PEPTIDE ALDEHYDE INHIBITORS OF CATHEPSIN-K INHIBIT BONE-RESORPTION BOTH IN-VITRO AND IN-VIVO

We have shown previously that cathepsin K, a recently identified membe r of the papain superfamily of cysteine proteases, is expressed select ively in osteoclasts and is the predominant cysteine protease in these cells, Based upon its abundant cell type-selective expression, potent endoprotease activity at low pH and cellular localization at the bone interface, cathepsin K has been proposed to play a specialized role i n osteoclast-mediated bone resorption, In this study, we evaluated a s eries of peptide aldehydes and demonstrated that they are potent cathe psin K inhibitors, These compounds inhibited osteoclast-mediated bone resorption in fetal rat long bone (FRLB) organ cultures in vitro in a concentration-dependent manner. Selected compounds were also shown to inhibit bone resorption in a human osteoclast-mediated assay in vitro, Cbz-Leu-Leu-Leu-H (in vitro enzyme inhibition K-i,K-app = 1.4 nM) inh ibited parathyroid hormone (PTH)-stimulated resorption in the FRLB ass ay with an IC-50 of 20 nM and inhibited resorption by isolated human o steoclasts cultured on bovine cortical bone slices with an IC-50 of 10 0 nM, In the adjuvant-arthritic (AA) rat model, in situ hybridization studies demonstrated high levels of cathepsin K expression in osteocla sts at sites of extensive bone loss in the distal tibia, Cbz-Leu-Leu-L eu-H (30 mg/kg, intraperitoneally) significantly reduced this bone los s, as well as the associated hind paw edema, In the thyroparathyriodec tomized rat model, Cbz-Leu-Leu-Leu-H inhibited the increase in blood i onized calcium induced by a 6 h infusion of PTH. These data indicate t hat inhibitors of cathepsin K are effective at reducing osteoclast-med iated bone resorption and may have therapeutic potential in diseases o f excessive bone resorption such as rheumatoid arthritis or osteoporos is.