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`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.
`
`Thu.- physical, chemical and biological proper—
`ties of boron ollcr mcdicinal chctnists a tart:
`opportunity to explore and pioneer its utility in
`chcmothempcutics. However. up until the last
`few years. boron has mostly bccn overlooked
`by medicinal chemists in their dcsign of drug
`molccules. In trying to discern why boron has
`not been widely considered. we Found a com-
`mon bclicf within the medicinal chemistry
`community that boron is toxic. However. as we
`have investigated this claim, we have Found it to
`be largely unfounded. Thc beliefthat boron is
`toxic most likcly comes from the fact that boric
`acid (BIOHI’) is an ingredient oFant poisons.
`However, buric acid, has an Ll)” ufZéISU mglkg
`(tat. oral), which is similar to regular table salt
`at 3000 mglkg (tat, oral) [IDI]. Anethcr source
`ofthc loxicityconcetn may have arisen from the
`toxicity oFVelcach {49). tlu: only boron-basal
`therapeutic currently on the market and widely
`prescribed by oncologists. Velcadc is approved
`For the treatment of multiple mycloma and
`works through inhibition ol‘ the proteasomc.
`Recently. rcscatch has shown that the toxicity
`of Vclcadc is due to its mechanism of action
`and not simply because [moon is present in the
`molecule [IL
`The overwhelming data for the safety of
`boron arc to be noted. Boric acid is the main
`
`ingredient in ‘Gnop’ the soft semi-solid, often
`brightly colored toy that children enjoy squccz—
`ing through their fingers; boric acid is used. as a
`preservative in eye wash and in vaginal creams;
`it is used as a buffer in biological assay solu-
`tions; boron is also found in high concentrations
`
`1 0.21155lFMC-DOJ'I © 2009 Future Sclence Lid
`
`in fruit. vegetables and nuts. We consume in
`the range of 0.3—4.2 mg of boron pct day [2|
`and it is considered an essential plant nutrient,
`although its biological Functions are currently
`unknown. Studies at Anacor Pharmaceuticals
`
`l'ound background concentrations of boron in
`mouse plasma samples of approximately 200
`nglml [Wmncn C. Um-umsuzn Dan]. Therefore,
`it does appear that the body is familiar with
`boron. Thc boronic acid group in Vclcadc 'has'
`been shown to hr.- metabolized to boric acid and
`the body seems to manage its metabolism and
`excretion [31. Patabotonophenylalaninc was used
`For boron neutron-capture therapy (BNCT) and
`found to be safe in multiple Species including
`human. The LU“ values of Free base pa rabo—
`ronopltcnylalanine were determined to be more
`than 3000 mglkg in rat by intrapcritoncal or
`subcutaneous administration. In rcpcal don't:
`studies in rats, parabotonophenylalaninc was
`administered subcutaneously for 28 days and
`the 380—mgl’kg group exhibited no signifi—
`cant finding when comparcd with the control
`group l-tl. In humans, paraboronophenylalan int:
`was administrcrcd via infusion as a complex
`with Fructose, up to 900 mglltg of bodywcight
`[5]. and BSH (NazBI2H m-SH]. another boron
`therapeutic for BNCT, was administered at the
`lDO-mglltg bodyweight Icvcl [Eli both proved to
`be safe and well tolerated. From all these data.
`we have concluded that boron is not an inhcr-
`cntly toxic clctncnt, such as mctcury, and can
`bc considered by medicinal chemists for use in
`therapeutics. The toxicology question now is not
`what happens to boron. but what happcns to thc
`
`FUfureMed. Chem. (2‘00?) l0) 12751288
`
`Stephen] Baker‘,
`Charles I Ding.
`Tsutornu Akama.
`Yong-Kang Zhang,
`Vincent Hernandez 8: Yi Xia
`*Author for correspondence
`Anacor Pharmaceuticals. Inc..
`lO'ZO East Meadow Circle.
`Palo Alto. CA 94303. USA
`~Te|.:+1650'543 7500
`Fax: H 650 S43 T660
`E-mail: shaker-@anacorazom
`
`FUTURE
`
`SCIENCE
`
`ISSN 115641919
`
`
`
`

`

`i thmmwm'm4 rrmmwemwtouse:s1
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`
`fisarnuoxma
`
`R-B(OH)-N=N-R', six-
`membered heterocycle ring
`containing a boron, two
`nitrogen and three
`carbon atoms
`
`re5t ofthe molecule should boron be eliminated
`
`from the parent molecule, a standard question
`for any drug candidate ofnonboron origin.
