Boron therapeutics on the horizon

Identification, crystallization, and first X-ray structure analyses of phenyl boronic acid-based inhibitors of human carbonic anhydrase-II

Human carbonic anhydrases (hCAs) play a central role in various physiological processes in the human body. HCAs catalyze the reversible hydration of CO2 into HCO3-, and hence maintains the fluid and pH balance. Overexpression of CA II is associated with diseases, such as glaucoma, and epilepsy. Therefore, CAs are important clinical targets and inhibition of different isoforms, especially hCA II is used in treatment of glaucoma, altitude sickness, and epilepsy. Therapeutically used CA inhibitors (CAI) are sulfonamide-based, such as acetazolamide, dichlorphenamide, methazolamide, ethoxzolamide, etc. However, they exhibit several undesirable effects such as numbness, tingling of extremities, malaise, metallic taste, fatigue, renal calculi, and metabolic acidosis. Therefore, there is an urgent need to identify safe and effective inhibitors of the hCAs. In this study, different phenyl boronic acids 1-5 were evaluated against bovine (bCA II) and hCA II. Among all, compound 1 (4-acetylphenyl boronic acid) was found to be active against bCAII and hCA II with IC50 values of 246 ± 0.48 and 281.40 ± 2.8 μM, respectively, while the remaining compounds were found in-active. Compound 1 was identified as competitive inhibitor of hCA II enzyme (Ki = 283.7 ± 0.002 μM). Additionally, compound 1 was found to be non-toxic against BJ Human fibroblast cell line. The X-ray crystal structure for hCA II in-complex with compound 1 was evaluated to a resolution of 2.6 Å. In fact, this the first structural analysis of a phenyl boron-based inhibitor bound to hCA II, allowing an additional structure-activity analysis of the compounds. Compound 1 was found to be directly bound in the active site of hCA II by interacting with His94, His119, and Thr199 residues. In addition, a bond of 3.11 Å between the zinc ion and coordinated boron atom of the boronic acid moiety of compound 1 was also observed, contributing to binding affinity of compound 1 for hCA II. PDB ID: 8IGF.

The Rise of Boron-Containing Compounds: Advancements in Synthesis, Medicinal Chemistry, and Emerging Pharmacology

Boron-containing compounds (BCC) have emerged as important pharmacophores. To date, five BCC drugs (including boronic acids and boroles) have been approved by the FDA for the treatment of cancer, infections, and atopic dermatitis, while some natural BCC are included in dietary supplements. Boron's Lewis acidity facilitates a mechanism of action via formation of reversible covalent bonds within the active site of target proteins. Boron has also been employed in the development of fluorophores, such as BODIPY for imaging, and in carboranes that are potential neutron capture therapy agents as well as novel agents in diagnostics and therapy. The utility of natural and synthetic BCC has become multifaceted, and the breadth of their applications continues to expand. This review covers the many uses and targets of boron in medicinal chemistry.

Gold-Catalyzed Cyclization/Hydroboration of 1,6-Enynes: Synthesis of Bicyclo[3.1.0]hexane Boranes

The gold-catalyzed cyclization/hydroboration of 1,6-enynes offers facile, versatile, and atom-economical one-step access to bicyclo[3.1.0]hexane boranes. This new protocol proceeds in moderate to good yields under mild conditions. Different from bicyclo[3.1.0]hexane borates, these products are stable in air and during chromatography. Moreover, the borane moiety of the products can readily undergo a diverse array of transformations. The kinetic isotope effect experiment indicates that the hydrogen-transfer step is a fast process, which is not involved in the rate-limiting step.

Design, development, synthesis, and crystal structure of the prototype of a new class of deep blue-fluorescing boron heterocycle estrogen mimics

To further the development of boron heterocyclic compounds that are useful to medicinal chemistry, we demonstrate how the class of compounds known as the diazaborines can be elaborated to produce an exceptionally close structural mimic of a natural estrogen. After building progressively closer models, a benzyloxy-substituted formylphenylboronic acid was ultimately condensed with a hydroxymethylated β-hydrazinocyclopentenone to give, after debenzylation, an isosteric mimic (diazaborine 1) of the naturally-occurring estrogen equilenin and the prototype of a new class of boron heterocycle estrogen mimics. X-ray crystallography revealed the prototype to be planar, with a transmolecular interoxygen distance virtually identical to that found in equilenin and with a strong hydrogen-bond-donating hydroxyl group. From this it can be anticipated that members of this unique class of boron heterocycle estrogen mimics will be found to possess useful biological properties. Furthermore, the prototype was found to fluoresce in the deep blue region of the visible spectrum, and so the development of members serving as light-emitting probes in biochemical and biological studies can also be anticipated.

