Dissolution kinetics of griseofulvin in sodium dodecylsulphate solutions

The rate-limiting step in the absorption of poorly water-soluble drugs is often the dissolution process of these compounds. Surface-active agents influence the dissolution process by wetting and solubilizing. In order to study the effect of solubilization, a rotating-disk apparatus was used with griseofulvin as a model drug substance and sodium dodecylsulphate (SDS) as a model surfactant. The dissolution kinetics of griseofulvin in SDS solutions could adequately be described by the convective-diffusion model and the phase-separation model for micellar systems. The calculated micellar diffusion coefficient of SDS was in close agreement with values reported in the literature.     

The application of the convective diffusion model and the film equilibrium model to surfactant-facilitated dissolution of gliclazide.

Gliclazide is practically insoluble in water, and has low dissolution rate. Therefore, it was of interest to improve its dissolution rate using anionic and cationic surfactants. The intrinsic dissolution rates of gliclazide in solutions of sodium dodecyl sulfate (SDS) and in solutions of tetradecyltrimethyl ammonium bromide (TDTMAB) were measured using the rotating disk method to study the convective diffusion transport of drug-loaded micelles. Two different approaches were applied to the experimental data; the convective diffusion model and the film equilibrium model. The two approaches are based on the same fundamental assumptions differing only in their interpretation of the diffusional boundary layer. The results obtained from the film equilibrium model were less satisfactory, and in case of TDTMAB the model was inapplicable (negative diffusion coefficient). While excellent results were obtained from the convective diffusion model. The free solute diffusion coefficient (D(s)) obtained experimentally was 2.47 x 10(-5) cm(2)/s, and the diffusion coefficient of the drug-loaded SDS micelle (D(sm)) estimated was 1.74 x 10(-6) cm(2)/s. The drug-loaded SDS micelle radius was 14 A. The thickness of the diffusional boundary layer was 54 and 22 microm for the free solute and the drug-loaded SDS micelle, respectively. TDTMAB showed lower effect in improving the dissolution rate of gliclazide than SDS. The drug-loaded TDTMAB micelle diffusion coefficient was 1.03 x 10(-6) cm(2)/s. The radius of the drug-loaded TDTMAB micelle and the boundary layer thickness were 24 A and 19 microm, respectively.     

Surfactant-mediated dissolution: contributions of solubility enhancement and relatively low micelle diffusivity

The objective of this study was to assess the contributions of surfactant-mediated solubility and micellar diffusivity on the ability of surfactant to enhance drug dissolution. The following model was derived to predict the degree to which surfactants enhance griseofulvin dissolution: phi = 1 + (fm/ff).((D(D-M)2/3)/(DD2/3)) where phi is the degree of surfactant-mediated dissolution enhancement, fm is the fraction of the drug in micelle, and ff is the fraction of free drug, and DD and D(D-M) are the diffusivities of free drug and drug-loaded micelles, respectively. The Wood apparatus was used to measure the dissolution of griseofluvin in the presence of the anionic surfactant sodium dodecyl sulfate (SDS), the cationic surfactant cetyl trimethyl ammonium bromide (CTAB), and the neutral surfactants Tween 80 and Cremophor EL. DD was estimated using the Levich equation. D(D-M) was measured using dynamic light scattering. Griseofulvin solubility was evaluated in SDS, CTAB, Tween 80, and Cremophor EL at the surfactant concentrations used in the dissolution studies. DD was 11.0 x 10(-6) cm2/s. D(D-M) was 1.29 x 10(-6) cm2/s, 0.956 x 10(-6) cm2/s, 0.569 x 10(-6) cm2/s, and 0.404 x 10(-6) cm2/s for griseofulvin-loaded micelles of SDS, CTAB, Tween 80, and Cremophor EL, respectively. At the highest surfactant concentrations studied, griseofulvin solubility increased 107-fold, 31-fold, fourfold, and threefold for SDS, CTAB, Tween 80, and Cremophor EL. Dissolution into SDS and CTAB were markedly enhanced, but only about one-third as much as solubility enhancement. Dissolution enhancement in the presence of SDS and CTAB were in excellent agreement with model predicted values, with prediction error less than 12%. The model predicted dissolution into Tween 80 and Cremophor EL to be minimally enhanced, as was observed, although the model underpredicted dissolution into these two neutral surfactants. The derived model predicted surfactant-mediated dissolution and reflects dissolution enhancement to be promoted by surfactant-enhanced solubility, but limited by the relatively slow diffusion of drug-loaded surfactant micelles.     

