Discovery of an orally active benzoxaborole prodrug effective in the treatment of Chagas disease in non-human primates.

Trypanosoma cruzi, the agent of Chagas disease, probably infects tens of millions of people, primarily in Latin America, causing morbidity and mortality. The options for treatment and prevention of Chagas disease are limited and underutilized. Here we describe the discovery of a series of benzoxaborole compounds with nanomolar activity against extra- and intracellular stages of T. cruzi. Leveraging both ongoing drug discovery efforts in related kinetoplastids, and the exceptional models for rapid drug screening and optimization in T. cruzi, we have identified the prodrug AN15368 that is activated by parasite carboxypeptidases to yield a compound that targets the messenger RNA processing pathway in T. cruzi. AN15368 was found to be active in vitro and in vivo against a range of genetically distinct T. cruzi lineages and was uniformly curative in non-human primates (NHPs) with long-term naturally acquired infections. Treatment in NHPs also revealed no detectable acute toxicity or long-term health or reproductive impact. Thus, AN15368 is an extensively validated and apparently safe, clinically ready candidate with promising potential for prevention and treatment of Chagas disease.

Macrofilaricidal Benzimidazole-Benzoxaborole Hybrids as an Approach to the Treatment of River Blindness: Part 2. Ketone Linked Analogs.

The optimization of a series of benzimidazole-benzoxaborole hybrid molecules linked via a ketone that exhibit good activity against Onchocerca volvulus, a filarial nematode responsible for the disease onchocerciasis, also known as river blindness, is described. The lead identified in this series, 21 (AN15470), was found to have acceptable pharmacokinetic properties to enable an evaluation following oral dosing in an animal model of onchocerciasis. Compound 21was effective in killing worms implanted in Mongolian gerbils when dosed orally as a suspension at 100 mg/kg/day for 14 days but not when dosed orally at 100 mg/kg/day for 7 days.

Macrofilaricidal Benzimidazole-Benzoxaborole Hybrids as an Approach to the Treatment of River Blindness: Part 1. Amide Linked Analogs.

A series of benzimidazole-benzoxaborole hybrid molecules linked via an amide linker are described that exhibit good in vitro activity against Onchocerca volvulus, a filarial nematode responsible for the disease onchocerciasis, also known as river blindness. The lead identified in this series, 8a (AN8799), was found to have acceptable pharmacokinetic properties to enable evaluation in animal models of human filariasis. Compound 8a was effective in killing Brugia malayi, B. pahangi, and Litomosoides sigmodontis worms present in Mongolian gerbils when dosed subcutaneously as a suspension at 100 mg/kg/day for 14 days but not when dosed orally at 100 mg/kg/day for 28 days. The measurement of plasma levels of 8a at the end of the dosing period and at the time of sacrifice revealed an interesting dependence of activity on the extended exposure for both 8a and the positive control, flubendazole.

Identification of a 4-fluorobenzyl l-valinate amide benzoxaborole (AN11736) as a potential development candidate for the treatment of Animal African Trypanosomiasis (AAT).

Novel l-valinate amide benzoxaboroles and analogues were designed and synthesized for a structure-activity-relationship (SAR) investigation to optimize the growth inhibitory activity against Trypanosoma congolense (T. congolense) and Trypanosoma vivax (T. vivax) parasites. The study identified 4-fluorobenzyl (1-hydroxy-7-methyl-1,3-dihydrobenzo[c][1,2]oxaborole-6-carbonyl)-l-valinate (5, AN11736), which showed IC50 values of 0.15 nM against T. congolense and 1.3 nM against T. vivax, and demonstrated 100% efficacy with a single dose of 10 mg/kg against both T. congolense and T. vivax in mouse models of infection (IP dosing) and in the target animal, cattle, dosed intramuscularly. AN11736 has been advanced to early development studies.

Treatment of Skin Inflammation with Benzoxaborole Phosphodiesterase Inhibitors: Selectivity, Cellular Activity, and Effect on Cytokines Associated with Skin Inflammation and Skin Architecture Changes.