`Why do we not have more US FDA-approved
`boron-containing therapeutics to date? Our pri-
`mary conclusion is that it is because the organo—
`boron chemistry field is still in its infancy and
`we do not have a very large portfolio ofchemical
`reactions to introduce boron into organic mol—
`ecules, nor do we have a good understanding of
`the compatibility ofboron-containing molecules
`in common synthesis. Over the last two decades,
`and since Suzuki—Miyaura coupling reactions
`have become more widespread, there has been a
`significant increase in organoboron chemistry,
`which has led to the introduction of new catalysts
`and new methods of incorporating boron into
`organic molecules and has provided more insight
`to the chemical compatibilities oforganoboron
`compounds. This chemistry development is now
`allowing medicinal chemists to build drug-like
`boron—containing molecules to finally explore
`the usefulness ofboron in chemotherapeutics.
`Similar to hydrogen, carbon, nitrogen and
`oxygen, boron is, quite simply, another use—
`ful atom! Boron can be considered the equal
`and opposite ofnitrogen. Nitrogen is a Lewis
`base, boron is a Lewis acid; nitrogen has a
`fullp-orbital (lone pair), boron has an empty
`p—mbitalmitrogen is nuclcophilic, boron is clcc-
`trophilic; nitrogen sits to the right ofcarbon in
`the periodic table, boron sits to the left. Boron
`in organic molecules is most commonly present
`as a boronic acid group (R—B[OH]2), where its
`pKa usually ranges from 7 to 9, considerably
`higher than carboxylic acids. This means that, at
`physiological pH, boronic acid is uncharged and
`in the trigonal planar sp2 form (APPENDIX, I). At
`pHs above its pKa, a hydroxy group coordinates
`to the empty p—orbital, forming a dative bond,
`and boron is in a tetrahedral sp3 form (2).
`The empty p-orbital can also be occupied by
`a lone pair from other nucleophilcs, including
`alcohols and amines, allowing boron to form a
`dative bond with biological nucleophiles such as
`enzyme residues, including serine, and hydroxy
`groups from carbohydrates and nucleic acids.
`Boron can form a bond with a therapeutic target
`that is neither ionic nor an irreversible covalent
`
`bond. Also, the pKa of boron can be tuned by
`chemical modification ofthc molecule to make
`
`borate esters with alcohols. However, these are
`usually unstable and hydrolyze easily in water.
`Under certain circumstances, the stability of
`these borate esters can be increased through
`intramolecular cyclization, (e.g., forming a dies—
`rcr with both hydroxyl groups ofa 1,2—cis—diol,
`such as ethylene glycol, to form a 5-membered
`dioxaborolane ring). Substituted cir—diols are
`even more stabilized, due to steric hindrance
`preventing water approaching the boron and
`subsequent hydrolysis. The properties, prepara-
`tion and applications ofboronic acids have been
`comprehensively reviewed [7].
`Boron in therapeutics has been reviewed in
`depth [7-9]. This review is intended to describe
`some of the recent advances since those earlier
`reports, as well as to describe some older chem-
`
`istry oftlie
`":
`,1 Ems and to make ajudicial
`prediction about the potential future of boron
`in drug discovery.
`
`Therapeutic areas containing
`boron-based therapeutics
`I Diazaborines & enoyl reductase
`Early history of diazaborines
`One oftlie first classes ofboron-containing com—
`pounds evaluated as therapeutics was the diaza—
`borines [10}. Diazaborines were first synthesized
`by Dewar who was investigating the ‘nonbenze-
`. noid’ aromaticity ofhetcrocyclcs |11|, Dewar did ‘
`not report their medicinal application, but rather
`that they were tool compounds to demonstrate
`the potential of replacing the C—C unit in aro-
`matic compounds with the isoelectronic B-N
`bond. However,
`it was Gronowitz who saw a
`similarity with the hydrazone-eontaining nitro—
`fu ran antibiotic nitrofurantoin (3); diazaborines,
`he reasoned, contained an internal hydrazone
`and might also share the same antibiotic activity.
