Microwave-assisted extraction of felodipine tablets ☆

A microwave-assisted extraction (MAE) method was developed to determine felodipine and its degradation product H152/37 in tablets. These substances were present at different concentration levels, differing with a factor of ca 600. The choice of extraction solvent was found to be of great importance. The optimized solvent consisted of 5% methanol in acetonitrile. Methanol was capable of dissolving the outer covering layer. Acetonitrile made the inner matrix swell, fragmenting the tablet into small pieces releasing the analytes. Temperature was also of great importance and 80°C was needed to get a method with acceptable precision. At lower temperatures tablet cracking was incomplete for some units. Another advantage using a higher temperature was that shorter extraction times could be used. For instance, at 80°C, an extraction time of 10 min was sufficient. With the final method 99.0%, and 99.2% was extracted of felodipine and H152/37, respectively, compared to values obtained with a validated ultrasonication method. The developed method involves a minimum of manual operations, since whole tablets can be directly used for the extraction. The method can be used for single tablets as well as for ten tablets. This enables use of the method for determining individual tablet variation, while also giving an average value of the drug amount in a tablet batch.