Dissolution Rate Enhancement by in Situ Micronization of Poorly Water-Soluble Drugs

Purpose. The purpose of this study was to evaluate a novel in situ micronization method avoiding any milling techniques to produce nano- or microsized drug particles by controlled crystallization to enhance the dissolution rate of poorly water-soluble drugs. Methods. Ibuprofen, itraconazole, and ketoconazole microcrystals were prepared by the association of the previously molecularly dispersed drug using a rapid solvent change process. The drug was precipitated in the presence of stabilizing agents, such as hydrocolloids. The obtained dispersion was spray-dried. Particle size, morphology, dissolution rate, specific surface area, and wettability were analyzed. Physicochemical properties were characterized using differential scanning calorimetry and X-ray diffractometry. Results. The obtained dispersions showed a homogeneous particle size distribution. Drugs are obtained in a mean particle size of approximately 2 m and below. A high specific surface area was created and in situ stabilized. Different stabilizers showed differences in protecting the precipitated drug from crystal growth. The surface was hydrophilized because of the adsorbed stabilizer. Thus, a drug powder with markedly enhanced dissolution rate was obtained. Conclusions. In situ micronization is a suitable method for the production of micro-sized drugs. This technique can be performed continuously or discontinuously and uses only common technical equipment. Compared to milled products drug properties are optimized as all particle surfaces are naturally grown, the particle size is more uniformly distributed and the powder is less cohesive.     

Development and Characterization of a Scalable Controlled Precipitation Process to Enhance the Dissolution of Poorly Water-Soluble Drugs

PURPOSE: Poorly water-soluble compounds are being found with increasing frequency among pharmacologically active new chemical entities, which is a major concern to the pharmaceutical industry. Some particle engineering technologies have been shown to enhance the dissolution of many promising new compounds that perform poorly in formulation and clinical studies (Rogers et. al., Drug Dev Ind Pharm 27:1003-1015). One novel technology, controlled precipitation, shows significant potential for enhancing the dissolution of poorly soluble compounds. In this study, controlled precipitation is introduced; and process variables, such as mixing zone temperature, are investigated. Finally, scale-up of controlled precipitation from milligram or gram to kilogram quantities is demonstrated. METHODS: Dissolution enhancement capabilities were established using two poorly water-soluble model drugs, danazol and naproxen. Stabilized drug particles from controlled precipitation were compared to milled, physical blend, and bulk drug controls using particle size analysis (Coulter), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), dissolution testing (USP Apparatus 2), and residual solvent analysis. RESULTS: Stabilized nano- and microparticles were produced from controlled precipitation. XRD and SEM analyses confirmed that the drug particles were crystalline. Furthermore, the stabilized particles from controlled precipitation exhibited significantly enhanced dissolution properties. Residual solvent levels were below FDA limits. CONCLUSIONS: Controlled precipitation is a viable and scalable technology that can be used to enhance the dissolution of poorly water-soluble pharmaceutical compounds.     

Development of Solid Dispersion by Hot Melt Extrusion Using Mixtures of Polyoxylglycerides With Polymers as Carriers for Increasing Dissolution Rate of a Poorly Soluble Drug Model

Various polyoxylglycerides have been researched extensively in the development of solid dispersions (SDs) for bioavailability enhancement of poorly water-soluble drugs. However, because of their low melting points (40°C-60°C), SDs produced are usually soft and semisolid. The objective of present study was to prepare SDs of a Biopharmaceutical Classification System class II drug, carvedilol, in mixtures of stearoyl polyoxylglycerides (Acconon^® C-50; m.p. ～50°C) with polymers by hot melt extrusion to obtain free-flowing powder upon grinding. Miscibility of carvedilol with Kollidon^® VA64, hydroxypropyl methylcellulose acetate succinate, and Klucel^? EXF was first evaluated by film casting, and Kollidon^® VA64 was selected for further study. SDs containing 5%-20% carvedilol, 0%-20% Acconon^® C-50, and the remaining Kollidon^® VA64 were prepared for hot melt extrusion. SDs were characterized by differential scanning calorimetry and powder X-ray diffraction analysis, and dissolution tests were conducted in 250 mL of pH 6.8 phosphate buffer by filling powders in capsules. Carvedilol was miscible with all polymers tested up to 50% and remained amorphous in SDs. The drug release from formulations containing 20% carvedilol and 0, 5%, 10%, and 20% Acconon^® C-50 were 30%, 30%, 70%, and 90%, respectively, in 60 min. SDs containing carvedilol and Acconon^® C-50, up to 20% each, as well as Kollidon^® VA64, were physically stable after 3 months of storage at 25°C/60% relative humidity.     

