Breathtaking results in Alzheimer's drug trial

Melbourne's Prana Biotechnology reported remarkable results from a pre-clinical trial of its second-generation Alzheimer's disease drug in mice at the International Conference on Alzheimer's Disease and Related Disorders in Madrid in July.

Prana's co-founder, Professor Ashley Bush, reported that the mice improved markedly in Morris water maze tests less than a week after commencing once daily oral doses of Prana's PBT2 metal-protein-attenuating compound (MPAC).

Showing evidence of learning after five days of treatment, the AD mice performed virtually as well as normal mice in the Morris maze, a standard test of memory in the amyloid precursor protein (APP) gene mouse model of human Alzheimer's disease.

"Usually, a therapeutic intervention like the Alzheimer's vaccine takes many weeks, even months to show an effect," Bush said.

"With PBT2, we saw profound rescue of memory function within five days. By 10 days, they were basically operating like normal mice. "The result was breathtaking, beyond anything we had expected."

Bush said the response of the mice to the experimental drug had provided new insights into the nature and progression of Alzheimer's disease. "It's teaching us something that we had theorised - that there are two components to the disease.

"There is a loss of neurons, which may be irreversible. We don't expect to see neurons growing back.

"But we know that these mice don't lose many neurons, even though they show clear memory impairment. Their brain tissues contain lots of amyloid plaque, and we see clear symptoms of memory impairment, as well as synaptic pruning - the neurons are not forming many synapses. "Most of the neurons remain alive; they're just not making as many connections as they should.

"We're seeing something like [cellular] intoxication before neurons die, and we think this happens in humans with the disease. They have good days and bad days."

Bush said the beta amyloid plaques produce hydrogen peroxide, which breaks down to highly reactive oxygen free radicals, causing oxidative damage to cellular proteins and nucleic acids (see below).

"We would expect that, depending on the brain's ability to fight back, people will move in and out of mental confusion," he said.

"It looks like our mice are exhibiting much more of that effect than we previously realised. We believe the drug shuts down hydrogen peroxide production and free radical attack, so we see this surprising sobering effect in the mice.

"The hope is that we might see something similar in human Alzheimer's patients, if we can diagnose and treat them early enough, before large numbers of their neurons begin to die."

Clinical trials in humans

The company's next step is to conduct clinical trials in humans. It is collaborating with Swedish researchers at the University of Uppsala and the University of Gothenburg.

"Our plans for human clinical trials emphasise trying to identify people developing Alzheimer's disease before they're too far gone," Bush said. "We have a proposed study in Sweden where we'll be looking at people relatively early after diagnoses, and treating them with PBT2 over a narrow time frame of only three months.

"If their Alzheimer's disease is biochemically similar to what we see in the mouse, we could possibly see a clinical improvement in that period. The idea of a rapid onset of therapeutic benefit is no longer far-fetched, providing the brain has not lost too many neurons."

Asked whether the drug, by detoxifying the brain, could create conditions conducive for brain stem cells to replace some of the lost neurons, particularly in the brain's memory centre, the hippocampus, Bush said this was possible. "Hypothetically, if you can put out the fire, the cells could begin to replenish."

Research by Huntington's disease (HD) researcher Dr Tony Hannan, at the Florey Institute in Melbourne, has shown that the brain's resident pool of neural stem cells grow new hippocampal neurons in transgenic HD mice provided with a mentally stimulating environment, and exercise. Bush said Prana has accelerated the trials process on the basis of the drug's highly promising performance in pre-clinical trials. "On all pre-clinical measures, it's at least an order of magnitude more potent than clioquinol," he said.

"It's fantastic to see an experimental drug have such a rapid therapeutic benefit - we believe it's as record for any effect seen for a chronic brain disorder."

Drugs in the pipeline

Ashley Bush said other experimental AD therapies, including the discontinued Elan Pharmaceuticals vaccine, had taken many weeks to months to begin producing an effect, and the effect was much smaller in magnitude.

(In January 2002, Irish company Elan Pharmaceuticals discontinued trials of an experimental vaccine against beta amyloid plaque protein, after 15 of 97 volunteer patients developed brain and spinal cord inflammation several weeks after a second injection of the vaccine. Previous tests showed the vaccine cleared amyloid plaques from the brain, and it appeared safe.)

