Effect of nicotinamide and urea on the solubility of riboflavin in various solvents

Self-association as a solubility limiting factor of riboflavin in aqueous media

Single crystal X-ray diffraction analysis of needle shaped riboflavin (RF) crystals revealed π-stacking of RF's isoalloxazine units (distance: 3.643-3.313 Å) with syn-orientated ribityl chains. In line with this, classical molecular dynamics (MD) (400 ns) using an isobaric-isothermal (NPT) ensemble of eight RF in a water box (〈V〉 ∼ 508.62 nm3, 〈p〉 = 1.11 bar) revealed anti-aligned aggregation of RF in water (COM-distance: 4 Å). Comparing umbrella sampling for the separation of two RF molecules to the separation of two lumichrome molecules, the similar mean potential force for the separation of RF and lumichrome (22.8 kJ mol-1; 24.4 kJ mol-1) proved dispersive interactions as the origin of RF's aggregation. Though stacking of RF is the major water-solubility limiting factor, the conformation of RF's ribityl chain may alter the solubility in water. Both, MD (in water) and COSMO-RS (in water continuum) predicted that conformations of RF with an extended ribityl chain are thermodynamically preferred over any conformations with internal hydrogen bonds between hydroxyl groups and nitrogen/oxygen atoms of the pyrimidine moiety of the flavin ring. Interestingly, COSMO-RS predicted the solubility of the extended conformation to be significantly lower than the latter leading to the very low average solubility of RF. Nuclear Overhauser effect measurements (NOESY) of the structurally related sodium riboflavin 5'-monophosphate (RF-PO4) in deuterium oxide confirmed π-stacking of the isoalloxazine rings. In conformity with the 350 times higher water-solubility of RF-PO4, NOESY also indicated a contorted conformation of the ribityl phosphate chain, whereas, for RF, indications for a contorted chain were not observed.

Exploration of the physical states of riboflavin (free base) by mechanical milling

Amorphous riboflavin (free base) could be produced for the first time via high energy ball milling of a commercial crystalline form (Form I). Importantly, this solid state amorphization process allowed to circumvent chemical degradation occurring during melting as well as the lack of suitable solvents, which are required for amorphization via spray- or freeze-drying. The amorphous state of riboflavin was thoroughly characterized, revealing a complex recrystallization pattern upon heating, involving two enantiotropic polymorphic forms (II and III) and a dihydrate. The glass transition temperature (Tg) and heat capacity (Cp) jump of the amorphous form were determined as 144 °C and 0.68 J/g/°C. Moreover, the relative physical stability of the different physical states has been elucidated, e.g., at room temperature: I > II > III. The following rank order was observed for the dissolution rates in water at 37 °C during the first 4 h: amorphous > III ≈ II > I. Afterwards, a dihydrate crystallized from the solutions of amorphous and metastable crystalline riboflavin forms, the solubility of which was well above the solubility of the stable FormI.

Elucidating the Hydrotropic Mechanism of the Antagonistic Salt PPh4Cl

PPh₄Cl is an antagonistic salt that recently showed promise as a hydrotropic agent. Here, we give mechanistic insights into the PPh₄Cl-assisted solubility of a dye molecule using molecular dynamics simulations. Our findings reveal that dye molecules aggregate into a cluster which leads to an accumulation of PPh₄⁺ ions in its vicinity and subsequent exclusion of water molecules from the region. The structural organization is attributed to the preferential interaction of dye molecules and PPh₄Cl. The origin of such preference arises from the difference in π-π and CH-π interaction among the pairs. The hydrodynamic radius of PPh₄Cl indicates a low propensity for cluster formation, which enhances its hydrotropic behavior. The process of dye dissolution is thermodynamically favored and occurs through a cooperative mechanism. Our studies provide molecular insight into experimental observations crucial for the design of novel hydrotropes with enhanced solubilizing properties.

Electrochemical direct air capture of CO2 using neutral red as reversible redox-active material

Direct air capture of carbon dioxide is a viable option for the mitigation of CO₂ emissions and their impact on global climate change. Conventional processes for carbon capture from ambient air require 230 to 800 kJ thermal per mole of CO₂, which accounts for most of the total cost of capture. Here, we demonstrate electrochemical direct air capture using neutral red as a redox-active material in an aqueous solution enabled by the inclusion of nicotinamide as a hydrotropic solubilizing agent. The electrochemical system demonstrates a high electron utilization of 0.71 in a continuous flow cell with an estimated minimum work of 35 kJ_e per mole of CO₂ from 15% CO₂. Further exploration using ambient air (410 ppm CO₂ in the presence of 20% oxygen) as a feed gas shows electron utilization of 0.38 in a continuous flow cell to provide an estimated minimum work of 65 kJ_e per mole of CO₂.