Psoriasis and atopic dermatitis are skin diseases affecting millions of patients. Here, we characterize benzoxaborole phosphodiesterase (PDE)-4 inhibitors, a new topical class that has demonstrated therapeutic benefit for psoriasis and atopic dermatitis in phase 2 or phase 3 studies. Crisaborole [AN2728, 4-((1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-5-yl)oxy)benzonitrile], compd2 [2-ethoxy-6-((1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-5-yl)oxy)nicotinonitrile], compd3 [6-((1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-5-yl)oxy)-2-(2-isopropoxyethoxy)nicotinonitrile], and compd4 [5-chloro-6-((1-hydroxy-1,3-dihydrobenzo[c][1,2]oxaborol-5-yl)oxy)-2-((4-oxopentyl)oxy)nicotinonitrile] are potent PDE4 inhibitors with similar affinity for PDE4 isoforms and equivalent inhibition on the catalytic domain and the full-length enzyme. These benzoxaboroles are less active on other PDE isozymes. Compd4 binds to the catalytic domain of PDE4B2 with the oxaborole group chelating the catalytic bimetal and overlapping with the phosphate in cAMP during substrate hydrolysis, and the interaction extends into the adenine pocket. In cell culture, benzoxaborole PDE4 inhibitors suppress the release of tumor necrosis factor-α, interleukin (IL)-23, IL-17, interferon-γ, IL-4, IL-5, IL-13, and IL-22, and these cytokines contribute to the pathologic changes in skin structure and barrier functions as well as immune dysregulation in atopic dermatitis and psoriasis. Treatment with compd3 or N(6),2'-O-dibutyryladenosine 3',5'-cyclic monophosphate increases cAMP response element binding protein phosphorylation in human monocytes and decreases extracellular signal-regulated kinase phosphorylation in human T cells; these changes lead to reduced cytokine production and are among the mechanisms by which compd3 blocks cytokine release. Topical compd3 penetrates the skin and suppresses phorbol myristate acetate-induced IL-13, IL-22, IL-17F, and IL-23 transcription and calcipotriol-induced thymic stromal lymphopoietin expression in mouse skin. Skin thinning is a major dose-limiting side effect of glucocorticoids. By contrast, repeated application of compd3 did not thin mouse skin. These findings show the potential benefits and safety of benzoxaborole PDE4 inhibitors for the treatment of psoriasis and atopic dermatitis.

Optimization of isoxazoline amide benzoxaboroles for identification of a development candidate as an oral long acting animal ectoparasiticide.

Novel isoxazoline amide benzoxaboroles were designed and synthesized to optimize the ectoparasiticide activity of this chemistry series against ticks and fleas. The study identified an orally bioavailable molecule, (S)-N-((1-hydroxy-3,3-dimethyl-1,3-dihydrobenzo[c][1,2]oxaborol-6-yl)methyl)-2-methyl-4-(5-(3,4,5-trichlorophenyl)-5-(trifluoromethyl)-4,5-dihydroisoxazol-3-yl)benzamide (23), with a favorable pharmacodynamics profile in dogs (Cmax=7.42ng/mL; Tmax=26.0h; terminal half-life t1/2=127h). Compound 23, a development candidate, demonstrated 100% therapeutic effectiveness within 24h of treatment, with residual efficacy of 97% against American dog ticks (Dermacentor variabilis) on day 30 and 98% against cat fleas (Ctenocephalides felis) on day 32 after a single oral dose at 25mg/kg in dogs.

Discovery of an orally bioavailable isoxazoline benzoxaborole (AN8030) as a long acting animal ectoparasiticide.

A novel series of isoxazoline benzoxaborole small molecules was designed and synthesized for a structure-activity relationship (SAR) investigation to assess the ectoparasiticide activity against ticks and fleas. The study identified an orally bioavailable molecule, (S)-3,3-dimethyl-5-(5-(3,4,5-trichlorophenyl)-5-(trifluoromethyl)-4,5-dihydroisoxazol-3-yl)benzo[c][1,2]oxaborol-1(3H)-ol (38, AN8030), which was long lasting in dogs (t1/2=22 days). Compound 38 demonstrated 97.6% therapeutic effectiveness within 24 h of treatment, with residual efficacy of 95.3% against American dog ticks (Dermacentor variabilis) on day 30% and 100% against cat fleas (Ctenocephalides felis) on day 32 after a single oral dose at 50 mg/kg in dogs.

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-1ß, 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.

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 of a novel class of boron-based antibacterials with activity against gram-negative bacteria.

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.