Boric acid suppresses cell proliferation by TNF signaling pathway mediated apoptosis in SW-480 human colon cancer line

BACKGROUND/AIM

Colon cancer is one of the most common cancers. Treatment success and survival rates are not high enough with current approaches. Therefore, there is a need to develop new agents and treatment methods. Boric acid is the most frequently observed form of boron. Some epidemiological data suggest that environmental exposure to boric acid reduces the incidence of prostate cancer in men, cervical and lung cancers in women. Experimental studies show, boric acid reduces cell proliferation and stimulates apoptosis in some prostate, melanoma, breast cancer cell lines. In this study, it was investigated whether boric acid could be a new candidate molecule that could be used in the treatment of colon cancer.

MATERIALS AND METHODS

The effects of boric acid on human colon adenocarcinoma cell line SW-480 were investigated with BrdU, TUNEL, Caspase-3, and AIF immunohistochemical studies in both 2D and 3D culture systems. In addition, a qRT-PCR study was carried out to determine the expression changes in key genes that take part in apoptosis.

RESULTS

We observed that boric acid suppresses cell proliferation and induces apoptosis both in 2D and 3D culture conditions. In addition, as a result of qRt-PCR studies, it was revealed that the observed apoptotic process was related to the TNF signaling pathway.

CONCLUSION

Boric acid can be considered as a potential anti-cancer agent candidate for colon cancer treatment.

DATA AVAILABILITY

All data generated or analyzed during this study are included in this published article.

Boronic Acids and Their Derivatives in Medicinal Chemistry: Synthesis and Biological Applications

Boron containing compounds have not been widely studied in Medicinal Chemistry, mainly due to the idea that this group could confer some toxicity. Nowadays, this concept has been demystified and, especially after the discovery of the drug bortezomib, the interest for these compounds, mainly boronic acids, has been growing. In this review, several activities of boronic acids, such as anticancer, antibacterial, antiviral activity, and even their application as sensors and delivery systems are addressed. The synthetic processes used to obtain these active compounds are also referred. Noteworthy, the molecular modification by the introduction of boronic acid group to bioactive molecules has shown to modify selectivity, physicochemical, and pharmacokinetic characteristics, with the improvement of the already existing activities. Besides, the preparation of compounds with this chemical group is relatively simple and well known. Taking into consideration these findings, this review reinforces the relevance of extending the studies with boronic acids in Medicinal Chemistry, in order to obtain new promising drugs shortly.

Recent advances in Cu-catalyzed C(sp3)-Si and C(sp3)-B bond formation

Numerous reactions generating C-Si and C-B bonds are in focus owing to the importance of incorporating silicon or boron into new or existing drugs, in addition to their use as building blocks in cross-coupling reactions en route to various targets of both natural and unnatural origins. In this review, recent protocols relying on copper-catalyzed sp3 carbon-silicon and carbon-boron bond-forming reactions are discussed.

Tunable Synthesis of α-Amino Boronic Esters from Available Aldehydes and Amines through Sequential One-Pot Dehydration and Copper-Catalyzed Borylacylation

Copper-catalyzed multicomponent borylacylation of imines with acid chlorides and bis(pinacolato)diboron was developed for the preparation of synthetically useful and pharmacologically relevant α-amino boronic acid derivatives. Starting from a range of acid chlorides and imines with aryl, heteroaryl, and alkyl substituents, most of these ligand-free reactions proceeded smoothly at room temperature in moderate to good yields. Furthermore, a facile and convenient one-pot, multistep access to the direct synthesis of α-amino boronic acid derivatives from available aldehydes and amines was also developed.

