Interplanetary scientific missions pose several challenges for trajectory analysis because a direct transfer is often not possible due to the high delta-v. On the other hand, flybys and deep-space maneuvers allow a reduction of this delta-v at the cost of increased complexity in the trajectory. A well-known example of such an ambitious mission is the Cassini-Huygens robotic spacecraft. The study here presented focuses on the optimization of high-thrust interplanetary transfers with multiple flybys and deep-space maneuvers. A direct optimization method including black boxes is used to minimize the total delta-v in the presence of mission constraints. The complexity of these trajectories poses several challenges to the optimization process, due to the overall non-convexity and the significant number of variables and constraints involved.