In Vitro Assessment of Supersaturation/Precipitation and Biological Membrane Permeation of Poorly Water-Soluble Drugs: A Case Study With Albendazole and Ketoconazole

An Artificial Gut/Absorption Simulator: Understanding the Impact of Absorption on In Vitro Dissolution, Speciation, and Precipitation of Amorphous Solid Dispersions

Upon dissolution, amorphous solid dispersions (ASDs) of poorly water-soluble compounds can generate supersaturated solutions consisting of bound and free drug species that are in dynamic equilibrium with each other. Only free drug is available for absorption. Drug species bound to bile micelles, polymer excipients, and amorphous and crystalline precipitate can reduce the drug solute's activity to permeate, but they can also serve as reservoirs to replenish free drug in solution lost to absorption. However, with multiple processes of dissolution, absorption, and speciation occurring simultaneously, it may become challenging to understand which processes lead to an increase or decrease in drug solution concentration. Closed, nonsink dissolution testing methods used routinely, in the absence of drug removal, allow only for static equilibrium to exist and obscure the impact of each drug species on absorption. An artificial gut simulator (AGS) introduced recently consists of a hollow fiber-based absorption module and allows mass transfer of the drug from the dissolution media at a physiological rate after tuning the operating parameters. In the present work, ASDs of varying drug loadings were prepared with a BCS-II model compound, ketoconazole (KTZ), and hypromellose acetate succinate (HPMCAS) polymer. Simultaneous dissolution and absorption testing of the ASDs was conducted with the AGS, and simple analytical techniques were utilized to elucidate the impact of bound drug species on absorption. In all cases, a lower amount of crystalline precipitate was formed in the presence of absorption relative to the nonsink dissolution "control". However, formation of HPMCAS-bound drug species and crystalline precipitate significantly reduced KTZ absorption. Moreover, at high drug loading, inclusion of an absorption module was shown to enhance ASD dissolution. The rank ordering of the ASDs with respect to dissolution was significantly different when nonsink dissolution versus AGS was used, and this discrepancy could be mechanistically elucidated by understanding drug dissolution and speciation in the presence of absorption.

Control of Dissolution and Supersaturation/Precipitation of Poorly Water-Soluble Drugs from Cocrystals Based on Solubility Products: A Case Study with a Ketoconazole Cocrystal

This study demonstrates in vitro and in vivo control of cocrystal dissolution with drug supersaturation/precipitation based on the solubility product of a cocrystal. As a cocrystal model, KTZ-4ABA (ketoconazole, KTZ, a poorly water-soluble drug cocrystal, with 4-aminobenzoic acid, 4ABA, a coformer) was used. The presence of 4ABA in the dissolution media dramatically reduced the dissolution rate of KTZ-4ABA and regulated the supersaturation/precipitation of KTZ, supported by the solubility product of KTZ-4ABA. In the in vitro dissolution study, the combined solid form of KTZ-4ABA and a ten-fold amount of 4ABA significantly lowered the degree of KTZ supersaturation without precipitation and further cocrystal dissolution. To confirm cocrystal dissolution control in the gastrointestinal tract with the same composition as the in vitro study, an in vivo oral administration study with rats was conducted. When KTZ was coadministered to rats in the cocrystal form, an excess of 4ABA coadministered with KTZ-4ABA in the solid form reduced the maximum plasma KTZ concentration (Cmax), prolonged the time to reach the Cmax, but did not influence the area under the plasma concentration-time curve. These results demonstrate that both in vitro and in vivo cocrystal dissolution can be regulated by adding an appropriate amount of coformer based on the solubility product, which can be one of the promising strategies for the oral use of cocrystal formulations.

The Effects of Degree and Duration of Supersaturation on In Vivo Absorption Profiles for Highly Permeable Drugs, Dipyridamole and Ketoconazole

The prediction of oral absorption from a supersaturating drug delivery system (SDDS) remains a significant challenge. Here we evaluated the effects of the degree and duration of supersaturation on in vivoabsorption for dipyridamole and ketoconazole. Various dose concentrations of supersaturated suspensions were prepared by a pH shift method, and in vitro dissolution and in vivo absorption profiles were determined. For dipyridamole, the duration of supersaturation decreased with the increase of the dose concentration owing to rapid precipitation. For ketoconazole, the initially constant dissolved concentrations due probably to the liquid-liquid phase separation (LLPS) as a reservoir were observed at high dose concentrations. However, the LLPS did not delay the peak plasma concentration of ketoconazole in rats, indicating that drug molecules were immediately released from the oil phase to the bulk aqueous phase. For both model drugs, the degree of supersaturation, but not the duration of supersaturation, correlated with systemic exposure, indicating quick drug absorption before precipitation. Therefore, the degree of supersaturation is an important parameter compared with the duration of supersaturation for enhancing the in vivo absorption of highly permeable drugs. These findings would help develop a promising SDDS.

