Pharmacokinetics of ceftriaxone after intravenous, intramuscular and subcutaneous administration to domestic cats

The pharmacokinetic properties of ceftriaxone, a third-generation cephalosporin, were investigated in five cats after single intravenous, intramuscular and subcutaneous administration at a dosage of 25 mg/kg. Ceftriaxone MICs for some gram-negative and positive strains isolated from clinical cases were determined. Efficacy predictor (t > MIC) was calculated. Serum ceftriaxone disposition was best fitted by a bicompartmental and a monocompartmental open models with first-order elimination after intravenous and intramuscular and subcutaneous dosing, respectively. After intravenous administration, distribution was fast (t₁/₂d 0.14 ± 0.02 h) and moderate as reflected by the volume of distribution (Vd₍ss₎) of 0.57 ± 0.22 L/kg. Furthermore, elimination was rapid with a plasma clearance of 0.37 ± 0.13 L/h·kg and a t₁/₂ of 1.73 ± 0.23 h. Peak serum concentration (Cmax), tmax and bioavailability for the intramuscular administration were 54.40 ± 12.92 μg/mL, 0.33 ± 0.07 h and 85.72 ± 14.74%, respectively; and for the subcutaneous route the same parameters were 42.35 ± 17.62 μg/mL, 1.27 ± 0.95 h and 118.28 ± 39.17%. Ceftriaxone MIC for gram-negative bacteria ranged from 0.0039 to >8 μg/mL and for gram-positive bacteria from 0.5 to 4 μg/mL. t > MIC was in the range 83.31-91.66% (10-12 h) of the recommended dosing interval (12 h) for Escherichia coli (MIC₉₀ = 0.2 μg/mL).