08 Dec 2017

As prevention trials for Alzheimer’s disease ramp up, researchers need fast and cheap ways to diagnose early, and to screen cognitively normal people. PET is expensive. Cerebrospinal fluid (CSF) biomarkers of amyloidosis and tauopathy could bring down costs, and the field is moving from manual, research-grade immunoassays to automated systems that robustly measure CSF Aβ peptides Aβ42, Aβ40, and tau within an hour’s time. At the 10th Clinical Trials on Amyloid Disease conference, held November 1–4 in Boston, several groups presented new data validating the automated tests to support their widespread clinical use. Excitingly, just as CSF tests are nearing this important finish line, the story on nascent blood tests is suddenly changing from one of failure to one of great promise. New results from two different blood tests for Aβ give a glimpse of a future with less-invasive tests speeding trial recruitment.

In the latest validation study of automated CSF assays, Chengjie Xiong, working with Anne Fagan at Washington University, St. Louis, asked if Roche’s Elecsys platform for analyzing CSF biomarkers—for which low variance continues to be established in various clinical cohorts—manages to predict clinical progression in cognitively normal older people. Xiong measured CSF Aβ42, total tau (tTau) and phosphotau (pTau) in 362 clinically normal volunteers, aged 65 or older, who had been followed for an average of five years with repeat cognitive assessments. In this time, 84 people progressed to a clinical dementia rating of 0.5 or higher. After adjusting for age, ApoE4 status, sex, and education, Xiong found that Aβ42 concentration, or the ratios of tTau/Aβ42 or pTau/Aβ42, best predicted clinical progression and cognitive decline. Concentrations of tTau or pTau alone were not associated with progression.

In a separate analysis, Fagan’s group determined cutoff values for biomarker positivity in 198 people who had undergone both lumbar punctures and PiB-PET scans. Applying the cutoffs to their new data, they found that people on the amyloid-positive side of the cutoff had twice the risk of clinical progression and faster rates of cognitive decline than those on its amyloid-negative side. The work shows that automated tests of CSF biomarkers can identify candidates for further screening and enrollment in clinical trials, the authors said. 

On a second poster, Fagan and colleagues asked how well a competing automated assay, Fujirebio’s Lumipulse G Aβ42, compared with that company’s research-grade standard, the INNOTEST ELISA, at predicting brain amyloid. Fagan analyzed CSF from a cohort selected to have varying amyloid levels by PiB-PET, and used both the Aβ42 Lumipulse and the Aβ40 ELISA. Both assays gave low and acceptable variability between 3 and 8 percent. Absolute values for Lumipulse were 50 percent higher than for ELISA, but the two measures correlated well. As expected, both Lumipulse and INNOTEST Aβ42 discriminated PiB-PET-positive from -negative people better than did Aβ40. The Aβ42/40 ratio was best of all, with AUCs of 0.88 and 0.91 for the INNOTEST and Lumipulse G Aβ42 assays, respectively. 

Fagan’s results broadly echo a recent clinical validation of Roche’s Elecsys assay, presented at the AD/PD conference last spring by Oskar Hansson of Lund University, Sweden (Apr 2017 conference news). In that study, cutoff values determined in the BioFinder cohort were applied to ADNI participants, in whom they successfully identified those with brain amyloid accumulation. In that study, too, the ratio of Aβ42/40 or Aβ42/tau outperformed Aβ42 alone in predicting amyloid-PET status.

A month ago, Fujirebio released its Lumipulse tTau assay, and Fagan told Alzforum that her lab is testing it currently. In the meantime, Didier Pitsi of the BARC Global Central Laboratory in Ghent, Belgium, shared his experience with both Lumipulse G Aβ42 and total tau assays in a large clinical lab. In Pitsi’s hands, Lumipulse had a 7–8 percent variance for Aβ42 and 7–11 percent for tau. That’s better than the 15–20 percent variability routinely reported for manual ELISAs in the Alzheimer’s Association Quality Control program, which sends samples to dozens of labs for parallel analysis.

Data from the limited number of quality-control runs of the automated tests available thus far are starting to show similarly consistent results. In five different labs, the Elecsys Aβ42 assay had a 5 percent variance, while Lumipulse G Aβ42 came in at 9.5 percent when compared in three labs. Floyd Sarsoza, University of California, San Diego, presented additional validation of the Lumipulse Aβ42 assay. He saw less than 3.4 percent variation on pooled CSF tested on different runs or days. The measurements remained stable over four weeks, and through three free-thaw cycles of the CSF.

Valeria Lifke, Roche Diagnostics, Penzberg, Germany, presented her company’s own validation data on the Elecsys tTau and pTau assays. They include a sensitivity of 60 pg/ml for tTau and 4 pg/ml pTau in CSF, good linearity and reproducibility, and little interference from a panel of drugs and endogenous compounds.

