Comparisons of pKa and log P values of some carboxylic and phosphonic acids: synthesis and measurement

Bioisoteres for carboxylic acids: From ionized isosteres to novel unionized replacements

Carboxylic acids are key pharmacophoric elements in many molecules. They can be seen as a problem by some, due to perceived permeability challenges, potential for high plasma protein binding and the risk of forming reactive metabolites due to acyl-glucuronidation. By others they are viewed more favorably as they can decrease lipophilicity by adding an ionizable center which can be beneficial for solubility, and can add enthalpic interactions with the target protein. However, there are many instances where the replacement of a carboxylic acid with a bioisosteric group is required. This has led to the development of a number of ionizable groups which sufficiently mimic the carboxylic acid functionality whilst improving, for example, the metabolic profile of the molecule in question. An alternative strategy involves replacement of the carboxylate by neutral functional groups. This review initially details carefully selected examples whereby tetrazoles, acyl sulfonamides or isoxazolols have been beneficially utilized as carboxylic acid bioisosteres altering physicohemical properties, interactions with the target and metabolism and/or pharmacokinetics, before delving further into the binding mode of carboxylic acid derivatives with their target proteins. This analysis highlights new ways to consider the replacement of carboxylic acids by neutral bioisosteric groups which either rely on hydrogen bonds or cation-π interactions. It should serve as a useful guide for scientists working in drug discovery.

Lanthanide Phosphonates and Phosphates in Molecular Magnetism

Phosphonate and phosphate ligands have historically received less attention when compared to the widely prevalent carboxylate ligand system. Phosphonates possess multiple donating sites, often leading to the formation of larger aggregates with limited solubility. Conversely, the P-O bond within phosphates is highly susceptible to hydrolysis, resulting in the precipitation of insoluble compounds, particularly when interacting with lanthanide metal ions. However, over the past few decades, various synthetic approaches have emerged for the preparation and characterization of lanthanide complexes involving both phosphonate and phosphate ligands. Consequently, researchers have delved into exploring the magnetic properties of these complexes, such as their potential as single molecule magnets (SMMs) and their ability to exhibit a magnetocaloric effect (MCE). This review will encompass an examination of the crystal structures and magnetic characteristics of lanthanide complexes featuring phosphonate and phosphate ligands.

Design, Synthesis, and Biological Evaluation of Small-Molecule-Based Radioligands with Improved Pharmacokinetic Properties for Imaging of Programmed Death Ligand 1

Small molecules offer some advantages for developing positron emission tomography (PET) tracers and are therefore a promising approach for imaging and therapy monitoring of programmed death ligand 1 (PD-L1) positive tumors. Here, we report six biphenyl PD-L1 radioligands using the NODA-GA-chelator for efficient copper-64 complexation. These radioligands contain varying numbers of sulfonic and/or phosphonic acid groups, serving as hydrophilizing units to lower the log D7.4 value down to -4.28. The binding affinities of compounds were evaluated using saturation binding and a real-time binding assay, with a highest binding affinity of 21 nM. Small-animal PET imaging revealed vastly different pharmacokinetic profiles depending on the quantity and type of hydrophilizing units. Of the investigated radioligands, [64Cu]Cu-3 showed the most favorable kinetics in vitro. This was also found in vivo, with a predominantly renal clearance and a specific uptake in the PD-L1-overexpressing tumor. With further modifications, this compound could be a promising candidate for the imaging of PD-L1 in the clinical setting.

