Limited sample stability of chenodeoxycholic acid in pharmaceutical quality control

The Amsterdam UMC makes chenodeoxycholic acid (CDCA) capsules to treat their patients with the rare metabolic disease cerebrotendinous xanthomatosis. During routine quality control analysis of the CDCA active pharmaceutical ingredient according to the European Pharmacopoeia (PhEur), an unknown impurity was found in the test results for related substances. After re-analysis using a fresh sample solution, the impurity was not detected, and it was hypothesised that the impurity was caused by sample instability. Sample stability was investigated using the PhEur high pressure liquid chromatography with refractive index detection (HPLC-RI) test for related substances on different CDCA sample solutions (dissolved in methanol R) stored for 0, 1, 3 and 7 days at 5°C and 25°C. An unknown impurity appeared when stored for 1 day and its concentration increased over time and at a higher temperature, conforming sample instability. It is recommended to perform the test on related substances of CDCA with a fresh sample solution.