Biomedical Engineering Reference
In-Depth Information
The.irst.pH-sensitive.liposomes.were.prepared.combining.DOPE.with.lipids.that.pos-
sess.titrable.acid.groups.(negatively.charged.at.physiological.pH),.such.as.palmitoylho-
mocysteine.(PHC),
38
.n-succinyldioleoylphosphatidylethanolamine,
39
.oleic.acid.(OA),
40-43
.
dipalmitoylsuccinylglycerol. (DPSG),. dioleoylsuccinylglycerol. (DOSG),
44
. or. cholesteryl.
hemisuccinate. (CHEMS).
45
. These. liposomes. possess. electrostatically. charged. bilayers.
with. high. interfacial. hydration. that. repel. each. other,. impairing. interaction. between.
DOPE.molecules..Such.repulsion.prevents.the.formation.of.the.H
II
.phase.and.stabilizes.
the. Lα. phase. at. physiological. pH. and. temperature.
40,46
. When. the. pH. is. decreased,. the.
protonation. of. carboxilic. groups. neutralizes. the. negative. charge. and. dehydrates. the.
interfacial. region.. The. adjacent. bilayers. get. close,. allowing. interaction. between. DOPE.
molecules,. with. subsequent. destabilization. of. the. Lα. phase. and. conversion. to. the. H
II.
phase.
45,47
Once.captured.by.cells,.liposomes.made.of.DOPE/OA,.DOPE/PHC,.DOPE/DPSG,
44
.and.
DOPE/CHEMS
48
.release.their.aqueous.content.to.the.cell.cytoplasm.within.5.and.15.min..
This.speed.is.compatible.with.release.occurring.between.early.and.late.endosomes,.since.
the.transport.up.to.lysosomes.takes.nearly.30.min.
49,50
The.liposomal.escape.is.thought.to.be.mediated.by.fusion.between.the.liposomal.and.
endosomal.bilayers.
51,52
.The.observation.by.freeze.fracture.of.lipid.particles.in.the.point.of.
union.between.membranes.in.fusion.has.been.interpreted.as.highly.curved.fusion.inter-
mediaries.(inverted.micelles,
53
.or.“stalk”
54
)..These.intermediaries,.in.the.facing.monolayers.
of.two.contacting.membranes,.favor.the.incorporation.of.lipids.with.a.negative.membrane.
curvature,
55
. such. as. DOPE.. In. the. case. of. liposomes,. the. two. contacting. membranes. in.
question.will.be.the.outer.monolayer.of.the.vesicles.and.the.inner.monolayer.of.the.endo-
somal.membrane.
The.pH.sensitivity.of.these.liposomes.depends.on.the.pK
a
.of.the.anionic.lipid.that.sta-
bilizes.the.lamellar.phase;.varying.their.molar.ratio.slightly.modiies.the.pH.sensitivity..
For. instance,. DOPE/OA. liposomes. are. the. most. labile,. becaming. unstable. at. pH. values.
between.6.9.(8:2.mol:mol).and.6.5.(7:3),
56
.whereas.the.other.liposomes.destabilize.at.lower.
pH:.DOPE/PHC.(8:2).destabilizes.at.pH.6.25,.DOPE/CHEMS.(6:4).at.5.5,.and.DOPE/DPSG.
(8:2).at.5.
44,45
.In.theory,.appropiate.formulations.with.different.pH.sensitivities.should.be.
chosen.according.to.the.different.minimal.intravesicular.pH.values.exhibited.by.different.
cells:.4.6.for.macrophages,
57,58
.5.5.for.ibroblasts,.and.5.6.for.epithelioid.CV-1.cells.
59,60
20.2.1 Plasma Stability
Since. pH-sensitive. liposomes. are. mainly. administered. i.v.,. structural. stability. must. be.
maintained.during.blood.circulation,.until.liposomes.reach.the.target.site,.where.they.will.
be. captured.. Leakage. of. inner. content,. as. well. as. aggregation. that. prematurely. leads. to.
phase.transition,.must.be.minimized..In.addition,.plasma.proteins.should.not.reduce.the.
pH.sensitivity..However,.in.blood,.DOPE/titrable.anionic.lipids.become.pH.insensitive.and.
experience.leakage.that.varies.according.to.the.nature.of.the.anionic.lipid..Formulations.
of.liposomes.containing.OA.are.less.stable..The.addition.of.cholesterol.and.Peg.does.not.
impair.the.loss.of.pH.sensitivity,.but.Peg.reduces.leakage.
DOPE/OA.liposomes.extensively.destabilize.(content.release.and.aggregation).in.mouse
61
.
and.human.plasma
62
.because.of.the.albumin.extraction.of.OA..DOPE/DPSG.and.DOPE/
DOSG.liposomes.are.more.stable,.releasing.less.than.20%.calcein.upon.overnight.incuba-
tion.in.human.plasma.
63,64
.However,.after.only.3.h.incubation.in.plasma,.the.pH.sensitivity.
of.these.liposomes.is.diminished.(instability.is.triggered.at.pH.4.2),.due.to.the.extraction.
of.DPSG.
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