Blocking Stromelysin-3 to Inhibit Breast Cancer Metastasis
Nurulain Zaveri, Ph.D.
SRI International
Award Type: ItaMoCA,
Innovative Treatments Modalities
Duration: 2 yrs., 1997-1999
Metastasis, the spread of cells from the primary cancer to distant sites in the body, is the most fearsome aspect of breast cancer. Despite significant advances in breast cancer detection and treatment, most deaths from breast cancer are due to metastasis that is resistant to conventional therapies. Occult or hidden micro-metastases may already exist in women diagnosed with tumors. Metastasis consists of a series of linked, sequential steps that enable tumor cells to secrete tissue-destroying enzymes, which allow cells to break away from the original cancer. Therefore, current drugs that only stop cancer growth may not always halt the metastatic spread of the cancer. Recent studies on breast cancer biopsies reported high levels of a new enzyme, stromelysin-3 (ST-3), particularly from patients who later developed metastasis. ST-3 was found to be involved in the early steps of metastasis, which convert a tumor cell to a metastatic cell. ST-3 belongs to a well-known class of zinc-containing enzymes (matrix metalloproteinases, MMPs), but it has several unique properties. Interestingly, it does not directly break down the extracellular matrix surrounding tumor cells, but instead appears to bind to and inactivate a natural inhibitor of other tissue-destroying enzymes. This then allows these other enzymes to degrade the matrix, leading to metastasis.
The goal of this proposed research is to target the unique aspects of ST-3 to design and develop a drug that will specifically block the action of ST-3 and thereby halt the process of metastasis in its early stages. We will use a multidisciplinary approach for drug design, which consists of organic synthesis, computer modeling and biological testing. We will target the active site of ST-3 by using small protein-based (dipeptide) inhibitors that incorporate a strong zinc-binding component. We will evaluate the effectiveness of these inhibitors by using both purified ST-3 and whole cell systems to better represent a natural, biological model. Feedback from this biological evaluation will be integrated into the drug design to obtain a therapeutically viable anti-metastatic compound.
Novel antimetastatic drugs would be especially valuable in combination with surgery and/or chemotherapy. Current widespread use of screening mammography has increased the detection of very small, early-stage breast cancers. Thus, drugs that prevent metastasis would serve to combat recurrence and assure a complete and lasting cure.
Final Report (2000)
Note: This grant was extended one year to complete the aims.
The objective of this research project is to design and develop small-molecule inhibitors of the enzyme stromelysin-3 (ST-3), a new matrix metalloproteinase (MMP) that has recently been implicated in the early events of breast cancer progression and metastasis. We used the integrated approach of rational drug design, organic synthesis and biological evaluation. Such inhibitors would be valuable not only as therapeutics, but also to study the role of ST-3 in metastasis as well as normal tissue function. ST-3 has some rather unusual properties for a MMP, described below, which make it an interesting target for controlling breast cancer.
ST-3, like other MMPs, contains a zinc atom in its active site. However, unlike other known MMPs that are active in metastasis and angiogenesis (blood vessel growth) mature ST-3 does not break down normal extracellular matrix such as collagen. Instead, two unique physiologic substrates have been identified for ST-3: (a) a proteinase inhibitor (a-1 PI), a physiologic inhibitor of several tissue-destroying enzymes and (b) insulin-like growth factor binding protein-1 (IGFBP-1), which binds insulin-like growth factor (IGF) and prevents cell proliferation induced by IGF. Thus, by inactivating physiologic inhibitors of malignant growth signaling proteins and tissue-destroying enzymes, ST-3 contributes to early events in breast tumor progression. Therefore, ST-3 represents a very attractive target for new anti-metastatic drug candidates, to be used in combination with chemotherapy and/or surgery.
We synthesized several new dipeptide small-molecule inhibitors of ST-3. These inhibitors contain strong zinc-binding components, which direct the inhibitors to the active-site zinc. First, we synthesized an inhibitor containing the phosphoric acid zinc ligand and the substrate residues on either side of the cleavable bond. Secondly, we synthesized several dipeptide inhibitors containing the hydroxamate and thiol zinc-binding ligands. After designing and synthesizing these first substrate-based dipeptide inhibitors, we focused our efforts on designing non-peptide inhibitors of ST-3, since we expect that such compounds will be less susceptible to breakdown by enzymes and thus make better candidates for further drug development. In this class of compounds, we synthesized compounds containing the hydroxamate and carboxylic acid zinc ligands. We further developed the synthesis of these compounds on solid support, because in the future we aim to apply combinatorial and parallel synthesis techniques to obtain several inhibitors in a shorter time, for structural optimization.
The target inhibitors were evaluated against isolated human ST-3 by our collaborator, Dr. Stephen J. Weiss at the University of Michigan, who originally isolated and purified human ST-3. These inhibitors were also evaluated for their inhibition of ST-3 as well as for their selectivity compared with other MMPs, such as collagenase, gelatinase, and stromelysin- 1. Initial results from these experiments indicate that only the hydroxamate ligand-containing inhibitors work well against ST-3. We will use this information to optimize the design of new, more potent inhibitors of ST-3.