A classification scheme for b-lactamases based on functional characteristics is presented. Three major groups of enzymes are defined by their substrate and inhibitor profiles: group 1 cephalosporinases that are not well inhibited by clavulanic acid; group 2 penicillinases, cephalosporinases, and broadspectrum b-lactamases that are generally inhibited by active site-directed b-lactamase inhibitors; and the group 3 metallob-lactamases that hydrolyze penicillins, cephalosporins, and carbapenems and that are poorly inhibited by almost all b-lactam-containing molecules. Functional characteristics have been correlated with molecular structure in a dendrogram for those enzymes with known amino acid sequences. b-Lactamases (EC 3.5.2.6) have been designated by the Nomenclature Committee of the International Union of Biochemistry as ‘‘enzymes hydrolysing amides, amidines and other CON bonds . . . separated on the basis of the substrate: . . . cyclic amides’’ (323). These enzymes are the major cause of bacterial resistance to b-lactam antibiotics and have been the subject of extensive microbiological, biochemical, and genetic investigations. Investigators have described more than 190 unique bacterial proteins with the ability to interact with the variety of b-lactam-containing molecules that can serve as sub-

A functional classification scheme for beta-lactamases and its correlation with molecular structure.

INTRODUCTION During the past 5 to 7 years, much attention has been given to the synthesis of new 4-quinolone-3-carboxylates and to the evaluation of these newly prepared agents for antibacterial activity, a revival of interest generated by the discovery of nalidixic and oxolinic acids mtny years ago. This renewed effort uncovered a. number of 6-fluoro-7piperazino-4-quinolones noteworthy for both the breadth and intensity of their activities against gram-negative bacilli and cocci in vitro and for the capacity to control experimentally induced systemic infections with selected bacteria when administered orally in well-tolerated doses (21). The most active representatives of this compound class, designated fluoroquinolones, include norfloxacin (AM-715, MK-0366), ofloxacin (DL-8280), ciprofloxacin (Bay o 9867), amifloxacin (WIN 49375), enoxacin (AT-2266, CI-919), and pefloxacin (1589-RB). The prim4ary target of nalidixic acid and probably also all the fluoroquinolones is DNA gyrase, an essential bacterial enzyme that maintains superhelical twists in DNA (13, 18, 24). The promise of the fluoroquinoloies led to their prompt evaluation for pharmacokinetics in human volunteers and cautious assessments of therapeutic potential, including tolerability in selected patients. The encouraging results of these early trials, with the burgeoning interest in further studies, have made it desirable to review the status of these agents as revealed in the published English literature. We have attempted this task in a two-part minireview. The first part here considers the structures, mechanisms of action and resistance, and spectra of activity of the six aforementioned fluoroquinolones with reference to nalidixic and oxolonic acids for comparison. The second part will consider pharmacology, clinical activity, and toxicities in humans (D. C. Hooper and J. S. Wolfson, Antimicrob. Agents Chemother., in press). Other quinolones, such as cinoxacin, pipemidic acid, rosoxacin, and flumequine, will not be discussed directly. Several reviews of cinoxacin have recently appeared (28, 63).

The fluoroquinolones: structures, mechanisms of action and resistance, and spectra of activity in vitro.

