Dissolution kinetics and solubilities of p-aminosalicylic acid and its salts

Investigating the Effect of Basic Amino Acids and Glucosamine on the Solubility of Ibuprofen and Piroxicam

Purpose

Poor aqueous solubility hampers the development of several compounds as pharmacological agents. Hence, preparing novel formulations with augmented absorption is a challenge in pharmaceutical industries. In this paper, we have examined the effect of basic amino acids including arginine (ARG), lysine (LYS), and glucosamine (GlucN) on the solubility of ibuprofen (IBU) and piroxicam (PXM) as drugs with limited solubility. We have also studied the effect of the dissolution media with the pH values 1.2 to 7.4.

Methods

The saturation shake-flask method was used for solubility studies in the presence of amino acids. Briefly, buffer solutions containing different concentrations of amino acids were prepared. Then, an excess amount of each drug with these buffers was shaken to reach equilibrium. After 48 hours, the upper phase was separated, and solubility was calculated by reading their UV-Vis absorbance.

Results

The results illustrated that amino acids increased solubility of both drugs with different ratios, which were pH and concentration-dependent. Solubility improved as the amount of amino acids went up, and this upward pattern was more robust with ARG than LYS. The presence of GlucN in citrate buffer significantly enhanced IBU solubility. The solubility of PXM in accompany of GlucN in water did not change significantly while in citrate buffer solubility enhanced specially at pH 6.

Conclusion

Overall, GlucN in citrate buffer and ARG in phosphate buffer could be introduced as the most suitable media for IBU and PXM solubility improvement, respectively.

New amino acid propyl ester ibuprofenates from synthesis to use in drug delivery systems

This study aimed to evaluate the effect of introducing structural modification of ibuprofen in the form of an ion pair on the permeability of ibuprofen through the skin and the properties of the adhesive layer of the medical patch produced. The active substances tested were the salts of ibuprofen obtained by pairing the anion of ibuprofen with organic cations such as propyl esters of amino acids such as tyrosine, tryptophan, histidine, or phenylalanine. For comparison, the penetration of unmodified ibuprofen and commercially available patches was also tested. Acrylate copolymers based on isobornyl methacrylate as a biocomponent and a monomer increasing the T g ("hard") were used to produce the adhesive layer of transdermal patches. The obtained patches were characterized in terms of adhesive properties and tested for the permeability of the active ingredient and the permeability of the active ingredient through the skin. This study demonstrates the possibility of developing acrylic-based photoreactive transdermal patches that contain biocomponents that can deliver a therapeutically appropriate dose of ibuprofen.

Salts of Therapeutic Agents: Chemical, Physicochemical, and Biological Considerations

The physicochemical and biological properties of active pharmaceutical ingredients (APIs) are greatly affected by their salt forms. The choice of a particular salt formulation is based on numerous factors such as API chemistry, intended dosage form, pharmacokinetics, and pharmacodynamics. The appropriate salt can improve the overall therapeutic and pharmaceutical effects of an API. However, the incorrect salt form can have the opposite effect, and can be quite detrimental for overall drug development. This review summarizes several criteria for choosing the appropriate salt forms, along with the effects of salt forms on the pharmaceutical properties of APIs. In addition to a comprehensive review of the selection criteria, this review also gives a brief historic perspective of the salt selection processes.

Calcium Coordination Solids for pH-Triggered Release of Olsalazine

Calcium coordination solids were synthesized and evaluated for delivery of olsalazine (H₄ olz), an anti-inflammatory compound used for treatment of ulcerative colitis. The materials include one-dimensional Ca(H₂ olz)⋅4 H₂ O chains, two-dimensional Ca(H₂ olz)⋅2 H₂ O sheets, and a three-dimensional metal-organic framework Ca(H₂ olz)⋅2DMF (DMF=N,N-dimethylformamide). The framework undergoes structural changes in response to solvent, forming a dense Ca(H₂ olz) phase when exposed to aqueous HCl. The compounds Ca(H₂ olz)⋅x H₂ O (x=0, 2, 4) were each pressed into pellets and exposed to simulated gastrointestinal fluids to mimic the passage of a pill from the acidic stomach to the pH-neutral intestines. All three calcium materials exhibited a delayed release of olsalazine relative to Na₂ (H₂ olz), the commercial formulation, illustrating how formulation of a drug within an extended coordination solid can serve to tune its solubility and performance.

