Infrared studies of the polymorphic states of the fenamates

Photolysis of tolfenamic acid in aqueous and organic solvents: a kinetic study

Tolfenamic acid (TA) is a non-steroidal anti-inflammatory drug that was studied for its photodegradation in aqueous (pH 2.0-12.0) and organic solvents (acetonitrile, methanol, ethanol, 1-propanol, 1-butanol). TA follows first-order kinetics for its photodegradation, and the apparent first-order rate constants (k obs) are in the range of 0.65 (pH 12.0) to 6.94 × 10-2 (pH 3.0) min-1 in aqueous solution and 3.28 (1-butanol) to 7.69 × 10-4 (acetonitrile) min-1 in organic solvents. The rate-pH profile for TA photodegradation is an inverted V (∧) or V-top shape, indicating that the cationic form is more susceptible to acid hydrolysis than the anionic form of TA, which is less susceptible to alkaline hydrolysis. The fluorescence behavior of TA also exhibits a V-top-shaped curve, indicating maximum fluorescence intensity at pH 3.0. TA is highly stable at a pH range of 5.0-7.0, making it suitable for formulation development. In organic solvents, the photodegradation rate of TA increases with the solvent's dielectric constant and solvent acceptor number, indicating solute-solvent interactions. The values of k obs decreased with increased viscosity of the solvents due to diffusion-controlled processes. The correlation between k obs versus ionization potential and solvent density has also been established. A total of 17 photoproducts have been identified through LC-MS, of which nine have been reported for the first time. It has been confirmed through electron spin resonance (ESR) spectrometry that the excited singlet state of TA is converted into an excited triplet state through intersystem crossing, which results in an increased rate of photodegradation in acetonitrile.

Analysis of Tolfenamic Acid using a Simple, Rapid, and Stability-indicating Validated HPLC Method

BACKGROUND

Tolfenamic acid (TA) belongs to the fenamates class of nonsteroidal anti-inflammatory drugs. Insufficient information is available regarding the availability of a reliable and validated stability-indicating method for the assay of TA.

OBJECTIVE

A relatively simple, rapid, accurate, precise, economical, robust, and stabilityindicating RP-HPLC method has been developed to determine TA in pure and tablet dosage forms.

METHODS

The method was validated according to the ICH guideline, and parameters like linearity, range, selectivity, accuracy, precision, robustness, specificity, and solution stability were determined. TLC and FTIR spectrometry were used to ascertain the purity of TA. The specificity was determined with known impurities and after performing forced degradation, while the robustness was established by Plackett-Burman's experimental design. The mobile phase used for the analysis was acetonitrile and water (90:10, v/v) at pH 2.5. The detection of the active drug was made at 280 nm using a C18 column (tR = 4.3 min.). The method's applicability was also checked for the yellow polymorphic form of TA.

RESULTS

The results indicated that the method is highly accurate (99.39-100.80%), precise (<1.5% RSD), robust (<2% RSD), and statistically comparable to the British Pharmacopoeia method with better sensitivity and specificity.

CONCLUSION

It was observed that the stress degradation studies do not affect the method's accuracy and specificity. Hence the proposed method can be used to assay TA and its tablet dosage form.

Preparation, Characterization and Pharmacokinetics of Tolfenamic Acid-Loaded Solid Lipid Nanoparticles

The clinical use of nonsteroidal anti-inflammatory drugs is limited by their poor water solubility, unstable absorption, and low bioavailability. Solid lipid nanoparticles (SLNs) exhibit high biocompatibility and the ability to improve the bioavailability of drugs with low water solubility. Therefore, in this study, a tolfenamic acid solid lipid nanoparticle (TA-SLN) suspension was prepared by a hot melt–emulsification ultrasonication method to improve the sustained release and bioavailability of TA. The encapsulation efficiency (EE), loading capacity (LC), particle size, polydispersity index (PDI), and zeta potential of the TA-SLN suspension were 82.50 ± 0.63%, 25.13 ± 0.28%, 492 ± 6.51 nm, 0.309 ± 0.02 and −21.7 ± 0.51 mV, respectively. The TA-SLN suspension was characterized by dynamic light scattering (DLS), fluorescence microscopy (FM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and Fourier transform infrared (FT-IR) spectroscopy. The TA-SLN suspension showed improved sustained drug release in vitro compared with the commercially available TA injection. After intramuscular administration to pigs (4 mg/kg), the TA-SLN suspension displayed increases in the pharmacokinetic parameters T_max, T_1/2, and MRT_0–∞ by 4.39-, 3.78-, and 3.78-fold, respectively, compared with TA injection, and showed a relative bioavailability of 185.33%. Thus, this prepared solid lipid nanosuspension is a promising new formulation.

