Solubility Behavior of Narcotic Analgesics in Aqueous Media: Solubilities and Dissociation Constants of Morphine,Fentanyl, and Sufentanil

The pH dependence of the aqueous solubility of morphine, fentanyl, and sufentanil was investigated at 35°C. Dissociation constants and corresponding pK _a values of the drugs were obtained from measured free-base solubilities (determined at high pH's) and the concentrations of saturated solutions at intermediate pH's. Morphine, fentanyl, and sufentanil exhibited pK _a values of 8.08, 8.99, and 8.51, respectively. Over the pH range of 5 to 12.5 the apparent solubilities are determined by the intrinsic solubility of the free base plus the concentration of ionized drug necessary to satisfy the dissociation equilibrium at a given pH. Consequently, the drug concentrations of saturated aqueous solutions fall off precipitously as the pH is raised and ionization is suppressed. Further, at low pH's the aqueous solubility of morphine increased in a linear fashion with increases in the molar strength of citric acid which was added to acidify the medium, suggesting the formation of a soluble morphine–citrate complex.     

Solubility Prediction of Satranidazole in Aqueous N,N-dimethylformamide Mixtures Using Extended Hildebrand Solubility Approach

The solubility of satranidazole in several water–N,N-dimethylformamide mixtures was analysed in terms of solute–solvent interactions and data were treated on the basis of extended Hildebrand solubility approach. The solubility profile of satranidazole in water–N,N-dimethylformamide mixtures shows a curve with a solubility maxima well above the ideal solubility of drug. This is attributed to solvation of the drug with the water–N,N-dimethylformamide mixture, and indicates that the solute–solvent interaction energy (W) is larger than the geometric mean (δ₁δ₂) of regular solution theory. The new approach provides an accurate prediction of solubility once the interaction energy (W) is obtained. In this case, the energy term is regressed against a polynomial in δ₁ of the binary solvent mixture. A quartic expression of W in terms of solvent solubility parameter was found for predicting the mole fraction solubility of satranidazole in the studied mixtures. The method has potential usefulness in preformulation and formulation studies during which solubility prediction is important for drug design.     

Phase Behavior of Amorphous Molecular Dispersions I: Determination of the Degree and Mechanism of Solid Solubility

PURPOSE: To understand the phase behavior and the degree and mechanism of the solid solubility in amorphous molecular dispersions by the use of thermal analysis. METHODS: Amorphous molecular dispersions of trehalose-dextran and trehalose-PVP were prepared by co-lyophilization. The mixtures were exposed to 23 degrees C, 40 degrees C, and 50 degrees C [75% relative humidity (RH)] and 23 degrees C (69% RH) storage conditions, respectively. Thermal analysis was conducted by modulated differential scanning calorimeter (MDSC). RESULTS: Upon exposure to moisture, two glass transition temperatures (TgS), one for phase-separated amorphous trehalose (Tg1) and the other for polymer-trehalose mixture (Tg2), were observed. With time, Tg2 increased and reached to a plateau (Tg(eq)), whereas Tg1 disappeared. The disappearance of Tg1 was attributed to crystallization of the phase-separated amorphous trehalose. It was observed that Tg(eq) was always less than Tg of pure polymer. The lower Tg(eq) when compared to Tg of pure polymer may be the result of solubility of a fraction of trehalose in the polymers chosen. The miscible fraction of trehalose was estimated to be 12% and 18% wt/wt in dextran at 50 degrees C/75% RH and 23 degrees C/75% RH, respectively, and 10% wt/wt in PVP at 23 degrees C/69% RH. CONCLUSIONS: Mixing behavior of trehalose-dextran and trehalose-PVP dispersions were examined both experimentally and theoretically. A method determining the "extent of molecular miscibility," referred to as "solid solubility," was developed and mechanistically and thermodynamically analyzed. Solid dispersions prepared at trehalose concentrations below the "solid solubility limit" were physically stable even under accelerated stability conditions.     

