Novel chemotherapeutics for treating multidrug-resistant (MDR) strains of Mycobacterium tuberculosis (MTB) are required to combat the spread of tuberculosis, a disease that kills more than 2 million people annually. Using structure-based drug design, we have developed a series of alkyl diphenyl ethers that are uncompetitive inhibitors of InhA, the enoyl reductase enzyme in the MTB fatty acid biosynthesis pathway. The most potent compound has a Ki' value of 1 nM for InhA and MIC99 values of 2-3 microg mL(-1) (6-10 microM) for both drug-sensitive and drug-resistant strains of MTB. Overexpression of InhA in MTB results in a 9-12-fold increase in MIC99, consistent with the belief that these compounds target InhA within the cell. In addition, transcriptional response studies reveal that the alkyl diphenyl ethers fail to upregulate a putative efflux pump and aromatic dioxygenase, detoxification mechanisms that are triggered by the lead compound triclosan. These diphenyl ether-based InhA inhibitors do not require activation by the mycobacterial KatG enzyme, thereby circumventing the normal mechanism of resistance to the front line drug isoniazid (INH) and thus accounting for their activity against INH-resistant strains of MTB.

Summary Understanding the basis of bacterial persistence in latent infections is critical for eradication of tuberculosis. Analysis of Mycobacterium tuberculosis mRNA expression in an in vitro model of non-replicating persistence indicated that the bacilli require electron transport chain components and ATP synthesis for survival. Additionally, low μM concentrations of aminoalkoxydiphenylmethane derivatives inhibited both the aerobic growth and survival of non-replicating, persistent M. tuberculosis. Metabolic labeling studies and quantitation of cellular menaquinone levels suggested that menaquinone synthesis, and consequently electron transport, is the target of the aminoalkoxydiphenylmethane derivatives. This hypothesis is strongly supported by the observations that treatment with these compounds inhibits oxygen consumption and that supplementation of growth medium with exogenous menaquinone rescued both growth and oxygen consumption of treated bacilli. In vitro assays indicate that the aminoalkoxydiphenylmethane derivatives specifically inhibit MenA, an enzyme involved in the synthesis of menaquinone. Thus, the results provide insight into the physiology of mycobacterial persistence and a basis for the development of novel drugs that enhance eradication of persistent bacilli and latent tuberculosis.

In earlier work structure-based design studies resulted in the discovery of alkyl substituted diphenyl ether inhibitors of InhA, the enoyl reductase from Mycobacterium tuberculosis. Compounds such as 5-hexyl-2-phenoxyphenol 19 are nM inhibitors of InhA and inhibit the growth of both sensitive and isoniazid-resistant strains of Mycobacterium tuberculosis with MIC 90 values of 1-2 µg/mL. However, despite their promising in vitro activity, these compounds have ClogP values of over 5. In efforts to reduce the lipophilicity of the compounds, and enhance potentially enhance compound bioavailability, a series of B ring analogues of 19 were synthesized that contained either heterocylic nitrogen rings or phenyl rings having amino, nitro, amide or piperazine functions. Compounds 3c, 3e and 14a show comparable MIC 90 values to that of 19, but have improved ClogP values. KeywordsTuberculosis; Diaryl ether; enoyl reductase; mycolic acids; InhA; isoniazid Tuberculosis (TB) is responsible for more than 1.6 million deaths per annum with 8.8 million new cases being reported each year. These numbers make TB one of the leading infectious causes of death, eclipsed only by AIDS. In addition, according to the World Health Organization, the number of multi-drug-resistant and extensively drug-resistant TB cases is growing with almost a half million new cases being reported each year. Therefore, there is an urgent need to develop novel TB chemotherapeutic agents. 1 *Corresponding authors, PJT: Tel.: (631) 632 7907; Email: peter.tonge@sunysb.edu, RAS: Tel.: (970) 491 1925; Email: richard.slayden@colostate.edu. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. In this study we describe two classes of molecules in which alterations have been made to the diphenyl ether 'B' ring. In one series of compounds we have replaced the B ring with isosteric heterocycles that incorporate nitrogen atoms within the ring, thereby causing little steric perturbation to the overall structure of the molecule (Scheme 2). The second series of compounds have nitro, amino, amide and piperazino functionalities incorporated at the ortho, meta, or para positions of the B ring (Scheme 3 and Scheme 4). This second series of compounds was synthesized not only to improve solubility but also to systematically identify positions on the B ring which could be substituted without diminishing biological activity. NIH Public AccessThe synthesis of the heterocyclic diaryl ether compounds was initiated either by nucleophilic aromatic substitution or by Buchwald-Hartwig cross-coupling of the appropriate nitrogen heterocycle with 4-b...

Structure-based design was used to develop a focused library of A-ring-modified diphenyl ether InhA inhibitors. From this library of analogs, two high-affinity alkyl-substituted diphenyl ethers, 6PP and 8PP, were selected for advanced study into their in vitro activity against Mycobacterium tuberculosis clinical isolates, their in vivo properties, and their signature response mode of action. 6PP and 8PP demonstrated enhanced activity against whole bacteria and showed activity in a rapid macrophage model of infection. In addition, transcriptional profiling revealed that the A-ring modifications of 6PP and 8PP increased the specificity of each analog for InhA. Both analogs had substantially longer half-lives in serum than did the parent compound, exhibited a fivefold reduction in cytotoxicity compared to the parent compound, and were well tolerated when administered orally at 300 mg/kg of body weight in animal models. Thus, the A-ring modifications increased the affinity and whole-cell specificity of the compounds for InhA and increased their bioavailability. The next step in optimization of the pharmacophore for preclinical evaluation is modification of the B ring to increase the bioavailability to that required for oral delivery.

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