New Formulation Technique for Solubility and Dissolution Rate Enhancement of Poorly Soluble Drugs

Pharmaceutical Chemistry Journal - Ebastine (EBS) is a second-generation non-sedating antihistamine used for the prevention and treatment of allergic rhinitis and chronic idiopathic urticaria. It...     

Application of Dissolution/Permeation System for Evaluation of Formulation Effect on Oral Absorption of Poorly Water-Soluble Drugs in Drug Development

Purpose

The aim of the present study is to evaluate the formulation effect on the oral absorption of poorly water-soluble drugs using a dissolution/permeation system (D/P system).

Methods

This D/P system, consisting of apical and basal chambers and a Caco-2 cell monolayer mounted between chambers, can be used to perform simultaneous analysis of drug dissolution and permeation process of drugs applied as various dosage forms. Oral administration study with rats was also performed for both drugs as the same dosage forms.

Results

When danazol, a low-soluble and high-permeable drug, was applied to the D/P system as various formulations, dissolved and permeated amounts were significantly high compared with those from a suspension form. On the other hand, whereas the dissolved amount of pranlukast, a low-soluble and low-permeable drug, was significantly increased by formulations, there were no significant changes observed in the permeated amount between suspension and formulation. The oral availability of danazol was significantly increased by formulations but not pranlukast, which corresponded well to in vitro evaluations.

Conclusion

These results indicated that the D/P system might be applicable for selection of formulation on the basis of physicochemical drug properties.     

In Vitro System to Evaluate Oral Absorption of Poorly Water-Soluble Drugs: Simultaneous Analysis on Dissolution and Permeation of Drugs

Purpose. The aim of the present work was to develop a new in vitro system to evaluate oral absorption of poorly water-soluble drugs by utilizing Caco-2 monolayers. Methods. Caco-2 monolayer was mounted between side-by-side chambers, which enabled the simultaneous assay of dissolution and permeation of drugs (dissolution/permeation system; D/P system). Apical and basal sides of the chamber were filled with buffer solutions. Drugs were applied to the apical side as powder, suspension, or solution, and then, the permeated amounts into the basal side were monitored for 2 h. At the same time, dissolved amounts of drugs at the apical side were detected. The amount of drug applied to the D/P system was based on its in vivo clinical dose. Results. Sodium taurocholate (5 mM, apical side) and bovine serum albumin (4.5% w/v, basal side) increased the permeated amount of poorly water-soluble drugs. Both additives were considered to be effective at mimicking in vivo conditions of intestinal drug absorption. From the correlation between the permeated amount of 13 drugs (% dose/2 h) in the D/P system and their percentage dose absorbed in humans in vivo, this system was found to be useful in evaluating oral absorption of poorly water-soluble drugs. Conclusions. With attempts made to mimic the physiologic conditions of the human GI tract, in vivo oral absorption of drugs was quantitatively assessed in the D/P system in vitro. This system is quite useful to predict the oral absorption of poorly water-soluble drugs after administration as solid dosage forms.     

Controlled Suspensions Enable Rapid Determinations of Intrinsic Dissolution Rate and Apparent Solubility of Poorly Water-Soluble Compounds

Purpose

To develop a small-scale set-up to rapidly and accurately determine the intrinsic dissolution rate (IDR) and apparent solubility of poorly water-soluble compounds.

