Solution-mediated phase transformation of salts during dissolution: investigation using haloperidol as a model drug

Soluble salts can undergo solution-mediated phase transformation to a lower solubility form due to pH gradients in the gastrointestinal tract. Therefore, dissolution rate rather than solubility may be the best predictor of bioavailability for such compounds. The purpose of this project was to examine the kinetics of the conversion of a basic compound, haloperidol, and its salt forms using a flow-through dissolution apparatus and rotating disk method in neutral conditions. The effects of buffer concentration, salt form, dissolution apparatus, and hydrodynamics were examined. Raman microscopy was used to characterize solids after dissolution. Haloperidol mesylate and haloperidol chloride showed a decrease in dissolution rate with time in the dissolution media. Haloperidol mesylate and haloperidol chloride dissolution rates also decreased with increasing buffer capacity. Raman microscopy confirmed phase conversion from the salt forms to the free base form in phosphate buffer. Hydrodynamics did not affect the time course of the solution-mediated phase transformation of salt forms. Dissolution and precipitation appear to be a function of pH close to the surface of the dissolving solid. In situations where equilibrium solubility of salts cannot be assessed experimentally, dissolution experiments are useful for examining the extent and duration of the dissolution rate enhancement.     

Pharmaceutical particle technologies: An approach to improve drug solubility,dissolution and bioavailability

Pharmaceutical particle technology is employed to improve poor aqueous solubility of drug compounds that limits in vivo bioavailability owing to their low dissolution rate in the gastrointestinal fluids following oral administration. The particle technology involves several approaches from the conventional size reduction processes to the newer, novel particle technologies that modify the solubility properties of the drugs and produce solid, powdered form of the drugs that are readily soluble in water and can be easily formulated into various dosage forms. This review highlights the solid particle technologies available for improving solubility, dissolution and bioavailability of drugs with poor aqueous solubility.     

Evolution of a physiological pH 6.8 bicarbonate buffer system: Application to the dissolution testing of enteric coated products

The use of compendial pH 6.8 phosphate buffer to assess dissolution of enteric coated products gives rise to poor in vitro-in vivo correlations because of the inadequacy of the buffer to resemble small intestinal fluids. A more representative and physiological medium, pH 6.8 bicarbonate buffer, was developed to evaluate the dissolution behaviour of enteric coatings. The bicarbonate system was evolved from pH 7.4 Hanks balanced salt solution to produce a pH 6.8 bicarbonate buffer (modified Hanks buffer, mHanks), which resembles the ionic composition and buffer capacity of intestinal milieu. Prednisolone tablets were coated with a range of enteric polymers: hypromellose phthalate (HP-50 and HP-55), cellulose acetate phthalate (CAP), hypromellose acetate succinate (HPMCAS-LF and HPMCAS-MF), methacrylic acid copolymers (EUDRAGIT® L100-55, EUDRAGIT® L30D-55 and EUDRAGIT® L100) and polyvinyl acetate phthalate (PVAP). Dissolution of coated tablets was carried out using USP-II apparatus in 0.1 M HCl for 2 h followed by pH 6.8 phosphate buffer or pH 6.8 mHanks bicarbonate buffer. In pH 6.8 phosphate buffer, the various enteric polymer coated products displayed rapid and comparable dissolution profiles. In pH 6.8 mHanks buffer, drug release was delayed and marked differences were observed between the various coated tablets, which is comparable to the delayed disintegration times reported in the literature for enteric coated products in the human small intestine. In summary, the use of pH 6.8 physiological bicarbonate buffer (mHanks) provides more realistic and discriminative in vitro release assessment of enteric coated formulations compared to compendial phosphate buffer.     

Characterisation of the thermal,spectroscopic and drug dissolution properties of mefenamic acid and polyoxyethylene–polyoxypropylene solid dispersions

