Role of Electrostatic Interactions on the Transport of Druglike Molecules in Hydrogel-Based Articular Cartilage Mimics: Implications for Drug Delivery.

In the field of drug delivery to the articular cartilage, it is advantageous to apply artificial tissue models as surrogates of cartilage for investigating drug transport and release properties. In this study, artificial cartilage models consisting of 0.5% (w/v) agarose gel containing 0.5% (w/v) chondroitin sulfate or 0.5% (w/v) hyaluronic acid were developed, and their rheological and morphological properties were characterized. UV imaging was utilized to quantify the transport properties of the following four model compounds in the agarose gel and in the developed artificial cartilage models: H-Ala-ß-naphthylamide, H-Lys-Lys-ß-naphthylamide, lysozyme, and α-lactalbumin. The obtained results showed that the incorporation of the polyelectrolytes chondroitin sulfate or hyaluronic acid into agarose gel induced a significant reduction in the apparent diffusivities of the cationic model compounds as compared to the pure agarose gel. The decrease in apparent diffusivity of the cationic compounds was not caused by a change in the gel structure since a similar reduction in apparent diffusivity was not observed for the net negatively charged protein α-lactalbumin. The apparent diffusivity of the cationic compounds in the negatively charged hydrogels was highly dependent on the ionic strength, pointing out the importance of electrostatic interactions between the diffusant and the polyelectrolytes. Solution based affinity studies between the model compounds and the two investigated polyelectrolytes further confirmed the electrostatic nature of their interactions. The results obtained from the UV imaging diffusion studies are important for understanding the effect of drug physicochemical properties on the transport in articular cartilage. The extracted information may be useful in the development of hydrogels for in vitro release testing having features resembling the articular cartilage.

PEGylation of phytantriol-based lyotropic liquid crystalline particles--the effect of lipid composition, PEG chain length, and temperature on the internal nanostructure.

Poly(ethylene glycol)-grafted 1,2-distearoyl-sn-glycero-3-phosphoethanolamines (DSPE-mPEGs) are a family of amphiphilic lipopolymers attractive in formulating injectable long-circulating nanoparticulate drug formulations. In addition to long circulating liposomes, there is an interest in developing injectable long-circulating drug nanocarriers based on cubosomes and hexosomes by shielding and coating the dispersed particles enveloping well-defined internal nonlamellar liquid crystalline nanostructures with hydrophilic PEG segments. The present study attempts to shed light on the possible PEGylation of these lipidic nonlamellar liquid crystalline particles by using DSPE-mPEGs with three different block lengths of the hydrophilic PEG segment. The effects of lipid composition, PEG chain length, and temperature on the morphology and internal nanostructure of these self-assembled lipidic aqueous dispersions based on phytantriol (PHYT) were investigated by means of synchrotron small-angle X-ray scattering and Transmission Electron Cryo-Microscopy. The results suggest that the used lipopolymers are incorporated into the water-PHYT interfacial area and induce a significant effect on the internal nanostructures of the dispersed submicrometer-sized particles. The hydrophilic domains of the internal liquid crystalline nanostructures of these aqueous dispersions are functionalized, i.e., the hydrophilic nanochannels of the internal cubic Pn3m and Im3m phases are significantly enlarged in the presence of relatively small amounts of the used DSPE-mPEGs. It is evident that the partial replacement of PHYT by these PEGylated lipids could be an attractive approach for the surface modification of cubosomal and hexosomal particles. These PEGylated nanocarriers are particularly attractive in designing injectable cubosomal and hexosomal nanocarriers for loading drugs and/or imaging probes.

Determination of liposome-buffer distribution coefficients of charged drugs by capillary electrophoresis frontal analysis.

