.delivery.to.the.eye.and.lung.for.
age-related.macular.degeneration.(AMD).and.respiratory.syncytial.virus.(RSV).infection.
(Juliano.et.al..2008,.Davis.2009,.Whitehead.et.al..2009).
For.systemic.administration.of.siRNA.for.delivery.to.solid.tumors,.there.are.four.major.
barriers,. as. illustrated. in. Figure. 24.2.. First,. the. siRNAs. must. avoid. rapid. degradation.
