South Korean neuroscientists and bioengineers have successfully developed a new blood test for Alzheimer's disease (AD), which was shown to be capable of rapidly distinguishing between AD patients and individuals with normal cognition, with selectivity and sensitivity exceeding 90%.

Early AD diagnosis is important, but "current AD diagnostic methods require patients to visit medical facilities, usually because patients or their families notice there is something wrong with their memory," said study co-leader YoungSoo Kim, a professor in the Integrated Science and Engineering Division and Department of Pharmacy at Yonsei University, Incheon.

"Onset of cognitive impairments means brain damage has already occurred and there are currently no clinical methods to reverse that damage, so the best therapeutic options are preventive approaches before brain atrophy begins."

While there have been many previous studies of AD markers, said Kim, "ours is the first to distinguish AD patients from normal controls within just two hours and with over 90% sensitivity and selectivity using a single plasma protein biomarker."

Abeta protein

That single plasma protein biomarker is amyloid beta (Abeta), "abnormality in levels of which precedes AD's pathological events, with blood tests being common methods to detect at-risk patients," Kim told BioWorld.

However, use of Abeta levels for diagnosing AD has been controversial, due to unreliable results concerning protein concentrations, with different studies indicating decreased, increased or unchanged Abeta levels.

Nevertheless, Abeta assessment is minimally invasive and widely accepted as the earliest diagnostic plasma biomarker for AD.

Abeta is transported from the brain, where it exists as soluble and insoluble aggregates, to the blood by low-density lipoprotein receptor-related protein 1.

It is unclear whether Abeta aggregates cross the blood-brain barrier, but studies have reported oligomeric Abeta to be a component of plasma Abeta. Oligomeric Abeta molecules may therefore exist as heterogeneous sizes and affect quantitative plasma measurements.

One objective of the collaborative study led by Kim and Kyo Seon Hwang, a professor in the Department of Clinical Pharmacology and Therapeutics, College of Medicine, Kyung Hee University, Seoul, was therefore to confirm Abeta presence as a heterogeneous mixture by developing a way to measure true plasma Abeta concentrations.

It is also unclear whether Abeta aggregate concentrations are kept at a consistent level since, if such homeostasis were not maintained, it would be important to establish a means to normalize concentrations, enabling comparative analysis of variations.

For plasma Abeta to be useful as an AD biomarker, the method of quantification and normalization of levels of heterogeneous Abeta species must minimize confounding variables, enabling precise clinical differentiation of AD patients.

Moreover, any Abeta detection device should also be highly sensitive, as plasma Abeta concentrations exist at picomolar concentrations, the researchers reported April 17, 2019, in Science Advances.

The South Korean researchers therefore devised an advanced interdigitated microelectrode (IME) sensor system and used it in conjunction with 4-(2-hydroxyethyl)-1- piperazinepropanesulfonic acid (EPPS), a small molecule helping to convert aggregated Abeta into its monomeric form.

"We originally tried commercially available enzyme-linked immunoabsorbent assays [ELISA], but these were insufficiently sensitive to show changes in plasma Abeta concentrations of 10-100 picomolars, while the IME sensors we developed can detect proteins at subpicomolar levels," said Kim.

That newly developed AD diagnostic technique was used to compare heterogeneous and monomerized forms of plasma Abeta in plasma, using EPPS to dissociate aggregated forms into monomers to enhance quantification accuracy.

The researchers then compared Abeta levels of EPPS-treated plasma with those of untreated samples, in order to minimize inter- and intra-individual variations. The IME sensor system was able to measure plasma Abeta levels at picomolar levels.

"EPPS treatment dissociates oligomeric Abeta, which will acutely and artificially increase the Abeta concentration as measured by antibodies, if an individual has oligomers in blood," said Kim.

"Conversely, in a non-AD patient, the plasma Abeta oligomer concentration should be lower and the EPPS-mediated Abeta concentration increase minimal, which is how we distinguish AD patients from normal subjects."

The implementation of that self-standard blood test was shown to result in substantial distinctions between AD patients and those with normal cognition, "with a sensitivity of 93% and a specificity of 97% and minimal overlaps between groups," said Kim.

"In this study, we reported results from 61 amyloid-positive AD patients and 45 amyloid-negative cognitively normal subjects, diagnosed using positron-emission tomography," said Kim. "Including our current ongoing investigation, the total subject number now exceeds 200 in each group."

Looking ahead, "we hope our new AD diagnostic technology could become one of the primary screening tools ahead of current techniques," said Kim. "We are now conducting a larger-scale clinical investigation and planning to expand the technology to other brain disorders, such as Parkinson's disease.