Overview of nanoparticulate strategies for solubility enhancement of poorly soluble drugs

Poor aqueous solubility and poor bioavailability are major issues with many pharmaceutical industries. By some estimation, 70-90% drug candidates in development stage while up-to 40% of the marketed products are poorly soluble which leads to low bioavailability, reduced therapeutic effects and dosage escalation. That's why solubility is an important factor to consider during design and manufacturing of the pharmaceutical products. To-date, various strategies have been explored to tackle the issue of poor solubility. This review article focuses the updated overview of commonly used macro and nano drug delivery systems and techniques such as micronization, solid dispersion (SD), supercritical fluid (SCF), hydrotropy, co-solvency, micellar solubilization, cryogenic technique, inclusion complex formation-based techniques, nanosuspension, solid lipid nanoparticles, and nanogels/nanomatrices explored for solubility enhancement of poorly soluble drugs. Among various techniques, nanomatrices were found a promising and impeccable strategy for solubility enhancement of poorly soluble drugs. This article also describes the mechanism of action of each technique used in solubilization enhancement.

The Impact of Size and Shape in the Performance of Hydrotropes: A Case-Study of Alkanediols

Inspired by the recently proposed cooperative mechanism of hydrotropy, where water molecules mediate the aggregation of hydrotrope around the solute, this work studies the impact of apolar volume and polar...

Influence of Hydrotropes on the Solubilities and Diffusivities of Redox-Active Organic Compounds for Aqueous Flow Batteries

In this study, we explored the extent to which hydrotropes can be used to increase the aqueous solubilities of redox-active compounds previously used in flow batteries. We measured how five hydrotropes influenced the solubilities of five redox-active compounds already soluble in aqueous electrolytes (≥0.5 M). The solubilities of the compounds varied as a function of hydrotrope type and concentration, with larger solubility changes observed at higher hydrotrope concentrations. 4-OH-TEMPO underwent the largest solubility increase (1.18 ± 0.04 to 1.99 ± 0.12 M) in 20 weight percent sodium xylene sulfonate. The presence of a hydrotrope in solution decreased the diffusion coefficients of 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate, which was likely due to the increased solution viscosity as opposed to a specific hydrotrope-solute interaction because the hydrotropes did not alter their molecules' hydraulic radii. The standard rate constants and formal potentials of both 4-OH-TEMPO and 4,5-dihydroxy-1,3-benzenedisulfonate remained largely unchanged in the presence of a hydrotrope. The results suggest that using hydrotropes may be a feasible strategy for increasing the solubilities of redox-active compounds in aqueous flow batteries without substantially altering their electrochemical properties.

Role of Hydrotropes in Sparingly Soluble Drug Solubilization: Insight from a Molecular Dynamics Simulation and Experimental Perspectives

Drug molecules' therapeutic efficacy depends on their bioavailability and solubility. But more than 70% of the formulated drug molecules show limited effectiveness due to low water solubility. Thus, the water solubility enhancement technique of drug molecules becomes the need of time. One such way is hydrotropy. The solubilizing agent of a hydrophobic molecule is generally referred to as a hydrotrope, and this phenomenon is termed hydrotropy. This method has high industrial demand, as hydrotropes are noninflammable, readily available, environmentally friendly, quickly recovered, cost-effective, and not involved in solid emulsification. The endless importance of hydrotropes in industry (especially in the pharmaceutical industry) motivated us to prepare a feature article with a clear introduction, detailed mechanistic insights into the hydrotropic solubilization of drug molecules, applications in pharma industries, and some future directions of this technique. Thus, we believe that this feature article will become an adequate manual for the pharmaceutical researchers who want to explore all of the past perspectives of the hydrotropic action of hydrotropes in pharmaceutics.

The impact of the counterion in the performance of ionic hydrotropes

The efficiency of an ionic hydrotrope is shown to increase with the hydrophobicity of its counterion, challenging the common view that ionic hydrotropes should possess a small, densely charged counterion such as sodium or chloride.