Quantum Chemical Study on Mefenamic Acid Polymorphic Forms

Three polymorphic structures of mefenamic acid, which is a very popular drug, have been studied using quantum chemical methods. It has been shown that the centrosymmetric dimer formed due to two O-H···O hydrogen bonds is a complex building unit in all of the polymorphic structures under study. On the basis of an analysis of the pairwise interaction energies between molecules, the polymorphic forms I and II are classified as columnar-layered while the polymorphic form III has a columnar structure. The stabilities of the three polymorphic forms of mefenamic acid under ambient conditions (I > II > III) correlate with the degree of anisotropy of the interaction energies between columns (primary basic structural motifs) formed due to stacking interactions. The shear deformation modeling of strongly bound layers in all of the polymorphic structures has not revealed any possibility for deformation of the crystal structure. The construction of the shift energy profiles and calculation of the energy barriers for the displacement along the (100) crystallographic plane in the [100], [010], and [011] crystallographic directions make it possible to explain the experimental data obtained for commercially available polymorphic structure I in a diamond anvil cell. The absence of any local minimum near the starting point on the shift energy profile and the extremely high energy barrier can be considered as criteria for the impossibility of a crystal structure deformation under pressure.

Liquid antisolvent recrystallization and solid dispersion of flufenamic acid with polyvinylpyrrolidone K-30

Flufenamic acid (FFA) is a Biopharmaceutical Classification System- II (BCS-II) class drug with poor bioavailability and a lower dissolution rate. Particle size reduction is one of the conventional approaches to increase the dissolution rate and subsequently the bioavailability. The use of the liquid antisolvent method for particle size reduction of FFA was studied in this work. Ethanol and water were used as solvent and antisolvent, respectively. Experimental parameters such as solution concentration (10–40 mg/ml), flow rate (120–480 ml/h), temperature (298–328 K) and stirring speed (200–800 rpm) were investigated. Furthermore, the solid dispersion of FFA was prepared with polyvinylpyrrolidone K-30 (PVP K-30) with different weight ratios (1:1, 1:2, 1:3 and 1:4) and samples were characterized using SEM, FTIR and XRD techniques. The experimental investigation revealed that higher values of concentration, injection rate, stirring speed, along with lower temperature favored the formation of fine particles. SEM analysis revealed that the morphology of raw FFA changed from rock-like to rectangular-like after liquid antisolvent recrystallization. FTIR analysis validated the presence of hydrogen bonding between FFA and PVP in solid dispersion. XRD analysis showed no significant change in the crystallinity of the processed FFA.

Development and validation of a spectrofluorimetric method for the analysis of tolfenamic acid in pure and tablet dosage form

Tolfenamic acid (TA) is commonly used in humans and animals because of its anti-inflammatory, antipyretic, and analgesic effects. So far, no study has been carried out to develop a validated spectrofluorimetric method for determination of TA. Therefore, the present study aimed to develop and validate a simple, accurate, rapid, economical, and precise spectrofluorimetric method to assay TA in its pure and dosage forms, and also in degraded solutions. The fluorimetric method had higher sensitivity compared with the spectrophotometric and high-performance liquid chromatography methods and could determine the drug at the microgram level. Optimum pH of TA for maximum fluorescence intensity was 3, and its two pK_a values were calculated as 1.95 and 4.05. The proposed method was validated according to the guidelines of the International Council for Harmonisation, and parameters such as linearity, range, accuracy, precision, sensitivity, robustness, specificity, and solution stability were tested. Stress-induced degradation studies on TA did not affect the accuracy and precision of the proposed method. The results obtained indicated that the method was linear over the concentration range 0.2-0.9 × 10^-3 M with good accuracy, precision, and robustness for assay of TA in its pure and its tablet dosage forms and was comparable statistically with the British Pharmacopoeia method.

