Pharmacokinetics and distribution of minocycline in mature horses after oral administration of multiple doses and comparison with minimum inhibitory concentrations L. V. SCHNABEL, M. G. PAPICH † , T. J. DIVERS, C. ALTIER ‡ , M. S. APREA § , T. M. McCARREL and L. A. FORTIER* [Corrections added after online publication 10 October 2011] Department of Clinical Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA † Clinical Pharmacology Laboratory, College of Veterinary Medicine, North Carolina State University, Raleigh, North Carolina, USA ‡ Department of Population Medicine and Diagnostic Sciences, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA § New York State Veterinary Diagnostic Laboratory, College of Veterinary Medicine, Cornell University, Ithaca, New York, USA. *Correspondence email: laf4@cornell.edu; Received: 05.04.11; Accepted: 28.06.11 Summary Reasons for performing study: Minocycline holds great potential for use in horses not only for its antimicrobial effects but also for its anti-inflammatory and neuroprotective properties. However, there are no pharmacokinetic or safety data available regarding the use of oral minocycline in horses. Objectives: To determine pharmacokinetics, safety and penetration into plasma, synovial fluid, aqueous humour (AH) and cerebral spinal fluid (CSF) of minocycline after oral administration of multiple doses in horses and to determine the minimum inhibitory concentrations (MIC) of minocycline for equine pathogenic bacteria. Methods: Six horses received minocycline (4 mg/kg bwt q. 12 h for 5 doses). Thirty-three blood and 9 synovial fluid samples were collected over 96 h. Aqueous humour and CSF samples were collected 1 h after the final dose. Minocycline concentrations were measured using high pressure liquid chromatography. The MIC values of minocycline for equine bacterial isolates were determined. Results: At steady state, the mean  s.d. peak concentration of minocycline in the plasma was 0.67  0.26 mg/ml and the mean half-life was 11.48  3.23 h. The highest trough synovial fluid minocycline concentration was 0.33  0.12 mg/ml. The AH concentration of minocycline was 0.09  0.03 mg/ml in normal eyes and 0.11  0.04 mg/ml in blood aqueous barrier-disrupted eyes. The mean CSF concentration of minocycline was 0.38  0.09 mg/ml. The MIC values were determined for 301 isolates. Minocycline concentrations were above the MIC50 and MIC90 for many gram-positive equine pathogens. Potential relevance: This study supports the use of orally administered minocycline at a dose of 4 mg/kg bwt every 12 h for the treatment of nonocular infections caused by susceptible (MIC0.25 mg/ml) organisms in horses. Further studies are required to determine the dose that would be effective for the treatment of ocular infections. Keywords: horse; minocycline; pharmacokinetics; synovial fluid; cerebral spinal fluid; aqueous humour; minimum inhibitory concentrations Introduction Minocycline and doxycycline have been used extensively in human medicine as antimicrobial and therapeutic agents for degenerative diseases such as rheumatoid and osteoarthritis due to their ability to inhibit matrix metalloproteinases (MMPs) [1–3]. Minocycline also has reported neuroprotective effects due to its anti-apoptotic and anti-oxidant properties [4–6]. Compared with doxycycline, minocycline has a broader antimicrobial spectrum and a longer half-life in man [7–9]. Minocycline is more lipophilic and less protein bound than doxycycline, resulting in enhanced penetration into tissues and body fluids including aqueous humour (AH) and cerebral spinal fluid (CSF) [8–11]. Pharmacokinetic and safety data on minocycline in horses are limited to a single i.v. dose study [12], but there are abundant data on orally administered doxycycline, which is commonly used in horses to treat infections caused by Borrelia burgdorferi, Leptospira and Ehrlichia as well as many gram-positive organisms [13–15]. Consistent with human literature, the plasma protein binding of doxycycline is higher than that of minocycline (81.76  2.43% [15] vs. 68.1  2.6% [12], respectively). Higher protein binding may limit tissue distribution of doxycyline to a greater extent than minocycline. However, we cannot rule out factors other than protein binding that may produce differences in drug distribution to tissues. In previous studies, doxycycline had limited penetration into the AH and CSF and was not detected in either fluid when administered orally at a dose of 10 mg/kg bwt q. 12 h [13–15]. When administered orally at a dose of 20 mg/kg bwt q. 12 h, doxycycline was detected in the AH, but only at 10.9% of corresponding plasma concentrations and with serious gastrointestinal side effects in one horse [15]. Minocycline also has more potent anti-catabolic effects in cartilage and synovium than doxycycline, suggesting that minocycline may have a role in the treatment of equine joint disease [16]. For these reasons, minocycline holds great potential for use in horses. However, there are no pharmacokinetic or safety data regarding the use of oral minocycline in horses. The purposes of this study were to determine the pharmacokinetics, safety and penetration of minocycline into plasma, synovial fluid, AH and CSF after oral administration of 5 doses in horses and to determine the minimum inhibitory concentrations (MIC) of minocycline for equine pathogenic bacteria. Materials and methods Pilot study A pilot study was performed to determine the safety of minocycline when administered at the dose of 2 mg/kg bwt [9] and to determine the effects of diet (hay  grain) on absorption. Four healthy mature horses (one Thoroughbred, one Quarter Horse and 2 Warmbloods) were housed in stalls for 48 h prior to drug administration and maintained on grass hay and water ad libitum. Minocycline was administered once at 2 mg/kg bwt via nasogastric intubation and venous blood samples were collected at t = 0, 0.08, 0.17, 0.25, 0.5, 0.75, 1, 2, 4, 6, 8, 10, 12, 18 and 24 h after drug administration. After a 2 week washout period, the study was repeated on the same 4 horses but they were also fed 1.25 g/kg bwt of 12% protein pellets twice daily. Minocycline concentrations were analysed by high pressure liquid chromatography (HPLC). Peak plasma concentration (Cmax), time to peak concentration (Tmax), and area under the plasma concentration time curve (AUC) for hay vs. hay and grain feeding were compared. Results were similar for both conditions so horses were fed hay and grain in the subsequent multiple dose study. The mean  s.d. Cmax was 0.37  0.22 mg/ml for horses fed hay only, and 0.37  0.15 mg/ml for horses fed hay Equine Veterinary Journal ISSN 0425-1644 DOI: 10.1111/j.2042-3306.2011.00459.x 453 Equine Veterinary Journal 44 (2012) 453–458 © 2011 EVJ Ltd