The fluoroquinolones, a new class of potent orally absorbed antimicrobial agents, are reviewed, considering structure, mechanisms of action and resistance, spectrum, variables affecting activity in vitro, pharmacokinetic properties, clinical efficacy, emergence of resistance, and tolerability. The primary bacterial target is the enzyme deoxyribonucleic acid gyrase. Bacterial resistance occurs by chromosomal mutations altering deoxyribonucleic acid gyrase and decreasing drug permeation. The drugs are bactericidal and potent in vitro against members of the family Enterobacteriaceae, Haemophilus spp., and Neisseria spp., have good activity against Pseudomonas aeruginosa and staphylococci, and (with several exceptions) are less potent against streptococci and have fair to poor activity against anaerobic species. Potency in vitro decreases in the presence of low pH, magnesium ions, or urine but is little affected by different media, increased inoculum, or serum. The effects of the drugs in combination with a beta-lactam or aminoglycoside are often additive, occasionally synergistic, and rarely antagonistic. The agents are orally absorbed, require at most twice-daily dosing, and achieve high concentrations in urine, feces, and kidney and good concentrations in lung, bone, prostate, and other tissues. The drugs are efficacious in treatment of a variety of bacterial infections, including uncomplicated and complicated urinary tract infections, bacterial gastroenteritis, and gonorrhea, and show promise for therapy of prostatitis, respiratory tract infections, osteomyelitis, and cutaneous infections, particularly when caused by aerobic gram-negative bacilli. Fluoroquinolones have also proved to be efficacious for prophylaxis against travelers9 diarrhea and infection with gram-negative bacilli in neutropenic patients. The drugs are effective in eliminating carriage of Neisseria meningitidis. Patient tolerability appears acceptable, with gastrointestinal or central nervous system toxicities occurring most commonly, but only rarely necessitating discontinuance of therapy. In 17 of 18 prospective, randomized, double-blind comparisons with another agent or placebo, fluoroquinolones were tolerated as well as or better than the comparison regimen. Bacterial resistance has been uncommonly documented but occurs, most notably with P. aeruginosa and Staphylococcus aureus and occasionally other species for which the therapeutic ratio is less favorable. Fluoroquinolones offer an efficacious, well-tolerated, and cost-effective alternative to parenteral therapies of selected infections.

Fluoroquinolone antimicrobial agents.

3.1. Chelation 564 3.2. Acid−Base Character 564 3.3. Photochemistry 565 4. In Vitro Antimicrobial Spectra 566 5. Structure−Activity Relationships 568 5.1. N-1 Ethyl Family 568 5.2. N-1 Cyclopropyl Family 568 5.3. N-1 to C-8 Bridged (Tricyclic) Family 568 5.4. N-1 Aryl Family 569 5.5. Positions C-2, C-3, and C-4 570 5.6. C-4a Substituted Analogues 571 5.7. C-5 Substituents 571 5.8. C-6 Substituents 571 5.9. C-7 Substituents 571 5.9.1. Piperazinyl and Related Moieties 572 5.9.2. Pyrrolidinyl and Related Moieties 572 5.9.3. Cyclobutylaminyl and Related Moieties 572 5.9.4. Bicycloaminyl Moieties 572 5.9.5. Carbon-Linked Substituents 572 5.10. Substituents at C-8 573 5.11. Resume of Structure−Activity Relationships of Quinolones 573

Bacterial topoisomerase inhibitors: quinolone and pyridone antibacterial agents.

The in vitro activity of Bay 09867, a new quinoline derivative, was compared with those of norfloxacin, nalidixic acid, ceftazidime, cefaclor, cefuroxime, gentamicin, and other antimicrobial agents, when appropriate, against 410 recent clinical isolates. The minimal inhibitory concentrations of Bay 09867 for 90% of Enterobacteriaceae, Pseudomonas aeruginosa, Haemophilus influenzae, Neisseria gonorrhoeae, streptococci, Staphylococcus aureus, and Bacteroides fragilis strains were between 0.008 and 2 micrograms/ml. Bay 09867 was considerably more active against the gram-negative bacteria tested than were other agents tested. Gentamicin-resistant Enterobacteriaceae, P. aeruginosa, and methicillin-resistant S. aureus were highly susceptible to Bay 09867. Strains less susceptible to nalidixic acid and norfloxacin tended to be less susceptible to Bay 09867. The protein binding of Bay 09867 was about 20%.

In vitro activity of Bay 09867, a new quinoline derivative, compared with those of other antimicrobial agents.

DL-8280, 9-fluoro-3-methyl-10-(4-methyl-1-piperazinyl)-7-oxo-2,3-dihydro-7H- pyrido-(1,2,3-de)1,4-benzoxazine-6-carboxylic acid, is a new nalidixic acid analog with a broad spectrum of antibacterial activity against gram-negative and gram-positive bacteria, including obligate anaerobes. The activity of DL-8280 against Enterobacteriaceae, Pseudomonas aeruginosa, Haemophilus influenzae, Neisseria gonorrhoeae, and Clostridium perfringens was roughly comparable to that of norfloxacin and far exceeded that of pipemidic acid and nalidixic acid. DL-8280 had greater activity against Staphylococcus spp., Streptococcus spp., Pseudomonas maltophilia, Acinetobacter spp., and Bacteroides fragilis than did norfloxacin, pipemidic acid, and nalidixic acid. Nalidixic acid-resistant Enterobacteriaceae, ampicillin-resistant gonococci, and clindamycin-resistant obligate anaerobes were also susceptible to DL-8280. The activity of DL-8280 was affected very little by inoculum size, and its action was bactericidal at two times the minimal inhibitory concentrations at most. Administered orally to mice experimentally infected with Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Proteus mirabilis, Serratia marcescens, or P. aeruginosa, DL-8280 was 2 to 7 times more effective than norfloxacin and 7 to more than 50 times more active than pipemidic acid.

