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Thermal analysis techniques, DSC and TG can advantageously be used in quality control of drug products.

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Abstract  

DSC purity analysis is based on thermodynamic phase diagrams for substances (purity ≥98%) which undergo a melting point. Impurities which have eutectic behaviour with the analyte are determined together. DSC purity analysis obtained from a single melting event of a 1–2 mg sample is, therefore, extremely attractive for the global assessment of eutectic impurities. The main advantages in early development lie in the very small amount of material necessary and the very fast analysis time. However, the DSC purity analysis cannot replace chromatographic methods which deliver specific individual levels of impurities. Furthermore, a complete validation of a DSC purity method is difficult and time consuming. Despite these limitations, DSC is the best support for the development of chromatographic methods, for purity profile and stability assessment during pharmaceutical development. Parameters of purity determination and validation aspects are discussed. Examples of use in pharmaceutical development are given.

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Use of sub-ambient DSC to complement conventional DSC and TG

The study of water adsorption of drug substances and excipients

Journal of Thermal Analysis and Calorimetry
Authors: D. Giron and C. Goldbronn

Abstract  

Several drug substances or excipients are hygroscopic. The uptake or loss of water of such substances is generally difficult to control during processing or storage of drug products. DSC instruments with sub-ambient temperature equipment allow the determination of the amount of freezable water by measuring the corresponding melting enthalpy. The determination of freezable water adds valuable information complementary to TG analysis for understanding the processing and storage of raw materials and drug products. Several substances were tested as is, without treatment, after storage at 92% r.h. and after equilibration with water. The results of these experiments showed that it was possible to demonstrate defined hydrate formation, to determine the upper level of binding of water in amorphous substances and to confirm reversible hydrate formations demonstrated by temperature resolved X-ray diffraction.

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Abstract  

The polymorphic behaviour of the purine derivative MKS 492 was studied with investigations of suspensions of selected samples in different solvents and of samples obtained by crystallizations. The samples were analyzed by DSC, TG and X-ray diffraction. Six different crystalline modifications called A, B, B, C, D and E and an amorphous form were identified. Four pure crystalline modifications, A, B, C and D have been manufactured and characterized by DSC, X-ray, IR, solubilities, densities, hygroscopicity and dissolution measurements. The four forms A, C, D and E are monotrop to the form B. The form B is enantiotrop to the form B, which revealed the highest melting point of all known polymorphs. This form B is only stable at high temperature. Temperature resolved X-ray diffraction was very helpful for proper interpretation of the thermal events. The melting peaks of the forms A and C and the endothermic peak corresponding to the enantiotropic transition B into B occur in a narrow range of temperature. The form B which is the most stable one at room temperature has been chosen for further development. Quantitative methods to determine the content of the forms A, C and D in samples of form B or to determine the content of form A, B and D in form C have been developed by using X-ray diffraction. Limits of detection are 1 or 2%. For the quantitative determination of the amorphous fraction, X-ray diffraction and microcalorimetry are compared. For high amounts of the amorphous fraction, the X-ray diffraction method is preferred because it is faster. Microcalorimetry is very attractive for levels below 10% amorphous content. The lowest limit of detection is obtained by microcalorimetry, about 1%.

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Abstract  

The local anesthetic drug tetracaine hydrochloride is described in the Europ. Pharmacopea with a melting point of 148°C or with a range of 134 to 147°C due to the melting points of two other forms. The polymorphic behaviour of tetracaine hydrochloride has been studied by using thermal treatments, storage at 92% r.h., crystallizations and equilibrations with saturated solutions. Samples were characterized by X-ray diffraction, IR, thermal analysis and elemental analysis. Since some findings were difficult to interpret, temperature resolved X-ray diffraction was used additionally for the understanding of the thermal behaviour of tetracaine hydrochloride. In this study the polymorphic behaviour of some other local anesthetic drugs is compared. Ten different forms of tetracaine hydrochloride: six anhydrous crystalline forms, an amorphous form, a hemihydrate, a monohydrate and a tetrahydrate were identified. The relationships between all forms are given. The heating curve of the commercial form 1 is very dependent on the heating rate. This anhydrous form 1 is the thermodynamic stable modification at ambient temperature. The form 2 is reversibly enantiotrope to form 1. The four other modifications called 3, 4, 5 and 6 are monotropes of form 1. Only forms 1 and 5 are stable at ambient temperature. Form 1 is hygroscopic only at high humidity level of 92% r.h., form 5 is hygroscopic at 61% r.h. Both transform into the monohy-drate. No polymorphic forms of tetracaine base, dibucaine hydrochloride, procaine hydrochloride or prilocaine hydrochloride were found. The commercial form of bupivacaine hydrochloride is a monohydrate. Thermal treatment at 200°C gives one anhydrous form. As demonstrated by temperature resolved X-ray diffraction two other forms are detected by heating and cooling processes between 100 and 170°C. Equilibrations and crystallization experiments show that solvates are easily obtained in different solvents. Temperature resolved X-ray diffraction is a very efficient tool as a support to DSC for the identification of the transition processes and interpretation of thermal events and thermodynamic relationships. Equilibration experiments are very adequate to find out the thermodynamically stable form at ambient temperature (solvent mediated transitions).

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Abstract  

The physico-chemical properties and polymorphism of a new active pharmaceutical ingredient entity has been analyzed and the gain of knowledge during the chemical development of the substance is described. Initial crystallization revealed an anhydrous crystal form with good crystallinity and a single, sharp DSC melting peak at 171C and a straightforward development of this crystal form seemed possible. However, during polymorphism screening, new crystalline forms were detected that were often analyzed as mixtures of crystal forms. The process of characterization and identification of the different crystalline forms and its thermodynamical relationship has been supported by a combination of experimental and computational work including determination of the three-dimensional structures of the crystal forms. The crystal structure of one polymorphic form was solved by single crystal X-ray structure analysis. Unfortunately, Mod B resisted in formation of suitable single crystals, but its structure could be solved by high resolution powder diffraction data analysis using synchrotron radiation. Calculation of the theoretical X-ray powder diffraction pattern from three dimensional crystal coordinates allowed an unambiguous identification of the different crystalline forms. Two polymorphic crystal forms of the API-CG3, named Mod A and Mod B, are enantiotropic whereas Mod B is the most stable polymorph at room temperature up to about 50C and Mod A at temperatures above 50C. The mechanism of the solid-solid transition can be explained by analyzing the molecular packing information gained from the single crystal structures. A third crystalline form with the highest melting peak turned out to be not a polymorphic or pseudopolymorphic crystal modification of our API-CG3 but a chemically different substance.

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