Pharmacodynamics

Pharmacodynamic aspects of antimicrobials describe the interaction between the drug and the microorganisms. Pharmacodynamics describe the mode of action, the in vitro activity with regard to Minimal Iinhibitory Concentration (MIC) and Minimal Bactericidal Concentration (MBC), the time-kill kinetics of an antimicrobial, and the Post-Antibiotic Effect (PAE).

The mode of action of an antimicrobial substance influences the dosage and the dosage regimen.

Time-dependent bactericidal action:
The effectiveness of therapy with these drugs (e.g., beta-lactam antibiotics) is largely influenced by the aggregate time over which effective plasma concentrations (>MIC of microorganisms involved) are maintained. The dosage regimen must be chosen in a manner that maintains plasma concentrations above the MIC for most of the dosage interval, because the effect of these drugs is a combination of bacterial killing (principal activity) and sub-MIC PAE (minor contribution) that is shown in susceptible gram-positive bacteria.

Concentration-dependent bactericidal action:
The effectiveness of drugs that produce a concentration-dependent bactericidal effect (e.g., aminoglycosides and fluoroquinolones) is influenced both by the height of the Inhibitory Quotient (IQ = C_max:MIC ratio) and by the area under the inhibitory plasma concentration-time curve (AUIC). The latter might be of importance in gram-positive bacteria and mycoplasma, whilst the Cmax influences the activity against gram-negative bacteria.

The maximum activity appears to be achieved when the IQ (C_max:MIC ratio) is in the range of 5 - 10 (Prescott et al., 2000).

This was assessed and a specific product for single injection (Baytril® Max) was developed to address this principle.

In addition to rapid bactericidal activity, fluoroquinolones induce a PAE that inhibits regrowth of susceptible bacteria and mycoplasma even after complete removal of drug.

Bacteriostatic antimicrobial action:
The category of bacteriostatic drugs (e.g., tetracyclines, phenicols, sulfonamides, macrolides, lincosamides) essentially requires a dosage regimen that maintains plasma concentrations above MIC throughout the dosage interval. The success of therapy also depends strongly on the host's defense mechanisms.

Due to its rapid bactericidal action, low MIC values for the relevant pathogens and high AUIC values, Baytril allows successful therapy in a convenient dosage regimen of a single daily dose or a simple "one shot only“ treatment with Baytril® Max.