Cobalt (III) Complexes as Novel Matrix Metalloproteinase-9 Inhibitors

Abstract

Matrix metalloproteinases (MMPs) are a family of zinc binding endopeptidases that play crucial roles in various physiological processes and diseases such as embryogenic growth, angiogenesis, arthritis, skin ulceration, liver fibrosis and tumor metastasis. Because of their implications in a wide range of diseases, MMPs are considered as intriguing drug targets. The majority of MMP inhibitors are organic small molecules containing a hydroxamate functionality for the zinc binding group. This hydroxamate group binds to a zinc(II) center in a bidentate fashion and creates a distorted trigonal bipyramidal geometry. Although the hydroxamate group is the most effective zinc binding moiety reported, it has shown two major limitations in clinical trials: low bioavailability and lack of specificity. Because the hydroxamate group is prone to rapid metabolism and has a high binding affinity towards various transition metals, many efforts have been made to develop novel zinc binding groups, including reverse hydroxamate, phosphate, and pyridinone. However, the in vitro activity of these newly developed inhibitors are not as high as the hydroxamate derivatives. It was previously reported that cobalt(III) acacen complexes could interact with histidine residues of proteins and model peptides. The cobalt(III) complexes bind histidine residues in active sites and on enzyme surfaces in a random fashion. Spectroscopic and chromatographic data suggested that the complexes bind to a histidine residue by axial ligand substitution. When an appropriate targeting group is attached, the cobalt(III) complexes can selectively inhibit the histidinecontaining enzymes such as thermolysin, human α-thrombin, and carbonic anhydrase. Since histidine is the most commonly found residue in the active sites of zinc enzymes and zinc-binding proteins, it has been demonstrated that the cobalt(III) complexes can interact with zinc finger proteins, such as HIV-1 nucleocapsid protein NCp7 and human zinc finger transcription factor Sp1. It was also reported that a Co(III) complex with a oligonucleotide targeting Snail family zinc finger transcription factor resulted in a selective inactivation of a transcriptional activity implicated in embryonic development and breast cancer. In this work, a series of the cobalt(III) complexes are prepared as novel MMP inhibitors. Since cobalt(III) complexes have high affinity towards histidine residues and zinc binding proteins, they can be engineered to disrupt the zinc binding active site of MMP. Furthermore, the cobalt(III) complexes may be able to provide better solubility and higher binding affinity than the known organic inhibitors. To test this hypothesis, a series of cobalt(III) complexes were designed and synthesized. For active site directing groups, biphenyl sulfonate and biphenyl amide group were chosen based on the structures of well known MMP inhibitors. In vitro activity of these complexes was evaluated for MMP-9 (gelatinase B), because MMP-9 is one of the most highly expressed MMPs in tumors and has been implicated in tumor aggressiveness. The syntheses of cobalt(III) complexes are shown in Schemes 1-2. The synthesis of 1 and 2 begins with the methylation of commercially available Cbz-Lys(Boc)-OH as