Combinatorial Chemistry & High Throughput Screening - Volume 10, Issue 5, 2007

One of the high impact events to celebrate during 2007 is the 40th anniversary of the publication of the first supramolecular chemistry paper (Pedersen, C.J. J. Am. Chem. Soc., 1967, 89, 2495-2496) and its coincidence with the 20th anniversary of the Nobel Prize recognizing the pioneers in this field, namely, Charles J. Pedersen (1904-1989), Donald J. Cram (1919-2001) and Jean-Marie Lehn (1939). The vigorous and impressive development of various supramolecular concepts since then has enabled the fashionable ‘nano’ boom of today and the current global investment in nanotechnology-based industry. Indeed, the basic supramolecular chemistry concept combined with the possibility to recognize, control and manipulate molecular information is still seminal and by dynamic exploration of the structure-energy hypersurface would lead to construction, testing and, quite soon, large-scale production of light driven autonomous artificial molecular machines and/or their natural-artificial hybrids for clinical analysis, drug delivery and more sophisticated therapeutic treatment related to tissue modification and engineering. Before this optimistic view will be realized, the supramolecular dynamics approach should enable construction of a range of high throughput screening (HTS) devices, arrays and robotics for systematic assay of vast amounts of synthetic and natural compounds to find candidates with desirable bioactive properties. Some of these HTS concepts will soon evolve to more complicated HTS machines enabling a fast, reproducible and high content screening of variable dynamic self-recognition, selforganization and self-optimization nanoscale phenomena leading to molecular and supramolecular diversity, activity and reproduction of interacting entities, e.g., receptors and drugs. Examples of such solutions from HTS progress will be illustrated in a three part special issue of Combinatorial Chemistry & High Throughput Screening entitled, Supramolecular devices for the high-throughput screening of drugs, nutrients and metabolites in postgenomic era, which commemorates the excellence of the scientific work, sophisticated achievements and vision of the noble supramolecular chemistry founders. Part 1, presented in the current issue, includes papers related to HTS that concern chiral and molecular recognition properties of synthetic and natural small molecules, as drug-like candidates or real metabolites and their macrocyclic nano-carriers. These considerations are discussed in the review paper by Schug which describes the current status and future prospects of electrospray ionization mass spectrometry that is enabling solution phase multiplexed HTS of nanosized samples of chiral analytes and quantitative characteristics of non-covalent enantiomeric discrimination and recognition processes observed in thousands of self-assembling and binding drug-carrier or drug-target systems. In addition, the minireview paper by Mangelings and Vander Heyden describes the recent advances in construction and use of microfluidic and microchip supramolecular devices employed in various chromatographic modes for the high resolution and accelerated HTS of minute amounts of drug enantiomers and labeled small molecule building blocks in time of miliseconds. Similarly, the review paper by Ali presents some in vitro and in vivo urgent needs and challenges related to homochiral drug design, selection, development, therapeutic application, and side effects monitoring, thus implying the importance of implementating dedicated, miniaturized, and high content HTS procedures for such these emerging studies. One approach to solving these problems is modification of the pharmacological properties of homochiral drugs by the use of macrocyclic compounds as drug nano-carriers and prodrug nano-scaffolds. This approach is elaborated in the mini-review paper by Cheng et al., describing the covalent and cleavable conjugation of various drugs to the surface of dendrimers followed by their modification, molecular tuning and nanopatterning to fit highly specific therapeutic demands.....

A review is given from the stand-point of applying methods utilizing electrospray ionization - mass spectrometry (ESI-MS) for quantitative binding (affinity and selectivity) determinations in small molecule host-guest (receptorligand, selector-selectand, etc.) systems. Advantages over more commonly utilized and traditional solution phase approaches, both in the context of developing new chiral selector molecules for separation and purification of enantiomers and in drug discovery applications, are presented. Although the majority of studies employing methods such as host-guest screening, competitive binding, and titrations have focused on large protein-ligand, DNA-ligand, and RNA-ligand systems, the use of ESI-MS for studying small molecule and chiral recognition systems is growing. For the latter case, greater care must be given to considering the effects of the ESI process on the ionization of the species involved in equilibria of interest. Some basic mechanistic and practical concerns for performing solution-phase-targeting quantitative measurements by ESI-MS are given in this light. Finally, an account of the application of these methods in a high throughput format is given, highlighting the potential of traditional and novel screening and titration approaches which allow scientists to screen the performance of hundreds, if not thousands, of compounds in a single day.

