An anti-amyloid antibody reduces levels of Aβ oligomers in the cerebrospinal fluid of most patients tested, according to late-breaking results presented at the Alzheimer’s Association International Conference in Chicago on July 25.
In collaboration with Genentech/Roche, Dennis Selkoe and Dominic Walsh, both at Brigham and Women’s Hospital, Boston, used an oligomer-specific ELISA assay they had developed to quantitate soluble Aβ assemblies in CSF from participants in two Phase 2 trials of crenezumab. After 69 weeks of treatment with this immunotherapy, oligomer levels had fallen almost in half in the majority of treated patients, Selkoe reported.
“The results strongly suggest principal target engagement of crenezumab with oligomeric Aβ,” said co-author Tobias Bittner of Roche. The results mark the first time that researchers have measured Aβ oligomers in clinical samples from trials of antibody therapies. Finding a decrease with treatment is encouraging, said Selkoe. “Theoretically, that could lead to therapeutic efficacy,” he told the audience. The result also suggests that crenezumab is well-positioned to test the Aβ oligomer hypothesis, Selkoe said.
That hypothesis holds that the neurotoxicity of amyloid comes mainly from small, soluble oligomeric assembles of Aβ (oAβ), which exist in equilibrium with larger amounts of insoluble Aβ in plaques. Despite growing interest from pharma in anti-oligomer therapies (Dec 2017 conference news), the toxic species have proved difficult to isolate and characterize, never mind to detect and quantitate in vivo.
Selkoe and Walsh recently demonstrated that oAβ extracted from postmortem brain tissue of AD patients represents a minority of the Aβ species in brain, but accounts for the majority of the toxic activity toward neurons (Jul 2018 news). They previously developed a high-sensitivity ELISA using their 1C11 antibody, which shows 30,000-fold selectivity for oAβ over monomeric Aβ (Yang et al., 2015). The optimized assay detects as little as 0.15 pg/ml of the oligomers, giving within-sample variances of less than 20 percent, and most often less than 15 percent, Selkoe told Alzforum.
The CSF for the new study came from the ABBY and BLAZE Phase 2 trials, both of which tested crenezumab in people with mild to moderate AD. In all, Selkoe’s lab tested CSF from 104 patients, drawn at baseline and again at 69 weeks into the trial. Concentrations of oAβ were quite low, with most samples reading out less than 10 pg/ml. Six of the samples tested below the 0.15 pg/ml limit of detection at baseline and were excluded. The investigators analyzed the remaining 98 baseline/endpoint pairs, which included 28 samples from the placebo group, and 35 each from people who had received the antibody by intravenous injection of 15 mg/kg every four weeks (high dose) or subcutaneous injection of 300 mg every two weeks (low dose). The testers were blind to patient treatment status.
According to the results presented at AAIC, there was no difference in oAβ between the placebo and treatment groups, or between the baseline and endpoint. Nonetheless, most of the people in the two dose arms posted a decrease in CSF oAβ over the course of the trial. In total, 86 percent of the intravenous group and 89 percent of the subcutaneous group registered a decline. In the placebo group, half of the subjects increased their oAβ and half decreased. “We were struck when we broke the blinding, that most of the patients who had been on crenezumab had a substantial decline in oligomer levels,” Selkoe said.
Treatment with crenezumab caused a reduction in the median oligomer concentration by 43 and 48 percent from baseline to week 69 in the two respective dose groups. As a result, 20 and 14 percent of participants in each arm fell below the 0.15 pg/ml detection limit of the assay. That exceeded the change in the placebo group, whose median value slipped, too, but just 13 percent, with no patients dipping below the assay detection level.
Why would the placebo group change? Selkoe noted that oAβ levels vary from person to person, and attributed their change to biological variation in a low-abundance analyte. “The intra-patient variance, i.e., same person on different days or months, and inter-patient variance in oAβ levels is considerable. I don’t think that a mean 13 percent change in the concentration of oAβ in either direction is surprising, as there may be quite a lot of dynamics of the low amounts of soluble oligomers in human CSF,” he wrote to Alzforum. “The key message is that despite biological variation, about 90 percent of the time crenezumab treatment lowers oAβ levels over 18 months,” Selkoe said.