The potential of using CE frontal analysis (CE-FA) to study the interactions between a range of charged low molecular weight drug substances and liposomes was evaluated. The liposomes used were net negatively charged and consisted of 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine and 1,2-dipalmitoyl-sn-glycero-3-phosphate monosodium salt in a ratio of 80/20 mol%. Apparent distribution coefficients (D(mem)), defined as the molar concentration of drug substance in the membrane phase divided by the molar concentration of drug substance in the aqueous phase, were successfully determined for six positively and eight negatively charged drug substances with log D(mem) ranging from 1.35 to 3.63. The extent of liposome-buffer distribution was found to be dependent on the drug concentration. The results obtained with the developed CE-FA method were in good agreement with results obtained by equilibrium dialysis. Furthermore, the CE-FA method was faster, less labor intensive and required smaller sample volumes (approximately 50 microL) compared with equilibrium dialysis. Thus, CE-FA is an efficient and useful tool for the characterization of interactions between liposomes and low molecular weight drug substances.

Drug release into hydrogel-based subcutaneous surrogates studied by UV imaging.

Upon subcutaneous administration, the distribution of drug between the delivery vehicle and the biological tissue critically affects the absorption of drug substances. Utilization of physical models resembling the native tissues appears promising for obtaining a detailed understanding of the performance of drug delivery systems based on in vitro experiments. The objective of this study was to evaluate a UV imaging-based method for real-time characterization of the release and transport of piroxicam in hydrogel-based subcutaneous tissue mimics/surrogates. Piroxicam partitioning from medium chain triglyceride (MCT) into 0.5% (w/v) agarose or 25% (w/v) F127-based hydrogels was investigated by monitoring the concentration profiles of the drug in the gels. The effect of pH on piroxicam distribution and diffusion coefficients was studied. For both hydrogel systems, the diffusion of piroxicam in the gels was not affected significantly by the pH change from 4.0 to 7.4 but a considerable change in the oil-gel distribution coefficients was found (24 and 34 times less at pH 7.4 as compared those observed at pH 4.0 for F127 and agarose gels, respectively). In addition, the release and transport processes of piroxicam upon the injection of aqueous or MCT solutions into an agarose-based hydrogel were investigated by UV imaging. The spatial distribution of piroxicam around the injection site in the gel matrix was monitored in real-time. The disappearance profiles of piroxicam from the injected aqueous solution were obtained. This study shows that the UV imaging methodology has considerable potential for characterizing transport properties in hydrogels, including monitoring the real-time spatial concentration distribution in vitro after administration by injection.

Real-time UV imaging of piroxicam diffusion and distribution from oil solutions into gels mimicking the subcutaneous matrix.

A novel real-time UV imaging approach for non-intrusive investigation of the diffusion and partitioning phenomena occurring during piroxicam release from medium chain triglyceride (MCT) solution into two hydrogel matrices is described. Two binary polymer/buffer gel matrices, 0.5% (w/v) agarose and 25% (w/v) Pluronic F127, were applied as simple models mimicking the subcutaneous tissue. The evolution of the absorbance maps as a function of time provided detailed information on the piroxicam release processes upon the exposure of the gel matrices to MCT. Using calibration curves, the concentration maps of piroxicam in the UV imaging area were determined. Regression of the longitudinal concentration-distance profiles, which were obtained using expressions derived from Fick's second law, provided the diffusivity and the distribution coefficients of piroxicam penetrated into the gels. The obtained MCT-agarose (pH 7.4) distribution coefficient of 1.4 was identical to the MCT-aqueous (pH 7.4) distribution coefficient determined by the shake-flask method whereas that of the MCT-Pluronic F127 system was four times less. The experimental data show that UV imaging may have considerable potential for investigating the transport properties of drug formulations intended for the subcutaneous administration.

Real-time UV imaging of drug diffusion and release from Pluronic F127 hydrogels.

