Pharmacokinetics and dissolution of two crystalline forms of carbamazepine

Revealing the early stages of carbamazepine crystallization by cryoTEM and 3D electron diffraction

Time-resolved carbamazepine crystallization from wet ethanol has been monitored using a combination of cryoTEM and 3D electron diffraction. Carbamazepine is shown to crystallize exclusively as a dihydrate after 180 s. When the timescale was reduced to 30 s, three further polymorphs could be identified. At 20 s, the development of early stage carbamazepine dihydrate was observed through phase separation. This work reveals two possible crystallization pathways present in this active pharmaceutical ingredient.

Biowaiver Monograph for Immediate-Release Solid Oral Dosage Forms: Carbamazepine

Literature relevant to assessing whether BCS-based biowaivers can be applied to immediate release (IR) solid oral dosage forms containing carbamazepine as the single active pharmaceutical ingredient are reviewed. Carbamazepine, which is used for the prophylactic therapy of epilepsy, is a non-ionizable drug that cannot be considered "highly soluble" across the range of pH values usually encountered in the upper gastrointestinal tract. Furthermore, evidence in the open literature suggests that carbamazepine is a BCS Class 2 drug. Nevertheless, the oral absolute bioavailability of carbamazepine lies between 70 and 78% and both in vivo and in vitro data support the classification of carbamazepine as a highly permeable drug. Since the therapeutic and toxic plasma level ranges overlap, carbamazepine is considered to have a narrow therapeutic index. For these reasons, a BCS based biowaiver for IR tablets of carbamazepine cannot be recommended. Interestingly, in nine out of ten studies, USP dissolution conditions (900 mL water with 1% SLS, paddle, 75 rpm) appropriately discriminated among bioinequivalent products and this may be a way forward to predicting whether a given formulation will be bioequivalent to the comparator product.

Dissolution enhancement of carbamazepine using twin-screw melt granulation

The current study explored the twin-screw melt granulation (TSMG) as a potential technology for the water solubility enhancement of biopharmaceutical classification system (BCS) class II drugs. As a model drug, carbamazepine (CBZ) was formulated with three different polymers as melt granules produced in a co-rotating twin-screw granulator. Polyethylene glycol 6000 (PEG 6000) and Kolliphor® (poloxamer) P407 were used as binding materials at two different granulation temperatures (T_max: 70 °C; 100 °C). Additionally, Soluplus® (polyvinyl caprolactam-polyvinyl acetate-polyethylene glycol graft copolymer) was chosen as binder of higher melting/ granulation temperature (T_max: 140 °C). Temperature dependent polymorphic transition of CBZ during melt granulation was observed and identified using XRPD- (X-ray powder diffraction) and FTIR- (Fourier transform infrared spectroscopy) analysis. The effects of polymer type, polymer content (10, 15, 20% (w/w)) and granulation temperature on polymorphic transition, their impact on wettability (contact angle via drop shape-analysis), and the resulting dissolution performance at non-sink conditions in phosphate buffer (pH 6.8), were studied. This study showed that TSMG led to a crystalline system facilitating supersaturation when brought in solution, even when high drug loads (up to 90% (w/w)) were used. In general, for all granules produced, the supersaturation level and its duration varied with the extent of polymorphic transition and binder concentration. The results of this study indicated the importance of temperature control and polymer selection for tailoring desired dissolution profiles.

Assessment of Biopharmaceutical Performance of Supersaturating Formulations of Carbamazepine in Rats Using Physiologically Based Pharmacokinetic Modeling