Experimental and Computational Screening Models for Prediction of Aqueous Drug Solubility

Purpose. To devise experimental and computational models to predict aqueous drug solubility. Methods. A simple and reliable modification of the shake flask method to a small-scale format was devised, and the intrinsic solubilities of 17 structurally diverse drugs were determined. The experimental solubility data were used to investigate the accuracy of commonly used theoretical and semiexperimental models for prediction of aqueous drug solubility. Computational models for prediction of intrinsic solubility, based on lipophilicity and molecular surface areas, were developed. Results. The intrinsic solubilities ranged from 0.7 ng/mL to 6.0 mg/mL, covering a range of almost seven log₁₀ units, and the values determined with the new small-scale shake flask method agreed well with published solubility data. Solubility data computed with established theoretical models agreed poorly with the experimentally determined solubilities, but the correlations improved when experimentally determined melting points were included in the models. A new, fast computational model based on lipophilicity and partitioned molecular surface areas, which predicted intrinsic drug solubility with a good accuracy (R ²of 0.91 and RMSE_tr of 0.61) was devised. Conclusions. A small-scale shake flask method for determination of intrinsic drug solubility was developed, and a promising alternative computational model for the theoretical prediction of aqueous drug solubility was proposed.     

Thermophysical Properties of Trehalose and Its Concentrated Aqueous Solutions

Purpose. To address the lack of fundamental thermophysical data for trehalose and its aqueous systems by measuring equilibrium and non-equilibrium properties of such systems. Methods/Results. Differential scanning calorimetry (DSC) and dynamic mechanical analysis were used to measure glass transition temperatures of trehalose and its solutions. X-ray diffractometry was used to verify the structure of amorphous trehalose. Controlled-stress rheometry was used to measure viscosity of several aqueous trehalose systems at ambient and sub-ambient temperatures. Over this temperature range, the density of these solutions was also measured with a vibrating tube densimeter. The equilibrium phase diagram of aqueous trehalose was determined by measuring the solubility and freezing point depression. Conclusions. Our solubility measurements, which have allowed long times for attainment of chemical equilibrium, are substantially different from those reported earlier that used different techniques. Our measurements of the glass transition temperature of trehalose are higher than reported values. A simple model for the glass transition is presented to describe our experimental observations.     

Phase Behavior of Binary and Ternary Amorphous Mixtures Containing Indomethacin, Citric Acid, and PVP

Purpose. To better understand the nature of drug-excipient interactions we have studied the phase behavior of amorphous binary and ternary mixtures of citric acid, indomethacin and PVP, as model systems. Methods. We have prepared amorphous mixtures by co-melting or coprecipitation from solvents, and have measured glass transition temperatures with differential scanning calorimetry. Results. Citric acid and indomethacin in the amorphous state are miscible up to 0.25 weight fraction of citric acid, equivalent to about 2 moles of citric acid and 3 moles of indomethacin. Phase separation, as reflected by two Tg values, occurs without crystallization leading to a saturated citric acid-indomethacin amorphous phase and one essentially containing only citric acid. PVP-citric acid and PVP-indomethacin form non-ideal miscible systems at all compositions. A ternary system containing 0.3 weight fraction of PVP produces a completely miscible system at all citric acid-indomethacin compositions. The use of 0.2 weight fraction of PVP, however, only produces miscibility up to a weight fraction of 0.4 citric acid relative to indomethacin. The two phases above this point appear to contain citric acid in PVP and citric acid in indomethacin, respectively. Conclusions. Two components of an amorphous solid mixture containing citric acid and indomethacin with limited solid state miscibility can be solublized as an amorphous solid phase by the addition of moderate levels of PVP.     

Intrinsic Solubility Estimation and pH-Solubility Behavior of Cosalane (NSC 658586), an Extremely Hydrophobic Diprotic Acid

