The ability of the plasmin protease to degrade amyloid-β has been known for some time (Tucker et al., 2000). Plasmin is activated by tissue plasminogen activator (tPA), and its commonly known function is to degrade fibrin clots. Despite upregulation of tPA in AD brain, plasmin activity remains low because levels of PAI-1 are also elevated (see ARF related news story).

Based on these results, Steve Jacobsen and colleagues at Wyeth in Princeton, New Jersey, have been working to develop CNS-penetrant PAI-1 inhibitors. Their paper describes the activity of their lead inhibitor, PAZ-417, to stimulate Aβ degradation in vitro, and lower brain and plasma Aβ in an AD mouse model. Mice treated with the compound showed improved memory performance in the contextual fear-conditioning test. The paper includes data showing that administration of PAZ-417 to Tg2576 mice 24 hours before hippocampal slice preparation reversed deficits in long-term potentiation seen in the slices. The in vivo activity of the PAI-1 inhibitor depended on its ability to inhibit PAI-1 in the CNS, as a non-brain-penetrant inhibitor did not reduce Aβ or affect behavior.

Activation of the plasmin system raises the fear of bleeding. Preclinical toxicology studies with PAZ-417 in mice, rats, and dogs indicated the compound did not affect bleeding times or show cardiovascular side effects, the authors write. Wyeth is now running three Phase 1 tests of PAZ-417 to establish safety, tolerability, and pharmacokinetics, company representative Michael Lampe told Alzforum. The trials are all listed on ClincalTrials.gov as currently recruiting. The first, NCT00366288, involving 56 healthy young and elderly adults, started in July 2006, and was supposed to wrap up March 2008. A second trial in 40 healthy elderly adults, NCT00663065, began in April 2008 and is scheduled to be completed in September 2008. The third Phase 1 trial, NCT00684710, started in April to recruit 56 healthy Japanese men, and is slated for completion July 2008. There is no data available yet on any of these trials, Lampe said, but he confirmed that the PAI-1 inhibitor continues to be an active clinical program at Wyeth.—Pat McCaffrey.

Reference:
Jacobsen JS, Comery TA, Martone RL, Elokdah H, Crandall DL, Oganesian A, Aschmies S, Kirksey Y, Gonzales C, Xu J, Zhou H, Atchison K, Wagner E, Zaleska MM, Das I, Arias RL, Bard J, Riddell D, Gardell SJ, Abou-Gharbia M, Robichaud A, Magolda R, Vlasuk GP, Bjornsson T, Reinhart PH, Pangalos MN. Enhanced clearance of A{beta} in brain by sustaining the plasmin proteolysis cascade. Proc Natl Acad Sci U S A. 2008 Jun 16. [Epub ahead of print] Abstract