This is sufficiently low that they are not highly recommended for CNS infections. Tetracycline and oxytetracycline took 60 minutes to reach ratios of 1.25 and 0.75, respectively. These values reflect the differences in ability of the drugs to cross membranes implied above. However, note that in all cases, the ratio approached 1 or more, a result not seen with such drugs as the beta-lactams or aminoglycosides. Tetracyclines typify the complexities of using Vd as an indicator of therapeutic concentrations in tissues.
Tetracyclines localize in bones, teeth, liver, speen, and tumors.
Because they are highly bound to these tissues and bone, they are non-homogeneously distributed outside the plasma. Volumes of distribution for animals are in the same range as for humans, but significant differences do occur. All tetracyclines are eliminated via renal and biliary pathways, but differ in their relative dependence on the two. All undergo significant enterohepatic circulation. Doxycycline and minocycline are primarily eliminated in the bile and less than a third is eliminated unchanged.