Enhancement in corneal permeability of riboflavin using cyclodextrin derivates complexes as a previous step to transepithelial cross-linking

Corneal cross-linking has been described as an effective treatment to slow the progression of keratoconus. The standard protocol entails corneal epithelial removal to allow the diffusion of riboflavin into the stroma. Although, de-epithelization can generate risks or complications that transepithelial cross-linking tries to solve or avoid. Different formulations were developed after verifying that hydroxypropyl-β-cyclodextrin (HPβCD) and sulfobuthylether-β-cyclodextrin (SBEβCD) in a 20% concentration, increased the solubility of practically insoluble in water drugs such as riboflavin from 0.12 mg/mL to 0.35 mg/mL and 0.29 mg/mL respectively. These values were higher when chitosan and arginine were added to the formulation, showing solubility of 0.78 mg/mL when HPβCD concentration was not modified. Ex vivo corneal permeability was measured after having kept in contact bovine corneas with intact epithelium for 5 h with the 0.1 mg/mL riboflavin solution, the formulations developed and a reproduced nanoemulsion from another work. Riboflavin's permeability was increased when cyclodextrins, chitosan, and arginine were part of the formulations, compared to the control drug solution. The best permeability coefficient was reached when riboflavin was combined with 40% (w/v) HPβCD, 0.5% (w/w) arginine, and 0.5% (w/w) chitosan. After having carried out toxicity studies as bovine corneal opacity and permeability (BCOP) and Heńs Egg Test - Chorioallantoic Membrane Test (HET-CAM) it was verified that both, the active ingredients and the excipients of the different formulations were not harmful without generating irritation, loss of transparency or corneal permeability alterations. The results show the great potential of the ocular developed solution for their use in transepithelial cross-linking for keratoconus treatment.

The Solubility-Permeability Interplay for Solubility-Enabling Oral Formulations

The Biopharmaceutical classification system (BCS) classifies the drugs based on their intrinsic solubility and intestinal permeability. The drugs with good solubility and intestinal permeability have good bioavailability. The drugs with poor solubility and poor permeability have solubility dependent and permeability dependent bioavailability, respectively. In the current pharmaceutical field, most of the drugs have poor solubility. To solve the problem of poor solubility, various solubility enhancement approaches have been successfully used. The effects of these solubility enhancing approaches on the intestinal permeability of the drugs are a matter of concern, and must not be overlooked. The current review article focuses on the effect of various solubility enhancing approaches viz. cyclodextrin, surfactant, cosolvent, hydrotropes, and amorphous solid dispersion, on the intestinal permeability of drugs. This article will help in the designing of the optimized formulations having balanced solubility enhancement without affecting the permeability of drugs.

Unveiling the mechanism of hydrotropy: evidence for water-mediated aggregation of hydrotropes around the solute

The mechanism of hydrotropy is experimentally proven in this work. Apolarity is shown to be the driving force of hydrotropy.

An overview of techniques for multifold enhancement in solubility of poorly soluble drugs

Poor water solubility of newly discovered compounds has become the most common challenge in the drug development process. Indeed, poor solubility is considered as the root cause of failure of drug during drug development phases. Moreover, it has also been reported to be the main reason for bioavailability issues such as poor, inconsistent, incomplete and highly variable bioavailability of the marketed products. As per an estimate, approximately 90% of drug molecules suffer with poor water solubility at early stage and approximately 40% of the marketed drugs have bioavailability problems mainly due to poor water solubility. Solubility enhancement of the newly discovered compounds is primary research area for the pharmaceutical industries and research institutions. The conventional techniques to improve aqueous solubility of drugs employ salt formation, prodrug formation, co-crystallization, complexation, amorphous solid dispersion and use of co-solvent, surfactants or hydrotropic agents. Current advancement in the science and technology has enabled the use of relatively new techniques under the umbrella of nanotechnology. These include the development of nanocrystals, nanosuspensions, nanoemulsions, microemulsions, liposomes and nanoparticles to enhance the solubility. This review focuses on the conventional and current approaches of multifold enhancement in the solubility of poorly soluble marketed drugs, including newly discovered compounds.

