Pharmacokinetics and distribution of minocycline in mature horses after oral administration of multiple doses and comparison with minimum inhibitory concentrations.
Authors: Schnabel L V, Papich M G, Divers T J, Altier C, Aprea M S, McCarrel T M, Fortier L A
Journal: Equine veterinary journal
Summary
# Editorial Summary: Oral Minocycline Pharmacokinetics in Horses Minocycline's appeal in equine practice extends beyond antimicrobial action to encompass anti-inflammatory and neuroprotective benefits, yet its oral use in horses has lacked foundational pharmacokinetic and safety data. Schnabel and colleagues administered 4 mg/kg minocycline orally every 12 hours for five doses to six horses, collecting plasma samples over 96 hours alongside synovial fluid, aqueous humour, and cerebrospinal fluid measurements to establish drug distribution and determine minimum inhibitory concentrations (MIC) against 301 equine bacterial isolates. At steady state, plasma peak concentrations reached 0.67 μg/ml with an 11.5-hour half-life, whilst synovial fluid achieved therapeutic levels (0.33 μg/ml at trough) and the drug demonstrated reasonable central nervous system penetration (0.38 μg/ml in cerebrospinal fluid); however, aqueous humour concentrations were subtherapeutic. Minocycline concentrations exceeded the MIC(50) and MIC(90) for susceptible gram-positive pathogens, supporting the 4 mg/kg q12h dosing schedule for systemic infections caused by organisms with MIC values ≤0.25 μg/ml. Practitioners should recognise that whilst this regimen appears effective for soft tissue and joint infections, additional research is needed to establish efficacy for ocular conditions where aqueous humour penetration remains inadequate.
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Practical Takeaways
- •Minocycline at 4 mg/kg orally twice daily is a viable antimicrobial option for equine nonocular infections caused by susceptible organisms, with the added benefit of anti-inflammatory and neuroprotective properties
- •The drug achieves adequate concentrations in synovial fluid and CSF, making it potentially useful for joint and neurological infections, though ocular use requires further investigation
- •Clinicians should culture and determine MIC values for bacterial isolates to ensure susceptibility (≤0.25 µg/ml) before relying on minocycline therapy, as this is the threshold for expected efficacy
Key Findings
- •Oral minocycline 4 mg/kg every 12 hours achieved steady-state peak plasma concentration of 0.67 ± 0.26 µg/ml with mean half-life of 11.48 ± 3.23 hours in horses
- •Minocycline penetrated synovial fluid (0.33 ± 0.12 µg/ml), CSF (0.38 ± 0.09 µg/ml), and aqueous humour (0.09-0.11 µg/ml), indicating good tissue distribution
- •Plasma concentrations exceeded MIC50 and MIC90 for many gram-positive equine pathogens, supporting clinical efficacy for susceptible organisms with MIC ≤ 0.25 µg/ml
- •MIC values were determined for 301 equine bacterial isolates to establish therapeutic targets