Strategies to address low drug solubility in discovery and development

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.

A discriminatory intrinsic dissolution study using UV area imaging analysis to gain additional insights into the dissolution behaviour of active pharmaceutical ingredients

For efficient and effective drug development it is desirable to acquire a deep understanding of the dissolution behaviour of potential candidate drugs and their physical forms as early as possible and with the limited amounts of material that are available at that time. Using 3-10mg sample quantities, the ability of a UV imaging system is investigated to provide deep mechanistic insight into the intrinsic dissolution profiling of a range of compounds and physical forms assessed under flow conditions. Physical forms of indomethacin, theophylline and ibuprofen were compressed and their solid-state form confirmed before and after compression with X-ray methods and/or Raman spectroscopy. Intrinsic dissolution rates (IDRs) were determined using the compact's UV-imaging profile. The ratio in the IDRs for theophylline anhydrate over hydrate was 2.1 and the ratio for the alpha form of indomethacin over the gamma form was approximately 1.7. The discriminatory power of the novel UV area visualisation approach was shown to be high in that process-induced solid-state dissolution differences post-micronisation could be detected. Additionally, the scale-down system was able to visualise a previously observed increase in ibuprofen IDR with an increase in concentration of sodium dodecyl sulphate. The mechanistic dissolution insights from the visualisation approach are evident.

Preparation and characterisation of novel spray-dried nano-structured para-aminosalicylic acid particulates for pulmonary delivery: impact of ammonium carbonate on morphology, chemical composition and solid state

Objectives

The objective of this work was to spray dry p-aminosalicylic acid (PAS) and its ammonium salt and to investigate the impact of the pore-forming agent, ammonium carbonate (AC), on the morphological, aerodynamic and physicochemical properties of the resulting powders.

Methods

Microparticles were prepared by spray drying from ethanol/water solvent systems. Their solid-state properties were evaluated by scanning electron microscopy, powder X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and in-vitro deposition, using the twin impinger.

Key Findings

The physicochemical properties of PAS were altered on spray drying with AC and a new solid state was produced. The solution composition impacted on the morphology of the resulting powders, which ranged from irregular crystal agglomerates to spherical crystal clusters and porous microparticles. The chemical composition, structure and morphology were dependent on process inlet temperature, low inlet temperatures resulting in a novel solid of stoichiometry; PAS : ammonia : water, 2 : 1 : 0.5. At higher temperatures pure PAS was obtained. In-vitro deposition studies showed an increase in emitted dose from spray dried drug, relative to the micronised PAS.

Conclusions

Under appropriate process conditions AC interacts with the acidic PAS, resulting in the formation of a novel solid-state drug phase. Spray-dried PAS powders have potential for pulmonary delivery.

Preparation and characterization of amino acids-based trimethoprim salts

Trimethoprim (TMP) is a dihydrofolate reductase (DHFR) inhibitor which prevents the conversion of dihydrofolic acid into tetrahydrofolic acid, resulting in the depletion of the latter and leading to bacterial death. Oral bioavailability of TMP is hindered by both its low solubility and low permeability. This study aims to prepare novel salts of TMP using anionic amino acids; aspartic and glutamic acid as counter ions in order to improve solubility and dissolution. TMP salts were prepared by lyophilisation and characterized using FT-IR spectroscopy, proton nuclear magnetic resonance (1HNMR), Differential Scanning Calorimetry (DSC) and Thermogravimetric analysis (TGA). Both the amino acids formed salts with TMP in a 1:1 molar ratio and showed a 280 fold improvement in solubility. Investigation of the microbiological activity of the prepared salts against TMP sensitive Escherichia coli showed that the new salts not only retained antibacterial activity but also exhibited higher zone of inhibition which was attributed to improved physicochemical characters such as higher solubility and dissolution. The results are an important finding that could potentially impact on faster onset of antibacterial activity and reduced therapeutic dose when administered to patients. Studies are underway investigating the effect of ion-pairing TMP with amino acids on the permeability profile of the drug.

