Cholinesterase inhibitors – the future of Alzheimer’s therapy?

Cholinesterase inhibitors – the future of Alzheimer’s therapy?
Moss argued that irreversible acetylcholinesterase (AChE) inhibitors offer a new opportunity for both therapy and prophylactics against the progression of Alzheimer’s disease © iStock

Speaking at an AD/PD focus meeting, Professor Donald Moss discussed cholinesterase inhibitors as a new and more effective treatment for Alzheimer’s disease.

In March, SciTech Europa travelled to Torino, Italy to the AAT-AD/PD focus meeting on ‘Advances in Alzheimer’s and Parkinson’s Therapies’. There, SciTech Europa attended a session entitled ‘clinical trials in AD, PD, and progressive supranuclear palsy’, and there listened to Donald Moss, Professor Emeritus at the University of Texas, USA, who recently retired his tenured full professor faculty position in 2010 and now devotes full time to the development of cholinesterase inhibitors for the more effective treatment of Alzheimer’s dementia.

He set up his speech by proclaiming that “this is an exciting time in the history of Alzheimer’s disease,” before explaining that his address would be used to explore the possibility that irreversible acetylcholinesterase (AChE) inhibitors which, he said, “offer a new opportunity for both therapy and prophylactics against the progression of Alzheimer’s disease.”

According to Moss, there is now evidence that cholinesterase inhibitors, which act by blocking the breakdown of acetylcholine, one of the severe deficiencies that occurs early in the progression of Alzheimer’s disease, have strong disease modifying, or even disease preventing, properties.

Donepezil

One of the most exciting things in the development of Alzheimer’s disease, Moss said, particularly with regard to the use of acetylcholinesterase inhibitors, has been demonstrated in new research which has shown that a one year course of treatment with donepezil will slow the loss of the cholinergic mid-brain system in prodromal Alzheimer’s disease patients.

“It is very interesting,” he went on, “that before the disease has progressed entirely into Alzheimer’s, it is in fact possible to slow the loss of the mid-brain cholinergic system. Donepezil, however, is a poor drug to start with, and it is amazing that it has any effects at all; but it does in this case.

“Of course, we already know that in people who have already been diagnosed with Alzheimer’s disease that cholinesterase inhibitor therapy will both slow the atrophy of the hippocampus as well as prevent white matter loss, and that long term cholinesterase therapy appears to slow the progression of the disease. But the problem that we have in Alzheimer’s disease at the moment in taking advantage of acetylcholinesterase inhibitors is that the ones we have are just awful.”

He went on to explain that in cholinesterase therapy, it is generally assumed that it takes at least a 50% inhibition in order to have a therapeutic effect, and that donepezil, as an example, produces only 25-35% inhibition as measured in vivo by CAT scans in Alzheimer’s disease patients. “There is thus a huge untapped potential for improving cholinesterase therapy,” he said.

One way that the potential for therapy can be increased is, for Moss, by using irreversible inhibitors. He explained: “This is a mechanism of action that is extremely important and which sets it apart from the other cholinesterase inhibitors, because the only way the brain can recover from irreversible acetylcholinesterase inhibition is through the synthesis of new enzymes, and fortunately that synthesis takes place 12 times more slowly than in the peripheral tissues.”

Methanesulfonyl fluoride (MSF)

The irreversible inhibitor Moss focused on in is presentation was methanesulfonyl fluoride (MSF), but, he said, any truly irreversible inhibitor would have the same effect.

Using a visual aid to illustrate his argument, Moss showed his audience a graph of the accumulation of inhibition from the beginning of therapy to 21 days. This demonstrated an overall increase in inhibition, as well as a dissipation of inhibition between doses, giving the graph a ‘saw-tooth’ appearance.

Essentially, the graph showed that with an irreversible inhibitor, inhibition accumulates in the brain to a very high level because it is being replaced at a very slow rate, whereas in the peripheral tissues it is replaced very quickly.

