By David N. Leff

Seeing is believing.

That prudent maxim has a special meaning in the diagnosis of Alzheimer's disease (AD). A skilled clinical neurologist, by dint of extensive behavioral testing, can render a verdict for a particular patient that's better than 90 percent accurate. The remaining 10 percent or so are confused with the spectrum of other senile dementias that superficially look alike - and like AD.

Their main difference is that unlike AD, these other cognitive deficits can be treated with more or less efficacious drugs. So far, there is no approved medicament for predicting, arresting, slowing or ameliorating the devastating laundry list of cognitive, behavioral and other symptoms that mark Alzheimer's disease.

The key hallmark of AD is invisible to the eye - even of the most experienced clinician. It's a peptide called amyloid-beta, which forms senile neuritic plaques around dying nerve cells in the brain and is associated with the loss of memory and orientation featured in authentic Alzheimer's disease.

The only way those telltale plaques can be seen - and thus a diagnosis of AD definitely confirmed - is by post-mortem examination of the deceased patient's brain. This limited form of pathological voyeurism has - as of now - limited utility. It can verify a neurological determination, thus sharpening the practitioner's expertise. It can lay to rest, or alert, the concerns of the patient's family as to the true nature of his or her dementia, hence its likelihood to afflict other relatives.

Modern scanning techniques - MRI (magnetic resonance imaging), PET (positron emission tomography) and SPECT (single positron emission computerized tomography) - can visualize the living brain, but not deep down and dirty enough to detect the neuritic plaques of AD. That need is becoming more urgent.

Two years ago, pioneer AD neurologist Dennis Selkoe said that "a number of pharmaceutical companies have publicly announced that they are finding candidate compounds" to stop amyloid-beta peptide from cutting free of its progenitor molecule, amyloid precursor protein (APP), and beginning to make AD plaques. (See BioWorld Today, May 21, 1998, p. 1.)

Drugs In Pipeline Need Human In Vivo Tests

Now, in the August 2000 issue of Nature Biotechnology, Selkoe points out that ". . . the past few months have witnessed the initiation of the first human clinical trials of compounds specifically designed to interfere with alpha-beta accumulation in the brain and thereby potentially slow or even prevent the disease. But such efforts need to be accompanied by sensitive and accurate methods to diagnose the disease."

His observation leads off a "News and Views" commentary in that journal, accompanying a research report titled: "Targeting Alzheimer amyloid plaques in vivo." Its senior author is molecular neuroscientist Joseph Poduslo, professor of biochemistry and molecular biology at the Mayo Clinic and Foundation in Rochester, Minn.

"I think one of the major obstacles to treating neurodegenerative disorders like AD, as well as for the diagnosis of these diseases," Poduslo told BioWorld Today, "is the restriction of the blood-brain barrier - where the brain is a privileged organ. It's good for the brain, but unfortunately if you want to get drugs into the brain, it's a pretty major obstacle.

"We've developed ways of getting around that," he continued, "specifically with peptides and proteins by polyamine modification. Our approach therefore is to use proteins that have high affinity for amyloid-beta peptide to see if we can image its plaques in a living patient. If we can show efficacy of delivery in transgenic mouse models of neurodegenerative diseases with regard to drug therapies, hopefully there'll be chances to apply this to patients for early diagnosis and treatment before cognitive decline begins."

For openers, Poduslo's strategy began with a polyamine molecule called putrecine, which has the property of traversing the blood-brain barrier (BBB). He and his co-authors coupled this genteel reverse brain burglar to a radioactive isotope of iodine, 125I, and a human protein called amyloid-beta 1 to 40, which binds with high affinity to amyloid-beta plaques. (Those numbers 1 to 40 signify the 40-amino-acid sequence that defines the plaque's fragment of APP.)

The team injected this double-threat BBB-bypassing, plaque-fingering construct into the veins of 27-week-old, young adult, transgenic mice innately prone to come down with AD in later life. Their genomes carry two AD-mutated human proteins - for APP and presenilin.

"We found by histological analysis," Poduslo recounted, "that deposition of neuritic-type plaques occurred at an accelerated rate starting around 12 weeks, reaching a quadrupled amyloid burden in cortex and hippocampus in one year.

"What this means," he pointed out, "is that we've been able to develop a probe that one would give systemically, that crosses the barrier and binds tightly to plaques. Consequently," he added, "we think that this might be useful for potential imaging of plaques down the road with SPECT, PET or MRI. I think that by some of these approaches we could probably label more plaques. Our thinking and direction now is to try to image them in living mouse brains by one of these techniques."

With Patients In Mind - Heavy Metal

Recently, he and his co-authors have taken on a non-radioactive, rare-earth, heavy-metal element called gadolinium. It's currently used as an opaque contrast medium in MRI scanning. "We're essentially repeating the same study that we described in the paper," Poduslo said, "but with gadolinium attached.

"Ultimately in a patient, for example," he went on, "one would use only the gadolinium. But our intent at this point is to see if we could just image the live animal by MRI. If that works, it sort of opens up the doors for patient studies."

Mayo has one patent issued a few years ago on Poduslo's polyamine modification technique, and several others pending. Talks with potential commercial partners, he vouchsafed, "are in development," and concluded, "I think if a company has a specific protein-based therapy that they are interested in exploiting for the central nervous system, they might want to use our delivery technology."