Fluoroquinolone MICs are increased through the acquisition of chromosomal mutations in the genes encoding gyrase (gyrA and gyrB) and topoisomerase IV (parC and parE), increased levels of the multidrug efflux pump AcrAB, and the plasmid-borne genes aac(6)-Ib-cr and the qnr variants in Escherichia coli. In the accompanying report, we found that ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs for fluoroquinolone-resistant E. coli clinical isolates were very high and widely varied (L. Becnel Boyd, M. J. Maynard, S. K. Morgan-Linnell, L. B. Horton, R. Sucgang, R. J. Hamill, J. Rojo Jimenez, J. Versalovic, D. Steffen, and L. Zechiedrich, Antimicrob. Agents Chemother. 53:229-234, 2009). Here, we sequenced gyrA, gyrB, parC, and parE; screened for aac(6)-Ib-cr and qnrA; and quantified AcrA levels in E. coli isolates for which patient sex, age, location, and site of infection were known. We found that (i) all fluoroquinolone-resistant isolates had gyrA mutations; (ii) ϳ85% of gyrA mutants also had parC mutations; (iii) the ciprofloxacin and norfloxacin MICs for isolates harboring aac(6)-Ib-cr (ϳ23%) were significantly higher, but the gatifloxacin and levofloxacin MICs were not; (iv) no isolate had qnrA; and (v) ϳ33% of the fluoroquinolone-resistant isolates had increased AcrA levels. Increased AcrA correlated with nonsusceptibility to the fluoroquinolones but did not correlate with nonsusceptibility to any other antimicrobial agents reported from hospital antibiograms. Known mechanisms accounted for the fluoroquinolone MICs of 50 to 70% of the isolates; the remaining included isolates for which the MICs were up to 1,500-fold higher than expected. Thus, additional, unknown fluoroquinolone resistance mechanisms must be present in some clinical isolates.Fluoroquinolones have become some of the most frequently prescribed antimicrobial agents worldwide. Fluoroquinolone MICs can be increased by mutations in the genes encoding the targets gyrase (gyrA and gyrB) and topoisomerase IV (parC and parE), by increased levels of the multidrug efflux pump AcrAB (12,27,29), and by the presence of plasmid-borne mechanisms QnrA, QnrB, QnrS, and Aac(6Ј)-Ib-cr (9, 23, 28). However, these mechanisms do not equally affect the MICs of all fluoroquinolones; in general, ciprofloxacin and norfloxacin MICs are affected by all of these mechanisms, and gatifloxacin and levofloxacin MICs are less affected or are not affected (14,23,26). In a companion study, we found tremendous variation among the MICs of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin, such that the four fluoroquinolones were affected to the same extent only in a small subset of isolates (ϳ5%) (4). In addition to the tremendous diversity in MICs of the four tested fluoroquinolones, we recently uncovered clear correlations between fluoroquinolone resistance in Escherichia coli isolates and patient age, patient sex, hospital location, and culture site (3). It is possible that these patient factors correlate with some of the MIC relationships or with some resis...

In a single quantitative study, we measured acrA, acrB, tolC, mdfA, and norE expression in Escherichia coli clinical isolates by using real-time PCR. acrA and acrB overexpression strongly correlated with fluoroquinolone and multidrug resistance; tolC, mdfA, and norE expression did not. The order of abundance of efflux pump transcripts in all fluoroquinolone-susceptible isolates was tolC (highest), then acrA and acrB, and then mdfA and norE. Our findings suggest acrAB overexpression is an indicator of multidrug resistance.

In defined, isogenic strains, at least three mutations, two of which must be in gyrA, were required to exceed the CLSI breakpoint for fluoroquinolone resistance. Strains with double mutations in both gyrA and parC had even higher MICs of fluoroquinolones than strains with totals of three mutations.

