Even though ApoE stands head and shoulders above other genetic risk factors for late-onset Alzheimer’s disease, few therapeutic strategies aimed at ApoE have made it to clinical trials. That may be about to change. At “ApoE, Alzheimer’s and Lipoprotein Biology,” a five-day Keystone symposium held 26 February-2 March 2012, researchers delved into ApoE biology and touted potential remedies to match. Boosting lipidated ApoE with retinoid X receptor agonists (see Part 1 of this series), elevating signaling through ApoE receptors (see Part 3), and blocking proteolytic fragmentation of the apolipoprotein (see Part 4) were among the ideas that could be put to the test. Here are some more, as well as updates on solanezumab and a BACE inhibitor presented at Keystone.

In her presentation, Kelly Bales of Pfizer in Groton, Connecticut, outlined a screening program to search for small molecules that raise ApoE levels. Bales uses an ApoE promoter-driven luciferase gene to measure expression changes in a human astrocyte cell line. Hits included RXR and liver X receptor agonists, and also histone deacetylase (HDAC) inhibitors (see ARF related news story). At Keystone, Bales talked about delving deeper into the role of HDACs to find specific deacetylases that regulate ApoE. While there are five classes of HDACs, astrocytes primarily express class I and II. A small interfering RNA that suppressed expression of all class I enzymes modestly raised ApoE expression, said Bales, whereas knocking down class II HDACs achieved more dramatic results. However, she noted that HDAC inhibition modulates expression of other genes as well. Blocking class I HDACs stimulated production of ABCA1, a cholesterol and phospholipid transporter. ABCA1 keeps ApoE lipidated, which seems crucial for ApoE-mediated clearance of Aβ from the brain (see Part 1). Blocking class I HDACs also suppressed astrocytic interleukin 6 (IL6), a proinflammatory cytokine. With these pleiotropic effects, HDAC inhibition might be a novel way to tackle neurodegenerative diseases, suggested Bales.

Keeping with the inflammation theme, Michael Vitek, Duke University Medical Center, Durham, North Carolina, outlined a strategy for tamping it down with ApoE mimetics. Vitek cited lines of evidence that point to ApoE3 being anti-inflammatory, while ApoE4 falls short in that respect. For example, macrophages from ApoE4 targeted replacement (TR) mice generate much more of the proinflammatory protein tumor necrosis factor α (TNFα) and IL6 than those from ApoE3 TR mice. Vitek founded the biotech company called Cognosci, Inc., to develop compounds that mimic ApoE3 anti-inflammatory properties. The company is evaluating candidate drugs for AD, multiple sclerosis, and traumatic brain injury.

The compounds are short peptides that correspond to the receptor-binding domain of ApoE3. The prototype, COG133 (amino acids 133-149 of the protein), reduced inflammatory responses in human blood ex vivo and in the CNS and the periphery of mice treated with lipopolysaccharide, which induces inflammation.

Two analogs, COG112 and COG1410, protected the CVND transgenic mouse model of AD developed by Vitek and Carol Colton at Duke (APPSwDI/NOS-/-; see ARF related news story). These mice develop robust plaques by 12 months of age and show substantial neuron loss, something other models recapitulate less successfully. Vitek and colleagues gave subcutaneous COGs to these animals at nine months. Three months later, the animals made less IL6 in the brain, maintained more of their neurons, grew fewer plaques and neurofibrillary tangles, and navigated better in the radial arm water maze than did untreated littermates (see Vitek et al., 2012). Vitek and colleagues also tested compounds in a model of traumatic brain injury. Given two hours after injury, the ApoE mimetics improved survival, strength on the rotarod, and cognition in the Morris water maze as compared to untreated animals, Vitek said.

What is the mechanism of action of these compounds? Researchers at Cognosci used biotin-labeled compounds to fish out binding partners and found strong binding to the protein SET/I2PPA, short for inhibitor No. 2 of protein phosphatase A (PP2A). The compounds appear to relieve inhibition of PP2A, said Vitek, because they reduce the amount of phosphorylated MAP kinase, p38, and JNK kinase in cells. The mechanism intrigued researchers at the meeting, who wondered whether the reductions in plaque and tangle pathology are due to toning down inflammation, or are more directly due to activation of PP2A. Vitek said that was not known, though he noted that PP2A inhibitors boost tau phosphorylation.

