Isoelectric buffers, part 3: Determination of pKa and pI values of diamino sulfate carrier ampholytes by indirect UV-detection capillary electrophoresis

Recent advances in testing of microsphere drug delivery systems

INTRODUCTION

This review discusses advances in the field of microsphere testing.

AREAS COVERED

In vitro release-testing methods such as sample and separate, dialysis membrane sacs and USP apparatus IV have been used for microspheres. Based on comparisons of these methods, USP apparatus IV is currently the method of choice. Accelerated in vitro release tests have been developed to shorten the testing time for quality control purposes. In vitro-in vivo correlations using real-time and accelerated release data have been developed, to minimize the need to conduct in vivo performance evaluation. Storage stability studies have been conducted to investigate the influence of various environmental factors on microsphere quality throughout the product shelf life. New tests such as the floating test and the in vitro wash-off test have been developed along with advancement in characterization techniques for other physico-chemical parameters such as particle size, drug content, and thermal properties.

EXPERT OPINION

Although significant developments have been made in microsphere release testing, there is still a lack of guidance in this area. Microsphere storage stability studies should be extended to include microspheres containing large molecules. An agreement needs to be reached on the use of particle sizing techniques to avoid inconsistent data. An approach needs to be developed to determine total moisture content of microspheres.

Protic organic ionic plastic crystals based on a difunctional cation and the triflate anion: a new solid-state proton conductor

A new family of ammonium based organic ionic plastic crystals exhibits exciting solid-state proton conductivity.

Separation of Recombinant Therapeutic Proteins Using Capillary Gel Electrophoresis and Capillary Isoelectric Focusing

Detailed step-by-step methods for protein separation techniques based on capillary electrophoresis (CE) are described in this chapter. Focus is placed on two techniques, capillary gel electrophoresis (CGE) and capillary isoelectric focusing (cIEF). CGE is essentially gel electrophoresis, performed in a capillary, where a hydrogel is used as a sieving matrix to separate proteins or peptides based on size. cIEF separates proteins or peptides based on their isoelectric point (pI), the pH at which the protein or peptide bears no charges. Detailed protocols and steps (including capillary preparation, sample preparation, CE separation conditions, and detection) for both CGE and cIEF presented so that readers can follow the described methods in their own labs.

Novel Instrument for Automated pK(a) Determination by Internal Standard Capillary Electrophoresis

The internal standard capillary electrophoresis method (IS-CE) has been implemented in a novel sequential injection-capillary electrophoresis instrument for the high-throughput determination of acidity constants (pK(a)) regardless of aqueous solubility, number of pK(a) values, or structure. This instrument comprises a buffer creation system that automatically mixes within a few seconds four reagents for in situ creation of the separation electrolyte with a pH range of 2-13, ionic strength of 10-100 mM and organic solvent content from 0% to 40%. Combined with 1.2 kV/cm and a short effective length (15 cm to the UV detector) fast 20 s electrophoretic separations can be obtained. The low standard deviation of the replicates and the low variation compared to reference values show that this system can accurately determine acidity constants of drugs by IS-CE. A single pK(a) can be determined in 2 min and a set of 20 measurements in half an hour, allowing rapid, simple, and flexible determination of pK(a) values of pharmaceutical targets.

Application of capillary electrophoresis in determination of acid dissociation constant values

The chemical groups undergoing protonation or deprotonation in solution are described by the acid dissociation constant value, the key parameter for physicochemical characterization of biologically- and pharmacologically-important compounds. Capillary electrophoresis (CE) proved to be suitable technique for its determination: it enables automated and accurate measurements even for minute amount of sample, does not require the information about concentration, and handle both the impure and complex samples. In this review, a number of contributions reporting on the application of CE in pKa prediction has been summarized and critically discussed. The reader will find herein the brief introduction of theory, summary of all works published in the last decade, considerations on the most important innovations and achievements, and the discussion of pKa-related issues as e.g. the role of pKa-shifts in the chiral separation mechanism or the elucidation of migration order reversals observed during CE-mediated separations.