Inhibition of Toll-like receptor-mediated inflammation in vitro and in vivo by a novel benzoxaborole.

Pro-inflammatory cytokines play a critical role in the development of autoimmune and inflammatory diseases. Targeting the cytokine environment has proven efficient for averting inflammation. In this study, we reported that 6-[4-(aminomethyl)-2-chlorophenoxyl]benzo[c][1,2]oxaborol-1(3H)-ol (AN3485), a benzoxaborole analog, inhibited TLR2-, TLR3-, TLR4-, and TLR5-mediated TNF-α, IL-1ß, and IL-6 release from human PBMCs and isolated monocytes with IC(50) values ranging from 18 to 580 nM, and the inhibition was mediated at the transcriptional level. Topical administration of AN3485 significantly reduced PMA-induced contact dermatitis and oxazolone-induced delayed-type hypersensitivity in mice, indicating its capability of penetrating skin and potential topical application in skin inflammation. Oral administration of AN3485 showed dose-dependent suppression of LPS-induced TNF-α and IL-6 production in mice with an ED(90) of 30 mg/kg. Oral AN3485, 35 mg/kg, twice a day, suppressed collagen-induced arthritis in mice over a 20-day period. The potent anti-inflammatory activity in in vitro and in vivo disease models makes AN3485 an attractive therapeutic lead for a variety of cutaneous and systemic inflammatory diseases.

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.

Early rapid identification of in vivo rat metabolites of AN6414, a novel boron-containing PDE4 inhibitor by QTRAP LC/MS/MS to support drug discovery.

There is an increasing interest in in vivo metabolite identification in early drug discovery in order to (i) give a more complete picture of metabolic profile in investigational animal models, (ii) propose phase I and phase II metabolites using the same pharmacokinetic/toxicokinetic study samples, (iii) expose metabolically labile groups where chemical modifications could improve stability, and (iv) enable early safety assessment of metabolites. In the early discovery stage of our anti-inflammatory program, one novel benzoxaborole, AN6414, exhibiting both PDE4 enzyme and TNFα inhibition activities, became our primary candidate for further investigation. The traditional metabolite identifications usually require high dosed samples with long data scans and analysis. In this study, we conducted quick and more selective core-structure related precursor scans followed by daughter ion scans and identified a total of 10 major phase I and phase II metabolites using rat plasma samples from a toxicokinetic study at an oral dosing of 30 mg/kg. Plasma samples were treated with solid phase extraction (SPE) prior to LC/MS/MS. An AB SCIEX API 4000 QTRAP mass spectrometer coupled with a Shimadzu LC system was used for LC/MS/MS analysis. We found the major metabolites of AN6414 to be oxidative deboronation, protodeboronation, oxidation products and their sulfate-conjugated species. This analysis drove analoging efforts which improved the pharmacokinetic profile, namely, lowering clearance and increasing exposure relative to AN6414. Toxicity predictions by the software program DEREK suggest the identified potential metabolites to be safe.

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 C(max) >10 µg/mL and AUC(0-24 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.

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.

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.

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.

In Vitro penetration of a novel oxaborole antifungal (AN2690) into the human nail plate.

Onychomycosis is a challenging fungal infection to treat topically, likely due to the unique properties of the nail plate. This seemingly impenetrable barrier has high resistance to the passage of antifungal drugs in sufficient concentrations to kill the causative fungi deep in the nail bed. Recently, a new class of antifungal agent was described, termed oxaboroles, which have broad-spectrum activity. These oxaboroles were designed with properties believed to be required to allow for easier transit through the nail plate. Herein, we report (i) the nail penetration results of four oxaboroles that led to the selection of AN2690, (ii) the results of the nail penetration of AN2690 from four vehicles, and (iii) the nail penetration of AN2690 in its chosen vehicle compared to a commercial control, ciclopirox. AN2690 has superior penetration compared to ciclopirox, and achieves levels within and under the nail plate that suggest it has the potential to be an effective topical treatment for onychomycosis.

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 tRNA(Leu)-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 tRNA's3'-terminal adenosine. The trapping of enzyme-bound tRNA(Leu) in the editing site prevents catalytic turnover, thus inhibiting synthesis of leucyl-tRNA(Leu) and consequentially blocking protein synthesis. This result establishes the editing site as a bona fide target for aminoacyl-tRNA synthetase inhibitors.