AUGUST 2006 | VOLUME 1 | NUMBER 3 15

RESEARCH NEWS

Functionalized nanoparticles (NPs) are ofconsiderable interest for targeted cancertherapy. One popular approach is to conjugateNPs to a ligand that targets receptors only foundon cancer cells. However, the number of NPsthat can bind to the cancer cells is limited by thenumber of free receptors. Researchers from Massachusetts Institute ofTechnology (MIT), Brigham and Women’sHospital, the University of California at SanDiego, and the Burnham Institute areinvestigating an alternative method of activetargeting that is not restricted by receptoravailability. As a first step, they have designedNPs that self-assemble in the presence of aprotease [Harris et al., Angew. Chem. Int. Ed.(2006) 4455, 3161]. If the self-assembly could betriggered to occur only within tumors, theclumped NPs should become fixed in position.“We have designed nanoparticles that actsimilarly to fibrin, the protein that polymerizesduring blood clotting. They are separate anddispersed in solution until a specific protease isadded, whereupon they rapidly self assemble,"says Geoffrey von Maltzahn of MIT. “In the body,this enzyme-actuated platelet/fibrin assemblyenables very rapid deposition of proteins in sitesof vascular injury. We’re hoping to use proteases

associated with tumor processes to do the samething with our NPs.”The team used 50 nm superparamagneticmagnetite (Fe3O4) particles functionalized witheither biotin or neutravidin. A polyethyleneglycol (PEG) coating ensures the mixture of NPsremains dispersed in solution. The addition ofmatrix metalloproteinase-2 (MMP-2), a proteaseover expressed by cancer cells, causes the PEGcoating to shed. The high affinity between biotin

and neutravidin leads to rapid NP self assembly.Further in vitro tests have shown that thetechnique can be used to image proteaseexpression with magnetic resonance imaging(MRI). As the NPs form clumps, the observed MRIsignal becomes stronger. This was demonstratedby incubating NP solutions with varyingconcentrations of MMP-2. Image data acquiredfrom a 4.7 T MRI scanner reveals that protease-triggered assembly can be detected frommeasured T2 changes with reliable sensitivity.“Protease levels can be correlated withmalignancy and invasiveness and could be usedfor diagnosis and prognosis, as well as forprescribing treatment and monitoring efficacy,”says Maltzahn. “Other applications may be a bitfurther off, but as self assembly is used to buildmaterials that have other emergent imaging ortherapeutic properties, one can imagine applyingthis strategy to build these at the site oftumors.”The next stage will be to show that NP selfassembly can be triggered in vivo. Experimentsinvolving mouse models are currently underway.“We are also interested in what other functionsof the surface of a NP could be temporarilyshielded by cleavable polymers,” says Maltzahn.Paula Gould

Tumor protease triggers self assemblyNANOPARTICLES

Nanoparticles make sense as antisense agentsNANOPARTICLES

Northwestern University researchers have shown that

Au nanoparticles (NPs) functionalized with

oligonucleotides can decrease gene expression and

protein production more effectively than existing

commercial agents [Rosi et al., Science (2006) 331122,

1027]. Their findings suggest a promising role for Au

NPs in the development of antisense drugs.

Antisense DNA can disrupt the production of specific

protein molecules by binding to messenger RNA

(mRNA). Efforts have, therefore, focused on targeting

the delivery of antisense DNA to mRNA that would

otherwise trigger production of cancer-causing

proteins. Options for delivering the nucleic acids have

included cationic lipids and polymers, modified viruses,

dendrimers, liposomes, and NPs.

The high affinity of oligonucleotide-tagged Au NPs for

DNA indicates that they could be suitable for the

carrier role. “Such particles cooperatively bind to

complementary DNA targets and are approximately

100 times better binders of such DNA than free

oligonucleotides in solution,” says Chad A. Mirkin. “So

we hypothesized that if we could get them into cells,

they would be better scavengers of mRNA and

therefore more effective antisense agents.”

The team used 13 nm Au NPs functionalized with one

of two different antisense oligodeoxynucleotides

(ASODNs). While one group of the antisense NPs

supported 45-50 ASODN strands, particles in the other

group were conjugated with 110-120 ASODN strands.

Experiments focused on mRNA sequences that code

for enhanced green fluorescent protein (EGFP)

expressed in a mouse cell. Confocal fluorescence

microscopy was performed before and after incubation

of the cells for 48 hours with antisense NPs. Those

cells treated with NPs show lower fluorescence

compared with untreated control cells, and cells

containing NPs with a higher number of bound

ASODNs exhibit the greatest decrease in fluorescence.

Both batches of Au NPs outperform commercial

antisense agents lipofectamine and cytofectin.

Functionalized Au NPs offer a number of key benefits

as antisense agents, according to Mirkin. The particles

are readily taken up by all cell lines studied to date

(over ten) and the antisense NPs resist degradation

within cells. Perhaps most importantly, the particles

show no signs of toxicity over the period studied,

unlike a number of commercial antisense agents.

“This represents an entire new class of antisense

agents and could dramatically accelerate efforts to

create novel and useful gene therapies,” says Mirkin.

“As we move to animal systems, this could have a very

big impact on cancer research and therapies.”

Research will now move in vivo to improve

understanding of how the systems work. This will help

researchers to engineer the NPs to fit specific

applications, says Mirkin.

Paula Gould

Schematic three-dimensional representation ofparticles assembling in response to proteases.Polymers (ribbons) are attached via peptidesubstrates (jagged stalk). Biotin is shown in blue,neutravidin in red. (Credit: Geoffrey von Maltzahn,MIT.)

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