Mass distribution, polydispersity, and focusing properties of carrier ampholytes for IEF. Part V: pH 9-11 interval

As a last part of an investigation on all 2-pH-unit intervals of carrier ampholytes (CAs) for IEF (see Electrophoresis 2006, 27, 3919-3934; 2006, 27, 4849-4858; 2007, 28, 715-723) two different lots of Servalyt CAs, in the pH 9-11 range, have been analyzed by a 2-D technique based on preparative Rotofor fractionation followed by capillary electrophoresis mass-spectrometry of 10 out of 20 fractions harvested, in the second dimension. The findings: the two lots contain 65 and 69 different M(r) compounds, in the M(r) interval of 232-667 Da, for a total of 341-387 isoforms, respectively. Since this is a chaotic organic synthesis, the high reproducibility (here demonstrated for the first time during the 40 years of existence of CAs) of the synthetic process (for two batches produced at 6 years of distance) is remarkable, considering that a 94% agreement for the individual chemicals and 88% agreement for the total number of isoforms for the two lots is found. It is additionally demonstrated that the lower pI species are accompanied by considerably more isoforms than the high pI forms and that in all cases such isoforms consist of family of compounds clustered around the pI of the parental form, with a pI spread of ca. 0.1-0.2 pH units.

Rapid isoelectric trapping in a micropreparative-scale multicompartment electrolyzer

A multicompartmental electrolyzer called membrane-separated wells for isoelectric focusing and trapping (MSWIFT) has been developed and tested for micropreparative-scale isoelectric trapping separations. In the MSWIFT, the length of the heat conduction path from the center of the compartments to the wall is less than 1 mm. The compartments are made from 99.8% nonporous alumina that has a high heat conductivity and a high specific heat capacity, leading to adequate Joule heat dissipation even at power loads as high as 5 W. The length of any compartment parallel to the electric field (the intermembrane distance) can be selected to be multiples of 1.5 mm, leading to compartment volumes that are multiples of about 60 muL. A maximum of 20 (1.5 mm long) separation compartments can be readily assembled in the current version of MSWIFT. The MSWIFT has been used to desalt samples, isolate small ampholytic components (amino acids, peptides, and dyes), prefractionate complex protein mixtures and enrich minor components; these separations were achieved in 20-60 min.

pH transients during salt removal in isoelectric trapping separations: A curse revisited

The pH transients that occur during isoelectric trapping separations as a result of the removal of nonampholytic ionic components have been re-examined. Salts containing strong electrolyte anions and cations, both with equal and dissimilar mobilities, have been studied using anodic and cathodic buffering membranes whose pH values were both equidistant and nonequidistant from pH 7. The direction and magnitude of the pH transient (acidic or basic) was found to depend on both the mobilities of the anion and cation (mu(anion)/mu(cation)) and the pH difference between pH 7 and the pH of the buffering membranes (|pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|). When |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1, mu(anion)/mu(cation)<1 leads to an acidic pH transient, mu(anion)/mu(cation) = 1 eliminates the pH transient and mu(anion)/mu(cation)>1 leads to a basic pH transient. When mu(anion)/mu(cation) = 1, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|<1 leads to a basic pH transient, |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| = 1 eliminates the pH transient and |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)|>1 leads to an acidic pH transient. By selecting appropriate anodic and cathodic buffering membranes to adjust the |pH(memb) (anodic) - 7|/|7 - pH(memb) (cathodic)| value, pH transients caused by dissimilar anion and cation mobilities can be avoided.

Determination of pKa values of diastereomers of phosphinic pseudopeptides by CZE

A CE method was used for the determination of acidity constants (pK(a)) of a series of ten phosphinic pseudopeptides, which varied in number and type of ionogenic groups. Effective electrophoretic mobilities were measured in the 1.8-12.0 pH range in the BGEs of constant ionic strength of 25 mM. Effective electrophoretic mobilities, corrected to standard temperature of 25 degrees C, were subjected to non-linear regression analysis and the obtained apparent pK(a) values were recalculated to thermodynamic pK(a)'s by extrapolation to zero ionic strength according to the extended Debye-Hückel model. The pK(a) values of the phosphinic acid group fell typically in the 1.5-2.25 interval, C-terminal carboxylic groups in the 2.94-3.50 interval, carboxylic groups of the lateral chain of glutamate and aspartate in the 4.68-4.97 interval, imidazolyl moiety of histidine in the 6.55-8.32 interval, N-terminal amino groups in the 7.65-8.28 interval and epsilon-amino group of the lateral chain of lysine in the 10.46-10.61 interval. Further, separation of diastereomers of the phosphinic pseudopeptides was investigated in achiral BGEs. Evaluation of the resolution of the diastereomers as a function of pH of the BGE revealed that most suitable pH region for separation of the diastereomers is around the pK(a) values of the central phosphinic acid group of the pseudopeptides. Successful separation of some diastereomers was, however, achieved in the neutral and alkaline BGEs as well.