Measuring the Accessible Surface Area within the Nanoparticle Corona Using Molecular Probe Adsorption

The corona phase-the adsorbed layer of polymer, surfactant, or stabilizer molecules around a nanoparticle-is typically utilized to disperse nanoparticles into a solution or solid phase. However, this phase also controls molecular access to the nanoparticle surface, a property important for catalytic activity and sensor applications. Unfortunately, few methods can directly probe the structure of this corona phase, which is subcategorized as either a hard, immobile corona or a soft, transient corona in exchange with components in the bulk solution. In this work, we introduce a molecular probe adsorption (MPA) method for measuring the accessible nanoparticle surface area using a titration of a quenchable fluorescent molecule. For example, riboflavin is utilized to measure the surface area of gold nanoparticle standards, as well as corona phases on dispersed single-walled carbon nanotubes and graphene sheets. A material balance on the titration yields certain surface coverage parameters, including the ratio of the surface area to dissociation constant of the fluorophore, q/KD, as well as KD itself. Uncertainty, precision, and the correlation of these parameters across different experimental systems, preparations, and modalities are all discussed. Using MPA across a series of corona phases, we find that the Gibbs free energy of probe binding scales inversely with the cube root of surface area, q. In this way, MPA is the only technique to date capable of discerning critical structure-property relationships for such nanoparticle surface phases. Hence, MPA is a rapid quantitative technique that should prove useful for elucidating corona structure for nanoparticles across different systems.

Crown Ethers: Novel Permeability Enhancers for Ocular Drug Delivery?

Crown ethers are cyclic molecules consisting of a ring containing several ether groups. The most common and important members of this series are 12-crown-4 (12C4), 15-crown-5 (15C5), and 18-crown-6 (18C6). These container molecules have the ability to sequester metal ions, and their complexes with drugs are able to traverse cell membranes. This study investigated 12C4, 15C5, and 18C6 for their ability to increase solubility of ocular drugs and enhance their penetration into the cornea. Phase solubility analysis determined crown ethers' ability to enhance the solubility of riboflavin, a drug used for the therapy of keratoconus, and these solutions were investigated for ocular drug permeation enhancing properties. Atomic absorption spectroscopy demonstrated crown ether solutions' ability to sequester Ca²⁺ from corneal epithelia, and crown ether mediated adsorption of riboflavin into the stroma was investigated. Induced corneal opacity studies assessed potential toxicity of crown ethers. Crown ethers enhanced riboflavin's aqueous solubility and its penetration into in vitro bovine corneas; the smaller sized crown ethers gave greatest enhancement. They were shown to sequester Ca²⁺ ions from corneal epithelia; doing so loosens cellular membrane tight junctions thus enhancing riboflavin penetration. Induced corneal opacity was similar to that afforded by benzalkonium chloride and less than is produced using polyaminocarboxylic acids. However, in vivo experiments performed in rats with 12C4 did not show any statistically significant permeability enhancement compared to enhancer-free formulation.

Computational analysis of the solvation of coffee ingredients in aqueous ionic liquid mixtures

In this paper, we investigate the solvation of coffee ingredients including caffeine, gallic acid as representative for phenolic compounds and quercetin as representative for flavonoids in aqueous mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate [C2mim][OAc] at various concentrations. Due to the anisotropy of the solutes we show that classical Kirkwood-Buff theory is not appropriate to study solvation effects with increasing ionic liquid content. However, excess coordination numbers as well as the mean residence time of solvent molecules at the surface of the solutes can be determined by Voronoi tessellation. Since the volume of the hydration shells is also available by this method, solvation free energies will be discussed as a function of the ionic liquid concentration to yield a physical meaningful picture of solvation for the anisotropic solutes. Hydrogen bonding capabilities of the solutes and their relevance for experimental extraction yields from spent coffee grounds are also discussed.

Hydrotropic Solubilization of Lipophilic Drugs for Oral Delivery: The Effects of Urea and Nicotinamide on Carbamazepine Solubility-Permeability Interplay

Hydrotropy refers to increasing the water solubility of otherwise poorly soluble compound by the presence of small organic molecules. While it can certainly increase the apparent solubility of a lipophilic drug, the effect of hydrotropy on the drugs’ permeation through the intestinal membrane has not been studied. The purpose of this work was to investigate the solubility–permeability interplay when using hydrotropic drug solubilization. The concentration-dependent effects of the commonly used hydrotropes urea and nicotinamide, on the solubility and the permeability of the lipophilic antiepileptic drug carbamazepine were studied. Then, the solubility–permeability interplay was mathematically modeled, and was compared to the experimental data. Both hydrotropes allowed significant concentration-dependent carbamazepine solubility increase (up to ∼30-fold). A concomitant permeability decrease was evident both in vitro and in vivo (∼17-fold for nicotinamide and ∼9-fold for urea), revealing a solubility–permeability tradeoff when using hydrotropic drug solubilization. A relatively simplified simulation approach based on proportional opposite correlation between the solubility increase and the permeability decrease at a given hydrotrope concentration allowed excellent prediction of the overall solubility–permeability tradeoff. In conclusion, when using hydrotropic drug solubilization it is prudent to not focus solely on solubility, but to account for the permeability as well; achieving optimal solubility–permeability balance may promote the overall goal of the formulation to maximize oral drug exposure.

