Combinatorial Chemistry & High Throughput Screening - Volume 13, Issue 7, 2010

Work in Analytical Chemistry is divided into two interrelated parts: The methodological part includes the study of physicochemical processes from the analytical point of view, construction of the related instrumentation and the development and optimization of analytical methods. The practical part adjusts these analytical methods to the necessities of the users. Analytical methods widely utilize the knowledge of all the other disciplines such as biology, pharmaceutical chemistry, pharmaceutical technology, biotechnology and many other fields. The various electroanalytical methods that permit the determination of drugs in their dosage forms, raw material, biological media etc, and also their metabolites to be separated, identified and quantitatively assayed are briefly reviewed. The sensitivity of the analytical assay has a direct impact on the validity of the pharmacokinetic model which is built up from plasma concentration data. The precision and accuracy of the assay is also important, and it is not always straightforwardly estimated. A new significant parameter is the speed of analysis, and the resulting massive production of analytical data. New drugs coming from biotechnology, and their dosage forms, like targeted drugs, may produce new analytical problems in the future. Pharmaceutical Sciences have contributed to drug development, synthesis, formulations and analysis through extensive studies in drug assays. Analytical and pharmaceutical chemists and pharmaceutical scientists play important roles in monitoring the drugs in their dosage forms and biological samples. From the viewpoints mentioned above, the title of this special issue, “Applications of Analytical Methods: Prospects for High Throughput Screening of Pharmaceutically Active Compounds”, was chosen so as to ask analytical chemists and pharmaceutical scientists to appreciate their great roles in pharmaceutical science. The second issue of this hot topic feature 4 Reviews and 7 Original Papers. Second issue mainly contains current progresses in DNA biosensors, sensors, electroanalytical methods and their applications to the pharmaceuticals and biological samples, new electrode materials such as carbon nanotubes, boron doped diamond, etc. In addition to other analytical methods, the use of electrochemical methods to gain key information about drug molecules and their mechanism of action is getting one of the important ways in drug discovery. Applications of electrochemical techniques to redox-active drug development and mechanistic studies are one of the recent interests in drug discovery. It should be noticed that many vital physiological processes are depending on redox reactions. So, it is easy to find connections between electrochemical and biological reactions regarding electron transfer pathways. Although electrochemical data do not give absolute relationship with biological activity, due to in vitro conditions it is still possible to consider that the oxidation mechanisms taking place at the electrode and in the body share related principles as a result of the existing similarities between electrochemical and biological reactions. Electrochemical DNA biosensors enable the study of the interaction of DNA immobilized on the electrode surface with analytes in solution. The investigation based on DNA interactions has great importance in understanding the mechanism of action of many drug compounds, designing of new DNA-drug biosensors, and screening of the drugs in vitro. Electrochemical investigations of nucleic acid binding molecules-DNA interactions can provide a useful complement to the spectroscopic techniques and yield information about the mechanism at intercalation and the confirmation of anti HIV-DNA adduct. Binding of drugs to DNA and a general DNA damage has been described through the variation of the electrochemical signal of guanine or adenine. The recent developments of application, evaluation and validation of electroanalytical methods are focused by key topics in drug developments and analysis by assessment of the distinguished authors of the second issue. Thus, I expect this special issue will assist readers to find out new information and to encourage them to contribute more to recent development on drug development and its analysis using different methods. The purpose of this special issue will be to serve as a guide to what analytical methods bring to analytical and medicinal chemistry and other pharmaceutical sciences as well as briefly review their role in drugs and the new developments and validation of assay methods of pharmaceutically active compounds. I hope that the reader will find a number of topics of interest, and that additional new ideas will emerge from this special issue. I would like to thank to all of the authors for their excellent contributions, and the Editor-in-Chief of Combinatorial Chemistry & High Throughput Screening for his kind invitation to act as guest editor for this special issue.

