Preformulation study of the inclusion complex irbesartan-beta-cyclodextrin

The aim of the present work was to improve the solubility and dissolution profile of irbesartan (IRB), a poorly water-soluble drug by formation of inclusion complex with beta-cyclodextrin (betaCD). Phase solubility studies revealed increase in solubility of the drug upon cyclodextrin addition, showing A(L)-type of graph with slope less than one indicating formation of 1:1 stoichiometry inclusion complex. The stability constant (K(s)) was found to be 104.39 M(-1). IRB-betaCD binary systems were prepared by cogrinding, kneading using alcohol, kneading using aqueous alcohol, and coevaporation methods. Characterization of the binary systems were carried out by differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and proton nuclear magnetic resonance. The dissolution profiles of inclusion complexes were determined and compared with those of IRB alone and physical mixture. Among the various methods, coevaporation was the best in which the solubility was increased and dissolution rate of the drug was the highest. The study indicated the usefulness of cyclodextrin technology to overcome the solubility problem of IRB.

Kinetic solubility and dissolution velocity of rutin nanocrystals

Lyophilized rutin nanocrystals were intensively evaluated regarding their physicochemical properties with respect to particle size analyses, crystallinity, kinetic solubility and dissolution behavior. The particle size was determined by photon correlation spectroscopy (PCS) and laser diffraction (LD). DSC and X-ray diffraction were used to study the crystalline state of rutin nanocrystals. In a period of 1 week, the kinetic solubility was determined using a shaker at 25 degrees C. DSC and X-ray diffraction analyses showed that lyophilized rutin nanocrystals prepared by high pressure homogenization remained in crystalline state. Lyophilized rutin nanocrystals could be re-dispersed completely in water and the kinetic solubility in water increased to 133 microg/ml.. Lyophilized rutin nanocrystals were almost completely dissolved within 15 min in water, buffer of pH 1.2 and buffer of pH 6.8. In contrast, only 70% of rutin raw material (rutin microcrystals) was dissolved within 15 min. The superior physicochemical properties of rutin nanocrystals should overcome the absorption problem in the gastrointestinal tract and increase the bioavailability.

New cyclodextrin dimers and trimers capable of forming supramolecular adducts with shape-specific ligands

Bridged cyclodextrin dimers and trimers, in which respectively two and three hydrophobic cavities lie in close proximity, display much higher binding affinities and molecular selectivities than do parent cyclodextrins (CDs). By joining betaCD units with links inserted at different positions (2-2', 3-2', 6-2' or 6-2'-6'') and interposing spacers of different lengths and shapes, multicavity structures can be synthesized that are precisely tailored to fit specific guest molecules. This enzyme-mimicking strategy can also be used to generate stable supramolecular adducts. A series of CD dimers and trimers was prepared in good yields by carrying out the critical synthetic steps under power ultrasound (US) or microwave (MW) irradiation. Starting from azide and acetylenic CD derivatives, we exploited an efficient MW-promoted Huisgen 1,3-dipolar cycloaddition in the presence of Cu(I) salts. The resulting bridged CD derivatives gave stable adducts with magnetic-resonance-imaging contrast agents (MRI CAs) containing gadolinium(III) chelates. These inclusion complexes were found to be 2 to 3 orders of magnitude more stable than those formed by betaCD and to be endowed with high relaxivity values.

Preparation and solid-state characterization of inclusion complexes formed between miconazole and methyl-beta-cyclodextrin

The aim of this study is to confirm the formation of inclusion complexes between miconazole (MCZ) and two derivatives of beta-cyclodextrin, methyl-beta-cyclodextrin (MbetaCD) and 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) in aqueous solution by phase solubility studies. Inclusion complexes with MbetaCD in the solid state were then prepared by different methods, i.e., kneading, coevaporation (COE), spray-drying (SD), and lyophilization (LPh). The physicochemical properties of these complexes were subsequently studied by means of differential scanning calorimetry, Fourier transform infrared spectroscopy, scanning electron microscopy, and X-ray diffraction techniques. Phase solubility diagrams with MbetaCD and HPbetaCD were classified as A(P) type, indicating the formation of 1:1 and 1:2 stoichiometric inclusion complexes. The apparent stability constants (K(S)) calculated from the phase solubility diagram were 145.69 M(-1) (K(1:1)) and 11.11 M(-1) (K(1:2)) for MbetaCD and 126.94 M(-1) (K(1:1)) and 2.20 M(-1) (K(1:2)) for HPbetaCD. The method of preparation of the inclusion complexes in the solid state was shown to greatly affect the properties of the formed complex. Hence, the LPh, SD, and COE methods produce true inclusion complexes between MCZ and MbetaCD. In contrast, crystalline drug was still clearly detectable in the kneaded (KN) product.