Purpose. The selection of cosalane (NSC 658586) by the National Cancer Institute for further development as a potential drug candidate for the treatment of AIDS led to the exploration of the solubility behavior of this extremely hydrophobic drug, which has an intrinsic solubility (S₀) approaching 1 ng/ml. This study describes attempts to reliably measure the intrinsic solubility of cosalane and examine its pH-solubility behavior. Methods. S₀ was estimated by 5 different strategies: (a) direct determination in an aqueous suspension; (b) facilitated dissolution; (c) estimation from the octanol/water partition coefficient and octanol solubility; (d) application of an empirical equation based on melting point and partition coefficient; and (e) estimation from the hydrocarbon solubility and functional group contributions for transfer from hydrocarbon to water. Results. S₀ estimates using these five methods varied over a 5 × 10⁹-fold range. Method (a) yielded the highest values, two-orders of magnitude greater than those obtained by method (b) (facilitated dissolution, 1.4 ± 0.5 ng/ml). Method (c) gave a value 20-fold higher while that from method (d) was in fair agreement with that from facilitated dissolution. Method (e) yielded a value several orders-of-magnitude lower than other methods. A molecular dynamics simulation suggests that folded conformations not accounted for by group contributions may reduce cosalane's effective hydrophobicity. Ionic equilibria calculations for this weak diprotic acid suggested a 100-fold increase in solubility per pH unit increase. The pH-solubility profile of cosalane at 25°C agreed closely with theory. Conclusions. These studies highlight the difficulty in determining solubility of very poorly soluble compounds and the possible advantage of the facilitated dissolution method. The diprotic nature of cosalane enabled a solubility enhancement of >10⁷-fold by simple pH adjustment.     

The Impact of Aqueous Solubility and Dose on the Pharmacokinetic Profiles of Resveratrol

Purpose  This study aimed at the investigation of the impact of aqueous solubility and dose manipulation on the pharmacokinetics of resveratrol. Methods  Water soluble intravenous and oral formulations of resveratrol were prepared with hydroxypropyl-β-cyclodextrin (HP-β-CD) and randomly methylated-β-cyclodextrin (RM-β-CD), respectively. Sodium salt and suspension of resveratrol in carboxymethyl cellulose (CMC) were used as the reference intravenous and oral formulations, respectively. The pharmacokinetics of resveratrol was assessed in Sprague–Dawley rats. Plasma resveratrol concentrations were measured by high performance liquid chromatography (HPLC). Results  Both HP-β-CD and RM-β-CD enhanced the aqueous solubility of resveratrol. After intravenous administration, rapid elimination of resveratrol was observed at all tested doses (5, 10, and 25 mg kg⁻¹) regardless of formulation types; with non-linear elimination occurring at the dose of 25 mg kg⁻¹. RM-β-CD significantly increased the maximal plasma concentration of orally administered resveratrol, but, it did not increase the oral bioavailability in comparison with the CMC suspension. Furthermore, the oral bioavailability remained unchanged among all tested doses (15, 25, and 50 mg kg⁻¹). Conclusions  Aqueous solubility barrier might affect the speed but not the extent of resveratrol absorption. Further, dose manipulation (up to 50 mg kg⁻¹) did not have a significant impact on the oral bioavailability of resveratrol. An erratum to this article can be found at     

Non-Sink Dissolution Behavior and Solubility Limit of Commercial Tacrolimus Amorphous Formulations

An increasing number of drugs with low aqueous solubility are being formulated and marketed as amorphous solid dispersions because the amorphous form can generate a higher solubility compared to the crystalline solid. The amorphous solubility of a drug can be determined experimentally using various techniques. Most studies in this area investigate the drug in its pure form and do not evaluate any effects from other formulation ingredients. In this study, we use 6 marketed amorphous oral drug products, capsules containing 5 mg of tacrolimus, and various excipients, consisting of 1 innovator product and 5 generics. The amorphous solubility of tacrolimus was evaluated using different techniques and was compared to the crystalline solubility of the drug. Dissolution of the different products was conducted under non-sink conditions to compare the maximum achieved concentration with the amorphous solubility. Diffusion studies were performed to elucidate the maximum flux across a membrane and to evaluate whether there was any difference in the thermodynamic activity of the drug released from the formulation and the pure drug. The amorphous solubility of tacrolimus was found to be a factor of 35 higher than the crystalline solubility. The maximum concentration obtained after dissolution of the capsule contents in non-sink conditions was found to match the experimentally determined amorphous solubility of the pure drug. Furthermore, the membrane flux of tacrolimus following dissolution of the various formulations was found to be similar and maximized. This study demonstrates a link between key physicochemical properties (amorphous solubility) and in vitro formulation performance.     