The effect of stereoisomerism on the 4D-QSAR study of some dipeptidyl boron derivatives

The electron conformational genetic algorithm (EC-GA) method had been employed by distinguishing between enantiomers for the first time as a 4D-QSAR approach to reveal the pharmacophore (Pha) and to predict the bioactivity of the dipeptidyl boron compounds. The Electron Conformational Matrices of Congruity (ECMCs) were prepared for all conformers of compounds in the data set based on the quantum chemical calculations at HF/3-21 G level in an aqueous medium. The comparison of the ECMCs within the certain tolerances by the EMRE program revealed the pharmacophore for some dipeptidyl boron derivatives. For the selection of the most influential parameters on the activity and the calculation of theoretical activities, the genetic algorithm with the non-linear least square method was used. The final model was validated by the cross-validation method with the division of the data set into training and test items. The 12-parameter model gave excellent statistical results (R²_training = 0.850, R²_test = 0.809, q² = 0.755, q²_ext1 = 0.776, q²_ext2 = 0.759, q²_ext3 = 0.735, CCC_tr = 0.922, CCC_test = 0.846, CCC_all = 0.905). Because of the inexistence of 4D-QSAR studies on the dipeptidyl boron derivatives and the stereoisomerism effect on the biological activity was examined for the first time for these compounds, this study plays an important role in the development of new boron-containing compounds.

Boron in drug design: Recent advances in the development of new therapeutic agents

Advances in the field of boron chemistry have expanded the application of this element in Medicinal Chemistry. Boron-containing compounds represent a new class for medicinal chemists to use in their drug designs. Bortezomib (Velcade^®), a dipeptide boronic acid approved by the FDA in 2003 for treatment of multiple myeloma, paved the way for the discovery of new boron-containing compounds. After its approval, two other boron-containing compounds have been approved, tavaborole (Kerydin^®) for the treatment of onychomicosis and crisaborole (Eucrisa^®) for the treatment of mild to moderate atopic dermatitis. A number of boron-containing compounds have been described and evaluated for a plethora of therapeutic applications. The present review is intended to highlight the recent advances related to boron-containing compounds and their therapeutic applications. Here, we focused only in those most biologically active compounds with proven in vitro and/or in vivo efficacy in the therapeutic area published in the last years.

Arylboronic acids inhibit P2X7 receptor function and the acute inflammatory response

The P2X7 receptor (P2X7R) is an ion channel which is activated by interactions with the extracellular ATP molecules. The molecular complex P2X7R/ATP induces conformational changes in the protein subunits, opening a pore in the ion channel macromolecular structure. Currently, the P2X7R has been studied as a potential therapeutic target of anti-inflammatory drugs. Based on this, a series of eight boronic acids (NO) analogs were evaluated on the biologic effect of this pharmacophoric group on the human and murine P2X7R. The boronic acids derivatives NO-01 and NO-12 inhibited in vitro human and murine P2X7R function. These analogs compounds showed effect better than compound BBG and similar to inhibitor A740003 for inhibiting dye uptake, in vitro IL-1β release and ATP-induced paw edema in vivo. In both, in vitro and in vivo assays the compound NO-01 showed to be the hit compound in the present series of the arylboronic acids analogs. The molecular docking suggests that the NO derivatives bind into the upper body domain of the P2X7 pore and that the main intermolecular interaction with the two most active NO derivatives occur with the residues Phe 95, 103 and 293 by hydrophobic interactions and with Leu97, Gln98 and Ser101 by hydrogen bonds.. These results indicate that the boronic acid derivative NO-01 shows the lead compound characteristics to be used as a scaffold structure to the development of new P2X7R inhibitors with anti-inflammatory action.

Morin P, Westcott SA. Synthesis, characterization, and anticancer properties of iminophosphineplatinum(II) complexes containing boronate esters

Three new iminophosphines containing pinacol-derived boronate esters have been prepared and ligated to dichloridoplatinum(II) fragments. All compounds have been characterized fully, including an X-ray diffraction study carried out for the platinum complex 8, which is derived from 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline. These three new platinum complexes, along with the non-boron containing control, have been examined for their initial cytotoxic properties against two glioma cell lines using the MTT method.

Crystal structure of pyridinium bis(naphthalane-2,3-diolato-κ2 O,O′)borate monohydrate, C25H20BNO5

C25H20BNO5, triclinic, P1̅ (no. 2), a = 7.0868(6) Å, b = 10.3522(6) Å, c = 15.3140(10) Å, α = 74.088(7)°, β = 89.996(7)°, γ = 79.190(1)°, V = 1059.71(13) Å3, Z = 2, R gt(F) = 0.0445, wR ref(F 2) = 0.1105, T = 298 K.

