Pharmacodynamic interaction between trimethoprim and sulphonamides for <i>A. pleuropneumoniae</i> : synergistic ratios, optimal potentiation, and dynamic responses assessed via time-kill curves

AIMS: Trimethoprim (TMP) and sulphonamides are widely used in combination to treat many conditions, including porcine respiratory infections caused by Actinobacillus pleuropneumoniae (APP), yet the pharmacodynamic basis of their synergy remains unclear. This study investigated the interactions between TMP and three sulphonamides (sulfamethoxazole, sulfadiazine, and sulfadimethoxine) against APP. METHODS AND RESULTS: Interactions were evaluated against three APP strains using MIC determination, static time-kill assays of individual drugs across five TMP: sulphonamide ratios (1:4, 1:19, 1:99, 1:499, 1:999), and semi-mechanistic pharmacodynamic modelling. Single-drug models showed greater TMP potency than sulphonamides (median EC50 0.018 µg mL-1 vs ∼3-10 µg mL-1). A General Pharmacodynamic Interaction model identified a unidirectional interaction in which TMP enhanced sulphonamide potency by reducing their EC50 at sub-MIC TMP concentrations (∼22-fold below the MIC), consistent with indirect antifolate action requiring folate depletion. Delayed sulphonamide effects and density-dependent growth limitation improved model performance. Maximal synergy, defined as the relative increase in bacterial killing rate, was 110-115% for all sulphonamides and occurred at a pharmacodynamic ratio of 1:250 (0.002 µg mL-1 TMP with 0.5 µg mL-1 sulphonamide). This represents a PD ratio and should not be confused with the classical 1:19 TMP: sulphonamide ratio reported for clinical effectiveness. Simulations using median porcine pharmacokinetics predicted rapid initial bacterial killing across intravenous and oral regimens despite pharmacokinetic ratio drift. However, sustained efficacy was regimen-dependent, with only the TMP-sulfadimethoxine combination achieving eradication, whereas others showed regrowth. CONCLUSIONS: These findings provide a mechanistic explanation of TMP-sulphonamide synergy against APP and highlight the value of semi-mechanistic PK/PD modelling for optimising veterinary combination therapy.