Pharmacokinetics of florfenicol in North American elk (Cervus elaphus)

Optimization of florfenicol dose against Piscirickettsia salmonis in Salmo salar through PK/PD studies

Salmonid Rickettsial Septicemia (SRS) is the disease of greatest economic importance in the Chilean salmon farming industry, causing high mortality in fish during the final stage of their productive cycle at sea. Since current, commercially available vaccines have not demonstrated the expected efficacy levels, antimicrobials, most commonly florfenicol, are still the main resource for the treatment and control of this pathogen. The aim of this study was to determine the most appropriate single dose of florfenicol, administered through medicated feed, for the treatment of Piscirickettsia salmonis (P. salmonis), using pharmacokinetic/pharmacodynamic (PK/PD) models. Previously, Minimum Inhibitory Concentrations (MICs) of florfenicol were determined for 87 P. salmonis isolates in order to define the epidemiological cut-off point (CO_WT). The most commonly observed MIC was 0.125 μg mL^-1 (83.7%). The CO_WT value was 0.25 μg mL^-1 with a standard deviation of 0.47 log₂ μg mL^-1 and 0.36 log₂ μg mL^-1, for Normalized resistance interpretation (NRI) method and ECOFFinder method, respectively. A MIC of 1 μg mL^-1 was considered the pharmacodynamic value (PD) to define PK/PD indices. Three doses of florfenicol were evaluated in fish farmed under controlled conditions. For each dose, 150 fish were used and blood plasma samples were collected at different time points (0–48 hours). PK parameters were obtained from curves representing plasma concentrations as a function of time. The results of Monte Carlo simulation indicate that at a dose of 20 mg/Kg l.w. of florfenicol, administered orally as medicated feed, there is 100% probability (PTA) of achieving the desired efficacy (AUC_0-24h/MIC>125). According to these results, we suggest that at the indicated dose, the PK/PD cut-off point for florfenicol versus P. salmonis could be 2 μg mL^-1 (PTA = 99%). In order to assess the indicated dose in Atlantic salmon, fish were inoculated with P. salmonis LF-89 strain and then treated with the optimized dose of florfenicol, 20 mg/Kg bw for 15 days.

Absorption and disposition of florfenicol after intravenous, intramuscular and subcutaneous dosing in alpacas

The objectives of this study were to define disposition and systemic availability of florfenicol in alpacas. Administration of 20 mg/kg doses to 8 male alpacas by i.v., i.m. and s.c. routes was performed by randomized, 3-way crossover design. Clearance and steady state volumes (Vdss) after i.v. injection were 5 ml/min/kg and 775 ml/kg respectively. Mean residence time (MRT) and terminal phase half-life (T1/2λz) were 2.8 h and 2 h respectively. Maximum serum concentrations (Cmax) after i.m. were higher than s.c. administration (p = 0.034). After s.c. dosing, T1/2λz and MRT were greater than after i.m. injection (p < 0.001; p = 0.006 respectively). Mean absorption time (MAT) after s.c. dosing was also prolonged (p = 0.006). Fractional absorption of florfenicol after i.m. and s.c. was not different (p > 0.05). Serum florfenicol concentrations remained >1.0 µg/ml for 20 h after i.m. dosing. Differences in rate and extent of florfenicol absorption after extravascular dosing could influence therapeutic outcomes.

Florfenicol pharmacokinetics in healthy adult alpacas after subcutaneous and intramuscular injection

A single dose of florfenicol (Nuflor(®)) was administered to eight healthy adult alpacas at 20 mg/kg intramuscular (i.m.) and 40 mg/kg subcutaneous (s.c.) using a randomized, cross-over design, and 28-day washout period. Subsequently, 40 mg/kg florfenicol was injected s.c. every other day for 10 doses to evaluate long-term effects. Maximum plasma florfenicol concentrations (C(max), measured via high-performance liquid chromatography) were achieved rapidly, leading to a higher C(max) of 4.31±3.03 μg/mL following administration of 20 mg/kg i.m. than 40 mg/kg s.c. (C(max): 1.95±0.94 μg/mL). Multiple s.c. dosing at 48 h intervals achieved a C(max) of 4.48±1.28 μg/mL at steady state. The area under the curve and terminal elimination half-lives were 51.83±11.72 μg/mL·h and 17.59±11.69 h after single 2 mg/kg i.m. dose, as well as 99.78±23.58 μg/mL·h and 99.67±59.89 h following 40 mg/kg injection of florfenicol s.c., respectively. Florfenicol decreased the following hematological parameters after repeated administration between weeks 0 and 3: total protein (6.38 vs. 5.61 g/dL, P<0.0001), globulin (2.76 vs. 2.16 g/dL, P<0.0003), albumin (3.61 vs. 3.48 g/dL, P=0.0038), white blood cell count (11.89 vs. 9.66×10(3)/μL, P<0.044), and hematocrit (27.25 vs. 24.88%, P<0.0349). Significant clinical illness was observed in one alpaca. The lowest effective dose of florfenicol should thus be used in alpacas and limited to treatment of highly susceptible pathogens.