Current data regarding the structure-toxicity relationship of boron-containing compounds

Boron is ubiquitous in nature, being an essential element of diverse cells. As a result, humans have had contact with boron containing compounds (BCCs) for a long time. During the 20th century, BCCs were developed as antiseptics, antibiotics, cosmetics and insecticides. Boric acid was freely used in the nosocomial environment as an antiseptic and sedative salt, leading to the death of patients and an important discovery about its critical toxicology for humans. Since then the many toxicological studies done in relation to BCCs have helped to establish the proper limits of their use. During the last 15 years, there has been a boom of research on the design and use of new, potent and efficient boron containing drugs, finding that the addition of boron to some known drugs increases their affinity and selectivity. This mini-review summarizes two aspects of BCCs: toxicological data found with experimental models, and the scarce but increasing data about the structure-activity relationship for toxicity and therapeutic use. As is the case with boron-free compounds, the biological activity of BCCs is related to their chemical structure. We discuss the use of new technology to discover potent and efficient BCCs for medicinal therapy by avoiding toxic effects.

Synthesis, characterization, and antibacterial activity of structurally complex 2-acylated 2,3,1-benzodiazaborines and related compounds

A set of 2-acylated 2,3,1-benzodiazaborines and some related boron heterocycles were synthesized, characterized, and tested for antibacterial activity against Escherichia coli and Mycobacterium smegmatis. By high-field solution NMR, the heretofore unknown class of 2-acyl-1-hydroxy-2,3,1-diazaborines has been found to be able to exist in several interconvertable structural forms along a continuum comprised of an open hydrazone a, a monomeric B-hydroxy diazaborine b, and an anhydro dimer c. X-Ray crystallography of one of the anhydro dimers, 17c, revealed it to have an unprecedented structure featuring a double intramolecular O→B chelation. The crystal structure of another compound, 37, showed it to be based on a new pentacyclic B heterocycle framework. Nine compounds were found to possess activities against E. coli, and two others were active against M. smegmatis. The finding that these two contain isoniazid covalently embedded in their structures suggests that they might possibly be acting as prodrugs of this well-known antituberculosis agent in vivo.

Synthesis and biological activity of arylspiroborate salts derived from caffeic Acid phenethyl ester

Two novel boron compounds containing caffeic acid phenethyl ester (CAPE) derivatives have been prepared and characterized fully. These new compounds and CAPE have been investigated for potential antioxidant and antimicrobial properties and their ability to inhibit 5-lipoxygenase and whether chelation to boron improves their biological activity. Sodium salt 4 was generally more active than ammonium salt 5 in the biological assays and surpassed the radical scavenging ability of CAPE. Compounds 4 and 5 were more active than CAPE and Zileuton in human polymorphonuclear leukocytes. These results clearly show the effectiveness of the synthesized salts as transporter of CAPE.

Tavaborole, a Novel Boron-Containing Small Molecule for the Topical Treatment of Onychomycosis, Is Noncarcinogenic in 2-Year Carcinogenicity Studies

Tavaborole, a cyclized boronic acid, has been approved by the Food and Drug Administration for the topical treatment of toenail onychomycosis. This novel, low-molecular-weight pharmaceutical compound has broad-spectrum antifungal activity against dermatophytes, yeasts, and molds responsible for the disease. Tavaborole was tested in 2-year carcinogenicity studies in mice (once daily dermal administration) and rats (once daily by oral gavage) as part of the extensive nonclinical safety program. There was no evidence of tavaborole-related neoplasms observed in either study. Based on the data gathered from these 2 carcinogenicity studies, tavaborole is considered noncarcinogenic.

Discovery of new heterocycles with activity against human neutrophile elastase based on a boron promoted one-pot assembly reaction

Herein we demonstrate for the first time that a boron promoted one-pot assembly reaction may be used to discover novel enzyme inhibitors. Inhibitors for HNE were simply assembled in excellent yields, high diastereoselectivities and IC50 up to 1.10 μM, based on components like salicylaldehyde, aryl boronic acids and amino acids. The combination of synthetic, biochemical, analytical and theoretical studies allowed the identification of the 4-methoxy or the 4-diethyl amino substituent of the salicylaldehyde as the most important recognition moiety and the imine alkylation, lactone ring opening as key events in the mechanism of inhibition.

