Current Cancer Drug Targets, 2005, 5, 285-298 285 Matrix Metalloproteinase Inhibitors as Anticancer Therapeutics F. Mannello *,1 , G. Tonti 1 and S. Papa 2 1 Institute of Histology and Laboratory Analysis; 2 Institute of Morphological Sciences; Faculty of Sciences MM. FF. NN., University of Urbino “Carlo Bo”, 61029 Urbino, Italy Abstract: Matrix metalloproteinases (MMPs), also designated as matrixins, play a central role in many biological processes and are involved both in physiologic cellular processes and in pathologic situations such as tumor growth, invasion and metastasis. For more than 30 years MMPs have been considered as promising targets for cancer therapy and a number of different synthetic and natural MMP inhibitors have been identified as cytostatic and anti-angiogenic agents and have begun clinical testing in view of their specific implication in malignant tissues. Although preclinical studies were so compelling to encourage several clinical trials, the past years have seen a consistent number of disappointments and limited success. The critical examination of previous studies shed light on new information about the cellular source, substrates and mode of action of MMPs, focusing the attention of future research on the identification of specific MMP targets in tumors at different stage of tumor progression, both in order to improve efficacy and to reduce the side effect profile. In this review we discuss the current view on the feasibility of MMPs as target for therapeutic intervention in cancer, taking into account that the perspective may be of great value for molecular medicine for the twenty-first century, providing intriguing information about the MMPs as mediators in biology and pathology, and as targets for disease therapies. Keywords: Matrix metalloproteinases, Gelatinases, Cancer, Protease Inhibitors, Therapeutic strategies, Clinical trials. INTRODUCTION constitute the zinc binding site and a “methionin-turn” motif that lies beneath the active site zinc ion [3]. The ion-binding motif reads HEBXHXHBGBXHZ, where histidine (H), glutamic acid (E) and glycine (G) are invariant, B is a bulky hydrophobic residue, X is a variable residue and Z is a family-specific amino acid (serine in MMPs). All MMPs have an N-terminal hydrophobic signal sequence (predomain) which leads their synthesis to the endoplasmic reticulum and their secretion into the extracellular space; this amino acid stretch is thereafter removed. This predomain is followed by a 77-87 amino acid-long prodomain that constitutes the N-terminus of the secreted enzyme and maintains it in its latent form until its removal or disruption. The prodomain keeps the enzyme inactive through a mechanism identified as “ cysteine switch” where the unpaired cysteine in the highly conserved “Pro-Arg-Cys- Gly-X-Pro-Asp” sequence forms a bridge with the catalytic zinc, thus preventing enzymatic activity. The enzyme acquires total proteolytic capacity when the prodomain becomes chemically removed by cleavage [1]. The active site is of great importance: it specifically binds to selective substrates by means of its active site cleft, through specificity sub-site pockets that bind amino acids adjacent to the scissile peptide bond, and through secondary substrate- binding exosites located outside the active site [5]. These domains represent the minimal structure of MMPs found in MMP-7 (matrilysin) and MMP-26 (endometase/matrylisin- 2) which lack any other domain. All the other MMPs have a hinge region varying in length and composition which also influences substrate specificity [6] , and a four-blade β- propeller structure representing the hemopexin/vitronectin- like domain [7]. Two metalloproteinases, Gelatinase A and B, are further characterized by the presence of three head-to- tail cystein-rich repeats within the catalytic domain. This structure resembles the collagen-binding type II repeats of fibronectin and is necessary for the binding and cleavaging Extracellular proteinases are involved in a wide variety of developmental and disease-associated processes proving to be key protagonists in many physiological and pathological mechanisms. The first observation made 40 years ago of an activity during metamorphosis in tadpole tails, able to degrade interstitial collagen, opened a new field of unexpected biomedical research and the way to study what has now become the matrix metalloproteinase (MMP) family [1]. These endopeptidases belong to the wider metzincin group, which in turn constitutes one of several metalloendopeptidase families [2-4]; all the metzincins are zinc-dependent proteinases and according to their structural characteristics are subdivided into serralysins, adamalysins, astacins and matrixins [3]. As our knowledge of the MMP family continues to grow, we are learning that their activities are not limited to matrix degradation [1]. MMPs seem to touch almost every aspect of mammalian biology and, through more than 1,000 new publications each year, certain exciting discoveries solidified the field, solving previous puzzles and pointing the research in new directions (starting from biochemical studies, discussing the flurry of activity that involved the gene regulation through molecular biology approaches, and finally focusing the interest to MMPs as therapeutic targets in several diseases). STRUCTURAL AND FUNCTIONAL COMPLEXITY OF MMPs All MMPs share a conserved structural topology, which consists of a catalytic domain containing three histidines that *Address correspondence to this author at the Istituto di Istologia ed Analisi di Laboratorio, Facoltà di Scienze MFN, Via E. Zeppi, Università degli Studi di Urbino “Carlo Bo”, 61029 Urbino (PU), Italy; Tel: +39- 0722-351479; Fax: +39-0722-322370; E-mail: f.mannello@uniurb.it 1568-0096/05 $50.00+.00 © 2005 Bentham Science Publishers Ltd.