A biomimetic redox flow battery based on flavin mononucleotide

The versatility in design of redox flow batteries makes them apt to efficiently store energy in large-scale applications at low cost. The discovery of inexpensive organic electroactive materials for use in aqueous flow battery electrolytes is highly attractive, but is thus far limited. Here we report on a flow battery using an aqueous electrolyte based on the sodium salt of flavin mononucleotide. Flavins are highly versatile electroactive molecules, which catalyse a multitude of redox reactions in biological systems. We use nicotinamide (vitamin B3) as a hydrotropic agent to enhance the water solubility of flavin mononucleotide. A redox flow battery using flavin mononucleotide negative and ferrocyanide positive electrolytes in strong base shows stable cycling performance, with over 99% capacity retention over the course of 100 cycles. We hypothesize that this is enabled due to the oxidized and reduced forms of FMN-Na being stabilized by resonance structures.

Combination of hydrotropic nicotinamide with nanoparticles for enhancing tacrolimus percutaneous delivery

Tacrolimus (FK506), an effective immunosuppressant for treating inflammatory skin diseases, hardly penetrates into and through the skin owing to its high hydrophobicity and molecular weight. The aim of this study was to develop a hybrid system based on nicotinamide (NIC) and nanoparticles (NPs) encapsulating FK506, such as FK506–NPs–NIC, for facilitating percutaneous delivery, which exploited virtues of both NIC and NPs to obtain the synergetic effect. Solubility and percutaneous permeation studies were carried out. The results showed that NIC could increase the solubility and permeability of FK506 and that 20% (w/v) NIC presented higher FK506 permeability and was thus chosen as the hydrotropic solution to solubilize FK506 and prepare FK506–NPs–NIC. Hyaluronic acid (HA) was chemically conjugated with cholesterol (Chol) to obtain amphiphilic conjugate of HA–Chol, which self-assembled NPs in 20% NIC solution containing FK506. The particle size, zeta potential, and morphology of NPs were characterized. The encapsulation efficiency and in vitro percutaneous permeation of NPs were evaluated in the presence and absence of NIC. The results demonstrated that hydrotropic solubilizing FK506 was readily encapsulated into NPs with a higher encapsulation efficiency of 79.2%±4.2%, and the combination of NPs with NIC exhibited a significantly synergistic effect on FK506 deposition within the skin (2.39±0.53 μg/cm²) and penetration through the skin (13.38±2.26 μg/cm²). The effect of the combination of NPs with NIC on drug permeation was further visualized by confocal laser scanning microscope through in vivo permeation studies, and the results confirmed that NPs–NIC synergistically enhanced the permeation of the drug into the skin. The cellular uptake performed in HaCaT cells presented a promoting effect of NPs on cellular uptake. These overall results demonstrated that HA–Chol–NPs–NIC can synergistically improve the percutaneous delivery of FK506, and it is a novel potential strategy based on a nano-sized carrier for FK506 to treat skin diseases.

Effect of Nicotinamide on the Photolysis of Riboflavin in Aqueous Solution

The photolysis of riboflavin (RF) in aqueous solution in the presence of nicotinamide (NA) by visible light has been studied in the pH range 1.0-12.0 and the various photoproducts have been identified as known compounds. RF has been determined in degraded solutions by a specific multicomponent spectrometric method in the presence of its photoproducts and NA. The second-order rate constants (k 2) for the bimolecular interaction of RF and NA range from 0.54 (pH 1.0) to 9.66 M(-1) min(-1) (pH 12.0). The log k 2-pH profile for the photolysis reaction follows a sigmoid curve showing a gradual increase in the rate of pH due to a change in the ionization behavior of the molecule. The lower rate in the acid region is probably due to protonation of the molecule since the cationic form of RF is less susceptible to photolysis than the neutral form. Similarly, a slowing of the rate in the alkaline region is due to anion formation of the molecule. NA is involved as an electron acceptor during the sequence of reactions and thus enhances the rate of photolysis of RF. Absorption and fluorescence measurements did not provide evidence for the complex formation between the two compounds under the present conditions.