There is an overgrowing emphasis on supersaturating drug delivery systems (SDDS) with increase in number of poorly water-soluble compounds. However, biopharmaceutical performance from these formulations is limited by phase transformation to stable crystalline form due to their high-energy physical form. In the present study, in vitro kinetic solubility in water and dissolution in biorelevant medium integrated with in silico physiologically based pharmacokinetic (PBPK) modeling was used to predict biopharmaceutical performance of SDDS of poorly water-soluble compound, carbamazepine (CBZ). GastroPlus™ with advanced compartmental absorption and transit model was used as a simulation tool for the study. Wherein, the model was developed using physicochemical properties of CBZ and disposition parameters obtained after intravenous administration of CBZ (20 mg/kg) into Sprague-Dawley (SD) rats. Biorelevant medium was selected by screening different dissolution media for their capability to predict oral plasma concentration-time profile of marketed formulation of CBZ. In vivo performance of SDDS was predicted with the developed model and compared to observed plasma concentration-time profile obtained after oral administration of SDDS into SD rats (20 mg/kg). The predictions, with strategy of using kinetic solubility and dissolution in the selected biorelevant medium, were consistent with observed biopharmaceutical performance of SDDS. Additionally, phase transformation of CBZ during gastrointestinal transit of formulations was evaluated and correlated with in vivo dissolution deconvoluted by Loo-Reigelman analysis.

Influences of Crystal Anisotropy in Pharmaceutical Process Development

Crystalline materials are of crucial importance to the pharmaceutical industry, as a large number of APIs are formulated in crystalline form, occasionally in the presence of crystalline excipients. Owing to their multifaceted character, crystals were found to have strongly anisotropic properties. In fact, anisotropic properties were found to be quite important for a number of processes including milling, granulation and tableting. An understanding of crystal anisotropy and an ability to control and predict crystal anisotropy are mostly subjects of interest for researchers. A number of studies dealing with the aforementioned phenomena are grounded on over-simplistic assumptions, neglecting key attributes of crystalline materials, most importantly the anisotropic nature of a number of their properties. Moreover, concepts such as the influence of interfacial phenomena in the behaviour of crystalline materials during their growth and in vivo, are still poorly understood. The review aims to address concepts from a molecular perspective, focusing on crystal growth and dissolution. It begins with a brief outline of fundamental concepts of intermolecular and interfacial phenomena. The second part discusses their relevance to the field of pharmaceutical crystal growth and dissolution. Particular emphasis is given to works dealing with mechanistic understandings of the influence of solvents and additives on crystal habit. Furthermore, comments and perspectives, highlighting future directions for the implementation of fundamental concepts of interfacial phenomena in the rational understanding of crystal growth and dissolution processes, have been provided.

Pharmaceutical solvates, hydrates and amorphous forms: A special emphasis on cocrystals

Active pharmaceutical ingredients (APIs) may exist in various solid forms, which can lead to differences in the intermolecular interactions, affecting the internal energy and enthalpy, and the degree of disorder, affecting the entropy. Differences in solid forms often lead to differences in thermodynamic parameters and physicochemical properties for example solubility, dissolution rate, stability and mechanical properties of APIs and excipients. Hence, solid forms of APIs play a vital role in drug discovery and development in the context of optimization of bioavailability, filing intellectual property rights and developing suitable manufacturing methods. In this review, the fundamental characteristics and trends observed for pharmaceutical hydrates, solvates and amorphous forms are presented, with special emphasis, due to their relative abundance, on pharmaceutical hydrates with single and two-component (i.e. cocrystal) host molecules.

Polymorph Impact on the Bioavailability and Stability of Poorly Soluble Drugs

Drugs with low water solubility are predisposed to poor and variable oral bioavailability and, therefore, to variability in clinical response, that might be overcome through an appropriate formulation of the drug. Polymorphs (anhydrous and solvate/hydrate forms) may resolve these bioavailability problems, but they can be a challenge to ensure physicochemical stability for the entire shelf life of the drug product. Since clinical failures of polymorph drugs have not been uncommon, and some of them have been entirely unexpected, the Food and Drug Administration (FDA) and the International Conference on Harmonization (ICH) has required preliminary and exhaustive screening studies to identify and characterize all the polymorph crystal forms for each drug. In the past, the polymorphism of many drugs was detected fortuitously or through manual time consuming methods; today, drug crystal engineering, in particular, combinatorial chemistry and high-throughput screening, makes it possible to easily and exhaustively identify stable polymorphic and/or hydrate/dehydrate forms of poorly soluble drugs, in order to overcome bioavailability related problems or clinical failures. This review describes the concepts involved, provides examples of drugs characterized by poor solubility for which polymorphism has proven important, outlines the state-of-the-art technologies and discusses the pertinent regulations.