Prana has other MPAC drug candidates in its development pipeline, including one that is showing considerable promise for Huntington's disease. Clioquinol has turned out to be a promising treatment for Parkinson's disease, and the company is also developing MPAC candidates for motor neuron disease.

The common thread to all these chronic brain disorders is that they are all associated with protein aggregation in specific regions of the brain. Bush said that the identity of the aggregating protein varies - in Parkinson's disease, for example, alpha synuclein protein forms aggregates called Lewy bodies.

He said that by matching the MPAC molecule to the offending protein aggregate, it may be possible to develop specific therapies for these chronic brain disorders.

Originally, clioquinol was thought to work by chelating the metal ions, rendering them non-reactive. But Bush said it now appeared that, in Alzheimer's brains, the drugs mediate a process that redistributes the metal ions from their abnormal compartment, outside neurons, back into the neurons themselves.

In the new picture, the amyloid plaques fall apart, and the metal ions to diffuse back into ailing, metal-deficient neurons, restoring their function and causing them to regrow new dendrites.

Still some doubters

PBT2 is furthest advanced in the clinical pipeline, and Bush said that if the therapeutic benefit seen in the AD mice is replicated in humans, without adverse effects, the drug could be on the market within five years.

The metal-ion hypothesis of Alzheimer's disease has gone from fringe to mainstream in the past decade, but Bush said there are still doubters. One reason for the skepticism, he said, is that no major medical illness has ever been linked to abnormalities of copper and zinc metabolism, and the new theory sounded like a re-run of the now-discredited hypothesis that linked AD to aluminium toxicity.

Menkes disease and Wilsons disease are major, hereditary disorders of copper metabolism, but outside these specialist areas, few researchers are investigating the biochemistry of copper, or the possibility that abnormalities in copper metabolism could underlie chronic disease.

The metals hypothesis of Alzheimer's

In the 1990s, Bush, a former PhD student of eminent Australian neurodegenerative diseases expert Professor Colin Masters of the University of Melbourne, developed a new hypothesis for the mechanism development of Alzheimer's disease, based on abnormal metal metabolism in the brain.

Beta amyloid plaques and fibre-like tangles accumulate in the brains of all ageing mammals, including humans; in hereditary, early-onset forms of AD, the process is accelerated.

The beta amyloid protein has long been presumed toxic to neurons. It is cleaved from a larger protein, Alzheimer's precursor protein (APP), encoded by a gene on chromosome 21.

Beta amyloid is normally a soluble serum peptide, but as the individual ages, it precipitates and aggregates in the brain, forming plaques that are resistant to breakdown.

The Bush hypothesis proposes that copper ions released from neurons abnormally combine with beta-amyloid to catalyse the production of hydrogen peroxide; as the hydrogen peroxide breaks down chemically, it generates oxygen free radicals that damage proteins and nucleic acids in neural tissue.

In a seminal experiment in 1996, Bush showed that clioquinol, an old 1960s remedy for traveller's diarrhoea, rapidly dissolves amyloid plaques by removing the copper and zinc ions that cause them to aggregate - the metal ions pin the peptide fragments together.

He later showed that treatment with clioquinol relieved the mental confusion of AD mice, and significantly improved their ability to swim water mazes, which test memory function in the mouse model of the disease.

A small-scale trial in human volunteers with advanced AD also produced a small but significant improvement in their mental clarity and memory - even though late-stage AD patients have probably suffered a major loss of neurons in the brain.

Last year Prana decided to move to a second-generation compound developed by its chemists, based on the structural theme of clioquinol - PBT2 showed much more potent activity than clioquinol in all pre-clinical tests, including its ability to disaggregate purified amyloid plaque in vitro. PBT2 also shows superior solubility to clioquinol, and other characteristics that suggest it should cross the blood-brain barrier in high concentrations.

Tested in AD mice, it proved significantly superior in reducing amyloid plaque concentrations in the brain.

Researchers at the University of Utrecht in the Netherlands successfully completed phase 1 (safety and tolerability) clinical testing of PBT2 in normal volunteers in March 2006, showing that it was well tolerated at the doses proposed for Alzheimer's therapy.