The emergence of multidrug-resistant (MDR) Gram-negative bacteria has become a global problem. Among MDR Gram-negative bacteria, carbapenem-resistant Enterobacteriaceae (CRE), MDR Pseudomonas aeruginosa, and MDR Acinetobacter baumannii have limited treatment options and present serious threats. Therefore, strong countermeasures must be taken against these bacteria immediately. Accordingly, the focus of this review is on recent advances in the development of promising antibacterial agents against MDR Gram-negative bacteria. These agents include novel tetracyclines, polymyxins, β-lactams, β-lactam/βlactamase inhibitors, aminoglycosides, and peptide mimetics that have been recently approved or have shown promising results in clinical and preclinical development. This review summarizes these potent antibiotics in terms of their development status, mode of action, spectra of activity, and structure-activity relationship. Key words antibiotic; drug candidate; Gram-negative bacteria; carbapenem-resistant 2. Novel Tetracycline-class Antibiotics for MDR Gramnegative Bacteria Tetracycline antibiotics derived from natural products do not contain substituents at the C-7, C-8, and C-9 positions. However, the development of totally synthetic methods to synthesize tetracyclines has made it possible to modify these three positions, 21,22) providing novel and fully synthetic tetracyclines such as XERAVA and TP-6076, developed by Tetraphase Pharmaceuticals Inc. (Fig. 1). The introduction of an electron-withdrawing group at the C-7 position, and substitution at the C-9 position, significantly improved the antimicrobial activity of these synthetic tetracyclines. 23,24) XERAVA (eravacycline, TP-434) 25-27) was the first fully synthetic fluorocycline produced by Michael-Dieckmann reaction. 28) XERAVA was approved by the U.S. Food and Drug Administration (FDA) in August 27, 2018 for the treatment of