Micellar Solubilization of Timobesone Acetate in Aqueous and Aqueous Propylene Glycol Solutions of Nonionic Surfactants

The micellar solubilization of timobesone acetate, a novel topical corticosteroid, was studied in aqueous and aqueous propylene glycol solutions of 1 to 5% nonionic surfactants at 25°C. The surfactants used were polyoxyethylene (POE) sorbitan monofatty acid esters (polysorbates), fatty acid esters (Myrj), and fatty alcohol ethers (Brij), as well as sucrose monolaurate (Crodesta SL40). The increase in the solubility of timobesone acetate in the micellar solutions was dependent on the type and concentration of surfactant. The solubilizing capacity of the surfactant micelles and the distribution coefficient of timobesone acetate in aqueous micellar solutions were found (1) to increase with increasing length of the hydrophobic fatty acid group; (2) to increase according to the structure of the hydrophilic group in the order of POE sorbitan ester, sucrose ester, POE ester, and POE ether; (3) to be unaffected by the increase in POE chain length; and (4) to tend to decrease in surfactant containing unsaturated fatty acid groups. In aqueous propylene glycol solution, the solubilizing capacity increased slightly, i.e., up to 1.5-fold in 50% propylene glycol solution, for the ester-type surfactants (polysorbates and Myrj). But this increase was not observed in the ether-type surfactant (Brij) solution. The distribution coefficient decreased logarithmically with increasing concentrations of propylene glycol in the solution. This was caused by the logarithmic increase in the timobesone acetate solubility in the bulk phase, while the solubility in the micellar phase was practically unchanged. The results support the equilibrium distribution model of micellar solubilization.     

Crystallization Behavior of Mannitol in Frozen Aqueous Solutions

Purpose. To study the effect of cooling rate, the influence of phosphate buffers and polyvinylpyrrolidone (PVP) on the crystallization behavior of mannitol in frozen aqueous solutions. Methods. Low-temperature differential scanning calorimetry and powder X-ray diffractometry were used to characterize the frozen solutions. Results. Rapid cooling (20°C/min) inhibited mannitol crystallization, whereas at slower cooling rates (10°C and 5°C/min) partial crystallization was observed. The amorphous freeze-concentrate was characterized by two glass transitions at -32°C and -25°C. When the frozen solutions were heated past the two glass transition temperatures, the solute crystallized as mannitol hydrate. An increase in the concentration of PVP increased the induction time for the crystallization of mannitol hydrate. At concentrations of 100 mM, the buffer salts significantly inhibited mannitol crystallization. Conclusions. The crystallization behavior of mannitol in frozen solutions was influenced by the cooling rate and the presence of phosphate buffers and PVP.     

Phase Transitions of Glycine in Frozen Aqueous Solutions and During Freeze-Drying

Purpose. To study the solid-state and phase transitions of glycine, (i) in frozen aqueous solutions, and (ii) during freeze-drying. Methods. X-ray powder diffractometry (XRD) and differential scanning calorimetry (DSC) were used to analyze the frozen systems. In situ freeze-drying in the sample chamber of the diffractometer enabled characterization of phase transitions during freeze-drying. Results. Transitions in frozen systems. Rapid (20°C/min) or slow (2°C/min) cooling of aqueous solutions of glycine (15% w/w) to –70°C resulted in crystallization of -glycine. Annealing at –10°C led to an increase in the amount of the crystalline phase. When quench-cooled by immersing in liquid nitrogen, glycine formed an amorphous freeze-concentrate. On heating, crystallization of an unidentified phase of glycine occurred at \-65°C which disappeared at –55°C, and the peaks of -glycine appeared. Annealing caused a transition of - to the - form. The extent of this conversion was a function of the annealing temperature. Slower cooling rates and annealing in frozen solutions increased the crystalline -glycine content in the lyophile. Freeze-drying of quench-cooled solutions led to the formation of -glycine during primary drying resulting in a lyophile consisting of a mixture of - and -glycine. The primary drying temperature as well as the initial solute concentration significantly influenced the solid-state of freeze-dried glycine only in quench-cooled systems. Conclusions. The cooling rate, annealing conditions and the primary drying temperature influenced the solid-state composition of freeze-dried glycine.     

