Antibiotic pharmacokinetics in newborns.

Non-metabolized polar antibiotics have a volume of distribution roughly equivalent to the extracellular fluid compartment and are cleared by the kidney. The volume of the ECF, therefore, affects the magnitude of Cmax when a constant dose is administered. Variations in glomerular filtration rate, or in the case of beta-lactams renal plasma flow, vary the rate at which the drug is cleared from the body. The pharmacokinetics of metabolizable antibiotics is complex; not only does prodrug pharmacokinetics affect the observed serum concentration of the active agent, but hepatic blood flow and biliary flow rate affect the rate at which such antibiotics (i.e., chloramphenicol) are removed from the serum. In these, general dosage guidelines are almost impossible in newborns; monitoring the serum concentrations is mandatory.