Synthesis and evaluation of carbaborane derivatives of indomethacin as cyclooxygenase inhibitors

Molecular engineering of AIE-active boron clustoluminogens for enhanced boron neutron capture therapy

The development of boron delivery agents bearing an imaging capability is crucial for boron neutron capture therapy (BNCT), yet it has been rarely explored. Here we present a new type of boron delivery agent that integrates aggregation-induced emission (AIE)-active imaging and a carborane cluster for the first time. In doing so, the new boron delivery agents have been rationally designed by incorporating a high boron content unit of a carborane cluster, an erlotinib targeting unit towards lung cancer cells, and a donor-acceptor type AIE unit bearing naphthalimide. The new boron delivery agents demonstrate both excellent AIE properties for imaging purposes and highly selective accumulation in tumors. For example, at a boron delivery agent dose of 15 mg kg-1, the boron amount reaches over 20 μg g-1, and both tumor/blood (T/B) and tumor/normal cell (T/N) ratios reach 20-30 times higher than those required by BNCT. The neutron irradiation experiments demonstrate highly efficient tumor growth suppression without any observable physical tissue damage and abnormal behavior in vivo. This study not only expands the application scopes of both AIE-active molecules and boron clusters, but also provides a new molecular engineering strategy for a deep-penetrating cancer therapeutic protocol based on BNCT.

Carborane Analogues of Fenoprofen Exhibit Improved Antitumor Activity

Fenoprofen is a widely used nonsteroidal anti-inflammatory drug (NSAID) against rheumatoid arthritis, degenerative joint disease, ankylosing spondylitis and gout. Like other NSAIDs, fenoprofen inhibits the synthesis of prostaglandins by blocking both cyclooxygenase (COX) isoforms, COX-1 the "house-keeping" enzyme and COX-2 the induced isoform from pathological stimuli. Unselective inhibition of both COX isoforms results in many side effects, but off-target effects have also been reported. The steric modifications of the drugs could afford the desired COX-2 selectivity. Furthermore, NSAIDs have shown promising cytotoxic properties. The structural modification of fenoprofen using bulky dicarba-closo-dodecaborane(12) (carborane) clusters and the biological evaluation of the carborane analogues for COX inhibition and antitumor potential showed that the carborane analogues exhibit stronger antitumor potential compared to their respective aryl-based compounds.

Carboranyl Analogues of Mefenamic Acid and Their Biological Evaluation

Mefenamic acid represents a widely used nonsteroidal anti-inflammatory drug (NSAID) to treat the pain of postoperative surgery and heavy menstrual bleeding. Like other NSAIDs, mefenamic acid inhibits the synthesis of prostaglandins by nonselectively blocking cyclooxygenase (COX) isoforms COX-1 and COX-2. For the improved selectivity of the drug and, therefore, reduced related side effects, the carborane analogues of mefenamic acid were evaluated. The ortho-, meta-, and para-carborane derivatives were synthesized in three steps: halogenation of the respective cluster, followed by a Pd-catalyzed B-N coupling and hydrolysis of the nitrile derivatives under acidic conditions. The COX inhibitory activity and cytotoxicity for different cancer cell lines revealed that the carborane analogues have stronger antitumor potential compared to their parent organic compound.

Carboranes in drug discovery, chemical biology and molecular imaging

There exists a paucity of structural innovation and limited molecular diversity associated with molecular frameworks in drug discovery and biomolecular imaging/chemical probe design. The discovery and exploitation of new molecular entities for medical and biological applications will necessarily involve voyaging into previously unexplored regions of chemical space. Boron clusters, notably the carboranes, offer an alternative to conventional (poly)cyclic organic frameworks that may address some of the limitations associated with the use of novel molecular frameworks in chemical biology or medicine. The high thermal stability, unique 3D structure and aromaticity, kinetic inertness to metabolism and ability to engage in unusual types of intermolecular interactions, such as dihydrogen bonds, with biological receptors make carboranes exquisite frameworks in the design of probes for chemical biology, novel drug candidates and biomolecular imaging agents. This Review highlights the key developments of carborane derivatives made over the last decade as new design tools in medicinal chemistry and chemical biology, showcasing the versatility of this unique family of boron compounds.

