Bile salt-fatty acid mixed micelles as nasal absorption promoters. III. Effects on nasal transport and enzymatic degradation of acyclovir prodrugs

Prediction of Oral Drug Absorption in Rats from In Vitro Data

PURPOSE

In drug discovery, rats are widely used for pharmacological and toxicological studies. We previously reported that a mechanism-based oral absorption model, the gastrointestinal unified theoretical framework (GUT framework), can appropriately predict the fraction of a dose absorbed (Fa) in humans and dogs. However, there are large species differences between humans and rats. The purpose of the present study was to evaluate the predictability of the GUT framework for rat Fa.

METHOD

The Fa values of 20 model drugs (a total of 39 Fa data) were predicted in a bottom-up manner. Based on the literature survey, the bile acid concentration (C_bile) and the intestinal fluid volume were set to 15 mM and 4 mL/kg, respectively, five and two times higher than in humans. LogP, pK_a, molecular weight, intrinsic solubility, bile micelle partition coefficients, and Caco-2 permeability were used as input data.

RESULTS

The Fa values were appropriately predicted for highly soluble drugs (absolute average fold error (AAFE) = 1.65, 18 Fa data) and poorly soluble drugs (AAFE = 1.57, 21 Fa data). When the species difference in C_bile was ignored, Fa was over- and under-predicted for permeability and solubility limited cases, respectively. High C_bile in rats reduces the free fraction of drug molecules available for epithelial membrane permeation while increasing the solubility of poorly soluble drugs.

CONCLUSION

The Fa values in rats were appropriately predicted by the GUT framework. This result would be of great help for a better understanding of species differences and model-informed preclinical formulation development.

Bile salts-containing vesicles: promising pharmaceutical carriers for oral delivery of poorly water-soluble drugs and peptide/protein-based therapeutics or vaccines

Most of the new drugs, biological therapeutics (proteins/peptides) and vaccines have poor performance after oral administration due to poor solubility or degradation in the gastrointestinal tract (GIT). Though, vesicular carriers exemplified by liposomes or niosomes can protect the entrapped agent to a certain extent from degradation. Nevertheless, the harsh GIT environment exemplified by low pH, presence of bile salts and enzymes limits their capabilities by destabilizing them. In response to that, more resistant bile salts-containing vesicles (BS-vesicles) were developed by inclusion of bile salts into lipid bilayers constructs. The effectiveness of orally administrated BS-vesicles in improving the performance of vesicles has been demonstrated in researches. Yet, these attempts did not gain considerable attention. This is the first review that provides a comprehensive overview of utilizing BS-vesicles as a promising pharmaceutical carrier with a special focus on their successful applications in oral delivery of therapeutic macromolecules and vaccines. Insights on the possible mechanisms by which BS-vesicles improve the oral bioavailability of the encapsulated drug or immunological response of entrapped vaccine are explained. In addition, methods adopted to prepare and characterize BS-vesicles are described. Finally, the gap in the scientific researches tackling BS-vesicles that needs to be addressed is highlighted.

Effective parameters for formulation of gastro adhesive nanoparticles: screening by design-of-experiments approach

The objective of this study was selection of most influential variable for the preparation of gastro adhesive nanoparticles of acyclovir. Effect of formulation and processing variables on various response variables were studied by a Taguchi standard orthogonal array L8 design. Independent variables studied were the amount of gelatin, amount of glutaraldehyde, amount of Pluronic F-68, acetone addition rate, pH, stirring time and stirring speed. The size of all nanoparticulate formulations prepared as per the experimental design (Taguchi screening design) varied between 165 and 1610 nm, PDI between 0.360 and 1.00, Q6 between 7.31-34.93%, T60% between 19.2-37.6 h, entrapment efficiency between 15.70 and 83.12%, loading efficiency between 39.72 and 80.49% and mucoadhesive strength between 3.959-11.02 g. Pareto ranking analyses showed that the two most important factors affecting the selected responses were amount of gelatin and amount of Pluronic F-68 (p < 0.05).

Screening of Most Effective Variables for Development of Gastroretentive Mucoadhesive Nanoparticles by Taguchi Design

The objective of this study was the selection of the most influential variable for the preparation of gastroretentive mucoadhesive nanoparticles of acyclovir. Nanoparticles were prepared by one-step desolvation method; effect of formulation and processing variables on various response variables were studied by a Taguchi standard orthogonal array L8 design. Independent variables studied were the amount of gelatin, amount of glutaraldehyde, amount of Pluronic F-68, acetone addition rate, pH, stirring time, and stirring speed. The dependent variables studied were the particle size, polydispersity index, amount of drug released in 6 h, time required to release 60% of drug, entrapment efficiency, loading efficiency, and mucoadhesiveness. The size of all nanoparticulate formulations prepared as per the experimental design (Taguchi screening design) varied between 165 and 1610 nm, PDI between 0.360 and 1.00, bioadhesiveness between 3.959 and 11.02 g, cumulative percent drug release in 24 h between 40.74 and 72.48, entrapment efficiency between 15.70 and 83.12, and loading efficiency between 39.72 and 80.49. Pareto ranking analyses showed that the two most important factors affecting the selected responses were amount of gelatin and amount of Pluronic F-68 (P<0.05).

