Characterisation and quantification of the polymorphic forms of stavudine
Strydom, Schalk Johannes
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Objective: Stavudine is a nucleoside reverse transcriptase inhibitor (NRTI) that is used in the treatment of human immunodeficiency virus (HIV) infections. Stavudine exhibits polymorphism and various polymorphic forms of stavudine are described in the literature, however the available information on these solid states, at the start and during this study, was limited. This study was conducted in order to (1) generate supplementary and/or possibly new information on the physicochemical properties of the various polymorphs of stavudine, (2) to possibly prepare and characterise a new polymorphic form of stavudine, (3) to determine and compare the dissolution behaviour of the stavudine polymorphs and (4) to investigate the possibility of applying analytical techniques to quantify the stavudine polymorphs in solid state mixtures. Methods: Various characterisation methods were used to determine the physicochemical properties of the polymorphic forms of stavudine, including X-ray powder diffraction (XRPD); variable temperature X-ray powder diffraction (VT-XRPD); diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS); differential scanning calorimetry (DSC); thermogravimetric analysis (TGA); polarising optical microscopy; hot-stage microscopy (HSM); scanning electron microscopy (SEM); as well as Karl Fischer (KF) analysis. The dissolution behaviour of the various polymorphic forms of stavudine, that were prepared during this study, was also determined, whilst quantitative XRPD and DRIFTS methods were developed for the quantitative study. Results: Polymorphic form I and form II of stavudine were prepared by recrystallisation of stavudine raw material from various solvents, whereas form Ill (hydrate) and the N-methyl-2-pyrrolidone (NMP) solvate of stavudine were recrystallised from water and NMP respectively. The results generated from the VT-XRPD analyses of form I and form II demonstrated that these solid states are monotropically related (supportive of the findings of Mirmehrabi et al. (2006:141)), and that form I and II do not interconvert to one another. The hydrate of stavudine was not observed to convert to polymorphic form I upon heating, as was determined by Gandhi et al. (2000:228). However, VT-XRPD analysis of form Ill and the NMP solvate showed that upon heating, both these pseudopolymorphs interconvert to form a polymorphic mixture consisting of form I and II. A glassy (amorphous) form of stavudine that was previously not described in the available literature was also prepared and characterised during this study. Dissolution testing of polymorphic form I, form II, the glassy (amorphous) stavudine and the form 1/11 mixture of stavudine revealed that a greater amount of the glassy stavudine dissolved within one minute compared to the other polymorphic forms. A comparison of the dissolution profiles, based on the requirements of the Medicines Control Council of South Africa, indicated that the profiles of form I and form II, form I and the glassy stavudine, and form I and the form 1/11 mixture are similar. Two different methods (based on the analytical techniques of XRPD and DRIFTS) were developed to quantify the amount of form I and form II of stavudine in solid state mixtures. Each method was validated, and the results indicated that the quantitative DRIFTS method showed the greatest agreement between the experimental and theoretical polymorphic content. Preferred orientation was assumed to be the reason for the deviation of the quantitative XRPD results, and it was suggested that this might be corrected by background subtraction, Ka2 stripping and smoothing of the X-ray diffraction peaks. A test sample with an unknown concentration was analysed using both methods, and the comparison between the XRPD and DRIFTS results revealed that the DRIFTS method might be more accurate when compared with the XRPD method. Conclusion: Stavudine exhibits polymorphism and this study confirmed that the physicochemical properties of the various polymorphs differ. A glassy (amorphous) form of stavudine was, according to available literature, prepared and characterised for the first time during this study. Two methods for quantifying the amount of form I and form II of stavudine in mixtures comprising these two polymorphs were successfully developed and tested. The DRIFTS method may have generated the more accurate results, since it shows the best correlation between the experimental and theoretical results.
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