Minicells, the spiky gold balls, emerge from the leaky blood vessel within the tumour mass. They proceed to disable the tumour cells with a one-two punch as shown in the enlarged tumour cell below. Following the arrows clockwise, the minicells first deliver siRNA to disable the MDR pump (small purple dumbbells). Then they deliver the cancer drugs (small red spheres).

Credit: Russell Kightley

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Other carrier systems for siRNA have been proposed and tested, such as fat bubbles known as liposomes, but these are more flimsy and difficult to fit with an antibody-based guidance system.

Opens door for new drugs

Another plus is that the targeted minicell does not trigger much of an immune response so the minicells can be injected over and over again, the scientists say. Their size makes the minicells more lethal to tumours than to healthy tissue. At 400nm in diameter, they tend to fall out of the slipshod new blood vessels that supply tumours. Blood vessels that supply healthy tissue are leak-proof.

The ability of minicells to deliver their cargo directly to tumours has Bruce Stillman, the President of the Cold Spring Harbour Laboratory in Long Island, New York excited, "This has the potential to deliver drugs that were [previously] considered undeliverable", he says.

Drug companies have developed innumerate cancer-targeting drugs that never made it into the market place because of toxicity or, in the case of siRNA, because they were too unstable in the bloodstream. "Most of the expense of drug development lies in the process of making things bio-available or low toxicity. This technique bypasses all of that."

In every case the tumours shrank

So far, pet dogs with terminal cancer have been the first patients to benefit from the power of minicells. In a study reported in Cancer Cell in 2007, the dogs were injected with minicells that delivered cancer drugs, such as doxorubicin.

Because very little of the drug was released into the bloodstream, the dogs suffered few side effects yet whopping doses were delivered to the tumours. In every case the dog's tumours shrank. "Many dogs are in remission; the owners are delighted", says MacDiarmid.

In the study released today, the minicells were used against drug-resistant tumours in a one, two punch strategy. This time the patients were mice that had been seeded with human tumours.

The tumours were drug-resistant because they ramped up their output of a gene called MDR1, which codes for a protein that pumps drugs out of the tumour cells.

In the first punch, minicells delivered siRNA to disable the MDR1 pump. Five days later, minicells delivered the coup de grace – drugs that the tumours could no longer resist. 100% of the mice recovered. MacDiarmid says they have seen similar as yet unpublished results in pet dogs with drug-resistant tumours.

In the next few months, the company will recruit 20 long-term cancer patients for a phase one safety trial at three Melbourne hospitals. Stillmann, though cautious about the road ahead, says the dog studies augur well: "Dogs are as close as you can get to human cancer." Adds Bryan Williams, the Director of the Monash Institute of Medical Research in Melbourne: "It's an exciting new technology but the big issue is safety."

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