Characterization of Polymorphs and Solvates of 3-Amino-1-(m-trifluoromethylphenyl)-6-methyl-1H-pyridazin-4-one

Reversibility of Enantiotropically Related Polymorphic Transformations from a Practical Viewpoint: Thermal Analysis of Kinetically Reversible/Irreversible Polymorphic Transformations

Although enantiotropically related polymorphic transitions are thermodynamically reversible with temperature and pressure, they may be observed as either reversible or irreversible in practical investigations. This behavior may be explained simply by a difference in the energy barrier for the transition. However, this difference may have a significant effect on strategy in formulation development. This article summarizes the characteristics of enantiotropic transitions with reference to the literature. Also provided are careful investigations of polymorphic transitions by thermal analysis, which is the most promising method to investigate transition behavior.

Trends in Solubility of Polymorphs

Polymorphism of drug substances has been the subject of intense investigation in the pharmaceutical field for over 40 years. Considering the multitude of reports on solubility or dissolution of polymorphs in the literature, an attempt is made in this study to answer the question: How big is the impact of polymorphism on solubility? A large number of literature reports on solubility or dissolution of polymorphs were reviewed and the data were analyzed for trends in solubility ratio of polymorphs. The general trend reveals that the ratio of polymorph solubility is typically less than 2, although occasionally higher ratios can be observed. A similar trend is also observed for anhydrate/hydrate solubility ratios, although anhydrate/hydrate solubility ratios appear to be more spread out and higher than the typical ratio for nonsolvated polymorphs. An attempt is also made in this commentary to estimate the ratio of solubilities of polymorphs from thermal data. The trend in estimated solubility ratio shows good agreement with the one observed with experimentally determined solubility values.

Solid state characterization of E2101, a novel antispastic drug

E2101, a novel antispastic drug, was found to exist in at least two polymorphs that were confirmed by X-ray powder diffraction (XRD). These two species are designated forms I and II. The physicochemical and thermodynamic properties of these polymorphs were characterized by variable temperature XRD, thermal analysis, hygroscopicity measurements, and dissolution studies. The transition temperature was also estimated from the solubilities determined at various temperatures. The E2101 polymorphs were anhydrous and adsorbed little moisture under high humidity conditions. The melting onsets and heats of fusion for form I were 148.1 +/- 0.2 degrees C and 38.2 +/- 1.0 kJ/mol, respectively, and for form II were 139.8 +/- 0.4 degrees C and 35.2 +/- 0.5 kJ/mol, respectively. The intrinsic dissolution rate of form II in JP 2 medium was 1.5-fold faster than that of form I, corresponding to the rank order of the aqueous solubility and the enthalpy of fusion. Accordingly, form I was thought to be thermodynamically more stable than form II and thus suitable for further development. According to the thermal analysis and variable temperature XRD results, the recrystallization of form I occurred at approximately 145 degrees C after form II melted, however, no crystal transition behavior was observed below the lower melting point. The DSC thermograms at various heating rates and van't Hoff plots from the solubility studies indicated that the polymorphic pair would be monotropic.

Neotame anhydrate polymorphs. II: Quantitation and relative physical stability

PURPOSE

To study the relative thermodynamic and kinetic stabilities of neotame anhydrate polymorphs A, D, F, and G, and to develop a quantitative method for analyzing polymorphic mixtures of A and G by powder X-ray diffractometry (PXRD).

METHODS

Based on the melting points, heats of fusion, and densities of the four polymorphs, thermodynamic rules were applied to study their thermodynamic relationships. The phase transition temperature of Forms A and G was estimated from their heats of solution and intrinsic dissolution rates (J) in 2-propanol. Using PXRD, a method for the quantitative analysis of polymorphic mixtures of Forms A and G was developed. Binary polymorphic mixtures of Forms A, D, F, or G were stored under zero relative humidity at 23 or 70 degrees C, and their compositions were monitored by PXRD.

RESULTS

The endothermic enthalpy of solution of A, D, F, and G follows the rank order: G (29.71 +/- 0.82 kJ/mol, n = 4) > A (28.48 +/- 0.51 kJ/mol, n = 4) > D (20.43 +/- 0.45 kJ/mol, n = 4) > F (18.77 +/- 0.31 kJ/mol, n = 4). The van't Hoff plots of ln(J) against 1/T for A and G show good linearity between 25 degrees C and 32 degrees C. At 23 degrees C polymorphic mixtures remain unchanged for 4 months. However, at 70 degrees C the phase transition is fast and the relative stability of the four polymorphs follows the rank order: G > D > F and G > A.

CONCLUSIONS

PXRD provides a reliable and accurate technique for the quantitative analysis of polymorphic mixtures of Forms A and G. Among the four polymorphs, A-G and A-D are enantiotropic pairs, whereas D-F, D-G, F-G are monotropic pairs. The phase transition temperature between A and G lies within the range 35-70 degrees C.

Inferring thermodynamic stability relationship of polymorphs from melting data

This study investigates the possibility of inferring the thermodynamic stability relationship of polymorphs from their melting data. Thermodynamic formulas are derived for calculating the Gibbs free energy difference (delta G) between two polymorphs and its temperature slope from mainly the temperatures and heats of melting. This information is then used to estimate delta G, thus relative stability, at other temperatures by extrapolation. Both linear and nonlinear extrapolations are considered. Extrapolating delta G to zero gives an estimation of the transition (or virtual transition) temperature, from which the presence of monotropy or enantiotropy is inferred. This procedure is analogous to the use of solubility data measured near the ambient temperature to estimate a transition point at higher temperature. For several systems examined, the two methods are in good agreement. The qualitative rule introduced this way for inferring the presence of monotropy or enantiotropy is approximately the same as The Heat of Fusion Rule introduced previously on a statistical mechanical basis. This method is applied to 96 pairs of polymorphs from the literature. In most cases, the result agrees with the previous determination. The deviation of the calculated transition temperatures from their previous values (n = 18) is 2% on average and 7% at maximum.