Ring Structure and Aromatic Substituent Effects on the pK a of the Benzoxaborole Pharmacophore

In this work, we present an investigation into the physical properties of a unique class of aromatic boronic acids, the benzoxaboroles. Using spectrophotometric methods, the ionization constants of a family of substituted benzoxaboroles are determined. Heterocyclic ring modifications are examined to determine their effects on the ionization of the boronic acid moiety. It is also shown that the substituent effects about the aromatic ring follow a Hammett relationship with the compounds' measured pK a values. Finally, these substituent effects are also shown to extend to the sugar binding properties of these compounds under physiologically relevant conditions. Combined, these data will inform medicinal chemists wishing to tailor the ionization and/or ability of this class of compound to bind diol-containing biomolecules.

Benzoxaboroles: a new class of potential drugs for human African trypanosomiasis

Human African trypanosomiasis, caused by the kinetoplastid parasite Trypanosoma brucei, affects thousands of people across sub-Saharan Africa, and is fatal if left untreated. Treatment options for this disease, particularly stage 2 disease, which occurs after parasites have infected brain tissue, are limited due to inadequate efficacy, toxicity and the complexity of treatment regimens. We have discovered and optimized a series of benzoxaborole-6-carboxamides to provide trypanocidal compounds that are orally active in murine models of human African trypanosomiasis. A key feature of this series is the presence of a boron atom in the heterocyclic core structure, which is essential to the observed trypanocidal activity. We also report the in vivo pharmacokinetic properties of lead compounds from the series and selection of SCYX-7158 as a preclinical candidate.

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.

Design, synthesis and in vitro evaluation of (R)-4-(2-(tert-butylamino)-1-hydroxyethyl)-2-(hydroxymethyl)phenyl hydrogen phenylboronate: a novel salbutamol derivative with high intrinsic efficacy on the β2 adrenoceptor

We tested a set of boron containing arylethanolamine derivatives on the human and guinea pig β(2) adrenoceptor (β(2)AR) 3-D structures by docking methodology. The compound with the highest affinity based on docking analysis, (R)-4-(2-(tert-butylamino)-1-hydroxyethyl)-2-(hydroxymethyl)phenyl hydrogen phenylboronate (boronterol) was synthesized, characterized and tested in guinea pig tracheal rings at basal tone and with histamine-induced contractions. Boronterol was at least eightfold more potent than salbutamol as a smooth muscle relaxant drug (judged by the EC(50) values) and showed a similar maximal relaxant effect as isoproterenol. ICI118,551 showed competitive antagonism on the relaxing effect of boronterol. These results suggest the β(2)AR agonist action of boronterol.

An NMR Study of the Bortezomib Degradation under Clinical Use Conditions

The (R)-3-methyl-1-((S)-3-phenyl-2-(pyrazine-2-carboxamido)propanamido)butyl-boronic acid, bortezomib (BTZ), which binds the 20S proteasome subunit and causes a large inhibition of its activity, is a peptidomimetic boronic drug mainly used for the treatment of multiple myeloma. Commercial BTZ, stabilized as mannitol derivative, has been investigated under the common conditions of the clinical use because it is suspected to be easily degradable in the region of its boronic moiety. Commercial BTZ samples, reconstituted according to the reported commercial instructions and stored at 4 degrees C, were analyzed by high-field nuclear magnetic resonance spectroscopy in comparison with identical samples bubbled with air and argon, respectively. All the samples remained unchanged for a week. After a month, the air filled samples showed the presence of two main degradation products (6% of starting material), the N-(1-(1-hydroxy-3-methylbutylamino)-1-oxo-3-phenylpropan-2-yl) pyrazine-2-carboxamide (BTZ1; 5%, determined from NMR integration) and the (S)-N-(1-(3-methylbutanamido)-1-oxo-3-phenylpropan-2-yl)pyrazine-2-carboxamide (BTZ2; 1%, determined from NMR integration), identified on the basis of their chemical and spectroscopic properties. The BTZ1 and BTZ2 finding suggests that, under the common condition of use and at 4 degrees C, commercial BTZ-mannitol is stable for a week, and that, in time, it undergoes slow oxidative deboronation which partially inactivates the product. Low temperature and scarce contact with air decrease the degradation process.