Mechanism of Hydrotropic Action of Hydrotrope Sodium Cumene Sulfonate on the Solubility of Di-t-Butyl-Methane: A Molecular Dynamics Simulation Study

Hydrotropes are special class of amphiphilic molecules that have an ability to solubilize the insoluble or sparingly soluble molecules in water. To find out the mechanism of hydrotropic action of hydrotropes on hydrophobic molecules, we have carried out classical molecular dynamics simulation of hydrophobic solute di-t-butyl-methane (DTBM) and hydrotrope sodium cumene sulfonate (SCS) in water with a regime of SCS concentrations. Our study demonstrates that, above the minimum hydrotrope concentration (MHC), the self-aggregation of SCS starts, and it creates a micellar-like environment in which the hydrophobic tail part of SCS points inward while its hydrophilic sulfonate group points outward to make favorable contact with water molecules. The formation of the hydrophobic core of SCS cluster creates a hydrophobic environment where the hydrophobic DTBM molecules are encapsulated. Interestingly, the determination of average water-SCS hydrogen bonds further suggests that the aggregate formation of SCS molecules has a negligible influence on it. Moreover, the calculations of Flory-Huggins interaction parameters also reveal favorable interactions between hydrotrope SCS and solute DTBM molecules. The implications of these findings on the mechanism of hydrotrope assisted enhanced solubility of hydrophobic molecules are discussed.

Water-soluble cationic poly(β-cyclodextrin-co-guanidine) as a controlled vitamin B2delivery carrier

Vitamin B₂(VB₂) is effectively incorporated into novel water-soluble cationic β-cyclodextrin (β-CD) polymers in order to improve its physiochemical properties.

Efficient production of lumichrome by Microbacterium sp. strain TPU 3598

Lumichrome is a photodegradation product of riboflavin and is available as a photosensitizer and fluorescent dye. To develop new efficient methods of lumichrome production, we isolated bacterial strains with high lumichrome productivity from soil. The strain with highest productivity was identified as Microbacterium sp. strain TPU 3598. Since this strain inductively produced lumichrome when cultivated with riboflavin, we developed two different methods, a cultivation method and a resting cell method, for the production of large amounts of lumichrome using the strain. In the cultivation method, 2.4 g (9.9 mmol) of lumichrome was produced from 3.8 g (10.1 mmol) of riboflavin at the 500-ml scale (98% yield). The strain also produced 4.7 g (19.4 mmol) of lumichrome from 7.6 g (20.2 mmol) of riboflavin (96% yield) by addition of riboflavin during cultivation at the 500-ml scale. In the resting cell method, 20 g of cells (wet weight) in 100 ml of potassium phosphate buffer, pH 7.0, produced 2.4 g of lumichrome from 3.8 g of riboflavin (98% yield). Since the lumichrome production by these methods was carried out in suspension, the resulting lumichrome was easily purified from the cultivation medium or reaction mixture by centrifugation and crystallization. Thus, the biochemical methods we describe here are a significant improvement in terms of simplicity and yield over the existing chemical, photolytic, and other biochemical methods of lumichrome production.

Advances in ophthalmic drug delivery

Various strategies for ocular drug delivery are considered; from basic formulation techniques for improving availability of drugs; viscosity enhancers and mucoadhesives aid drug retention and penetration enhancers promote drug transport into the eye. The use of drug-loaded contact lenses and ocular inserts allows drugs to be better placed where they are needed for more direct delivery. Developments in ocular implants gives a means to overcome the physical barriers that traditionally prevented effective treatment. Implant technologies are under development allowing long-term drug delivery from a single procedure, these devices allow posterior chamber diseases to be effectively treated. Future developments could bring artificial corneas to eliminate the need for donor tissue and one-off implantable drug depots lasting the patient's lifetime.

Hydrotrope accumulation around the drug: the driving force for solubilization and minimum hydrotrope concentration for nicotinamide and urea

A rigorous statistical thermodynamic theory explains how urea and nicotinamide can solubilize hydrophobic drugs in water.

