| William Stetler-Stevenson |
One particular family of enzymes, called matrix metalloproteinases (MMPs), may hold much of the key to such a weapon. When first studied in the 1940s, matrix degradation was actually more important to the leather industry than it was to the biomedical industry. Because proteases seemed to affect the integrity of leather, they were studied at several leather institutes around the world. Only with the explanation of this useful quality in the early 1960s did what came to be called MMPs get introduced to developmental and molecular biology. Amphibian experiments showed that MMPs digest collagens, a major component of skin (leather is essentially tanned skin).
In fact, MMPs actually degrade several substances in the extracellular matrix--like cartilage, tendons, fibrin clots--to allow for the migration of cells, the deposition of new extracellular matrix, and the development of new tissue. As such, they're involved in any process, like cancer, wound repair, or inflammation, that involves tissue reorganization and any developmental event that involves tissue remodeling.
But this classic notion of an MMP functioning solely as an extracellular matrix Pac-Man that clears the way for cell migration has begun to evolve in light of recent findings. Scientists have found that MMPs can also modify cell attachment and that the products of their action on the extracellular matrix can actually influence cell behavior--a new role that has increased the number of known MMP targets. "It's opened up a whole new field of matrix metalloproteinase biology that we hadn't seen before," says William Stetler-Stevenson, senior investigator in the laboratory of pathology at the National Cancer Institute (NCI) and chief of the extracellular matrix pathology section. "We are starting to identify substrates for these enzymes that we didn't anticipate."
Stetler-Stevenson's lab is one of many that are concentrating on using MMPs, MMP-2 (gelatinase A), and MMP-9 (gelatinase B) in particular, for the development of anticancer therapies. MMPs were initially thought to be of primary importance in cancer cell invasion. But the recent emphasis on angiogenesis research has led several groups to demonstrate functional similarities between tumor cell invasion and the process of endothelial cell invasion during angiogenesis. The same enzymes that facilitate normal cell migration by degrading the extracellular matrix can therefore potentially facilitate the spread of cancer by clearing a path for tumor cells, and by enabling the growth of new blood vessels (angiogenesis) that provide nutrients for those tumor cells. Stop the MMPs with inhibitors, and you might be able to impede cancerous growth.
This seemingly straightforward task has become the mission of many a pharmaceutical company. And NCI classifies MMP inhibitors as antiangiogenic agents, making them frequent candidates for clinical trials. Says Stetler-Stevenson, "I think that from animal models there's at least proof of principle that targeting metalloprotease activity is effective in reducing metastasis and growth of the primary tumor."
But the role of MMPs in tumorigenesis remains to be fully dissected and it is, as yet, unclear whether they're a critical enough link in the chain of biochemical events that leads to cancer to be a viable therapy. "You can show proteases are involved, you can find animal models in which they're definitely involved, but at this point, the question is whether there are mechanisms by which the ever-changeable and elusive tumors know how to get around that problem," comments Zena Werb, a professor of anatomy at the University of California, San Francisco. Werb's basic research focuses on several projects that look to MMPs to examine the parallels between development and regeneration, as well as development and cancer. "It's clear that a lot of what's happening in cancer makes use of paradigms that are used only during development or embryogenesis," she comments. In a recently published study Werb and collaborators reported, for example, that the activation of mast cells, a type of white blood cell, and MMP-9 coincides with the start of angiogenesis in the skin of mice with squamous carcinoma.1
Werb notes that although the majority of MMP anticancer trials boast positive outcomes, there are a few that report that the inhibition of proteases actually accelerates the growth of tumors. "I think both are true," remarks Werb. "It may be a question of time and space." Certain MMPs may have different functions at different times. She cites, for example, the observed varying functions of MMPs in the mammary gland in the course of its development, a system which her lab continues to study. Says Werb, "[It] really begs the question of how you're going to decide on what you do for treatment."
Despite the irregularities of MMP function, clinical MMP cancer and arthritis trials are under way. But although a handful of small-molecule inhibitors have entered preclinical and even Phase III trials, few if any have demonstrated the desired specificity and lack of side effects. Compounds tested are too far-reaching, and often cause joint pain or deterioration because of unwanted interactions with the wrong MMPs. As in many chemotherapy treatments, insufficient specificity and unwanted toxicity are a problem. "Efficacy with the safety--that's the goal," comments Archie W. Prestayko, president and chief executive officer of BioStratum Inc., Research Triangle Park, N.C.
MMP-related arthritis treatments have similar ends. A recent paper in Science reported the identification of two proteases that belong to a family of ADAMTSs, an offshoot of MMPs. Called aggrecanase-1 and aggrecanase-2, these enzymes could be integral to the development of arthritis therapies with the advent of appropriate inhibitors.2 Both enzymes interact with aggrecan, a component of cartilage whose degradation causes cartilage to lose its compressibility and resilience. Arthritis develops as matrix proteins in the cartilage tissue break down.
As with cancer, the hope is to design a potent, selective small molecule--in this case, one that blocks aggrecanase-1 and -2 and stops the progression of cartilage degradation in arthritis. "Our number-one objective is to show that a small molecule ... has efficacy in animal models of cartilage degradation, and ultimately ... in human beings," explains the lead author of the aggrecanase study, Micky D. Tortorella, a research scientist at DuPont Pharmaceuticals in Wilmington, Del. His company is among several working on X-ray crystallography projects aimed at aiding in the search for more specific inhibitors by elucidating the precise structure of MMPs. Seven years of painstaking X-ray crystallography research at Biostratum recently yielded the entire X-ray structure of MMP-2, information that should be useful in designing more selective therapies.3,4 But processing the data gained from such accomplishments and finding the perfect inhibitor takes time. "We need to get additional information on comparing the structures of the MMPs," comments Stetler-Stevenson.
"There's a big story left to tell, a lot of work to do once you have the crystal structure, once you have the parameters and the design of the molecule," remarks David Cheresh, a professor of immunology and vascular biology at the Scripps Research Institute in La Jolla, Calif., and an angiogenesis researcher. The right inhibitor, says Cheresh, must be orally active and not easily degraded.