Preclinical Pharmacokinetics and Acute Toxicity in Rats of 5-{[(2E)-3-Bromo-3-carboxyprop-2-enoyl]amino}-2-hydroxybenzoic Acid: A Novel 5-Aminosalicylic Acid Derivative with Potent Anti-Inflammatory Activity

Compound 5-{[(2E)-3-bromo-3-carboxyprop-2-enoyl]amino}-2-hydroxybenzoic acid (C1), a new 5-aminosalicylic acid (5-ASA) derivative, has proven to be an antioxidant in vitro and an anti-inflammatory agent in mice. The in vivo inhibition of myeloperoxidase was comparable to that of indomethacin. The aim of this study was to take another step in the preclinical evaluation of C1 by examining acute toxicity with the up-and-down OECD method and pharmacokinetic profiles by administration of the compound to Wistar rats through intravenous (i.v.), oral (p.o.), and intraperitoneal (i.p.) routes. According to the Globally Harmonized System, C1 belongs to categories 4 and 5 for the i.p. and p.o. routes, respectively. An RP-HPLC method for C1 quantification in plasma was successfully validated. Regarding the pharmacokinetic profile, the elimination half-life was approximately 0.9 h with a clearance of 24 mL/min after i.v. administration of C1 (50 mg/kg). After p.o. administration (50 mg/kg), the maximum plasma concentration was reached at 33 min, the oral bioavailability was about 77%, and the compound was amply distributed to all tissues evaluated. Therefore, C1 administered p.o. in rats is suitable for reaching the colon where it can exert its effect, suggesting an important advantage over 5-ASA and indomethacin in treating ulcerative colitis and Crohn’s disease.

Carboranyl Derivatives of Rofecoxib with Cytostatic Activity against Human Melanoma and Colon Cancer Cells

Owing to the involvement of cyclooxygenase-2 (COX-2) in carcinogenesis, COX-2-selective inhibitors are increasingly studied for their potential cytotoxic properties. Moreover, the incorporation of carboranes in structures of established anti-inflammatory drugs can improve the potency and metabolic stability of the inhibitors. Herein, we report the synthesis of carborane-containing derivatives of rofecoxib that display remarkable cytotoxic or cytostatic activity in the micromolar range with excellent selectivity for melanoma and colon cancer cell lines over normal cells. Furthermore, it was shown that the carborane-modified derivatives of rofecoxib showed different modes of action that were dependent on the cell type.

Boron Chemistry for Medical Applications

Boron compounds now have many applications in a number of fields, including Medicinal Chemistry. Although the uses of boron compounds in pharmacological science have been recognized several decades ago, surprisingly few are found in pharmaceutical drugs. The boron-containing compounds epitomize a new class for medicinal chemists to use in their drug designs. Carboranes are a class of organometallic compounds containing carbon (C), boron (B), and hydrogen (H) and are the most widely studied boron compounds in medicinal chemistry. Additionally, other boron-based compounds are of great interest, such as dodecaborate anions, metallacarboranes and metallaboranes. The boron neutron capture therapy (BNCT) has been utilized for cancer treatment from last decade, where chemotherapy and radiation have their own shortcomings. However, the improvement in the already existing (BPA and/or BSH) localized delivery agents or new tumor-targeted compounds are required before realizing the full clinical potential of BNCT. The work outlined in this short review addresses the advancements in boron containing compounds. Here, we have focused on the possible clinical implications of the new and improved boron-based biologically active compounds for BNCT that are reported to have in vivo and/or in vitro efficacy.

Carboranyl Analogues of Ketoprofen with Cytostatic Activity against Human Melanoma and Colon Cancer Cell Lines

Ketoprofen is a widely used nonsteroidal anti-inflammatory drug (NSAID) that also exhibits cytotoxic activity against various cancers. This makes ketoprofen an attractive structural lead for the development of new NSAIDs and cytotoxic agents. Recently, the incorporation of carboranes as phenyl mimetics in structures of established drugs has emerged as an attractive strategy in drug design. Herein, we report the synthesis and evaluation of four novel carborane-containing derivatives of ketoprofen, two of which are prodrug esters with an nitric oxide-releasing moiety. One of these prodrug esters exhibited high cytostatic activity against melanoma and colon cancer cell lines. The most pronounced activity was found in cell lines that are sensitive to oxidative stress, which was apparently induced by the ketoprofen analogue.