Mild and scalable synthesis of phosphonorhodamines

Since their discovery in 1887, rhodamines have become indispensable fluorophores for biological imaging. Recent studies have extensively explored heteroatom substitution at the 10' position and a variety of substitution patterns on the 3',6' nitrogens. Although 3-carboxy- and 3-sulfono-rhodamines were first reported in the 19th century, the 3-phosphono analogues have never been reported. Here, we report a mild, scalable synthetic route to 3-phosphonorhodamines. We explore the substrate scope and investigate mechanistic details of an exogenous acid-free condensation. Tetramethyl-3-phosphonorhodamine (phosTMR) derivatives can be accessed on the 1.5 mmol scale in up to 98% yield (2 steps). phosTMR shows a 12- to 500-fold increase in water solubility relative to 3-carboxy and 3-sulfonorhodamine derivatives and has excellent chemical stability. Additionally, phosphonates allow for chemical derivatization; esterification of phosTMR facilitates intracellular delivery with localization profiles that differ from 3-carboxyrhodamines. The free phosphonate can be incorporated into a molecular wire scaffold to create a phosphonated rhodamine voltage reporter, phosphonoRhoVR. PhosRhoVR 1 can be synthesized in just 6 steps, with an overall yield of 37% to provide >400 mg of material, compared to a 6-step, ∼2% yield for the previously reported RhoVR 1. PhosRhoVR 1 possesses excellent voltage sensitivity (37% ΔF/F) and a 2-fold increase in cellular brightness compared to RhoVR 1.

Bimetallic Anionic Organic Frameworks with Solid-State Cation Conduction for Charge Storage Applications

A new phosphonate-based anionic bimetallic organic framework, with the general formula of A4 -Zn-DOBDP (wherein A is Li+ or Na+ , and DOBDP6- is the 2,5-dioxido-1,4-benzenediphosphate ligand) is prepared and characterized for energy storage applications. With four alkali cations per formula unit, the A4 -Zn-DOBDP MOF is found to be the first example of non-solvated cation conducting MOF with measured conductivities of 5.4×10-8  S cm-1 and 3.4×10-8  S cm-1 for Li4 - and Na4 - phases, indicating phase and composition effects of Li+ and Na+ shuttling through the channels. Three orders of magnitude increase in ionic conductivity is further attained upon solvation with propylene carbonate, placing this system among the best MOF ionic conductors at room temperature. As positive electrode material, Li4 -Zn-DOBDP delivers a specific capacity of 140 mAh g-1 at a high average discharge potential of 3.2 V (vs. Li+ /Li) with 90 % of capacity retention over 100 cycles. The significance of this research extends from the development of a new family of electroactive phosphonate-based MOFs with inherent ionic conductivity and reversible cation storage, to providing elementary insights into the development of highly sought yet still evasive MOFs with mixed-ion and electron conduction for energy storage applications.

Enlightening the Path to Protein Engineering: Chemoselective Turn-On Probes for High-Throughput Screening of Enzymatic Activity

Many successful stories in enzyme engineering are based on the creation of randomized diversity in large mutant libraries, containing millions to billions of enzyme variants. Methods that enabled their evaluation with high throughput are dominated by spectroscopic techniques due to their high speed and sensitivity. A large proportion of studies relies on fluorogenic substrates that mimic the chemical properties of the target or coupled enzymatic assays with an optical read-out that assesses the desired catalytic efficiency indirectly. The most reliable hits, however, are achieved by screening for conversions of the starting material to the desired product. For this purpose, functional group assays offer a general approach to achieve a fast, optical read-out. They use the chemoselectivity, differences in electronic and steric properties of various functional groups, to reduce the number of false-positive results and the analytical noise stemming from enzymatic background activities. This review summarizes the developments and use of functional group probes for chemoselective derivatizations, with a clear focus on screening for enzymatic activity in protein engineering.

Quantum and Classical Evaluations of Carboxylic Acid Bioisosteres: From Capped Moieties to a Drug Molecule

