The antibacterial efficacy of
azlocillin against Pseudomonas aeruginosa was quantitatively assessed in short-term growth curves in vitro and in a short-term thigh muscle
infection in granulocytopenic mice.
Ticarcillin was used as the reference
drug. The antibacterial effect in vitro was expressed as the difference between the logarithms of the numbers of cfu in the presence and absence of
antibiotics (log ratio). The log ratio-time curves could be described by a concentration-dependent parameter, i.e., the inhibition constant (i). According to a standard parallel line bio-assay, based on values of i,
azlocillin was 3.80 times more active than
ticarcillin (coefficient of variation 27%).
Tobramycin in a concentration of 0.19 mg/l, which is not effective in itself, potentiated both drugs by
a factor of 3.85 (coefficient of variation 26%). In the in vivo model
ticarcillin showed a dose-dependent antibacterial effect, whereas the effect of
azlocillin was limited and not dose dependent. The effect of
tobramycin in a dose of 4 mg/kg was additive for both
penicillins. The results obtained in vivo were less favourable for
azlocillin than would be predicted on the basis of the in-vitro results.