INHIBITION OF ISCHEMIC HIPPOCAMPAL NEURONAL DEATH IN PRIMATES WITH CATHEPSIN-B INHIBITOR CA-074 - A NOVEL STRATEGY FOR NEUROPROTECTION BASED ON CALPAIN-CATHEPSIN HYPOTHESIS

Although Cornu Ammonis (CA) 1 neurons of the hippocampus are known to be vulnerable to transient ischaemia, the mechanism of ischaemic neuro nal death is stilt unknown, and there are very few strategies to preve nt neuronal death at present, in a previous report we demonstrated mu- calpain activation at the disrupted lysosomal membrane of postischaemi c CA1 neurons in the monkey undergoing a complete 20 min whole brain i schaemia, Using the same experimental paradigm, we observed that the e nzyme activity of the lysosomal protease cathepsin B increased through out the hippocampus on days 3-5 after the transient ischaemia. Further more, by immunocytochemistry cathepsin B showed presence of extralysos omal immunoreactivity with specific localization to the cytoplasm of C A1 neurons and the neuropil of the vulnerable CA1 sector. When a speci fic inhibitor of cathepsin B, the epoxysuccinyl peptide CA-074 (C18H29 N3O6) was intravenously administered immediately after the ischaemic i nsult, approximate to 67% of CA1 neurons were saved from delayed neuro nal death on day 5 in eight monkeys undergoing 20 min brain ischaemia: the extent of inhibition was excellent in three of eight and good in five of eight monkeys. The surviving neurons rescued by blockade of ly sosomal activity, showed mild central chromatolysis and were associate d with the decreased immunoreactivity for cathepsin B. These observati ons indicate that calpain-induced cathepsin B release is crucial for t he development of the ischaemic neuronal death, and that a specific in hibitor of cathepsin B is of potential therapeutic utility in ischaemi c injuries to the human CNS.