The objective of this study was to introduce and evaluate UV imaging technology for real-time characterization of drug diffusion in and release from hydrogels. Piroxicam and human serum albumin diffusion in Pluronic F127 hydrogel was monitored by measuring the absorbance of light passing through the diffusion cell at 26°C, thus providing real-time concentration maps (7×3 mm imaging area) within the gel as a function of time. Apparent diffusion coefficients were obtained on the basis of Fick's second law. Piroxicam and human serum albumin diffusivities in 20% (w/w) F127 gel were 8 and 24 times lower than those determined in the phosphate buffer (pH 7.4). The effect of increasing polymer concentration (20%, 25% and 30% (w/w)) on piroxicam diffusion was further investigated. The decreasing diffusion rate with increasing F127 concentration agreed well with small-angle X-ray scattering (SAXS) measurements. UV imaging was also successfully applied to monitor piroxicam release from 30% (w/w) F127 gel into a stirred aqueous buffer solution, providing simultaneous information on gel dissolution rate, change in thickness of gel-aqueous boundary layer as well as the release of piroxicam into bulk aqueous phase. The current study indicates that UV imaging has great potential for measuring drug diffusion in and release from gel matrices. Compared to the currently used conventional techniques, this technology has several advantages including high information content, non-intrusive measurements without the need for labeling, flexibility with respect to experimental design and simplicity of operation.

In vitro and in vivo characteristics of celecoxib in situ formed suspensions for intra-articular administration.

The objective of the present study was to explore the potential of using an in situ suspension forming drug delivery system of celecoxib to provide sustained drug exposure in the joint cavity following intra-articular administration. In vitro, precipitates were formed upon addition of a 400 mg/mL solution of celecoxib in polyethylene glycol 400 (PEG 400) to phosphate buffer, pH 7.4, or synovial fluid. The in vitro release profiles of the in situ formed suspensions were characterized by an initial fast release followed by a slower constant flux. In buffer solutions, these fluxes were comparable to those determined for a preformed suspension containing celecoxib in its most stable crystal form despite the in situ formed precipitates contained a mixture of two crystal forms of celecoxib as determined by X-ray powder diffraction. In situ suspension formation in synovial fluid was subject to considerable variation. A relatively high dose of celecoxib, corresponding to 1.25 mg/kg, in the form of PEG 400 solution (400 mg/mL) was injected into the radiocarpal joint in four horses. Celecoxib was present in serum samples taken over 10 days and in the joint tissue (post mortem), strongly indicating that joint sustained celecoxib exposure can be achieved using in situ suspension formation.

Drug-liposome distribution phenomena studied by capillary electrophoresis-frontal analysis.

The potential of using CE frontal analysis (CE-FA) for the study of low-molecular-weight drug-liposome interactions was assessed. The interaction of bupivacaine, brompheniramine, chlorpromazine, imipramine, and ropivacaine with net negatively charged 80/20 mol% 1-oleoyl-2-palmitoyl-sn-glycero-3-phosphocholine/egg yolk phosphatidic acid liposome suspensions in HEPES buffer at pH 7.4 was investigated. The fraction of free drug as a function of lipid concentration was measured and apparent liposome-buffer distribution coefficients were determined for the basic drug substances. The distribution coefficients increased in the order ropivacaine, bupivacaine, brompheniramine, imipramine, and chlorpromazine. The developed CE method was relatively fast allowing estimates of drug-liposome affinity to be obtained within 15 min. CE-FA may have the potential to become a valuable tool for the characterization of drug-liposome interactions in relation to estimation of drug lipophilicity and for the evaluation of drug distribution in liposomal drug delivery systems.

Complexation between low-molecular-weight cationic ligands and negatively charged polymers as studied by capillary electrophoresis frontal analysis.

The potential of using capillary electrophoresis frontal analysis for the study of low-molecular-weight ligand-polyelectrolyte interactions was assessed. The interaction of the ligands 1-propylpyridinium bromide, 2-propylisochinolinium bromide, and paraquat with the polymer dextran sulfate was investigated as a function of polymer concentration and ionic strength of the buffer solution. Linear binding isotherms were obtained and association constants were determined. The complex formation was independent of the dextran sulfate concentration at low ionic strength. Ligand-polyelectrolyte interactions were strongly dependent on the ionic strength. The interaction of the divalent cation paraquat with the dextran sulfate was much stronger than the interactions of the monovalent cationic ligands with the polyelectrolyte. The binding data obtained were in accord with results obtained by equilibrium dialysis. Capillary electrophoresis frontal analysis has the potential to become a valuable tool for characterization of ligand-polyelectrolyte interactions in drug design as well as in other areas.