In vitro and in vivo activity of DL-8280, a new oxazine derivative.

DR-3355, the S-(-)-isomer of ofloxacin, was generally twice as potent as ofloxacin against a variety of gram-positive and gram-negative pathogens, and its action was bactericidal. The compound was characterized by having the highest level of activity against staphylococci, Bacteroides fragilis, and Peptococcus spp. of the fluorinated quinolones tested, including ofloxacin, ciprofloxacin, fleroxacin, and NY-198. The activity of DR-3355 was not affected by different media, inoculum size, or human serum, but decreased under acidic conditions at pH 5.0 or in human urine.

In vitro activity of DR-3355, an optically active ofloxacin.

A highly penicillin-resistant strain of Bacteroides fragilis, strain GN11499, was found among 80 clinical isolates of the B.fragilis group and appears to produce a new type of penicillinase. Penicillinase activity was detected in crude extracts and had a specific activity of 0.25 U/mg of protein. About 20% of the enzyme was released into the surrounding medium during growth. The enzyme hydrolyzed ampicillin and cloxacillin more rapidly than it did penicillin G, carbenicillin, and cephaloridine. Relative rates in a crude extract with penicillin G as 100 were ampicillin, 357; carbenicillin, 57; cloxacillin, 271; and cephaloridine, 71. Enzyme activity was inhibited by clavulanic acid, CP-45, 899, N-formimidoyl thienamycin, cefoxitin, moxalactam, and p-chloromercuribenzoate. The enzyme had a molecular weight of approximately 41,500 and an isoelectric point of 6.9. Penicillinase production and tetracycline resistance were transferred from B.fragilis GN11499 to two susceptible strains of B.fragilis and Bacteroides vulgatus by filter mating.

Properties of a new penicillinase type produced by Bacteroides fragilis.

Preparation of monoclonal antibody to V antigen from Yersinia pestis.

DN-9550 [(6R, 7R)-7-[(Z)-2-(2-aminothiazol-4-yl)-2-(1H-imidazol-4-yl) methoxyiminoacetamido]-3-[(1-pyridinio)methyl]-8-oxo-5-thia -1-azabicyclo methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate hydrochloride] is a new semisynthetic cephalosporin with a broad spectrum of antibacterial activity against gram-positive and gram-negative bacteria. The activity of DN-9550 against most species of the family Enterobacteriaceae was roughly comparable to that of ceftazidime, slightly lower than that of cefotaxime, and far exceeded that of cefoperazone. Against Citrobacter freundii, Enterobacter cloacae, and Serratia marcescens, DN-9550 was more active than ceftazidime and cefotaxime. DN-9550 and ceftazidime were significantly more active than cefotaxime against Pseudomonas aeruginosa, but DN-9550 and cefotaxime were clearly more active than ceftazidime against staphylococci and streptococci. Haemophilus influenzae and Neisseria gonorrhoeae were also highly susceptible to DN-9550, but Bacteroides fragilis was generally not susceptible to the compound. DN-9550 was stable to various types of beta-lactamases and had high affinities for penicillin-binding protein 3 of both Escherichia coli and P. aeruginosa. When DN-9550 was administered subcutaneously to mice experimentally infected with Staphylococcus aureus, Streptococcus pyogenes, Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, or Pseudomonas aeruginosa, its efficacy well reflected its in vitro potency.

K Sato

Papers

In vitro and in vivo activity of DL-8280, a new oxazine derivative.

In vitro activity of DR-3355, an optically active ofloxacin.

Properties of a new penicillinase type produced by Bacteroides fragilis.

Preparation of monoclonal antibody to V antigen from Yersinia pestis.

In vitro and in vivo activities of DN-9550, a new broad-spectrum cephalosporin.