Interactions between a poorly soluble cationic drug and sodium dodecyl sulfate in dissolution medium and their impact on in vitro dissolution behavior

In the pharmaceutical industry, in vitro dissolution testing ofsolid oral dosage forms is a very important tool for drug development and quality control. However, ion-pairing interaction between the ionic drugand surfactants in dissolution medium often occurs, resulting in inconsistent and incomplete drug release. The aim of this study is toevaluate the effects ofsodium dodecyl sulfate (SDS) mediated medium onthe dissolution behaviors of a poorly soluble cationic drug (Drug B). The study was carried out by measuring solubility of Drug B substance and dissolution rate of Drug B product in media containing SDS.Desolubilization of Drug B substance was observed at pH 4.5 in the presence of SDS at concentrations below critical micelle concentration (CMC) which is attributed to the formation of an insoluble di-dodecyl sulfate salt between SDS and Drug B. This ion-pairing effect is less significant with increasing medium pH where Drug B is less ionized and CMC of SDS is lower. In medium at pH 4.5, dissolution of Drug B product was found incomplete with SDS concentration below CMC due to the desolubilization of Drug B substance. In media with SDS level above CMC, the dissolution rate is rather slower with higher inter-vessel variations compared to that obtained in pH 4.5 medium without SDS. The dissolution results demonstrate that the presence of SDS in medium generates unexpected irregular dissolution profiles for Drug B which are attributed to incompatible dissolution medium for this particular drug. Therefore, non-ionic surfactant was selected for Drug B product dissolution method and ion-pairing effect in SDS mediated medium should be evaluated when developing a dissolution method for any poorly soluble cationic drugs.     

Mass transport in dissolution kinetics. I: Convective diffusion to assess the role of fluid viscosity under forced flow conditions

The quantitative influence of viscosity on dissolution kinetics is assessed under laminar flow conditions by utilizing a convective diffusion model for drug dissolution. Functional dependency of three types of viscosity inducing agents is established with respect to the parameters of fluid flow rate, diffusivity, and solubility. Studies of aqueous solutions of sucrose and of glycerol demonstrate that the decrease in dissolution rate of ethyl p-aminobenzoate is related to the decrease in solute diffusivity in these solutions, whereas the solubility change in the glycerol solutions has an additional independent simultaneous effect. Dissolution in hydroxypropyl cellulose solutions remains constant under fixed fluid flow conditions because of the negligible effect of the polymer upon the drug diffusivity. A change in fluid flow rate, however, alters the dissolution rate and correlates quantitatively with the rate of shear in the convective diffusion model. The interpretation of the effect of viscosity on dissolution kinetics with the convective diffusion model explains these phenomena quantitatively in terms of the fundamental mass transport processes.     

Estimation of dissolution rate of salicylamide in complexing media using a theoretical diffusion model

Dissolution rates of salicylamide in water and caffeine solutions under perfect sink conditions were predicted by theoretical diffusion equations applicable to dissolution in complexing media. Experimental dissolution rates were measured using a compartmentalized rotating-basket apparatus under two sets of conditions. Agreement was found between experimental and predicted rates. Use of the theoretical equation for estimating dissolution rates involves simple calculations of diffusion coefficients and diffusion layer thickness under the operative dissolution conditions. The increase in dissolution rate caused by addition of the complexant can be calculated for diffusion-controlled dissolution directly if the stability constant and the drug solubility in water are known or measured.     

Slower dissolution rates of sulphamerazine in aqueous sodium dodecyl sulphate solutions than in water

The solubility of sulphamerazine in 0.1 M sodium dodecyl sulphate solution (SDS) was found to be 2.8-times that in water (at standard reporting conditions). The thermodynamics of transfer to the micelle were indicative of a spontaneous, enthalpy-driven, process. Contact angle and surface energy data revealed a favoured interaction between drug and SDS micelles, however, the dissolution rate of the drug into SDS decreased with increasing SDS concentration. The activation thermodynamics revealed a slight activation barrier to solubilisation. Apparent diffusion coefficients were calculated from initial dissolution rates from rotating disks, with different rotating speeds. The greatly reduced diffusion coefficient in the presence of micelles was taken as an explanation for the reduced rate of dissolution. The changes in solubility and diffusion rate have been considered in terms of the Noyes-Whitney equation, to show the combined effect on dissolution rate is as seen, i.e., reduced dissolution rate, despite the increased solubility. Care should be taken before assuming that an SDS solution is an appropriate medium for improving dissolution test data for poorly soluble drugs.     

