Design and synthesis of small-molecule inhibitors of cathepsin L bearing the thiosemicarbazone warhead as potential anti-metastatic agents.

Mestastasis is a devastating component associated with tumor progression which claims the majority of lives lost to cancer. Upregulation of cathepsin L, a powerful cysteine protease capable of degrading components in the extracellular matrix, is correlated with increased invasiveness of cancer cells and poor prognosis. Consequently, cathepsin L has emerged as a target for the development of new anticancer agents designed to mitigate the invasion and metastasis of malignant cancer cells. The development of a focused, small library of thiosemicarbazone analogues was pursued in an effort to discover potent and selective inhibitors of cathepsin L, which possess the potential to bind to the active sit of cathepsin L through a proposed covalent interaction with the catalytic amino acid residue Cys25. Incorporation of the thiosemicarbazone moiety with a variety of aryl based molecular scaffolds led to a functionally diverse series of thiosemicarbazone analogues that were evaluated, through collaborative efforts, for their ability to inhibit cathepsin L and thus to potentially function as anti-metastatic agents. Benzophenone, benzoylbenzophenone, di-2-naphthalenylmethanone, 1,3-diphenylpropan-2-one, dibenzylideneacetone, and 1,5-diphenyl-3-pentanone aryl-based molecular scaffolds were all explored. From the thirty-five compounds in this thiosemicarbazone based library, fifteen analogues were determined to be potent inhibitors of cathepsin L (IC₅₀ < 10 μM). 1,3-Bis(2-fluorobenzoyl)-5-bromobenzene thiosemicarbazone (KGP312) demonstrated the greatest inhibitory activity against cathepsin L with an IC₅₀ value of 8.1 nM. Furthermore, KGP312 inhibited the invasion and migration of both MDA-MB-231 breast cancer cells and PC-3ML prostate cancer cells. In addition to the design and synthesis of new thiosemicarbazone analogues, improvement of the synthetic route for our previously discovered lead cathespin L inhibitor, KGP94, was achieved. Derivatization designed to increase aqueous solubility and potentially oral bioavailability was performed and ultimately led to the preparation of KGP420, the phosphate prodrug of KGP94. Owing to high cathepsin L inhibitory activity and favorable outcomes in initial biological studies, on-going collaborative efforts have been aimed towards the advancement of these preclinical candidates as potential anti-metastatic agents