Tsutomu Akama

An antifungal agent inhibits an aminoacyl-tRNA synthetase by trapping tRNA in the editing site

Aminoacyl–transfer RNA (tRNA) synthetases, which catalyze the attachment of the correct amino acid to its corresponding tRNA during translation of the genetic code, are proven antimicrobial drug targets. We show that the broad-spectrum antifungal 5-fluoro-1,3-dihydro-1-hydroxy-2,1-benzoxaborole (AN2690), in development for the treatment of onychomycosis, inhibits yeast cytoplasmic leucyl-tRNA synthetase by formation of a stable tRNALeu-AN2690 adduct in the editing site of the enzyme. Adduct formation is mediated through the boron atom of AN2690 and the 2′- and 3′-oxygen atoms of tRNAs3′-terminal adenosine. The trapping of enzyme-bound tRNALeu in the editing site prevents catalytic turnover, thus inhibiting synthesis of leucyl-tRNALeu and consequentially blocking protein synthesis. This result establishes the editing site as a bona fide target for aminoacyl-tRNA synthetase inhibitors.

SCYX-7158, an Orally-Active Benzoxaborole for the Treatment of Stage 2 Human African Trypanosomiasis

Background Human African trypanosomiasis (HAT) is an important public health problem in sub-Saharan Africa, affecting hundreds of thousands of individuals. An urgent need exists for the discovery and development of new, safe, and effective drugs to treat HAT, as existing therapies suffer from poor safety profiles, difficult treatment regimens, limited effectiveness, and a high cost of goods. We have discovered and optimized a novel class of small-molecule boron-containing compounds, benzoxaboroles, to identify SCYX-7158 as an effective, safe and orally active treatment for HAT. Methodology/Principal Findings A drug discovery project employing integrated biological screening, medicinal chemistry and pharmacokinetic characterization identified SCYX-7158 as an optimized analog, as it is active in vitro against relevant strains of Trypanosoma brucei, including T. b. rhodesiense and T. b. gambiense, is efficacious in both stage 1 and stage 2 murine HAT models and has physicochemical and in vitro absorption, distribution, metabolism, elimination and toxicology (ADMET) properties consistent with the compound being orally available, metabolically stable and CNS permeable. In a murine stage 2 study, SCYX-7158 is effective orally at doses as low as 12.5 mg/kg (QD×7 days). In vivo pharmacokinetic characterization of SCYX-7158 demonstrates that the compound is highly bioavailable in rodents and non-human primates, has low intravenous plasma clearance and has a 24-h elimination half-life and a volume of distribution that indicate good tissue distribution. Most importantly, in rodents brain exposure of SCYX-7158 is high, with Cmax >10 µg/mL and AUC0–24 hr >100 µg*h/mL following a 25 mg/kg oral dose. Furthermore, SCYX-7158 readily distributes into cerebrospinal fluid to achieve therapeutically relevant concentrations in this compartment. Conclusions/Significance The biological and pharmacokinetic properties of SCYX-7158 suggest that this compound will be efficacious and safe to treat stage 2 HAT. SCYX-7158 has been selected to enter preclinical studies, with expected progression to phase 1 clinical trials in 2011.

Therapeutic potential of boron-containing compounds

Relative to carbon, hydrogen, nitrogen and oxygen, very little is currently known about boron in therapeutics. In addition, there are very few boron-containing natural products identified to date to serve as leads for medicinal chemists. Perceived risks of using boron and lack of synthetic methods to handle boron-containing compounds have caused the medicinal chemistry community to shy away from using the atom. However, physical, chemical and biological properties of boron offer medicinal chemists a rare opportunity to explore and pioneer new areas of drug discovery. Boron therapeutics are emerging that show different modes of inhibition against a variety of biological targets. With one boron-containing therapeutic agent on the market and several more in various stages of clinical trials, the occurrence of this class of compound is likely to grow over the next decade and boron could become widely accepted as a useful element in future drug discovery.

Discovery and structure–activity study of a novel benzoxaborole anti-inflammatory agent (AN2728) for the potential topical treatment of psoriasis and atopic dermatitis

A series of phenoxy benzoxaboroles were synthesized and screened for their inhibitory activity against PDE4 and cytokine release. 5-(4-Cyanophenoxy)-2,3-dihydro-1-hydroxy-2,1-benzoxaborole (AN2728) showed potent activity both in vitro and in vivo. This compound is now in clinical development for the topical treatment of psoriasis and being pursued for the topical treatment of atopic dermatitis.