Understanding pharmaceutical polymorphic transformations II: crystallization variables and influence on dosage forms

Excipients or formulation variables have often been exploited to improve stability, modify release, or improve physicochemical properties of dosage forms. In pharmaceutical field, it is generally expected that excipients work at macromolecular level where they might influence the crystal structure of a solid. These polymers/colloidal particles may modify the rate and direction of crystal growth. It has also been observed, that different polymorphic crystals exhibit different colors on exposure to same colorant, predominantly due to difference in surface pH of different crystal lattices. Apart from physicochemical affect, crystal habit also influences pharmacokinetic parameters of the dosage form. Crystals with smaller size or lower lattice energy have shown to exhibit higher bioavailability with faster rate of release.

In situ monitoring of carbamazepine-nicotinamide cocrystal intrinsic dissolution behaviour

Cocrystals have shown huge potential to improve the dissolution rate and absorption of a poorly water soluble drug. However, solution mediated phase transformation of cocrystals could greatly reduce the enhancement of its apparent solubility and dissolution rate. The aim of this study is to gain a deep understanding of the phase transition behaviour of cocrystals during dissolution and to investigate the improvement of dissolution rate. Dissolution and transformation behaviour of carbamazepine-nicotinamide (CBZ-NIC) cocrystal, physical mixture and different forms of carbamazepine: form I (CBZ I), form III (CBZ III) and dihydrate (CBZ DH) were studied by different in situ techniques of UV imaging and Raman spectroscopy. It has been found that compared with CBZ III and I, the rate of intrinsic dissolution rate (IDR) of CBZ-NIC cocrystal decreases slowly during dissolution, indicating the rate of crystallisation of CBZ DH from the solution is slow. In situ solid-state characterisation has shown the evolution of conversion of CBZ-NIC cocrystal and polymorphs to its dihydrate form. The study has shown that in situ UV imaging and Raman spectroscopy with a complementary technique of SEM can provide an in depth understanding during dissolution of cocrystals.

The influence of various excipients on the conversion kinetics of carbamazepine polymorphs in aqueous suspension

The influence of various excipients on the conversion of carbamazepine polymorphs to the dihydrate in aqueous suspension has been investigated. Ten excipients having functional groups which were potentially able to form hydrogen bonds with carbamazepine (group 1: methylcellulose, hypromellose (hydroxypropyl methylcellulose), hydroxypropylcellulose (HPC), 2-hydroxyethylcellulose (HEC), carmellose sodium (sodium carboxymethylcellulose), cellobiose; group 2: povidone (polyvinylpyrrolidone), povidone-vinyl acetate copolymer (povidone/VA) and N-methyl-2-pyrrolidone; group 3: macrogol (polyethylene glycol) and polyethylene oxide-polypropylene oxide copolymer (PEO/PPO)) were selected. Carbamazepine polymorphic forms III and I were dispersed separately into each aqueous excipient solution (0.1%, w/v) for 30 min at room temperature. The inhibition effect of each excipient was quantified using Raman spectroscopy combined with multivariate analyses. The solubility parameter of each excipient was calculated and used for categorizing excipients. Excipients in groups 1 and 2, which had both low solubility parameters (< 27.0 MPa½) and strong hydrogen bonding groups, inhibited the conversion completely. With increasing solubility parameter, the inhibition effect decreased for group 1 excipients, especially for carbamazepine form I, which had a higher specific surface area. Also, the excipients of group 3, lacking strong hydrogen bonding groups, showed poor inhibition although they had low solubility parameters (< 21.0 MPa½). This study indicated the importance of both hydrogen bonding interaction and a suitable hydrophobicity (expressed by the solubility parameter) in the inhibition of the conversion of carbamazepine to the dihydrate.