Fluoroquinolones are some of the most prescribed antibiotics in the United States. Previously, we and others showed that the fluoroquinolones exhibit a class effect with regard to the CLSI-established breakpoints for resistance, such that decreased susceptibility (i.e., an increased MIC) to one fluoroquinolone means a simultaneously decreased susceptibility to all. For defined strains, however, clear differences exist in the pharmacodynamic properties of each fluoroquinolone and the extent to which resistance-associated genotypes affect the MICs of each fluoroquinolone. In a pilot study of 920 clinical Escherichia coli isolates, we uncovered tremendous variation in norfloxacin MICs. The MICs for all of the fluoroquinolone-resistant isolates exceeded the resistance breakpoint, reaching 1,000 g/ml. Approximately 25% of the isolates (n ‫؍‬ 214), representing the full range of resistant norfloxacin MICs, were selected for the simultaneous determinations of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs. We found that (i) great MIC variation existed for all four fluoroquinolones, (ii) the ciprofloxacin and levofloxacin MICs of >90% of the fluoroquinolone-resistant isolates were higher than the resistance breakpoints, (iii) ciprofloxacin and levofloxacin MICs were distributed into two distinct groups, (iv) the MICs of two drug pairs (ciprofloxacin and norfloxacin by Kendall's Tau-b test and gatifloxacin and levofloxacin by paired t test) were similar with statistical significance but were different from each other, and (v) ϳ2% of isolates had unprecedented fluoroquinolone MIC relationships. Thus, although the fluoroquinolones can be considered equivalent with regard to clinical susceptibility or resistance, fluoroquinolone MICs differ dramatically for fluoroquinolone-resistant clinical isolates, likely because of differences in drug structure.Fluoroquinolones, some of the most frequently prescribed antimicrobial agents worldwide, target the bacterial type II topoisomerases gyrase and topoisomerase IV. Type II topoisomerases are essential, ubiquitous enzymes involved in virtually every aspect of DNA metabolism. These enzymes cleave one DNA double helix, pass a second DNA molecule (or a different region of the first DNA molecule) through the break, and religate the broken DNA. Fluoroquinolones increase the longevity of the normally short-lived cleaved DNA-topoisomerase intermediates (reviewed in reference 7). DNA tracking machinery somehow is affected by these intermediates, resulting in multiple subsequent effects, such as chromosome fragmentation, the inhibition of DNA synthesis, and death (reviewed in reference 6).With regard to susceptibility or resistance defined by CLSI breakpoints (Table 1), the fluoroquinolones appear to exemplify a class effect, such that any decrease in susceptibility (i.e., increased MIC) to one drug means a simultaneous decrease for all (2). The fluoroquinolones, however, vary with regard to pharmacokinetic and pharmacodynamic parameters, including potency (reviewed in refer...

BACKGROUND: EZN‐2208 is a water‐soluble, polyethylene glycol drug conjugate of SN38, which is the active moiety of irinotecan. In this study, the authors evaluated the tolerability, pharmacokinetics (PK), and activity of EZN‐2208 in adult patients with advanced solid tumors. METHODS: Patients in sequential cohorts (3 + 3 design) received intravenous EZN‐2208 at doses between 1.25 mg/m2 and 25 mg/m2 once every 21 days. RESULTS: Thirty‐nine patients received EZN‐2208. The median number of prior therapies was 2 (range, 0‐10 prior therapies). Seventeen patients received prior irinotecan. Two maximum tolerated doses (MTDs) were defined: EZN‐2208 with (16.5 mg/m2) and without (10 mg/m2) granulocyte‐colony–stimulating factor (G‐CSF). The dose‐limiting toxicity (DLT) was febrile neutropenia. Two of 19 patients who were heterozygous for a polymorphism in the uridine diphosphate glucuronosyltransferase 1 family, polypeptide A1 (UGT1A1) gene (UGT1A1*28) developed DLTs (dose, 25 mg/m2 with G‐CSF), and 2 patients who were homozygous for UGT1A1*28 were treated without DLTs (dose, 5 mg/m2). PK analysis indicated a mean terminal half‐life of 19.4 ± 3.4 hours. Sixteen patients (41%) achieved stable disease, including 6 of 39 patients (15%) who had stable disease that lasted ≥4 months. One patient with cholangiocarcinoma (no prior irinotecan) achieved a short‐lived 32% tumor regression. Among 6 patients who had stable disease that lasted for ≥4 months, 3 had received prior irinotecan, and 1 had KRAS‐positive colorectal cancer. CONCLUSIONS: EZN‐2208 was well tolerated and produced stable disease that lasted for ≥4 months/unconfirmed partial responses in 7 of 39 heavily pretreated patients (18%) with advanced solid tumors, including those who had failed prior irinotecan therapy. Cancer 2012. © 2012 American Cancer Society.