Mucoadhesive microspheres for gastroretentive delivery of acyclovir: in vitro and in vivo evaluation

The aim of the present investigation was to evaluate the potential use of mucoadhesive microspheres for gastroretentive delivery of acyclovir. Chitosan, thiolated chitosan, Carbopol 71G and Methocel K15M were used as mucoadhesive polymers. Microsphere formulations were prepared using emulsion-chemical crosslinking technique and evaluated in vitro, ex-vivo and in-vivo. Gelatin capsules containing drug powder showed complete dissolution (90.5 +/- 3.6%) in 1 h. The release of drug was prolonged to 12 h (78.8 +/- 3.9) when incorporated into mucoadhesive microspheres. The poor bioavailability of acyclovir is attributed to short retention of its dosage form at the absorption sites (in upper gastrointestinal tract to duodenum and jejunum). The results of mucoadhesion study showed better retention of thiolated chitosan microspheres (8.0 +/- 0.8 h) in duodenal and jejunum regions of intestine. The results of qualitative and quantitative GI distribution study also showed significant higher retention of mucoadhesive microspheres in upper GI tract. Pharmacokinetic study revealed that administration of mucoadhesive microspheres could maintain measurable plasma concentration of acyclovir through 24 h, as compared to 5 h after its administration in solution form. Thiolated chitosan microsphere showed superiority over the other formulations as observed with nearly 4.0-fold higher AUC(0-24) value (1,090 +/- 51 ng h/ml) in comparison to drug solution (281 +/- 28 ng h/ml). Overall, the result indicated prolonged delivery with significant improvement in oral bioavailability of acyclovir from mucoadhesive microspheres due to enhanced retention in the upper GI tract.

Physicochemical Characteristics of Molecules and Their Diffusion across Human Vaginal Mucosa

The diffusion rate of permeant molecules through mucosal tissue depends on the physicochemical characteristics of the molecules themselves as well as the properties of the tissue. In this study the diffusion kinetics of various molecules was examined through intact as well as de-epithelialised human vaginal mucosa. The molecules studied included tritium-labelled water, 17β-estradiol, reduced-arecoline, vasopressin, oxytocin, bradykinin, tacrolimus, cyclosporin A, dihydro-alprenolol, cimetidine and benzylpenicillin. Freshly harvested human vaginal tissue was frozen in liquid nitrogen and stored at −85°C. A flow-through diffusion apparatus was used for the in vitro permeability studies (24 h, 20°C, 1.5 ml/h). The mean estimated – or mean steady-state flux values for all the molecules studied across intact human vaginal mucosa, were generally found to be lower than those of the corresponding de-epithelialised tissue. Using an F-test and comparing whole curves, statistically significant differences in the diffusion rates of tacrolimus, reduced-arecoline, vasopressin, bradykinin, benzylpenicillin, water and cimetidine were found when comparing intact and de-epithelialised vaginal mucosa. Generally, smaller permeant molecules diffused at a higher rate than larger molecules. The epithelial layer retarded the diffusion rate of molecules carrying charges at physiological pH. Damage to the epithelial layer did not necessarily increase the diffusion rate of all molecules tested and small lipophilic molecules did not necessarily diffuse at higher rates than hydrophilic molecules.