Methods

The IDR and apparent solubility (S_app) were measured in fasted state simulated intestinal fluid (FaSSIF) for six model compounds using wet-milled controlled suspensions (1.0% (w/w) PVP and 0.2% (w/w) SDS) and the μDISS Profiler. Particle size distribution was measured using a Zetasizer and the total surface area was calculated making use of the density of the compound. Powder and disc dissolution were performed and compared to the IDR of the controlled suspensions.

Results

The IDR values obtained from the controlled suspensions were in excellent agreement with IDR from disc measurements. The method used low amount of compound (μg-scale) and the experiments were completed within a few minutes. The IDR values ranged from 0.2–70.6 μg/min/cm² and the IDR/S_app ratio ranged from 0.015 to 0.23. This ratio was used to indicate particle size sensitivity on intestinal concentrations reached for poorly water-soluble compounds.

Conclusions

The established method is a new, desirable tool that provides the means for rapid and highly accurate measurements of the IDR and apparent solubility in biorelevant dissolution media. The IDR/S_app is proposed as a measure of particle size sensitivity when significant solubilization may occur.

     

Study of Dissolution Kinetics for Poorly Water-Soluble Drugs from Ternary Interactive Mixtures in Comparison with Commercially Available Capsules

Objective

The main objective of this work was to study the dissolution kinetics of poorly water-soluble drugs, indomethacin and ibuprofen, from formulated capsules or interactive mixtures containing fine lactose (FL), as ternary additive, and coarse lactose as carrier compared with selected commercially indomethacin capsules and to investigate the role of FL-drug size ratio on the dissolution.

Results and Discussion

It was found that the addition of FL in lactose-indomethacin capsules enhanced the dissolution of indomethacin while it has decreased the dissolution of ibuprofen from the lactose-ibuprofen mixtures. The particle size distributions for drugs and fine lactose used in this study suggested that the difference in dissolution behaviour for the two drugs could be due to the FL-drug ratio. Results obtained from the application of different dissolution kinetic equations showed that the first-order equation can best describe the kinetic of the dissolution for Rothacin®, Indylon®, Indomin® and ternary-formulated capsules of indomethacin, while the dissolution from the binary-formulated indomethacin capsules showed that the dissolution cannot be described by zero-order or first-order equations. For ibuprofen mixtures, the results showed that the release followed the first-order kinetic for both systems, binary and ternary mixtures. Results obtained from Peppas equation showed that all indomethacin formulations used in this study released the drug by Fickian release with release exponent (n) <0.45, while all ibuprofen formulations used in this study released the drug by non-Fickian (anomalous) release with release exponent (n) >0.45 and >0.89.

Conclusion

The FL-drug ratio could give an explanation to the enhanced dissolution of indomethacin and decreased dissolution of ibuprofen from interactive mixtures.     

Evaluation of Using Dogs to Predict Fraction of Oral Dose Absorbed in Humans for Poorly Water-Soluble Drugs

Dogs have been widely used to study the oral absorption of a drug in drug discovery. However, there has been no quantitative validation of using dogs to predict the fraction of oral dose absorbed (Fa) in humans (Fa_h) for poorly water-soluble drugs. Here, we report the results of using dogs for quantitative Fa_h prediction, focusing on poorly water-soluble free acid and neutral drugs. The Fa values of 4 acidic and 1 neutral proprietary compounds were measured in humans and dogs. Extensive literature survey was also performed to increase the number of Fa data. Fa_h and Fa in dogs (Fa_d) were then compared at equivalent body weight–normalized doses. In the case of neutral compounds, Fa_d was found to be similar to Fa_h. In the case of acidic compounds, Fa_d significantly overestimated Fa_h in most cases. A difference in intestinal pH was suggested as the main reason for this discrepancy. In conclusion, the use of dogs would not be appropriate to predict Fa_h for acidic compounds, but more work is required to know about neutral compounds.     

