Expression and Manufacture of Pharmaceutical Proteins in Genetically Engineered Horticultural Plants - Transgenic Horticultural Crops: Challenges and Opportunities - page 88

plants.are.increasingly.being.grown.in.containment.or.in.areas.with.natural.geographic.
barrier.to.isolate.transgenic.plants.from.their.agricultural.or.wild.relatives..Once.
the.transgenic.plant.lines.are.selected.and.the.“master.seed.banks”.are.established,.
however,.stable.transgenic.plants.will.provide.the.most.scalable.technology.when.
large.amounts.of.pharmaceutical.protein.are.required.for.commercial.production..
Since.there.are.thousands.of.chloroplasts.in.each.cell,.the.copies.of.transgene.per.
cell.in.transplastomic.plants.are.signiicantly.higher.than.that.of.nuclear.transfor-
mation,. which,. in. turn,. drive. higher. level. of. transgenic. protein. production. 32-34 .
The.prokaryotic.origin.of.chloroplasts.also.supports.facile.homologous.recombina-
tion.and.polycistronic.transcript.production..These.features.allow.the.production.
of. multiple. transgenic. proteins. from. one. polycistronic. mRNA. as. well. as. better.
transgene.targeting.in.chloroplast.genome.to.avoid.the.“position.effect”.and.trans-
gene. silencing,. which. cause. inconsistent. level. of. transgenic. protein. production..
Chloroplasts.are.maternally.transmitted.in.most.crops,.which.exclude.chloroplast.
transgene.escape.from.pollen.transmission..Therefore,.transplastomic.expression.
of.transgene.is.considered.a.superb.alternative.for.regulatory.compliance.in.ensur-
ing.transgene.containment.and.improving.the.biosafety.of.transgenic.plants. 35 .Due.
to.lack.of.posttranslational.machineries.in.chloroplasts,.however,.plant.lines.with.
stably.transformed.chloroplast.genomes.will.mostly.be.applicable.to.produce.pro-
teins,.which.do.not.require.posttranslational.modiications.for.their.function.or.pro-
teins. for. diagnostic.purposes. 36 . The. transient,. viral-based. expression. systems 21,23 .
are. focused. on. production. speed,. and. therefore,. the. most. convenient. technology.
for. obtaining. the. initial. research. material. (mg-g. level). used. in. preclinical. stud-
ies.. These. transient. expression. systems. rely. on. Agrobacteria's . ability. to. deliver.
the.“deconstructed”.viral.genome.to.majority.of.plant.cells,.therefore,.eliminating.
the.need.for.viral.systemic.spreading.function..This.characteristic.not.only.elimi-
nates.the.concern.of.transgene.loss.during.systemic.spreading.but.also.allows.the.
application. of. this. technology. to. a. diversity. of. plant. species. beyond. the. natural.
virus.hosts..Since.nontransgenic.plant.materials.are.used.for.viral.vector.delivery,.
they.can.be.readily.stocked.up.for.semi-large-scale.productions..Overall,.a.rapid.
evaluation.of.pharmaceutical.candidates.and.transition.to.a.large-scale.commercial.
production.platform.can.be.accomplished.by.employing.the.combination.of.both.
transient.and.stable.transgenic.plant.technologies.
PROTEIN PHARMACEUTICAL PRODUCTION
USING PLANT SYSTEMS
Unlike. mammalian. cell. cultures. or. bacterial. fermentation. systems,. which. require.
capital-demanding. bioreactors. and. expensive. tissue. culture. media. and. operations,.
pharmaceutical. protein. production. by. transient. or. stable. transgenic. plants. is. more.
economical.in.both.initial.setup.and.subsequent.scale-up..This.is.due.to.decrease.in.
large. hardware. investment. and. expensive. culture. media. regardless. of. whether. it. is.
produced.in.the.ield.or.greenhouses..As.a.result,.a.variety.of.pharmaceutical.proteins.
have.been.expressed.and.characterized.in.plant.systems..Here,.we.will.focus.on.the.
production.of.the.two.most.important.classes.of.pharmaceuticals:.mAbs.and.vaccines.