Hydrotropy: monomer-micelle equilibrium and minimum hydrotrope concentration

Drug molecules with low aqueous solubility can be solubilized by a class of cosolvents, known as hydrotropes. Their action has often been explained by an analogy with micelle formation, which exhibits critical micelle concentration (CMC). Indeed, hydrotropes also exhibit "minimum hydrotrope concentration" (MHC), a threshold concentration for solubilization. However, MHC is observed even for nonaggregating monomeric hydrotropes (such as urea); this raises questions over the validity of this analogy. Here we clarify the effect of micellization on hydrotropy, as well as the origin of MHC when micellization is not accompanied. On the basis of the rigorous Kirkwood-Buff (KB) theory of solutions, we show that (i) micellar hydrotropy is explained also from preferential drug-hydrotrope interaction; (ii) yet micelle formation reduces solubilization effeciency per hydrotrope molecule; (iii) MHC is caused by hydrotrope-hydrotrope self-association induced by the solute (drug) molecule; and (iv) MHC is prevented by hydrotrope self-aggregation in the bulk solution. We thus need a departure from the traditional view; the structure of hydrotrope-water mixture around the drug molecule, not the structure of the aqueous hydrotrope solutions in the bulk phase, is the true key toward understanding the origin of MHC.

Hydrotropic Solubilization by Urea Derivatives: A Molecular Dynamics Simulation Study

Hydrotropy is a phenomenon where the presence of a large quantity of one solute enhances the solubility of another solute. The mechanism of this phenomenon remains a topic of debate. This study employed molecular dynamics simulation to investigate the hydrotropic mechanism of a series of urea derivatives, that is, urea (UR), methylurea (MU), ethylurea (EU), and butylurea (BU). A poorly water-soluble compound, nifedipine (NF), was used as the model solute that was solubilized. Structural, dynamic, and energetic changes upon equilibration were analyzed to supply insights to the solubilization mechanism. The study demonstrated that NF and urea derivatives underwent significant nonstoichiometric molecular aggregation in the aqueous solution, a result consistent with the self-aggregation of urea derivatives under the same conditions. The analysis of hydrogen bonding and energy changes revealed that the aggregation was driven by the partial restoration of normal water structure. The energetic data also suggested that the promoted solubilization of NF is favored in the presence of urea derivatives. While the solutes aggregated to a varying degree, the systems were still in single-phase liquid state as attested by their active dynamics.

Cyclodextrin-mediated enhancement of riboflavin solubility and corneal permeability

Cyclodextrins are water-soluble cyclic oligosaccharides consisting of six, seven, and eight α-(1,4)-linked glucopyranose subunits. This study reports the use of different cyclodextrins in eye drop formulations to improve the aqueous solubility and corneal permeability of riboflavin. Riboflavin is a poorly soluble drug with a solubility up to 0.08 mg mL(-1) in deionized water. It is used as a drug topically administered to the eye to mediate UV-induced corneal cross-linking in the treatment of keratoconus. Aqueous solutions of β-cyclodextrin (10-30 mg mL(-1)) can enhance the solubility of riboflavin up to 0.12-0.19 mg mL(-1), whereas the higher concentration of α-cyclodextrin (100 mg mL(-1)) achieved a lower level of enhancement of 0.11 mg mL(-1). The other oligosaccharides were found to be inefficient for this purpose. In vitro diffusion experiments performed with fresh and cryopreserved bovine cornea have demonstrated that β-cyclodextrin enhances riboflavin permeability. The mechanism of this enhancement was examined through microscopic histological analysis of the cornea and is discussed in this paper.

Mechanism of hydrophobic drug solubilization by small molecule hydrotropes

Drugs that are poorly soluble in water can be solubilized by the addition of hydrotropes. Albeit known for almost a century, how they work at a molecular basis is still controversial due to the lack of a rigorous theoretical basis. To clear up this situation, a combination of experimental data and Fluctuation Theory of Solutions (FTS) has been employed; information on the interactions between all the molecular species present in the solution has been evaluated directly. FTS has identified two major factors of hydrotrope-induced solubilization: preferential hydrotrope-solute interaction and water activity depression. The former is dominated by hydrotrope-solute association, and the latter is enhanced by ionic dissociation and hindered by the self-aggregation of the hydrotropes. Moreover, in stark contrast to previous hypotheses, neither the change of solute hydration nor the water structure accounts for hydrotropy. Indeed, the rigorous FTS poses serious doubts over the other common hypothesis: self-aggregation of the hydrotrope hinders, rather than promotes, solubilization.

Improving the chemical stability of amorphous solid dispersion with cocrystal technique by hot melt extrusion

PURPOSE

To explore in-situ forming cocrystal as a single-step, efficient method to significantly depress the processing temperature and thus minimize the thermal degradation of heat-sensitive drug in preparation of solid dispersions by melting method (MM) and hot melt extrusion (HME).

