The goal of BACE1 inhibition is to stop the production of toxic Aβ. However, a new study reports that at very high doses, the inhibitors may harm the very neurons they are meant to protect. Initial findings were presented by senior author Jochen Herms of Ludwig-Maximilians-Universität, Munich, at a conference last year (see Dec 2013 conference story). In press in Biological Psychiatry, the full study now reports that BACE inhibitors quash synaptic plasticity and cause memory loss in normal adult mice. The findings suggest that such inhibitors could pose a threat to neurons if used at high enough doses, but also offer reassurance that low doses could block Aβ production without affecting memory. The paper is available online.
BACE1 performs the rate-limiting step of processing amyloid precursor protein (APP) into amyloidogenic Aβ species, making the enzyme a prime therapeutic target. However, developing inhibitors to block the enzyme has not been easy. The enzyme’s sizable catalytic site acts on myriad substrates, and researchers walk a fine line when seeking to block the enzyme without unleashing undue side effects. Measuring neuronal reactions to the inhibitors, particularly reactions that might cause gradual cognitive decline, is a tall order in the context of clinical trials, Herms said.
First author Severin Filser and colleagues decided to monitor potential effects on synaptic plasticity in mice. The researchers treated normal adult mice with two different BACE inhibitors—SCH1682496 from Merck, or LY2811376 from Eli Lilly—twice daily for 16 days. Herms said the Merck inhibitor is likely similar to MK-8931, which currently is being tested in clinical trials. The Lilly compound was abandoned in a Phase 1 trial due to a retinal pigmentation defect in rats (see Mar 2011 conference story). The German researchers confirmed that the inhibitors reduced Aβ40 in the plasma and cortex of mice in a dose-dependent manner. Neither inhibitor caused weight loss or overt health problems.
To monitor the inhibitors’ effect on dendritic spine growth—a structural measure of synaptic plasticity—the researchers fitted cranial windows onto mice expressing green fluorescent protein (GFP) and monitored the spines on cortical neurons via two-photon imaging. Mice treated with a high dose (hereafter 100 mg/kg) of Merck’s inhibitor formed half as many spines as untreated mice by the last day of treatment; mice treated with the same dose of Lilly’s inhibitor produced about a third fewer spines. Spine growth returned to normal after removal of the inhibitors, and was unaffected by a low dose (hereafter 30 mg/kg) of either inhibitor. Adult BACE1 knockout mice had normal dendritic spine growth that was unaffected by the inhibitors, suggesting that the inhibitors’ effect on spines was specific to BACE1, and that mice deprived of BACE1 throughout life develop compensatory mechanisms.
The researchers then looked for electrophysiological effects of BACE1 inhibition in cortical neurons in brain slices. The high dose of both inhibitors abolished more than half of the spontaneous excitatory postsynaptic currents (sEPSCs). The inhibitors also reduced the frequency and amplitude of miniature EPSCs (mEPSCs), which are triggered by the release of small amounts of neurotransmitters that don’t require action potentials. The authors speculated that this might correlate directly with the reduction in dendritic spines. In BACE1 knockout mice, both sEPSCs and mEPSCs were unaffected by the inhibitors, once again suggesting that the BACE inhibitor was specific. However, the knockouts had a higher frequency of mEPSCs than wild-type mice. This hints that the dynamics of presynaptic vesicle release is altered in BACE1 knockouts, but more research is needed to understand how, Herms said.
The researchers measured long-term potentiation (LTP)—the prolonged period of heightened excitability following stimulation that promotes learning and memory. In hippocampal slices, they found that CA1 neurons from mice treated with high doses of either inhibitor had striking defects, losing their sensitivity to stimulation. Low doses of the inhibitors did not cause these electrophysiological defects. Neither high nor low doses of the inhibitors affected LTP in BACE1 knockout mice; however, neurons in these mice had lower initial responses to stimulation than did wild-type mice.
The effects of the inhibitors on synaptic plasticity played out in the form of mild memory deficits as well. While the treated mice performed normally on measures of anxiety, their performance on the Y-maze test of spatial memory indicated they may have problems with working memory, the authors report.
The study points to the importance of selecting the correct therapeutic dosage in trials, said Filser. “A low dose was able to significantly lower the amount of Aβ, but didn’t cause any of the synaptic deficits we see with the higher dose,” he said. Other researchers weighed in with similar comments (see below). Participants in Merck’s Phase 2/3 trial are receiving daily doses of 12, 40, or 60 mg of MK-8931. However, a direct comparison between mouse and human doses is not practical, Filser said, because the human brain retains BACE1 inhibitors longer than the mouse brain does. He believes researchers should keep these synaptic defects in mind when choosing the lowest effective dose in humans. The human equivalent of the high dose used in these mouse studies would be would be about 8 mg/kg, or roughly 600 mg per day, according to FDA guidance.
In the context of AD, where synapses already have taken a hit from Aβ pathology, losing additional synaptic plasticity may be particularly problematic, Herms said. On the flip side, he added that BACE1 activity ramps up in AD brains, especially in axons co-mingling with plaques. In this situation, perhaps BACE1 inhibition would bring the enzyme’s activity down to normal levels rather than wiping it out completely. This would theoretically cause fewer synaptic side effects—something that Herms’ lab is currently testing in AD mouse models.
Fred van Leuven of the Catholic University of Leuven in Belgium was not surprised by the results. “When you inhibit an enzyme with so many substrates, it is bound to have side effects,” he said. “But if the therapeutic activity is more beneficial than the negative effects, we should push ahead with clinical trials. My attitude is we should try everything we’ve got against this disease, but at the same time be very aware of potential side effects.”—Jessica Shugart
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No Available Further Reading
- Filser S, Ovsepian SV, Masana M, Blazquez-Llorca L, Brandt Elvang A, Volbracht C, Müller MB, Jung CK, Herms J. Pharmacological inhibition of BACE1 impairs synaptic plasticity and cognitive functions. Biol Psychiatry. 2015 Apr 15;77(8):729-39. Epub 2014 Oct 29 PubMed.