NM394 is a new 6-fluoroquinolone antibacterial agent with a tricyclic structure which has a bridge that connects the N-1 and C-2 positions of the quinolone. The antibacterial activity of NM394 against clinical isolates of staphylococci, streptococci, enterococci, members of the family Enterobacteriaceae, and Pseudomonas aeruginosa was equal to or one-half that of ciprofloxacin. NM394 was as active as ofloxacin against gram-positive bacteria and was two to eight times more active against gram-negative bacteria, including P. aeruginosa. NM394 was two to eight times more active than enoxacin against gram-positive and gram-negative bacteria. The MICs of NM394 against Escherichia coil and P. aeruginosa at pH 5.5 were reduced 4 to 16 times compared with those at pH 7.0. Ciprofloxacin, ofloxacin, and enoxacin were 2 to 32 times less active against these two bacteria and Staphylococcus aureus at an acidic pH than they were at pH 7.0. In the presence of 5 mM Mg2+, the MICs of aUl of these drugs increased 2 to 32 times, but they were only slightly affected by 5 mM Ca2, type of medium, serum, or size of inoculum. NM394 showed potent bactericidal activity and inhibited the supercoiling activity of E. coli DNA gyrase. The in vitro antibacterial profile of NM394 is similar to that of other 6-fluoroquinolones.NM394 is a new 6-fluoroquinolone with the chemical (Fig. 1). It has broad antibacterial activity against gram-positive and gramnegative bacteria. 6-Fluoroquinolone derivatives such as norfloxacin (6), ofloxacin (15), enoxacin (9), and ciprofloxacin (17) have been reported to have significant antibacterial activities, but 6-fluoroquinolones with a substituent at the C-2 position have rarely been reported (2). We have tested quinolone derivatives with a substituted sulfur atom at the C-2 position and found 5-oxo-1,2-dihydro-5H-thiazolo[3,2-a]quinoline carboxylic acid derivatives to have potent antibacterial activity (8). We also found that 4-oxo-4H-[1,3]thiazeto[3,2-a]quinoline carboxylic acid derivatives have broader-spectrum and more potent antibacterial activities than those of 5-oxo-1,2-dihydro-5H-thiazolo[3,2-a]quinoline carboxylic acid derivatives, leading to the synthesis of NM394. In this study, we compared the in vitro antibacterial activity of NM394 with those of ciprofloxacin, ofloxacin, and enoxacin.(A part of this work was presented at the 29th Interscience Conference on Antimicrobial Agents and Chemotherapy [11].) MATERIALS AND METHODS Antimicrobial agents. NM394 was synthesized at the chemistry laboratories of Nippon Shinyaku Co., Ltd. The other drugs used in this study were ofloxacin (Daiichi Seiyaku Co., Ltd., Tokyo, Japan), ciprofloxacin (Bayer Yakuhin Co., Ltd., Osaka, Japan), enoxacin (Dainippon Seiyaku Co., Ltd., Osaka, Japan), and norfloxacin (Kyorin Seiyaku Co., Ltd. Tokyo, Japan).Bacterial strains. The bacterial strains used in this study were standard strains and clinical isolates that were obtained * Corresponding author. from 15 hospitals in Japan from 1986 to 1989 and that were maintained in the ...