The aim of this study was to investigate the solubility of mefenamic acid (MA), a highly cohesive, poorly water-soluble drug in a copolymer of polyoxyethylene–polyoxypropylene (Lutrol F68®), and to understand the effect drug polymer solubility has on in vitro dissolution of MA. Solid dispersions (SD) of MA were prepared by a hot melt method, using Lutrol F68® as a thermoplastic polymeric platform. High-speed differential scanning calorimetry (Hyper-DSC), Raman spectroscopy, powder X-ray diffractometry (PXRD) and hot-stage/fluorescence microscopy were used to assess the solubility of the drug in molten and solid polymer. Drug dissolution studies were subsequently conducted on single-phase solid solutions and biphasic SD using phosphate buffer pH 6.8 as dissolution media. Solubility investigations using Hyper-DSC, Raman spectroscopy and hot-stage microscopy suggested MA was soluble in molten Lutrol F68® up to a concentration of 35% (w/w). Conversely, the solubility in the solid-state matrix was limited to <15% (w/w); determined by Raman spectroscopy, PXRD and fluorescence microscopy. As expected the dissolution properties of MA were significantly influenced by the solubility of the drug in the polymer matrix. At a concentration of 10% (w/w) MA (a single phase solid solution) dissolution of MA in phosphate buffer 6.8 was rapid, whereas at a concentration of 50% (w/w) MA (biphasic SD) dissolution was significantly slower. This study has clearly demonstrated the complexity of drug–polymer binary blends and in particular defining the solubility of a drug within a polymeric platform. Moreover, this investigation has demonstrated the significant effect drug solubility within a polymeric matrix has upon the in vitro dissolution properties of solid polymer/drug binary blends. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 98:4545–4556, 2009     

Aqueous Dissolution and Dispersion Behavior of Polyvinylpyrrolidone Vinyl Acetate-based Amorphous Solid Dispersion of Ritonavir Prepared by Hot-Melt Extrusion with and without Added Surfactants

In this study, the lack of complete drug release from amorphous solid dispersions (ASDs), as observed in most published reports, was investigated. ASDs with 20% ritonavir were prepared by HME using polyvinylpyrrolidone vinyl acetate (PVPVA) alone and in combination with 10% poloxamer 407 or Span 20 as carriers. It was established by the film casting technique that ritonavir was molecularly dispersed in formulations, and accelerated stability testing confirmed that extrudates were physically stable. Dissolution of ASDs (100-mg ritonavir equivalent) was performed in 250 mL 0.01 N HCl (pH 2), pH 6.8 phosphate buffer and FeSSIF-V2. Drug concentrations were measured by filtration through 0.45-μm pores and in unfiltered media; the latter gave total amounts of drug present in dissolution media, both as solution and dispersion. Because of low solubility, ritonavir did not dissolve completely in aqueous media. Rather, it formed supersaturated solutions, and the excess drug dispersed in the oily amorphous form with low particle sizes that could crystallize with time. Due to higher drug solubility, the dissolved drug in FeSSIF-V2 was much higher than that in the phosphate buffer. Complete drug release could be observed by accounting for drug both in solution and as phase-separated dispersion. Thus, the present study provides a complete picture of in vitro drug dissolution and dispersion from ASDs.     

Microenvironmental pH control of drug dissolution

Microenvironmental pH control has been used to modify the dissolution of pharmaceutical formulations in a predictive manner. An internal buffer system comprising disodium hydrogen orthophosphate and citric acid was incorporated at the 10% w/w level into a frusemide-polyvinylpyrrolidone (PVP) solid dispersion system which was X-ray amorphous, or non-crystalline, in nature. The dissolution rate, determined from constant surface area discs, was shown to be dependent on the phosphate/citric acid ratio of the internal buffer system. The approach proved successful for increasing the dissolution rate of a weakly acidic model drug in acidic media and retarding the dissolution rate in alkaline media. In an attempt to measure the pH at the “surface” of a dissolving compact a technique was developed that utilised a modified dissolution apparatus and a micro-pH probe. The data confirmed that the internal buffer system produced controlled changes in the measured surface pH. The surface pH-dissolution profiles for a series of internally buffered solid dispersions in two dissolution media (0.01 M sodium acetate and 0.01 M acetic acid) displayed a similar pattern to the pH-dissolution profile of an unbuffered X-ray amorphous frusemide-PVP solid dispersion in buffered dissolution media. This approach is proposed to be a useful method for producing controlled changes in the dissolution behaviour of pharmaceutical formulations and may be also applied to the prevention of crystallisation of drugs at the solid/liquid interface.     

