Mecillinam is a β-lactam antibiotic.1 It is active against Gram-negative bacteria, including E. coli, K. pneumoniae, and N. gonorrhoeae (MIC90s = 4, 8, and 1 μg/ml, respectively). It is also active against clinical isolates of E. coli, K. pneumoniae, and P. mirabilis isolated from human urine.2 It binds to K. pneumoniae penicillin-binding protein 2 (PBP2) over PBP1A/B, PBP3, PBP4, and PBP5/6 (IC50s = <0.0075, >256, 128, >256, and >256 mg/L, respectively) in a competitive binding assay.3 Mecillinam is efficacious against susceptible E. coli strains in a systemic mouse model of infection with 50% protective dose (PD50) values ranging from less than or equal to 0.39 to 6.4 mg/kg.4

|1. Neu, H.C. Amdinocillin: A novel penicillin. Antibacterial activity, pharmacology and clinical use. Pharmacotherapy 5(1), 1-10 (1985).|2. Fuchs, F., and Hamprecht, A. Results from a prospective in vitro study on the mecillinam (amdinocillin) susceptibility of Enterobacterales. Antimicrob. Agents Chemother 63(4), e02402-02418 (2019).|3. Sutaria, D.S., Moya, B., Green, K.B., et al. First penicillin-binding protein occupancy patterns of β-lactams and β-lactamase inhibitors in Klebsiella pneumoniae. Antimicrob. Agents Chemother. 62(6), e00282-00218 (2018).|4. Anderson, J.D., Eftekhar, F., Cleeland, R., et al. Comparative activity of mecillinam and ampicillin singly and in combination in the urinary bladder model and experimental mouse model. J. Antimicrob. Chemother. 8(2), 121-131 (1981).