Enhancement of the Solubility and Efficacy of Poorly Water-Soluble Drugs by Hydrophobically-Modified Polysaccharide Derivatives

Purpose This work was intended to develop and evaluate a new polymeric system based on amphiphilic carboxymethylpullulans (CMP₄₉C₈ and CMP₁₂C₈) that can spontaneously self-assemble in aqueous solutions and efficiently solubilize hydrophobic drugs. Methods The self-assembling properties of CMP₄₉C₈ and CMP₁₂C₈ were characterized by fluorescence spectroscopy and surface tension measurements. The solubilization of benzophenone and docetaxel was assessed from surface tension measurements, UV spectrometry and HPLC assays. The in vitro cytoxicity of CMP₄₉C₈ solutions and the docetaxel commercial vehicle (Tween 80?/Ethanol–water) were evaluated in the absence and in the presence of docetaxel. Results Compared to CMP₁₂C₈, CMP₄₉C₈ in aqueous solutions appeared to self-organize into monomolecular aggregates containing hydrophobic nanodomains, and to significantly increase the apparent solubility of benzophenone. Docetaxel solubility could also be improved in the presence of CMP₄₉C₈ but to a lower extent due to the surface properties of the drug. Nevertheless, in vitro, the cytotoxicity studies revealed that against cancer cells, the CMP₄₉C₈-docetaxel formulation was equipotent to the commercial docetaxel one. Furthermore, in the absence of the drug, CMP₄₉C₈ appeared less cytotoxic against macrophages than the Tween? 80/Ethanol–water. Conclusions CMP₄₉C₈ is a good candidate for solubilizing hydrophobic drugs and could be applied to docetaxel formulations.     

Rate-Limiting Steps of Oral Absorption for Poorly Water-Soluble Drugs in Dogs; Prediction from a Miniscale Dissolution Test and a Physiologically-Based Computer Simulation

PURPOSE: Nonlinear oral absorption due to poor solubility often impedes drug development. The purpose of this study was to elucidate the rate-limiting process in oral absorption of Biopharmaceutical Classification System (BCS) class II (low solubility-high permeability) drugs in order to predict nonlinear absorption of dose caused by solubility-limited absorption. METHODS: Oral absorption of danazol, griseofulvin, and aprepitant was predicted from a miniscale dissolution test and a physiologically-based model. The effect of particle size reduction and dose increase on absorption was investigated in vitro and in vivo to clarify the rate-limiting steps of dissolution-rate-limited and solubility-limited absorption. RESULTS: The rate-limiting steps of oral absorption were simulated and increase in the dissolution rate and administration dose showed a shift from dissolution rate-limited to solubility-limited absorption. In the study in dogs, particle size reduction improved the oral absorption of large particle drugs but had little effect on small particle drugs. Dose nonlinearity was observed with small particles at a high dose. Our model quantitatively predicted results observed in vivo, including but not exclusively, dissolution-rate-limited and solubility-limited absorption. CONCLUSION: The present study provides a powerful tool to predict dose nonlinearity and will aid in the success of BCS class II drug development.     

Increasing the Dissolution Rate of Itraconazole Processed by Gas Antisolvent Techniques using Polyethylene Glycol as a Carrier

Purpose The purpose of this study was to increase the dissolution rate of the poorly water soluble, antifungal drug Itraconazole. Methods Itraconazole was successfully micronized using both the gas antisolvent (GAS) and aerosol solvent extraction systems (ASES) using Acetone as the solvent. The affects of operating conditions such as temperature, pressure and solvent choice on variables such as morphology, particle size and dissolution were investigated. The influence of temperature in the range 25 to 40°C and pressure between 90 and 190 bar were investigated. Results Solvent choice was found to have the largest affect on particle production, with acetone found to be the optimal solvent choice when compared with dimethyl formamide (DMF), tetrahydrofuran (THF) and dichloromethane (DCM). Itraconazole particles with an average particle size of 6.9 μm were formed at the optimal ASES processing conditions of 40°C and 190 bar. More significantly, in the first 100 minutes of dissolution 71.1% of the dense gas processed itraconazole was dissolved compared with 52.5% of Sporonox (the commercially available formulation) and 14.6% of the unprocessed material. Additional studies demonstrated that the formation of an itraconazole/PEG composite resulted in a 6-fold increase in dissolution rate in the first 100 min, to 89.8%, when compared to the unprocessed material. Conclusions Using ASES, microparticles of itraconazole were produced with an increased dissolution rate compared with raw material and commercially available product.     