Using the Quantum Theory of Atoms in Molecules, the average electron density (AED) tool was developed and employed to quantitatively evaluate the similarities between bioisosteric moieties in drug design. Bioisosteric replacements are valuable in drug molecules to fine-tune their pharmacokinetic and pharmacodynamic properties while maintaining their biological activity. This study was performed on non-classical bioisosteres of carboxylic acid. It was found that the AED of a given bioisostere is generally transferable, within less than 5% difference, irrespective of its environment. It was shown that the AED tool succeeds at depicting not only the similarities of bioisosteric groups but also at highlighting, as counter examples, the differences in non-bioisosteric groups. For the first time, the AED was used to evaluate bioisosterism in an FDA-approved drug molecule, furosemide, and in five analogues of this medicine. In one of the analogues, non-bioisosteric moieties were exchanged, and in four of the analogues, carboxylic acid was replaced with either furan or sulfonamide, and vice versa. It was also found that irrespective of the pH, the AED tool consistently reproduced experimental predictions. The distinct power of the AED tool in quantitatively and precisely measuring the similarity among bioisosteric groups is contrasted with the relatively ambiguous bioisosteric evaluations through the classical qualitative electrostatic potential (ESP) maps. The ESP maps were demonstrated to fail, even qualitatively, in depicting the similarities, in some cases.

Carboxylic Acid Bioisosteres in Medicinal Chemistry: Synthesis and Properties

Lead optimization represents the tedious process of fine-tuning lead compounds from biologically active hits to suitable drug candidates for clinical trials. By chemically modifying a hit structure, an improved compound can be obtained in terms of activity, selectivity, and pharmacokinetic ADME (absorption, distribution, metabolism, and excretion) properties. The carboxylic acid moiety is known to be a crucial functionality in many pharmaceutically active compounds. Despite its common use as a key functionality in drugs, its presence in a lead molecule is often associated with poor pharmacokinetic properties and toxicity. In this literature overview, we discuss how the shortcomings of a carboxylic acid can be circumvented by replacing this functionality with bioisosteres. In this way, the positive aspects of this moiety, such as its activity, for example, by virtue of its capacity to form hydrogen bonds, can be maintained or even improved. To that end, we provide an overview of the most promising carboxylic acid bioisosteres and discuss a selection of synthetic routes towards the main functionalities.

Facile Synthesis of Solubilizing a Group-Free, Solution-Processable p-Type Ladder Conjugated Polymer and Its Thermoelectric Properties

Here we report the synthesis of a new solubilizing group-free, solution-processable p-type ladder conjugated polymer, 6H-pyrrolo[3,2-b:4,5-b']bis[1,4]benzothiazine ladder (PBBTL) polymer by using a polyphosphoric acid (PPA) and phenylphosphonic acid (PhPO₃H₂) 1:1 binary acid solvent system together with careful control of reaction kinetics. With a good intrinsic viscosity of 3.69 dL/g in methanesulfonic acid (MSA), good quality PBBTL films can be obtained via spin-coating. Intrinsic thin film properties and thermoelectric performance of PBBTL were evaluated, making it the second solubilizing group-free, solution-processable ladder-type conjugated polymer after BBL to be used for thin-film polymer electronics. While our preliminary thermoelectric performance of the FeCl₃-doped PBBTL films is modest, we believe that many opportunities lie ahead for PBBTL and hope that its successful synthesis using the new PPA:PhPO₃H₂ binary acid solvent system will inspire synthetic organic chemists to relook into solubilizing group-free, solution-processable ladder-type conjugated polymer systems.

A novel approach in revealing mechanisms and particular step predictors of pH dependent tartrazine catalytic degradation in presence of Oxone®

The degradation of tartrazine in the presence of cobalt activated Oxone® (potassium peroxymonosulfate) was investigated at different initial pH values. Aluminum pillared clay had the role of a support for catalytically active cobalt oxide species. The degradation of tartrazine and the formation of a mixture of degradation products were monitored using the Ultraviolet-Visible (UV-Vis) spectroscopy and gas chromatography-mass spectrometry (GC-MS). The exact qualitative composition of this mixture and the determination of the most probable mechanism of degradation (the primary goal) were obtained using GC-MS. Besides, the main reaction pathway (reaction with SO₄˙- radical anion) and secondary pathways were proposed depending on the pH value. At pH = 6 the reaction with HO˙ radical was proposed. At pH = 11 decarboxilation was suggested as the first step of the secondary proposed reaction pathway. The combination of results acquired from the deconvolution of UV-Vis spectra and the theoretical UV-Vis spectra of degradation products, whose occurrence was predicted by quantum-chemical calculations, was proven to be beneficial for the identification of tartrazine degradation products and for defining UV-Vis predictors of particular degradation steps. An additional contribution of this paper, from the reactivity aspect, was the establishment of the critical structural demand for the radical degradation of any diazo compound. The existence of a hydrogen atom bound to a diazo group was found to be the essential prerequisite for the radical cleavage of diazo compounds.