Diflunisal salts of bupivacaine, lidocaine and morphine

The present work describes the characterization of diflunisal salts of the analgesic agents bupivacaine, lidocaine, and morphine including their solubility behaviour and release characteristics from solutions and selected salt suspensions in vitro using the rotating dialysis cell model. The solubility of the 1:1 salts at pH 7.4 differed by a factor of 9 with the bupivacaine and lidocaine salts representing the poorest and most soluble salt (0.73 and 6.6mM, respectively). Common ion effects were observed for the diflunisal salts of bupivacaine and morphine when various concentrations of the lidocaine-diflunisal salt were present in aqueous buffer (pH 7.4). The most pronounced salting-out effect was observed for the poorest soluble salt. From Setschenow type plots apparent salting-out constants of 265 M(-1) (bupivacaine) and 54.7 M(-1) (morphine) were calculated. After instillation of mixed salt suspensions comprising the diflunisal salts of bupivacaine and lidocaine into the donor cell of the release model, lidocaine appeared rapidly in the acceptor phase. After clearance of lidocaine from the donor cell, equal and constant fluxes of bupivacaine and diflunisal were observed. The residence times of bupivacaine within the donor compartment was prolonged with increasing lidocaine-diflunisal salt load in the mixed suspensions whereas the slopes of the linear part of the bupivacaine release profiles were affected to a minor extent only. The obtained data indicate that local multimodal analgesia, characterized by rapid onset and extended duration of action, can be achieved upon injection of mixed suspensions of salts differing with respect to aqueous solubility comprising a common ion into a small body compartment (such as the joint cavity).

QSPR prediction of aqueous solubility of drug-like organic compounds

A quantitative structure property relationship (QSPR) study was performed to develop a model that relates the structures of 150 drug organic compounds to their aqueous solubility (log S(w)). Molecular descriptors derived solely from 3D structure were used to represent molecular structures. A subset of the calculated descriptors selected using stepwise regression that used in the QSPR model development. Multiple linear regression (MLR) is utilized to construct the linear QSPR model. The applied multiple linear regression is based on a variety of theoretical molecular descriptors selected by the stepwise variable subset selection procedure. Stepwise regression was employed to develop a regression equation based on 110 training compounds, and predictive ability was tested on 40 compounds reserved for that purpose. The final regression equation included three parameters that consisted of octanol/water partition coefficient (log P), molecular volume (MV) and hydrogen bond forming ability (HB), of the drug molecules, all of which could be related to solubility property. Application of the developed model to a testing set of 40 drug organic compounds demonstrates that the new model is reliable with good predictive accuracy and simple formulation. The use of descriptors calculated only from molecular structure eliminates the need for experimental determination of properties for use in the correlation and allows for the estimation of aqueous solubility for molecules not yet synthesized. The prediction results are in good agreement with the experimental values. The root mean square error of prediction (RMSEP) and square correlation coefficient (R(2)) of prediction of log S(w) were 0.0959 and 0.9954, respectively.

Prediction of the aqueous solubility of benzylamine salts using QSPR model

Models predicting aqueous solubility of benzylamine salts were developed using multivariate partial least squares (PLS) and artificial neural network (ANN). Molecular descriptors, including binding energy (BE) and surface area of salts (SA), were calculated by the use of Hyperchem and ChemPlus QSAR programs for Windows. Other physicochemical properties, such as hydrogen acceptor for oxygen atoms, hydrogen acceptor for nitrogen atoms, hydrogen bond donors, hydrogen bond forming ability, molecular weight (MW), and calculated log partition coefficient (clog P) of p-substituted benzoic acids, were also used as descriptors. In this study, the predictive ability of ANN, especially multilayer perceptron (MLP) architecture networks, was founded to be superior to PLS models. The best ANN model derived, a 6-1-1 architecture, had an overall R(2) of 0.850 and root mean square error (RMSE) for cross-verification and test set of 0.189 and 0.185 log units, respectively. Since all the utilized descriptors are readily obtained from calculation, these derived models offer the advantage of not requiring the experimental determination of some descriptors.

Prediction of aqueous solubility of organic salts of diclofenac using PLS and molecular modeling

Organic salts of diclofenac were predicted by using computed molecular descriptors and multivariate partial least squares (PLS). The molecular descriptors including binding energy and surface area of salts were calculated by the use of Hyperchem and ChemPlus QSAR programs for Windows. Other physicochemical properties such as hydrogen acceptor for oxygen atoms, hydrogen acceptor for nitrogen atoms, hydrogen bond donors, hydrogen bond-forming ability, molecular weight, and log partition coefficient (logP) of bases were also used as descriptors. Good statistical models were derived that permit simple computational prediction of salt solubility of a same parent structure. The final models derived had R2 value = 0.96 and root mean square error for prediction (RMSEP) values ranging from 0.021 to 0.054 (log scale). Preferably all utilized descriptors in the final models can readily obtain from the chemical structure of salt and base. Molecular weight of base is one of the important factors associated with salt solubility. While increased molecular weight of base, surface area of salt and hydrogen bonding ability of base increase solubility, and increased binding energy and logP of base have negative effect on salt solubility.