Development and Characterization of Nanostructured Mists with Potential for Actively Targeting Poorly Water-Soluble Compounds into the Lungs

Purpose

To formulate nanoemulsions (NE) with potential for delivering poorly water-soluble drugs to the lungs.

Method

A self nanoemulsifying composition consisting of cremophor RH 40, PEG 400 and labrafil M 2125 CS was selected after screening potential excipients. The solubility of carbamazepine, a poorly water-soluble drug, was tested in the formulation components. Oil-in-water (o/w) NEs were characterized using dynamic light scattering, electrophoretic light scattering, transmission electron microscopy (TEM) and differential scanning calorimetry. NEs were nebulized into a mist using a commercial nebulizer and characterized using laser diffraction and TEM. An aseptic method was developed for preparing sterile NEs. Biocompatibility of the formulation was evaluated on NIH3T3 cells using MTT assay. In vitro permeability of the formulation was tested in zebra fish eggs, HeLa cells, and porcine lung tissue.

Results

NEs had neutrally charged droplets of less than 20 nm size. Nebulized NEs demonstrated an o/w nanostructure. The mist droplets were of size less than 5 μm. Sterility testing and cytotoxicity results validated that the NE was biocompatible and sterile. In vitro tests indicated oil nanodroplets penetrating intracellularly through biological membranes.

Conclusion

The nanoemulsion mist has the potential for use as a pulmonary delivery system for poorly water-soluble drugs.     

Formulation Development and Dissolution Rate Enhancement of Efavirenz by Solid Dispersion Systems

The aim of this study was to enhance the dissolution rate of efavirenz using solid dispersion systems (binary and ternary). A comparison between solvent and fusion method was also investigated. Solid dispersions of efavirenz were prepared using polyethylene glycol 8000, polyvinylpyrrolidone K30 alone and combination of both. Tween 80 was incorporated to obtain a ternary solid dispersion system. Dissolution tests were conducted and evaluated on the basis of cumulative percentage drug release and dissolution efficiency. Physicochemical characterizations of the solid dispersions were carried out using differential scanning calorimetric, powder X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. Dissolution was remarkably improved in both systems compared to pure efavirenz (P<0.05). An optimum ratio was identified at a drug:polymer of 1:10. Incorporation of Tween 80 to 1:10 formulations formed using solvent method showed further improvement in the dissolution rate. Physicochemical characterization results suggested that efavirenz existed in the amorphous form in all the solid dispersion systems providing evidence of improvement in dissolution. No statistically significant difference (P>0.05) in dissolution was observed between the two methods. Binary and ternary solid dispersion systems both have showed a significant improvement in the dissolution rate of efavirenz. Formulations with only polyvinylpyrrolidone K30 showed best dissolution profile and 1:10 was identified as an optimum drug-polymer weight ratio.     

Mechanistic Analysis of Cocrystal Dissolution,Surface pH,and Dissolution Advantage as a Guide for Rational Selection

The dissolution behavior of a dibasic drug ketoconazole under the influence of pH has been evaluated and compared to its three 1:1 cocrystals with diacidic coformers, fumaric acid, succinic acid (SUC), and adipic acid. Mass transport models were developed by applying Fick's law of diffusion to dissolution with simultaneous chemical reactions in the hydrodynamic boundary layer adjacent to the dissolving surface to predict the interfacial pH and flux of the parent drug and cocrystals. All 3 cocrystals have the ability to modulate the interfacial pH to different extents compared to the parent drug due to the acidity of the coformers. Dissolution pH dependence of ketoconazole is significantly reduced by the cocrystallization with acidic coformers. Due to the different dissolution pH dependence, there exists a transition pH where the flux of the cocrystal is the same as the parent drug. Below this transition pH, the drug flux is higher, but above it, the cocrystal flux is higher. The development of these mass transport models provide a mechanistic understanding of the dissolution behavior and help identify cocrystalline solids with optimal dissolution characteristics.     

