Population pharmacokinetic modeling of sulfamethoxazole, sulfadiazine and sulfadimethoxine combined with trimethoprim in calves.

The standard 1:5 TMP:sulfonamide dose ratio rarely achieves synergistic plasma concentrations in calves.

This study characterized the parenteral pharmacokinetics (PK) of three trimethoprim/sulfonamide (TMP/S) combinations — TMP/sulfamethoxazole (SMX), TMP/sulfadiazine (SDZ), and TMP/sulfadimethoxine (SDMX) — in calves using population PK modeling. The current veterinary dosing ratio of TMP:S (1:5) is derived from human medicine and targets an in vivo free-concentration ratio of 1:19 for synergistic antibacterial activity. This study questioned whether this ratio is pharmacokinetically appropriate across different sulfonamides used in food-producing animals. Two-compartment models best described SMX, SDZ, and TMP data, while SDMX required a three-compartment model. TMP exhibited the highest clearance (1.41 L/h/kg) and shortest elimination half-life (1.39 h), contrasting sharply with SDMX, which had the lowest clearance (0.02 L/h/kg) and longest half-life (19.67 h). Monte Carlo simulations (n = 50,000 calves) revealed that at marketed doses, only 0.2% (TMP/SDZ), 11.6% (TMP/SMX), and 21% (TMP/SDMX) of simulated animals achieved target synergistic free-concentration ratios (1:10–1:50). These findings suggest that the conventional 1:5 dose ratio is inadequate for achieving the intended synergistic plasma ratios in calves, particularly for TMP/SDZ combinations. The authors emphasize the need for pharmacodynamic (PD) and PK/PD investigations to optimize dosing regimens and improve antimicrobial efficacy while supporting responsible antibiotic use in food-producing animals.

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