The role of solid state properties on the dissolution performance of flufenamic acid

Flufenamic acid is a nonsteroidal anti-inflammatory drug characterized by a low solubility and a variable oral bioavailability. Flufenamic acid is present in the commercial products in two polymorphic enantiotropic forms (Form I and III). Bioinequivalence was observed for commercial solid dosage forms due to the different dissolution rate of batches. Aim of this work is the full characterization of the solid state properties of flufenamic acid in order to evidence reasons of its variable dissolution properties. Two different batches of pure drug obtained by different suppliers were fully characterized. In order to evaluate the effect of the technological processes used for tablet production, the powders were submitted to grinding, kneading, and compression. Thermal analysis and X-ray diffraction studies proved that the drug was provided by both suppliers as Form I, Form III is obtained by recrystallization from ethanol or ethanol/water of both batches and no changes were observed after the different mechanical treatments. No difference was observed between the two forms in terms of equilibrium solubility values. Dissolution rate studies evidenced a difference between the two batches due to their different particle size, which disappeared after sieving. Interestingly, a significant difference in terms of intrinsic dissolution rate and surface wettability of the two compacted powders was observed, even after sieving, probably related to a different behavior of the two powder samples under compaction. These results should be taken into account, during a tablet formulation, in order to obtain a reproducible dissolution performance of the drug, regardless of its original supplier.

Polymorphism in Solid Dispersions

Solid dispersions embed active pharmaceutical ingredients in polymeric carriers to improve their solubility. Three solid dispersion preparation techniques are typically employed: solvent evaporation, solvent-fusion, and fusion methods. Although these are also widely recommended as preparative methods for phase diagram determination, few examples exist concerning their effect on the resulting polymorph, once the solid dispersion is produced. In this study, the influence of these methods on the polymorphic form obtained in crystalline solid dispersions (CSDs) composed of flufenamic acid (FFA) and poly(ethylene glycol) was investigated. The physical mixtures and CSDs were characterized by powder X-ray diffraction, infrared spectroscopy, and differential scanning calorimetry. The results reveal that the fusion method leads to concomitant polymorphs (mainly FFA I and III) in the CSDs. In contrast, the solvent evaporation and solvent-fusion methods lead to FFA III. Collectively, these results demonstrate that preparative methods have a significant influence on the phase diagrams determined (average relative deviation ≤8%), which are often used to justify the design space of manufacturing processes, including those deemed "continuous." Consequently, choosing a preparation method that results in the desired polymorph is crucial to ensure accurate determination of phase diagrams and critical quality attributes of formulations.

In the Context of Polymorphism: Accurate Measurement, and Validation of Solubility Data

Solubility measurements for polymorphic compounds are often accompanied by solvent-mediated phase transformations. In this study, solubility measurements from undersaturated solutions are employed to investigate the solubility of the two most stable polymorphs of flufenamic acid (FFA forms I and III), tolfenamic acid (TA forms I and II), and the only known form of niflumic acid (NA). The solubility was measured from 278.15 to 333.15 K in four alcohols of a homologous series (methanol, ethanol, 1-propanol, n-butanol) using the polythermal method. It was established that the solubility of these compounds increases with increasing temperature. The solubility curves of FFA forms I and III intersect at ~315.15 K (42 °C) in all four solvents, which represents the transition temperature of the enantiotropic pair. In the case of TA, the solubility of form II could not be reliably obtained in any of the solvents because of the fast solvent-mediated phase transformation. The solubility of the only known form of NA was also determined, and no other polymorphs of NA were observed. The experimental solubility data of FFA (forms I and III), TA (form I), and NA in these four solvents was correlated using the modified Apelblat and λh model equations. The correlated and experimentally determined solubility data obtained serves to (i) guide the accurate determination of the solubility for polymorphic compounds, (ii) assess the role of the solvent in mediating transformations, and (iii) provide a route to engineer advanced crystallization processes for these pharmaceutical compounds.

