Impurity profiling of siRNA by two-dimensional liquid chromatography-mass spectrometry with quinine carbamate anion-exchanger and ion-pair reversed-phase chromatography.

A short RNA with the sequence of the antisense strand of Patisiran has been selected as test material for the investigation of its common impurities using three different two-dimensional liquid chromatography (2D-LC) platforms. On the one hand, a quinine (QN) carbamate-based weak anion-exchange (AX) stationary phase (QN-AX) and a classical C18 reversed phase (RP) stationary phase in ion-pair (IP) mode with tripropylammonium acetate, respectively, have been used in the first dimension (D) to provide the selectivity for impurities formed during the synthesis of the RNA. In the next step, certain peaks of interest from D have been transferred by multiple-heart-cutting (MHC) into a D in which an ESI-MS-compatible non-ionpairing RP method has been used for desalting via a diverter valve to remove non-volatile phosphate buffer components and ion-pair agents, respectively. Thus, a sensitive electrospray-ionization quadrupole time of flight mass spectrometry (ESI-TOF-MS) analysis of resolved impurity peaks of the siRNA has become possible under MS-friendly conditions. With both 2D-LC setups, peak purity of the ON has been evaluated by selective comprehensive (high resolution) sampling of the main peak. In a third MHC 2D-LC approach, the QN-AX LC mode was online coupled with the IP-RPLC in the D using UV detection. It allows the separation of additional impurities which coeluted in the first dimension. The potential of these methods for comprehensive impurity profiling of ON therapeutics is illustrated and discussed.