Effect of polymer additives on the transformation of BMS-566394 anhydrate to the dihydrate form

PURPOSE

To investigate the effect of polymer additives on the transformation of BMS-566394 anhydrate to the dihydrate form and to propose the possible mechanisms for inhibition of conversion of the anhydrate to the dihydrate form.

MATERIALS AND METHODS

The conversion of anhydrate to dihydrate was monitored using differential scanning calorimetry, powder X-ray diffraction and polarized light microscopy. Solubility and intrinsic dissolution studies were performed on anhydrate and dihydrate. IR and NMR spectroscopy were used to probe the molecular interactions between BMS-566394 and cellulose ether polymers.

RESULTS

The anhydrate form of BMS-566394 was readily transformed into the more stable dihydrate form in aqueous suspension. The kinetic solubility and intrinsic dissolution rate of the anhydrate were ca. fourfold that of the dihydrate. Addition of cellulose ether polymers (HPC, HPMC, MC) inhibited anhydrate to dihydrate transformation in aqueous suspensions. Hydrogen bonding interaction between the polar groups of the drug and polymers was inferred from infrared spectroscopy. Solution NMR also indicated a hydrophobic interaction between the drug and polymer backbone.

CONCLUSIONS

The anhydrate form of BMS-566394 is stabilized in the presence of cellulose ether polymers. Spectroscopic evidence is offered to postulate a molecular interaction between drug and polymers.

Influence of Polymorphic Form, Morphology, and Excipient Interactions on the Dissolution of Carbamazepine Compacts

To gain a deeper understanding of the behavior of carbamazepine (CBZ) and CBZ dihydrate (DH) compacts during in vitro dissolution tests various factors were investigated: hydrate formation of CBZ, crystal morphology, surface area, and excipient influence. Dissolution tests were performed in three different dissolution media: distilled water, hydroxypropyl methylcellulose (HPMC), and polyethylene glycol (PEG) solutions. For the CBZ compacts, the dissolution rate of CBZ in water was fastest (0.338 mg L(-1) min(-1)). With increasing ability of the excipients to inhibit the hydration of CBZ (PEG < HPMC), surprisingly the dissolution rate of CBZ compacts decreased: PEG solution (0.314 mg L(-1) min(-1)) > HPMC solution (0.257 mg L(-1) min(-1)). This implies that DH formation resulted in an apparent increase in the dissolution rate rather than slowing it down. For the DH compacts, the dissolution rate in water (0.055 mg L(-1) min(-1)) was slower than that of PEG and HPMC solutions (0.174 and 0.178 mg L(-1) min(-1), respectively). The contact angle measurements showed a significantly higher value in water (61.0 degrees) than in PEG and HPMC solutions (44.8 degrees and 43.1 degrees, respectively). Although the dissolution of CBZ and DH compacts in various dissolution media are complex processes, the influence and relative importance of these factors were clearly detected providing better understanding of the dissolution behavior of the drug.

Towards a universal dissolution medium for carbamazepine

The aim of this study was to develop a dissolution medium for assessment of various carbamazepine (CBZ) formulations with different strengths. The design of a system inhibiting transformation of the anhydrous CBZ (CBZ A) to the dihydrate form (CBZ D), with minimum surface-active properties and suitable sink was investigated. The effect of pH, different concentrations of sodium lauryl sulphate (SLS), polyvinylpyrrolidone (PVP), and methyl cellulose (MC) on dissolution rate, solubility, dissolution solubility, and polymorphic transformation of CBZ was assessed. Solution-mediated transformation of CBZ A into CBZ D was monitored using optical microscopy, Fourier transform infrared spectroscopy and differential scanning calorimetry. Results showed that different strengths (100, 200, 400 mg) of the same CBZ tablet formulation exhibited different dissolution patterns, in 1% SLS (USP system). Such differences were reduced in 0.5% SLS solution which provided sufficient sink for up to 200 mg CBZ. It was also shown that solubility of CBZ A could not be detected in the media under study (water, SGF, SIF, and SLS solutions) due to its rapid transformation into CBZ D. The use of 3% PVP solution protected CBZ A from conversion for 75 min, while 0.01% MC completely inhibited the transformation up to 24 h. Therefore, a medium consisting of 0.5% SLS and 0.01% MC was selected. The medium provided: a) protection against transformation of CBZ A to CBZ D, b) increased solubility of CBZ A (204 mg % compared to 128 mg % of CBZ D in 0.5% SLS), c) suitable sink for up to 400 mg CBZ and d) overlapping dissolution profiles of various strengths of the same CBZ formulation. The suggested system may be a step in the way of solving CBZ dissolution problems that forced the USP to specify two similar dissolution tests with two different limits for conventional 200 mg CBZ tablets.

