A series of 1-cyclopropyl-8-methoxy-quinazoline-2,4-diones was synthesized and evaluated for lowering the ratio of the antimicrobial MIC in gyrase resistance mutants to that in the gyr ؉ (wild type) using isogenic strains of Escherichia coli. Dione features that lowered this ratio were a 3-amino group and C-7 ring structure (3-aminomethyl pyrrolidinyl < 3-aminopyrrolidinyl < diazobicyclo < 2-ethyl piperazinyl). The wild-type MIC was also lowered. With the most active derivative tested, many gyrA resistance mutant types were as susceptible as, or more susceptible than, wild-type cells. The most active 2,4-dione derivatives were also more active with two quinolone-resistant gyrB mutants than with wild-type cells. With respect to lethality, the most bacteriostatic 2,4-dione killed E. coli at a rate that was affected little by a gyrA resistance mutation, and it exhibited a rate of killing similar to its cognate fluoroquinolone at 10؋ the MIC. Population analysis with wild-type E. coli applied to agar showed that the mutant selection window for the most active 2,4-dione was narrower than that for the cognate fluoroquinolone or for ciprofloxacin. These data illustrate a new approach to guide early-stage antimicrobial selection. Use of antimutant activity (i.e., ratio of the antimicrobial MIC in a mutant strain to the antimicrobial MIC in a wild-type strain) as a structure-function selection criterion can be combined with traditional efforts aimed at lowering antimicrobial MICs against wild-type organisms to more effectively afford lead molecules with activity against both wild-type and mutant cells.Fluoroquinolones are lethal antibacterial agents that are widely used for many bacterial infections; with some diseases, such as multidrug-resistant tuberculosis, they are sometimes considered to be agents of last resort. However, fluoroquinolone use is threatened by an increasing prevalence of resistance, now seen with almost every bacterial species treated. Even highly susceptible species, such as Haemophilus influenzae, Neisseria gonorrhoeae, and Streptococcus agalactiae, are exhibiting quinolone resistance (11,21,35,36). A common strategy to bypass resistance is to seek new derivatives with increased ability to kill wild-type (susceptible) cells. Unfortunately, even highly lethal compounds can leave resistant mutants alive and able to amplify (13). As an alternative, we suggested that the choice of lead compounds in antibiotic discovery be guided toward those that have a very narrow mutant selection window, i.e., the MIC approximates the mutant prevention concentration (MPC), a measure of the mutant subpopulation MIC (5, 40, 41). With some gram-positive pathogens, particularly Streptococcus pneumoniae, this criterion has been approached using dual-targeted fluoroquinolones that have similar activities against both gyrase and DNA topoisomerase IV (8, 22-25, 30, 31). In this situation, the MIC of the less-susceptible target approximates the MPC, which creates a narrow window and restricts the recovery of resistant mutants...