Enhancement of the dissolution rate and oral absorption of a poorly water soluble drug by formation of surfactant-containing microparticles

The slow dissolution rate exhibited by poorly water-soluble drugs is a major challenge in the drug development process. Following oral administration, drugs with slow dissolution rates generally show erratic and incomplete absorption which may lead to therapeutic failure. The aim of this study was to improve the dissolution rate and subsequently the oral absorption and bioavailability of a model poorly water-soluble drug. Microparticles containing the model drug (griseofulvin) were produced by spray drying the drug in the absence/presence of a hydrophilic surfactant. Poloxamer 407 was chosen as the hydrophilic surfactant to improve the particle wetting and hence the dissolution rate. The spray dried particles were characterized and in vitro dissolution studies and in vivo absorption studies were carried out. The results obtained showed that the dissolution rate and absolute oral bioavailability of the spray dried griseofulvin/Poloxamer 407 particles were significantly increased compared to the control. Although spray drying griseofulvin alone increased the drug's in vitro dissolution rate, no significant improvement was seen in the absolute oral bioavailability when compared to the control. Therefore, it is believed that the better wetting characteristics conferred by the hydrophilic surfactant was responsible for the enhanced dissolution rate and absolute oral bioavailability of the model drug.     

缬沙坦固体分散体的制备及体外溶出度研究

目的采用冷冻干燥法制备缬沙坦（Valsartan）速释固体分散体（SD）来提高其体外溶出度。方法分别以羟丙甲基纤维素（HPMC）、聚乙二醇6000（PEG6000）、聚乙烯吡咯烷酮k30（PVPk30）为载体,十二烷基硫酸钠（SDS）为表面活性剂来制备不同比例的缬沙坦固体分散体,通过测定体外溶出度,来选择最优辅料及比例,结果当以PEG6000载体,SDS为表面活性剂时,且药物：PEG6000：SDS=1：5：1％时药物呈现了很好的水溶性。结论在5min时即可溶出90％以上,很大程度上提高了缬沙坦的体外溶出度。     

Mass transport in dissolution kinetics. II: Convective diffusion to assess role of viscosity under conditions of gravitational flow

Dissolution kinetics was studied in a laminar flow cell, through which aqueous solutions of increasing viscosity flowed under the force of gravity, in order to help elucidate the mechanism of drug dissolution. The viscosity was varied by addition of either hydroxypropyl cellulose, sucrose, or glycerin. The dissolution data were evaluated quantitatively in terms of a convective diffusion model for dissolution. It was demonstrated that the decrease in dissolution rate of a test compound due to viscosity in the polymer solution occurs primarily because of a decrease in the rate of shear over the dissolving surface. In solutions of sucrose or glycerin, the decrease in dissolution rate due to viscosity results because of a decrease in the diffusivity of the solute in addition to the decreased rate of shear. Also, the model accounts for the increase in solubility in the glycerin solution. Thus, the influence of viscosity on dissolution depends on whether the viscosity-inducing agent primarily affects only the flow properties of the liquid, or whether it affects diffusivity (i.e., a "micro" viscosity effect, in addition to the flow properties).     

Dissolution of anecortave acetate in a cylindrical flow cell: re-evaluation of convective diffusion/drug dissolution for sparingly soluble drugs

Steady-state drug release rates were measured from a model cylindrical implant, comprised mainly of the sparingly soluble drug anecortave acetate, suspended as an obstacle in a cylindrical flow cell. Dissolution medium was delivered at a steady, slow flow rate (0.05-0.7 mLs/min) using an HPLC pump, and samples from the outflow were analyzed by direct injection onto an HPLC column. Release rates were determined as a function of flow rate for three different implant orientations--vertical, elevated to the center of the dissolution cell; horizontal, elevated; and horizontal, resting directly upon the flat porous inlet frit. Release rates were ranked as follows: horizontal, floor > horizontal, elevated>vertical, elevated. The steady, laminar flow enabled use of the finite element method (FEM) to simulate the dissolution process using convective diffusion/drug dissolution theory. Simulations predicted the absolute magnitude of the release rate to within < 10% for all situations, and predicted the power law exponent of the dependence of release rate on flow rate with great accuracy. The current method is more general than compendial methods that provide a dissolving surface that is uniformly accessible to the dissolution medium, or a shear rate that is uniform across the entire dissolving surface. The current approach may be utilized to provide estimates of dissolution rates for any geometry and set of hydrodynamic conditions that can be numerically calculated.     