Investigating a Modified Apparatus to Discriminate the Dissolution Capacity In Vitro and Establish an IVIVC of Mycophenolate Mofetil Tablets in the Fed State

In this study, a modified dissolution apparatus was developed by equipping a USP apparatus Ⅰ with an open-loop system to discriminate the dissolution capacity in vitro and establish an in vitro and in vivo correlation (IVIVC) for mycophenolate mofetil (MMF) tablets. MMF had strong pH-dependent solubility that could influence the dissolution rate in vivo after the meal. Dissolution tests involving reference (Cellcept®) and test formulations (F1 and F2) were conducted using pH 4.5 acetate buffer to simulate gastric fluids in the fed state. The dissolution profiles of the reference and test formulations were distinguished by using the modified dissolution apparatus and compared with those determined using the USP apparatuses Ⅱ and Ⅳ, and the dissolution capacities of the formulations were discriminated at different sampling time-points. The results of human bioequivalence (BE) studies in the fed state were consistent with in vitro evaluations that the maximum concentrations (C_{max, in vivo}) of both F1 and F2 fell below the acceptable range (80.00%). A level A IVIVC between the absorption fraction in vivo and dissolution in vitro, and a level C correlation between C_{max, in vivo} and C_{max, in vitro}, were established to guide the optimization of the tablet formulation containing MMF.     

具有区分力的罗红霉素片体外溶出度方法研究

目的 研究罗红霉素片的溶出行为,确定有区分力的溶出度检查方法。方法 考察罗红霉素在不同pH溶液中的稳定性及溶解度,测定不同处方自研制剂与参比制剂在4种不同溶出介质中的溶出度,绘制溶出曲线,用相似因子法进行拟合。结果 罗红霉素在pH6.8和7.4磷酸盐缓冲液中8 h内稳定性较好,自研制剂A与参比制剂在4种溶出介质中的溶出曲线均具有较好的相似性,自研制剂B与参比制剂在pH 6.8磷酸盐缓冲液中的溶出曲线不相似。结论 以pH 6.8磷酸盐缓冲液为溶出介质,采用浆法测定,75 r/min对本品溶出具有较好的区分力,可为本品的质量控制和一致性评价提供参考。     

Dissolution improvement of RS-8359 by the solid dispersion prepared by the solvent method

Study on dissolution improvement of a water-insoluble compound, RS-8359 ((±)-4-(4-cyanoanilino)-5,6-dihydro-7-hydroxy-7H-cyclopenta[d]pyrimidine) was carried out. A hydrochloric acid salt of RS-8359 had much higher solubility than its free form. However, the free form separated out of a neutral buffer solution instantaneously once the salt form was dissolved. We found that the dissolution properties were greatly improved by preparing a solid dispersion of the salt form with a water-soluble polymer mainly because the dissolved water-soluble polymer included in the solid dispersion retarded the precipitation rate of the free form. The crystallinity of the salt form in the solid dispersion did not affect the dissolution properties greatly. Furthermore, the importance of microenvironmental pH in the solid dispersion was suggested by a significant increase in the maximum concentration of RS-8359 in the dissolution process of the solid dispersion as compared with the case that the simple salt form was dissolved in the buffer solution that included the water-soluble polymer.     

磺胺二甲嘧啶的溶解度及油水分配系数的研究

目的:研究磺胺二甲嘧啶(SM2)在不同pH值下的平衡溶解度和油水分配系数(Ko/w),为SM2制剂的开发提供基础研究。方法:配制不同pH值的磷酸盐缓冲液,采用饱和法测定其表观溶解度;通过SM2分配平衡后在油相(正辛醇)和水相的浓度比,计算油水分配系数。结果:SM2在pH2·0和pH9·0时平衡溶解度较高,分别为1·916、1·375g·L-1,油水分配系数在pH8·0时最高,为3·9070。结论:SM2在水中的平衡溶解度及油水分配系数与介质的pH值有关,pH<3·0或pH>6·8时溶解度较好。pH3～8时SM2在油相中分配较多,较易被机体吸收。     

In vivo dissolution of poorly water-soluble drugs: Proof of concept based on fluorescence bioimaging

In vitro‒in vivo correlation (IVIVC) of solid dosage forms should be established basically between in vitro and in vivo dissolution of active pharmaceutical ingredients. Nevertheless, in vivo dissolution profiles have never been accurately portrayed. The current practice of IVIVC has to resort to in vivo absorption fractions (F_a). In this proof-of-concept study, in vivo dissolution of a model poorly water-soluble drug fenofibrate (FNB) was investigated by fluorescence bioimaging. FNB crystals were first labeled by near-infrared fluorophores with aggregation-caused quenching properties. The dyes illuminated FNB crystals but quenched immediately and absolutely once been released into aqueous media, enabling accurate monitoring of residual drug crystals. The linearity established between fluorescence and crystal concentration justified reliable quantification of FNB crystals. In vitro dissolution was first measured following pharmacopoeia monograph protocols with well-documented IVIVC. The synchronicity between fluorescence and in vitro dissolution of FNB supported using fluorescence as a measure for determination of dissolution. In vitro dissolution correlated well with in vivo dissolution, acquired by either live or ex vivo imaging. The newly established IVIVC was further validated by correlating both in vitro and in vivo dissolution with F_a obtained from pharmacokinetic data.KEY WORDS: In vivo dissolution, Fenofibrate, Fluorescence, Aggregation-caused quenching, Bioimaging, IVIVC     