Effect of beta-cyclodextrin and hydroxypropyl beta-cyclodextrin complexation on physicochemical properties and antimicrobial activity of cefdinir

The solid-state properties, dissolution profile and antimicrobial activity of inclusion complexes of cefdinir (CEF) with beta-cyclodextrin (betaCD) and hydroxypropyl beta-cyclodextrin (HPbetaCD) were investigated. The phase solubility profiles of cefdinir with betaCD and HPbetaCD were classified as A(L)-type, which indicates the formation of 1:1 stoichiometry inclusion complexes. Stability constants with 1:1 molar ratio obtained from the phase solubility diagrams were 120.38+/-1.07 and 58.60+/-1.20 M(-1) for betaCD and HPbetaCD, respectively. Binary systems of CEF with betaCD and HPbetaCD prepared by kneading method were characterized by Fourier transformation-infrared spectroscopy (FTIR) and X-ray powder diffractometry (XRD). The aqueous solubility of CEF was enhanced by 101% for betaCD and 23.4% for HPbetaCD, respectively. The dissolution profiles of inclusion complexes were determined and compared with those of CEF alone and their physical mixtures. The dissolution rate of CEF was increased by betaCD and HPbetaCD inclusion complexation moderately. However, the antimicrobial activity of CEF was increased significantly (p<0.001) by betaCD and HPbetaCD inclusion complexation against S. aureus and E. coli. In all these studies, HPbetaCD had superior antimicrobial activity than that of betaCD while betaCD had greater effect on solubility enhancement of CEF.

The utility of cyclodextrins for enhancing oral bioavailability

Cyclodextrins (CD) have been utilized extensively in pharmaceutical formulations to enhance oral bioavailability. A critical review of the literature in which cyclodextrins were utilized for this purpose was conducted. The goal of this review was to determine if quantitative guidelines for drug and cyclodextrin properties necessary for bioavailability enhancement using cyclodextrins could be extracted. Twenty-eight studies were examined in which the focus was on the use of cyclodextrins as solubilizers to enhance bioavailability. Commonly observed factors included: utilization of pre-formed complex rather than physical mixtures, drug hydrophobicity (logP > 2.5), low drug solubility (typically< 1 mg/ml), moderate binding constant (< 5000 M(-1)), low dose (< 100 mg), and low CD:drug ratio (< 2:1). These general guidelines, however, did not apply to all studies. Quantitative guidelines useful to a formulation scientist considering the use of cyclodextrins were difficult to develop due to missing information and the complicated manner in which drug and cyclodextrin properties interact to influence key drug delivery processes (e.g., dissolution, absorption). The mechanisms by which cyclodextrins influence these processes, again emphasizing solubilization capabilities, are discussed to provide further insight into why cyclodextrins will increase bioavailability in certain cases but not influence or possibly decrease bioavailability in others.

Effects of Cyclodextrins on Drug Delivery Through Biological Membranes

Cyclodextrins have proven themselves to be useful functional excipients. Cyclodextrin derivatives can be hydrophilic or relatively lipophilic based on their substitution and these properties can give insight into their ability to act as permeability enhancers. Lipophilic cyclodextrins such as the methylated derivatives are thought to increase drug flux by altering barrier properties of the membrane through component extraction or fluidization. The hydrophilic cyclodextrin family also modulate drug flux through membranes but via different mechanisms. The current effort seeks to provide various explanations for these observations based on interactions of hydrophilic cyclodextrins with the unstirred water layer that separates the bulk media from biological membranes such as the gastric mucosa, cornea and reproductive tract. Theories on the serial nature of resistances to drug flux are used to explain why hydrophilic cyclodextrins can enhance drug uptake in some situation (i.e., for lipophilic material) but not in others. In addition, the nature of secondary equilibria and competition between cyclodextrins and rheologically important biopolymers such as mucin are assessed to give a complete picture of the effect of these starch derivatives. This information can be useful not only in understanding the actions of cyclodextrin but also in expanding their application and uses.

