NEWS FEATURE

Cancer nanomedicine, reengineeredAfter recent setbacks, researchers hope to find approaches more attuned to the

complexities of cancer biology.

Katherine Bourzac, Science Writer

The miniature craft cruises silently through a bloodvessel. Sneaking through a hole in the vessel wall, itinfiltrates a tumor and uses an on-board antibody keyto gain entry into a cancer cell. Once inside, the sleekship deploys its anticancer cargo to destroy the tumor.Mission accomplished.

This vision of nanomedicine, commonly portrayedin animations from the early 2000s, promised thatnanotechnology would be cancer’s magic bullet, giv-ing us therapies that could identify the site of disease,travel there, and wipe it out.

Omid Farokhzad, a physician and biomaterials re-searcher at the Brigham and Women’s Hospital in Bos-ton, says he now feels embarrassed when he sees suchvideos. “Fifteen years ago, our concept was that if weput some tumor-targeting ligands on the surface of a

nanoparticle, it would get into a cell,” says Farokhzad.“That is incredibly naïve.”

In principle, nanomedicine could deliver a drug(or even a combination of drugs) precisely where it’sneeded in the body. This increases a drug’s effective-ness while avoiding side effects caused by flooding theentire body with a conventional chemotherapy, all thisby virtue of their size. One of the field’s core assump-tions is that nanoparticles are small enough to leakthrough shoddily built blood vessels around tumors,but too big to pass through normal vasculature intohealthy tissues. Based on this passive targeting princi-ple, the field has had a handful of clinical successes thatrely on bulky nanoparticle carriers to keep toxic cancerdrugs out of the heart and other off-target tissues.

But at the same time, researchers want to improvethe nanoparticle’s active targeting of tumor tissue,which has been much more difficult to achieve. Untilrecently, the messy biology of real human tumors hasrepulsed most attempts.

So researchers are trying a fresh approach,reengineering nanomedicine using data about hownanoparticles interact with tissues and cells. They’re alsostriving to understand why nanomedicine works in somepatients and tumors but fails in others.While shrugging offthe field’s more fantastical visions, nanomedicine re-searchers now hope to make more effective, targetedtherapies that live up to their name.

Mixed ResultsThis year, nanomedicine suffered a setback that seemsto have intensified the introspective mood of nano-technologists. InMay, one of the leading nanomedicinecompanies, BIND Therapeutics of Cambridge, Mas-sachusetts, declared bankruptcy after disappointingclinical trial results and poorly performing stock in-creased pressure on the company to repay debts.Cofounded by Farokhzad and prominentMassachusettsInstitute of Technology (MIT) chemical engineer andserial entrepreneur Robert Langer, BIND had received alot of attention. Observers lauded its aim of usingsophisticated chemical engineering and computer-automated design techniques to make polymer nano-carriers that would actively target cancer cells withtumor-specific binding molecules on their surfaces.

Nanomedicine researchers had high hopes forBIND’s strategy, and were disappointed when it fell

One experimental nanomedicine approach employs a rat model of glioblastoma inwhich nanoparticles (red), injected intracranially, are taken up by tumor cells(green). Image courtesy of Eric Song (Yale University, New Haven, CT).

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short. More consternation followed with the publica-tion of a critical meta-analysis of the field in April (1).University of Toronto nanotechnologist Warren Chanand colleagues reviewed more than 100 nanomedi-cine papers from the past 10 years, and found that anaverage of just 0.7% of any nanoparticle dose—whether actively targeted or not—gets into tumors. “Iwas surprised it was that low,” says Chan. “When youthink ‘nano’ you think ‘targeted’—it’s a psychologicalassumption.”

The review has had its fair share of criticism, withsome arguing that it unfairly lumps together data onmany different tumors, drugs, and nanoparticles. Andalthough one percent or so doesn’t sound like much,“that level of accumulation can actually be useful if ithappens in the right cells,” says Sangeeta Bhatia, abiomedical engineer at MIT.

Still, others have welcomed Chan’s full-throatedcall for convincing demonstrations that nanoparticlescan significantly improve targeted delivery of drugs,and make a marked difference in clinical outcomes forpatients. Chan’s results “have been presented as avery dark, negative thing, but we have to be realisticand move on,” says Shanta Dhar, a biomedical engi-neer at the University of Miami Medical School.

There has been good news for nanomedicine thisyear as well. In June, for example, Celator, a subsidiary ofJazz Pharmaceuticals of Dublin, Ireland, announcedpositive results of a phase 3 nanomedicine trial in acutemyeloid leukemia. The company’s combination of twocancer drugs carried in a liposome nanoparticle mod-estly improved outcomes. Because overall survival forpatients with this disease is poor, even a small im-provement is noteworthy, says Piotr Grodzinski, directorof the Office of Cancer Nanotechnology Research at theNational Cancer Institute in Bethesda, Maryland.

Grodzinski thinks that nanomedicine could make abigger impact by focusing on diseases that lack con-ventional therapy options. “The field needs to establishsome niche applications where therapies or diagnosticsare almost nonexistent,” he says.

