Design and synthesis of functionalized small-molecule inhibitors of cathepsins L and K.

Inhibition of a class of cysteine proteases known as the cathepsins has received increasing attention in recent years, since these enzymes play key roles in cancer metastasis. Cathepsins B, K, and L have been of particular interest to this research study. These proteases have also been implicated in various disease states, including Alzheimer’s disease, osteoporosis, and the severe acute respiratory syndrome (SARS). Cathepsin L is a ubiquitous cysteine protease found primarily within the lysosomes of most tissues; it is involved in normal protein turnover. The increased expression of this enzyme in cancer cells and increased secretion into the extracellular matrix aids in cancer growth and invasion. Within this study, a small library of inhibitors containing the thiosemicarbazone moiety was designed, synthesized, and evaluated for inhibition against these cathepsin proteases. The thiosemicarbazone moiety was contained on the thiochromanone, tetralone, chromanone, and 2,3-dihydroquinolinone molecular frameworks. Several of these analogues demonstrated potent inhibition of cathepsin L or cathepsin K. For example, 6-bromo- 2,3-dihydroquinolinone thiosemicarbazone strongly inhibited cathepsin L (IC50 = 164 nM), while 6-isopropylthiochromanone thiosemicarbazone proved to be a potent inhibitor of cathepsin K (IC50 = 21 nM). Each of the thiosemicarbazone analogues in this study was inactive against cathepsin B (IC50 >10,000 nM). This study further elucidates the structure activity relationship between these small molecules and the active sites of these enzymes.