At physiological pH, tulathromycin (a weak base) is approximately 50 times more soluble in hydrophilic than lipophilic media. This solubility profile is consistent with the extracellular pathogen activity typically associated with the macrolides.1 Markedly higher tulathromycin concentrations are observed in the lung parenchyma as compared to the plasma, and these elevated concentrations can remain in lung tissue for several days beyond that which can be measured in the plasma. However the clinical relevance of these elevated lung concentrations is undetermined.
As a class, macrolides tend to be primarily bacteriostatic, but may be bactericidal against some pathogens. 2 When acting as a cidal compound, they tend to exhibit concentration independent killing; the rate of bacterial eradication does not change once serum drug concentrations reach 2 to 3 times the minimum inhibitory concentration (MIC) of the targeted pathogen. Under these conditions, the time that serum concentrations remain above the MIC becomes the major determinant of antimicrobial activity.
Macrolides also exhibit a post-antibiotic effect (PAE), the duration of which tends to be both drug and pathogen dependent. In general, by increasing the macrolide concentration and the exposure time, the PAE will increase to some maximal duration. 3 Tulathromycin is eliminated from the body primarily unchanged via biliary excretion.
1 Carbon, C. 1998. Pharmacodynamics of Macrolides, Azalides, and Streptogramins: Effect on Extracellular Pathogens. Clin. Infect. Dis., 27:28-32.
2 Nightingale, C.J. 1997. Pharmacokinetics and Pharmacodynamics of Newer Macrolides. Pediatr. Infect. Dis. J., 16:438-443.
3 Andes D, Anon J, Jacobs MR, Craig WA. (2004). Application of pharmacokinetics and pharmacodynamics to antimicrobial therapy of respiratory tract infections. Clin Lab Med., 24:477-502.
Swine
Following intramuscular (IM) administration to feeder pigs at a dosage of 2.5 mg/kg BW, tulathromycin is nearly completely absorbed, with peak plasma concentrations achieved within ~0.25 hr. The volume of distribution exceeds 15 L/kg, which is consistent with extensive tissue binding.
This large distribution volume results in a long terminal elimination half-life (60 to 90 hours) despite a rapid systemic free drug clearance (187 mL/kg/hr). There are no gender differences in swine tulathromycin pharmacokinetics.
Comparative Bioavailability Summary
Despite slightly lower peak concentrations with tulathromycin injection 25 mg/mL, a single IM dose of 2.5 mg tulathromycin/kg BW of either tulathromycin injection 100 mg/mL or tulathromycin injection 25 mg/mL resulted in comparable tulathromycin total systemic exposure.
Therefore, tulathromycin injection 25 mg/mL is to be considered therapeutically equivalent to tulathromycin injection 100 mg/mL when administered to swine by IM injection at a dose of 2.5 mg tulathromycin/kg BW.
Calves
Following subcutaneous (SC) administration into the neck of feeder calves at a dosage of 2.5 mg/kg BW, tulathromycin is nearly completely absorbed, with peak plasma concentrations achieved within ~0.25 hr. The volume of distribution exceeds 11 L/kg 4, which is consistent with extensive tissue binding. This large distribution volume results in a long terminal elimination half-life of more than 100 hours, despite a rapid systemic free drug clearance (170 mL/kg/hr). No pharmacokinetic differences are observed in castrated male versus female calves.
Comparative Bioavailability Summary
Despite slightly lower peak concentrations with tulathromycin injection 25 mg/mL, a single SC dose of 2.5 mg tulathromycin/kg BW of either tulathromycin injection 100 mg/mL or tulathromycin injection 25 mg/mL resulted in comparable total systematic tulathromycin exposure.
Therefore, tulathromycin injection 25 mg/mL is considered therapeutically equivalent to tulathromycin injection 100 mg/mL when administered to calves by SC injection at a dose of 2.5 mg tulathromycin/kg BW.
4 Clearance and volume estimates are based on intersubject comparisons of 2.5 mg/kg BW administered by either subcutaneous or intravenous injection.