Recent studies have shown the feasibility of differential dynamic programming (DDP) in optimizing Earth-centered solar-sail trajectories. In order to further demonstrate the ability of DDP in the optimization of solar-sail trajectories, this work investigates the performance of DDP for optimizing interplanetary solar-sail trajectories. The selected dynamical framework is based on the two-body problem, augmented with an ideal solar-sail force model. A superior numerical performance is obtained for the optimization algorithm by propagating the state in modified equinoctial elements and applying a Sundman transformation to change the independent variable from time to the true anomaly. The developed algorithm finds similar or more optimal solutions than locally optimal steering laws for the maximization of different orbital elements. In addition, constrained time-optimal Earth-Mars orbital transfers are investigated for different sail performance levels. The DDP algorithm is proven to be efficient and robust for different optimization settings and initial guesses for solar-sail trajectory optimization in the interplanetary regime.