`His work demonstrated the first reported
`antimicrobial activity for a boron—containing
`compound [11,13]. Starting first with structur-
`ally similar nitrothiophene (4), Gronowitz
`quickly established that the nitro substitution
`on the ring was not necessary and that the most
`dramatic impact on activity was observed when
`changing the hydrazine component. A strong
`preference for 2-N—sulfonyl substitution in am:
`logs possessing antibacterial activity was noted
`and served as the template for future research.
`Several patents followed, demonstrating the
`apparent interest in this area and culminating
`in the largest such evaluation being published
`by Sandoz Pharmaceuticals [14]. In this study,
`the MIC activity of 80 different diazahorincs
`we.
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`it a stronger or weaker electrophile. Modulation
`of electronic and peripheral substitutions can
`allow boron to improve its selectivity towards
`its desired target. Boronic acids can also form
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`
`future science group
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`was Consistent with previous observations that
`activity was confined almost exclusively to
`Gram-negative bacteria. It was generally estab-
`lished that the ranking of potency, relative to
`the arene ring, followed the order: thienodiaza—
`borines, benzodiazaborines, furanodiazaborines,
`with pyrolodiazaborines being inactive. In the
`2-N-sulfonylalkyl-thienodiazahorine series,
`the effect of homologating the alltyl chain was
`striking, with 5 having a high MIC against
`Ercbericbirt calf of more than 50 pg/ml, com-
`pared with 6.25 pg/ml for 6. Mcthylation ofthe
`thiophenc ring gave a slight boost in potency to
`provide the most promising diazaborine reported
`(Sa 84474 [7]) with an MIC of 1.25 pg/ml.
`To better understand the
`require—
`ments
`for activity.
`a nonboron analog,
`4-hydroxy—3-(p-tolylsufononylhsoquinoline.
`was prepared and found to be inactive. It is
`important to note that these synthetic efforts
`were not guided by knowledge ofthe mechanism
`ofaction. In facr, the target was initially believed
`to be lipopolysaccharidc synthesis [15], which was
`consistent with the observation that activity was
`confined almost exclusively to Gram-negative
`bacteria. Subsequent studies would reveal the
`actual target to be fatty acid biosynthesis, but it
`took another 12 years until the crystal structure
`was reported [16,17].
`" "Investigation into the structure—activity rela-
`tionships of dialaborines during this time was
`almost exclusively limited to diazaborines con-
`taining the sulfonyl side chain. Problems with
`this particular class of diazaborinc are evident
`from the literature as little progress has been
`made since. This is possibly due to two reported
`cases oftoxicity.
`Forbes and Davies reported toxicology studies
`ofa furano derivative (ICI 78911 {81) that was in
`development for the treatment ofGram-negative
`infections. Toxicology studies in rodents yielded
`no abnormal findings. However. corneal ulcer-
`ation in dogs was evident following administra-
`tion ofthree daily doses of 25 mg/kg [13] and was
`cited as the reason to step further development.
`Grassberger er rd. cautioned against
`the
`potential toxicity associated with this class and
`openly speculated that boron c0uld be involved
`[14]. However, no toxicity data were published
`and no proof (or testable hypothesis) that boron
`was the origin of toxicity was offered. A retro-
`spective on Grassherger's work then misinter—
`preted these comments as proofthat boron can-
`not be used clinically because of the 'inherent
`toxicity of boron—containing Compounds’ [19].
`
`This has had a most unfortunate consequence as
`subsequent articles have referenced this review
`propagating the notion that boron is toxic {19].
`More recent studies [20—24] with other classes of
`diazaborines have not mentioned any reports
`of toxicity.
`
`Enoyi reductase is the target for
`N-sulfonyl diazaborines
`Enoyl reductase (ENR) is an enzyme involved
`in fatty acid biosynthesis. ENR is a target of
`a front-line anti-TB drug isoniazid and the
`antimicrobial agent triclosan [91. N—sulfonyl—
`substituted diazaborine inhibitors ofENR form
`
`a covalent B—O ester with the 2—hydroxyl group
`of the cofactor nicotinamide adenosine ribose,
`forming an inhibitor—substrate adduct bound
`in the enzyme active site. Co-crystal structures
`for a variety of sulfonyldiazaborines have been
`'published that explain the preference for the
`sulfonyl group and confirms boron is critical
`to the mechanism ofinhibition [25]. In addition
`to accepting an intramoIeCUlar hydrogen bond
`from the boron hydroxyl group, the electron-
`withdrawing nature ofthe sulfonyl group stabi—
`lizes the negative charge on the boron atom and
`induces a conformational bend into the molecule
`
`that orients the sulfonyl substituent into a cavity
`of the active site.