The electrochemical behavior of triflusal (TRF) and aspirin (ASA), before and after hydrolysis in water and in alkaline medium using two different electrode surfaces, glassy carbon and boron doped diamond, was studied by differential pulse voltammetry over a wide pH range. The hydrolysis products were 2-(hydroxyl)-4-(trifluoromethyl)- benzoic acid (HTB) for triflusal and salicylic acid (SA) for aspirin, which in vivo represent their main metabolites. The hydrolysis processes were also followed by spectrophotometry. The UV results showed complete hydrolysis after one hour for TRF and after two hours for ASA in alkaline solution. The glassy carbon electrode enables only indirect determination of TRF and ASA through the electrochemical detection of their hydrolysis products HTB and SA, respectively. The oxidation processes of HTB and SA are pH dependent and involve different numbers of electrons and protons. Moreover, the difference between the oxidation peak potential of SA and HTB was equal to 100 mV in the studied pH range from 1 to 8 due to the CF3 of the aromatic ring of HTB molecule. Due to its wider oxidation potential range, the boron doped diamond electrode was used to study the direct oxidation of TRF and ASA, as well as of their respective metabolites HTB and SA.

A sensitive aptamer based electrochemical biosensor to detect human immunoglobulin E (IgE) is presented in this study. 5' Biotin labeled 45 mer DNA aptamer sequence was immobilized onto streptavidin coated graphite surfaces. Interaction between human IgE and DNA aptamer was monitored by Electrochemical Impedance Spectrometry (EIS) in a 0.48nM detection limit of IgE. EIS analyses are based on electron transfer resistance (Rct) in the presence of 5mM [Fe(CN)6]3-/4-.

The interaction of anticancer drug irinotecan (CPT-11), which is the inhibitor of the Topoisomerase I enzyme, with fish sperm double stranded deoxyribonucleic acid (dsDNA) and synthetic short oligonucleotides were studied electrochemically based on the oxidation signals of guanine and CPT-11 by using differential pulse voltammetry (DPV) and cyclic voltammetry (CV) at pencil graphite electrode (PGE). In this work, three types of methods, such as adsorption, covalent attachment and electrostatic binding were used for the immobilization of DNA onto the PGE surface. It is found that an effective modification method for DNA on the electrode surface is very important because it effects the drug and DNA interaction. As a result of the interaction, the electrochemical signal of guanine and CPT-11 greatly decreased. Experimental parameters, such as the effect of buffer solution on the interaction between CPT-11 and DNA, the concentration of CPT-11/DNA, the immobilization time of DNA and the accumulation time of CPT-11 were studied in DPV; in addition, the interaction of CPT-11 with oligonucleotides was evaluated for using as a hybridization indicator in CV and DPV. The detection limit and the reproducibility were also determined.

A rapid electrochemical procedure for the determination of dipyrone was successfully developed at a carbon nanotube modified graphite-epoxy resin composite (GrEC) electrode. The composite electrode was used as support on which multi-walled carbon nanotubes (MWCNT) were immobilised by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS) in a chitosan (Chit) matrix. The electrochemical behaviour of dipyrone at this electrode in different buffer electrolytes with pH values between 5.0 and 8.0 was explored using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy, and comparison with a conventional glassy carbon electrode was made. Dipyrone was best determined by differential pulse voltammetry with a low limit of detection of 1.4 μM. Application to commercial samples was demonstrated.

A simple, reliable and selective differential pulse (DP) and square wave (SW) voltammetric methods at glassy carbon (GC) and boron-doped diamond (BDD) electrodes of lornoxicam in pharmaceutical dosage form and in spiked human serum samples have been developed and evaluated. The possible oxidation mechanism was discussed. The voltammetric study of the model compounds allowed elucidating the possible oxidation mechanism of lornoxicam. The dependence of the peak current and peak potentials on pH, concentration, nature of the buffer, and scan rate were investigated. The oxidation of lornoxicam gave a single and irreversible peak at both electrodes. The process was found diffusion controlled. For glassy carbon electrode, the linearity of the calibration curve was obtained in the range of 4×10-7 M to 2×10-5 M for DPV method and 4×10-7 M to 4×10-5 M for SWV method in 0,1 M sulphuric acid solutions. Using boron-doped diamond electrode, the plot of the calibration curve was linear between 6×10-7 M and 1×10-4 M for both voltammetric methods in pH 2 BR buffer. The repeatability, reproducibility, precision and accuracy of the proposed methods were investigated.