A Sustainable Improvement of ω-Bromoalkylphosphonates Synthesis to Access Novel KuQuinones

Owing to the attractiveness of organic phosphonic acids and esters in the pharmacological field and in the functionalization of conductive metal-oxides, the research of effective synthetic protocols is pivotal. Among the others, ω-bromoalkylphosphonates are gaining particular attention because they are useful building blocks for the tailored functionalization of complex organic molecules. Hence, in this work, the optimization of Michaelis–Arbuzov reaction conditions for ω-bromoalkylphosphonates has been performed, to improve process sustainability while maintaining good yields. Synthesized ω-bromoalkylphosphonates have been successfully adopted for the synthesis of new KuQuinone phosphonate esters and, by hydrolysis, phosphonic acid KuQuinone derivatives have been obtained for the first time. Considering the high affinity with metal-oxides, KuQuinones bearing phosphonic acid terminal groups are promising candidates for biomedical and photo(electro)chemical applications.

How to not build a cage: endohedral functionalization of polyoxometalate-based metal-organic polyhedra

Introducing functionalities into the interior of metal-organic cage complexes can confer properties and utilities (e.g. catalysis, separation, drug delivery, and guest recognition) that are distinct from those of unfunctionalized cages. Endohedral functionalization of such cage molecules, for decades, has largely relied on modifying their organic linkers to covalently append targeted functional groups to the interior surface. We herein introduce an effective coordination method to bring in functionalities at the metal sites instead, for a set of polyhedral cages where the nodes are in situ formed polyoxovanadate clusters, [VIV 6O6(OCH3)9(μ6-SO4)(COO)3]2-. Replacing the central sulfates of these hexavanadate clusters with more strongly coordinating phosphonate groups allows the installation of functionalities within the cage cavities. Organophosphonates with phenyl, biphenyl, and terphenyl tails were examined for internalization. Depending on the size/shape of the cavities, small phosphonates can fit into the molecular containers whereas larger ones inhibit or transform the framework architecture, whereby the first non-cage complex was isolated from a reaction that otherwise would lead to entropically favored regular polyhedra cages. The results highlight the complex and dynamic nature of the self-assembly process involving polyoxometalates and the scope of molecular variety accessible by the introduction of endo functional groups.

Phosphonofluoresceins: Synthesis, Spectroscopy, and Applications

Xanthene fluorophores, like fluorescein, have been versatile molecules across diverse fields of chemistry and life sciences. Despite the ubiquity of 3-carboxy and 3-sulfonofluorescein for the last 150 years, to date, no reports of 3-phosphonofluorescein exist. Here, we report the synthesis, spectroscopic characterization, and applications of 3-phosphonofluoresceins. The absorption and emission of 3-phosphonofluoresceins remain relatively unaltered from the parent 3-carboxyfluorescein. 3-Phosphonofluoresceins show enhanced water solubility compared to 3-carboxyfluorescein and persist in an open, visible light-absorbing state even at low pH and in low dielectric media while 3-carboxyfluoresceins tend to lactonize. In contrast, the spirocyclization tendency of 3-phosphonofluoresceins can be modulated by esterification of the phosphonic acid. The bis-acetoxymethyl ester of 3-phosphonofluorescein readily enters living cells, showing excellent accumulation (>6x) and retention (>11x), resulting in a nearly 70-fold improvement in cellular brightness compared to 3-carboxyfluorescein. In a complementary fashion, the free acid form of 3-phosphonofluorescein does not cross cellular membranes, making it ideally suited for incorporation into a voltage-sensing scaffold. We develop a new synthetic route to functionalized 3-phosphonofluoresceins to enable the synthesis of phosphono-voltage sensitive fluorophores, or phosVF2.1.Cl. Phosphono-VF2.1.Cl shows excellent membrane localization, cellular brightness, and voltage sensitivity (26% ΔF/F per 100 mV), rivaling that of sulfono-based VF dyes. In summary, we develop the first synthesis of 3-phosphonofluoresceins, characterize the spectroscopic properties of this new class of xanthene dyes, and utilize these insights to show the utility of 3-phosphonofluoresceins in intracellular imaging and membrane potential sensing.

