In pharmaceutical research and development of drugs, discovering new drugs from a large number of potential alternatives (chemical compounds), which are theoretically as many as 10⁶⁰, is an important challenge. Here the new-drug discovery process was explored through a case study on Rozerem from Takeda Pharmaceutical Company Limited. Although the recent mainstream approach is to generate and test a large number of alternatives using automation technologies, Rozerem was discovered through exploration of a small number of alternatives with rational chemical compound design, based on researchers’ experience and knowledge.