Impact of the Amount of Excess Solids on Apparent Solubility

Purpose The impact of excess solids on the apparent solubility is examined.Methods The apparent solubility of some model drugs was measured in various buffered solutions, with various amounts of excess solid. To help understand the dependence of the solubility on the amount of solid, we evaluated the dissolution and crystallization rates of indomethacin (IDM), one of the model drugs, at near-equilibrium conditions.Results In the case of IDM, the apparent solubility decreased with an increase in the solid amount at pH 5 and 6. On the other hand, it increased with an increase in the solid amount at pH 6.5 and 7. The crystallization and dissolution rates of IDM decreased and increased, respectively, with an increase in pH values, and became equal at between pH 6 and 7. Therefore, the apparent solubility was most likely to be affected by the balance between the crystallization and dissolution rates. The apparent solubility of other model drugs showed the same trend, although the dependency on the solid amount was not as significant as in the case of IDM.Conclusions The apparent solubility was affected by the amount of solid for all the model drugs investigated. This was most likely to be caused by a competition between the crystallization and dissolution rates.     

Evolution of Choice of Solubility and Dissolution Media After Two Decades of Biopharmaceutical Classification System

The introduction of the biopharmaceutics drug classification system (Biopharmaceutics Classification System (BCS)), in 1995, provided a simple way to describe the biopharmaceutics behavior of a drug. Solubility and permeability are among the major parameters, which determine the fraction dose absorbed of a drug substance and consequently its chances to be bioavailable. The purpose of this review is to summarize the evolution of the media used for determining solubility and dissolution and how this can be used in modern drug development. Over the years, physiologically adapted media and buffers were introduced with the intention to better predict the in vivo solubility and dissolution of drug substances. Water, buffer solutions, compendial media, micellar solubilization media, and biorelevant media are reviewed. At this time point, there is no universal medium available which can be used to predict every drug substance’s solubility or a drug product’s in vivo dissolution behavior. However, there have been many improvements and additions made to media to optimize their in vivo predictability; for example, the current phosphate concentrations in buffers seem to be too high to correlate with the carbonate buffer concentrations in vivo. Biorelevant media were updated to correlate them better with the composition of human intestinal fluids. The BCS was introduced into regulatory sciences as a scientific risk management tool to waive bioequivalence studies under certain conditions. Today’s different guidance documents define the dose-solubility ratio differently. As shown for amoxicillin, this can cause more confusion than certainty for globally operating companies. Harmonization of BCS guidelines is highly desirable.     

Effect of antacids on aspirin dissolution and bioavailability

The in vitrodissolution profile, in vitroand in vivobuffering characteristics, and single-dose bioavailability of various buffered aspirin tablet formulations were studied. Buffering agents,such as magnesium and aluminum hydroxides (formulations B and C) or magnesium carbonate and aluminum glycinate (formulation D), significantly increased the rate of aspirin dissolution from solid dosage forms as compared to an unbuffered tablet (formulation A). The extent of aspirin absorption was equivalent with all formulations;however, the faster rate of dissolution (t₅₀ and t₉₀)with buffered formulations resulted in earlier and higher peak concentration of salicylate compared to that with unbuffered formulation, following a two-tablet dose in the fasting state. A comparison of the in vivobuffer capacity of a four-tablet dose of formulations B and D was performed in the postcibal state at the time of maximal meal-induced acid secretion, using a radiotelemetry procedure for determination of pH. Formulation B prolonged the interval of elevation of intragastric pH > 3 for 32 min as compared to 12 min for D.     

用多条溶出曲线评价盐酸氟桂利嗪胶囊的质量

目的 研究国产盐酸氟桂利嗪胶囊与参比制剂在4种溶出介质中的溶出曲线,以评价该制剂的质量。方法 考察盐酸氟桂利嗪原料在pH 1.2盐酸溶液、pH 4.0醋酸盐缓冲液、pH 6.8磷酸盐缓冲液和水中的溶解度与稳定性,并采用篮法,转速为100 rpm,优化盐酸氟桂利嗪胶囊溶出度测定的溶出介质,测定溶出曲线,并用f₂因子法进行比较分析。结果 以A企业某进口本地化的上市产品为参比制剂,B、E企业只有在pH 1.2盐酸溶液中与参比制剂溶出过程相似,C企业在pH 1.2盐酸溶液和水中与参比制剂溶出过程相似,D企业在pH 1.2盐酸溶液和pH 4.0醋酸盐缓冲液中与参比制剂溶出过程相似。选择pH 4.0醋酸盐缓冲液作为测定盐酸氟桂利嗪胶囊溶出度的溶出介质。结论 国内仿制药与参比制剂在内在质量上存在差距,仍需在处方工艺和生产过程控制方面有所改进。     