Poly(2-Ethyl-2-Oxazoline) as an Alternative to Poly(Vinylpyrrolidone) in Solid Dispersions for Solubility and Dissolution Rate Enhancement of Drugs

Poly(2-ethyl-2-oxazoline) (PEOX), a biocompatible polymer considered as pseudopolypeptide, was introduced as a potential alternative to the commonly used polymer, poly(vinylpyrrolidone) (PVP) for the preparation of solid dispersion with a poorly soluble drug. Glipizide (GPZ), a Biopharmaceutical Classification System class II model drug, was selected for solubility and dissolution rate study. GPZ-polymer solid dispersions and physical mixtures were characterized and investigated by X-ray diffractometry, differential scanning calorimetry, scanning electron microscopy, and FTIR spectroscopy. The impact of polymers on crystal nucleation kinetics was studied, and PEOX exhibited strong inhibitory effect compared with PVP. Solubility and dissolution behavior of the prepared solid dispersions and their physical blends were in vitro examined and evaluated. A significant enhancement in GPZ solubility was obtained with PEOX compared with the pure drug and solid dispersion with PVP. A big improvement in the intrinsic dissolution rate (45 times) and dissolved amount of GPZ (58 times) was achieved with PEOX in fasted state simulated intestinal fluid, against comparable enhancement observed with PEOX and PVP in phosphate buffer at pH 6.8. Lower molecular weight of PEOX-5K (5000 g/mol) was found to be superior to higher molecular weight PEOX-50K (50,000 g/mol) in the improvement of dissolution behavior. The findings of this study with GPZ as a model drug introduce lower molecular weight PEOX as a promising polymeric carrier toward better oral bioavailability of poorly soluble drugs.     

Polar Molecular Surface as a Dominating Determinant for Oral Absorption and Brain Penetration of Drugs

Purpose. To study oral absorption and brain penetration as a function of polar molecular surface area. Methods. Measured brain penetration data of 45 drug molecules were investigated. The dynamic polar surface areas were calculated and correlated with the brain penetration data. Also the static polar surface areas of 776 orally administered CNS drugs that have reached at least Phase II efficacy studies were calculated. The same was done for a series of 1590 orally administered non-CNS drugs that have reached at least Phase II efficacy studies. Results. A linear relationship between brain penetration and dynamic polar surface area (Ų) was found (n = 45, R = 0.917, F_1,43 = 229). Brain penetration decreases with increasing polar surface area. A clear difference between the distribution of the polar surface area of the 776 CNS and 1590 non-CNS drugs was found. It was deduced that orally active drugs that are transported passively by the transcellular route should not exceed a polar surface area of about 120 Ų. They can be tailored to brain penetration by decreasing the polar surface to <60–70 Ų. This conclusion is supported by the inverse linear relationship between experimental brain penetration data and the dynamic polar surface area of 45 drug molecules. Conclusions. The polar molecular surface area is a dominating determinant for oral absorption and brain penetration of drugs that are transported by the transcellular route. This property should be considered in the early phase of drug screening.     

