Effect of solubilizing excipients on permeation of poorly water-soluble compounds across Caco-2 cell monolayers.

The purpose of this study was to evaluate the effects of solubilizing excipients on Caco-2 transport parameters of poorly water-soluble NCEs (new chemical entities), and determine their permeability class under the BCS guidance (Biopharmaceutics Classification System). The effect of solubilizing excipients on soluble donor concentration of Sch 56592, Sch-X and Sch-Y was estimated. The transport of reference compounds and NCEs was studied across Caco-2 monolayers in absence or presence of solubilizing agents. The Caco-2 permeability of reference compounds showed good correlation with their extent of human oral absorption data. Sch 56592, Sch-X and Sch-Y exhibited high baseline Caco-2 permeability (>10(-5) cm/s). Povidone (1%) improved soluble donor concentration and flux of Sch 56592 by 40%. Other solubilizing excipients predominantly improved Sch 56592 soluble donor concentration, with either no change or a decrease in flux. With Sch-X, 1% povidone, pluronic F68, gelucir 44/14, and 3:2 propylene glycol/Tween-80 markedly improved soluble donor concentration, while increasing Sch-X flux by 40-65%. The soluble donor concentration of Sch-Y was also enhanced by excipients; however, only 1% pluronic F68 and PEG 300 increased Sch-Y flux by 35-50%. Sch 56592, Sch-X and Sch-Y are low solubility-high permeability compounds under the BCS guidance. For such poorly water-soluble NCEs, solubilizing excipients should be carefully screened based on their effects on solubility profiles and membrane transport.

Adjuvancy enhancement of muramyl dipeptide by modulating its release from a physicochemically modified matrix of ovalbumin microspheres. I. In vitro characterization.

The weak immunogenicity of subunit vaccines has necessitated research into the development of novel adjuvants and methods to enhance the adjuvancy associated with vaccine delivery systems. The purpose of the present study was to modulate the release of muramyl dipeptide (MDP) from a physicochemically modified matrix of ovalbumin microspheres (OVA-MSs). A two-component MS vaccine delivery system was fabricated, which utilized OVA as the antigen and delivery matrix, and MDP as the adjuvant. The MSs were prepared from OVA using a water/oil emulsion method, followed by suspension cross-linking using glutaraldehyde. The MS matrix was modified with respect to the degree of cross-linking by varying the concentration of glutaraldehyde and matrix density, a function of disulfide-bond formation. The modifications in the MS matrix were characterized using SDS-PAGE, scanning electron microscopy, differential scanning calorimetry, and thin layer wicking (TLW). The in vitro release of MDP and OVA from the various preparations of OVA-MSs exhibited triphasic and biphasic profiles, respectively. The degree of cross-linking and the matrix density were found to be significant physicochemical parameters that affected the release profiles of MDP and OVA through two mechanisms: controlled surface erosion and bulk degradation of the MSs.

Release of bovine serum albumin from preformed porous microspheres of poly(L-lactic acid).

Preformed porous microspheres of poly(L-lactic acid) (Accurel have been shown to sustain the release of highly water soluble solutes, like dextran and mannitol, for a time period of more than 4 months. The purpose of this investigation was to mechanistically characterize the release of a model protein, bovine serum albumin (BSA), from these highly porous microspheres. The microspheres were loaded with [14C]BSA in three different concentrations of 0.06, 0.26 and 0.59% w/w. The rate of release of [14C]BSA from microspheres was correlated to media ([3H]PBS) uptake. The release of BSA showed a biphasic pattern; an initial rapid release, followed by a sustained release. The initial burst of BSA was found to be inversely proportional to BSA loading and highly correlated to water penetration. The sustained release phase was independent of water penetration kinetics. Washing the microspheres did not remove either the surface bound BSA or the BSA incorporated in the microsphere matrix, indicating the tight binding of BSA to highly porous microspheres. Furthermore, addition of a surfactant induced a dramatic increase in the amount of BSA released, suggesting that the release is controlled by the surface binding of BSA to the polymer. Also, the release rate of BSA beyond the initial burst was found to be much slower than for the lower MW macromolecules like dextran at a similar level. The data from the present work suggests the BSA-polymer interaction to be a major contributing factor in explaining the overall BSA release kinetics.

Analysis of intestinal perfusion data for highly permeable drugs using a numerical aqueous resistance--nonlinear regression method.

PURPOSE: To develop, validate and apply a method for analyzing the intestinal perfusion data of highly permeable compounds using the Numerical Aqueous Resistance (NAR) theory and nonlinear regression (NAR-NLR) and to compare the results with the well-established Modified Boundary Layer (MBL) Analysis. METHODS: The NAR-NLR method was validated and the results were compared to the MBL analysis results using previously reported cephradine jejunal perfusion data. Using the Single Pass Intestinal Perfusion (SPIP) method, the concentration dependence of intestinal permeability was investigated for formycin B, proline, and thymidine, three compounds reported to be absorbed by carrier-mediated transport processes. The MBL and NAR-NLR analyses were then applied to the three sets of SPIP data. RESULTS: The results demonstrate that the intrinsic MBL transport parameters were highly variable and, in one case, the analyses failed to give a statistically significant Michaelis constant. The MBL mean dimensionless wall permeabilities (P*w) were greater than the NAR-NLR P*w and were also highly variable. In all cases, the NAR-NLR variability was significantly lower than the MBL variability. The extreme variability in the MBL-calculated P*w is due to the sensitivity of P*w when the fraction of unabsorbed drug (Cm/Co) is low or, alternatively, when P*w approached the aqueous permeability, P*aq. CONCLUSIONS: The NAR-NLR method facilitates the analysis of intestinal perfusion data for highly permeable compounds such as those absorbed by carrier-mediated processes at concentrations below their Km. The method also allows for the use of a wider range of flow conditions than the MBL analysis resulting in more reliable and less variable estimates of intestinal transport parameters as well as intestinal wall permeabilities.

