Improving method capability of a drug substance HPLC assay.

The assay of a drug substance (DS) is one of the tests required to confirm the active pharmaceutical ingredient (API) quality at release. In the past, usually volumetric titration methods were performed, that were precise, but often non-specific. Nowadays specific chromatographic assay procedures are preferred. However, high performance liquid chromatographic (HPLC) methods, the way they are usually executed, tend to be less precise and have a larger total method variation compared to titration methods. The capabilities of fully validated titration and HPLC assay methods were determined and compared. It was studied which factors had the largest effects on the capability of chromatographic HPLC methods in order to improve their precision and precision-to-tolerance ratio. This was done using multiple Gage R&R (repeatability & reproducibility) studies and an experimental design approach. The investigations showed that it was feasible to define an HPLC method with a similar capability as the titration method. The most important factor determining the precision was demonstrated to be higher sample and reference material weights. When low weights are to be used, increasing the number of sample preparations and the number of reference solutions may enhance the method capability.