Current Pharmaceutical Biotechnology - Volume 9, Issue 1, 2008

Extensive research has gone into the area of metalloproteinases since the introduction of captopril as an inhibitor of angiotensin converting enzyme for the treatment of hypertension. Matrix metalloproteinases were favorite targets of pharmaceutical companies, with over 2,000 patents issued. Many of the MMPs are required for normal physiological processes such as bone growth, tissue remodeling, morphogenesis and regeneration. In some instances, however, production of active MMPs can lead to disease states that include cancer, arteriosclerosis, osteoarthritis, osteoporosis, chronic obstructive pulmonary disease, and multiple sclerosis. For cancer, MMP inhibition has been met mostly with failures, as the clinical candidates either caused unwanted side effects such as fibroplasias, or were ineffective. Knockout experiments done after the clinical failures, indicate that MMP activities can both promote and inhibit cancer and they have demonstrated that certain MMPs should not be targeted such as MT1-MMP; this MMP has an arthritis like phenotype in the knockout mice. The studies have paved the way for the future which is to develop selective inhibitors of MMPs for therapeutic use. MMP13, for example, is a target for osteoarthritis and/or osteoporosis. Selective inhibitors have been made and tested in preclinical trials. However, none have so far entered the clinic. This is principally due to the fact that MMP13 inhibitors are not selective enough and still cause tendonitis. MMP9 inhibition is also being considered for treating diseases such as multiple sclerosis and chronic obstructive pulmonary disorder. Knockout mice for MMP9 have been characterized, and used in an animal model of multiple sclerosis, experimental autoimmune encephalomyelitis. The severity of disease was reduced in young MMP9 knockout mice, suggesting that targeting of MMP9 could be beneficial. Other metalloproteinases such as members of the disintegrin and metalloproteinase family (ADAM family) could make excellent pharmaceutical targets as they are associated with disease states such as cancer, arthritis, asthma, and obesity. Knockout experiments have also been done with ADAMs and the closely related ADAMs containing a thrombospondin motif (ADAM TSs). These studies have confirmed that targeting members of both families can be of therapeutic value. ADAM10 is currently being investigated as a target for cancer while ADAM TS 4 and 5 are aggrecanases and therefore make excellent targets for osteoarthritis. In addition since most of the transgenic animals have been made, it has become clear as to which metalloproteinases to investigate for intervention and which to avoid. Finally, other metalloproteinases that are of bacterial origin are also popular, as compounds can be made that are selective over mammalian metalloproteinases such as those described above. For example, pharmaceutical companies are working on making inhibitors of anthrax lethal factor as a bio-defense tool. Anthrax lethal factor is a metalloproteinase that causes lethality in organisms infected with Bacillus Anthracis. Other programs focus on making LPXC inhibitors as antibacterial agents as LPXC is an enzyme important for gram-negative bacterial membrane synthesis.Now that the human genome has been sequenced, we realize, that the number of metalloproteinases is small, and that with proper selectivity, drugs can be made from targeting a select few. This issue focuses on different metalloproteinases that possess properties that render them as useful targets for drug discovery.

There is a great unmet medical need in the area of cancer treatment. A potential therapeutic target for intervention in cancer is ADAM10. ADAM10 is a disintegrin-metalloproteinase that processes membrane bound proteins from the cell surface to yield soluble forms. Pharmaceutical companies are actively seeking out inhibitors of ADAM10 for treatments in cancer as the enzyme is known to release the ErbB receptor, HER2/ErbB2 from the cell membrane, an event that is necessary for HER2 positive tumor cells to proliferate. ADAM10 is also capable of processing betacellulin indicating that an inhibitor could be used against EGFR/ErbB1 and/or HER4/ErbB4 receptor positive tumor cells that are betacellulin- dependent. ADAM10 is the principle sheddase for several other molecules associated with cancer proliferation, differentiation, adhesion and migration such as Notch, E-cadherin, CD44 and L1 adhesion molecule indicating that targeting ADAM10 with specific inhibitors could be beneficial.

Multi-drug resistant (MDR), pathogenic Gram-negative bacteria pose a serious health threat, and novel antibiotic targets must be identified to combat MDR infections. One promising target is the zinc-dependent metalloamidase UDP-3-O-(R-3-hydroxymyristoyl)-N-acetylglucosamine deacetylase (LpxC), which catalyzes the committed step of lipid A (endotoxin) biosynthesis. LpxC is an essential, single copy gene that is conserved in virtually all Gram-negative bacteria. LpxC structures, revealed by NMR and X-ray crystallography, demonstrate that LpxC adopts a novel ‘β-α-α-β sandwich’ fold and encapsulates the acyl chain of the substrate with a unique hydrophobic passage. Kinetic analysis revealed that LpxC functions by a general acid-base mechanism, with a glutamate serving as the general base. Many potent LpxC inhibitors have been identified, and most contain a hydroxamate group targeting the catalytic zinc ion. Although early LpxC-inhibitors were either narrow-spectrum antibiotics or broad-spectrum in vitro LpxC inhibitors with limited antibiotic properties, the recently discovered compound CHIR-090 is a powerful antibiotic that controls the growth of Escherichia coli and Pseudomonas aeruginosa, with an efficacy rivaling that of the FDA-approved antibiotic ciprofloxacin. CHIR-090 inhibits a wide range of LpxC enzymes with sub-nanomolar affinity in vitro, and is a two-step, slow, tight-binding inhibitor of Aquifex aeolicus and E. coli LpxC. The success of CHIR-090 suggests that potent LpxCtargeting antibiotics may be developed to control a broad range of Gram-negative bacteria.