Chiral separations facilitated using microchip devices are reviewed in this paper. The first research paper on this topic was published in 1999. It was seen that analysis times are greatly reduced compared with more conventional techniques such as liquid chromatography and capillary electrophoresis, and that these devices enable the separation of chiral molecules. Method optimization can be conducted in a rather easy manner, reducing the total method development time. Finally, minute amounts of sample and buffer are used during analysis, which makes the systems ultra-economical. Although the number of applications in the chiral separation field on these miniaturized systems is still rather limited, they exhibit much potential towards high-throughput screening. Some efforts are, however, still needed regarding detection modes, because derivatisation of the samples is often needed to enable their detection.

One of the enantiomers of a racemic drug may be pharmacologically inactive or toxic or ballast and, hence, U.S. FDA and European agencies have issued certain guidelines for marketing of optically active (homochiral) drugs. However, some homochiral drug enantiomers racemize in to human body leading to the generation of other antipodes, which may be toxic or ballast to the human beings. In addition, racemization reduces the administrated dosage concentration when the optically active enantiomer is converted into its inactive form. Therefore, racemization studies of homochiral drugs are the important and urgent need of today. This article reviews in vitro and in vivo racemization of homochiral drugs. The racemization of some homochiral drugs is described considering the affect of different variables such as temperature, concentration of the drug, ionic concentration, pH, addition of cyclodextrins, formation of inclusion complexes, etc. Efforts have also been made to discuss the mechanisms of the racemization process. Attempts have been made to suggest safe dosages of such drugs.

Dendrimers are a new class of artificial macromolecules with well-defined hyperbranched structures which enable bio-active molecules such as drugs to be presented in a highly multi-valent fashion. Covalent conjugation of drugs to the surface of dendrimers can be easily achieved either by direct chemical reactions between dendrimers and drug molecules including esterification and amidation or through cleavable linkers, depending on the functional groups on the surface of dendrimers. The pharmacological properties of these dendrimer-based prodrugs such as biocompatibility, biodistribution, biostability, bioadhesion and biopermeability can be modulated by further modifying dendrimers with specific functional molecules to fit a specific medicinal purpose. In this mini-review, recent advances on the use of dendrimers as prodrug nano-scaffolds were briefly demonstrated. The design and synthesis of dendrimer-based prodrugs as well as screening their intrinsic properties in biological systems were fully discussed.

The molecular recognition behaviors of some representative bile salts by three 3,6'-bridged β-cyclodextrin dimers with oligo(ethylenediamino) linkers in different lengths, i.e. 3,6'-(ethylenediamino-bridged) β-cyclodextrin dimer (1), 3,6'-(diethylenetriamino-bridged) β-cyclodextrin dimer (2), and 3,6'-(triethylenetetraamino-bridged) β-cyclodextrin dimer (3), were investigated in aqueous phosphate buffer solution (pH 7.20) at 25°C by means of 2D NMR spectroscopy and isothermal titration microcalorimetry. Owing to the cooperative host-linker-guest binding mode between host and guest, these 3,6'-bridged β-cyclodextrin dimers showed significantly enhanced binding abilities and molecular selectivities as compared with native β-cyclodextrin through the simultaneous contributions of hydrophobic, hydrogen bond, and electrostatic interactions. Thermodynamically, the inclusion complexations of these β-cyclodextrin dimers with bile salts were mainly driven by large enthalpic gain, accompanied by slight to moderate entropic loss. An enthalpy-entropy compensation analysis demonstrated that these β-cyclodextrin dimers experienced large conformational changes and extensive desolvation effect upon inclusion complexation with guest molecules.

