An in vitro pharmacokinetic model was used to study the comparative antibacterial activities of multiple-dose regimens of enoxacin and netilmicin. Strains of Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, and Staphylococcus aureus were exposed to changing drug concentrations, mimicking human twocompartment pharmacokinetics. Oral administration was simulated for the quinolone, and intravenous administration was simulated for the aminoglycoside. Similar ratios of peak concentration to MIC resulted in similar changes in bacterial concentrations over time with both compounds. Following the initial dose, a rapid bactericidal effect occurred, with a >99% reduction of the bacterial counts within 4 h at peak concentrations more than three times the MIC. However, bacterial regrowth occurred within 24 h un,less the peak concentration/MIC ratio exceeded 8:1 (P < 0.01). For the regrowing bacteria, MICs were four-to eightfold higher, and little or no bactericidal effect occurred following the second and subsequent doses. These data demonstrate the equally potent bactericidal activity of orally administered enoxacin and intravenously administered netilmicin. Selection of resistant subpopulations was similar with each drug. The peak concentration/MIC ratio may be an important parameter in the clinical use of quinolone and aniinoglycoside antibiotics.

A two compartment in-vitro model was designed to simulate human pharmacokinetics and to expose bacterial cultures to changing drug concentrations, thereby avoiding limitations of conventional antibiotic testing at constant drug levels. Serially placed bacterial compartments, representing extravascular infection sites, interface with a central compartment through artificial capillaries. Drug concentrations within the culture chambers closely mimic interstitial concentrations in vivo. Simultaneous first order elimination kinetics of two drugs with different half-lives were simulated to study antibacterial effects of drug combinations. This in-vitro model is an efficient tool for optimal dosage regimen design and the study of synergistic/antagonistic effects of antibiotic combinations.

Several aminoglycoside dosage regimens were studied in a kinetic in vitro model. Pseudomonas aeruginosa, Escherichia coli, Klebsieila pneumoniae, and Staphylococcus aureus were exposed in serially placed artificial capillary units to netilmicin concentrations that changed based on human two-compartment pharmacokinetics. The same total dose per 24 h was administered as a continuous infusion (3.7 ,ug/ml) or in 1-h infusions given every 24 (24 ,.g/ml) or 8 h (8 ,Lg/ml). The once daily administration showed the best response in terms of either faster killing of E. coli, K. pneumoniae, and S. aureus or greater reduction of the inocula ofP. aeruginosa. After 28 h of treatment, however, all regimens reduced the nonpseudomonads by more than 99.99%, whereas all three P. aeruginosa strains regrew to >108 CFU/ml due to selection of resistant subpopulations. In contrast to the bactericidal effect of the first dose, no killing occurred after subsequent doses if the ratio of peak drug concentration to MIC was low (c6). These results support the concept of administering high doses of aminoglycosides once every 24 h.

The relative contribution of carbohydrate and lipid energy metabolism in liver tissue of temperature-acclimated striped bass was examined in vitro. Respirometry experiments were conducted to assess the role of various endogenous foodstuffs in providing reduced two-carbon fragments for aerobic metabolism. Liver composition was measured as a reflection of foodstuff flux and storage. Catabolism of 14C-labeled substrates to 14CO2 was monitored to estimate the tissue capacity for utilization of various classes of compounds. When measured at the temperature of acclimation, oxygen uptake (VO2) by liver slices shows near perfect compensation between 15 and 25 degrees C, whereas a 2.75-fold increase is found between 5 and 15 degrees C. Respiratory quotients (R.Q.) near unity are found at 5 degrees C and decrease to 0.85 and 0.82 at 15 and 25 degrees C, respectively. Inhibition of VO2 by iodoacetic acid, a glycolytic inhibitor, is near 60% at 5 degrees C and decreases to 30-40% at 15 and 25 degrees C. Evolution of 14CO2 from 14C-labeled glucoses and palmitate confirms a conservation of liver tissue capacity for carbohydrate utilization between acclimation temperatures of 5 and 15 degrees C, and an increasing capacity for utilization of fatty acids as temperature increases. Ratios of 14CO2 from 14C-6-and 14C-1-labeled glucoses indicate a relative increase in the participation of the pentose shunt in carbohydrate metabolism with cold acclimation. The results are consistent with an increasing reliance on carbohydrates for energy metabolism in the cold, whereas lipid substrates are utilized more at warm temperatures. These changes may be adaptive in partitioning energy reserves for seasonal activities of migration and reproduction.