Impact of the antibiotic-cargo from MSNs on Gram-positive and Gram-negative bacterial biofilms

Mesoporous silica nanoparticles (MSNs) are promising drug nanocarriers for infection treatment. Many investigations have focused on evaluating the capacity of MSNs to encapsulate antibiotics and release them in a controlled fashion. However, little attention has been paid to determine the antibiotic doses released from these nanosystems that are effective against biofilm during the entire release time. Herein, we report a systematic and quantitative study of the direct effect of the antibiotic-cargo released from MSNs on Gram-positive and Gram-negative bacterial biofilms. Levofloxacin (LVX), gentamicin (GM) and rifampin (RIF) were separately loaded into pure-silica and amino-modified MSNs. This accounts for the versatility of these nanosystems since they were able to load and release different antibiotic molecules of diverse chemical nature. Biological activity curves of the released antibiotic were determined for both bacterial strains, which allowed to calculate the active doses that are effective against bacterial biofilms. Furthermore, in vitro biocompatibility assays on osteoblast-like cells were carried out at different periods of times. Albeit a slight decrease in cell viability was observed at the very initial stage, due to the initial burst antibiotic release, the biocompatibility of these nanosystems is evidenced since a recovery of cell viability was achieved after 72 h of assay. Biological activity curves for GM released from MSNs exhibited sustained patterns and antibiotic doses in the 2-6 μg/mL range up to 100 h, which were not enough to eradicate biofilm. In the case of LVX and RIF first-order kinetics featuring an initial burst effect followed by a sustained release above the MIC up to 96 h were observed. Such doses reduced by 99.9% bacterial biofilm and remained active up to 72 h with no emergence of bacterial resistance. This pioneering research opens up promising expectations in the design of personalized MSNs-based nanotherapies to treat chronic bone infection.

Phosphine Oxides from a Medicinal Chemist's Perspective: Physicochemical and in Vitro Parameters Relevant for Drug Discovery

Phosphine oxides and related phosphorus-containing functional groups such as phosphonates and phosphinates are established structural motifs that are still underrepresented in today's drug discovery projects, and only few examples can be found among approved drugs. In this account, the physicochemical and in vitro properties of phosphine oxides and related phosphorus-containing functional groups are reported and compared to more commonly used structural motifs in drug discovery. Furthermore, the impact on the physicochemical properties of a real drug scaffold is exemplified by a series of phosphorus-containing analogs of imatinib. We demonstrate that phosphine oxides are highly polar functional groups leading to high solubility and metabolic stability but occasionally at the cost of reduced permeability. We conclude that phosphine oxides and related phosphorus-containing functional groups are valuable polar structural elements and that they deserve to be considered as a routine part of every medicinal chemist's toolbox.

Fe-HBED Analogs: A Promising Class of Iron-Chelate Contrast Agents for Magnetic Resonance Imaging

Contrast-enhanced magnetic resonance imaging is an essential tool for disease diagnosis and management; all marketed clinical magnetic resonance imaging (MRI) contrast agents (CAs) are gadolinium (Gd) chelates and most are extracellular fluid (ECF) agents. After intravenous injection, these agents rapidly distribute to the extracellular space and are also characterized by low serum protein binding and predominant renal clearance. Gd is an abiotic element with no biological recycling processes; low levels of Gd have been detected in the central nervous system and bone long after administration. These observations have prompted interest in the development of new MRI contrast agents based on biotic elements such as iron (Fe); Fe-HBED (HBED = N,N′-bis(2-hydroxyphenyl)ethylenediamine-N,N′-diacetic acid), a coordinatively saturated iron chelate, is an attractive MRI CA platform suitable for modification to adjust relaxivity and biodistribution. Compared to the parent Fe-HBED, the Fe-HBED analogs reported here have lower serum protein binding and higher relaxivity as well as lower relative liver enhancement in mice, comparable to that of a representative gadolinium-based contrast agent (GBCA). Fe-HBED analogs are therefore a promising class of non-Gd ECF MRI CA.