Interaction of the antipsychotic drug risperidone with hydroxypropyl-beta-cyclodextrin (HPBCD) in solution and in the solid state was studied with the aim of overcoming the limitations associated with nasal administration of low solubility drugs. Risperidone solubility studies revealed inclusion complex formation with a 1:1 stoichiometry. Low concentrations (0.1 w/v %) of hydroxypropylmethyl cellulose (HPMC) and carbomer affected risperidone solubility in water. No formation of a ternary complex was detected. The solid inclusion complex was prepared by spray drying and was characterised by thermal (DSC) and spectral (FTIR) analyses. Risperidone and the inclusion complex were loaded into microparticles by spray drying using HPMC, carbomer and HPMC/carbomer interpolymer complex (IPC) as mucoadhesive components. The microparticles were characterised with respect to drug loading, particle size distribution, thermal analysis, and zeta potential measurements. Mucoadhesive properties of the microparticles were studied by measuring the work of adhesion. Carbomer and IPC based microparticles revealed superior mucoadhesive microparticles compared to HPMC based microparticles. Drug incorporation into microparticles reduced their mucoadhesive properties, while incorporation of the cyclodextrin complex caused no additional reduction in mucoadhesion. The in vitro dissolution studies showed that formation of the inclusion complex significantly increased the risperidone dissolution rate from the microparticles, thus providing sustained drug release.

Biomimetic poly(amidoamine)-poly(benzyl L-glutamate) (PAMAM-PBLG) amphiphiles with multi-armed architecture were synthesized by the ring-opening polymerization (ROP) of β-benzyl-L-glutamate N-carboxyanhydride (BLG-NCA) using primary amine-terminated PAMAM dendrimer as the macroinitiator. Both 1H NMR and 13C NMR demonstrated that all the primary amines of PAMAM participated in initiating the ROP of BLG-NCA monomer, and the chain length of PBLG can be adjusted linearly by the molar ratio of BLG-NCA monomer to PAMAM. These multi-armed PAMAM-PBLG amphiphiles exhibited both α-helix and β-sheet conformations similar to linear analogues, while their multi-armed architecture could enhance the secondary conformation content of PBLG segments. Meanwhile, the PAMAM-PBLG amphiphiles showed weaker liquid crystalline textures than the linear analogues. Moreover, spherical nanoparticles could be generated by direct injection of these PAMAM-PBLG solutions into distilled water, and their average size (38 - 275 nm) could be adjusted through the multi-armed architecture, the PBLG composition, and the concentration of the amphiphiles. These nanoparticles were stable in aqueous solution for up to 64 days at room temperature and 16 days at 37°C. Consequently, this will provide a convenient method not only to synthesize multi-armed polypeptides amphiphiles, but also to generate biodegradable and biocompatible nanoparticles with adjustable size for drug/gene release.

The changes in antioxidant compounds of Brassica oleracea L. var. costata DC seeds were monitored during the first twelve days of seedling development. Sprouts were screened at time intervals of two days for phenolic compounds and organic acids. The identified phenolic compounds included esters of sinapic acid with glucose, gentiobiose and kaempferol, as well as sinapoylcholine. The organic acids were oxalic, aconitic, citric, pyruvic, malic, shikimic, and fumaric acids. During germination, a depletion of phenolic compounds was observed, although no qualitative changes were seen. Among individual compounds, kaempferol, choline and glucose esters of sinapic acid showed a marked decrease between days two and six, whereas the changes in gentiobiose esters of sinapic acid were smaller. The total organic acids content increased rapidly during the first four days, with less significant variations thereafter. Malic acid, the major organic acid found in sprouts, greatly contributed to this result though oxalic, pyruvic, and fumaric acids also increased in the same manner. In contrast, aconitic, citric and shikimic acids showed decreases between days two and twelve of germination.

The effect of different chromatographic conditions, such as buffer concentration and type of organic modifier, on the retention behavior of nine tricyclic neuroleptics on three different RP-HPLC columns was investigated. Two recently developed columns, calixarene-bonded (CALTREX® AIII) and monolithic (Chromolith® Performance RP-18e) columns, were compared with a conventional RP-C18 HPLC column (LiChrospher®). The results showed how the mobile phase conditions had different effects on the analyte retention on these three columns. For example, the elution order of some analytes and the initiation of separation of the geometric isomers of the three analytes - which have E/Z-isomers (cis/trans-isomers) - could be altered by changing the conditions and the column type. Under identical conditions, a calixarene- bonded phase was the best for this separation, a monolithic phase gave comparable results and the conventional RPcolumn was the least effective. Concerning the geometric isomers separation, the Chromolith® Performance RP-18e was superior.