Potent, reversible cathepsin K inhibitor (IC50 = 0.25 nM) that displays > 4000-fold selectivity over cathepsins B, L and S. Displays reduced selectivity in cell-based assays possibly due to lysosomal accumulation. Reduces collagen breakdown and promotes bone deposition in vivo. Orally active and has intrinsic fluorescence. Purity 98%. MW (observed): 466.6