Structure activity relationships, multidrug resistance reversal and selectivity of heteroarylphenyl ABCG2 inhibitors

An overexpression of the transmembrane ATP-binding cassette transporter G2 (ABCG2, BCRP) in cancer tissues is supposed to play a role in the multidrug resistance (MDR) of tumors resulting in an inefficient chemotherapy. Therefore, co-administration of selective and non-toxic ABCG2 inhibitors is a promising strategy for improving the efficacy of chemotherapy by blocking ABCG2-mediated export of the cytostatic drugs. In the present study, we designed a small library of 38 novel compounds containing a heteroaryl-phenyl scaffold possessing several (bioisosteric) moieties, and twelve new precursors. We investigated the library for ABCG2 inhibition, for the selectivity against MDR-involved efflux pump ABCB1 (P-gp) and for toxicity. Structure activity relationship (SAR) studies revealed that, at least a phenylheteroaryl-phenylamide scaffold is necessary for observing an ABCG2 inhibition. 4-Methoxy-N-(2-(2-(6-methoxypyridin-3-yl)-2H-tetrazol-5-yl)phenyl)benzamide (43) exhibited a high potency (IC₅₀ = 61 nM)), selectivity, low intrinsic toxicity and reversed the ABCG2-mediated drug resistance in presence of only 0.1 μM.

Interactions of Boron Clusters and their Derivatives with Serum Albumin

Boron clusters are polyhedral boron hydrides with unique properties, and they are becoming increasingly widely used in biology and medicine, including for boron neutron capture therapy (BNCT) of cancers and in the design of novel bioactive molecules and potential drugs. Among boron cluster types, icosahedral boranes, carboranes, and metallacarboranes are particularly interesting, and there is a need for basic studies on their interaction with biologically important molecules, such as proteins. Herein, we report studies on the interaction of selected boron clusters and their derivatives with serum albumin, the most abundant protein in mammalian blood. The interaction of boron clusters with albumin was examined by fluorescence quenching, circular dichroism, dynamic and static light scattering measurements and MALDI-TOF mass spectrometry. Our results showed that metallacarboranes have the strongest interaction with albumin among the tested clusters. The observed strength of boron cluster interactions with albumin decreases in order: metallacarboranes [M(C₂B₉H₁₁)₂]⁻ > carboranes (C₂B₁₀H₁₂) >> dodecaborate anion [B₁₂H₁₂]²⁻. Metallacarboranes first specifically interact with the binding cavity of albumin and then, with increasing compound concentrations, interact non-specifically with the protein surface. These findings can be of importance and are useful in the development of new bioactive compounds that contain boron clusters.

Challenges and Opportunities for the Application of Boron Clusters in Drug Design

There are two branches in boron medicinal chemistry: the first focuses on single boron atom compounds, and the second utilizes boron clusters. Boron clusters and their heteroatom counterparts belong to the family of cage compounds. A subset of this extensive class of compounds includes dicarbadodecaboranes, which have the general formula C2B10H12, and their metal biscarboranyl complexes, metallacarboranes, with the formula [M(C2B10H12)2(-2)]. The unique properties of boron clusters have resulted in their utilization in applications such as in pharmacophores, as scaffolds in molecular construction, and as modulators of bioactive compounds. This Perspective presents an overview of the properties of boron clusters that are pertinent for drug discovery, recent applications in the design of various classes of drugs, and the potential use of boron clusters in the construction of new pharmaceuticals.

Recent developments with boron as a platform for novel drug design

INTRODUCTION

After decades of development, the medicinal chemistry of compounds that contain a single boron atom has matured to the present status of having equal rights with other branches of drug discovery, although it remains a relative newcomer. In contrast, the medicinal chemistry of boron clusters is less advanced, but it is expanding and may soon become a productive area of drug discovery.

AREAS COVERED

The author reviews the current developments of medicinal chemistry of boron and its applications in drug design. First generation boron drugs that bear a single boron atom and second generation boron drugs that utilize boron clusters as pharmacophores or modulators of bioactive molecules are discussed. The advantages and gaps in our current understanding of boron medicinal chemistry, with a special focus on boron clusters, are highlighted.