`
`Diazaborines for TB
`
`Renewed interest in this area followed the pub-
`lication of diazahorine’s mechanism of action
`with evaluation of new classes of diazaborincs
`
`including the isosteric 2,4,1-benzoleldiazabo-
`tines, targeting Mycobacrerz‘um tuberculous [24].
`Comparisons were made with two front—line TB
`drugs: isoniazitl and pyrazinamide.
`While none of the diazaborines tested had
`
`two
`activity near the potency of isoniazid,
`derivatives (9 8: I0) had MIC values in the
`range of 8—16 pg/ml. which is superior to
`pyrazinamide (~200 lag/ml).
`
`Diazaborines as steroid mimiCS
`
`Another potential application of diazahorines
`is in the design of‘uItra-high' fidelity estrogen
`structural mimics (ll).
`Crystalographic data for compound II, which
`contains an inti-amolecular hydrogen bond, con—
`firm the estrogen—like conformation of these
`boron-containing hererocycles [23]. Screening for
`antiproliferative activity againsr MCF-7 human
`breast cancer cells demonstrated an IC50 value
`for H ofapproximately 5 pM l9l.
`
`r53 future science group
`
`www.fulure-science.com
`
`'I 27 7
`
`

`

`
`
` ifsflfifiiii’tfa 3 Baker, Ding, Akama, Zhang. Hernandez & Xiamumiwwwmmm mmmmrmaram-mm an»Wmmmmvwem maiaszxxxxwyamamas-mm”WWW“
`
`
`
`I Boronic acid hepatitis C virus serine
`protease inhibitors
`Hepatitis C virus (HCV) infection is a major
`cause ofhuman liver disease. It is estimated that
`
`over200 million people worldwide are chronically
`infected with HCV, HCV was first identified by
`molecular cloning in 1989 [26] and is an enveloped
`- virus containing a single-strand RNA molecule
`of positive polarity with approximately 9600
`base pairs. The HCV serine prorease NSSI4A
`is considered to he an essential enzyme for the
`replication of the virus and has been a clinically
`validated drug target by BEN-2061 [27].
`I’eptide boronic acid derivatives, targeting the
`NS5/4A serine protease by trapping the catalytic
`See-159 hydroxyl functional group with its empty
`p-orbital of boron, have been investigated for
`more than a decade in the quest for novel agents
`for the treatment of HCV infection. The func—
`
`the biological data have not been disclosed, their
`enzyme potencies are likely to be better than the
`corresponding acyclic analogs, as observed with
`other nonhoronic acid protease inhibitors, due to
`the reduced rotational freedom and the known
`
`SAR in the HCV protease inhibitor field. It is
`speculated that one of their derivatives might
`have entered clinical development [107].
`Schering-Plough scientists recently pub-
`lished their work on boronic acid derivatives of
`
`SCH-505034 (23). SCH—503034 is in advanced
`clinical development
`for
`the treatment of
`HCV [29].
`As illustrated in the appendices, the replace-
`ment of P1 ethyl side chain in 24, 26 and 23
`with cyclobutylmethyl improves enzyme poten-
`cies by 50-, 68- and 260-fold, respectively, giv-
`ing 25 (Ki = 1011M), 21 (Ki = 0.5 nM) and 29
`(Ki = 0.2 nM). Enzyme potencies of boronic
`acids are not significantly different from their
`pinanediol esters in comparison of 26 with 28
`and 27 with 29. Evaluation ofthcse compounds
`in the cell-based replicon assay gave an EC90 of
`more than 5 pM, The poor potency suggests
`that these inhibitors may have very limited
`cell permeability.
`incorporation of boronic
`In summary,
`acid into HCV setine protease inhibitors has
`been a successful strategy in finding novel
`HCV therapeutics.