主动靶向脂质体的相变温度与靶细胞摄取的相关性研究

分别采用二硬脂酰磷脂酰胆碱(DSPC)、氢化大豆磷脂(HSPC)、二棕榈酰磷脂酰胆碱(DPPC)和二肉豆蔻酰磷脂酰胆碱(DMPC)为载体,制备了NGR肽(Asn-Gly-Arg)修饰的主动靶向脂质体,并用其负载香豆素-6.结果表明,4种脂质体的粒径为120～160 nm,粒度分布均匀,ζ电位接近中性,包封率均在95％以上.差示扫描量热分析(DSC)表明,4种脂质体的相变温度(Tm)值由高至低分别为DSPC脂质体＞HSPC脂质体＞DPPC脂质体＞DMPC脂质体.以CD13高表达的HT1080细胞为模型,采用流式细胞仪、激光共聚焦显微镜观察评价脂质体的入胞效果.结果显示,脂质体的入胞效率与Tm值呈正相关.     

Biorelevant Media Slows the Solution-Mediated Phase Transformation of Amorphous Spironolactone

Solution-mediated phase transformation (SMPT) can reduce the high drug concentration expected from amorphous formulations, eliminating the improvement in drug absorption one hoped to gain from this high energy drug state. The differences in SMPT of a supersaturating system were compared in biorelevant media (fasted state simulated intestinal fluid and fed state simulated intestinal fluid) and United States Pharmacopeia compendial medium, simulated intestinal fluid without pancreatin. Amorphous spironolactone underwent SMPT to the same hydrate of spironolactone in all 3 media which was confirmed by the decrease in dissolution rates assessed in a flow-through dissolution apparatus, as well as by the appearance of crystals on the amorphous solid surface detected by polarized light microscopy. Longer duration of supersaturation which may lead to greater in vivo oral drug absorption was found in both biorelevant media, compared to compendial (average > 90 vs. 20 min), indicating that the presence of surfactants in biorelevant media delays crystal growth. Surface profiles and polarized light micrographs suggest that (1) a significant increase in surface area due to 3D crystal formation, (2) amorphous areas remaining exposed on the surface, and (3) a lower nucleation rate are potential reasons for an elevated dissolution rate even after SMPT.     

Solubility studies on valdecoxib in the presence of carriers,cosolvents, and surfactants

Enhancement of the solubility of valdecoxib was examined using a series of hydrophilic carriers (mannitol, polyethylene glycol (PEG) 4000, PEG 6000, PEG 8000, and urea), surfactants (Tween‐20, Tween‐80, and sodium lauryl sulfate [SLS]) and cosolvents (ethanol, methanol, and glycerol) at 37°C. The solubility of valdecoxib could be enhanced significantly using PEG 4000 as a carrier, ethanol as cosolvent, and SLS as a surfactant. Because the solubility of valdecoxib increased dramatically in the presence of polyethylene glycols, these are suitable dispersing agents for preparing solid dispersions containing valdecoxib. Gibbs free energy (ΔG) values were all negative for all hydrophilic carriers tested, indicating the spontaneous nature of valdecoxib solubilisation. Among the cosolvents, ethanol exhibited higher solubilization potential than methanol and glycerol. As the dielectric constant of the cosolvent–water mixtures decreased, the solubility of valdecoxib increased. Finally, SLS exerted maximum solubilization of valdecoxib when compared to Tween‐20 or Tween‐80. The crystallinity of valdecoxib was explored by X‐ray diffraction study and showed numerous crystalline peaks. Examination of surface morphology by scanning electron microscopy depicted a near spherical shape of valdecoxib with irregular surface characteristics. Drug Dev. Res. 62:41–48, 2004. © 2004 Wiley‐Liss, Inc.     