Fluorescence studies of the transformation of carbamazepine anhydrate form III to its dihydrate phase

It has been found that both the anhydrous Form III and dihydrate phases of carbamazepine exhibit fluorescence in the solid state. The fluorescence intensity associated with the dihydrate phase was determined to be significantly more intense than that associated with the anhydrate phase, and this difference was exploited to develop a method for study of the kinetics of the aqueous solution-mediated phase transformation between these forms. Studies were conducted at temperatures over the range 18-40 degrees C, and it was found that the phase transformation was adequately characterized by first-order reaction kinetics. The temperature dependence in the calculated rate constants was used to calculate an activation energy of 11.2 kcal/mol (47.4 cal/g) for the anhydrate-to-dihydrate phase conversion.

Solution-mediated phase transformation of anhydrous to dihydrate carbamazepine and the effect of lattice disorder

This paper describes the kinetics of the solution-mediated phase transformation of the anhydrous monoclinic polymorph of carbamazepine (CBZ(A)) to the dihydrate crystal form (CBZ(D)). Monitoring both solution concentration and solid phase composition identified the steps and mechanisms that control the kinetic processes, and regulate the concentration of drug achieved during dissolution of the metastable solid phase, CBZ(A). The results show that the kinetics and the rate-controlling step for the transformation depend on grinding and storage conditions of CBZ(A). Grinding CBZ(A) shortened the transformation times and changed the rate-controlling step from crystallization of CBZ(D) to dissolution of CBZ(A). Grinding may cause various degrees of disorder in the form of lattice defects and/or amorphous regions. These disordered regions promote the anhydrous to dihydrate transformation by facilitating the surface nucleation of CBZ(D) on freshly ground CBZ(A) and on amorphous CBZ. The concentration-time profiles revealed aging effects on the solution-mediated transformation of ground CBZ(A) that were undetectable by diffraction and thermal analysis. These results have significant consequences on the concentration-time profiles of active pharmaceutical ingredients during dissolution of metastable solid phases, crystalline or amorphous.

Physicochemical characterization of solid dispersions of carbamazepine formulated by supercritical carbon dioxide and conventional solvent evaporation method

Solid dispersions of carbamazepine (CBZ) were formulated by supercritical fluid processing (SCP) and conventional solvent evaporation in polyethylene glycol (PEG) 8000 with either Gelucire 44/14 or vitamin E TPGS NF (d-alpha-tocopheryl PEG 1000 succinate). Formulations were evaluated by dissolution, scanning electron microscopy, powder X-ray diffraction, and differential scanning calorimetry, and excipient cytotoxicity in Caco-2 cells by MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt] assay. CBZ release was enhanced from supercritical fluid-treated CBZ and the CBZ/PEG 8000 (1:5), CBZ/PEG 8000/TPGS or Gelucire 44/14 (1:4:1) solid dispersions. The radically altered morphologies of SCP samples seen by scanning electron microscopy suggested polymorphic change that was confirmed by the X-ray diffraction and differential scanning calorimetry. Disappearance of the characteristic CBZ melting peak indicated that CBZ was dissolved inside the carrier system. Polymorphic change of CBZ during SCP led to faster dissolution. Therefore, SCP provides advantages over solid dispersions prepared by conventional processes.