`
`tional boronic acid is positioned at the peptide—1
`(Pl) position ofthe pcptidomimetic. Compound
`I2 is an example ofthc class of peptide boronic
`acids discovered in 1996 and shows an IC50 of
`34 nM against the NS3/4A enzyme [2m].
`A less—polar analog,
`l3, was also made, pre—
`sumably with the intention to improve cellular
`penetration. Afterwards, shorter peptide boronic
`acids and their esters with proline scaffold, such
`as l4, l5 and i6, were synthesized [202.203]. The
`.. (+)-pinancdiol moiety is needed for the chiral
`synthesis ofthe P1 amino boronic acid and may
`also promote the cellular penetration due to
`its lipophilicity.
`A follow—up study of l4 Pl—variablc analogs
`reveals that compound I? has a Ki on nM against
`N33 protease, lOOO-fold selectivity over clastase
`and 40-fold selectivity over chymotrypsin [28].
`The enzyme potency of I7 is remarkable. The
`large borate ester present at the P1 site might
`have been hydrolyzed to expose the functional
`boronic acid. More recent examples have quino-
`line and isoincloline structures at the P2“ posi-
`tion. Examples include compounds IB and I9,
`which have borate ester functionaliries at the Pl
`
`I Boronic acid as B-lactamase inhibitors
`B-lactam antibiotics remain the most used
`antibacterial agents in clinical practice. Thcir
`mechanism of action consists of interfering
`with cell wall assembly by binding to penicil—
`lin-binding proteins that insert the pcptidogly-
`can precursors into the nascent cell wall and
`inhibiting bacterial grOWth [so]. However, the
`continuous development of resistance repre—
`sents a serious threat to the clinical utility of
`B—Iactams, leading to an urgent requirement for
`new compounds [3||.
`fl—lactamases represent the most common sin-
`glc cause ofbacterial resistance to B-lactam anti—
`biotics, especially in Gram-negative bacteria [32].
`B-iactamases act by catalyzing the hydrolysis of
`the amide bond ofthe B-lactam ring, thus lead-
`ing to biologically inactive products [33]. There
`are more than 450 members of the B—lactamase
`superfamily, divided into four classes (A, B, C
`and 1)). Classes A, C and D are serine proteases
`and class Bis a metallo-B-lactamase. An impor-
`tant strategy that has been successfully utilized
`ful synthesis of macrocyclic boronic acid proter
`asc inhibitors, for example 20—22 [207]. Although
`for overcoming fl-lactamasc—mcdiatcd rcsista ncc
`esnwnuwww
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`site and quinoline and isoindoline at the [’2”‘ site,
`respectively. Both inhibitors exhibited increased
`molecular interaction with the N53 protease, as
`reflected in their potency enhancement [205.206].
`This also resulted in the lower peptide character
`of the inhibitors compared with previous com-
`pounds, such as l7, and is a progress towards the
`goal ofdiscovering inhibitors for oral use.
`A Further advance in this area was the success-
`
`1278
`
`Future Med. Chem. (2009) 1(7)
`
`future science group
`
`

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`.:‘\r"rv_‘/~i’;>1a Therapeutic potential of boron-containing compounds
`
`
`Gmoosmu
`
`R-B(OH)-OR'. five-membered
`heterocyelic ring containing a
`boron. an oxygen and three
`carbon atoms
`
`to B-lactams has been the co—administration of
`the B—lactam antibiotic together with a B—lacra—
`mase inhibitor [34}. in these combinations, the
`B—lactamase inhibitor forms a covalent adduct
`with the enzyme, preventing it from hydro-
`lyzing the fi-lactam antibiotic. Three widely
`spread clinical B-lactamasc inhibitors, Clavulanic
`acid, tazohactam and sulbactam, are effective
`only against Class A scrinc B—lactamases [55].
`Therefore,
`there is a clear medical need for
`broad-spectrum inhibitors that include activity
`against class C and D enzymes [36].
