Potent active site-directed inhibition of steroid sulphatase by tricyclic coumarin-based sulphamates

Background: There is now abundant evidence that inhibition of steroid sulph atase alone or in conjunction with inhibition of aromatase may enhance the response of postmenopausal patients with hormone-dependent breast cancer to this type of endocrine therapy. Additionally, sulphatase inhibition has be en proposed to be of potential therapeutic benefit in the immune system and for neuro-degenerative diseases. After the finding that our first highly p otent active site-directed steroid sulphatase inhibitor, oestrone-3-O-sulph amate (EMATE), was highly oestrogenic, we proposed non-steroidal coumarin s ulphamates such as 4-methylcoumarin-7-O-sulphamate (COUMATE) as alternative non-steroidal steroid sulphatase inhibitors. In this work, we describe how tricyclic coumarin-based sulphamates have been developed which are even mo re potent than COUMATE, are non-oestrogenic and orally active. We also disc uss potential mechanisms of action. Results: 4-Ethyl- (4), 4-(n-propyl)- (6), 3-ethyl-4-methyl- (8), 4-methyl-3 -(n-propyl)coumarin-7-O-sulphamate (11), the tricyclic derivatives 655COUMA TE (13), 666COUMATE (15), 667COUMATE (17), 668COUMATE (20) and the tricycli c oxepin sulphamate (22) were synthesised. In a placental microsome prepara tion. all of these analogues were found to be more active than COUMATE in t he inhibition of oestrone sulphatase, with the most potent inhibitor being 667COUMATE which has an IC50 of 8 nM, some 3-fold lower than that for EMATE (25 nM). In addition, 667COUMATE was also found to inhibit DHEA-sulphatase some 25-fold more potently than EMATE in a placental microsome preparation . Like EMATE, 667COUMATE acts in a time- and concentration-dependent manner , suggesting that it is an active site-directed inhibitor. However, in cont rast to EMATE, 667COUMATE has the important advantage of not being oestroge nic. In addition, we propose several diverse mechanisms of action for this active site-directed steroid sulphatase inhibitor in the light of recent pu blications on the crystal structures of human arylsulphatases A and B and t he catalytic site topology for the hydrolysis of a sulphate ester, Conclusions: A highly potent non-steroidal, non-oestrogenic and irreversibl e steroid sulphatase inhibitor has been developed. Several mechanisms of ac tion for an active site-directed steroid sulphatase inhibitor are proposed. With 667COUMATE now in pre-clinical development for clinical trial, this s hould allow the biological and/or clinical significance of steroid sulphata se inhibitors in the treatment of postmenopausal women with hormone-depende nt breast cancer and other therapeutic indications to be fully evaluated.