Collision-induced dissociation of the negative ions of simvastatin hydroxy acid and related species

Simvastatin hydroxy acid (1) is a well-known, potent HMG-CoA reductase inhibitor for the treatment of hypercholesterolemia. Its lactone, simvastatin (commercial name Zocor) (a prodrug of 1), has been widely prescribed in the USA and throughout the world. In this work, collision-induced dissociation (CID) of the negative ion of 1 (m/z 435), a carboxylic anion, was analyzed in detail. The major fragmentation pathway of this ion is a novel de-esterification to form the negative product ions at m/z 319 and 115. The ion at m/z 319 undergoes further collision-induced rearrangements to form the negative ions at m/z 215, 159 and 85. Possible mechanisms of the de-esterification are discussed in terms of both charge-initiated and charge-remote fragmentations. The de-esterification of the negative ion of 1 and the rearrangements of the ion at m/z 319 are rationalized by charge transfer and negative-charge initiated fragmentation. This study deepens our understanding of collision-induced fragmentations of carboxylic anions with multi-functional groups. A comparison of the CID data for the negative ions of 1 and 5 (a major oxidation degradate of 1) indicates that the analysis of the CID data for 1 can serve as a basis for identification of oxidation degradation products or metabolites of 1. The analysis of the CID data for the negative ion of 1 also reveals the fundamental characteristics of the CID data for the negative ions of other statin hydroxy acids such as lovastatin (3) and pravastatin (4).