Far-Infrared Spectroscopy as a Probe for Polymorph Discrimination

Pharmaceutical crystalline polymorph and amorphous form detection and quantification is a standard requirement in the pharmaceutical industry. Infrared (IR) spectroscopy provides an important probe for the characterization of polymorphs. Nonetheless, characterization and discrimination among polymorphs using mid-IR spectroscopy is not always possible in part because the technique mainly probes vibrational modes arising from functional groups in the sample. In the present work, far-IR spectroscopy is demonstrated for the discrimination of polymorphs. This region is influenced by delocalized lattice vibrational modes derived from intermolecular forces and packing arrangements in the crystal structure. A total of 10 polymorphic pharmaceuticals were prepared to conduct a critical evaluation of the question, does this far-IR region add value for polymorph differentiation? It is demonstrated that the far-IR region offers high discriminating power for polymorphs compared to the mid-IR spectral region. In addition, structural similarity and dissimilarity in polymorphic packing arrangements can be derived from this analysis.

Protection of normal cells from irradiation bystander effects by silica-flufenamic acid nanoparticles

The development of a myriad of nanoparticles types has opened new possibilities for the diagnostics and treatment of many diseases, especially for cancer. However, most of the researches done so far do not focus on the protection of normal cells surrounding a tumor from irradiation bystander effects that might lead to cancer recurrence. Gap-junctions are known to be involved in this process, which leads to genomic instability of neighboring normal cells, and flufenamic acid (FFA) is included in a new group of gap-junction blockers recently discovered. The present work explores the use of mesoporous silica nanoparticles MCM-41 functionalized with 3-Aminopropyltriethoxysilane (APTES) for anchoring the flufenamic acid for its prolonged and controlled release and protection from radiation bystander effects. MCM-41 and functionalized samples were structurally and chemically characterized with multiple techniques. The biocompatibility of all samples was tested in a live/dead assay performed in cultured MRC-5 and HeLa cells. HeLa cells cultured were exposed to 50 Gy of gamma-rays and the media transferred to fibroblast cells cultured separately. Our results show that MCM-41 and functionalized samples have high biocompatibility with MCR-5 and HeLa cells, and most importantly, the FFA delivered by these NPs was able to halt apoptosis, one of main bystander effects.

Revealing Polymorphic Phase Transformations in Polymer-Based Hot Melt Extrusion Processes

The inadvertent occurrence of polymorphic phase transformations in active pharmaceutical ingredients (APIs) during hot melt extrusion (HME) processes has been claimed to limit the application of this technique. Hence, the control of polymorphism would need to be addressed if there is any prospect of HME to be successfully implemented as an alternative solid dosage formulation strategy in integrated, continuous end-to-end pharmaceutical manufacturing settings. This work demonstrates that flufenamic acid (FFA), one of the most polymorphic APIs known, thus far, can be processed using temperature-simulated HME with polyethylene glycol (PEG) as polymeric carrier. At temperatures above the transition point of FFA forms III and I (42 °C), the induction time of the polymorphic phase transformation is longer than the average reported residence time in conventional HME processes (5 min). Moreover, it was demonstrated that thorough understanding of the thermodynamic and kinetic design space for the PEG-FFA system leads to polymorphic control in the produced crystalline solid dispersions. Ultimately, this investigation helps to gain fundamental understanding of the processing needs of crystalline solid dispersions, which will lead to the broader application of HME as a continuous manufacturing strategy for drug products containing APIs prone to polymorphism, representing about 80% of all APIs.

Tolfenamic Acid

Tolfenamic acid (TA) is a nonsteroidal antiinflammatory drug and belongs to the group of fenamates. It is used as a potent pain reliever in the treatment of acute migraine attacks, and disorders like dysmenorrhea, rheumatoid, and osteoarthritis. TA has shown excellent in vitro antibacterial activity against certain ATCC strains of bacteria when complexed with bismuth(III). It has also been reported to block pathological processes associated with Alzheimer's disease. In the recent past, TA has also been used as a novel anticancer agent for the treatment of various cancers. In view of the clinical importance of TA, a comprehensive review of the physical and pharmaceutical properties and details of the various analytical methods used for the assay of the drug in pharmaceutical and biological systems has been made. The methods reviewed include identification tests and titrimetric, spectrophotometric, chromatographic, electrochemical, thermal, microscopic, enzymatic, and solid-state techniques. Along with the analytical profile, the stability and degradation of TA, its pharmacology and pharmacokinetics, dosage forms and dose, adverse effects and toxicity, and interactions have been discussed.