Synthesis of boronic acid derivatives of tyropeptin: proteasome inhibitors

Boronic acid derivatives of tyropeptin were synthesized with TP-110 as the lead compound. Due to the lability of the aminoboronic acid moiety, careful design of the deprotection and coupling sequence was required. Liquid-liquid partition chromatography was found to be a powerful tool for purification of compounds of this class. The obtained derivatives showed potent inhibitory activities against the human 20S proteasome in vitro.

Phenylboronic acid selectively inhibits human prostate and breast cancer cell migration and decreases viability

We compared the in vitro effect of boric acid (BA) versus phenylboronic acid (PBA) on the migration of prostate and breast cancer cell lines and non-tumorigenic cells from the same tissues. Treatment at 24 hours with BA (< or =500 microM) did not inhibit chemotaxis on fibronectin in any cell line. However, treatment over the same time course with concentrations of PBA as low as 1 muM significantly inhibited cancer cell migration without effecting non-tumorigenic cell lines. The compounds did not affect cell adhesion or viability at 24 hours but did alter morphology; both decreased cancer cell viability at eight days. These results suggest that PBA is more potent than BA in targeting the metastatic and proliferative properties of cancer cells.

Therapeutic applications of carbon monoxide-releasing molecules

Carbon monoxide (CO), which is formed in mammalian cells through the oxidation of haem by the enzyme haem oxygenase, actively participates in the regulation of key intracellular functions. Emerging evidence reveals that an increased generation of haem oxygenase-derived CO plays a critical role in the resolution of inflammatory processes and alleviation of cardiovascular disorders. The authors have identified a novel class of substances, CO-releasing molecules (CO-RMs), which are capable of exerting a variety of pharmacological activities via the liberation of controlled amounts of CO in biological systems. A wide range of CO carriers containing manganese (CORM-1), ruthenium (CORM-2 and -3), boron (CORM-A1) and iron (CORM-F3) are currently being investigated to tailor therapeutic approaches for the prevention of vascular dysfunction, inflammation, tissue ischaemia and organ rejection.

AMBER force field implementation of the boronate function to simulate the inhibition of beta-lactamases by alkyl and aryl boronic acids

Boronic acids are a very appealing class of serine proteases inhibitors whose rational design suffers, in spite of their therapeutic potential, from the lack of boron-related parameters in force fields commonly used for proteins. We introduced bonded, non-bonded and point charges in the MacroModel/Amber force field, as well as GB/SA solvation parameters, to model boronic acids as tetrahedral adducts formed after protease's serine Ogamma coordination. With the aim to check the implemented force field, flexible docking studies were performed on three crystallographic complexes of beta-lactamases with boronic acids that output the crystallographic conformation of the complexes as the global minimum energy structure. Although the used approach was basic, nevertheless the resultant force field seems to be efficient and suitable for the structure-based design of new boronic inhibitors of beta-lactamases.

RELATIVE CONTRIBUTIONS OF THE FIVE MAJOR HUMAN CYTOCHROMES P450, 1A2, 2C9, 2C19, 2D6, AND 3A4, TO THE HEPATIC METABOLISM OF THE PROTEASOME INHIBITOR BORTEZOMIB

VELCADE (bortezomib, PS-341), reversibly inhibits the 20S proteasome and exhibits cytotoxic and antitumor activities. Pretreatment of cancer cells with bortezomib increases the chemosensitivity of these cells, suggesting that bortezomib may be used in combination chemotherapy. The relative contributions of the five major human cytochromes P450 (P450s), 1A2, 2C9, 2C19, 2D6, and 3A4 (the focus of the present study), to the metabolism of bortezomib are an important aspect of potential drug interactions. Relative activity factor (RAF), chemical inhibition, and immunoinhibition using monoclonal antibodies were three approaches employed to determine the relative contributions of the major human P450s to the net hepatic metabolism of bortezomib. RAFs for the P450 isoform-selective substrates were determined; the ratio of the rate of metabolism of bortezomib with cDNA-expressed P450s versus rate of metabolism with human liver microsomes was normalized with respect to the RAF for each P450 isoform to determine the percentage contributions of the P450s to the net hepatic metabolism of bortezomib. CYP3A4 followed by CYP2C19 were determined to be the major contributors to the metabolism of bortezomib. Chemical inhibition and immunoinhibition confirmed that CYP3A4 and CYP2C19 were the major P450s responsible for the hepatic metabolism of bortezomib. The studies were conducted with 2 muM bortezomib, and the disappearance of bortezomib, rather than appearance of a specific metabolite, was quantified to determine the contributions of the P450s to the overall hepatic metabolism of bortezomib in humans.