Small Molecule-Based Fluorescent Organic Nanoassemblies with Strong Hydrogen Bonding Networks for Fine Tuning and Monitoring Drug Delivery in Cancer Cells

Bright supramolecular fluorescent organic nanoassemblies (FONs), based on strongly polar red-emissive benzothiadiazole fluorophores containing acidic units, are fabricated to serve as theranostic tools with large colloidal stability in the absence of a polymer or surfactant. High architectural cohesion is ensured by the multiple hydrogen-bonding networks, reinforced by the dipolar and hydrophobic interactions developed between the dyes. Such interactions are harnessed to ensure high payload encapsulation and efficient trapping of hydrophobic and hydrogen-bonding drugs like doxorubicin, as shown by steady state and time-resolved measurements. Fine tuning of the drug release in cancer cells is achieved by adjusting the structure and combination of the fluorophore acidic units. Notably delayed drug delivery is observed by confocal microscopy compared to the entrance of hydrosoluble doxorubicin, demonstrating the absence of undesirable burst release outside the cells by using FONs. Since FON-constituting fluorophores exhibit a large emission shift from red to green when dissociating in contact with the lipid cellular content, drug delivery could advantageously be followed by dual-color spectral detection, independently of the drug staining potentiality.

DMSO as a Switchable Alkylating Agent in Heteroarene C-H Functionalization

Herein, we report a novel strategy for the activation of DMSO to act as a versatile alkylating agent in heteroarene C-H functionalization. This direct, simple, and mild switch between methylation/trideuteromethylation and methylthiomethylation of heteroarenes was achieved under reagent-controlled photoredox catalysis conditions. The proposed mechanism is supported by both experimental and computational studies.

Sugar-Assisted Photogeneration of Didehydrotoluenes from Chlorobenzylphosphonic Acids

Irradiation of the three isomeric chlorobenzylphophonic acids in aqueous buffer led to a pH-dependent photochemistry. Under acidic conditions (pH = 2.5), photocleavage of the Ar-Cl bond occurred and a phenyl cation chemistry resulted. Under basic conditions (pH = 11), a photoinduced release of the chloride anion followed by the detachment of the metaphosphate anion gave α,n-didehydrotoluene diradicals (α,n-DHTs), potential DNA cleaving intermediates. At a physiological pH (pH = 7.2), both a cationic and a diradical reactivity took place depending on the phosphonic acid used. It is noteworthy that the complexation exerted by a monosaccharide (glucose or methylglucopyranoside) present in solution induced an exclusive formation of α,n-DHTs. The mechanistic scenario of the different photoreactivities occurring when changing the pH of the solution and the role of the various intermediates (phenyl cations, diradicals, etc.) in the process was studied by computational analysis.

Phosphonic acid: preparation and applications

The phosphonic acid functional group, which is characterized by a phosphorus atom bonded to three oxygen atoms (two hydroxy groups and one P=O double bond) and one carbon atom, is employed for many applications due to its structural analogy with the phosphate moiety or to its coordination or supramolecular properties. Phosphonic acids were used for their bioactive properties (drug, pro-drug), for bone targeting, for the design of supramolecular or hybrid materials, for the functionalization of surfaces, for analytical purposes, for medical imaging or as phosphoantigen. These applications are covering a large panel of research fields including chemistry, biology and physics thus making the synthesis of phosphonic acids a determinant question for numerous research projects. This review gives, first, an overview of the different fields of application of phosphonic acids that are illustrated with studies mainly selected over the last 20 years. Further, this review reports the different methods that can be used for the synthesis of phosphonic acids from dialkyl or diaryl phosphonate, from dichlorophosphine or dichlorophosphine oxide, from phosphonodiamide, or by oxidation of phosphinic acid. Direct methods that make use of phosphorous acid (H3PO3) and that produce a phosphonic acid functional group simultaneously to the formation of the P-C bond, are also surveyed. Among all these methods, the dealkylation of dialkyl phosphonates under either acidic conditions (HCl) or using the McKenna procedure (a two-step reaction that makes use of bromotrimethylsilane followed by methanolysis) constitute the best methods to prepare phosphonic acids.