A rapid and sensitive voltammetric sensor based on reduction of betamethasone has been developed using single wall carbon nanotube modified edge plane pyrolytic graphite electrode (SWNT/EPPGE). The reduction of betamethasone occurred in a well-defined, pH dependent peak. Linear calibration curve was obtained in the range 1 to 25 nM in 1.0 M phosphate buffer solution (PBS) of pH 7.2 with the limit of detection (3σ/slope) as 0.50 nM. The analytical utility of the developed method has been demonstarted by sensing the drug in human body fluids and for the determination of betamethasone content in several commercially available pharmaceutical preparations. Interfering effect of some common metabolites including ascorbic acid, uric acid, albumin and hypoxanthine has also been evaluated. A comparison of the observed results of proposed method with HPLC clearly indicates that the results of both methods are essentially similar.

Electrochemical techniques provide valuable information about drug molecules and their mechanisms in the body such as metabolism, which is one of the important actions in drug discovery. Since, many physiological processes depend on oxido-reduction reactions, it is not difficult to find associations between electrochemical and biological reactions regarding electron transfers. To investigate this proposal two compounds namely 1-methylindole-3- carboxaldehyde izonicotinoyl hydrazone and 5-chloro-1H-indole-3-carboxaldehyde izonicotinoyl hydrazone were synthesized, characterized and examined electrochemically using different voltammetric techniques in order to evaluate the possible biological behavior. The characteristics of the corresponding electrode reaction were discussed. A linear response was obtained in the different media for the compounds with low detection limits of the synthesized compounds.

In recent years increased attention has been focused on the ways in which drugs interact with DNA, with the goal of understanding the toxic as well as chemotherapeutic effects of many molecules. The development of fast and accurate methods of DNA damage detection is important, especially caused by anticancer drugs or hazard compounds. The DNA-electrochemical biosensor is a very good model for evaluation of nucleic acid damage, and electrochemical detection a particularly sensitive and selective method for the investigation of specific interactions. The electrochemical sensor for detecting DNA damage consists of a glassy carbon electrode with DNA immobilized on its surface. The starting materials or the redox reaction products can be pre-concentrated on the dsDNA-biosensor surface, enabling electrochemical probing of the presence of short-lived radical intermediates and of their damage to dsDNA. AFM images were used to characterize different procedures for immobilization of nanoscale DNA surface films on carbon electrodes before and after interaction with hazard compounds. The electrochemical transduction is dynamic in that the electrode is itself a tuneable charged reagent as well as a detector of all surface phenomena, which greatly enlarges the electrochemical biosensing capabilities.

Aptamers are single stranded DNA or RNA ligands which can be selected for different targets starting from a huge library of molecules containing randomly created sequences. Aptamers have been selected to bind very different targets, from proteins to small organic dyes. In the last years great progress has been accomplished in the development of aptamer-based bioanalytical assays with different detection techniques. This review will describe some recent aptamer-based biosensors which have been developed for the detection of small molecules that could be interesting in the pharmaceutical field. The use of aptamers to develop assays for small molecules has not been extensively studied as for protein targets. This is mainly due to difficulties in selecting aptamers for small molecules which present fewer binding possibilities for the aptamers with respect to proteins. Despite these difficulties, a few works aiming at developing aptamer-based biosensor for small molecules have been reported which take advantage of the versatility and the flexibility of aptamers.

The measurement of antiepileptic drugs (AEDs) in different samples has received considerable attention due to the direct correlation between clinical effects and plasma concentration. Numerous methods have been extensively applied to the analysis of AEDs since many years ago providing reliable and accurate results. This paper provides an overview of electrochemical techniques used for the analysis of different AEDs. More than sixty papers from refereed analytical chemistry journals on the analysis of AEDs in pharmaceutical formulations and biological samples are included. The present review shows the development of novel measurement electrochemical based on the use of different types of electrodes including mercury electrodes, screen-printed electrodes (SPEs) and electrodes modified with metal films and nanoparticles. Electrochemical biosensors and immunosensors developed for the analysis of AEDs have been also reviewed.

Recently, the use of electrochemistry and combination of this method with spectroscopic and other analytical techniques are becoming one of the important approaches in drug discovery and research as well as quality control, drug stability, determination of physiological activity, and measurement of neurotransmitters. Many fundamental physiological processes depend on oxido-reduction reactions in the body. Therefore, it may be possible to find connections between electrochemical and biochemical reactions concerning electron transfer pathways. Applications of electrochemical techniques to redox-active drug development and studies are one of the recent interests in drug discovery. In this review, the latest developments related to the use of electrochemical techniques in drug research will be surveyed in order to evaluate possible combinations of spectrometric methods with electrochemical techniques.