Boron neutron capture therapy (BNCT) is expected to be a promising next-generation cancer treatment. In 2020, Japan, which has led the research on this treatment modality, was the first country in the world to approve BNCT. The boron agents that have been clinically applied in BNCT include a caged boron compound (mercaptoundecahydrododecaborate: BSH) and a boron-containing amino acid (p-boronophenylalanine: BPA). In particular, the BPA preparation Steboronine ® is the only approved drug for BNCT. However, the problem with BPA is that it is poorly retained in the tumor and has very low solubility in water. This cannot be overlooked for BNCT, which requires large amounts of boron in the tumor. The high dosage volume, together with low tumor retention, leads to reduced therapeutic efficacy and increased physical burden on the patient. In the case of BSH, its insufficient penetration into the tumor is problematic. Based on drug delivery system (DDS) technology, we have developed a next-generation boron pharmaceutical superior to Steboronine ® . Our approach involves the redevelopment of BPA using innovative ionic liquid formulation technology. Here, we describe previous boron agents and introduce our recent efforts in the development of boron compounds.