Effects of a phosphonate anchoring group on the excited state electron transfer rates from a terthiophene chromophore to a ZnO nanocrystal

Relative to carboxyl-anchored chromophores, phosphonate-anchored dyes are bound more strongly but slow the excited state electron transfer to ZnO nanocrystals.

A second-generation phosphohistidine analog for production of phosphohistidine antibodies

Protein histidine phosphorylation plays a crucial role in cell signaling and central metabolism. However, its detailed functions remain elusive due to technical challenges in detecting and isolating proteins bearing phosphohistidine (pHis), a labile posttranslational modification (PTM). To address this issue, we previously developed the first pHis-specific antibodies using stable, synthetic triazole-based pHis analogs. A second-generation, pyrazole-based pHis analog that enabled the development of a pan-pHis antibody with much improved pHis specificity is now reported.

Carboxylic acid (bio)isosteres in drug design

The carboxylic acid functional group can be an important constituent of a pharmacophore, however, the presence of this moiety can also be responsible for significant drawbacks, including metabolic instability, toxicity, as well as limited passive diffusion across biological membranes. To avoid some of these shortcomings while retaining the desired attributes of the carboxylic acid moiety, medicinal chemists often investigate the use of carboxylic acid (bio)isosteres. The same type of strategy can also be effective for a variety other purposes, for example, to increase the selectivity of a biologically active compound or to create new intellectual property. Several carboxylic acid isosteres have been reported, however, the outcome of any isosteric replacement cannot be readily predicted as this strategy is generally found to be dependent upon the particular context (i.e., the characteristic properties of the drug and the drug-target). As a result, screening of a panel of isosteres is typically required. In this context, the discovery and development of novel carboxylic acid surrogates that could complement the existing palette of isosteres remains an important area of research. The goal of this Minireview is to provide an overview of the most commonly employed carboxylic acid (bio)isosteres and to present representative examples demonstrating the use and utility of each isostere in drug design.

Hybrid inorganic-organic adsorbents Part 1: Synthesis and characterization of mesoporous zirconium titanate frameworks containing coordinating organic functionalities

A series of functional hybrid inorganic-organic adsorbent materials have been prepared through postsynthetic grafting of mesoporous zirconium titanate xerogel powders using a range of synthesized and commercial mono-, bis-, and tris-phosphonic acids, many of which have never before been investigated for the preparation of hybrid phases. The hybrid materials have been characterized using thermogravimetric analysis, diffuse reflectance infrared (DRIFT) and 31P MAS NMR spectroscopic techniques and their adsorption properties studied using a 153Gd radiotracer. The highest level of surface functionalization (molecules/nm2) was observed for methylphosphonic acid (∼3 molecules/nm2). The level of functionalization decreased with an increase in the number of potential surface coordinating groups of the phosphonic acids. Spectral decomposition of the DRIFT and 31P MAS NMR spectra showed that each of the phosphonic acid molecules coordinated strongly to the metal oxide surface but that for the 1,1-bis-phosphonic acids and tris-phosphonic acids the coordination was highly variable resulting in a proportion of free or loosely coordinated phosphonic acid groups. Functionalization of a porous mixed metal oxide framework with the tris-methylenephosphonic acid (ATMP-ZrTi-0.33) resulted in a hybrid with the highest affinity for 153Gd3+ in nitric acid solutions across a wide range of acid concentrations. The ATMP-ZrTi-0.33 hybrid material extracted 153Gd3+ with a Kd value of 1×10(4) in 0.01 M HNO3 far exceeding that of the other hybrid phases. The unfunctionalized mesoporous mixed metal oxide had negligible affinity for Gd3+ (Kd<100) under identical experimental conditions. It has been shown that the presence of free or loosely coordinated phosphonic acid groups does not necessarily translate to affinity for 153Gd3+. The theoretical cation exchange capacity of the ATMP-ZrTi-0.33 hybrid phase for Gd3+ has been determined to be about 0.005 mmol/g in 0.01 M HNO3. This behavior and that of the other hybrid phases suggests that the surface-bound ATMP ligand functions as a chelating ligand toward 153Gd3+ under these acidic conditions.