Evaluation of a convective diffusion drug dissolution rate model.

A recently introduced drug dissolution rate model based on convective diffusion was evaluated by experimentally determining dissolution rates. Alkyl p-aminobenzoates were used as the test compounds in a dissolution cell which promoted laminar flow of the liquid past the dissolving surface. The parameters evaluated were diffusivity, solubility, rate of shear, dissolving surface shape, and orientation of the surface relative to flow. The agreement between theory and experiment was quite satisfactory with respect to the functional dependence of the rate on these parameters as well as the actual magnitude of the rates.     

尼群地平固体分散体体外溶出度研究

目的:选择尼群地平固体分散体适宜的体外溶出介质。方法:测定尼群地平在多种溶出介质中的溶解度，通过体外溶出度试验比较尼群地平固体分散体在不同溶出介质中的溶出行为。结果:溶出介质1.0%和0.5%的十二烷基硫酸钠(SDS)水溶液可以满足“漏槽”条件，但药物释放较快，固体分散体间的溶出行为差异不明显；0.3%的SDS水溶液可以保证药物的全部溶出，且可以明显区别各固体分散体之间的溶出差异。结论:选用0.3%的SDS水溶液作为溶出介质，便于通过溶出度试验来筛选固体分散体处方。     

Some factors affecting the release of imipramine from gel-precipitated aluminium hydroxide spheres

Changing the pH of the dissolution medium has been found to affect the release of imipramine from gel-precipitated aluminium hydroxide spheres. Release from unwashed, unheated spheres into solutions of pH 1.2 was controlled by dissolution of the gel matrix, whereas that into solutions of pH 3 and pH 5 appeared to be under diffusion control. The liberation of drug from unwashed, heated spheres into the media of higher pH exhibited more complex kinetics. Washed spheres failed to release significant amounts of imipramine into the solutions of pH 3 and 5. Changing the ionic strength of the media had little effect on drug release. These phenomena have been explained with reference to model theories of the precipitation and ageing of aluminum hydroxide gels and their pH-solubility profiles.     

离子强度对丙基甲基纤维素骨架片释药的影响

目的：探讨溶出介质的离子强度对难溶性药物的丙基甲基纤维素（HPMC）骨架片释药的影响。方法：以甲氧苄胺嘧啶、卡马西平、磺胺甲恶唑和茶碱4种难溶性药物为模型药物，测定5种不同离子强度介质（水、0．20％氯化钠溶液、0．50％氯化钠溶液、0．90％氯化钠溶液和1．80％氯化钠溶液）下的药物释放度和溶出参数。结果：难溶性药物的HPMC骨架片释药随着其溶出介质离子强度的增加而减慢。结论：释药速率与离子强度之间存在较好的线性负相关关系。     

Inulin and poly(acrylic acid) grafted inulin for dissolution enhancement and preliminary controlled release of poorly water-soluble Irbesartan drug

In this article, inulin and poly(acrylic acid) grafted inulin copolymer were used to enhance the dissolution of poorly water-soluble Irbesartan drug and to control its drug release rate, respectively. Topological structure of inulin showed sleazy separable flower-like platelets and granules accumulated above each other, which adapt it to physically bind Irbesartan drug and enhance its dissolution. Consequently, the increase of inulin content in the polymeric matrix was found to increase the drug dissolution gradually until it reaches its maximum (～90%) within the first 60 min. The release rate had followed zero-order transport mechanism. On the other hand, the poly(acrylic acid) grafted inulin copolymer, characterized using (1)H NMR, FTIR, TGA, and SEM techniques, was found to form highly consistent amorphous systems of two-dimensional surfaces with some voids topology. Such features adapted it to control Irbesartan drug dissolution (～33%) and show Fickian diffusion mechanism.     