Stereospecific chemical and enzymatic stability of phosphoramidate triester prodrugs of d4T in vitro

The phosphoramidate triester prodrug approach is widely used to deliver nucleotide forms of nucleoside analogues into target cells. We investigated the stereoselective stability of a series of prodrugs of the anti-HIV agent 2',3'-didehydro-2',3'-dideoxythymidine (d4T). Chemical stability was evaluated in phosphate buffer at pH values of biological relevance (i.e. pH 2.0, 4.6, 7.4). Enzymatic stability was tested in human plasma, in Caco-2 cell homogenates and monolayers and in rat liver. The compounds were relatively stable to chemical hydrolysis. Between 50 and 70% of unchanged prodrug was recovered after 16h incubation in human plasma, with no stereoselective preference for phosphate diastereoisomers. The p-OMe phenyl derivative, however, was an exception and only 5% of one diastereoisomer was recovered. In Caco-2 cells the stability and stereoselectivity largely depended on the experimental conditions: high enzymatic activity and stereoselectivity was observed in cell homogenates, but not in monolayers. In rat liver S9 fractions the stability profile was similar to that in Caco-2 cells and carboxyl ester cleavage appeared to be the sole mechanism of degradation in both media. The large and unpredictable differences in stereoselective metabolic rate of the pronucleotide series here presented suggest that in vivo circulating levels of intact prodrug could exert profoundly different activity or toxicity due to preferential body distribution of one diastereoisomeric form.

Alteration in the radiation-induced LD release in HeLa cells by acyclovir

The aim of the present investigation was to study the effect of various concentrations of acyclovir (ACV) alone or the effect of 0.1 micromol/l acycloir pretreatment on the radion-induced lactate dehydrogenase (LD) release by HeLa cells exposed to 0-3 Gy of gamma-radiation. The plateau-phase cells were treated with 0-100 micromol/l ACV for 8 h or 0.1 micromol/l ACV before exposure to 0-3 Gy of gamma-radiation. The whole media was removed from the culture and the LD release was measured at 0, 0.5, 1, 2, and 4 h post-drug treatment or post-irradiation. The ACV treatment resulted in a concentration-dependent elevation in the LD release at all the post-treatment time periods. A peak level for LD release was observed at 0 h up to 1 micromol/l ACV and declined thereafter. However, at higher doses a highest elevation in LD release was observed at 1 h post-treatment. Pretreatment of HeLa cells with 0.1 micromol/l ACV before irradiation to different doses of gamma-radiation resulted in significant elevation in the release of LD in the medium. The LD release was twofold greater in the ACV+irradiation group at 0 h post-irridiation when compared with the concurrent PBS+gamma-radiation group. The highest increase in LD activity was observed at 0 h post-irradiation and a gradual decline thereafter, without restoration to non-drug treated control level up to 4 h post-irradiation. The LD measurements can serve as a short-term assay to evaluate the cytotoxicity of ACV or radiation in vitro.

Powder dynamic contact angle data in the pharmaceutical industry

Formulation scientists generate physicochemical data that are used in process modelling and in the prediction of end-product quality. In this respect, contact angles are of particular importance as the surface energy of substances influences their processability and bioavailability. Dynamic contact angle (DCA) analysis represents a straightforward wettability assay. Now fully adapted to powders, DCA analysis also proves to be more efficient than other methods for wettability determination.

Effect of various lipid-bile salt mixed micelles on the intestinal absorption of amphotericin-B in rat

Amphotericin-B (AmB), a lipophilic polyene antibiotic, is the drug of choice in the treatment of many serious mycotic diseases. The solubility and gastrointestinal membrane permeability (Papp) of AmB in mixed micellar systems were examined. Membrane permeability was determined using a rat gut perfusion method. The mixed micellar systems studied contained the bile salt in association with fatty acid. All mixed micellar systems enhanced the absorption of AmB relative to the simple micelle. These results have shown that mixed micelles can enhance the absorption of AmB to a greater extent relative to nonmicellar and simple micellar systems. Maximum enhancement (> 20-fold) in the rate of AmB absorption was obtained with the sodium desoxycholate/soya lecithin 40:40 mM system. These results offer a possible explanation for the reported enhancement in gastrointestinal absorption of AmB when coadministered with lipid-bile salt mixed micelles, and these systems can be used as a vehicle for designing novel drug delivery systems for poorly absorbable drug(s).

Prodrugs for nasal drug delivery

Recently, the delivery of xenobiotics via the nasal route has received increasing attention as this offers several advantages, i.e. high systemic availability, rapid onset of action. Both charged and uncharged forms of drugs can be transported across the nasal epithelium. This mucosa is rich in various metabolizing enzymes such as aldehyde dehydrogenase, glutathione transferases, epoxide hydrolases, cyt-P450-dependent monooxygenases. The presence of these enzymes may make it possible for pharmaceutical scientists to design prodrugs for better absorption and high systemic availability. Recent advances in peptide nasal delivery through prodrug modification has been thoroughly discussed in this paper. Finally, nasally delivered therapeutic agents targeted to various disease states have been examined.

The physicochemical properties, plasma enzymatic hydrolysis, and nasal absorption of acyclovir and its 2'-ester prodrugs

A series of 2'-(O-acyl) derivatives of 9-(2-hydroxyethoxymethyl)guanine (acyclovir) was synthesized by acid anhydride esterification. Aqueous solubilities in isotonic phosphate buffer (pH 7.4), partition coefficients in 1-octanol/phosphate buffer, and hydrolysis kinetics in rat plasma were determined. The ester prodrugs showed consistent increases in lipophilicity with corresponding decreases in aqueous solubility as a function of side-chain length. The bioconversion kinetics of the prodrugs appear to depend on both the apolar and the steric nature of the acyl substituents. When perfused through the rat nasal cavity using the in situ perfusion technique, acyclovir showed no measurable loss from the perfusate. Nasal uptake of acyclovir prodrugs, on the other hand, were moderately improved. Furthermore, the extent of nasal absorption appears to depend on the lipophilicity of the prodrugs in the descending order hexanoate > valerate > pivalate > butyrate. Simultaneous prodrug cleavage by nasal carboxylesterase was also noted in the case of hexanoate.