SSAO

The natural ligand for the amine oxidase activity is not known for certain. While SSAO/VAP-1 will oxidize endogenous molecules such as methylamine and tyramine, the substrates associated with diapedesis are unknown. It has been speculated that leukocyte cell surface lysine residues or amino sugars, such as mannosamine residues (5) known to be associated with cell/cell recognition may be involved [14,15]. 

AOC-3 Amine oxidase, copper-containing 3 (vascular adhesion protein 1) HPAO, 17q21 SSAO, VAP1, VAP-1, 38,256,727 -38,263, 667 6941 4 4026 763... 

For those interested in the discovery of drug candidates to attenuate SSAO/ VAP-1 activity there are two properties that need to be considered. First, as mentioned above, SSAO/VAP-1 exists as a membrane bound protein and a truncated version is found in the plasma [10,11]. Second, there is tremendous species variation which is revealed in a very large range of the second order rate constant V/K, using benzylamine as substrate, [22,23], and that inhibitor potencies vary widely according to the species [24,25]. Furthermore, within a single species specific activity varies from tissue to tissue [26]. 

In contrast to the flavin-dependent monoamine oxidases, SSAO/VAP-1 has evolved to hydroxylate a tyrosine residue in the active site which is further oxidized to the quinone state by oxygen in the presence of copper ion releasing hydrogen peroxide [28-30]. The primary amine in the substrate (R-NH2, Scheme 1) forms a Schiff-base with the quinone carbonyl group, which through a series of steps ultimately releases the aldehyde product. 

Scheme 1 SSAO/VAP-1 TPQ-catalyzed amine oxidase mechanism.

The symptoms of many chronic diseases, such as rheumatoid arthritis, multiple sclerosis, asthma and chronic obstructive pulmonary disease (COPD) are caused in large part by an excessive and chronic inflammatory response and are therefore potential human diseases for drugs which inhibit the SSAO/VAP-1 activity. Notably, it has been recently shown that patients suffering from either atopic eczema or psoriasis, both chronic inflammatory skin disorders, demonstrate an increase in SSAO/VAP-1 positive vessels in their skin compared to skin from healthy controls [47,48]. 

In an important study, Kirton et al. [5] engineered a mouse-human chimeric antibody and demonstrated that this was able to reduce leukocyte migration in various in vitro and in vivo mouse models. In particular, leukocyte migration to the peritoneal cavity was reduced by 40% in the thioglycollate inflammation model using mice expressing human SSAO/VAP-1 protein. The Finnish company Biotie Therapies Corporation, is in clinical development with an antibody to VAP-1 [54],... 

A group from La Jolla Pharmaceuticals has released data on their novel hydrazines in recent scientific [20,59,60] and patent literature [61,62], A series of arylallyl hydrazines (e.g., 8 and 9) were shown to be potent, irreversible inhibitors of rat and human SSAO/VAP-1 [59]. LJP-1207 (8, IC50 = 2nM, human) was evaluated in a series of in vivo inflammation models. Significant efficacy was observed in a mouse ulcerative colitis, mouse LPS-induced septic shock, and the rat carrageenan foot models [20], in a mouse model that resembles human multiple sclerosis [63], and in a transient forebrain ischemia model in estrogen-treated ovariectomized female rats [60]. 

Two laboratories have independently disclosed an interesting series of mechanism-based inhibitors. The dihydropyrrole 31, which appeared in a patent application [61], was reported to inhibit rat lung SSAO/VAP-1 with an IC50 = 500 nM. Recently, the Sayre team extended earlier work [74] and showed that these inhibitors, exemplified by 32, covalently bound to the enzyme with the cofactor in the reduced form [75]. Presumably, aromatization of the dihydropyrrole moiety accounts for the observed potencies. 

Patent applications from Astellas Pharma [77,78] and Fujisawa [79] have disclosed series of guanine, imidates and aminoimidazoles built on a bis arylethane core structure. Some of these are very potent. Compound 36 inhibits the human and rat enzymes with IC50 = 150 nM and 12nM, respectively compounds 37, 38 and 39 inhibit human SSAO/VAP-1 with IC50 = 5.7 nM, 5.3 nM and 2.4 nM, respectively. 

Other inhibitors include alginic acid hydroxamates [80], the natural products myricetin galloylglycosides [81], polyaminoglycosides [82] and a number of antidepressant drugs [83] which demonstrate various degree of SSAO/VAP-1 inhibitory property in rats, humans and cows. 

Although most work to date has focused on the enzyme inhibition aspect, it is possible that the monoclonal VAP-1 antibody discussed above may well be closer to the clinic. The task of designing small molecules to interfere with cell-cell recognition is certainly feasible, but this will not be a trivial effort, more akin to the search for selective selectin antagonists which has proven to be very challenging. As confidence grows in the pharmaceutical industry that SSAO/VAP-1 is a validated target, it is inevitable that considerable resources will be directed to all avenues to block the functional action of this protein. 

Semicarbazide-sensitive amine oxidase (SSAO) (rat) SSAO can generate vasoactive H2O2. HA (1 mM) inhibits staining of SSAO in rat aorta, and is thought to antagonize H202-induced vasoconstriction 26, 48... 

Aromatic side chain-fluorinated MAO and SSAO inhibitors 671... 

Effects of fluorine substitution on inhibition of SSAO by cyclopropyl amines 683... 

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