Tomopenem (formerly CS-023) is a 1β-methylcarbapenem with improved activity against diverse hospital pathogens, including Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus (MRSA), and has a half-life about twice longer than the half-lives of other carbapenems such as imipenem (IPM) and meropenem (MEM). In vitro activity of tomopenem is comparable to that of IPM against most isolates of Gram-positive pathogens and similar to that of MEM against Gram-negative pathogens. Furthermore, tomopenem displayed improved activity against not only P. aeruginosa but also MRSA compared to IPM and MEM. In addition to the improved activity, tomopenem showed a half-life about twice longer than that of IPM or MEM in humans. The affinity of tomopenem for penicillin-binding protein (PBP) 2a might be higher than that of IPM. Tomopenem had been in phase II clinical trial for the treatment of Gram-positive and Gram-negative bacterial infection. However, this development was discontinued.