Effects of dehydration temperature on water vapor adsorption and dissolution behavior of carbamazepine

Anhydrous carbamazepine was prepared by heating carbamazepine dihydrate at 60, 80, 100, 120, and 140 degrees C and used to investigate the effects of dehydration temperature on water vapor adsorption and dissolution behavior. The hydration rate of anhydrous carbamazepine at 75, 83, and 95% relative humidity and 25 degrees C decreased with increasing heating temperature. From the dissolution study by the rotating disk method, the calculated solubility of anhydrous carbamazepine was about 2.5 times higher than that of the dihydrate. The rate of phase transformation from the anhydrous form into the dihydrate during the dissolution process decreased with an increase in sample preparation temperature. These phenomena were further studied by thermal analysis, specific surface area measurement, density measurement, small-angle X-ray scattering, and wide-angle powder X-ray diffraction. As the heating temperature was raised, the specific surface area was reduced and the density was increased; furthermore, the average of the solid part calculated by the Debye method with small-angle X-ray scattering increased. The anhydrous carbamazepine prepared at lower heating temperatures was found to have a more porous structure and was seen by wide-angle powder X-ray diffraction to comprise both anhydrous forms I and II.

Effects of dehydration temperatures on moisture absorption and dissolution behavior of theophylline

Anhydrous theophylline was prepared by heating theophylline monohydrate at temperatures between 60 degrees C and 140 degrees C. The effects of dehydration temperatures on the moisture absorption and dissolution behavior of anhydrous theophylline were investigated in this study. The hydration rate of anhydrous theophylline at 95% relative humidity and 25 degrees C decreased with increasing dehydration temperatures. From the fitting analysis of solid-state reaction models, the hydration reaction was found to be governed by the phase boundary reaction model for samples prepared at lower dehydration temperatures (<100 degrees C) but the reaction obeyed the growth of nuclei reaction model when samples were dehydrated at higher temperatures. The dissolution rates of various anhydrous theophylline samples were measured by the rotating disk method. The calculated solubility of anhydrous theophylline prepared by heating was about 2.5 times higher than that of theophylline monohydrate. The phase transformation rate from the anhydrous form to the monohydrate during dissolution tests decreased with higher dehydration temperatures. It was found that the anhydrous theophylline prepared at different dehydration temperatures transformed to the monohydrate by way of different growth of hydrate nuclei mechanism.

Time-resolved X-ray scattering using synchrotron radiation applied to the study of a polymorphic transition in carbamazepine

The thermodynamic status of alpha-carbamazepine has been clarified using equilibrium solubility measurements, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), heated X-ray powder diffraction (XRPD), and temperature-controlled X-ray scattering techniques. alpha-Carbamazepine is the least stable of the three well-characterized anhydrous polymorphs of carbamazepine at 25 degrees C. In addition, it was confirmed that alpha-carbamazepine undergoes an exothermic transition to gamma-carbamazepine at 130 degrees C. The novel technique of time-resolved simultaneous small- and wide-angle X-ray scattering has been successfully applied to monitor this transition in situ. It was concluded that alpha-carbamazepine has a monotropic relationship with gamma-carbamazepine.

Formulation and evaluation of vinylpyrrolidone/vinylacetate copolymer microspheres with carbamazepine

Vinylpyrrolidone/vinylacetate copolymer (VP/VAc) was used for the enhancement of dissolution rate of carbamazepine, an antiepileptic drug characterized by very low water solubility. Microspheres containing different drug-to-polymer ratios were prepared using a solvent-evaporation technique and their physical characterizations were carried out by differential scanning calorimetry, x-ray diffractometry, and scanning electron microscopy. Through the solubilization kinetics and dissolution rates studies, the in vitro drug availability of the microspheres was evaluated. These results showed that the dissolution of carbamazepine in gastrointestinal fluids from all of the prepared formulations was increased with respect to the drug itself. However, the best dissolution parameters were obtained from 1:10 w/w system because of the presence of the drug in amorphous form and its efficient encapsulation.

