We used electrospray ionization and ion mobility spectrometry–mass spectrometry to detect and characterize the three anticancer drugs palbociclib, copanlisib, and olaparib. Ion mobility–mass spectrometry and density functional theory revealed that these compounds generate isomers during ionization (protomers) due to the presence of multiple protonation sites within their chemical structures. Our work has implications for understanding the solution- and gas-phase chemistry of these molecules during spray-based ionization processes.