`Boronic acid derivatives have proven to bc
`promising selecrive inhibitors ofthe serine pro-
`‘ tease family of B-lactamases. The electrophilic
`boron atom acts as a mimic ofthe carbonyl car—
`bon ofthe B-lactam ringand forms a tetrahedral
`addUCt with the Catalytic serine, which closely
`resembles one of the‘ transition states of the
`
`hydrolytic mechanism [37]. Compounds 30-32
`were discovered as potent inhibitors ofAmpC
`B—lactamase. They were designed to gain inter-
`actions with highly conserved residues, Such as
`Asn343, in addition to catalytic serinc, and to
`bind more tightly to the enzymes. Compound 30
`has a Ki value of420 nM in Alan. The stereo—
`controlled introduction of the plienyl group,
`mimicking the dihydrothiazinc ring as well as
`the configuration at the C7 of ccphalosporins,
`led to a tenfold improvement in affinity (com-'
`pound 3], Ki = 35 nM). Addition ofa m—car—
`boxyphenyl moiety further improved affinity
`againstAmpC B-lactamase (32, Ki: 1 nM) [37}.
`Another scrics ofglycylboronic acids bearing
`the side chains of cephalosporins and penicil—
`lins have proven to be reversible and com—
`petitive inhibitors of CTX—M B-lactamase.
`Compound 33, containing the side chain ofnaf-
`cillin, has Ki values of 1.2 and 3.0 pM against
`CTX—M—9 and CTX—M—lé, respectively. The
`2-aminothiazole inhibitor 34, containing the
`side chain from eeftaziclime, has Ki values ONE
`and 4 nM against CTX—M-9 and CTX—M-IG,
`respectively. Both 33 and 34 adopted a conforma-
`tion in the active site consistent with acylation
`transition state analogues [381.
`In summary, the unique ability ofthe boronic
`acid functionality to accept an active site serine
`into its electophilicp-otbtal has provided a novel
`series of B-lactamase inhibitors.
`
`I Amino-acyl tRNA synthetase inhibitors
`There is a clear need to develop new efficacious
`therapeutics to treat fungal infections. One of
`the strategies is to discover and develop novel
`
`chemotype agents. Amino-aeyl t—RNA synthe-
`tases are crucial for protein synthesis, and target-
`ing the editing domain ofthis enzyme is a new
`approach to its inhibition. A new class ofboron—
`containing compounds, known as 1,3—dihydro—l-
`hydroxy—2,l-benzoxaboroles, has been identified
`as inhibitors of fungal leucyl t-RNA synthetase
`and have potent antifungai activities with MICs
`as low as 0.25 rig/ml against the major derma-
`tophytes I’i’icbaplaytan rubrum and Wichapbymn
`mmmgrop/Jyte: and the yeasts and molds Candida
`albimm, Cryptomccu: nmfirmam and Asprrgillu:
`fumigatu: [39.203] . AN2690 (35) and AN2718 (36)
`are two examples of this class of compounds.
`A penetration study indicated that these
`benm-ctbomie compounds can effectively pen—
`etrate through human nail plate and reach the
`nail bed in sufficient concentration to inhibit
`
`I fungal pathogens [4o]. AN2690 (35) is currently
`in clinical development for the topical treatment
`ofonychomycosis, a fungal infection of the nail
`and nail bed. AN2718 (36) is also in clinical trials
`to treat skin and other topical fungal infections.
`Mechanism investigation with AN2690 (35)
`demonstrates that this compound inhibits yeasr
`cytoplasmic leucyl-tRNA synthetase by forma—
`tion ofa stable AN2690—tRNAL” adduct (38) in
`the editing site ofthe enzyme [41]. The AN2690—
`tRNALE“ adduct (38) is formed through the boron
`atom of the AN2690 (35) and the cis-diol an
`the 3’—terminal adenosine (37) of the tRNA, as
`proposed below.
`The trapping of enzyme-bound tRNAL‘“ in
`the editing site prevents catalytic turnover, thus
`inhibiting synthesis ofleucyl-tRNAL‘“ and con-
`sequentially blocking protein synthesis. This
`result establishes the editing site as a novel target
`for aminoacyl-tRNA synthetase inhibitors.
`In summary, the recent discovery of boron
`therapeutics as amino acyl
`t—RNA synthe—
`tase inhibitors, acting by trapping the tRNA
`in the enzyme-editing domain, is expected to
`be a promising field for the discovery of novel
`antifungal therapeutics. The combination of
`the unique boron chemistry, molecular-level
`knowledge gained from crystal structure stud—
`ies and rational drug design has established a
`powerful drug—discovery machinery to feed the
`development pipeline.