Loss of aggrecan from articular cartilage is an early and critical event in the pathogenesis of osteoarthritis (OA) and is enzymatically mediated by aggrecanase activity. Since the discovery of aggrecanase-1 (ADAMTS-4) and aggrecanase- 2 (ADAMTS-5), both members of the “a disintegrin and metalloproteinase with thrombospondin motif” family of proteinases, other members of the family have been reported to have aggrecanase activity, as currently defined, including ADAMTS-1, -8, -9, -15 and -16. Understanding whether these other ADAMTS members are in fact genuine in vivo aggrecanases will be important for the development of therapeutic agents that aim to block aggrecan degradation. The goal of this review is to look at the current definition of “aggrecanase activity”, and define its strengths, weaknesses and suitability for determining which ADAMTS, are aggrecanases that participate in aggrecan catabolism in OA. In addition, we propose a more comprehensive definition of aggrecanase activity, based on 6 criteria that encompass both biochemical and biological characteristics of the endogenous aggrecanase activity detected in vitro and in vivo. Finally, using these criteria, we propose which ADAMTSs should be classified as aggrecanases and therefore be considered as drug targets for the development of chondroprotective OA treatments.

Anthrax is caused by infection with Bacillus anthracis, a spore forming, rod-shaped, encapsulated gram positive bacteria. The disease manifests itself in distinct ways depending on the route of entry of infective bacterial spores: cutaneous, inhalational, and gastrointestinal. Though rare in humans, inhalational anthrax has become a major concern due to the capacity for spores to be weaponized. The limited success of antibiotic therapy has motivated investigation of complementary therapeutic strategies that target the bacteria's secreted toxin. The zinc-dependent metalloproteinase lethal factor (LF) is a critical component of anthrax toxin and an important potential target for small molecule drugs. In the past few years, a number of approaches have been taken to identify LF inhibitors, from generating conventional metal chelating substrate analogs to random screening of diverse compound libraries. These efforts have produced several different classes of specific nanomolar range inhibitors. Some compounds have fared well in animal models for anthrax toxemia and infection, and these inhibitors and their derivatives may form the basis for future therapies to treat the disease in humans.

Matrix metalloproteinases (MMPs) are a large family of proteolytic enzymes involved in an array of physiological and pathological processes from development, morphogenesis, reproduction, wound healing, and aging to inflammation, angiogenesis, neurological disorders, and cancer cell invasion and metastasis. The imbalance between MMP activity and the inhibitory action of tissue inhibitors of metalloproteinases (TIMPs) are implicated in multiple diseases. Secreted in the body in a latent form, upon activation MMP-9 (gelatinase B) acts on many inflammatory substrates, and thus is suspected of contributing to the progression of cardiovascular disease, rheumatoid arthritis, and the subjects of this review, chronic obstructive pulmonary disease (COPD) and multiple sclerosis (MS). COPD is the fourth most common cause of death in the United States. In COPD, increased expression of MMP-9 by inflammatory cells e.g. neutrophils and macrophages is correlated with a variety of processes that cause lung damage. MMP-9 is also important in cytokine and protease modulation; it degrades the serine protease inhibitor α1-antitrypsin, which thus may lead to lung destruction. MS affects approximately 400,000 Americans and over a million people worldwide. Upregulation of MMP-9 increases the permeability of the blood brain barrier (BBB), facilitates the infiltration of leukocytes into the central nervous system, and causes myelin sheath degradation and neuronal damage. Early stage clinical trials have shown promising results when MMP-9 is inhibited in MS. These observations lead to the hypothesis that MMP-9 is a potential drug target for both COPD and MS and further development of highly potent and specific MMP-9 inhibitors is warranted.

The role of matrix metalloproteinases in disease has been investigated over the last two decades. A focus on this family of proteases is particularly emphasized in two major arthritides in humans, osteoarthritis and rheumatoid arthritis. Early work described the presence of multiple MMP family members in the joint of the disease state and recent advances in the development of new knockout mice and disease models have allowed investigators to directly test the role of the MMP proteases in arthritis. MMP-13 is expressed by chondrocytes and synovial cells in human OA and RA and is thought to play a critical role in cartilage destruction. The recent development of an MMP-13 knockout mouse has documented the important role for this enzyme in cartilage formation and further studies under disease conditions promise to reveal the function of this enzyme in disease pathology. This review describes a body of research that supports the development of novel selective MMP-13 inhibitors with the hope of developing these compounds in clinical trials for the treatment of arthritis.