Going PlacesYale University biomedical engineer Mark Saltzmanbelieves glioblastoma is just such a disease. This braincancer has dismal survival figures, and most peopledie within a year of diagnosis. Saltzman hopes to de-sign a nanomedicine that targets cancer stem cells,which can linger deep in the brain after surgery orconventional chemotherapy, and eventually restart atumor. Getting a drug deep into the brain is critical fortreating glioblastoma.

However, delivering drugs into the brain is difficult.The cells lining the brain’s blood vessels are connectedby much tighter junctions than in the rest of the cir-culatory system, creating a highly policed entrywaycalled the blood–brain barrier. It lets in glucose andother essential supplies, but tends to keeps out toxinsand nanoparticles. The brain is also adept at flushingout drugs once they get in.

The stalwart blood–brain barrier spurred Saltzman toabandon the usual approach of injecting nanoparticlesinto the bloodstream and hoping they get to the rightplace. Instead, the Yale researcher aims to infuse nano-carriers behind the barrier, directly into the interstitial fluidthat bathes the brain. Then, a process called convection-enhanced delivery would carry the drug deeper. Themethod involves applying positive pressure when infusingthe drug, which moves the liquid surrounding the brainand sets up a convection current.

Others had shown that this principle works with li-posome nanoparticles, and that the particles were bigenough to get stuck inside the brain. But liposomes donot excel at long-term drug release, so Saltzman turnedto nanoparticles made of a polymer called poly(lactic-coglycolic acid) (PLGA) that is already used in sutures andother medical implants. He had to make the PLGAnanoparticles smaller than in previous nanomedicineformulations so they could fit through the interstitialspaces of the brain, which are as narrow as tens ofnanometers in tumors.

In 2013, the Yale group reported that PLGA nano-particles about 70 nanometers wide were small enough

These confocal laser scanning microscope images show nanoparticles (red) releasing a payload of gene-silencing RNAinto cervical cancer cells. Image courtesy of Xia-Ding Xu (Harvard Medical School, Boston).

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to move around in the brain, yet still capable of carryingand releasing drugs (2). The researchers screened a li-brary of anticancer compounds for effectiveness againstglioblastoma cancer stem cells, and combined promisingcandidates with the carrier. In mice, this brain-targeted,glioblastoma stem cell-specific therapy shrank tumorsderived from human brain cancer stem cells.

Mouse brains are much smaller than human ones,though. In people, glioblastomas spread out to locationsup to two centimeters from the initial site, which meansthe tiny nanoparticles have a lot of ground to cover. Testsin pig brains have confirmed that convection-enhanceddelivery carries their nanodrugs into volumes over 1,000cubic millimeters. The Yale group is now working on anapplication to run a clinical trial at the university’smedicalcenter.

One appeal of this delivery vehicle is that it could bea platform for any sort of drug that needs to get deep

“Nobody’s infused nanoparticles into the human brain inthe presence of a tumor before, so there is a lot to learn.”

—Mark Saltzman

into the brain and be released in a controlled fashionover time, at which PLGA nanoparticles excel. If thebrain-specialized drug carrier shows clinical promise,scientists could try using it to deliver new anticancercompounds and other neurological drugs. “Nobody’sinfused nanoparticles into the human brain in thepresence of a tumor before, so there is a lot to learn,”says Saltzman. “But we’re confident it worksin animals.”

Sweating the Small StuffEveryone working in nanomedicine should apply thissort of back-to-basics approach, says Chan. He arguesthat the nanomedicine community should focus onmaking sure their results are repeatable, and performfundamental studies of how nanoparticles actuallywork inside the human body. “If you don’t know whatan alternator does and you try to build a car, it’s notgoing to work,” he says. “If you understand how asystem works, you can engineer your system.”

The fundamental challenges are many. Take, forexample, the near-universal problem of nanoparticle-clogging proteins. “Anytime you introduce a nanoparticleto the bloodstream, a lot of proteins bind weakly to theparticle and form a cloud around it,” says Kim Hamad-Schifferli, a biological engineer at MIT. “It’s impossible tomake a particle that doesn’t form aprotein corona.” Theseprotein clouds can cover the tumor-binding moleculesthat nanoengineers have painstakingly designed andplaced on the surface of their drug carriers.

Because there’s no good way to prevent the coronafrom forming, engineers are figuring out how to use itto their advantage. Farokhzad thinks that gaining con-trol of the protein coating could be key: the proteins ona nanoparticle are part of what determine how long itcan circulate, how the immune system reacts to it, andwhere it goes. Often, a coated nanoparticle is eitherexcreted through the kidneys or taken up by immune

cells called macrophages and marched to the liver,lungs, or spleen. But these pathways are not inevitable,and Farokhzad is working on systematic studies ofhow the size of a nanoparticle affects the chemistry ofthe protein corona. It’s possible the right nanoparticlecould attract a protein corona that instructs the body tosend it to a particular tissue, or to a tumor. And Hamad-Schifferli has shown in vitro that the protein coating canimprove the function of a nanoparticle that can switchblood clotting on and off (3).