Aqueous solubility study of salts of benzylamine derivatives and p-substituted benzoic acid derivatives using X-ray crystallographic analysis

Twenty two p-substituted benzoic acid derivates were used to prepare salts of N-methylbenzylamine (II) and N,N-dimethylbenzylamine (III), respectively. Only five salts of (II) and two salts of (III) were obtained in a crystalline state. The solubility of these salts was orders of magnitude higher than those reported for the corresponding salts of benzylamine (I). Thermal analysis indicated that the increased solubility was caused by reduced crystal lattice energy, which was most likely due to the reduced number of strong hydrogen bonds of the salt of (II) and (III). X-ray crystallographic analysis of p-hydroxybenzoic acid salt of (I), (II) and (III) suggested that the reduced number of hydrogen bonds caused the apparent higher solubility. Further analyses of seven salts of (I) were performed. It was not possible to identify any relationship between the number of hydrogen bonds and the corresponding solubility of the salts.

Colonic delivery of 4-aminosalicylic acid using amylose-ethylcellulose-coated hydroxypropylmethylcellulose capsules

BACKGROUND

4-Aminosalicylic acid has the potential for use in the treatment of diseases of the colon.

AIM

To assess the feasibility of delivering 4-aminosalicylic acid directly to the colon using a hydroxypropylmethylcellulose capsule coated with a mixture of amylose, a polysaccharide metabolized by bacterial enzymes in the colon, and ethylcellulose.

METHODS

Seven healthy male volunteers received, on three separate occasions, an uncoated or amylose-ethylcellulose-coated hydroxypropylmethylcellulose capsule containing 4-aminosalicylic acid Na (550 mg), or an intravenous injection of 4-aminosalicylic acid Na (135 mg). The capsules were radiolabelled with 99mTc to allow their positions in the gastrointestinal tract to be followed using a gamma camera. Plasma and urine samples were collected and assayed for 4-aminosalicylic acid and metabolite concentrations.

RESULTS

The uncoated capsules broke down within 10 min in the stomach, allowing rapid and complete absorption of the drug. The coated capsules remained intact in the upper gastrointestinal tract, and had a median gastric emptying time of 61 min (interquartile range, 77 min) and a median colon arrival time of 363 min (interquartile range, 185 min). For the coated capsules, only the metabolite was detected in the plasma and/or urine after the capsules had reached the colon.

CONCLUSIONS

The specific coating protected the drug until the capsule reached the colon, where 4-aminosalicylic acid was slowly released and absorbed. Thus, such a formulation has the potential for use in the treatment of inflammatory bowel disease.

Preparation and characterisation of a range of diclofenac salts

Physicochemical properties of diclofenac salts prepared using eight different counterions and including five novel salts, obtained with the bases 2-amino-2-methyl-1,3-propanediol, 2-amino-2-methylpropanol, tert-butylamine, benzylamine and deanol, were compared. Four of the bases used to prepare these salts were related in their chemical structure, differing only in the number of hydroxy groups. Characterisation techniques included X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis, thermomicroscopy, Karl Fischer titration, FT-IR spectroscopy and elemental analysis. In the case of salts prepared from 2-amino-2-methylpropanol and benzylamine, two polymorphic forms of each salt were identified. For the 2-amino-2-methyl-1,3-propanediol salt, a pseudopolymorphic form was identified. The aqueous solubilities of the salts studied ranged from 3.95 mM (tris(hydroxymethyl)aminomethane salt) to 446 mM (deanol salt), corresponding to a 113-fold difference in solubility. The solubility of diclofenac deanol was higher than the solubilities for diclofenac salts reported earlier. Correlation was found between the inverse of the salt melting point and the logarithm of salt solubility. A log-log relationship was observed between salt solubility and hydrogen ion concentration in the salt solution. Relationships between the properties of the salt-forming agents and those of the resulting diclofenac salts were explored. Reasonable correlation was found between the free base melting point and the salt melting point.