In vitro and in vivo antipseudomonal activity, acute toxicity, and mode of action of a newly synthesized fluoroquinolonyl ampicillin derivative WEN-PO LIN, DAR-DER JI, CHIA-YANG SHIAU, TSE-CHUN YANG, YUNG-WEN YANG, TAI-LI TSOU, SHANG-TAO TANG, CHI-HSING CHEN, and YU-TIEN LIU TAIPEI, TAIWAN, REPUBLIC OF CHINA Compounds N-(6,7-difluoroquinolonyl)-ampicillin (AU-1) and N-(6-fluoroquinolo- nyl)-ampicillin (FQ-1), synthesized by coupling of the carboxyl group of 6,7-difluo- roquinolone (FP-3) and 6-fluoroquinolone (FP4), respectively, with the -amino- group of ampicillin side chain, exhibit antipseudomonal activity similar to and lower acute toxicity than that of norfloxacin, whereas neither ampicillin nor the fluoro- quinolone moieties, compound FP-3 or FP4, alone have such activity. Also, AU-1 and FQ-1 are active against tested clinical isolates of Pseudomonas aeruginosa that are highly resistant to norfloxacin, gentamicin, or both. The therapeutic efficacies of FQ-1 and norfloxacin were assessed and compared in neutropenic mice infected with a 90% lethal dose of P aeruginosa. Mice intraperitoneally administered FQ-1 (10 mg/kg) 4, 8, 24, and 48 hours after infection had survival rates as high as 80%, comparable to those of mice treated with norfloxacin at the same dosage and dosing schedule. The study of protoplast formation revealed that FQ-1 did not inhibit cell-wall biosynthesis but did induce cell filamentation of Bacillus subtilis at a level close to its minimal inhibition concentration. Both AU-1 and FQ-1 were able to intercalate into the double-stranded DNA. However, that FQ-1 lost such activity after it was treated with penicillinase suggests that the lactam-ring structure in ampicillin moiety of FQ-1 was hydrolyzed by penicillinase and that the hydrolyzed structure of FQ-1 does not own DNA-intercalation activity. (J Lab Clin Med 2003;142:158-65) Abbreviations: AU-1 = N-(6,7-difluoroquinolonyl)-ampicillin; ccc DNA = covalently closed circular DNA; CFU = colony-forming unit; DMSO = dimethylsulfoxide; FP-3 = 6,7-difluoroquino- lonic acid; FP-4 = 6-fluoroquinolonic acid; FQ-1 = N-(6-fluoroquinolonyl)-ampicillin; LD 50 = lethal dose for 50% of population; LD 90 = lethal dose for 90% of population; MIC = minimal inhibition concentration; PBPs = penicillin-binding proteins; PBS = phosphate-buffered saline solution; TSGH = Tri-Service General Hospital. P seudomonas aeruginosa is a major problem as a multidrug-resistant nosocomial pathogen; espe- cially in burn patients and other immunocom- promised subjects in hospitals, 1 and most infections caused by this bacterium are associated with high rates of morbidity and mortality. 2-4 Ampicillin and fluoroquinolones are antibacterial agents with potent action against a broad spectrum of From the Institutes of Microbiology and Immunology, Preventive Medicine, and Medical Science, and the Section of Bacteriology, Division of Clinical Pathology, Tri-Service General Hospital, Na- tional Defense Medical Center. Supported by grants NSC 88-2314-B-016-051, DOD-89-16, and NSC 89-2320-B-016 from the National Science Council, ROC, and the Department of National Defense, ROC. Submitted for publication July 30, 2002; accepted May 21, 2003. Reprint requests: Yu-Tien Liu, PhD, Institute of Microbiology and Immunology, National Defense Medical Center, PO Box 90048-505, Neihu, Taipei 114, Taiwan, Republic of China; e-mail: ytliu@ndmctsgh.edu.tw. Copyright © 2003 by Mosby, Inc. All rights reserved. 0022-2143/2003/$30.00 + 0 doi:10.1016/S0022-2143(03)00112-4 158