Antibiotics
Distribution
Improves (higher concentrations in cells, CNS, prostate) as lipophilicity increases.
- For tetracyclines (in order of increasing lipophilicity and Volume of distribution/tissue penetration):tetracycline, oxytetracycline << doxycydine < minocycline.
- For sulfonamides: behavior depends on pKa and surrounding conditions. Sulfisoxazole has a low pKa (= 5.0) so it is ionized in blood stream and has a low Vz. Sulfamethazine has a higher pKa (= 7.0) so it is less ionized and has a higher Vz.
Absorption
Tetracyclines are well absorbed orally, im, and sc. There are “sustained release” dose forms available. Sulfonamides a usually well absorbed orally though some forms are not absorbed well by design. Sulfonamides are usually well absorbed when given by im or sc routes.