`
`I Boron-containing anticoagulants
`Thrombin and Factor Xa have been promising
`targets for anticoagulant agents for more than
`a decade. A number of boro-Lys- and boro‘
`Arg—bascd (boronic acid analogs oflysine and
`
`
`
`future science group
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`www.foiure-science.com
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`1279
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`awn.-M.st-Jnizficfirfnfinrgrfl ,“ammo:
`E Baker, Ding. Akama, Zhang, Hernandez & Xia
`1
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`strategy to design potent DPP4 inhibitors and,
`following extensive SAR, they identified Ala-
`boroPro (4|) and Pro—boroPto (42) as very potent
`inhibitors of DPP4 (2 nM for 4| and 3 nM for
`42) 151.52”. The NHZ-PZ-boroPl structure is the
`essential pharmacophore For the DI’1’4 inhi-
`bition. Simple boroPro (43) and N—Boc—Ala—
`horol’ro (44) were not active, while the P2 residue
`has some flexibility. As For the P2 residue, Ala,
`Glu, Lily and Pro showed nanomolar to picomo-
`lar Ki values. The boroPro moiety can be sub-
`stituted with boroAla as well, although boroAla
`derivatives are less active than boroPro. There
`
`is a concern of selectivity to Dl’l’4 over DI’PS,
`DPP9 and potentially other enzymes for these
`boropeptides. It could be overcome by the struc-
`tural modifications oFthe molecule [53}. Recently,
`in viva blood glucose—lowering activity of com-
`pounds 4I, 45 and 46 (Glu-boroPro) was reported
`[54] and the safety of these three closely related
`dipeptide boronic acid inhibitors (4|, 45 8: 46)
`was determined. Just like the nonboron inhibi-
`tors ofthe DPP4 enzyme, their toxicity is related
`to their inhibition ofrelated isozymes DPP8 and
`DPP9. A tight correlation was observed between
`intracellular inhibition of DPP9 and the maxi-
`mum tolerated dose (MTD) (TABLE i). The Glu-
`boroAla (46) is a selective inhibitor of DPPé,
`and it is a safe compound, while the toxicity of
`the other compounds is- related to their nons'e—
`lective nature (TAaLE l). Therefore, the author
`concluded that boronic acid—based inhibitors of
`DPP4 do not exhibit unique or untoward tox—
`icities. While Val—boroPro (45, talabostat) is a
`potent D1’1’4 inhibitor [52], this compound is
`also in clinical trials For colorectal cancer as a
`
`fibroblast activation protein (FAP) inhibitor [54].
`Very recently, clinical trial results of Gly—
`borol’ro derivative DPP4 inhibitor PHX1149
`
`arginine) tlirombin inhibitors with nanomolar
`to picomolar potency have been identified [8.9].
`Recently, a potent thrombin inhibitor TRISOc
`(39, Ki = 10 nM) has been developed and is in
`clinical trials for the treatment of thrombo-
`sis [209,210]. It is Formulated for either oral (Ca
`salt: TGN167) [:02] or intravenous (Na salt:
`TGNZSS) [103) administration.
`In addition, phenylboronic acid derivatives,
`represented by 40, were reported as inhibitors
`of another scrine protease in the blood coagula-
`tion cascade, Factor Xla (FXIa) [-91]. While the
`potency of 40 is Still in the micromolar range,
`it showed a nine- to 31-Fold selectivity to FXia
`over FXa and thrombin, respectively, and more
`than 140-fold over trypsin.
`These examples further support the utility of
`boronic acids against a variety of serine prote—
`ases by making use ofthe ability of the boron to
`form a tetrahedral transition state mimic with
`
`the active site nucleophile.
`
`I Boron-containing dlpeptidyl
`peptidase 4 inhibitors
`Dipeptidyl peptidase 4 (DPP4, also known as
`CID-26) is a serine protease that specifically
`removes Xaa-l’ro dipeptides from the N-terminus
`ofpolypeptides and proteins [43]. DPP4 is Found
`in a variety of mammalian cells and tissues [44].
