Current direction and advances in analytical sample extraction techniques for drugs with special emphasis on bioanalysis

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Executive Summary 1. Introduction Good analysis begins with best sample collection procedures. Extracting the desired analytes from the matrices is a difficult task as each matrix has its distinctive challenges. Selection of proper sample extraction procedure has significant contribution in quality of analytical and bioanalytical study results. 2. Sample preparation in bioanalysis The complexity of biological matrix (tissue, whole blood, plasma, serum, urine, saliva) varies from each other. Measuring the drug concentration in biological matrices is an essential aspect of medicinal drug product development. 3. Conventional sample extraction procedure The protein precipitation method does not remove most of the matrix interferences, increase the back pressure of high performance liquid chromatography system and permanently affect the column performance. This technique can be chosen in case of pharmacokinetic or other type of screening of biological sample in the drug discovery process where huge numbers of molecules are required to be tested in a faster rate. Liquid-liquid extractions (LLE) and Solid-phase extraction (SPE) are the basic methods used for the sample preparation for many years. LLE generates relatively cleaner samples than the protein precipitation method. However, the limitations include less selectivity than SPE, requirement of significant user intervention, use of large volumes of organics and substantial solvent evaporation requirement. Additionally, this method is not suitable for extraction of highly polar drugs and metabolites. SPE is a better alternative that provides high recovery. The limitation of this procedure includes the requirement of significant involvement in method development. On a large scale, it is better to avoid SPE as it is tedious, time consuming and complex methodology. 4. Advancement in sample preparation techniques 4.1 Microextraction techniques 4.1.1 Solid-phase microextraction (SPME) SPME is a very efficient and simple technique which avoids the use of much solvent. It reduces the overall sample preparation method by decreasing the number of steps needed. Headspace SPME can be used as a high throughput technique for the extraction of low molecular weight compounds. SPME with nonpolar coating should be used for the extraction of non-polar components. Polyacrylate coating is more suitable for the polar components. 4.1.2 Single drop micro extraction (SDME) SDME uses relatively less organic solvents compared to traditional techniques like LLE and SPE. It provides high extraction efficiency and permits automation. Advantages include improved precision and increased sample throughput due to less extraction time. Direct immersion SDME is generally applied to liquid samples containing non-polar or moderately polar analytes. 4.1.3 Dispersive liquid-liquid microextraction (DLLME) DLLME consumes very less organic solvent and is a fast procedure. Due to the large surface area between the aqueous sample and extracting phase, equilibrium is attained very quickly and the extraction is time independent. Surfactant-assisted DLLME (SA-DLLME) avoid the use of dispersed organic solvent by adding a surfactant to the known amount of organic solvent. SA-DLLME should be used where analyte recovery is not satisfactory in conventional techniques. 4.2 Microwave assisted magnetic extraction Ultrasound, magnetically assisted and microwave assisted magnetic extraction works in different principles. Microwave radiations are absorbed significantly by magnetic nanoparticles. Therefore, microwave assists extraction of the drugs from the biological samples. Adsorbent particles with lower size in nanometer range are used in microwave assisted magnetic extraction due to the advantage of their higher surface area. 4.3 Electro membrane extraction (EME) EME used electric current gradient as a principle for dragging the drug out of the complex biological matrices. Microfluidic system of on-chip EME found to be advantageous in terms of less use of toxic organic solvents and sample and also serves with a good extraction efficiency. Application of potential difference across the membrane in EME technique has advanced the extraction efficiency for most drug molecules possessing ionic properties. 4.4 Dried blood spot (DBS) DBS is an established technique of collecting the blood sample on paper for screening the in-vivo changes. The sample that is collected in DBS technique is very less and the repeat analysis of the compounds is difficult. The interference of hematocrit value causes poor distribution of the analytes on the DBS card. However, DBS technique can play a vital role in the quantitative analysis of metabolites in blood. 5. Conclusion The steps in the sample preparation are gradually decreasing with the emergence of the recently developed novel methods. With the introduction of advanced technology in the sample pretreatment, new sorbents (immunosorbents, graphene oxides and carbon nanotubes) have been developed which lead to provide higher extraction efficiencies. Magnetically assisted bioanalytical sample extraction technique is also one of the best emerging techniques. The drug molecules need to be conjugated with magnetic components to induce magnetism on analyte so that it can be separated from the complex matrices with the help of external magnetic field. These methods are able to give clean and interfering substance free samples after extraction. The matrix effects are very less or negligible in magnetically assisted bioanalytical sample extraction technique. Ultrasound assisted extraction can be utilized for the determination of metals or organic compounds in biological matrices.

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