Complexation of Apigenin and Oxymatrine Leading to Enhanced Anti-inflammatory Activity

Apigenin (APG) is a well-known dietary flavonoid with multiple bioactivities, but its poor aqueous solubility may result in low oral bioavailability and thus compromised therapeutic effects. In the present study, APG was complexed with oxymatrine (OMT), a natural quinolizidine alkaloid, for enhanced anti-inflammatory activity, and the related mechanisms in the interaction of APG with OMT were investigated. Fourier transform-infrared spectroscopy, fluorescence spectroscopy, Raman spectroscopy, and proton nuclear magnetic resonance spectroscopy characterizations demonstrated the occurrence of an APG-OMT complex formed at a molar ratio of 1:2. Then, molecular dynamics simulations and quantum chemical calculations were utilized to elucidate that hydrogen bonding, van der Waals forces, and hydrophobic effects were the main forces acting in the formation of the APG-OMT complex. Pharmacokinetic studies in rats demonstrated that the oral bioavailability of APG in the APG-OMT complex was significantly higher than that of APG alone. Finally, bioactivity evaluation in the lipopolysaccharide-induced acute inflammatory injury mouse models showed that the APG-OMT complex exhibited more potent anti-inflammatory effects than APG alone. This study confirmed that APG and OMT exerted enhanced anti-inflammatory effects through self-complexation, which may provide a novel strategy for improving the bioavailability and bioactivity of natural product mixtures.

The effect of anionic Eudragit polymers on drug supersaturation and in vitro permeation improvement

OBJECTIVES

In the present study, Cinnarizine was selected as a weakly basic drug with poor aqueous solubility to investigate the supersaturation maintaining the ability of different types of anionic Eudragit polymers (Eudragits L100-55, L100 and S100). Furthermore, the interplay between polymer-mediated supersaturation maintenance and in vitro permeation enhancement was studied.

METHODS

The effect of Eudragit polymers on the pH-induced supersaturation of Cinnarizine was examined under different pHs (6.4, 6.8 and 7.8). Moreover, the effect of Eudragit polymers on the permeation of Cinnarizine through the Caco-2 membrane was investigated.

RESULTS

The aggregate size of Eudragit polymers in solution was determined and it was found that the size of polymer aggregate was bigger when lower pH or more hydrophobic polymer was used, which corresponded strongly with improved drug supersaturation. Based on the findings, hydrophobic Cinnarizine-polymer interactions seemed to be essential in determining the impact of Eudragit polymers on maintaining the Cinnarizine supersaturation. The permeation study demonstrated that the rate of drug permeation through the Caco-2 membrane increased in the presence of Eudragit polymers, but their effect on maintaining supersaturation was more significant than their effect on the drug permeation rate. Moreover, the highest level of Cinnarizine supersaturation observed in a non-permeation condition did not correlate with the optimal absorption in a permeation condition.

CONCLUSION

This study revealed that the integration of permeation and supersaturation assays is needed to reliably predict the impact of supersaturation maintenance by polymers on the absorption of poorly soluble drugs.

Supersaturation and phase behavior during dissolution of amorphous solid dispersions

Amorphous solid dispersion (ASD) is a promising strategy to enhance solubility and bioavailability of poorly water-soluble drugs. Due to higher free energy of ASD, supersaturated drug solution could be generated during dissolution. When amorphous solubility of a drug is exceeded, drug-rich nanodroplets could form and act as a reservoir to maintain the maximum free drug concentration in solution, facilitating the absorption of the drug in vivo. Dissolution behavior of ASD has received increasing interests. This review will focus on the recent advances in ASD dissolution, including the generation and maintenance of supersaturated drug solution in absence or presence of liquid-liquid phase separation. Mechanism of drug release from ASD including polymer-controlled dissolution and drug-controlled dissolution will be introduced. Formation of amorphous drug-rich nanodroplets during dissolution and the underlying mechanism will be discussed. Phase separation morphology of hydrated ASD plays a critical role in dissolution behavior of ASD, which will be highlighted. Supersaturated drug solution shows poor physical stability and tends to crystallize. The effect of polymer and surfactant on supersaturated drug solution will be demonstrated and some unexpected results will be shown. Physicochemical properties of drug and polymer could impact ASD dissolution and some of them even show opposite effect on dissolution and physical stability of ASD in solid state, respectively. This review will contribute to a better understanding of ASD dissolution and facilitate a rational design of ASD formulation.