Effect of receiver fluid pH on in vitro skin flux of weakly ionizable drugs.

In in vitro skin permeation experiments, the pH of viable epidermis is readily conditioned by the receiver fluid. For weakly ionizable compounds, the flux determined experimentally thus depends on the receiver fluid pH. The purpose of the present work is to characterize this pH effect, since nonphysiological conditions have often been used in the receiver fluid to enhance the solubility of the subject compounds. A transport model was developed to analyze the above-mentioned pH effect of the receiver fluid on the steady state flux of weakly ionizable drugs. The results showed that the skin flux had a strong dependence on pH for those compounds with high intrinsic partition coefficients. Experimentally, this pH effect was observed with a model acid and a model base. The skin flux was found to have a profound dependence on the receiver fluid pH. This dependence also correlates with the octanol/water partition coefficient of the molecule. It was concluded that the use of a physiological receiver fluid would be crucial for a realistic estimation of transdermal potential. The results also suggested that, for weakly ionizable compounds with high partition coefficients, the viable epidermis could be a significant transport barrier for systemic absorption.

Calculation of the aqueous diffusion layer resistance for absorption in a tube: application to intestinal membrane permeability determination.

The single-pass intestinal perfusion technique has been used extensively to estimate the wall permeability in rats. The unbiased membrane parameters can be obtained only when the aqueous resistance is properly accounted for. This aqueous resistance was calculated numerically from a convective diffusive mass transfer model, including both passive and carrier-mediated transport at the intestinal wall. The aqueous diffusion layer resistance was shown to be best described by a function of the form, [formula: see text] where G zeta, P*m, P*c, Km, and Co are, respectively, Graetz number, passive permeability, carrier-mediated permeability, Michaelis constant, and the drug concentration entering the tube. Asterisked are dimensionless quantities obtained by multiplying the permeability constants with R/D, where R and D being radius and drug diffusivity, respectively. A, B, C, D and E were obtained by a least-squares nonlinear regression method, giving values of 1.05, 1.74, 1.27, 0.0659, and 0.377, respectively, over the range of 0.001 less than or equal to G zeta less than or equal to 0.5, 0.01 less than or equal to P*m less than or equal to 10, 0.01 less than or equal to P*c less than or equal to 10, and 0.01 less than or equal to Km/Co less than or equal to 100. This aqueous resistance was found to converge to those calculated from Levich's boundary layer solution in low Graetz range, indicating the correct theoretical limit. Using an iteration method, the equation was shown to be useful in extracting the intrinsic membrane permeability from the experimental data.

Phenytoin interaction with enteral feedings administered through nasogastric tubes.

Inadequate drug plasma levels have been associated with the administration of phenytoin with enteral feedings through nasogastric (NG) tubes. It is demonstrated in this study that loss of phenytoin to tubing is a function of pH. Nonionized phenytoin is irreversibly bound to NG tubing from solution at the pH of enteral nutrient solutions while this is not the case for anionic phenytoin in unbuffered water or saline. In experiments pulsing phenytoin through glass vs NG tubing perfused with buffer at varying pH, reversible loss to tubing was observed at high pH while irreversible loss was observed at low pH. In addition, the irreversible loss of phenytoin was greater in NG tubing than glass particularly at low pH. It is suggested that in those cases where tubing is placed into the duodenum, inadequate gastrointestinal residence time for dissolution of phenytoin solid and suspension dosage forms coupled with irreversible drug loss from solution to NG tubing will result in decreased phenytoin absorption and subsequently lower drug plasma levels.

pH-dependent swelling and solute diffusion characteristics of poly(hydroxyethyl methacrylate-co-methacrylic acid) hydrogels.

Poly(hydroxyethyl methacrylate-co-methacrylic acid) hydrogels can swell extensively in a high-pH medium where the carboxyl groups are ionized. The swelling equilibrium is a strong function of the methacrylic acid composition of the polymer and pH of the medium. The nonionized gel structure was found to be rather insensitive to the amount of cross-linker, tetraethylene glycol dimethacrylate (TEGDMA), incorporated, within the range of 0.5 to 3%. This result is supportive of the existence of secondary interactions that shield the effect of covalent cross-links. Phenylpropanolamine (PPA) was used as a probe solute to study the diffusion characteristics of the poly(HEMA-co-MA) gels. Its diffusion coefficient in the swollen matrices of different methacrylic acid compositions at various pH's was measured via a desorption method. It is evident that these diffusion coefficients follow Yasuda's free volume theory, which expresses an exponential relationship between the solute diffusivity in a swollen polymer membrane and the reciprocal of the membrane hydration. Although interactions exist between PPA and the hydrogel matrix, these interactions are not significant enough to perturb the free volume relationship established. This observation can be explained by the high ionic strength of the system.