Reinhard Neubert

Modification of drug penetration into human skin using microemulsions

Abstract The objective of this study was to investigate the penetration of the hydrophilic substance diphenhydramine hydrochloride from a W/O-microemulsion into human skin under ex vivo conditions. The focus of the study was to determine the amount of a model substance in the different skin layers, rather than measure the transdermal rate. Modifications of the vehicle components clarified the extent, to which it is possible to control the penetration of a hydrophilic drug incorporated in a microemulsion (ME) system. A standard ME showed an accumulation of penetrated drug in the dermis, indicating a potential following high absorption rate. Incorporation of cholesterol into the system leads to an even higher penetration rate and a shifting of the concentration profile further towards the epidermis. In comparison, addition of oleic acid had no effect. The results are in concordance with the assumption that diphenhydramine hydrochloride follows hydrophilic structures into the stratum corneum. Therefore, an alteration of the barrier properties can be achieved obviously only by influencing this hydrophilic pathway.

Structure of stratum corneum lipids characterized by FT-Raman spectroscopy and DSC. III. Mixtures of ceramides and cholesterol

The thermotropic and lyotropic phase behaviour of mixtures of ceramides type IV and cholesterol was investigated using Fourier transform (FT) Raman spectroscopy and differential scanning calorimetry (DSC). In the dry and in the fully hydrated state of these mixtures the DSC-curves exhibit an eutectic melting followed by the melting of the residual solid component. The Raman spectrum of the mixtures is complex, nevertheless, the appearance of the conformationally dependent bands indicates the ordered structure of the hydrocarbon chains. The temperature dependence of the conformationally sensitive bands in the CH2 stretching region (2800-2975 cm-1) and in the chain C-C stretching region (1050-1150 cm-1) was used to estimate the degree of order in terms of the relative population of trans and gauche conformers. The spectrum of the pure cholesterol shows only a weak temperature dependence in the CH2 stretching region and, therefore, the decrease of the intensity of the asymmetric CH2 stretching mode at 2880 cm-1 can be attributed to the melting of the alkyl chains of ceramides. The temperature and width of the phase transition, derived from Raman data, are similar to those of the DSC study.

Structure of stratum corneum lipids characterized by FT-Raman spectroscopy and DSC. II. Mixtures of ceramides and saturated fatty acids

Fourier transform (FT) Raman spectroscopy and differential scanning calorimetry (DSC) were used to study the thermotropic phase behaviour of mixtures of ceramides type IV (CER) and stearic acid (SA). For comparison the melting behaviour of SA was re-examined. The Raman spectra of all mixtures in the solid state show sharp bands associated with trans sequencies of the alkyl chain residues of both lipids. These features demonstrate that the hydrocarbon chains are highly ordered in the mixtures, too. The temperature dependence of the conformationally sensitive bands is used to estimate the degree of order in terms of the relative population of trans and gauche conformations. The DSC heating curves for the mixtures show two endothermic transitions which are typical for eutectic melting. The factor group splitting of the CH2 scissoring mode, arising from the orthorhombic subcell packing of SA, disappears in the course of the eutectic melting of samples with a SA content lower than 90 mol%. Both DSC and Raman spectroscopic studies reveal that CER and SA are immiscible in the solid state. The phase diagram of the system is a simple eutectic type one. The addition of SA to CER shifts the melting temperature of ceramides to lower values. However, though SA is a major component of stratum corneum (SC) it is not efficient enough to increase the fluidity of ceramides.

Analytical methods for measuring urea in pharmaceutical formulations

Two new methods are described for the routine determination of urea that utilize HPTLC-densitometry and colorimetry. The methods involve derivatization of urea with p-dimethylaminobenzaldehyde to a yellow-coloured compound. Validation of the methods was accomplished with respect to linearity, accuracy, reproducibility and limit of detection/quantification. Both methods were compared with an enzymatic method previously described in the literature and were found to be in close agreement. The proposed methods have the advantages of being simple, rapid and involve a single step sample preparation. Under experimental conditions HPTLC was the most sensitive method.