The results will support applications for marketing approval of the test for diagnostic use in Europe and the U.S. Roche scientists declined to answer questions about when their tests would be broadly available. So far, the Roche Elecsys Aβ42 test is approved in Europe, with the tTau and pTau assays expected in early 2018. Among Fujirebio’s Lumipulse assays, the CSF Aβ42 and tTau are approved in Europe, with Aβ40 expected soon. The tests are available for research use only in the U.S.; FDA approval will take longer.

Smelling Blood: Companies Catch on to Promise of Plasma Testing
Blood tests for Aβ and tau would enable screening of much larger numbers of potential participants for prevention trials. Like CSF tests, they likely would be cheaper than a PET scan, and many doctors and patients who demur when asked about a lumbar puncture readily agree to a blood draw. In the past, efforts to develop blood assays have foundered because Aβ concentrations in blood were 50 times lower than in CFS. Some blood cells make Aβ, and antibody-based plasma assays have been noisy. Together, this has stymied efforts to correlate blood Aβ levels with clinical status. Now, new techniques are “resurrecting blood assays,” said Fagan, and researchers are comparing blood Aβ levels directly with brain amyloid by PET.

For example, Virginia Perez-Grijalba, of Araclon Biotech, Zaragoza, Spain, presented data on her company’s Aβ test ELISA kit for detecting the Aβ40/Aβ42 ratio in blood. In a multicenter study carried out in Spain, France, and Italy, Perez-Grijalba and colleagues measured total plasma Aβ42 and Aβ40 in 228 people at baseline, and then again one and two years later. At the start, 83 of the group tested cognitively normal and 145 had amnestic MCI. The MCI group had a lower plasma Aβ42/40 ratio than the cognitively normal folks at all time points. After two years, 43 percent of those with aMCI had progressed to AD dementia, and their Aβ42/40 ratios dropped further. Those with a lower ratio at baseline also had lower FDG-PET and faced a 70 percent higher risk of progression to dementia over the next two years. Finally, Perez-Grijalba showed that the plasma Aβ42/40 ratio correlated with CSF Aβ42, and inversely correlated with brain amyloid levels as per PET. 

The Spanish group recently published similar data evaluating their blood test in the Australian Imaging, Biomarker and Lifestyle study of aging (AIBL). In this longitudinal cohort, a low plasma Aβ42/40 ratio in cognitively normal people predicted cortical amyloid PET positivity with more than 80 percent accuracy (Fandos et al., 2017). The study has limitations: The researchers have yet to determine universal cutoff values, and there is some overlap between groups. Further studies are underway to optimize the test, Perez-Grijalba said.

Even so, her data dovetails with work by the group of Randy Bateman at Washington University in St. Louis, who measured the plasma Aβ42/40 ratio by mass spectrometry. This test is being commercially developed by C2N Diagnostics in St. Louis, a company Bateman co-founded. He found that PiB-PET-positivity came with a 15 percent lower Aβ42/40 ratio in blood (Jul 2017 conference news). At CTAD, Bateman presented follow-up data, telling the audience that the plasma test was able to assign brain amyloid status with 88 percent accuracy. “That is remarkable, because they use a different strategy to measure Aβ, and yet we see very good agreement,” said Perez-Grijalba. “We think measuring Aβ42/40 ratios in blood has good prospects as a prescreening tool in AD trials,” she concluded.

At CTAD, the advent of blood tests to reveal who has brain amyloid generated considerable buzz. Beyond Araclon and C2N, big pharma appears to have caught the whiff of opportunity. Word in the hallways had it that Roche Diagnostics has already developed automated Elecsys blood tests for Aβ42, Aβ40, and tTau assays, and is evaluating them in clinical cohorts. Richard Batrla-Utermann of Roche Diagnostics confirmed the company’s interest. “Based on the published data we are interested in this combination for identification of patients with AD pathology at a general practitioner level, to funnel them into further assessment in memory centers,” he emailed to Alzforum.

Whoever makes a good blood test first will hand the field a tool to screen large populations of cognitively normal people and speed up recruitment for prevention trials. And once an effective therapeutic is finally found, “demand for a blood test will be very high,” Bateman said.

Importantly, though, the automated CSF tests have nearly completed extensive validation studies and are nearing routine clinical use, whereas all this still awaits the nascent blood tests. They remain investigational for a while longer.—Pat McCaffrey and Gabrielle Strobel

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References

News Citations

1. Are CSF Assays Finally Ready for Prime Time? 15 Apr 2017
2. Finally, a Blood Test for Alzheimer’s? 28 Jul 2017

Paper Citations

1. Fandos N, Pérez-Grijalba V, Pesini P, Olmos S, Bossa M, Villemagne VL, Doecke J, Fowler C, Masters CL, Sarasa M, AIBL Research Group. Plasma amyloid β 42/40 ratios as biomarkers for amyloid β cerebral deposition in cognitively normal individuals. Alzheimers Dement (Amst). 2017;8:179-187. Epub 2017 Sep 12 PubMed.

External Citations

1. Araclon Biotech

Further Reading

News

1. Finally, a Blood Test for Alzheimer’s? 28 Jul 2017