EXPERT OPINION

Boron is not a panacea for every drug discovery problem, but there is a good chance that it will become a useful addition to the medicinal chemistry tool box. The present status of boron resembles the medicinal chemistry status of fluorine three decades ago; indeed, currently, approximately 20% of pharmaceuticals on the market contain fluorine. The fact that novel boron compounds, especially those based on abiotic polyhedral boron hydrides, are currently unfamiliar could be advantageous because organisms may be less prone to developing resistance against boron cluster-based drugs.

nido-Dicarbaborate Induces Potent and Selective Inhibition of Cyclooxygenase-2

Carbaboranes are increasingly studied as pharmacophores, particularly as replacements for aromatic systems. However, especially ortho-carbaborane is prone to degradation of the cluster, which hampers biological application. This study demonstrates that deboronation of the cluster may not only lead to a more active analogue, but can also improve the solubility and stability of a carbaborane-containing inhibitor. Notably, introduction of a nido-dicarbaborate cluster into the cyclooxygenase (COX) inhibitor indomethacin results in remarkably increased inhibitory potency and selectivity for COX-2 relative to the respective phenyl analogue. The first crystal structure of a carbaborane-containing inhibitor bound to COX-2 further reveals a novel binding mode for the inhibitor that is strikingly different from that of indomethacin. These results indicate that nido-dicarbaborate is a promising pharmacophore that exhibits properties which are also highly beneficial for its introduction into other inhibitor classes.

One-pot synthesis of an indole-substituted 7,8-dicarba-nido-dodecahydroundecaborate(-1)

Carbaboranes are increasingly used as pharmacophores to replace phenyl substituents in established drug molecules. In contrast to traditional organic chemistry, elaborate procedures to introduce functionality frequently fail in the case of carbaboranes and their chemistry is often hampered by degradation of the cluster. Herein, the development of a one-pot synthesis of a water-soluble N-nido-dicarbaborato indole is presented, including a proposed mechanism for the reaction sequence. These studies provide useful synthetic tools for the conjugation of two important pharmacophores, indoles and carbaboranes.

Boron clusters in medicinal chemistry: perspectives and problems

Boron clusters have been employed successfully as constituents in bioactive substances. In this review, the perspectives of boron clusters for drug design and problems to be solved for a broader application are discussed, and a list of actions is given for overcoming the problems.

Carbaboranes – more than just phenyl mimetics

Dicarba-closo-dodecaboranes(12) (C2B10H12, carbaboranes) are highly hydrophobic and stable icosahedral carbon-containing boron clusters. The cage framework of these clusters can be modified with a variety of substituents, both at the carbon and at the boron atoms. Substituted carbaboranes are of interest in medicine as boron neutron capture therapy (BNCT) agents or as pharmacophores. High and selective accumulation in tumour cells is an important requirement for a BNCT agent and is achieved by incorporating boron-rich, water-soluble carbaborane derivatives into breast tumour-selective modified neuropeptide Y, [F7, P34]-NPY. Preliminary studies showed that the receptor binding affinity and signal transduction of the boron-modified peptides were very well retained. Use of carbaboranes as pharmacophores was shown by replacement of Bpa32 (Bpa=benzoylphenylalanine) in the reduced-size NPY analogue [Pro30, Nle31, Bpa32, Leu34]-NPY 28–36 by ortho-carbaboranyl propanoic acid. The inclusion of the carbaborane derivative resulted in a short NPY agonist with an interesting hY2R/hY4R preference. This might be a promising approach in the field of anti-obesity drug development.

2-Carbaborane-3-phenyl-1H-indoles--synthesis via McMurry reaction and cyclooxygenase (COX) inhibition activity

Cyclooxygenase-2 (COX-2) inhibitors have been the focus of medicinal chemistry efforts for years, and many compounds that exhibit high selectivity and affinity have been developed. As carbaboranes represent interesting pharmacophores as phenyl mimetics in drug development, this paper presents the synthesis of carbaboranyl derivatives of COX-2-selective 2,3-disubstituted indoles. Despite the lability of carbaboranes under reducing conditions, 2-carbaborane-3-phenyl-1H-indoles could be synthesized by McMurry cyclization of the corresponding amides. Whereas the meta-carbaboranyl-substituted derivatives lacked COX inhibitory activity, an ortho-carbaboranyl analogue was active, but showed a selectivity shift toward COX-1.