Effect of Cations and Anions on Glass Transition Temperatures in Excipient Solutions

The purpose of this study was to investigate the effects of cations and anions of various electrolytes on the glass transition temperature (Tg') of frozen solutions of excipients commonly used in freeze-drying. The effect of electrolyte concentration on freezable water content was also investigated by measuring the enthalpy of melting (ΔH) using Differential Scanning Calorimetry (DSC). Cations and anions induce changes in Tg' of frozen solutions of commonly used parenteral excipients. These changes are dependent on the properties of the excipients used. Tg' values of 5% w/v solutions of maltose, trehalose, sucrose, dextran 40, and polyvinylpyrrolidone (PVP, 17K) were determined as a function of sodium chloride (NaCl) or potassium chloride (KCl) concentrations. In general, a significant decrease in Tg' was observed as a function of increasing the electrolyte concentration. For the disaccharide solutions, the decrease in Tg' due to the addition of NaCl or KCl was similar in magnitude, indicating that changing the cation from K⁺ to Na⁺ had no effect on Tg'. However, the decrease in Tg' for the PVP solution due to the addition of KCl was greater than that observed by the addition of NaCl. The differences in the electrolyte-induced changes on Tg' between the disaccharides and PVP may be potentially attributed to the formation of complexes between the cations and the properly oriented hydroxyl groups in the sugars leaving the anions (Cl^? ions) to exert their effect on Tg'. While zero cation effect would be consistent with these results for the disaccharides, these results do not mean that the cation effects are zero; they only mean that the cation effects are the same. For the PVP solution, K⁺ and Na⁺ ions are not engaged in complex formation with PVP due to the lack of hydroxyl groups. We hypothesize that the structure-breaking K⁺ ions increase the fluidity of water and exert a greater plasticizing effect on Tg', leading to a more significant decrease in Tg' than the structure-making Na⁺ ions, which increase the viscosity of water. The decrease in Tg' of frozen solutions of pharmaceutical excipients caused by the addition of electrolytes may be primarily attributed to an increase in the unfrozen plasticizing water surrounding the excipient molecules. Formulation scientists should evaluate the use of electrolytes in the formulation development of lyophilized products containing commonly used excipients. Electrolytes are often needed as stabilizers for protein formulations; however, their selection and use should be properly evaluated. Because electrolytes cause a decrease in Tg' as a function of electrolyte concentration, it is recommended that the minimum electrolyte concentration needed to maintain product stability should be used to minimize the effect of the electrolyte on lowering the Tg'.     

Forecasting the Oral Absorption Behavior of Poorly Soluble Weak Bases Using Solubility and Dissolution Studies in Biorelevant Media

Pharmaceutical Research -     

Solubility and lonization Behavior of the Antiarrhythmic 4-hydroxy-N-phenyl-3,5-bis(l-pyrrolidinylmethyl)benzamide dihydrochloride (DuP 923)

Pharmaceutical Research -     

Solubility of Small-Molecule Crystals in Polymers: <Emphasis Type="SmallCaps">d</Emphasis>-Mannitol in PVP,Indomethacin in PVP/VA,and Nifedipine in PVP/VA

Objective  Amorphous pharmaceuticals, a viable approach to enhancing bioavailability, must be stable against crystallization. An amorphous drug can be stabilized by dispersing it in a polymer matrix. To implement this approach, it is desirable to know the drug’s solubility in the chosen polymer, which defines the maximal drug loading without risk of crystallization. Measuring the solubility of a crystalline drug in a polymer is difficult because the high viscosity of polymers makes achieving solubility equilibrium difficult. Method  Differential Scanning Calorimetry (DSC) was used to detect dissolution endpoints of solute/polymer mixtures prepared by cryomilling. This method was validated against other solubility-indicating methods. Results  The solubilities of several small-molecule crystals in polymers were measured for the first time near the glass transition temperature, including d-mannitol (β polymorph) in PVP, indomethacin (γ polymorph) in PVP/VA, and nifedipine (α polymorph) in PVP/VA. Conclusion  A DSC method was developed for measuring the solubility of crystalline drugs in polymers. Cryomilling the components prior to DSC analysis improved the uniformity of the mixtures and facilitated the determination of dissolution endpoints. This method has the potential of providing useful data for designing physically stable formulations of amorphous drugs.