Surface electrochemistry of mesoporous silicas as a key factor in the design of tailored delivery devices

The fundamental mechanisms of biologically active molecule adsorption and release from ordered mesoporous silica are discussed in terms of the variation of surface electrochemistry after functionalization. Specifically, ordered mesoporous SBA-15 has been grafted with aminopropyl, etilenediamine, phosphatoethyl, propyl methacrylate, and carboxylic acid groups at different degrees of functionalization. To test the molecular adsorption and release features, three molecules of clinical interest have been selected, namely, antiresorptive zoledronic acid, amino acid L-tryptophan, and protein bovine serum albumin. Molecular loading and delivery aspects have been studied by emphasizing the host-guest interactions, which determine the adsorption and release behavior. It has been found that careful control of surface electrochemistry by functionalization determines the bioactive molecule adsorption whereas the release can be mainly thought of as a diffusion matter dependent on the surface area and molecule size. This enhanced approach opens up new ways to optimize molecule loading for specific clinical needs.

N-succinimidyl 3-[(131)I]iodo-4-phosphonomethylbenzoate ([(131)I]SIPMB), a negatively charged substituent-bearing acylation agent for the radioiodination of peptides and mAbs

An important criterion in design of acylation agents for the radioiodination of internalizing monoclonal antibodies (mAbs) is to maximize the retention of radioiodine in the tumor following mAb intracellular processing. We have previously shown that labeling methods that generate positively charged catabolites have enhanced tumor retention. Herein we have extended this strategy to investigate the potential utility of labeling internalizing mAbs with an acylation agent that yielded labeled catabolites that would be negatively charged at lysosomal pH. The negatively charged acylation agent, N-succinimidyl 3-[(131)I]iodo-4-phosphonomethylbenzoate ([(131)I]SIPMB), was prepared from its tin precursor, N-succinimidyl 4-di-tert-butylphosphonomethyl-3-trimethylstannylbenzoate (tBu-SPMTB), in 40% radiochemical yield. The free acid, 3-[(131)I]iodo-4-phosphonomethylbenzoic acid ([(131)I]IPMBA), was also prepared from the corresponding precursor, 4-di-tert-butylphosphonomethyl-3-trimethylstannylbenzoic acid (tBu-PMTBA), in 80% radiochemical yield. The rapidly internalizing mAb L8A4 was conjugated to [(131)I]SIPMB in 25-40% yield with preservation of its immunoreactivity. Internalization and processing in the U87DeltaEGFR glioma cell line was studied in a paired label format with L8A4 labeled with (125)I using the Iodogen method. Retention of initially bound radioactivity in these cells at 24 h from [(131)I]SIPMB-labeled mAb was approximately 6-fold higher than that for directly labeled mAb. Catabolite analysis demonstrated that this difference reflected an order of magnitude higher retention of low molecular weight species in these cells. The [(131)I]SIPMB-L8A4 conjugate was intact over the first 2 h; thereafter, lysine-[(131)I]SIPMB was the predominant catabolite. In contrast, L8A4 labeled using Iodogen rapidly gave rise to mono-[(125)I]iodotyrosine within 2 h, which then cleared rapidly from the cells. These results suggest that SIPMB could be a potent candidate for labeling internalizing mAbs and warrant further study.