Though their candidates are not directly related to ApoE or lipoproteins, researchers from Lilly and Merck reviewed the status of some of their drugs in clinical trials. Eric Siemers from Lilly first spoke about the failure of semagacestat, the company’s γ-secretase inhibitor (see ARF related news story). Siemers emphasized how important it is to learn as much as possible from that trial. His take-home message was that, while the company had struggled with getting optimal dosing of the drug in Phase 2, it eventually took appropriate doses into Phase 3, where biomarker analysis confirmed that the drug indeed reached its target in the brain. CSF levels of Aβ1-16 rose in patients taking the drug, a finding in keeping with predictions from Kaj Blennow’s lab at University of Gothenburg, Sweden. This short fragment appears when β-cleavage of APP is followed, unusually, by α-, not γ-cleavage, which could happen when the latter is blocked (see ARF related news story). Despite engaging its target as predicted, the drug caused a decline in cognition. Siemers believes that was likely due to blocking γ-secretase cleavage of substrates other than APP, of which some 50 are known. Companies are now pursuing γ-secretase modulators that tweak APP cleavage while allowing processing of other substrates, including Notch.

Lilly’s solanezumab, a humanized mouse monoclonal antibody to Aβ, is in Phase 3 (see ARF related news story). Noting that the trials will end shortly, Siemers presented no new data at Keystone. Reviewing some pre-Phase 3 biomarker data, he noted that in mouse models and humans, the antibody dose-dependently raised plasma and CSF Aβ, implying that the antibody pulls some Aβ out of the brain (see ARF related conference story). In mice, it reduced plaque load. What about humans? Tantalizingly, Siemers said that pyroglutamate-modified Aβ appears in plasma of people who received the antibody. “You don’t normally see pyroglu-Aβ in the blood, so this is an indication that bits of plaque are ending up there,” said Siemers.

Eric Parker of Merck Research Laboratories, Kenilworth, New Jersey, gave an update on his company’s BACE program. BACE has been a tough nut to crack from a pharmaceutical perspective. Parker said the key is saturating BACE in the brain. Merck has achieved that goal now with the drug MK-8931, Parker claimed. In a two-week-long Phase 1 trial in healthy volunteers, MK-8931 reduced Aβ in the CSF by 90 percent. The company presented those data at an investors' meeting in November 2011 (see slides 152-155 of the presentation).

So far, the drug appears to be safe enough for further human testing, though other BACE inhibitors have failed in late Phase 1 (see ARF related conference story). In mice, rats, and monkeys, MK-8931 does not affect nerve conductance or prepulse inhibition, which are suppressed in BACE knockout animals and are likely related to myelination defects during development, Parker said. The company is planning a Phase 2 trial to start this year. For that, they are using a modeling strategy for dose finding since the drug’s inhibition of BACE during Phase 1 was too strong for appropriate dose ranges to be determined. Merck is currently recruiting for a second Phase 1 dose-finding trial in Alzheimer’s patients to determine a mean inhibitory concentration. This will overlap with the planned Phase 2 study. The company hopes this parallel-phase strategy will save development time, according to a company spokesman.—Tom Fagan.

This concludes a five-part story. See also Part 1, Part 2, Part 3, Part 4. Download a PDF of the entire series.


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News Citations

  1. Keystone: Symposium Emphasizes Key Aspects of ApoE Biology
  2. Keystone: ApoE Receptors and Ligands in Memory and AD
  3. Keystone: Does ApoE Fragmentation Drive Pathology?
  4. San Francisco: Tweaking Brain ApoE Reduces Aβ, Symptoms
  5. No NOS Promotes Tau Pathology in APP Transgenic Mice
  6. Lilly Halts IDENTITY Trials as Patients Worsen on Secretase Inhibitor
  7. Sweet 16: Novel APP Processing Pathway and a New Biomarker?
  8. Solanezumab, Gammagard Trials Survive Futility Analysis
  9. Sink or Swim?—New Take on Aβ Antibody’s Modus Operandi
  10. Barcelona: Out of Left Field—Hit to The Eye Kills BACE Inhibitor
  11. Keystone: Probing the Function of Lipoprotein and Related Receptors

Paper Citations

  1. . APOE-Mimetic Peptides Reduce Behavioral Deficits, Plaques and Tangles in Alzheimer's Disease Transgenics. Neurodegener Dis. 2012;10(1-4):122-6. PubMed.

Other Citations

  1. Download a PDF of the entire series.

External Citations

  1. Phase 3
  2. presentation
  3. Phase 1

Further Reading