Indole based cyclooxygenase inhibitors: synthesis, biological evaluation, docking and NMR screening

The close structural similarity between the two cyclooxygenase (COXs) isoforms and the absence of selective inhibitors without side effects continues to stimulate the development of novel approaches towards selective anti-inflammatory drugs. In the present study a small library of new indolic compounds involving two different substitutions patterns at the indole scaffold was synthesized. In order to establish a relation between the spatial distribution of known functional groups related with inhibitory activity, two substitution patterns were explored: one with substituents at N-1, C-3, C-5 positions and another at C-2, C-3 and C5 positions. Accordingly, indole positions C-5, C-3 and N-1 were substituted with: sulfonamide or methylsulfone at C-5, p-halo-benzyl group at C-3, and an alkyl chain with a trifluoromethyl group at N-1. Alternatively, a p-halo-benzyl group was introduced at C-2, leaving the indolic nitrogen free. Inhibitory studies were performed and the activity results obtained against both COXs isoforms were rationalized based on docking and NMR studies. Docking studies show that dialkyation at C-2 and C-3 favors a binding with an orientation similar to that of the known selective inhibitor SC-558. From the tested compounds, this substitution pattern is correlated with the highest inhibitory activity and selectivity: 70% COX-2 inhibition at 50 μM, and low COX-1 inhibition (18 ± 9%). Additionally, Saturation Transfer Difference NMR experiments reveal different interaction patterns with both COXs isoforms that may be related with different orientations of the sulfonamide group in the binding pocket. Despite the moderated inhibitory activities found, this study represents an innovative approach towards COXs inhibitory activity rationalization and to the design of anti-inflammatory drugs.

Ortho-carbaborane derivatives of indomethacin as cyclooxygenase (COX)-2 selective inhibitors

A series of novel indomethacin analogues with carbaboranes as three-dimensional substitutes for the chlorophenyl ring have been prepared. Their cyclooxygenase (COX) inhibition and enzyme selectivity has been tested and compared to the corresponding adamantyl analogues. Surprisingly, only the ortho-carbaborane derivatives were active compounds. Preliminary biological studies gave an interesting insight into the validity of employing carbaboranes as pharmacophores.

Imitation and modification of bioactive lead structures via integration of boron clusters

In medicinal chemistry, carbaboranes can be employed either as boron carriers for boron neutron capture therapy (BNCT) or as scaffolds for radiodiagnostic or therapeutic agents. We have developed a suitable synthesis employing the phosphoramidite method to connect meta-carbaboranyl bis-phosphonites with the 6'-OH group of isopropylidene-protected galactose, followed by oxidation or sulfurization to give the corresponding bis-phosphonates. Deprotection yielded water-soluble compounds. The corresponding disodium salts exhibit especially low cytotoxicity. Preliminary results on the in vivo toxicity and biodistribution of two compounds in mice indicated a lack of selectivity for the cotton rat lung (CRL) tumor chosen for the experiment. For the incorporation of carbaboranes into breast tumor-selective modified neuropeptide Y, [F7, P34]-NPY, a synthesis of a carbaborane-modified lysine derivative was developed. Linkage of the lysine to the boron cluster was achieved by using a propionic acid spacer. Incorporation of the amino acid derivatives into NPY and [F7, P34]-NPY by solid-phase peptide synthesis was successful. Preliminary studies showed that the receptor binding affinity and signal transduction of the boron-modified peptides were very well retained. Asborin, the carbaborane analogue of aspirin, is a rather weak inhibitor of cyclooxygenase-1 (COX-1) and COX-2, but a highly potent aldo/keto reductase 1A1 (AKR1A1) inhibitor. Modification either at the carboxyl group or at the chlorophenyl ring in indomethacin with ortho- and meta-carbaboranyl derivatives gave active derivatives only for the ortho-carbaborane directly attached to the carboxyl group, while the corresponding adamantyl and meta-carbaboranyl derivatives were inactive.