5′-O-[N-(Salicyl)sulfamoyl]adenosine (Sal-AMS, 1) is a nucleoside antibiotic that inhibits incorporation of salicylate into siderophores required for bacterial iron acquisition and has potent activity against Mycobacterium tuberculosis (Mtb). Cinnolone analogues exemplified by 5 were designed to replace the acidic acyl-sulfamate functional group of 1 (pKa = 3) by a more stable sulfonamide linkage (pKa = 6.0) in an attempt to address potential metabolic liabilities and improve membrane permeability. We showed 5 potently inhibited the mycobacterial salicylate ligase MbtA (apparent Ki = 12 nM), blocked production of the salicylate-capped siderophores in whole-cell Mtb, and exhibited excellent antimycobacterial activity under iron-deficient conditions (minimum inhibitor concentration, MIC = 2.3 μM). To provide additional confirmation of the mechanism of action, we demonstrated the whole-cell activity of 5 could be fully antagonized by the addition of exogenous salicylate to the growth medium. Although the total polar surface area (tPSA) of 5 still exceeds the nominal threshold value (140 Å) typically required for oral bioavailability, we were pleasantly surprised to observe introduction of the less acidic and conformationally constrained cinnolone moiety conferred improved drug disposition properties as evidenced by the 7-fold increase in volume of distribution in Sprague-Dawley rats.
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finding supported by computational and crystallographic analyses. However, a formal negative charge is deleterious to passive diffusion of 1 across mammalian membranes. Thus, a delicate balance of the linker pKa is required to maintain potent antitubercular activity and acceptable membrane permeability. In the present work, we report on the design of new conformationally constrained analogues 4−7 (Figure 1C) containing a cinnolone as a closer isosteric mimic of 1 that possess a range of pKas at N-1 through strategic incorporation of fluorines at C-6 and C-7.
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- conformationally constrained analogues