a Departamento de Farmacia, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, (5000) Ciudad Universitaria, Córdoba; b Hospital Tránsito Cáceres de Allende, Córdoba, Argentina
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Abstract |
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Introduction |
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In this study, we compared the in vitro antibacterial activity of several BSFQs with that of NSFQ-105 and ciprofloxacin (Figure 1) against medically significant Grampositive strains such as S. aureus, Streptococcus pneumoniae and Enterococcus faecalis because Gram-positive bacteria are rapidly becoming the most important pathogens in hospital infections,1 adding new data to our earlier study.4,6
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Materials and methods |
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Clinical isolates were collected and identified by standard techniques at the Hospital Tránsito Cáceres de Allende, Córdoba, Argentina, from 1996 to 2000.
Susceptibility tests
The in vitro activity was assayed by the agar dilution technique as recommended by the NCCLS,8 using Mueller Hinton agar (Merck Química Argentina SAIC, Buenos Aires, Argentina). NSFQ-105 and BSFQs IIII were synthesized in our laboratory as described previously.3,5,7 Ciprofloxacin was obtained from Amifarma (Madrid, Spain). The effect of pH on activity was determined against S. aureus ATCC 29213 by the macrodilution tube method with MuellerHinton broth as reported previously.4 The final inoculum size was 5 x 105 cfu/mL.
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Results and discussion |
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NSFQ-105 and BSFQ-I were the most active compounds tested. The MIC90s of both NSFQ-105 and BSFQ-I for ciprofloxacin-susceptible or intermediate S. aureus, S. epidermidis and E. faecalis were 0.015, 0.015 and 0.125 mg/L, respectively. These compounds were 16- to 64-fold more active than ciprofloxacin. Among the 46 S. aureus tested, 14 were methicillin resistant (MRSA). However, for both NSFQ-105 and BSFQ-I no change in MIC profiles was noticeable between the two populations.
MICs of all newly tested compounds were increased for ciprofloxacin-resistant staphylococci and enterococci. However, NSFQ-105 and BSFQ-I yielded MICs of 1 mg/L and
2 mg/L, against ciprofloxacin-resistant MRSA or methicillin-resistant S. epidermidis (MRSE) and E. faecalis, respectively.
The Table also includes 11 S. pneumoniae isolates; some of them exhibited intermediate susceptibility or low-level ciprofloxacin resistance (MIC range 24 mg/L). Nevertheless, no changes were observed in the MICs of NSFQ-105 and BSFQ-I, which remained as low as 0.03 and 0.125 mg/L, respectively. This behaviour is in agreement with published results regarding the activity of these derivatives against a panel of S. pneumoniae strains with defined resistance mutations.10
Although BSFQ-II was more active than NSFQ-105 and BSFQ-I against a few ciprofloxacin-resistant coagulase-negative staphylococci, in general its MICs were increased two- to 16-fold in comparison with those of NSFQ-105 and BSFQ-I. The MIC range of BSFQ-II for ciprofloxacin-resistant Gram-positive isolates was 0.12516 mg/L. However, BSFQ-II is more active than ciprofloxacin against Gram-positive bacteria.
BSFQ-III was less active than the other BSFQs against Staphylococcus spp. Nevertheless, its MICs were 0.25 mg/L for all quinolone-susceptible staphylococci, which means that BSFQ-III was two- to eight-fold more active than ciprofloxacin. However, BSFQ-III MICs rose to values similar to those of ciprofloxacin against quinolone-resistant isolates. Furthermore, BSFQ-III did not show good anti-pneumococcal activity.
The antibacterial activities of NSFQ-105 and other BSFQs against staphylococci were affected similarly by changes in the pH of MuellerHinton broth between 8.5 and 5.5. All BSFQs were four- to eight-fold more active at pH 5.5 than at pH 7.4. In addition, the activities were reduced by shifting the pH from 7.4 to 8.5 (data not shown). Previously, we had related such behaviour to the high proportion of the uncharged form of the NSFQs at pH 5.5.4,6 It is also documented that the activity of zwitterionic fluoroquinolones drops in acidic media.11 Hence, NSFQ-105 and BSFQ-I were 128-fold more active than ciprofloxacin at pH 5.5. This observation may have clinical relevance for the use of the new derivatives at sites of infection such as macrophages, abscesses or the urinary tract.
In order to compare the activity of the test compounds, log MICs of each were correlated with those of ciprofloxacin. High degrees of correlation between quinolone log MICs have been reported previously for other fluoroquinolones, with slopes of regression close to unity and regression coefficients usually >0.85.12Figure 2 shows a plot of BSFQ-I log MIC versus ciprofloxacin log MIC for 91 Staphylococcus spp., including MSSA, MRSA and coagulase-negative staphylococci. Such a plot reveals a bimodal distribution of the strains assayed; a linear regression analysis shows a slope of 1.05 with r2 = 0.89. Similar distributions and linear correlations were also observed for NSFQ-105, BSFQ-II and BSFQ-III. The same behaviour was observed for E. faecalis but not for pneumococci. These results indicate cross-resistance between new BSFQs and ciprofloxacin in S. aureus, S. epidermidis and E. faecalis, but not in pneumococci. Previously, we have shown that the primary target of NSFQ-105 and other BSFQs in pneumococci is DNA gyrase rather than topoisomerase IV (the primary target for ciprofloxacin in pneumococci).10 This observation, together with the present results, could indicate a different target affinity in pneumococci compared with the other bacteria.
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Acknowledgements |
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Notes |
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References |
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2
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7 . Nieto, M. J., Alovero, F. L., Manzo, R. H. & Mazzieri, M. R. (1999). A new class of fluoroquinolones: Benzenesulfonamidefluoroquinolones (BSFQs). Antibacterial activity and SAR studies. European Journal of Medicinal Chemistry 34, 20914.[ISI]
8 . National Committee for Clinical Laboratory Standards. (1995). Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria that Grow AerobicallyThird Edition: Approved Standard M7-A3. NCCLS, Villanova, PA.
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Alovero, F. L., Pan, X-S., Morris, J., Manzo, R. H. & Fisher, L. M. (2000). Engineering the specificity of antibacterial fluoroquinolones: Benzenesulfonamide modifications at C-7 of ciprofloxacin change its primary target in Streptococcus pneumoniae from topoisomerase IV to gyrase. Antimicrobial Agents and Chemotherapy 44, 3205.
11 . Smith, J. T. & Ratcliffe, N. T. (1986). Effect of pH and magnesium on the in vitro activity of ciprofloxacin. Excerpta Medica Current Clinical Practice Service 34, 126.
12 . Fass, R. J. (1997). In vitro activity of Bay12-8039, a new 8-methoxyquinolone. Antimicrobial Agents and Chemotherapy 41, 181824.[Abstract]
Received 16 January 2001; returned 24 April 2001; revised 25 June 2001; accepted 13 August 2001