Use of a screening method to determine excipients which optimize the extent and stability of supersaturated drug solutions and application of this system to solid formulation design

Assessing the effect of excipients on the ability to attain and maintain supersaturation of drug-based solution may provide useful information for the design of solid formulations. Judicious selection of materials that affect either the extent or stability of supersaturating drug delivery systems may be enabling for poorly soluble drug candidates or other difficult-to-formulate compounds. The technique suggested herein is aimed at providing a screening protocol to allow preliminary assessment of these factors based on small to moderate amounts of drug substance. A series of excipients were selected that may, by various mechanisms, affect supersaturation including pharmaceutical polymers such as HMPC and PVP, surfactants such as Polysorbate 20, Cremophor RH40 and TPGS and hydrophilic cyclodextrins such as HPbetaCD. Using a co-solvent based method and 25 drug candidates, the data suggested, on the whole, that the surfactants and the selected cyclodextrin seemed to best augment the extent of supersaturation but had variable benefits as stabilizers, while the pharmaceutical polymers had useful effect on supersaturation stability but were less helpful in increasing the extent of supersaturation. Using these data, a group of simple solid dosage forms were prepared and tested in the dog for one of the drug candidates. Excipients that gave the best extent and stability for the formed supersaturated solution in the screening assay also gave the highest oral bioavailability in the dog.

Effect of the unstirred water layer on permeability enhancement by hydrophilic cyclodextrins

Saturated solutions of three test compounds, carbamazepine, griseofulvin and hydrocortisone, were prepared in aqueous buffer (pH 7.4) containing 0, 1, 5 and 10% HPbetaCD. The permeability and flux of the drugs though a PAMPA membrane at different unstirred water layer (UWL) thicknesses was determined. In absence of HPbetaCD, permeability coefficients increased two- to three-fold with decreasing UWL thickness to a certain minimum values of about 40 microm. Addition of HPbetaCD to systems exhibiting larger UWL thicknesses significantly increased compound flux. The effect of HPbetaCD was linked to its association constant (K(1:1)) with the model drugs and decreased with decreasing UWL thickness to a certain minimum value. This suggests that hydrophilic cyclodextrins enhance flux when the UWL resistance significantly contributes to the overall barrier resistance.

Influence of sulfobutyl ether beta-cyclodextrin (Captisol) on the dissolution properties of a poorly soluble drug from extrudates prepared by hot-melt extrusion

The aim of this study was to investigate the influence of sulfobutyl ether beta-cyclodextrin (SBE(7)-beta-CD; Captisol on the dissolution properties of a poorly water-soluble drug from extrudates prepared by hot-melt extrusion. Ketoprofen was employed as a model drug. Extrudates containing the parent beta-cyclodextrin (beta-CD) were also produced for comparative evaluation to assess the benefits of SBE(7)-beta-CD. Hot-melt extrudates were produced at 100 degrees C, which was close to the melting point of ketoprofen. The physiochemical properties and the in vitro drug release properties of ketoprofen from extrudates were investigated and compared with samples prepared by physical mixing, co-grinding, freeze-drying and heat-treatment. The solubilizing effects and the interactions of ketoprofen with SBE(7)-beta-CD and beta-CD were investigated using phase solubility and NMR studies, respectively. The dissolution rate of ketoprofen from samples prepared by hot-melt extrusion with SBE(7)-beta-CD was significantly faster than both the physical mixture and the hot-melt extrudates prepared with the parent beta-CD. Moisture absorption studies revealed that the hygroscopic nature of SBE(7)-beta-CD led to particle aggregation and a corresponding decrease in drug release rate for all samples. However, the samples prepared by melt extrusion were least affected by exposure to elevated humidity.

Cyclodextrins as pharmaceutical solubilizers

Cyclodextrins are useful functional excipients that have enjoyed widespread attention and use. The basis for this popularity from a pharmaceutical standpoint, is the ability of these materials to interact with poorly water-soluble drugs and drug candidates resulting in an increase in their apparent water solubility. The mechanism for this solubilization is rooted in the ability of cyclodextrin to form non-covalent dynamic inclusion complexes in solution. Other solubilizing attribute may include the ability to form non-inclusion based complexes, the formation of aggregates and related domains and the ability of cyclodextrins to form and stabilize supersaturated drug solutions. The increase in solubility also can increase dissolution rate and thus improve the oral bioavailability of BCS Class II and IV materials. A number of cyclodextrin-based products have reached the market based on their ability to camouflage undesirable physicochemical properties. This review is intended to give a general background to the use of cyclodextrin as solubilizers as well as highlight kinetic and thermodynamic tools and parameters useful in the study of drug solubilization by cyclodextrins.

