Absorption

Macrolides are readily absorbed from the Gl tract if not inactivated by gastric acid. Erythromycin and tylosin may also be administered IV or IM.

Tilmicosin, gamithromycin, and tulathromycin are administered SC.

Absorption after injection is rapid, but pain and swelling can develop at the injection sites.

Distribution

Macrolides are widely distributed in tissues, and concentrations are about the same as in plasma, or even higher in some instances. Their accumulation important for the long dosing interval that characterizes some macrolides (eg, tilmicosin).

Spiramycin tissue concentrations remain high even when the plasma concentrations become lower.

Macrolides tend to concentrate in all tissues specially in the spleen, liver, kidneys, and particularly the lungs.

They concentrate in the bile and milk. Up to 75% of the dose.

Biotransformation

After administration PO, 80% of an erythromycin dose undergoes metabolic inactivation, whereas tylosin appears to be eliminated in an active form.

Excretion

Macrolide antibiotics and their metabolites are excreted mainly in bile (>60%) and often undergo enterohepatic cycling. Urinary clearance may be slow and variable (often <10%). The concentration of macrolides in milk often is several times greater than in plasma, especially in mastitis.