In contrast, treatment increased soluble, monomeric CSF Aβ. Values before treatment ranged from several hundred to 1,000 pg/ml, and in three-quarters of the treated group, soluble Aβ rose, with a median change of 9–10 percent. That treatment response jibes with previously presented trial data (Cummings et al., 2018; Dec 2014 conference news). The Phase 2 BAN2401 trial that was also presented at AAIC posted an increase of more than 300 pg/ml CSF Aβ42 in the highest-dose group, but gave no percentages (Jul 2018 conference news). This antibody binds Aβ protofibrils, a larger species than Selkoe and Walsh focus on, but smaller than fibrils.
Measuring minute quantities of oligomeric Aβ is a challenge. “The assay is certainly pushing the limit of what can be measured with an immunoassay and is therefore less precise compared to the Elecsys Aβ42 assay,” Bittner wrote to Alzforum. “It gave reproducible results if the baseline and week 69 samples were measured on the same plate,” Bittner added.
Generally speaking, the target of a therapeutic antibody can be difficult to quantify by immunoassay in the presence of that antibody, because the therapeutic and the assay antibody could compete for the same target. In this case, the reduction in oAβ did not seem to be due to crenezumab in the CSF of treated people interfering with the assay, Selkoe believes. As a control, the investigators spiked crenezumab into human CSF, and this did not affect the ELISA’s ability to detect oAβ in subsequent testing.
This late-afternoon session on the fourth day of AAIC was sparsely attended, but some scientists liked the work even from afar. David Brody at Uniformed Services University of the Health Sciences, Bethesda, Maryland, works on purifying and measuring oligomeric Aβ (May 2016 conference news). Brody wasn’t at AAIC, but reading the abstract inspired him to rattle off research ideas. “The data in the abstract is exciting and nicely presented. This may be just the kind of target-engagement biomarker the field needs. There are, of course, many questions: How do CSF levels of soluble Aβ aggregates relate to those present in the brain and, most importantly, to those engaging their targets? Do changes in CSF levels of soluble Aβ aggregates correlate with changes in cognitive function? Is the approximately 50 percent reduction in CSF soluble Aβ aggregate levels the maximum possible response, or would higher doses of crenezumab produce even greater reductions? Do other therapeutics that have been unsuccessful in Phase 3 also lower CSF levels of soluble Αβ aggregates?”
Susan Catalano of Cognition Therapeutics, Pittsburgh, was there, and raised many of the same questions (see full comment below). “The results presented at AAIC are exciting and have major implications for the AD therapeutics field,” she wrote in an email. However, she cautioned, “The amount and even the direction of change in oligomer concentrations in the CSF that would reflect effective oligomer target engagement in the brain is still emerging.” Going forward, it will be important to correlate such measures with synaptic structure and function, as well as with cognition, she wrote.
What next? Bittner noted that Roche intends to replicate Walsh and Selkoe’s testing in the ongoing, fully enrolled crenezumab Phase 3 CREAD studies that test a fourfold higher dose of crenezumab in early AD patients.
How about the ongoing Alzheimer’s Prevention Initiative trial of crenezumab in Colombian presenilin1 E280A mutant autosomal-dominant AD families? Currently there is no plan to test their CSF samples, although Bittner and Selkoe agreed that would be worthwhile. “I think oligomeric Aβ should be looked at in all amyloid-targeting AD trials that have a significant number of CSF samples. Roche and we are working to try to make the assay widely available, though this will take some time,” Selkoe said.—Pat McCaffrey
- Elusive or Not, Aβ Oligomers Are in BioPharma Crosshairs
- A Minority of Human Aβ Species are Toxic, Good Drug Targets
- Immunotherapy I: Baby Steps, but No Breakthroughs
- BAN2401 Removes Brain Amyloid, Possibly Slows Cognitive Decline
- Aβ Oligomers Purified from Human Brain
- Yang T, O'Malley TT, Kanmert D, Jerecic J, Zieske LR, Zetterberg H, Hyman BT, Walsh DM, Selkoe DJ. A highly sensitive novel immunoassay specifically detects low levels of soluble Aβ oligomers in human cerebrospinal fluid. Alzheimers Res Ther. 2015;7(1):14. Epub 2015 Mar 22 PubMed.
- Cummings JL, Cohen S, van Dyck CH, Brody M, Curtis C, Cho W, Ward M, Friesenhahn M, Rabe C, Brunstein F, Quartino A, Honigberg LA, Fuji RN, Clayton D, Mortensen D, Ho C, Paul R. ABBY: A phase 2 randomized trial of crenezumab in mild to moderate Alzheimer disease. Neurology. 2018 May 22;90(21):e1889-e1897. Epub 2018 Apr 25 PubMed.
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