Interactions between carbamazepine and polyethylene glycol (PEG) 6000: characterisations of the physical, solid dispersed and eutectic mixtures

The influence of a hydrophilic carrier (PEG 6000) on the polymorphism of carbamazepine, an antiepileptic drug, was investigated in binary physical mixtures and solid dispersions by means of differential scanning calorimetry (DSC), thermal gravimetry, hot-stage microscopy (HSM), and X-ray diffractometry, respectively. This study provides also an attempt to develop a method to calculate more precisely the eutectic composition. In rather ideal physical mixtures, carbamazepine was found as monoclinic Form III. In solid dispersions, the drug was found to crystallize as trigonal Form II; a eutectic invariant in the PEG 6000-rich composition domain (6% of carbamazepine mass) was evidenced by DSC experiments and confirmed by HSM observations. In the binary phase diagram the ideal carbamazepine liquidus curve was located at temperatures higher than the respective experimental ones. This suggests that drug can be maintained in the liquid state in the temperature-mass fraction (T--x) region between the two carbamazepine liquidus curves. This indicates in turn that attractive interactions occur between carbamazepine and PEG 6000-chains. These interactions have been also claimed to prevent carbamazepine from degradation into iminostilbene (a compound resulting from the chemical degradation of carbamazepine which is postulated to be responsible for the idiosyncratic toxicity of the drug) and thought to lead to the crystallization of metastable Carbamazepine II from melt. The negative excess entropy for eutectic mixtures indicated that the drug crystals are finely dispersed in the bulk of polymer chains.

Physicochemical properties and bioavailability of carbamazepine polymorphs and dihydrate

The dissolution behaviors of carbamazepine (CZP) polymorphs and pseudopolymorphs (form I, form III and dihydrate) and the bioavailabilities (BA) of each form in dogs after oral administration were investigated. Bioavailability tests were carried out at a dose of either 40 mg/body or 200 mg/body. The results of dissolution tests in JP13 first fluid (pH 1.2) at 37 degrees C indicated that the initial dissolution rate was in the order of form III>form I>dihydrate, while form III was transformed to dihydrate more rapidly than form I, resulting in decrease of the dissolution rate. The solubilities of both anhydrates (form I and form III), calculated from the initial dissolution rate of each anhydrate, were 1.5--1.6 times that of the dihydrate. At the dose of 40 mg/body, there were no significant differences in the area under the curve (AUC) between forms; their AUCs were nearly equal to that of CZP solution using polyethyleneglycol 400. These findings suggested that most crystalline powder of each form administered at the low dose was rapidly dissolved in gastrointestinal (GI) fluid. On the other hand, for the dose of 200 mg/body, significant differences in plasma concentration--time curves of CZP among polymorphic forms and dihydrate were observed. The order of AUC values was form I>form III>dihydrate. The inconsistency between the order of initial dissolution rates and that of AUC values at the high dose may have been due to rapid transformation from form III to dihydrate in GI fluids.

Effect of packaging and storage on the stability of carbamazepine tablets

The effect of packaging and storage on carbamazepine (CBZ) tablets was examined using Tegretol and Tegral, dispensed in strip seals, and Finlepsin, dispensed in bottles. Tegretol and Tegral tablets were stored in their original strips at 40 degrees C, 50 degrees C, and 60 degrees C for 6 months, 3 months, and 1 month, respectively, at 75% relative humidity (RH). Also, tablets were removed from their strips, placed in bottles, and exposed daily to 97% RH at 40 degrees C for 5 min for 30 days. Finlepsin tablets were exposed to 97% RH at 25 degrees C or 40 degrees C for 1 month by removing bottle caps daily for 5 min. Dissolution was used to assess in vitro tablet performance, and high-performance liquid chromatography (HPLC) was used to evaluate the chemical stability of CBZ. Results show that Tegretol tablets were not affected by the tested stress conditions. Tegral tablets, stored in their strips at 50 degrees C or 60 degrees C and 75% RH, showed increased disintegration and dissolution. The effect of 40 degrees C/75% RH for 6 months was similar to 1-month storage at 40 degrees C/97% RH; the tablets hardened and dissolved less than fresh Tegral tablets. Removal of Tegral tablets from their original strips resulted in only 7% dissolved in 60 min. For Finlepsin, the effect of 97% RH at 40 degrees C was more profound than 97% RH at 25 degrees C, but both conditions caused a decrease in dissolution, the extent of which was dependent on tablet position in the bottle. Stressed CBZ tablets, however, showed no change in the chemical stability of CBZ under all tested conditions.