METHODS

Carbamazepine (CBZ)-nicotinamide (NIC) cocrystal solid dispersions were prepared with polymer carriers PVP/VA, SOLUPLUS and HPMC by MM and/or HME. The formation of cocrystal was investigated by differential scanning calorimetry and hot stage polarized optical microscopy. State of CBZ in solid dispersion was characterized by X-ray powder diffraction and optical microscopy. Interactions between CBZ, NIC and polymers were investigated by FTIR. Dissolution behaviors of solid dispersions were compared with that of pure CBZ.

RESULTS

CBZ-NIC cocrystal with melting point of 160°C was formed in polymer carriers during heating process, and the preparation temperature of amorphous CBZ solid dispersion was therefore depressed to 160°C. The dissolution rate of CBZ-NIC cocrystal solid dispersion was significantly increased.

CONCLUSIONS

By in-situ forming cocrystal, chemically stable amorphous solid dispersions were prepared by MM and HME at a depressed processing temperature. This method provides an attractive opportunity for HME of heat-sensitive drugs.

Application of hydrotropy to transdermal formulations: hydrotropic solubilization of polyol fatty acid monoesters in water and enhancement effect on skin permeation of 5-FU

OBJECTIVES

A hydrotropic formulation containing a percutaneous enhancer was developed for the transdermal formulation of a water-soluble drug and the solubilizing mechanisms of a percutaneous enhancer in water by a hydrotropic agent were investigated. The enhancement effect was also compared with the hydrotropic formulation and the other formulations using ethanol, propylene glycol or mixed micelles.

METHODS

Sodium salicylate (SA) and sodium benzoate (BA) were selected as hydrotropic agents, and polyol fatty acid ester (POFE) and 5-fluorouracil (5-FU) were selected as a percutaneous enhancer and a water-soluble drug, respectively. Near-infrared (NIR) spectrophotometric and ¹H NMR spectroscopic studies were carried out to investigate the solubilizing mechanisms. The mean particle size in the hydrotropic formulation was measured. The in-vitro skin permeation of 5-FU and the accumulation in the skin of propylene glycol monocaprylate (PGMC), one of the monoesters of POFE, from the hydrotropic formulation or the other formulations were investigated by using Franz-type diffusion cell.

KEY FINDINGS

The presence of SA and BA had a visible effect on the O-H stretching band of water in the NIR region. The surface tension of SA and BA aqueous solutions was found to decrease with an increase in SA or BA concentration. Although SA interacted with PGMC in the presence of water, it did not interact with PGMC in the absence of water. Mean particle size in a solution consisting of 5% (v/v) PGMC and 30% SA aqueous solution was approximately 14 nm. ¹H NMR spectroscopic studies indicated that the hydrotropic salts formed aggregates with which PGMC interacted from the outside. The hydrotropic formulation prepared in this study enhanced skin permeation of 5-FU when compared with the other formulations.

CONCLUSIONS

SA and BA solubilized monoesters of POFE in water, and SA interacted with PGMC in water. The hydrotropic formulation prepared in this study significantly enhanced skin permeation of 5-FU compared with the other formulations. The results suggest that a hydrotropic formulation containing PGMC may be a useful transdermal formulation for water-soluble drugs.

Formulation and stability of a novel artificial human sweat under conditions of storage and use

A limitation of most artificial sweat formulations used for in vitro assessment of chemical release from materials in contact with skin have little biological relevance to human sweat. The purposes of this paper are to provide guidance for preparation of a novel artificial sweat with chemical constituents at concentrations that match human sweat and to characterize chemical stability. The artificial sweat was characterized under conditions of use (with and without sebum at 36 degrees C) and storage (without sebum at -4, 4, and 23 degrees C) over 28 days by gas chromatography-mass spectroscopy, high-performance liquid chromatography, enzymatic assay kits, and ion-selective electrodes. Seven indicator constituents were tracked: sodium, chloride, glucose, lactic acid, urea, pantothenic acid, and alanine. With or without sebum at 36 degrees C, the sweat solvent was chemically stable for 14 days. Storage by refrigeration at 4 degrees C retained the chemical integrity of the solvent longest. Based on these results, the solvent should be used within 14 days of preparation. The artificial sweat model presented herein is most similar to human sweat and has applications as a dissolution solvent, donor solution in diffusion cells, or vehicle for patch testing. This sweat model may aid researchers in understanding potential release and percutaneous absorption of chemicals in contact with human skin surface liquids.