Response of Fibroblasts MRC-5 to Flufenamic Acid-Grafted MCM-41 Nanoparticles

Recently, flufenamic acid (FFA) was discovered among fenamates as a free radical scavenger and gap junction blocker; however, its effects have only been studied in cancer cells. Normal cells in the surroundings of a tumor also respond to radiation, although they are not hit by it directly. This phenomenon is known as the bystander effect, where response molecules pass from tumor cells to normal ones, through communication channels called gap junctions. The use of the enhanced permeability and retention effect, through which drug-loaded nanoparticles smaller than 200 nm may accumulate around a tumor, can prevent the local side effect upon controlled release of the drug. The present work, aimed at functionalizing MCM-41 (Mobil Composition of Matter No. 41) silica nanoparticles with FFA and determining its biocompatibility with human fibroblasts MRC-5 (Medical Research Council cell strain 5). MCM-41, was synthesized and characterized structurally and chemically, with multiple techniques. The biocompatibility assay was performed by Live/Dead technique, with calcein and propidium–iodide. MRC-5 cells were treated with FFA-grafted MCM-41 for 48 h, and 98% of cells remained viable, without signs of necrosis or morphological changes. The results show the feasibility of MCM-41 functionalization with FFA, and its potential protection of normal cells, in comparison to the role of FFA in cancerous ones.

Improvement of the water solubility of tolfenamic acid by new multiple-component crystals produced by mechanochemical methods

Tolfenamic acid (HTA) is a drug characterized by very poor solubility in water. By mechanochemical methods, new solid-state forms of HTA were obtained, showing better thermal stability than pure HTA and an improved dissolution rate.

Development and characterization of novel polyurethane films impregnated with tolfenamic acid for therapeutic applications

The present study deals with the preparation of polyurethane (PU) films impregnated with a nonsteroidal anti-inflammatory drug, tolfenamic acid (TA). Solvent evaporation technique has been employed for the preparation of TA-PU films in two different ratios of 1 : 2 and 1 : 5 in Tetrahydrofuran (THF) or THF-ethanol mixtures. The prepared films were characterized using X-Ray Diffraction (XRD), Differential Scanning Calorimetry (DSC), Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and release studies. The results indicate transformation of crystalline TA to its amorphous form. The degree of crystallinity changes both by increasing the polymer concentration and solvent used for the film preparations. The release profiles of TA were also found to be affected, showing a decrease from approximately 50% to 25% from 1 : 2 to 1 : 5 ratios, respectively.

Polymorph Discrimination using Low Wavenumber Raman Spectroscopy

Characterization of crystalline polymorphs and their quantitation has become an integral part of the pre-clinical drug development process. Raman spectroscopy is a powerful technique for the rapid identification of phases of pharmaceuticals. In the present work we demonstrate the use of low wavenumber Raman vibrational spectroscopy (including phonon measurement) for discrimination among polymorphs. A total of 10 polymorphic pharmaceuticals were employed to conduct a critical assessment. Raman scattering in the low frequency region (10-400 cm-1), which includes crystal lattice vibrations, has been analyzed and the results indicate lattice phonon Raman scattering can be used for rapid discrimination of polymorphic phases with additional discriminating power compared to conventional collection strategies. Moreover structural insight and conformational changes can be detected with this approach.

Studies on tolfenamic acid-chitosan intermolecular interactions: effect of pH, polymer concentration and molecular weight

Solid-state properties of tolfenamic acid (TA) and its complexes with chitosan (CT) have been studied. Effect of medium pH, molecular weight of polymer and its different concentrations on these TA-CT complexes were studied in detail. Low and medium molecular weight CT have been used in different ratios at pH ranging from 4 to 6 and freeze-drying technique has been employed to modify the appearance of crystalline TA. Physical properties of the formed complexes have been studied by employing X-ray diffraction, differential scanning calorimetry and scanning electron microscopy; chemical structure has been studied using Fourier transform infrared spectroscopy. The results showed that both forms of the polymer exhibited complete conversion in 1:8 ratio at pH 4, 1:4 at pH 5 and 1:1 at pH 6 indicating a marked effect of pH on drug-polymer complexation. The percent crystallinity calculations indicated low molecular weight CT slightly more effective than the other form. No changes in the complexes have been observed during the 12 week storage under controlled conditions. Both forms of CT at different pH values indicated retardation of recrystallization in TA during cooling of the melt from 1:1 ratios exhibiting formation of strong intermolecular hydrogen bonding between the drug and the polymer.