Crystalline properties of carbamazepine in sustained release hydrophilic matrix tablets based on hydroxypropyl methylcellulose

The influence of hydroxypropyl methylcellulose (HPMC) on the crystal habit properties of carbamazepine in sustained release matrix tablets and in aqueous solutions was investigated using differential scanning calorimetry (DSC), X-ray powder diffraction and scanning electron microscopy (SEM). The results suggest that HPMC inhibits the transformation of carbamazepine to carbamazepine dihydrate in the gel layer of hydrated tablets and in aqueous solutions (depending on HPMC concentration), participates in its crystallization process and induces amorphism of carbamazepine crystals. The mechanism which explains these effects envisages the polymer serving as a template or microsubstrate for nucleation in the crystallization process. We assume that the interaction between the drug and polymer occurs by hydrogen bonding. The hydroxyl groups of the polymer may attach to the drug at the site of water binding, and thus its transformation to the dihydrate form, is inhibited. A more specific interaction involves structural matching (similar bond spacing distances) between inter-atomic distances in the crystal lattice of carbamazepine dimer and intra-atomic distances along the polymer chain.

Characterization of dihydrates prepared from carbamazepine polymorphs

To clarify earlier literature suggesting its existence in more than one solid form, carbamazepine dihydrate was prepared from individual anhydrous carbamazepine polymorphs I and III. The resultant materials were characterized by thermal and spectroscopic techniques, including variable-temperature Fourier transform (FT) Raman spectroscopy. On thermal dehydration, the carbamazepine dihydrates formed different anhydrous polymorphs depending on both starting material and dehydration conditions. Under conditions where liberated hydrate water cannot readily escape from around the sample, the dihydrate originating from polymorph III reverted to polymorph III, but under conditions of lower humidity/moisture, this dihydrate reverted to polymorph I. The dihydrate originating from polymorph I reverted to polymorph I irrespective of dehydration conditions. These observations could be due to a trace amount of nonhydrated original polymorph existing in the dihydrate phase that acts as a seed to regenerate the original polymorph under conditions where liberated hydrate water cannot readily escape from around the sample during dehydration. No evidence was generated that supported the existence of true polymorphs of carbamazepine dihydrate.

Pharmaceutical evaluation of carbamazepine modifications: comparative study for photostability of carbamazepine polymorphs by using Fourier-transformed reflection-absorption infrared spectroscopy and colorimetric measurement

The tablet surface was evaluated without physical damage by means of Fourier-transform infrared reflection-absorption spectroscopy (FT-IR-RAS) and colorimetric measurement (colour difference, delta E) of the carbamazepine polymorphs I, II and III, after photodegradation at two irradiation intensities (3.0 and 12.0 J cm-2s-1) under a near-UV fluorescent lamp. The surface of sample pellets of all crystalline forms turned gradually from white to yellow-orange upon exposure to light, and the discoloration rate of form II was faster than that of forms I and III, indicating that form II was the most unstable of the three. The major photoproducts were identified by HPLC, NMR and MS analyses. The carbamazepine content on the surface of the tablet was determined based on the absorption at 1685 cm-1 attributable to C=O stretch vibration in the FT-IR-RAS spectra before and after irradiation by a near-UV fluorescent lamp. The semilogarithmic plots of the photodegradation profiles of the various polymorphs were straight lines, including the induction period, indicating that degradation of the drug on the surface followed first-order kinetics. The induction periods of all forms were not significantly different. However, the degradation rate constant of form II at 12.0 J cm-2s-1 was 5.1 and 1.5 times larger than those of forms I and III, respectively.