Pharmacokinetics/pharmacodynamics analysis of trimethoprim and sulfamethoxazole for methicillin-resistant Staphylococcus aureus infection after intravenous and per os administration in Thoroughbred horses.
Authors: Kuroda Taisuke, Minamijima Yohei, Niwa Hidekazu, Mita Hiroshi, Nomura Motoi, Ohta Minoru
Journal: Journal of equine veterinary science
Summary
Methicillin-resistant *Staphylococcus aureus* infections in horses present a significant therapeutic challenge, yet dosing recommendations for trimethoprim/sulfamethoxazole (TMP-SMX) lack robust pharmacokinetic/pharmacodynamic evidence specific to equine patients. Kuroda and colleagues conducted a rigorous PK/PD analysis in six Thoroughbreds, measuring plasma concentrations following both intravenous (15 mg/kg) and oral (30 mg/kg) dosing, then modelled the data using nonlinear mixed-effects analysis to predict probability of target attainment across a simulated population of 5000 horses. The current approved dosing interval of every 12 hours failed to achieve the PK/PD target (free drug area-under-the-curve to MIC ratio ≥25 over 24 hours) for MRSA, with the IV and oral regimens achieving only 90% target attainment against a lower MIC of 0.125/2.38 mg/L and falling short entirely against the CLSI breakpoint used in human medicine (2.0/38 mg/L). These findings indicate that practitioners treating MRSA infections in horses may need to consider more frequent dosing intervals—every eight hours rather than every 12 hours—to reliably achieve adequate drug exposure, though this warrants clinical validation and careful consideration of practical administration constraints in equine practice.
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Practical Takeaways
- •Current approved trimethoprim/sulfamethoxazole dosing intervals (q12h) may be insufficient for treating MRSA infections in horses; consider q8h dosing when treating confirmed MRSA
- •Clinicians should perform susceptibility testing and consider MIC values when selecting antimicrobial dosing strategies for equine MRSA infections
- •Higher or more frequent dosing may be required to achieve adequate drug exposure compared to standard recommendations
Key Findings
- •Standard dosage regimens of trimethoprim/sulfamethoxazole at 15 mg/kg IV and 30 mg/kg PO administered every 12 hours failed to achieve therapeutic PK/PD targets for MRSA in horses
- •More frequent administration every 8 hours may be necessary to achieve adequate fAUC/MIC ratios for MRSA treatment
- •Standard dosage achieved only 90% probability of target attainment against MIC 0.125/2.38 mg/L and did not achieve targets against CLSI human breakpoint MIC of 2.0/38 mg/L