Feature: Dosing the poison

The great 16th century Swiss physician and alchemist Philippus Aureolus Theophrastus Bombastus von Hohenheim – understandably often known by his moniker, Paracelsus – observed nearly 500 years ago: “All things are poison, and nothing is without poison; only the dose permits something to be not poisonous.”

Oncologists deal daily with a Paracelsian dilemma: most front-line chemotherapy drugs are potentially lethal to patients at the dosage needed to eliminate their cancer, but at lower dosage surviving tumour cells can become resistant to the drug; yet the maximum tolerated dose unavoidably causes collateral damage to healthy, rapidly dividing cells, resulting in side effects including hair loss, kidney damage, irreparable hearing damage and nausea.

Associate Professor Martina Stenzel, a polymer chemist with the University of New South Wales’ Centre for Advanced Macromolecular Design, is developing polymer nanoparticles that will deliver chemotherapy drugs in ways that will concentrate their cell-killing power within tumours, sparing healthy cells. The idea is not new, but making it work is technically daunting.

Nanoparticles

Stenzel is applying her expertise in polymer synthesis to develop nanoparticles targeting established front-line drugs like cisplatin and doxorubicin, using both active and passive delivery systems.

She is also investigating novel delivery systems for promising anti-cancer compounds like albendazole and curcumin, the latter being better known as the popular Indian spice turmeric.

Active targeting systems exploit some unique or distinctive properties of the tumour itself, such as the over-expression of certain hormone receptors in cancers of reproductive tissues.

Nanoparticle-encapsulated drugs can also be targeted to tumours using monoclonal antibodies against particular proteins over-expressed on the surface of the tumour cells.

Passive targeting exploits a tumour’s dependence upon a high rate of blood flow to fuel its rapid growth, and loss of some of the barriers that protect healthy organs against toxins as a result of its reduced gene-expression repertoire.

One of Stenzel’s primary interests is cisplatin, the most widely used chemotherapy agent for solid tumours. It comprises a central platinum atom linked to two ammonium groups and two chlorine atoms and is delivered intravenously.

It then diffuses throughout the body, even crossing the blood-brain barrier, which means it can be used to treat brain tumours, but its solubility in serum is relatively poor.

Working with St George Hospital oncologist Dr Paul de Souza, Stenzel’s team is seeking to target cisplatin to prostate and ovarian tumours, relying on the fact that both cancers over-express hormone receptors: androgen receptors, in the case of prostate tumours; and estrogen or progesterone receptors in ovarian tumours.

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“We try to decorate the nanoparticles carrying the drug with molecules designed to bind to the receptors,” says Stenzel. “The idea is not to mimic the natural hormone ligand but to attach functional groups to the nanoparticle that complement some aspect of the receptor’s chemistry.”

“There are quite big holes leading into the interior of most tumours, and they have a ‘leaky’ endothelium that lets nanoparticles up to 500 nanometres in diameter through, but to be safe, we keep our particle sizes below 100 nanometres,” she says.

The nanoparticles are built up from hundreds of self-assembling polymer subunits that are amphiphilic – meaning they have a hydrophobic part and a hydrophilic part – which are rapidly taken up through cell membranes.

The chemistry of the nanoparticles can be ‘tweaked’ to deliver more highly toxic drugs in inactive, pro-drug form to the cellular compartments where they will perform their executioner’s role.

Like many chemotherapy agents, cisplatin operates in the cell nucleus, where it wedges itself into the DNA helix, causing a kink that jams the cell’s DNA repair mechanisms (there’s a nifty high-definition YouTube animation that shows it in action – just search “cisplatin”). The damage activates the tumour-suppressor gene P53, causing cell death by apoptosis.

Cisplatin is inactive while it remains encapsulated in the polymer nanoparticle, so after being taken up by the cell, the polymer nanoparticle must release the drug to activate it and allow cisplatin to migrate into the nucleus.

Sturdy RAFT

Stenzel’s team creates its biopolymer nanoparticles by RAFT (Reversible Addition-Fragmentation chain Transfer) polymerisation, a technique pioneered by eminent CSIRO polymer chemist Dr Ezio Rizzardo of the Division of Materials Science and Engineering in Clayton.

“RAFT polymerisation builds fairly large molecules from small subunits that are both water soluble and oil soluble,” says Stenzel.

“The sub-units spontaneously self-assemble into larger structures that are hydrophilic on one side, and hydrophobic on the other. The advantage of RAFT polymerisation is that it is reversible and can be used to make a very wide range of molecules with tailored properties, there’s a RAFT polymer solution for virtually every drug-delivery problem,” she says.

“There are a few pathways we can use to liberate the drug from the polymer carrier. You can use a leaching mechanism to release the drug from the nanoparticle just before or after it enters the cancerous cell.

“We are also looking at exploiting different pH values in different cell compartments, that break the links between the subunits, allowing the drug to burst free of the nanoparticle.

“Inside the cell, the particles are taken up by lysosomes, enzyme-laden garbage-disposal organelles, which are more acidic than the cytosol. The higher acidity causes changes in the polymer structure, breaking it down and releasing the drug.”

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Stenzel’s team is also looking to repackage albendazole, originally developed as an oral anti-worm treatment in the 1970s. Albendazole shows promising activity against both prostate and ovarian tumours but is highly hydropobic and does not dissolve readily in the body.

“We are working to encapsulate albendazole in a polymer nanoparticle that will allow it to be delivered intravenously. It will remain encapsulated and inactive as long as it remains in the bloodstream, but will be released when it is carried into a tumour.”

Polymer nanoparticles also offer new options for ‘double whammy’ cancer therapies, combining frontline metal-based drugs like cisplatin with other molecules like doxorubicin or albendazole.

Combining drugs with independent, complementary modes of action should increase cell-killing power at reduced dosages, while reducing side effects and the risk of resistance.

Stenzel’s team, in collaboration with clinician Professor David Morris and pharmacologist Dr Mohammad Pourgholami, of the St.George hospital, is seeking ethical approval for the first animal trial of albendazole embedded in a polymer nanonparticle.

Morris and Pourgholami have already tested the encapsulated drug on tumour cell cultures at St George Hospital. Stenzel says it induced cell death at very low concentrations because of its increased solubility.

Another possibility is curcumin, a phenolic molecule extracted from turmeric, the bright yellow spice derived from the tubers of Curcumin longa, an Asian relative of ginger. It shows considerable potential as an anti-cancer compound but, like albendazole, it is very hydrophobic.

Turmeric is a common ingredient in Indian cuisine and news of its anti-cancer properties has led to people to experiment with it as a folk remedy for cancer. Stenzel says ingesting curcumin is unlikely to be of any benefit in suppressing cancer because of its extremely poor bioavailabilty. Even a 10-gram oral dose produces almost undetectable levels of curcumin in serum.

Stenzel and her colleagues are developing a nanoparticle to encapsulate curcumin and improve its solubility in serum. If curcumin can be safely delivered and released inside cancerous cells, it acts as a potent disruptor of the NF kappa-beta inflammatory pathway. NF kappa-beta plays a key role in driving cancer cell growth and replication, so its disruption triggers cell death by apoptosis.

If Stenzel’s work continues to see success, there’s a very real possibility of improving the existing anti-cancer drugs we have today, and doing so while reducing their undesirable side-effects. For such tiny things, nanoparticles can have great rewards.