Enhanced oral delivery of antimony from meglumine antimoniate/beta-cyclodextrin nanoassemblies

The composition comprising the highly water-soluble drug meglumine antimoniate (MA) and beta-cyclodextrin (beta-CD) was shown previously to enhance the absorption of Sb by oral route and render MA orally active in a murine model of cutaneous leishmaniasis. This unexpected behaviour was attributed, in part, to the fact that the heating of equimolar mixture of MA and beta-CD (first step of preparation of MA/beta-CD composition) induced the depolymerization of MA from high-molecular weight Sb complexes into 1:1 Sb-meglumine complex, resulting in an enhanced oral bioavailability of Sb. In the present work, we demonstrate that the heated MA+beta-CD mixture still produced significantly lower serum Sb levels when compared to the MA/beta-CD composition, indicating that the freeze-drying process (second step of preparation of MA/beta-CD composition) is required for achieving a high absorption of Sb by oral route. To get insight into the physicochemical alterations induced by the freeze-drying step, the MA/beta-CD composition was further characterized by circular dichroism, (1)H NMR and ESI(-)-MS and photon correlation spectroscopy. The freeze-drying process was found to promote the formation of supramolecular nanoassemblies with a mean hydrodynamic diameter of 190 nm, comprising 1:2:1, 2:2:1 and 2:2:2 NMG-Sb-beta-CD complexes. Another important observation was the ability of the MA/beta-CD composition to act as a sustained release system of the antimonial drug MA, suggesting that this property may result in the change of the drug absorption site in the gastrointestinal tract. A model is proposed for the mechanisms involved in the enhanced absorption of Sb from the MA/beta-CD composition.

Effects of the antiepileptic drug carbamazepine on human erythrocytes

The structural effects of the antiepileptic drug carbamazepine (CBZ) on the human erythrocyte membrane and molecular models have been investigated in the present work. This report presents the following evidence that CBZ interacts with red cell membranes: (a) X-ray diffraction and fluorescence spectroscopy of phospholipid bilayers showed that CBZ perturbed a class of lipids found in the outer moiety of the erythrocyte membrane; (b) in isolated unsealed human erythrocytes (IUM) the drug induced a disordering effect on the polar head groups and acyl chains of the membrane lipid bilayer; (c) in scanning electron microscopy (SEM) studies on human erythrocytes the formation of echinocytes was observed, due to the preferential insertion of CBZ in the outer monolayer of the red cell membrane. The effects of the drug detected in the present work were observed at concentrations of the order of those currently appearing in serum when it is therapeutically administered. This is the first time that toxic effects of carbamazepine on the human erythrocyte membrane have been described.

Permeability assessment of poorly water‐soluble compounds under solubilizing conditions: The reciprocal permeability approach

The objective of this study was to develop a general method to assess the intestinal permeability of poorly water-soluble drugs where low-aqueous drug solubility requires conduct of experiments under solubilizing experimental conditions. The permeability (Papp) of diazepam (DIA) was assessed across excised rat jejunum in the absence (Pappcontrol) and presence (Pappuncorr) of polysorbate-80 (PS-80). The micellar association constant (Ka) of DIA, estimated via equilibrium solubility studies, was used to correct Pappuncorr data and obtain an estimate of the true permeability coefficient (Pappcorr). An alternate approach was also developed (the reciprocal permeability approach) to allow direct estimation of Pappcorr without the need for independent estimation of Ka. The approach was further examined experimentally using a range of model drugs. DIA Pappcorr values obtained using the Ka from equilibrium solubility studies deviated from Papp(control) values, especially at PS-80 concentrations above 0.1% w/v. In contrast, data obtained using the reciprocal permeability method were consistent with Pappcontrol across the PS-80 concentration range. Similar trends were observed with propranolol (PRO), antipyrine (ANT), naproxen (NAP), and cinnarizine (CIN). The reciprocal permeability approach therefore provides a simple and accurate method by which the permeability of poorly water-soluble compounds may be estimated under solubilizing conditions.