In Vitro, Ex Vivo, and In Vivo Evaluation of Nanoparticle-Based Topical Formulation Against Candida albicans Infection

Ketoconazole is commonly used in the treatment of topical fungal infections. The therapy requires frequent application for several weeks. Systemic side effects, allergic reactions, and prolonged treatment are often associated with non-compliance and therapy failure. Hence, we developed an optimized topical antifungal gel that can prolong the release of drug, reduce systemic absorption, enhance its therapeutic effect, and improve patient compliance. Ketoconazole-loaded PLGA nanoparticles were prepared by the emulsion/solvent evaporation method and were characterized with respect to colloidal properties, surface morphology, and drug entrapment efficiency. The optimized ketoconazole-loaded PLGA nanoparticles and commercially available silver nanoparticles were incorporated into a Carbopol 934P-NF gel base. This arrangement was characterized and compared with commercially available 2% ketoconazole cream to assess physical characteristics of the gel, in vitro drug release, ex vivo skin permeation and retention, and in vivo studies on Wister male albino rats. The results showed that polymeric PLGA nanoparticles were very effective in extending the release of ketoconazole in our optimized formulation. Nanoparticles were smooth, spherical in shape, and below 200 nm in size which is consistent with the data obtained from light scattering and SEM images. The ex vivo data showed that our gel formulation could strongly reduce drug permeation through the skin, and more than 60% of the drug was retained on the upper surface of the skin in contrast to 38.42% of the commercial cream. The in vivo studies showed that gel formulation could effectively treat the infection. This study demonstrates that our topical gel could be effective in sustaining the release of drug and suggests its potential use as a possible strategy to combat antifungal-resistant Candida albicans.

An Insight into Eudragit S100 Preserving Mechanism of Cinnarizine Supersaturation

Generally, supersaturation of weakly basic drug solution in the gastrointestinal tract can be followed by precipitation, and this can compromise the bioavailability of drugs. The purpose of this study was to evaluate the effect of Eudragit® S100 on the pH-induced supersaturation of cinnarizine and to examine the preserving mechanism of cinnarizine supersaturation by Eudragit®. Variables, including pH of media, ionic strength, and degree of supersaturation, were studied to investigate the effects of these parameters on cinnarizine supersaturation in the presence and absence of Eudragit®. The size of the Eudragit® aggregate in solution using dynamic light scattering was determined. The effect of Eudragit® on the transport of cinnarizine through the Caco-2 membrane was also investigated. The particle size study of Eudragit® aggregates showed that the size of these aggregates become large when the pH was lowered. Supersaturation experiments also demonstrated that Eudragit® preserved higher cinnarizine supersaturation with increasing ionic strength of the solution. The phase separation behavior of cinnarizine solution as a function of the degree of the supersaturation could be readily explained by considering the drug amorphous solubility. In vitro permeation studies revealed that the rate of cinnarizine permeation across Caco-2 cells increased in the presence of Eudragit®. According to the obtained results, the aggregation status of Eudragit® and nonspecific hydrophobic cinnarizine-Eudragit® interactions seemed to be essential in determining the effect of Eudragit® on cinnarizine supersaturation.

A combined in vitro in-silico approach to predict the oral bioavailability of borderline BCS Class II/IV weak base albendazole and its main metabolite albendazole sulfoxide

The aim of this study was to use a combined in vitro-in silico approach to develop a physiologically based pharmacokinetic model (PBPK) that predicts the bioavailability of albendazole (ABZ), a BCS class II/IV lipophilic weak base, and simulates its main metabolite albendazole sulphoxide (ABZSO) after oral administration of the current marketed dose of 400 mg in the fasted state. In vitro data was collected from solubility and dissolution tests performed with biorelevant media and transfer tests were carried out to evaluate the supersaturation and precipitation characteristics of ABZ upon gastric emptying. These in vitro results were used as biopharmaceutical inputs together with ABZ physicochemical properties including also permeability and in vitro metabolism data and information gathered from different clinical trials reported in the literature, were used to enable PBPK models to be developed using GastroPlus™ (version 9.7). As expected for this weak base with pKa = 3.6, ABZ exhibited a pronounced pH dependent solubility, with the solubility and extent of dissolution being greater at gastric pH and dropping significantly in the intestinal environment suggesting supersaturation and precipitation upon gastric emptying, which was confirmed by the transfer model experiments. PBPK models were set up for heathy volunteers using a full PBPK modeling approach and by implementing dynamic fluid volumes in the ACAT gut physiology in GastroPlus™. When coupling in vitro data (solubility values, dissolution rate and precipitation rate constant, etc.) for ABZ and with fitted values for the Vdss and liver systemic clearance of the sulfoxide metabolite to the PBPK model, the simulated profiles successfully predicated plasma concentrations of ABZ at 400 mg dose and simulated ABZSO at different ABZ dose levels and with different study populations, indicating the usefulness of combing in vitro biorelevant tools with PBPK modeling for the accurate prediction of ABZ bioavailability. The results obtained in this study also helped confirm that ABZ behaves as a BCS class IV compound.