`' However,
`it was not recognized as an impor-
`tant drug target and] 1995, when glucagon-like
`peptide-1 (GLP—l) was identified as one of the
`substrates of DPPé {45.46}. GLP—l stimulates
`glucose-induced insulin biosynthesis and secre—
`tion [47], and increasing blood GLI’-l concentra-
`tion seemed to be a promising approach to treat
`diabetes. However, GLP—1 is rapidly inactivated
`by DPP4 in viva and discovery of DPN inhibi-
`tors therefore became a promising concept for
`treatment of diabetes {43]. Indeed, the small-
`molecule DPP4 inhibitor Januvia® (sitagliptin)
`was approved by the US FDA For the treatment
`onypc 2 diabetes [49].
`Bachovchin er a]. reported that peptide prolyl
`boronic acids are potent inhibitors of bacterial
`IgAl proteinases [50]. They applied a similar
`
`.
`
`,
`
`(41) were reported [SS-57.104]. PHX1149 was given
`orally at the doses of 200 or 400 mg. Patients
`were allowed to continue either metformin or
`thiazolidinedion, or a combination of the two.
`PHX1149 showed statistically significant reduc—
`tions on hemoglobin A1c(HbAlc) in both 200-
`and 400-mg groups. PHXl 149 also demonstrated
`”B‘s
`
`
`
`
`
`lC°"’-IC50 (FM)
`l; Compound
`KIM"
`KIM”
`KID?”
`MTD (mg/kg) -
`E
`6.8
`0.025
`l
`:EVaI-borono (45)
`0.18
`1.5
`0.75
`iAIa-boroPro (4!)
`0.027
`2.0
`0.53
`360
`5.0 s MTD < 38
`l
`Glu-boroAla (4a)
`8.3
`880
`2100
`ll
`7000
`500 s MTD < 900
`2 'KI is in nM.
`il
`
`j.DDPLgipepwlpeptidasezfljgMaximum-tolerated dose.
` 4.....w-mmmmrnw “M.“
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`
`
`1230
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`Fuiure Med. Chem. (2009) 1(7)
`
`future science group fSS
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`

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`hmxr’fmaamgxxamamwaywmmmmmmwzm:
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`
`statistically significant efficacy on the secondary
`end point including change in fasting and post-
`meal blood glucose levels. There was no substan—
`tial difference obserVed between active arms and
`
`placebo in terms oisalety and tolerability.
`In summary, boronic acid inhibitors ofDPPé
`have made a substantial contribution to this
`
`important field.
`
`i
`
`l Boron-containing
`phosphodiesterase 4 inhibitors
`Phosphodiesterases (PDEs) are a family of
`enzymes responsible For the hydrolysis of sec-
`ond—messenger CAMP and CGMP [58]. The PDE
`superfarnily comprises at least 11 members,
`including approximately 100 isoforms [59,60].
`Among those, PDE4 specifically catalyzes the
`hydrolysis of cAMP and is the predominant
`phosphodiesterase enzyme in immune and
`inflammatory cells [61.62]. Therefore, PDE4
`has been considered as a promising therapeutic
`target and a number of inhibitors have entered
`clinical
`trials For
`the treatment of chronic
`
`obstrucrive pulmonary disease, asthma, various
`kinds of arthritis, inflammatory bowel disease,
`psoriasis and atopic dermatitis. Indeed, positive
`results from clinical trials of PDE4 inhibitors
`
`give the validation to the target [631. Several drug
`candidates are close to filing for approval.
`Of the thousands of pttblic'ations‘on PDET
`inhibitors, no boronic acid inhibitors had been
`reported until recently, when a phenoxybenzoxa—
`horole derivative, AN2728 (48), was reported
`to be a PDE4 inhibitor (ICW = 0.49 pM) [64].
`This compound shoxvs anti-inflammatory activ-
`ity and has a structure distinct From existing
`PDE4 inhibitors. AN2728 (48) inhibits not only
`PDE4 but also PDEI A3 (ICW = 6.] pM), PDE3
`(IC50 = 6.4 pM) and PDE7A1 “(:50 = 0.73 pM).
`ANT/'28 is currently under Phase II clinical trial
`For the topical treatment of psoriasis [s4], and is
`showing positive results [105].
`In summary, the versatility shown by boron
`for different modes ofinhibition has provided a
`novel therapeutic lead For PDE4.
`
`it is safe when used in doses ofup to 900 mg/kg
`in humans [5]. Approval of Velcade (49) by the
`FDA as an anticancer agent in 2003 [106} has
`assured pharmaceutical developers that boron is
`a druggable clement. There are numerous boron
`compounds that have entered va