Unique Pharmacokinetic and Pharmacodynamic Parameters of Antimicrobials in Goats

Pharmacokinetics, the process that involves drug absorption, distribution, metabolism and excretion (ADME) of antimicrobials, determines pharmacodynamic response, that is, what drugs do to the body. Therefore, of all the pharmacokinetic parameters, elimination half-life (T 1 / 2 β ), volume of distribution (Vd), maximum plasma concentration (Cmax) and maximum time reached (Tmax) are the most important parameters. Hence, the parameters are unique in determining pharmacokinetic and pharmacodynamic response of antimicrobials. However, it is elimination half-life and minimum inhibitory concentration (MIC) that determine the dosing interval of antimicrobials. The dose range of 2.5 mg/kg for gentamicin passing through 4 mg/kg (ciprofloxacin), 4.2 mg/kg (ampicillin L/A), 5 mg/kg (kanamycin, enrofloxacin, gatifloxacin and norfloxacin), 7 mg/kg (mequindox), 10 mg/kg (amikacin, enrofloxacin, lincomycin, pefloxacin, cefpirome, erythromycin and isoniazid), 20 mg/kg (oxytetracycline) and 30 mg/kg (metronidazole) have elimination half-life of 1.2 – 67.2 h, Cmax of 0.12 – 54.4 μ g/ml, Tmax of 0.2 – 24 h, bioavailability of 16 – 99.8% and plasma protein binding of 0 – >80% when administered intramuscularly, intravenously and orally. Human equivalent dose formula could be used to extrapolate human-goat therapeutic doses of antimicrobials. How-ever, some antimicrobials such as sulfadimidine, tulathromycin, oxytetracycline and azithromycin may have high residues in the milk, kidneys, liver, intestines, brain and skeletal muscles and may portend high risk of antimicrobial resistance, hypersensitivity reaction, epidermal necrolysis, Stevens-Johnson syndrome and other adverse drug reactions.