“With donepezil,” he explained, “there is no separation between the brain and intestines, but with an irreversible inhibitor there is a huge separation between them, and this gets us way from the problem of gastrointestinal toxicity and opens the door to high level cholinesterase inhibition.”

Referring to his next slide, he went on to explain that this was the real result of an experiment in which rats were treated with MSF over 21 days. The rats were then sacrificed and the cholinesterase was assayed in the various tissues. In the brain, as Moss had anticipated, there was a very high level of cholinesterase inhibition, “way better than 25-35%, as is currently available,” he said.

Clinical trials

Moss revealed that MSF has been through three clinical trials so far, but he has still not been able to probe the upper limits of what might be possible with an irreversible cholinesterase inhibitor.  As such. Moss and his team conducted a further experiment involving Macaca fascicularis (Crab Eating Cynomolgus) monkeys, which were administered escalating doses of MSF over three months.

In the first instance, the monkeys were given very low levels of MSF, just a third of the human clinical dose, and this rose to five times the human clinical dose by the end of the experiment, at which point the animals were maintained on that schedule for 10 more weeks.

“At the end of the experiment,” Moss said, “we found that there was, as expected, no real toxicity.” Nor was there any diarrhoea or weight loss in the monkeys; and there were no behavioural changes, and no outward signs of toxicity.

“At the end of the 10 weeks of the highest doses,” Moss went on, “we took cortical biopsies two and a half days after the last MSF injection. We did this to see what level of cholinesterase inhibition was in the brain. As far as acetylcholinesterase inhibition in the cortex, we found 80% inhibition, very generous amount and well above what is necessary for a therapeutic effect.”

Enzyme synthesis

Methanesulfonyl fluoride requires new synthesis of enzyme to overcome inhibition, and this enzyme in the monkeys, which was synthesised in the 2.5 days between the end of the MSF dosing and the time of the biopsies, was estimated by Moss and his team as being probably around 90% inhibited in the cortex at the peak at the last MSF injection, and over the following 2.5 days it declined to 80%. “What is really valuable about this is that it allows us to estimate the acetylcholinesterase is synthesised in the cortex with a half time of 12 days; it is important to know how quickly the enzyme is being replaced in the brain in order to predict its therapeutic effect.”

Using this data, it is possible to predict what will happen in long term therapy when patients are kept on irreversible cholinesterase inhibitor for a period of two months or longer after the drug has had an opportunity to get into steady state inhibition in the brain. According to Moss, even at a low dose of MSF 50% inhibition in the brain is witnessed, “and if we are talking about using cholinesterase inhibitors as a prophylactic,” he added, “inhibition is way above the 25-35% that’s currently available.” Indeed, the level of cholinesterase inhibition in Moss’s human trials was 66%, and in the monkeys was 90%.

Obtaining this data and understanding these levels of inhibition is crucial, Moss went on to explain, because there is no guidance from toxicity when using irreversible cholinesterase inhibitors. “You get no guidance for dosing from gastrointestinal toxicity, which means that it is possible to essentially give any dose, within reason, so how do you know where to start? “ he asked.

The therapeutic window

The therapeutic window starts at 50% and extends up to around 85-90% inhibition in the brain, Moss said, with and the upper limit of the therapeutic window being limited by 2 things:

  • Diminishing returns with increasing dose; and
  • The accumulation of inhibition in the intestines up to a near toxic level, after which point toxicity begins to manifest.

For Moss, irreversible ACHE inhibitors open the door to real therapy in Alzheimer’s disease, both symptomatic therapy and, given the recent developments with slowing the disease process, there is also a sense that they have a role to play here too.

“Their prophylactic potential has also been widely tested, and, because of the wide range in the therapeutic window, we can walk patients up that curve as necessary in order to maintain their cognitive function or in response to changes in neural imaging,” he said.

Currently, MSF has been the subject of three successful clinical trials and is now freely available to anyone who wants to work on it to further their research in this area.

This article will appear in SciTech Europa Quarterly issue 27, which will be published in June, 2018.

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