Nowadays, the number of poorly water soluble drug candidates has increased tremendously. More than 40% of newly discovered drugs are poorly water soluble. It has to be kept in mind that up to now there is no universal science-based formulation design for low water soluble drugs [1]. Co-crystal formation is one of the methods for improving their solubility [2]. This presentation is about the formulation development of SDP-17, a co-crystal drug substance at Shionogi [3]. In the formulation research of this co-crystal, a series of formulation development challenges were found, such as ① crystal transition to a hydrate crystal with low solubility, ② increase in mutagenic impurities, and ③ crystal transition to a metastable form of the co-crystal. By ingenuity in the formulation design and process selection, all of these challenges were overcome and the co-crystal drug substance was successfully formulated. In this presentation, the history of the development of this formulation will be introduced along with some of the challenges and how to address with data.