BAL9141, a novel extended-spectrum cephalosporin active against methicillin-resistant Staphylococcus aureus in treatment of experimental endocarditis

Abstract

The therapeutic efficacy of BAL9141 (formerly Ro 63-9141), a novel cephalosporin with broad in vitro activity that also has activity against methicillin-resistant Staphylococcus aureus (MRSA), was investigated in rats with experimental endocarditis. The test organisms were homogeneously methicillin-resistant S. aureus strain COL transformed with the penicillinase-encoding plasmid pI524 (COL Bla+) and homogeneously methicillin-resistant, penicillinase-producing isolate P8-Hom, selected by serial exposure of parent strain P8 to methicillin. The MICs of BAL9141 for these organisms (2 mg/liter) were low, and BAL9141was bactericidal in time-kill curve studies after 24 h of exposure to either two, four, or eight times the MIC. Rats with experimental endocarditis were treated in a three-arm study with a continuous infusion of BAL5788 (formerly Ro 65-5788), a carbamate prodrug of BAL9141, or with amoxicillin-clavulanate or vancomycin. The rats were administered BAL9141 to obtain steady-state target levels of 20, 10, and 5 mg of per liter or were administered either 1.2 g of amoxicillin-clavulanate (ratio 5:1) every 6 h or 1 g of vancomycin every 12 h at changing flow rates to simulate the pharmacokinetics produced in humans by intermittent intravenous treatment. Treatment was started 12 h after bacterial challenge and lasted for 3 days. BAL9141 was successful in the treatment of experimental endocarditis due to either MRSA isolate COL Bla+ or MRSA isolate P8-Hom at the three targeted steady-state concentrations and sterilized >90% of cardiac vegetations (P < 0.005 versus controls; P < 0.05 versus amoxicillin-clavulanate and vancomycin treatment groups). These promising in vivo results with BAL9141 correlated with the high affinity of the drug for PBP 2a and its stability to penicillinase hydrolysis observed in vitro.

FIG. 1.

Chemical structures of BAL9141 and its prodrug, BAL5788.

FIG. 2.

Population analysis profiles of homogeneously methicillin-resistant, penicillinase-producing strains COL Bla+ (A) and P8-Hom (B). Bacteria (>10⁸ CFU) were spread onto agar plates containing increasing concentrations of methicillin (closed triangles), amoxicillin-clavulanate (closed squares), or BAL9141 (closed circles). Points indicate the numbers of colonies growing on the plates after 48 h of incubation at 35°C.

FIG. 3.

Time-kill curves for the two penicillinase-producing MRSA strains used for the in vivo studies. The two strains were exposed to all three target steady-state concentrations of BAL9141 produced in the serum of rats (5, 10, and 20 mg/liter) or the peak concentrations of amoxicillin-clavulanate (90 mg of amoxicillin per liter and 20 mg of clavulanate per liter) or vancomycin (40 mg/liter) produced in humans and rats after the administration of therapeutic doses of the drugs. Each time-kill experiment was done on two separate occasions, with similar results on each occasion. Symbols: |, control; •, BAL9141 at 20 mg/liter; ⧫, BAL9141 at 10 mg/liter; ▴, BAL9141 at 5 mg/liter; ▪, vancomycin at 40 mg/liter; □, amoxicillin-calvulanate at 90 mg of amoxicillin per liter and 20 mg of clavulanate per liter.

FIG. 4.

In vitro degradation of BAL9141 (circles), amoxicillin-clavulanate (squares), and methicillin (triangles) exposed to large bacterial inocula (>10⁹ CFU) of strain COL Bla+ (penicillinase producer; closed symbols) or COL Bla− (penicillinase negative; open symbols).

FIG. 5.

Outcome of experimental endocarditis caused by homogeneously methicillin-resistant, penicillinase-producing S. aureus strains COL Bla+ (A) and P8-Hom (B) after 3 days of therapy. Each dot represents the bacterial density in the vegetation of a single animal. The closed dots represent the data for animals that died before the end of treatment. Panel A presents the results of two experiments pooled together. In one of them, the BAL9141 regimen with the highest concentration (20 mg/liter) was compared both to the control treatment and to the amoxicillin-clavulanate and vancomycin treatments. In the second experiment, the BAL9141 regimen with the highest concentration was compared to the control regimen and to the two lower-dose regimens. This explains the larger number of animals used as controls and the 20-mg/liter steady-state concentration in serum. P was <0.05 when the results were compared by Fisher’s exact test. Contr, control; AMC, amoxicillin-clavulanate; VAN, vancomycin; ND, not done.