Raman and IR spectroscopic studies of fenamates--conformational differences in polymorphs of flufenamic acid, mefenamic acid and tolfenamic acid

Solid-state Raman and IR spectra of two polymorphic forms of each of three fenamates (flufenamic acid, mefenamic acid and tolfenamic acid) display subtle but highly reproducible differences. Many of these spectral differences can be ascribed to different conformations of these molecules, involving two of four possible orientations of one substituted benzene ring with respect to the other. Interpretation of the vibrational spectra in terms of conformational differences has been facilitated by DFT calculations at the B3LYP/cc-pVDZ level for each conformer. The calculated spectra are compared with the experimental spectra in order to identify the conformers present in two polymorphic forms in each case, and detailed band assignments are obtained from the DFT calculations.

Development of mucoadhesive films for buccal administration of flufenamic acid: Effect of cyclodextrin complexation

A new mucoadhesive film for topical administration in the oral cavity of flufenamic acid, a poorly soluble anti-inflammatory drug, has been developed, using complexation with hydroxypropyl-beta-cyclodextrin (HPbetaCD) to improve drug dissolution and release rate. Buccal films were prepared utilising chitosan as mucoadhesive polymer, KollicoatIR as film-forming polymer and glycerol as plasticiser. Different combinations of these components were used and the obtained films were characterised for weight, thickness, swelling, mucoadhesive and mechanical properties. The film containing chitosan 2%, glycerol 7.5% and KollicoatIR 1% showed the best properties for the development of the film formulation. The selected film was loaded with the plain drug and its colyophilised and coground products with HPbetaCD, and in vitro release studies in simulated saliva were performed. The improved drug dissolution properties, obtained by complexation with HPbetaCD, were critical to achieve complete release from film formulation during 4-5 h. On the contrary, film loaded with the plain drug showed incomplete release, not exceeding 70% release after 5 h. The developed film formulation containing the drug as complex with HPbetaCD can assure a prolonged drug release directly at the inflammation site and can be proposed as a new therapeutic tool in the treatment of oral mucosa inflammations.

Characterization and Physical Stability of Tolfenamic Acid-PVP K30 Solid Dispersions

Obtaining a stable formulation with high bioavailability of a poorly water-soluble drug often presents a challenge to the formulation scientist. Transformation of the drug into its more soluble high-energy amorphous form is one method used for improving the dissolution rate of such compounds. The present study uses the spray-drying technique for preparation of solid dispersions (SDs) of tolfenamic acid (TA) and polyvinylpyrrolidone K-30 (PVP). The SDs and TA in the form of a spray-dried powder were initially characterized and compared with a physical mixture and starting materials. Stability of the SDs was monitored over 12 weeks at 25 degrees C and 60% RH. XRPD studies revealed changes in solid state during the formation of the SDs and indicated the presence of TA in the amorphous state. FTIR, together with TGA, suggested molecular interactions (hydrogen-bonding) in the SDs. Dissolution studies proved an increase in the dissolution rate of TA from all SDs. The SDs with higher content of PVP retained TA in the amorphous state throughout the stability study. However, SDs with lower content showed recrystallization of TA after 1 week. Thus, this study reveals the possibility of preparing stable SDs of amorphous TA in PVP with improved dissolution rate.

Polymorphism and solvatomorphism 2005

Papers and patents that deal with polymorphism (crystal systems for which a substance can exist in structures characterized by different unit cells, but where each of the forms consists of exactly the same elemental composition) and solvatomorphism (systems where the crystal structures of the substance are defined by different unit cells, but where these unit cells differ in their elemental composition through the inclusion of one or molecules of solvent) have been summarized in an annual review. The works cited in this review were published during 2005, and were drawn primarily from the major physical, crystallographic, and pharmaceutical journals. The review is divided into sections that cover articles of general interest, computational and theoretical studies, preparative and isolation methods, structural characterization and properties of polymorphic and solvatomorphic systems, studies of phase transformations, effects associated with secondary processing, and United States patents issued during 2005.