Discovery of a Novel Class of Boron-Based Antibacterials with Activity against Gram-Negative Bacteria

ABSTRACT Gram-negative bacteria cause approximately 70% of the infections in intensive care units. A growing number of bacterial isolates responsible for these infections are resistant to currently available antibiotics and to many in development. Most agents under development are modifications of existing drug classes, which only partially overcome existing resistance mechanisms. Therefore, new classes of Gram-negative antibacterials with truly novel modes of action are needed to circumvent these existing resistance mechanisms. We have previously identified a new a way to inhibit an aminoacyl-tRNA synthetase, leucyl-tRNA synthetase (LeuRS), in fungi via the oxaborole tRNA trapping (OBORT) mechanism. Herein, we show how we have modified the OBORT mechanism using a structure-guided approach to develop a new boron-based antibiotic class, the aminomethylbenzoxaboroles, which inhibit bacterial leucyl-tRNA synthetase and have activity against Gram-negative bacteria by largely evading the main efflux mechanisms in Escherichia coli and Pseudomonas aeruginosa. The lead analogue, AN3365, is active against Gram-negative bacteria, including Enterobacteriaceae bearing NDM-1 and KPC carbapenemases, as well as P. aeruginosa. This novel boron-based antibacterial, AN3365, has good mouse pharmacokinetics and was efficacious against E. coli and P. aeruginosa in murine thigh infection models, which suggest that this novel class of antibacterials has the potential to address this unmet medical need.

Boron-based phosphodiesterase inhibitors show novel binding of boron to PDE4 bimetal center.

We have used boron‐based molecules to create novel, competitive, reversible inhibitors of phosphodiesterase 4 (PDE4). The co‐crystal structure reveals a binding configuration which is unique compared to classical catechol PDE4 inhibitors, with boron binding to the activated water in the bimetal center. These phenoxybenzoxaboroles can be optimized to generate submicromolar potency enzyme inhibitors, which inhibit TNF‐α, IL‐2, IFN‐γ, IL‐5 and IL‐10 activities in vitro and show safety and efficacy for topical treatment of human psoriasis. They provide a valuable new route for creating novel potent anti‐PDE4 inhibitors.

Design and synthesis of boron-containing PDE4 inhibitors using soft-drug strategy for potential dermatologic anti-inflammatory application

PDE4 inhibitors are a validated approach as anti-inflammatory agents but are limited by systemic side effects including emesis. We report a soft-drug strategy incorporating a carboxylic ester group into boron-containing PDE4 inhibitors leading to the discovery of a series of benzoxaborole compounds with good potency (for example IC(50)=47 nM of compound 2) and low emetic activity. These compounds are intended for dermatological use further limiting possible systemic side effects.

Linking Phenotype to Kinase: Identification of a Novel Benzoxaborole Hinge-Binding Motif for Kinase Inhibition and Development of High-Potency Rho Kinase Inhibitors

Benzoxaboroles are a novel class of drug-like compounds that have been rich sources of novel inhibitors for various enzymes and of new drugs. While examining benzoxaborole activity in phenotypic screens, our attention was attracted by the (aminomethylphenoxy)benzoxaborole family, which potently inhibited Toll-like receptor–stimulated cytokine secretion from leukocytes. After considering their structure-activity relationships and the central role of kinases in leukocyte biology, we performed a kinome-wide screen to investigate the members of the (aminomethylphenoxy)benzoxaborole family. This technique identified Rho-activated kinase (ROCK) as a target. We showed competitive behavior, with respect to ATP, and then determined the ROCK2–drug cocrystal structure. The drug occupies the ATP site in which the oxaborole moiety provides hydrogen bond donors and acceptors to the hinge, and the aminomethyl group interacts with the magnesium/ATP-interacting aspartic acid common to protein kinases. The series exhibits excellent selectivity against most of the kinome, with greater than 15-fold selectivity against the next best member of the AGC protein kinase subfamily. Medicinal chemistry efforts with structure-based design resulted in a compound with a Ki of 170 nM. Cellular studies revealed strong enzyme inhibition rank correlation with suppression of intracellular phosphorylation of a ROCK substrate. The biochemical potencies of these compounds also translated to functional activity, causing smooth muscle relaxation in rat aorta and guinea pig trachea. The series exhibited oral availability and one member reduced rat blood pressure, consistent with ROCK’s role in smooth muscle contraction. Thus, the benzoxaborole moiety represents a novel hinge-binding kinase scaffold that may have potential for therapeutic use.

Structure-activity relationships of 6-(aminomethylphenoxy)-benzoxaborole derivatives as anti-inflammatory agent.

A series of novel 6-(aminomethylphenoxy)benzoxaborole analogs was synthesized for the investigation of the structure-activity relationship of the inhibition of TNF-alpha, IL-1beta, and IL-6, from lipopolysaccharide stimulated peripheral blood mononuclear cells. Compounds 9d and 9e showed potent activity against all three cytokines with IC50 values between 33 and 83nM. Chloro substituted analog 9e (AN3485) is considered to be a promising lead for novel anti-inflammatory agent with a favorable pharmacokinetic profile.

Discovery and structure-activity relationships of 6-(benzoylamino)benzoxaboroles as orally active anti-inflammatory agents.

Structure-activity relationships of 6-(benzoylamino)benzoxaborole analogs were investigated for the inhibition of TNF-α, IL-1β, and IL-6 from lipopolysaccharide stimulated peripheral blood mononuclear cells. Compound 1q showed potent activity against all three cytokines with IC50 values between 0.19 and 0.50μM, inhibited LPS-induced TNF-α and IL-6 elevation in mice and improved collagen-induced arthritis in mice. Compound 1q (AN4161) is considered to be a promising lead for novel anti-inflammatory agent with an excellent pharmacokinetic profile.