Influence of Dissolution Media and Presence of Alcohol on the In Vitro Performance of Pharmaceutical Products Containing an Insoluble Drug

The purpose of this investigation is to determine how the dissolution media may influence the release rate of an insoluble drug in in vitro conditions. Some oral dosage forms containing ibuprofen, a molecule that shows pH-dependent solubility, are tested. They are evaluated in different media to simulate the gastrointestinal transit at paddle rotation speeds of 50 and 100 rpm. Moreover, the potential effect of different ethanol concentrations on drug release is tested. The dissolution profiles of the tablets show a similar behavior in water (pH 1.0) and phosphate buffer (pH 4.5) where the 2 doses are not completely dissolved. The soft capsules show a different behavior: a certain amount of ibuprofen, which is in solution inside the capsule, reprecipitates in water and in the pH 4.5 buffer. Instead, ibuprofen dissolves rapidly in the pH 6.8 buffer from all the formulations. In the water-ethanol solutions, the dissolution curves show a valuable increase in the drug dissolved at higher ethanol concentrations.     

布洛芬平衡溶解度及表观油水分配系数的测定

目的：测定布洛芬在不同pH磷酸盐缓冲溶液中的平衡溶解度与表观油水分配系数。方法采用紫外－可见分光光度法测定布洛芬的质量浓度,气浴恒温振荡－摇瓶法测定布洛芬的平衡溶解度及表观油水分配系数。结果37℃时布洛芬在pH2．0、3．0、4．0、5．0、6．0、6．8、7．4、8．0、9．0缓冲溶液中的平衡溶解度分别为0．05874、0．04078、0．06404、0．1930、1．027、2．551、4．613、3．866、4．409mg? mL －1;表观油水分配系数分别为10．19、17．11、12．55、6．390、4．911、5．976、4．925、2．599、1．716。结论布洛芬在磷酸盐缓冲溶液中的平衡溶解度及表观油水分配系数与介质的pH值相关,介质的pH值的增大,布洛芬的平衡溶解度增大;表观油水分配系数减小。     

伊潘立酮片溶出度一致性评价研究

目的 考察自制伊潘立酮片（1 mg规格）与参比制剂的溶出度一致性。方法 用HPLC法测定伊潘立酮在不同pH溶出介质中的溶解度,绘制伊潘立酮“pH-溶解度”曲线,测定自研制剂与参比制剂在4种不同pH溶出介质（0.1 mol/L HCl溶液、pH 4.5醋酸盐缓冲液、pH 6.8磷酸盐缓冲液、水）中的溶出度,绘制溶出曲线,用相似因子法进行拟合。结果 在所选4种溶出介质下,自研制剂与参比制剂的溶出曲线相似因子f₂值均大于50。结论 自研制剂与参比制剂能够达到体外溶出一致。     

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 °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 μg/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.     

In vitro evaluation of the dissolution behaviour of itraconazole in bio-relevant media

Drugs in the gastrointestinal tract are exposed to a medium of partially digested food, comprising mixtures of fat, protein and carbohydrate. The dissolution behaviour of itraconazole was evaluated in bio-relevant media which were developed to take this into account. Media containing milk with different fat contents, protein (albumin, casein, gluten and gelatin), carbohydrates (glucose, lactose and starch) and amino acids (lysine, glycine, alanine and aspartic acid) to mimic a digested meal and bile components (sodium taurocholate and lecithin) to represent a key endogenous digestive material were investigated. The effect of medium composition on the intrinsic dissolution rate of itraconazole was evaluated as this drug has extremely poor solubility and its bioavailability is affected by food. Dissolution tests were carried out in simple compendial media based on dilute solutions of hydrochloric acid or neutral solutions of phosphate buffer and in more complex media containing the dietary components. The data obtained showed that most of the dietary components enhanced the solubility compared to simulated gastric fluid (SGF) but to differing extents. The greatest increase in dissolution was observed with the addition of milk and albumin although an increase was also seen with other proteins, amino acids and simulated gastrointestinal fluids.