Quantitation of polymorphic impurity in entecavir polymorphic mixtures using powder X-ray diffractometry and Raman spectroscopy.

Entecavir was used for the treatment of chronic hepatitis B through inhibiting hepatitis B virus. The anhydrous form of entecavir (ENT-A) often appeared as impurity polymorph in the manufacturing process of entecavir monohydrate (ENT-H) such as granulation, drying and compression. Since different crystal forms might affect drug bioavailability and therapeutic effect, it was vital to control the ENT-A content of the drug product. The work aimed to develop useful methods to assess ENT-A weight percentage in ENT-H. Powder X-ray diffractometry (PXRD) and Raman spectrometric methods were applied. Binary mixtures with different ratios of pure ENT-H and pure ENT-A were scanned using PXRD and Raman to obtain spectra. Then peak heights and peak areas versus weight percentage were used to construct calibration curves. The best linear regression analysis data for PXRD and Raman method were found to be R2 = 0.9923 and R2 = 0.9953, in the weight ratio range of 2.1-20.2% w/w% of ENT-A in binary mixtures. Limit of detection (LOD) of ENT-A was 0.38% and limit of quantitation (LOQ) was 1.15% for PXRD method. LOD and LOQ for Raman method were 0.48% and 1.16%. The results showed that PXRD and Raman methods: both were precise and accurate, and could be used for measurement of ENT-A content in the selected weight percentage range. Partial least squares (PLS) algorithm with four data pre-processing methods: including multiplicative scatter correlation (MSC), standard normal variate (SNV), first and second derivatives were applied and evaluated using prediction errors. The best performance of PLS was R2 = 0.9958 with RMSEC (0.44%) and RMSEP (0.65%). Multivariate analysis for Raman spectra showed similar good results with univariate analysis, and would be an advantageous method when there were overlapped peaks in the spectra. In summary, the proposed PXRD and Raman method could be developed for the quality control of ENT-H. And Raman was a more promising method in industrial practice due to its slightly better precision, accuracy and time-saving advantage.