The Cysteine Protease Inhibitor, E64d, Reduces Brain Amyloid-β and Improves Memory Deficits in Alzheimer's Disease Animal Models by Inhibiting Cathepsin B, but not BACE1, β-Secretase Activity

The cysteine protease cathepsin B is a potential drug target for reducing brain amyloid-beta (A beta) and improving memory in Alzheimer's disease (AD), as reduction of cathepsin B in transgenic mice expressing human wild-type amyloid-beta protein precursor (A beta PP) results in significantly decreased brain A beta. Cathepsin B cleaves the wild-type beta-secretase site sequence in A beta PP to produce A beta, and cathepsin B inhibitors administered to animal models expressing A beta PP containing the wild-type beta-secretase site sequence reduce brain A beta in a manner consistent with beta-secretase inhibition. But such inhibitors could act either by direct inhibition of cathepsin B beta-secretase activity or by off-target inhibition of the other beta-secretase, the aspartyl protease BACE1. To evaluate that issue, we orally administered a cysteine protease inhibitor, E64d, to normal guinea pigs or transgenic mice expressing human A beta PP, both of which express the human wild-type beta-secretase site sequence. In guinea pigs, oral E64d administration caused a dose-dependent reduction of up to 92% in brain, CSF, and plasma of A beta(40) and A beta(42), a reduction of up to 50% in the C-terminal beta-secretase fragment (CTF beta), and a 91% reduction in brain cathepsin B activity, but increased brain BACE1 activity by 20%. In transgenic AD mice, oral E64d administration improved memory deficits and reduced brain A beta(40) and A beta(42), amyloid plaque, brain CTF beta, and brain cathepsin B activity, but increased brain BACE1 activity. We conclude that E64d likely reduces brain A beta by inhibiting cathepsin B and not BACE1 beta-secretase activity and that E64d therefore may have potential for treating AD patients.