19 Oct 2004

Inhibiting ACAT, a cholesterol-modifying enzyme, may prove a viable therapy for preventing or slowing the progression of Alzheimer disease, according to a report in the October 14 Neuron. A multi-institution collaboration led by Dora Kovacs at Massachusetts General Hospital, Boston, shows that inhibiting the enzyme limits production of amyloid-β (Aβ), reduces plaque load, and prevents learning and memory losses in a mouse model of the disease.

ACAT (Acyl coenzyme A: cholesterol transferase) catalyzes the esterification of cholesterol, redistributing the lipid from the plasma membrane into cytoplasmic droplets. Kovacs and colleagues have previously reported that blocking the enzyme attenuates production of Aβ in primary neurons (see ARF related news story). Now, joint first authors Birgit Hutter-Paier at JSW-Research Forschungslabor GmbH, Graz, Austria, and Henri Huttunen at MGH, along with other researchers from these institutions and the Mayo Clinic, Jacksonville, Florida, extend those observations to transgenic animals. They tested CP 113,818, an ACAT inhibitor developed by Pfizer, in mice expressing human amyloid precursor protein harboring two different kinds of mutation—the London mutation, which results in an isoleucine instead of a valine at amino acid number 717, and the Swedish double mutation, methionine and leucine for lysine and asparagine at positions 670 and 671, respectively. These animals develop amyloid plaques in the brain, and by six months old show signs of cognitive decline.

Because CP 113,818 is so poorly absorbed, the authors administered it in slow-release capsules, surgically implanting a two-month supply into four and a half-month-old mice. When the authors checked brain pathology at six and a half months, they found a considerable reduction in plaque load, as judged by both thioflavin S and antibody (6E10 antibody) staining. Treated animals had only about 26 plaques per square micrometer of cortex, while placebo-treated animals had over 200. The benefit was greatest in the hippocampuses of female animals, where the plaque load was only one percent of that found in sham-treated animals. In addition, when the authors measured soluble Aβ by ELISA, they found that levels of soluble Aβ1-42 were reduced by over 30 percent, a result that was statistically significant.

These particular transgenic mice only begin to show a hint of cognitive decline at six months. Nevertheless, the authors found that those given the ACAT inhibitor preformed consistently better in the Morris water maze—particularly the female animals. In these mice, improvement was statistically significant, which suggests that males, where the AD-like symptoms develop more slowly, may also benefit as they age. “We are presently examining the effect of the inhibitor on older animals,” Kovacs revealed.

These findings may put ACAT inhibitors on the fast track for development for AD. Already Pfizer has Avasimibe® in phase III clinical trials for the treatment of atherosclerosis, and it is already “considered safe for human use with a good therapeutic window,” write the authors.—Tom Fagan