Evaluation of Bioequivalence of Highly Variable Drugs Using Monte Carlo Simulations. I. Estimation of Rate of Absorption for Single and Multiple Dose Trials Using Cmax

Purpose. A Monte Carlo simulation study was done to investigate the effects of high intrasubject variation in clearance (CL), and volume of distribution (V) on the calculation of the 90% confidence interval (CI) for Cmax for single dose and multiple dose studies. Methods. Simulations were done for both immediate release and sustained release scenarios. The simulated data were compared with clinical data from bioequivalence studies performed on indomethacin and verapamil. Results. Previous reviews and simulations have shown that the probability of failure for the Cmax for single dose studies was always greater than that for multiple dose studies. However, the results for the simulated scenarios currently investigated indicate that if intrasubject (period-to-period) variation in CL and V is high (% CV's above 25%, and 12%, respectively), multiple dose studies can exhibit a higher probability of failure for Cmax than do single dose studies. Furthermore, Cmax values from studies performed with a sustained release scenario are more sensitive to changes in Ka, CL, and V than are results of studies on immediate release products. As an example, the probability of failure for immediate release products in simulated single dose studies is about 11% and 21% when the mean difference in Ka is 10% and 20%, respectively; while, the probability of failure for multiple dose studies is about 36% regardless of the difference in Ka. The corresponding values for the probability of failure for sustained release products were 25%, 53% for single dose studies and 39% for multiple dose studies. The simulations also indicate that changes in the fraction absorbed have a greater effect on the estimation of Cmax in multiple dose regimens than in single dose studies. Conclusions. The results from these investigations indicate that multiple dose studies do not necessarily always reduce variability in Cmax.     

Predictions of the In Vivo Clearance of Drugs from Rate of Loss Using Human Liver Microsomes for Phase I and Phase II Biotransformations

Purpose The utility of in vitro metabolism to accurately predict the clearance of hepatically metabolized drugs was evaluated. Three major goals were: (1) to optimize substrate concentration for the accurate prediction of clearance by comparing to K_m value, (2) to prove that clearance of drugs by both oxidation and glucuronidation may be predicted by this method, and (3) to determine the effects of nonspecific microsomal binding and plasma protein binding. Methods The apparent K_m values for five compounds along with scaled intrinsic clearances and predicted hepatic clearances for eight compounds were determined using a substrate loss method. Nonspecific binding to both plasma and microsomal matrices were also examined in the clearance calculations. Results The K_m values were well within the 2-fold variability expected for between laboratory comparisons. Using both phase I and/or phase II glucuronidation incubation conditions, the predictions of in vivo clearance using the substrate loss method were shown to correlate with published human clearance values. Of particular interest, for highly bound drugs (>95% plasma protein bound), the addition of a plasma protein binding term increased the accuracy of the prediction of in vivo clearance. Conclusions The substrate loss method may be used to accurately predict hepatic clearance of drugs.     

Chitosans as Absorption Enhancers for Poorly Absorbable Drugs. 1: Influence of Molecular Weight and Degree of Acetylation on Drug Transport Across Human Intestinal Epithelial (Caco-2) Cells

Purpose. Chitosan has recently been demonstrated to effectively enhance the absorption of hydrophilic drugs such as peptides and proteins across nasal and intestinal epithelia (1–3). In this study, the effect of the chemical composition and molecular weight of chitosans on epithelial permeability and toxicity was investigated using monolayers of human intestinal epithelial Caco-2 cells as a model epithelium. Methods. Eight chitosans varying in degree of acetylation (DA) and molecular weight were studied. The incompletely absorbed hydrophilic marker molecule ¹⁴C-mannitol was used as a model drug to assess absorption enhancement. Changes in intracellular dehydrogenase activity and cellular morphology were used to assess toxicity. Results. Chitosans with a low DA (1 and 15%) were active as absorption enhancers at low and high molecular weights. However, these chitosans displayed a clear dose-dependent toxicity. Chitosans with DAs of 35 and 49% enhanced the transport of ¹⁴C-mannitol at high molecular weights only, with low toxicity. One chitosan (DA = 35%; MW = 170kD) was found to have especially advantageous properties such as an early onset of action, very low toxicity, and a flat dose-absorption enhancement response relationship. Conclusions. The structural features of chitosans determining absorption enhancement are not correlated with those determining toxicity, which makes it possible to select chitosans with maximal effect on absorption and minimal toxicity.