Determination of cephalosporins in urine and bile by capillary zone electrophoresis

Abstract A capillary zone electrophoresis (CZE) method has been developed for the determination of new cephalosporin antibiotics (cephalexin, cefadroxil, cefaclor, ceftazidim, cefsulodin, cefotaxim, cefamandol, cefuroxim, cefodizim) in urine and in bile. The determination of cephalosporins was performed as a direct method. The samples were only diluted in buffer and were injected into the apparatus without any further sample preparation. The determination and the separation of cephalosporins in both media was performed at pH 6. Urine and bile samples were analyzed using a calibration curve for cephalosporin concentrations between 10 and 500 μg/ml. The detection limit, the effective mobility and the relative standard deviation of the migration times and of the peak areas in urine and in bile were determined.

Application of capillary zone electrophoresis in cephalosporin analysis.

Cephalosporins have structures and antibiotic activity similar to those of penicillins which represent a class of compounds with closely related structures. Most of the cephalosporins contain aromatic groups and show distinctive UV spectra. Separating the different types of cephalosporins is a challenging task for HPLC. but the resolving power of capillary zone electrophoresis (CZE) makes this separation fast and simple. The present study reports the application of CZE for cephalosporin analysis and the separation of cephalosporins from plasma. Both field strength and temperature were shown to influence the plate number. The influence of injection time on the peak height was studied. Furthermore, the influence of pH value on the separation of cephalosporins by CZE was investigated. The low sample amount required and the relatively short analysis time are the main advantages of this method.

Application of micellar electrokinetic chromatography for analyzing antiviral drugs in pharmaceutical semisolid formulations.

The application of micellar electrokinetic chromatography (MEKC) to the determination of the antiviral drugs brivudin (BV) and aciclovir (AC) in pharmaceutical semisolid formulations was studied. A method was developed for separating AC and BV both from hydrophilic and from lipophilic semisolid formulations and for the rapid determination of BV and AC using MEKC. The detection limit, the effective mobility and the relative standard deviation of the migration times and of the peak areas were determined.

Interactions between food components and drugs. Part 5: Effect of acetylation and amidation of pectins on the interaction with drugs

Interactions between acetylated and amidated pectins and different drugs were studied using affinity capillary electrophoresis (ACE). It is shown that ACE is a suitable method to characterize these interactions at the molecular level. Calculating the equilibrium binding constants it was found that the structure and size of the drugs used have a stronger influence on the binding of drugs to pectins than charge and hydrophilic character. Tetracycline (TT) followed by propranolol (PP) showed the strongest interactions with the pectin derivatives studied. In contrast to these results, only the permeation of the lipophilic drug PP across artificial lipid membranes was significantly reduced by interaction with the pectin derivatives. The very low membrane transport of the hydrophilic drug TT was not influenced by the interaction. Reduction of the permeation of PP by the pectins could further influence absorption and bioavailability.

Release of Urea from Semisolid Formulations Using a Multilayer Membrane System

A method for the determination of in vitro release of urea from semisolid formulations using a multilayer membrane system (MMS) has been developed. The artificial model membranes consisted of collodion as the matrix and glycerol as the hydrophilic acceptor phase. The method can be employed as a tool for comparison of in vitro release profile of semisolid formulations and, thus, can be used as a quality control procedure for assuring lot-to-lot uniformity.

Mathematical assessment of different penetration mechanisms from vehicles with propylene glycol

A compartmental model describing the penetration of drugs into lipophilic membranes is developed. The model is derived to simulate the role of dissolution and two different penetration mechanisms, i.e. simple penetration and cotransport, for the penetration from suspension-type vehicles containing propylene glycol (PG). Simple penetration is referred to the drug transport based on the gradient of the chemical potentials of the drug in the formulation and the acceptor. Cotransport is drug penetration together with the penetrating cosolvent (PG). Using experimental data for the penetration of the glucocorticoids betamethasone valerate (BMV) and hydrocortisone (HC) from PG/water formulations into dodecanol (DD) membranes as acceptor, the time-dependent mass alterations in all compartments are assessed by means of the model equations. Thus, the rate-limiting step of the penetration process can be determined. Covering substances with different solubilities in PG, the study indicates that the fraction of drug penetrating by cotransport increases as the solubility in PG increases. Furthermore, a higher PG content in the vehicle leads to a markedly higher importance of the cotransport pathway.