Chan is already working on translating detailedstudies of nanoparticle fates to people. “We’re tryingto design for people but we can’t test on them,” saysChan. So his laboratory has started a collaborationwith the University of Toronto’s medical center to workon the next best thing: patient samples. Resected livertumors, for example, can be perfused with nourishingfluid and kept alive long enough to study nanoparticlepaths through these tissues.

Projects like these are logistically difficult for mostbiomaterials researchers to do on their own, but Chanbelieves they could easily be incorporated into clini-cal trials. By monitoring patients for clinical outcomes,and correlating them with information about how aninvestigational nanodrug moves through a particularpatient’s tumor biopsy, companies could learn why adrug works, or doesn’t work. That knowledge canthen feed back into future nanoparticle designs. Thisstrategy promises to reveal fundamental informationabout the realities of human tumors, and a growingnumber of researchers are now pursuing this approach.

Watch CloselyIf the goal is to improve clinical outcomes immedi-ately, though, there may be an easier way: usingimaging methods to assess how receptive a patientwill be before giving them a nanomedicine. Person-alized medicine is a central doctrine of just aboutevery other aspect of cancer research, and it’s in-creasingly being applied to nanomedicine as well.No breast cancer patient is put on Herceptin unlessher tumor has the receptor it binds to; nor should apatient be put on a nanomedicine if her tumor won’tlet it in.

Merrimack Pharmaceuticals of Cambridge, Mas-sachusetts, is using a Food and Drug Administration-approved MRI agent called ferumoxytol, which containsiron oxide nanoparticles, as a sort of reconnaissance op-erative to reveal whether a tumor will let in nanoparticles.If the ferumoxytol gets into the tumor, the companyreasons, the therapeutic nanoparticle probably will too.At the American Association for Cancer Research annualmeeting in San Diego in 2014, the company presentedresults from a pilot trial of 12 patients with advanced tu-mors. Tumors that accumulated the imaging nanoparticlealso tended to take up their nanotherapy MM-398, a li-posome carrying the cancer drug irinotecan. The com-pany now hopes to prove the predictive effect of itsimaging screen in two phase 1 clinical trials: one forpatients with advanced breast cancer and other solidtumors, and another for patients whose tumors aremetastatic, or cannot be surgically removed.

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Meanwhile, Kazunori Kataoka at the University ofTokyo is using imaging to better understand passivetargeting. His goal is to design nanoparticles that aresuited to particular types of tumors and different kindsof tumor blood vessel dynamics.

Kataoka has previously developed four nano-medicines that are now in clinical trials being run byChiba, Japan-based nanomedicine maker NanoCarrierand Nippon Kayaku, a Tokyo-based multinationalchemical and drug company (4). One of these drugs,which is in phase 3 clinical trials in pancreatic cancer,responds to the relatively acidic environment of thetumor to trigger drug release.

In a study published this February, Kataoka fo-cused instead on how nanoparticles of different sizesget into pancreatic tumors, and what happens whenthey do (5). Pancreatic cancer is particularly difficultto treat, and the tumors are dense, making toughgoing for nanoparticles. His team imaged pancreatictumors and their vasculature every 10 minutes for 10hours after dosing mice with different-sized calciumphosphate nanoparticles. These nanoparticles don’tcarry any drugs, but they are visible under micros-copy in living animals.

The researchers found that small particles of 30nanometers or so can leak into pancreatic tumors, butlarger particles only enter during random “nano-eruptions” of blood. “The mechanism is not so simpleas people believe,” says Kataoka. And whereas thesmaller particles can migrate into the tumor’s densetissue, larger particles—even when they enter throughan eruption—get stuck at the periphery. However, theresearchers did not see the nanoeruptions in colorectaland ovarian tumor vasculature, the blood vessels ofwhich are naturally leakier and let in larger nanoparticles.This kind of information should inform the selection ofnanocarriers, smaller ones for pancreatic cancer, largerones for other tumors, says Kataoka. In the right pa-tient and the right tumor, the ideal nanomedicine canbe “like a spaceship carrying tiny fighters,” he says.

This newwave ofmechanistic studies, saysGrodzinski,will help tailor nanoparticles to the tumor, the tissue, andthe patient. Gone are the notions of sleek and precisenano-sized silver bullets that effortlessly home in ontumor targets. But by focusing on the basics, researchersare optimistic they can design tailored nanotherapiesthat come close to their science-fiction visions.

1 Wilhelm S, et al. (2016) Analysis of nanoparticle delivery to tumours. Nat Rev Mat 1:1–12.2 Zhou J, et al. (2013) Highly penetrative, drug-loaded nanocarriers improve treatment of glioblastoma. Proc Natl Acad Sci USA 110(29):11751–11756.

3 de Puig H, Cifuentes Rius A, Flemister D, Baxamusa SH, Hamad-Schifferli K (2013) Selective light-triggered release of DNA from goldnanorods switches blood clotting on and off. PLoS One 8(7):e68511.

4 Cabral H, Kataoka K (2014) Progress of drug-loaded polymeric micelles into clinical studies. J Control Release 190:465–476.5 Matsumoto Y, et al. (2016) Vascular bursts enhance permeability of tumour blood vessels and improve nanoparticle delivery.Nat Nanotechnol 11(6):533–538.

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