界面活性剂对阿斯匹林片剂溶出速度的影响及其机制的研究

本文考察了三种不同类型界面活性剂月桂醇硫酸钠(SDS),吐温-80(TEN)及十六烷基三甲基溴化铵(CAB),对难溶性固体药物阿斯匹林(ASPN)片剂溶出速度的影响,并进一步探讨了其作用机制。实验表明界面活性剂CAB和TEN对ASPN具有改善润湿及反絮凝作用,可明显增加水中ASPN的溶解及其片剂的溶出,胶团增溶作用经测定是很有限的,不能说明ASPN水中溶解的显著增加。界面活性剂SDS对ASPN的润湿、增溶及其片剂的溶出均没有明显作用。认为与界面活性剂及药物分子的结构和性质密切相关。     

A novel in vitro release method for submicron-sized dispersed systems

Sink conditions are often violated when using conventional release methods for dispersed systems. A novel reverse dialysis bag method was designed to overcome this problem. Model drug transport rates from submicron emulsions obtained using the conventional diffusion cell method and this novel method were compared. In the side-by-side diffusion cell method, emulsions were placed in the donor chamber and surfactant/buffer solutions in the receiver chamber. In the novel dialysis bag method, emulsions were diluted infinitely in the donor phase and surfactant/buffer solutions were placed in the receiver phase (dialysis bags). Slow release rates and linear release profiles were obtained using the side-by-side diffusion cell method apparently due to limited model drug solubility in the donor chamber resulting in violation of sink conditions. Biphasic release profiles were obtained using the dialysis bag method apparently due to an initial rapid release of free and micellar solubilized model drug from the donor to the receiver chambers followed by slow release from the oil droplets. Using both release methods, an initial increase and latter decrease in release rates were observed with increase in surfactant concentration. The initial increase was considered to be due to a decrease in the model drug oil-in-water partition coefficients and the subsequent decrease in release rates was due to micellar shape change (spheres to rods) causing a decrease in diffusion rates. Sink conditions were violated using the side-by-side diffusion cell method but were maintained in the dialysis bag method since emulsions were diluted infinitely in the donor phase.     

氢氯噻嗪HPMC骨架片释药机制影响因素研究

目的研究影响HPMC骨架释药机制的因素。方法以氢氯噻嗪为模型药物,HPMC为骨架材料,制备缓释片,测定释放度并描述其释放机制。考察各处方因素和释放条件对释放机制的影响。结果 HPMC用量越大、难溶性填充剂,释放机制越趋向于零级释放。较低的HPMC用量、水溶性填充剂,释放机制趋向于Higuchi和Peppas方程,释药行为以溶蚀和扩散相结合。结论 HPMC型号、片剂的硬度、制备方法、溶出度测定方法、转速对释放机制影响不大。HPMC用量、填充剂的性质、溶出介质的离子强度影响释放机制。     

SDS-aided immobilization and controlled release of camptothecin from agarose hydrogel.

Camptothecin (CPT), known to be an effective anticancer drug, has a limited therapeutic utility because of its poor water solubility. In this work, an approach has been made to overcome the limitation. CPT was first incorporated into the micelles formed from an ionic surfactant, sodium dodecyl sulfate (SDS) and the micellar drug aqueous solution was then used in preparation of the agarose hydrogel. It has been found that the presence of SDS greatly increased the solubility of CPT in water. For example, in 1 ml of 1.0 wt.% SDS water solution, 0.11 mg CPT could be solubilized (0.318 mM), which was 83 times the solubility in pure water. It was the hydrophobic cores of the SDS micelles that were able to accept the lipophilic drug to form stable drug-immobilized micelles. The formulation of a hydrogel using the drug-immobilized micelles has allowed us to obtain a unique and novel drug release system where the drug molecules are encapsulated by the micelles and the drug-containing micelles are dispersed in the gel network. The release of CPT from the so deliberately fabricated agarose hydrogel system has been studied as a function of surfactant concentration at 37 degrees C. The diffusion coefficients of CPT obtained by fitting to Fick's law ranged from 2.12 to 7.36 x 10(-7)cm(2)s(-1). The results showed that SDS prolonged the drug release by reducing the diffusion coefficient of CPT in the gel.