In Vitro Activities of Novel Des-Fluoro(6) Quinolone BMS-284756 against Mutants of Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus Selected with Different Quinolones

BMS-284756 (T-3811) is a novel quinolone that lacks a fluorine at the C-6 position. BMS-284756 has a broad spectrum of antibacterial activity (3, 7). The purpose of the present study was (i) to analyze the in vitro activity of the novel quinolone against mutants of Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus selected with marketed quinolones and (ii) to investigate whether this compound is a substrate for efflux pumps.

In order to analyze these therapeutically important aspects, we repeatedly exposed six clinical strains of each species to ciprofloxacin, gatifloxacin, moxifloxacin, and BMS-284756, as described previously for several other quinolones (1).

MICs were determined using microdilution methodology according to NCCLS guidelines (5). MIC determinations for BMS-284756 were conducted in the presence and absence of reserpine (20μg/ml; tests were repeated three times) for all of the original isolates (n = 18) as well as for all of the selected mutants (n = 72) (1, 2).

In addition, isolates were analyzed before and after transfers for mutations in the quinolone-resistance determining regions of parC or grlA and gyrA (4, 6, 8).

The MIC results from subculturing as well as the alterations in the quinolone resistance-determining regions are listed in Table 1,

TABLE 1.

Resistance selection results for six Streptococcus pneumoniae, six Streptococcus pyogenes, and six Staphylococcus aureus isolates^a

Selected drug (initial MIC [μg/ml])	Stepsb	MIC (μg/ml) upon retesting				Alteration(s) inc:
		Cipro	Gati	Moxi	BMS	GyrA	ParC or GrlA
S. pneumoniae
Cipro (0.25-1)	1-8	2-64	0.5-8	0.06-2	0.03-1	S81F	S79Y-D83G, S79Y, or —
Gati (0.06-0.25)	3-10	4-64	1-32	0.5-16	0.125-4	S81F or S81Y	S79Y, S79F, D83G
Moxi (0.03-0.125)	4-10	8-64	4-64	1-64	0.25-8	S81F	S79Y-D83N, S79F, or —
BMS (0.015-0.03)	3-8	4-64	1-64	0.5-32	0.125-4	S81F or —	S79Y, S79F, or —
S. pyogenes
Cipro (0.125-2)	2-7	2-32	1-4	0.5-4	0.06-1	S81F, E85K, or —	S79Y, S79A, D78N, or —
Gati (0.06-0.5)	2-6	2-16	2-4	1-4	0.125-1	S81F or —	S79Y, S79A, D78N, or —
Moxi (0.06-0.25)	3-8	2-32	1-32	1-16	0.25-2	E85K or —	S79Y, S79A, D78N, D84N, or —
BMS (0.004-0.06)	2-8	2-32	1-8	1-4	0.25-2	S81Y, E85G, or —	S79A, S79F, —
S. aureus
Cipro (0.125-0.25)	5-11	8-256	0.5-8	0.125-4	0.06-2	S84L	S80Y, E84K, or S80F-E84K
Gati (0.06)	4-11	4-128	1-128	0.25-32	0.06-4	S84L, S84L-E88K, or —	S80F, S80Y, or S80F-E84K
Moxi (0.06)	0-10	0.25-64	0.06-128	0.06-64	0.015-4	S84L, S84A, or —	S80F, E84K, S80F-E84K, or —
BMS (0.015-0.03)	3-9	4-128	0.5-64	0.25-32	0.125-8	S84L	S80Y, S80F, or E84K

^aCipro, ciprofloxacin; Gati, gatifloxacin; Moxi, moxifloxacin; BMS, BMS-284756.

^bFold increase in dilution steps between the MIC for the quinolone-resistant strain selected in the quinolone-containing medium and the MIC for the parental strain. For example, the MIC of ciprofloxacin for the parent strain was 0.25 μg/ml. After passages in ciprofloxacin-containing medium, the ciprofloxacin MIC had increased to 64 μg/ml, i.e., an eightfold increase in dilution steps.

^cAll parent strains had wild-type ParC or GrlA and GyrA enzyme. The ParE or GrlB and GyrA enzymes have not been analyzed. —, no alterations detectable.

BMS-284756 (range, 0.004 to 0.06 μg/ml) exhibited the best in vitro activities against all 18 original isolates, followed by moxifloxacin (range, 0.03 to 0.25 μg/ml) and gatifloxacin (range, 0.06 to 0.5 μg/ml). Ciprofloxacin (range, 0.125 to 2 μg/ml) showed the lowest in vitro activities. Furthermore, BMS-284756 (range, 0.015 to 8 μg/ml) exhibited the best in vitro activities against all 72 selected mutants, followed by moxifloxacin (range, 0.06 to 64 μg/ml), gatifloxacin (range, 0.06 to 128 μg/ml), and ciprofloxacin (range, 0.25 to 256 μg/ml) (Table 1).

None of the original 18 strains and none of the 72 selected mutants displayed lower MICs for BMS-284756 in the presence of reserpine. These results illustrate that the novel quinolone is not a good substrate for efflux pumps.

Based on the results presented, i.e., two- to ninefold increases in MICs after passages in BMS-284756-containing medium, the new quinolone seems to have the same potential for resistance development as that of the C-8 methoxy quinolones gatifloxacin and moxifloxacin. Alterations in ParC (S79Y or S79F) and GyrA (S81F) contributed to the resistance seen in S. pneumoniae mutants selected by BMS-284756. In Streptococcus pyogenes mutants, alterations in ParC (S79A and S79F) and GyrA (S81Y or E85G) were found. In addition, classical alterations in GrlA (S80Y, S80F, or E84K) and GyrA (S84L) contributed to the resistance in S. aureus mutants selected by BMS-284756. These classical alterations are identical to those observed in isolates selected by other quinolones (4, 6, 8).

In summary, the new quinolone BMS-84756 has good in vitro activity against wild-type isolates and selected mutants of Streptococcus pneumoniae, Streptococcus pyogenes, and Staphylococcus aureus. The novel quinolone is not a good substrate for efflux. BMS-84756 seems to have the same potential for resistance development as that of the C-8 methoxy quinolones gatifloxacin and moxifloxacin.