General Inverse Solid-Phase Synthesis Method for C-Terminally Modified Peptide Mimetics

Peptide mimetics are of considerable interest as bioactive agents and drugs. C-terminally modified peptide mimetics are of particular interest given the synthetic versatility of the carboxyl group and its derivatives. A general approach to C-terminally modified peptide mimetics, based on a urethane attachment strategy and amino acid t-butyl ester-based N-to-C peptide synthesis, is described. This approach is compatible with the reaction conditions generally employed for solution-phase peptide mimetic synthesis. To develop and demonstrate this approach, it was employed for the solid-phase synthesis of peptide trifluoromethyl ketones, peptide boronic acids, and peptide hydroxamic acids. The development of a versatile general approach to C-terminally modified peptides using readily available starting materials provides a basis for the combinatorial and parallel solid-phase synthesis of these peptide mimetic classes for bioactive agent screening and also provides a basis for the further development of solid-phase C-terminal functional group elaboration strategies.

Stereospecific activity of two glutamate analogs

Two glutamic acid analogs, (+)-(S)- and (-)-(R)-4-(2,2-diphenyl-1,3,2-oxazaborolidin-5-oxo)propionic acid ((+)-(S)- and (-)-(R)-Trujillon, respectively), were prepared. The stereospecific activity of their pharmacological properties was studied. The median convulsant dose (CD(50)) and median lethal dose (LD(50)) were analyzed in female Swiss Webster mice and their effects in vivo on unitary electrical activity in globus pallidus neurons were elucidated in male Wistar rats. Compounds were characterized by (1)H, (13)C, and (11)B nuclear magnetic resonance. The LD(50) of (+)-(S)-Trujillon was 449.08 mg/kg and it increased spontaneous motor activity, while with (-)-(R)-Trujillon there was no mortality up to 1,000 mg/kg and it decreased spontaneous motor activity. The CD(50) in experiments with (+)-(S)-Trujillon was 199.34 mg/kg. Unitary recording in globus pallidus neurons showed i.v. administration (+)-(S)-Trujillon (50 mg/kg) increased frequency 79.0 +/- 23.0% in relation to basal response. (-)-(R)-Trujillon and (+)-(S)-glutamate (50 mg/kg each) did not provoke changes in spontaneous basal firing. Local infusion of (+)-(S)-Trujillon (1 nMol) increased spontaneous firing in most neurons tested by 269.0 +/- 83.0% in relation to basal values. Intrapallidal infusion of (-)-(R)-Trujillon (1 nMol) and saline solution did not cause statistically significant changes in globus pallidus spiking. Results showed that (+)-(S)-Trujillon crosses the blood-brain barrier and has stereospecific activity.

The Structural Basis for Hydrolysis Resistance in the Esters of (2-Formylphenyl)boronic Acid 2,4-Dinitrophenylhydrazones

Under conditions that typically afford bicyclic boron heterocycles directly, (2-formylphenyl)boronic acids react with 2,4-dinitrophenylhydrazine in ethanol to give highly waterresistant diethyl boronate esters. Two such 2,4-dinitrophenylhydrazones were prepared and their X-ray crystal structures determined. Contrary to a previous suggestion that their unusual stability is due to an intramolecular N→B coordination giving a six-membered BN2C3 ring system based on a (Z)-hydrazone, these compounds instead were found to be (E)-hydrazones internally stabilized by a weak intramolecular interaction between nitrogen and boron from within a five-membered ring. Further study revealed that the electron deficiency of the starting hydrazine reagent plays a key role in determining the structure of the hydrazone isolated, and that the water-resistant boronate esters can be hydrolyzed under forcing conditions to the boronic acids.