Evaluation of cyclodextrin solubilization of drugs

The most common stoichiometry of drug/cyclodextrin complexes is 1:1, i.e. one drug molecule forms a complex with one cyclodextrin molecule, and the most common method for stoichiometric determination during formulation studies is the phase-solubility method. However, in recent years it has becoming increasingly clear that solubilizing effects of cyclodextrins are frequently due to the formation of multiple inclusion and non-inclusion complexes. The aqueous solubility of 38 different drugs was determined in pure aqueous solution, aqueous buffer solutions and aqueous cyclodextrin solutions, and the apparent stability constant (K1:1) of the 1:1 drug/cyclodextrin complexes calculated by the phase-solubility method. For poorly soluble drugs (aqueous solubility <0.1mM) the intrinsic solubility (S0) is in general much larger than the intercept of the phase-solubility diagram (Sint) resulting in non-linearity of otherwise linear (AL-type) phase-solubility diagram. This can lead to erroneous K(1:1)-values. A more accurate method for determination of the solubilizing efficiency of cyclodextrins is to determine their complexation efficiency (CE), i.e. the concentration ratio between cyclodextrin in a complex and free cyclodextrin. CE is calculated from the slope of the phase-solubility diagrams, it is independent of both S0 and Sint, and more reliable when the influences of different pharmaceutical excipients on the solubilization are being investigated.

Investigation of drug-cyclodextrin complexes by a phase-distribution method: some theoretical and practical considerations

The purpose of the study was to evaluate an octanol-water phase distribution method for investigation of drug/cyclodextrin (D/CD) complexes and to compare stability constant values obtained by this method to values obtained by the phase solubility method. A general equation for determination of 1 : 1 D/CD complex stability constant (K1 : 1) from the slope of a phase-distribution diagram (a diagram of the reciprocal of the apparent partition coefficient vs. the total CD concentration) was derived. The equation accounted for the possible inclusion of the organic solvent in the CD cavity and the gradual saturation of the CD binding with increasing concentration of the guest compound. This method was used to determine K1 : 1 for 2-hydroxypropyl-beta-cyclodextrin (HPbetaCD) complexes of hydrocortisone, prednisolone, diazepam, beta-estradiol and diethylstilbestrol. These values were comparable to K1 : 1 values determined by the phase-solubility method. The phase-distribution method could also be applied to determine stability constants for the neutral and ionic forms of the weakly acidic drugs, naproxen and triclosan and the weakly basic drug lidocaine. The phase-distribution method is a very versatile and fast method and has the advantage, compared to the phase-solubility method, that it only requires very small drug samples. Thus, this method would be suitable for screening of new drug candidates.

Cyclodextrins in drug delivery

Cyclodextrins are a family of cyclic oligosaccharides with a hydrophilic outer surface and a lipophilic central cavity. Cyclodextrin molecules are relatively large with a number of hydrogen donors and acceptors and, thus, in general they do not permeate lipophilic membranes. In the pharmaceutical industry cyclodextrins have mainly been used as complexing agents to increase aqueous solubility of poorly soluble drugs, and to increase their bioavailability and stability. Studies in both humans and animals have shown that cyclodextrins can be used to improve drug delivery from almost any type of drug formulation. However, the addition of cyclodextrins to existing formulations without further optimisation will seldom result in acceptable outcome. Currently there are approximately 30 different pharmaceutical products worldwide containing drug/cyclodextrin complexes on the market.

Cyclodextrin-based pharmaceutics: past, present and future

Cyclodextrins are cyclic oligomers of glucose that can form water-soluble inclusion complexes with small molecules and portions of large compounds. These biocompatible, cyclic oligosaccharides do not elicit immune responses and have low toxicities in animals and humans. Cyclodextrins are used in pharmaceutical applications for numerous purposes, including improving the bioavailability of drugs. Current cyclodextrin-based therapeutics are described and possible future applications discussed. Cyclodextrin-containing polymers are reviewed and their use in drug delivery presented. Of specific interest is the use of cyclodextrin-containing polymers to provide unique capabilities for the delivery of nucleic acids.