We evaluated the pharmacokinetics and therapeutic efficacy of ampicillin combined with sulbactam in a rabbit model of meningitis due to a 1-lactamase-producing strain of Escherichia coli K-1. Ceftriaxone was used as a comparison drug. The MIC and MBC were 32 and >64 ,ug/ml (ampicillin), >256 and >256 ,ug/ml (sulbactam), 2.0 and 4.0 jig/ml (ampicillin-sulbactam [2:1 ratio, ampicillin concentration]) and 0.125 and 0.25 ,ug/ml (ceftriaxone). All antibiotics were given by intravenous bolus injection in a number of dosing regimens. Ampicillin and sulbactam achieved high concentrations in cerebrospinal fluid (CSF) with higher dose regimens, but only tnoderate bactericidal activity compared with that of ceftriaxone was obtained. CSF bacterial titers were rVduced by 0.6 ± 0.3 log1o CFU/ml/h with the hihest ainpicillin-sulbactam dose used (500 and 500 mg/kg of body weight, two doses). This was'similar to the tivctericidal activity achieved by low-dose ceftriaxone (10 mg/kg), while a higher ceftriaxone dose (100 mg/kg) p,roduced a significant increase in bactericidal activity (1.1 0.4 log1o CFU/ml/h). It appears that ampicillin-sulbactam, despite favorable CSF pharmnacokinetics in animals with meningitis, may be of limited value in tlg treatment of difficult-to-treat ,l-lactamase-producing bacteria, against which the combination shows only moderate in vitro activity.Bacterial meningitis remains a serious disease associated with significant morbidity and mortality despite antibiotic and supportive therapy (10,15,25,28). The infecting organism is a major determinant of the prognosis, which is particularly poor in gram-negative-bacillus meningitis (1, 13, 23). The increasing rate of P-lactamase-producing gramnegative pathogens has prompted a search for effective agents to treat meningitis caused by these bacteria. Newer cephalosporins are generally highly effective in this regard (3,4,12,(19)(20)(21)30). As an alternative approach, the combination of a penicillin-derivative drug and an agent that inhibits the P-lactamase elaborated by the pathogen has attracted interest. Recently, we have examined the combination of piperacillin and tazobactam, a P-lactamase inhibitor, in experimental meningitis due to a ,-lactamase producing Escherichia coli strain (9). We found favorable pharmacokinetic properties for both compounds and showed that the addition of the P-lactamase inhibitor protected the bactericidal activity of piperacillin in cerebrospinal fluid (CSF). However, because of the relatively weak in vitro activity of piperacillin against the test strain, maximal CSF bactericidal activity of the combination was achieved only with extremely high drug concentrations.Sulbactam, a penicillanic acid sulfone, is an irreversible inhibitor of ,-lactamase that extends the spectrm of ampicillin to include strains of Staphylococcus dureus, Haemophilus sp., E. coli, and other P-lactamase-producing gram-negative bacteria (8, 16). Ampicillin-sulbactam is extensively used in clinical practice (11). In the present study, we examined ampicilli...